METHOD FOR MANUFACTURING HIGH-STRENGTH AND DUCTILE TRIP STEEL

Abstract

A method for manufacturing a high-strength transformation induced plasticity steel is disclosed. The method may include cold rolling a low-alloy steel sheet, annealing the cold-rolled low-alloy steel sheet, cooling the annealed low-alloy steel sheet, applying an intercritical annealing to the low-alloy steel sheet, and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority from pending U.S. Provisional Patent Application Ser. No. 62/460,820, filed on Feb. 19, 2017, and entitled "HIGH STRENGTH AND DUCTILE THERMOMECHANICALLY-PROCESSED TRIP STEEL," which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to transformation induced plasticity (TRIP) steels and particularly relates to a method for manufacturing a high-strength and ductile TRIP steel.

BACKGROUND

[0003] Transformation induced plasticity (TRIP) steels have both high strength and high ductility. TRIP steels are particularly suitable for automotive structural and safety components due to their high energy absorption capacity and fatigue strength. Besides, unique microstructures and metallurgical properties of the TRIP steels make it possible to make the automobile steel structure lighter while, at the same time, ensuring safety during potential collisions.

[0004] TRIP steels may be considered as one of the leading types of advanced high-strength steels (AHSSs). TRIP steels have a microstructure consisting of ferrite, bainite, retained austenite, and sometimes martensite. The usual route for obtaining TRIP microstructures is by performing intercritical annealing (IA) followed by bainitic isothermal transformation (BIT) on a cold-rolled ferritic-pearlitic sheet. During deformation, the metastable retained austenite transforms to martensite, which is the basis of TRIP effect. This transformation, in turn, increases the local strain hardening rate and delays necking which results in enhanced strength-ductility combination compared to conventional steel grades. There is an ongoing demand for obtaining sheets with improved strength as well as excellent formability based on simpler steel compositions. There is, therefore, a need in both automotive and steel industries for new TRIP steels that possess higher strength and better ductility.

SUMMARY

[0005] This summary is intended to provide an overview of the subject matter of the present disclosure, and is not intended to identify essential elements or key elements of the subject matter, nor is it intended to be used to determine the scope of the claimed implementations. The proper scope of the present disclosure may be ascertained from the claims set forth below in view of the detailed description below and the drawings.

[0006] In an exemplary embodiment consistent with the present disclosure, a method for manufacturing a high-strength transformation induced plasticity steel is disclosed. The method may include cold rolling a low-alloy steel sheet to a thickness of 1 mm, annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a first predetermined amount of time between 45 and 90 minutes, air cooling the annealed low-alloy steel sheet to room temperature, applying an intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C., and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature.

[0007] According to some exemplary embodiments, cold rolling a low-alloy steel sheet to a thickness of 1 mm may include cold rolling a low-allow steel sheet including not more than 0.2 wt. % carbon, 1 wt. % manganese, 1.4 wt. % copper, 0.75 wt. % aluminum, and 1.34 wt. % silicon with the balance being iron and unavoidable impurities.

[0008] According to some exemplary embodiments, continually heating the annealed low-alloy steel sheet at 460.degree. C. for the second predetermined amount of time may include heating the annealed low-alloy steel sheet at 460.degree. C. for 6 minutes.

[0009] In an exemplary embodiment consistent with the present disclosure, a method for manufacturing a high-strength transformation induced plasticity steel is disclosed. The method may include cold rolling a low-alloy steel sheet, annealing the cold-rolled low-alloy steel sheet, cooling the annealed low-alloy steel sheet, applying an intercritical annealing to the low-alloy steel sheet , and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet.

[0010] According to an exemplary embodiment, annealing the cold-rolled low-alloy steel sheet may include annealing the cold-rolled low-alloy steel sheet at a temperature below an austenite region of the low-alloy steel sheet. According to another exemplary embodiment, annealing the cold-rolled low-alloy steel sheet includes annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C.

[0011] According to an exemplary embodiment, annealing the cold-rolled low-alloy steel sheet may include annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a first predetermined amount of time between 45 and 90 minutes.

