COLD ROLLED AND COATED STEEL SHEET AND A METHOD OF MANUFACTURING THEREOF

Abstract

A cold rolled and heat treated steel sheet having a composition having the following elements: 0.13%.ltoreq.Carbon.ltoreq.0.18%, 1.1%.ltoreq.Manganese.ltoreq.1.8%, 0.5%.ltoreq.Silicon.ltoreq.0.9%, 0.6%.ltoreq.Aluminum.ltoreq.1%, 0.002%.ltoreq.Phosphorus.ltoreq.0.02%, 0%.ltoreq.Sulfur.ltoreq.0.003%, 0%.ltoreq.Nitrogen.ltoreq.0.007% and can contain one or more of the following optional elements: 0.05%.ltoreq.Chromium.ltoreq.1%, 0.001%.ltoreq.Molybdenum.ltoreq.0.5%, 0.001%.ltoreq.Niobium.ltoreq.0.1%, 0.001%.ltoreq.Titanium.ltoreq.0.1%, 0.01%.ltoreq.Copper.ltoreq.2%, 0.01%.ltoreq.Nickel.ltoreq.3%, 0.0001%.ltoreq.Calcium.ltoreq.0.005%, 0%.ltoreq.Vanadium.ltoreq.0.1%, 0%.ltoreq.Boron.ltoreq.0.003%, 0%.ltoreq.Cerium.ltoreq.0.1%, 0%.ltoreq.Magnesium.ltoreq.0.010%, 0%.ltoreq.Zirconium.ltoreq.0.010%, the remainder composition being iron and unavoidable impurities caused by processing, the microstructure of the steel sheet being in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein the cumulated amounts of Residual Austenite and Ferrite is between 70% and 80%.

Description

[0001] The present invention relates to cold rolled and coated steel sheets suitable for use as steel sheet for automobiles.

BACKGROUND

[0002] Automotive parts are required to satisfy two inconsistent necessities: ease of forming and strength. However in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability to fit the criteria of ease of fit in the intricate automobile assembly and at the same time improve strength for vehicle crashworthiness and durability while reducing the weight of the vehicle to improve fuel efficiency.

[0003] Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in a car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.

[0004] Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for appreciation of the present invention:

[0005] US20140234657 is a patent application that describes a hot-dip galvanized steel sheet having a microstructure, by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of martensite and bainite, a residual structure contains one or two of ferrite, residual austenite of less than 8% by volume fraction, and pearlite of equal to or less than 10% by volume fraction. Further the steels sheet of US20140234657 reaches a tensile strength of 980 MPa but is unable to reach an elongation of 25%.

[0006] U.S. Pat. No. 8,657,969 describes a high strength galvanized steel sheet having a Tensile Strength of 590 MPa or more and excellent processability. The component composition contains, by mass %, C: 0.05% to 0.3%, Si: 0.7% to 2.7%, Mn: 0.5% to 2.8%, P: 0.1% or lower, S: 0.01% or lower, Al: 0.1% or lower, and N: 0.008% or lower, and the balance: Fe or inevitable impurities. The microstructure contains, in terms of area ratio, ferrite phases: 30% to 90%, bainite phases: 3% to 30%, and martensite phases: 5% to 40%, in which, among the martensite phases, martensite phases having an aspect ratio of 3 or more are present in a proportion of 30% or more.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a cold-rolled steel and coated sheets that simultaneously have:

[0008] an ultimate tensile strength greater than or equal to 600 MPa and preferably above 620 MPa, and

[0009] a total elongation greater than or equal to 31% and preferably above 33%.

[0010] In a preferred embodiment, the steel sheets according to the invention may also present a yield strength 320 MPa or more.

[0011] In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of 0.6 or more.

[0012] Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability.

[0013] The present invention provides a cold rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:

[0014] 0.13%.ltoreq.Carbon.ltoreq.0.18%

[0015] 1.1%.ltoreq.Manganese.ltoreq.1.8%

[0016] 0.5%.ltoreq.Silicon.ltoreq.0.9%

[0017] 0.6%.ltoreq.Aluminum.ltoreq.1%

[0018] 0.002%.ltoreq.Phosphorus.ltoreq.0.02%

[0019] 0%.ltoreq.Sulfur.ltoreq.0.003%.

[0020] 0%.ltoreq.Nitrogen.ltoreq.0.007%

[0021] and can contain one or more of the following optional elements

[0022] 0.05%.ltoreq.Chromium.ltoreq.1%

[0023] 0.001%.ltoreq.Molybdenum.ltoreq.0.5%

[0024] 0.001%.ltoreq.Niobium.ltoreq.0.1%

[0025] 0.001%.ltoreq.Titanium.ltoreq.0.1%

[0026] 0.01%.ltoreq.Copper.ltoreq.2%

[0027] 0.01%.ltoreq.Nickel.ltoreq.3%

[0028] 0.0001%.ltoreq.Calcium.ltoreq.0.005%

[0029] 0%.ltoreq.Vanadium.ltoreq.0.1%

[0030] 0%.ltoreq.Boron.ltoreq.0.003%

[0031] 0%.ltoreq.Cerium.ltoreq.0.1%

[0032] 0%.ltoreq.Magnesium.ltoreq.0.010%

[0033] 0%.ltoreq.Zirconium.ltoreq.0.010%

[0034] the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein the cumulated amounts of Residual Austenite and Ferrite is between 70% and 80%.

[0035] Another object of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.

[0036] The present invention thus also provides method of production of a cold rolled steel sheet comprising the following successive steps includes:

[0037] providing the steel composition above;

[0038] reheating said semi-finished product to a temperature between 1150.degree. C. and 1280.degree. C.;

[0039] rolling the said semi-finished product in the austenitic range wherein the hot rolling finishing temperature shall be between Ac1+50.degree. C. and Ac1+250.degree. C. to obtain a hot rolled steel sheet;

[0040] cooling the sheet at a cooling rate above 30.degree. C./s to a coiling temperature which is below 625.degree. C.; and coiling the said hot rolled sheet;

[0041] cooling the said hot rolled sheet to room temperature;

[0042] optionally performing scale removal process on said hot rolled steel sheet;

[0043] optionally annealing is performed on hot rolled steel sheet at temperature between 400.degree. C. and 750.degree. C.;

[0044] optionally performing scale removal process on said hot rolled steel sheet;

[0045] cold rolling the said hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;

[0046] then performing a annealing at soaking temperature between Ac1+30.degree. C. and Ac3 for a duration between 10 and 500 seconds by heating the said cold rolled steel sheet by a two step heating wherein:

[0047] in step one of heating, the cold rolled steel sheet is heated at a heating rate between 10.degree. C./s and 40.degree. C./s to a temperature range between 550.degree. C. and 650.degree. C.;

[0048] then in step two, the cold rolled steel sheet is heated at a heating rate between 1.degree. C./s and 5.degree. C./s from a temperature range between 550.degree. C. and 650.degree. C. to the annealing soaking temperature at which it is maintained,

[0049] then cooling the cold rolled steel sheet in a two step cooling wherein:

[0050] in step one of cooling, the cold rolled steel sheet is cooled at a cooling rate less 5.degree. C./s to temperature range between 600.degree. C. and 720.degree. C.

[0051] then in step two, the sheet is cooled at a cooling rate between 10.degree. C./s to 100.degree. C./s from a temperature range between 600.degree. C. and 720.degree. C. to an overaging temperature

[0052] then the said cold rolled steel sheet is overaged at a temperature range between 250.degree. C. and 470.degree. C. during 5 to 500 seconds and

[0053] then cooled to room temperature to obtain a cold rolled steel sheet.

DETAILED DESCRIPTION

[0054] The cold rolled and heat treated steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminium or aluminium alloys to improve its corrosion resistance.