[0012] According to some exemplary embodiments, applying an intercritical annealing to the low-alloy steel sheet may include applying the intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C. According to other exemplary embodiments, applying the intercritical annealing to the low-alloy steel sheet may include applying the intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C. for at least 6 minutes.

[0013] According to an exemplary embodiment, inducing the bainetic isothermal transformation in the annealed low-alloy steel sheet may include quenching the annealed low-alloy steel sheet to a temperature within the bainite nose, continually heating the annealed low-alloy steel sheet at the temperature within the bainite nose for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature.

[0014] According to an exemplary embodiment, inducing the bainetic isothermal transformation in the annealed low-alloy steel sheet may include quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for 6 minutes, and air cooling the low-alloy steel sheet to room temperature.

[0015] In an exemplary embodiment consistent with the present disclosure, a method for manufacturing a high-strength transformation induced plasticity steel is disclosed. The method may include hot rolling a low-alloy steel sheet at 1000.degree. C., air cooling the hot-rolled steel sheet to room temperature, cold rolling the low-alloy steel sheet to a thickness of 1 mm, annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a first predetermined amount of time between 45 and 90 minutes, air cooling the annealed low-alloy steel sheet to room temperature, applying an intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C., and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawing figures depict one or more implementations in accord with the present teachings, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements.

[0017] FIG. 1 illustrates a method for manufacturing transformation induced plasticity (TRIP) steels.

[0018] FIG. 2A illustrates a method for manufacturing high-strength TRIP steels, consistent with one or more exemplary embodiments of the present disclosure.

[0019] FIG. 2B illustrates a method for manufacturing high-strength TRIP steels, consistent with one or more exemplary embodiments of the present disclosure.

[0020] FIG. 3 illustrates a temperature versus time graph of an implementation of a method for manufacturing a high-strength TRIP steel, consistent with one or more exemplary embodiments of the present disclosure.

[0021] FIG. 4 illustrates a temperature versus time graph of an implementation of a method for manufacturing a conventional TRIP steel.

[0022] FIG. 5A is an optical micrograph of a conventional TRIP steel sample manufactured as described in detail in connection with EXAMPLE 3.

[0023] FIG. 5B is an optical micrograph of a conventional Cu-bearing TRIP steel sample manufactured as described in detail in connection with EXAMPLE 2.

[0024] FIG. 5C is an optical micrograph of a high-strength TRIP steel sample manufactured as described in detail in connection with EXAMPLE 1.

[0025] FIG. 6 shows engineering stress-strain curves for a high-strength TRIP steel, a conventional Cu-bearing TRIP steel, and a conventional TRIP steel, consistent with exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION

[0026] The following detailed description is presented to enable a person skilled in the art to make and use the methods and devices disclosed in exemplary embodiments of the present disclosure. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that these specific details are not required to practice the disclosed exemplary embodiments. Descriptions of specific exemplary embodiments are provided only as representative examples. Various modifications to the exemplary implementations will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other implementations and applications without departing from the scope of the present disclosure. The present disclosure is not intended to be limited to the implementations shown but is to be accorded the broadest possible scope consistent with the principles and features disclosed herein.

[0027] FIG. 1 illustrates a method 100 for manufacturing transformation induced plasticity (TRIP) steels according to conventional methods. Method 100 may include step 101 of cold rolling a steel sheet; step 102 of subjecting the cold-rolled steel sheet to an intercritical annealing; and step 103 of subjecting the annealed sheet to a bainitic isothermal transformation.

[0028] The method 100 may be utilized for obtaining TRIP microstructures in the steel sheet. The TRIP microstructures may consist of ferrite, bainite, retained austenite, and sometimes martensite. Referring to FIG. 1, in step 102, the intercritical annealing is carried out in the austenite region between the austenite formation starting temperature (Ac.sub.1) and the austenite formation ending temperature (Ac.sub.3). The intercritical annealing between Ac.sub.1 and Ac.sub.3 leads to incomplete austenitization. With respect to step 103, the annealed steel sheet may then be cooled down or otherwise quenched to the bainite nose temperature and held at the bainite nose temperature for bainite to form in the steel structure. In addition, holding the quenched steel sheet at the bainite nose temperature stabilizes the retained austenite in the steel structure. There is a need in the art for improving properties of TRIP steels including strength, ability to deal with a range of temperatures, flexibility, and lightness, among other metallurgical properties to meet the diverse functional requirements of modem industries.

[0029] FIG. 2A illustrates a method 200 for manufacturing high-strength TRIP steels, consistent with one or more exemplary embodiments of the present disclosure. The method 200 may include step 201 of cold rolling a low-alloyed steel sheet; step 202 of annealing the cold-rolled low-alloy steel sheet; step 203 of cooling the annealed low-alloy steel sheet; step 204 of applying an intercrtical annealing to the low-alloy steel sheet; and step 205 of inducing a bainitic isothermal transformation in the annealed sheet.

[0030] Referring to FIG. 2A, in an exemplary embodiment, step 201 may include introducing or inputting the low-alloyed steel sheet between rollers to compress and squeeze the low-alloy steel sheet and reduce its thickness. The amount of strain that is put on the low-alloy steel sheet may determine the hardness of the cold-rolled low-alloy steel sheet.

[0031] With respect to step 202, in an exemplary embodiment, the cold-rolled low-alloy steel sheet may be annealed at a temperature below the austenite region of the low-alloy steel for a first predetermined amount of time. In other words, the cold-rolled low-alloy steel sheet may be annealed at a temperature below Ac.sub.1 for a first predetermined amount of time. IN an exemplary embodiment, the first predetermined amount of time may be between 45 and 90 minutes. The cementite dissolution during step 201 of cold rolling the low-alloyed steel sheet and the precipitation of cementite during step 202 of annealing the cold-rolled low-alloy steel sheet may lead to the presence of spherical carbides in the steel structure. In step 202, an equiaxed ultrafine-grained microstructure may be developed that are similar to the microstructures obtained after tempering of a cold-rolled martensite.

[0032] Referring to FIG. 2A, after annealing the cold-rolled low-alloy steel, method 100 may then proceed to step 203, where the annealed low-alloy steel sheet may be air cooled to ambient temperature. With respect to step 204, in an exemplary embodiment, the low-alloy steel sheet may be annealed in the intercritical region or the austenite region of the low-alloy steel at a temperature between Ac.sub.1 and Ac.sub.3. The Ac.sub.1 and Ac.sub.3 temperatures may be determined by a dilatometry method, and then a temperature between Ac.sub.1 and Ac.sub.3 may be selected for carrying out the intercritical annealing.

[0033] With reference to FIG. 2A, after step 204 of applying the intercritical annealing to the low-alloy steel sheet, the method 100 may proceed to step 205 of inducing a bainitic isothermal transformation in the annealed sheet. In an exemplary embodiment, inducing the bainitic isothermal transformation in the annealed sheet may include quenching the annealed low-alloy steel sheet to a temperature within the bainite nose, continually heating the annealed low-alloy steel sheet at the temperature within the bainite nose for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature. In an exemplary embodiment, the second predetermined amount of time may be six minutes. In step 205, the retained austenite formed in the steel structure during step 204 may be stabilized, leading to bainite formation in the steel structure.

[0034] FIG. 2B illustrates a method 220 for manufacturing high-strength TRIP steels, consistent with one or more exemplary embodiments of the present disclosure. Method 220 may include a step 206 of hot rolling a low-alloyed steel sheet; a step 207 of air cooling the hot-rolled low-alloy steel sheet; a step 208 of cold rolling a low-alloyed steel sheet; a step 209 of annealing the cold-rolled low-alloy steel sheet; a step 210 of cooling the annealed low-alloy steel sheet; a step 211 of subjecting the low-alloy steel sheet to an intercritical annealing; and a step 212 of subjecting the annealed sheet to a bainitic isothermal transformation.