[0055] Carbon is present in the steel between 0.13% and 0.18%. Carbon is an element necessary for increasing the strength of the steel sheet by producing low-temperature transformation phases such as bainite, further Carbon also plays a pivotal role in Austenite stabilization hence a necessary element for securing Residual Austenite. Therefore, Carbon plays two pivotal roles one in increasing the strength and another in retaining austenite to impart ductility. But Carbon content less than 0.13% will not be able to stabilize Austenite in an adequate amount required by the steel of the present invention. On the other hand, at a Carbon content exceeding 0.18%, the steel exhibits poor spot weldability which limits its application for the automotive parts.

[0056] Manganese content of the steel of the present invention is between 1.1% and 1.8%. This element is gammagenous. The purpose of adding Manganese is essentially to obtain a structure that contains Austenite and impart strength to the steel. An amount of at least 1.1% by weight of Manganese has been found to provide the strength and hardenability of the steel sheet as well as to stabilize Austenite. But when Manganese content is more than 1.8% it produces adverse effects such as retarding transformation of Austenite to Bainite during the over-aging holding for Bainite transformation. In addition Manganese content of above 1.8% also reduces the ductility and also deteriorates the weldability of the present steel hence elongation targets may not be achieved. A preferable content for the present invention may be kept between 1.2% and 1.8%, further more preferably 1.3% and 1.7%.

[0057] Silicon content of the steel of the present invention is between 0.5% and 0.9%. Silicon is a constituent that can retard the precipitation of carbides during overageing, therefore, due to the presence of Silicon, carbon rich Austenite is stabilized at room temperature. Further, due to poor solubility of Silicon in carbide it effectively inhibits or retards the formation of carbides, hence also promotes the formation of Bainitic structure which is sought as per the present invention to impart steel with its essential features. However, disproportionate content of Silicon does not produce the mentioned effect and leads to a problem such as temper embrittlement. Therefore, the concentration is controlled within an upper limit of 0.9%. A preferable content for the present invention may be kept between 0.6% and 0.8%

[0058] Aluminum is an essential element and is present in the steel between 0.6% and 1%. Aluminum is an alphagenous element and imparts total elongation to the steel of the present invention. A minimum of 0.6% of Aluminum is required to have a minimum Ferrite thereby imparting the elongation to the steel of the present invention. Aluminum is also used for removing oxygen from the molten state of the steel to clean steel of the present invention by and it also prevents oxygen from forming a gas phase. But whenever the Aluminum is more than 1% it forms AlN which is detrimental for the steel of the present invention therefore preferable range for the presence of the Aluminum is between 0.6% and 0.8%.

[0059] Phosphorus constituent of the steel of the present invention is between 0.002% and 0.02%. Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02% and preferably lower than 0.014%.

[0060] Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulfur content is preferably as low as possible, but is 0.003% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the steel of the present invention.

[0061] Nitrogen is limited to 0.007% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel.

[0062] Chromium is an optional element for the present invention. Chromium content may be present in the steel of the present invention is between 0.05% and 1%. Chromium is an essential element that provides strength and hardening to the steel but when used above 1% it impairs surface finish of steel. Further Chromium contents under 1% coarsen the dispersion pattern of carbide in Bainitic structures, hence; keep the density of carbides low in Bainite.

[0063] Molybdenum is an optional element that constitutes 0.001% to 0.5% of the Steel of the present invention; Molybdenum plays an effective role in determining hardenability and hardness, delays the appearance of Bainite and avoids carbides precipitation in Bainite. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%.

[0064] Niobium is an optional element for the present invention. Niobium content may be present in the steel of the present invention between 0.001 and 0.1% and is added in the Steel of present invention for forming carbo-nitrides to impart strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus finer microstructure formed at the end of the holding temperature and as a consequence after the completion of annealing that will lead to the hardening of the Steel of the present invention. However, Niobium content above 0.1% is not economically interesting as a saturation effect of its influence is observed this means that additional amount of Niobium does not result in any strength improvement of the product.