[0035] Referring to FIG. 2B, in an exemplary embodiment, step 206 of method 220 may include passing the low-alloy steel sheet or slab through pairs of rollers at a temperature above the recrystallization temperature of the low-alloy steel. Hot rolling the low-alloy steel sheet or slab may be carried out to reduce the thickness of the low-alloy steel sheet and to make the thickness of the low-alloy steel sheet uniform. With respect to step 207, in an exemplary embodiment, the hot-rolled low-alloy steel sheet may be air cooled to room temperature once its thickness is reduced in step 206.

[0036] With reference to FIGS. 2A and 2B, once the hot-rolled low-alloy steel sheet is cooled down to room temperature, method 220 may proceed to the subsequent steps 208 to 212 that may be similar to steps 201 to 205 of method 200.

[0037] As used herein, a low-alloy steel may have a carbon (C) content between 0.15 and 0.25 wt. % and other alloying elements. The Other alloying elements may include up to 2 wt. % manganese (Mn) and small quantities of copper (Cu), aluminum (Al), silicon (Si), and the like. According to an exemplary embodiment of the present disclosure, a low-alloy steel may include 0.05-0.2 wt % C, up to 2 wt. % Mn, and small quantities of Cu, Al, and Si with the remaining being iron and unavoidable impurities. According to other exemplary embodiments, the low-alloy steel may include at least one alloying component selected from the group consisting of Si, Mn, and Al.

EXAMPLE 1

[0038] In this example, a low-alloy steel with 0.2 wt. % C, 1 wt. % Mn, 1.4 wt. % Cu, 0.75 wt. % Al, and 1.34 wt. % Si was prepared as an ingot with a thickness of 15 mm. The low-alloy steel ingot was then used as a feedstock to manufacture a high-strength TRIP steel, according to one or more exemplary embodiments of the present disclosure.

[0039] FIG. 3 illustrates a temperature versus time graph 300 of an implementation of method 220 for manufacturing a high-strength TRIP steel, as was described in detail in connection with FIG. 2B. In an exemplary embodiment, Step 301 represents implementation of step 206 of method 220, step 302 represents implementation of step 207 of method 220, step 303 represents implementation of step 208 of method 220, step 304 represents implementation of step 209 of method 220, step 305 represents implementation of step 210 of method 220, step 306 represents implementation of step 211 of method 220, and step 307 represents implementation of step 212 of method 220. Referring to FIG. 3, the low-alloy steel ingot was hot rolled at a temperature of approximately 1000.degree. C., which is above the recrystallization temperature of the low-alloy steel (hot rolling step 301). Hot rolling the low-alloy steel ingot at 1000.degree. C. reduces the thickness of the low-alloy steel from 15 mm to approximately 7 mm. After a last rolling pass, the low-alloy steel sheet was air cooled to the room temperature (Air cooling step 302). Afterward, the low-alloy steel sheet was cold rolled down to a thickness of approximately 1 mm (cold rolling step 303). Then, a recrystallization annealing was applied to the cold-rolled low-alloy steel sheet at a temperature of approximately 650.degree. C. for a soaking time of approximately 1 hour (annealing step 304). The annealed low-alloy steel sheet was then air cooled to the room temperature (cooling step 305). An intercritical annealing was then applied to the low-alloy steel at a temperature between the lower and the upper boundaries of the low-alloy steel, which are referred to herein as Ac.sub.1 and Ac.sub.3, respectively. The Ac.sub.1 and Ac.sub.3 temperatures were determined by dilatometry. The Ac.sub.1 temperature was determined to be approximately 765.degree. C., and the Ac.sub.3 temperature was determined to be approximately 930.degree. C.

[0040] With reference to FIG. 3, to obtain a TRIP microstructure, the low-alloy steel sheet was annealed at 790.degree. C. for 6 minutes (intercritical annealing step 306). After that, a bainitic isothermal transformation was induced in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet in a salt bath to a temperature of approximately 460.degree. C., which is within the bainite nose, continually heating the annealed low-alloy steel sheet at 460.degree. C. for a soaking time of approximately 6 min, and air cooling the low-alloy steel sheet to room temperature (step 307). For ease of reference, the high-strength TRIP steel manufactured as was described in this example is referred to as "high-strength TRIP steel."

EXAMPLE 2

[0041] In this example, the low-alloy steel with 0.2 wt. % C, 1 wt. % Mn, 1.4 wt. % Cu, 0.75 wt. % Al, and 1.34 wt. % Si was prepared as an ingot with a thickness of 15 mm. The low-alloy steel ingot was then used as a feedstock to manufacture a conventional TRIP steel.

[0042] FIG. 4 illustrates a temperature versus time graph 400 of an implementation of method 220 without performing step 209 of annealing the cold-rolled low-alloy steel sheet for manufacturing a reference Cu-bearing TRIP steel. In an exemplary embodiment, Step 401 represents implementation of step 206 of method 220, step 402 represents implementation of step 207 of method 220, step 403 represents implementation of step 208 of method 220, step 404 represents implementation of step 211 of method 220, and step 405 represents implementation of step 212 of method 220. Referring to FIG. 4, the low-alloy steel ingot was hot rolled at a temperature of approximately 1000.degree. C. (hot rolling step 401). Hot rolling the low-alloy steel ingot at 1000.degree. C. reduces the thickness of the low-alloy steel from 15 mm to approximately 7 mm. After a last rolling pass, the low-alloy steel sheet was air cooled to the room temperature (Air cooling step 402). Afterward, the low-alloy steel sheet was cold rolled down to a thickness of approximately 1mm (cold rolling step 403). An intercritical annealing was then applied to the cold-rolled low-alloy steel at 790.degree. C. for 6 minutes (intercritical annealing step 404). After that, a bainitic isothermal transformation was induced in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet in a salt bath to a temperature of approximately 460.degree. C., which is within the bainite nose, continually heating the annealed low-alloy steel sheet at 460.degree. C. for a soaking time of approximately 6 min, and air cooling the low-alloy steel sheet to room temperature (step 405). For ease of reference, the high-strength TRIP steel manufactured as was described in this example is referred to as "reference Cu-bearing TRIP steel." which may have similar properties as a conventional TRIP steel and is manufactured herein for purposes of comparison with the high-strength TRIP steel manufactured as was described in EXAMPLE 1.

EXAMPLE 3

[0043] In this example, the low-alloy steel with 0.2 wt. % C, 1 wt. % Mn, 0.75 wt. % Al, and 1.34 wt. % Si was prepared as an ingot with a thickness of 15 mm. The low-alloy steel ingot was then used as a feedstock to manufacture a reference TRIP steel. It should be mentioned that the low-alloy steel ingot of this example does not include copper as an alloying element.

[0044] FIG. 4 illustrates a temperature versus time graph 400 of an implementation of method 210 for manufacturing a conventional TRIP steel. Referring to FIG. 4, the low-alloy steel ingot was hot rolled at a temperature of approximately 1000.degree. C. (hot rolling step 401). Hot rolling the low-alloy steel ingot at 1000.degree. C., reduces the thickness of the low-alloy steel from 15 mm to approximately 7 mm. After a last rolling pass, the low-alloy steel sheet was air cooled to the room temperature (Air cooling step 402). Afterward, the low-alloy steel sheet was cold rolled down to a thickness of approximately 1 mm (cold rolling step 403). An intercritical annealing was then applied to the cold-rolled low-alloy steel at 790.degree. C. for 6 minutes (intercritical annealing step 404). After that, a bainitic isothermal transformation was induced in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet in a salt bath to a temperature of approximately 460.degree. C., which is within the bainite nose, continually heating the annealed low-alloy steel sheet at 460.degree. C. for a soaking time of approximately 6 min, and air cooling the low-alloy steel sheet to room temperature (step 405). For ease of reference, the high-strength TRIP steel manufactured as was described in this example is referred to as "reference TRIP steel."

MICROSTRUCTURAL EVOLUTIONS

[0045] The microstructures of the high-strength TRIP steel sample, reference Cu-bearing TRIP steel sample, and reference TRIP steel sample were observed by an Olympus Vanox optical microscope and a Camscan MV2300 scanning electron microscope. The steel samples were first etched by a 2% Nital solution and LePera's reagent (1g Na.sub.2S.sub.2O.sub.5 in 100 ml H.sub.2O+4 g picric acid in 100 ml ethanol).