[0065] Titanium is an optional element and may be added to the Steel of the present invention between 0.001% and 0.1%. As Niobium, it is involved in carbo-nitrides formation so plays a role in hardening of the Steel of the present invention. In addition Titanium also forms Titanium-nitrides which appear during solidification of the cast product. The amount of Titanium is so limited to 0.1% to avoid formation of coarse Titanium-nitrides detrimental for formability. In case the Titanium content is below 0.001% it does not impart any effect on the steel of the present invention.

[0066] Copper may be added as an optional element in an amount of 0.01% to 2% to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01% of Copper is required to get such effect. However, when its content is above 2%, it can degrade the surface aspects.

[0067] Nickel may be added as an optional element in an amount of 0.01 to 3% to increase the strength of the steel and to improve its toughness. A minimum of 0.01% is required to produce such effects. However, when its content is above 3%, Nickel causes ductility deterioration.

[0068] Calcium content in the steel of the present invention is between 0.0001% and 0.005%. Calcium is added to steel of the present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of Steel by arresting the detrimental Sulfur content in globular form, thereby, retarding the harmful effects of Sulfur.

[0069] Vanadium is effective in enhancing the strength of steel by forming carbides or carbo-nitrides and the upper limit is 0.1% due to the economic reasons. Other elements such as Cerium, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Cerium.ltoreq.0.1%, Boron.ltoreq.0.003%, Magnesium.ltoreq.0.010% and Zirconium.ltoreq.0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

[0070] The microstructure of the Steel sheet comprises:

[0071] Ferrite constitutes from 60% to 75% of microstructure by area fraction for the Steel of the present invention. Ferrite constitutes the primary phase of the steel as a matrix. In the present invention, Ferrite cumulatively comprises Polygonal ferrite and acicular ferrite Ferrite and imparts high strength as well as elongation to the steel of the present invention. To ensure an elongation of 31% and preferably 33% or more it is necessary to have 60% of Ferrite. Ferrite is formed during the cooling after annealing in steel of present invention. But whenever ferrite content is present above 75% in steel of present invention the strength is not achieved.

[0072] Bainite constitutes from 20% to 30% of microstructure by area fraction for the Steel of present invention. In the present invention, Bainite cumulatively consists of Lath Bainite and Granular Bainite. To ensure tensile strength of 620 MPa and preferably 630 MPa or more it is necessary to have 20% of Bainite. Bainite is formed during over-aging holding.

[0073] Residual Austenite constitutes from 10% to 15% by area fraction of the Steel. Residual Austenite is known to have a higher solubility of Carbon than Bainite and, hence, acts as an effective Carbon trap, therefore, retarding the formation of carbides in Bainite. Carbon percentage inside the Residual Austenite of present invention is preferably higher than 0.9% and preferably lower than 1.1%. Residual Austenite of the Steel according to the invention imparts an enhanced ductility.

[0074] Martensite is an optional constituent and may be present between 0% and 5% of microstructure by area fraction and found in traces. Martensite for present invention includes both fresh martensite and tempered martensite. The present invention forms martensite due to the cooling after annealing and is tempered during overaging holding. Fresh Martensite also form during cooling after the coating of cold rolled steel sheet. Martensite imparts ductility and strength to the Steel of present invention when it is below 5%. When Martensite is in excess of 5% it imparts excess strength but diminishes the elongation beyond acceptable limit. The preferable limit for martensite is between 0% and 3%.

[0075] A total amount of Ferrite and Residual Austenite must always be between 70% and 80% to have total elongation of 31% and a minimum of 70% is required to ensure the total elongation above 31% while having a tensile strength of 600 MPa. Ferrite and residual austenite are soft phase in comparison to martensite and bainite therefore imparts for elongation and ductility but whenever the cumulative presence is more than 80% the strength drops beyond the acceptable limits.