[0046] FIG. 5A is an optical micrograph 501 of the reference TRIP steel sample manufactured as described in detail in connection with EXAMPLE 3. FIG. 5B is an optical micrograph 502 of the reference Cu-bearing TRIP steel sample manufactured as described in detail in connection with EXAMPLE 2. FIG. 5C is an optical micrograph 503 of the high-strength TRIP steel sample manufactured as described in detail in connection with EXAMPLE 1. Referring to FIGS. 5A to 5C, the microstructures of the three steel samples contain bright, small, and dispersed islands. These islands are the retained austenite phase. The distribution of the retained austenite in the high-strength TRIP steel sample as shown in the optical micrograph 503 is more uniform in comparison with the distribution of the retained austenite in the reference Cu-bearing TRIP steel sample as shown in the optical micrograph 502 and the distribution of the retained austenite in the conventional TRIP steel sample as shown in the optical micrograph 501. The areas without the austenite islands are fewer in the high-strength TRIP steel sample as shown in the optical micrograph 503 in comparison with the reference Cu-bearing TRIP steel sample as shown in the optical micrograph 502 and the reference TRIP steel sample as shown in the optical micrograph 501.

[0047] The retained austenite phase governs the high strength as well as good ductility of TRIP steels by inhibition of necking as a result of the strain-induced martensitic transformation. A proper distribution of the retained austenite might have some advantageous effects on the mechanical properties and usefulness of TRIP steels, such as improving mechanical strength and ductility. With reference to FIGS. 5A to 5C, the high-strength TRIP steel that was annealed at 650.degree. C. for a soaking time of approximately 1 hour before the intercritical annealing shows a more uniform distribution of the retained austenite in comparison with the samples and may have better mechanical properties. As a result of this additional annealing step, the high-strength TRIP steel shows a tensile strength of approximately 1.2 GPa and a total elongation of approximately 37%, which falls within the range reported for the third generation advanced high-strength steels.

MECHANICAL PROPERTIES

[0048] Tensile test samples were prepared according to the JIS-Z-2201 standard with a gage length of 8 mm. The tensile tests were conducted at room temperature under an initial strain rate of 0.001 s.sup.-1.

[0049] FIG. 6 shows engineering stress-strain curves for the high-strength TRIP steel, reference Cu-bearing TRIP steel, and reference TRIP steel. The engineering stress-strain curve for the high-strength TRIP steel sample prepared as described in detail in connection with example 1 is designated by reference numeral 601; the engineering stress-strain curve for the conventional Cu-bearing TRIP steel sample prepared as described in detail in connection with example 2 is designated by reference numeral 602; and the engineering stress-strain curve for the reference TRIP steel sample prepared as described in detail in connection with example 3 is designated by reference numeral 603.

[0050] Referring to FIG. 6, it can be observed that the tensile properties of Cu-containing steel samples (the high-strength TRIP steel sample and reference Cu-bearing TRIP steel sample) are better than those of the reference TRIP steel that does not include copper. Moreover, it can be seen that the high-strength TRIP steel sample obtained from the cold rolled and annealed low-alloy steel sheet shows much better strength-ductility combination compared to the reference Cu-bearing TRIP steel sample obtained from the cold rolled low-alloy steel sheet. This may be related to the additional annealing step at 650.degree. C. for 1 hour, during which an equiaxed ultrafine-grained microstructure with average ferrite grain size of approximately 700 nm and dispersed cementite particles with average size of approximately 200 nm was developed. The presence of these spherical carbides may be related to the cementite dissolution during cold rolling and its subsequent precipitation during annealing. The mechanism of cementite dissolution can be explained based on the binding energy between carbon and iron atoms in cementite and the interaction energy between carbon atoms and dislocations in the ferrite. The former seems to be higher than the latter, and hence, carbon atoms leave cementite and spread in the ferrite. The obtained microstructure is similar to that obtained after tempering of cold rolled martensite, where the mechanisms responsible for the formation of the ultrafine-grained microstructure. After TRIP heat treatment, this resulted a better distribution of the retained austenite phase in the microstructure of the high-strength TRIP steel, as was discussed in more detail in connection with FIGS. 5A to 5C.

[0051] According to an exemplary embodiment, the present disclosure is directed to a method for manufacturing a high-strength TRIP steel. The method may include the following steps. The low-alloy steel ingot may be hot rolled at a temperature of approximately 1000.degree. C. After a last rolling pass, the low-alloy steel sheet may be air cooled to the room temperature. Afterward, the low-alloy steel sheet may be cold rolled down to a thickness of approximately 1 mm. The cold-rolled low-alloy steel sheet may then be subjected to a recrystallization annealing at a temperature between 625.degree. C. and 675.degree. C. for a soaking time between 45 and 90 minutes. The annealed low-alloy steel sheet may then be air cooled to the room temperature. The low-alloy steel may then be subjected to an intercritical annealing at a temperature between the lower and the upper boundaries of the low-alloy steel. After that the annealed low-alloy steel sheet was subjected to a bainitic isothermal transformation by quenching the annealed low-alloy steel sheet in a salt bath to a temperature within the bainite nose, continually heating the annealed low-alloy steel sheet at that temperature for a soaking time (the second predetermined amount of time) of approximately 6 min, and air cooling the low-alloy steel sheet to room temperature.

[0052] While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings.

[0053] Unless otherwise stated, all measurements, values, ratings, positions, magnitudes, sizes, and other specifications that are set forth in this specification, including in the claims that follow, are approximate, not exact. They are intended to have a reasonable range that is consistent with the functions to which they relate and with what is customary in the art to which they pertain.

[0054] The scope of protection is limited solely by the claims that now follow. That scope is intended and should be interpreted to be as broad as is consistent with the ordinary meaning of the language that is used in the claims when interpreted in light of this specification and the prosecution history that follows and to encompass all structural and functional equivalents. Notwithstanding, none of the claims are intended to embrace subject matter that fails to satisfy the requirement of Sections 101, 102, or 103 of the Patent Act, nor should they be interpreted in such a way. Any unintended embracement of such subject matter is hereby disclaimed.

[0055] Except as stated immediately above, nothing that has been stated or illustrated is intended or should be interpreted to cause a dedication of any component, step, feature, object, benefit, advantage, or equivalent to the public, regardless of whether it is or is not recited in the claims.

[0056] It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. Relational terms such as first and second and the like may be used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "a" or "an" does not, without further constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0057] The Abstract of the Disclosure is provided to allow the reader to ascertain the nature of the technical disclosure quickly. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various implementations. This is for purposes of streamlining the disclosure and is not to be interpreted as reflecting an intention that the claimed implementations require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed implementation. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

[0058] While various implementations have been described, the description is intended to be exemplary, rather than limiting, and it will be apparent to those of ordinary skill in the art that many more implementations and implementations are possible that are within the scope of the implementations. Although many possible combinations of features are shown in the accompanying figures and discussed in this detailed description, many other combinations of the disclosed features are possible. Any feature of any implementation may be used in combination with or substituted for any other feature or element in any other implementation unless specifically restricted. Therefore, it will be understood that any of the features shown and/or discussed in the present disclosure may be implemented together in any suitable combination. Accordingly, the implementations are not to be restricted except in light of the attached claims and their equivalents. Also, various modifications and changes may be made within the scope of the attached claims.

Claims

1. A method for manufacturing a high-strength transformation induced plasticity steel, the method comprising: cold rolling a low-alloy steel sheet to a thickness of 1 mm; annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a first predetermined amount of time between 45 and 90 minutes; air cooling the annealed low-alloy steel sheet to room temperature; applying intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C.; and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet by: quenching the annealed low-alloy steel sheet to 460.degree. C.; continually heating the annealed low-alloy steel sheet at 460.degree. C. for a second predetermined amount of time; and air cooling the annealed low-alloy steel sheet to room temperature.