[0076] In addition to the above-mentioned microstructure, the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite and cementite, without impairing the mechanical properties of the steel sheets.

[0077] A steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220 mm for slabs up to several tens of millimeters for thin strip.

[0078] For example, a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.

[0079] The temperature of the slab, which is subjected to hot rolling, is at least 1150.degree. C. and must be below 1280.degree. C. In case the temperature of the slab is lower than 1150.degree. C., excessive load is imposed on a rolling mill. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac1+50.degree. C. to Ac1+250.degree. C. and preferably between Ac1+50.degree. C. and Ac1+200.degree. C. while always having final rolling temperature remains above Ac1+50.degree. C. Reheating at temperatures above 1280.degree. C. must be avoided because they are industrially expensive.

[0080] A final rolling temperature range between Ac1+50.degree. C. to Ac1+250.degree. C. is preferred to have a structure that is favorable to recrystallization and rolling. It is necessary to have final rolling pass to be performed at a temperature greater than Ac1+50.degree. C., because below this temperature the steel sheet exhibits a significant drop in rollability. The sheet obtained in this manner is then cooled at a cooling rate above 30.degree. C./s to the coiling temperature which must be below 625.degree. C. Preferably, the cooling rate will be less than or equal to 200.degree. C./s.

[0081] The hot rolled steel sheet is then coiled at a coiling temperature below 625.degree. C. to avoid ovalization and preferably below 600.degree. C. to avoid scale formation. The preferred range for such coiling temperature is between 350.degree. C. and 600.degree. C. The coiled hot rolled steel sheet may be cooled down to room temperature before subjecting it to optional hot band annealing.

[0082] The hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then subjected to an optional Hot Band Annealing at temperatures between 400.degree. C. and 750.degree. C. for at least 12 hours and not more than 96 hours, the temperature remaining below 750.degree. C. to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel sheet may performed through, for example, pickling of such sheet. This hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%. The cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.

[0083] In the annealing, the cold rolled steel sheet subjected to two steps of heating to reach the soaking temperature between Ac1+30.degree. C. and Ac3 wherein Ac1 and Ac3 for the present steel is calculated by using the following formula:

Ac1=723-10.7[Mn]-16[Ni]+29.1[Si]+16.9[Cr]+6.38[W]+290[As]

Ac3=910-203[C]{circumflex over ( )}(1/2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11[Cr]-20[Cu]+70- 0[P]+400[Al]+120[As]+400[Ti]

wherein the elements contents are expressed in weight percent.

[0084] In step one cold rolled steel sheet is heated at a heating rate between 10.degree. C./s and 40.degree. C./s to a temperature range between 550.degree. C. and 650.degree. C. Thereafter in subsequent second step of heating the cold rolled steel sheet is heated at a heating rate between 1.degree. C./s and 5.degree. C./s to the soaking temperature of annealing.

[0085] Then the cold rolled steel sheet is preferably held at the soaking temperature during 10 to 500 seconds to ensure at least 30% transformation to Austenite microstructure of the strongly work-hardened initial structure. Then the cold rolled steel sheet is then cooled in two step cooling to an over-aging holding temperature. In step one of cooling the cold rolled steel sheet is cooled at cooling rate less than 5.degree. C./s and preferably less than 3.degree. C./s to a temperature range between 600.degree. C. and 720.degree. C. and preferably between 625.degree. C. and 720.degree. C. During this step one of cooling, the ferrite matrix of the present invention is formed. Thereafter in a subsequent second cooling step the cold rolled steel sheet is cooled to an overaging temperature range between 250.degree. C. and 470.degree. C. at a cooling rate between 10.degree. C./s and 100.degree. C./s. Then the cold rolled steel sheet is held in the over-aging temperature range during 5 to 500 seconds. The cold rolled steel sheet is then brought to the temperature to a coating bath temperature range of 400.degree. C. and 480.degree. C. to facilitate coating of the cold rolled steel sheet. Then the cold rolled steel sheet is coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, Hot dip(GI) etc.