2. The method according to claim 1, wherein cold rolling the low-alloy steel sheet to the thickness of 1 mm includes cold rolling the low-allow steel sheet including not more than 0.2 wt. % carbon, 1 wt. % manganese, 1.4 wt. % copper, 0.75 wt. % aluminum, and 1.34 wt. % silicon with the balance being iron and unavoidable impurities.

3. The method according to claim 1, wherein continually heating the annealed low-alloy steel sheet at 460.degree. C. for the second predetermined amount of time includes heating the annealed low-alloy steel sheet at 460 .degree. C. for 6 minutes.

4. A method for manufacturing a high-strength transformation induced plasticity steel, the method comprising: cold rolling a low-alloy steel sheet; annealing the cold-rolled low-alloy steel sheet; cooling the annealed low-alloy steel sheet; applying intercritical annealing on the low-alloy steel sheet; and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet.

5. The method according to claim 4, wherein annealing the cold-rolled low-alloy steel sheet includes annealing the cold-rolled low-alloy steel sheet at a temperature below an austenite region of the low-alloy steel sheet.

6. The method according to claim 4, wherein annealing the cold-rolled low-alloy steel sheet includes annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C.

7. The method according to claim 4, wherein annealing the cold-rolled low-alloy steel sheet includes annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a predetermined amount of time between 45 and 90 minutes.

8. The method according to claim 4, wherein applying the intercritical annealing to the low-alloy steel sheet includes applying the intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C.

9. The method according to claim 4, wherein applying the intercritical annealing to the low-alloy steel sheet includes applying the intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C. for at least 6 minutes.

10. The method according to claim 4, wherein inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet includes quenching the annealed low-alloy steel sheet to a temperature within the bainite nose, continually heating the annealed low-alloy steel sheet at a temperature within the bainite nose for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature.

11. The method according to claim 4, wherein inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet includes quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for 6 minutes, and air cooling the low-alloy steel sheet to room temperature.

12. The method according to claim 4, wherein cold rolling the low-alloy steel sheet includes cold rolling the low-allow steel sheet including not more than 0.2 wt. % carbon, 1 wt. % manganese, 1.4 wt. % copper, 0.75 wt. % aluminum, and 1.34 wt. % silicon with the balance being iron and unavoidable impurities.

13. The method according to claim 4, wherein cold rolling the low-alloy steel sheet includes cold rolling the low-alloy steel sheet to a thickness of 1 mm.

14. A method for manufacturing a high-strength transformation induced plasticity steel, the method comprising: hot rolling a low-alloy steel sheet at 1000.degree. C.; air cooling the hot-rolled steel sheet to room temperature; cold rolling the low-alloy steel sheet to a thickness of 1 mm; annealing the cold-rolled low-alloy steel sheet at a temperature between 625.degree. C. and 675.degree. C. for a first predetermined amount of time between 45 and 90 minutes; air cooling the annealed low-alloy steel sheet to room temperature; applying an intercritical annealing to the low-alloy steel sheet at a temperature between 765.degree. C. and 930.degree. C.; and inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet to by quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for a second predetermined amount of time, and air cooling the low-alloy steel sheet to room temperature.

15. The method according to claim 14, wherein hot rolling the low-alloy steel sheet includes hot rolling the low-allow steel sheet including not more than 0.2 wt. % carbon, 1 wt. % manganese, 1.4 wt. % copper, 0.75 wt. % aluminum, and 1.34 wt. % silicon with the balance being iron and unavoidable impurities.

16. The method according to claim 14, wherein applying the intercritical annealing to the low-alloy steel sheet includes applying the intercritical annealing to the low-alloy steel sheet for 6 minutes.

17. The method according to claim 14, wherein inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet includes inducing a bainitic isothermal transformation in the annealed low-alloy steel sheet by quenching the annealed low-alloy steel sheet to 460.degree. C., continually heating the annealed low-alloy steel sheet at 460.degree. C. for 6 minutes, and air cooling the low-alloy steel sheet to room temperature.