EXAMPLES

[0086] The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.

[0087] Steel sheets made of steels with different compositions are gathered in Table 1, where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties.

TABLE-US-00001 TABLE 1 Other Sample elements Steels C Mn Si Al P S N present A 0.155 1.54 0.696 0.728 0.014 0.002 0.003 -- B 0.157 1.54 0.690 0.721 0.014 0.002 0.003 -- C 0.148 1.54 0.698 0 0.013 0.0027 0.0044 -- D 0.114 1.62 0.293 0.031 0.027 0.0028 0.005 --Ni: 0.025, Cr: 0.345

underlined values: not according to the invention.

Table 2

[0088] Table 2 gathers the annealing process parameters implemented on steels of Table 1. The Steel compositions A and B serve for the manufacture of sheets according to the invention. This table also specifies the reference steels which are designated in table as C and D. Table 2 also shows tabulation of Ac1 and Ac3. These Ac1 and Ac3 are defined for the inventive steels and reference steels as follows:

Ac1=723-10.7[Mn]-16[Ni]+29.1[Si]+16.9[Cr]+6.38[W]+290[As]

Ac3==910-203[C]{circumflex over ( )}(1/2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11[Cr]-20[Cu]+70- 0[P]+400[Al]+120[As]+400[Ti]

wherein the elements contents are expressed in weight percent.

[0089] All sheets were cooled at a cooling rate of 34.degree. C./s after hot rolling and were finally brought at a temperature of 460.degree. C. before coating. All the sheets have a cold rolled reduction of 65%.

[0090] The table 2 is as follows:

TABLE-US-00002 TABLE 2 Step one Heating rate Step two for fast Fast Slow Heating Reheating HR Finish HR Coiling heating before heating Rate before Soaking Soaking Steel T T T annealing stop T annealing Temperature time Trial Sample (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C./s) (.degree. C.). (.degree. C./s) (.degree. C.) (s) I1 A 1200 850 500 12 600 1.6 770 179 I2 B 1200 870 520 19 600 3.1 800 110 I3 A 1200 850 500 12 600 1.9 800 179 R1 A 1200 850 500 12 600 1.9 800 293 R2 C 1200 850 500 12 600 1.9 800 179 R3 D 1200 920 585 9 600 1.2 770 238 Step one Step two Slow cooling Slow Fast Fast cooling rate after cooling stop cooling stop temperature Holding annealing temperature rate temperature for overaging time Ac3 Ac1 Trial (.degree. C./s) (.degree. C./s) (.degree. C./s) (.degree. C.) (.degree. C.) (s) (.degree. C.) (.degree. C.) I1 0.6 700 30 410 410 129 1117 727 I2 1.5 700 39 460 460 79 1112 727 I3 0.9 700 31 400 400 129 1112 727 R1 -- -- 34 460 460 129 1117 727 R2 0.9 700 31 400 400 129 827 727 R3 0.5 700 27 350 350 172 835 720

I=according to the invention; R=reference; underlined values: not according to the invention.

Table 3

[0091] Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels.

The results are stipulated herein:

TABLE-US-00003 Ferrite + Residual Residual Sample Ferrite Bainite Austenite Martensite Austenite Steels (%) (%) (%) (%) (%) I1 67 22 11 0 78 I2 65 24 10 1 75 I3 63 27 10 0 73 R1 53 37 10 0 63 R2 62 32 5 1 67 R3 62 33 5 0 67

I=according to the invention; R=reference; underlined values: not according to the invention.

Table 4

[0092] Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance of JIS Z2241 standards.

[0093] The results of the various mechanical tests conducted in accordance to the standards are gathered

TABLE-US-00004 TABLE 4 Tensile Sample Strength YS Total Steels (MPa) (MPa) YS/TS Elongation(%) I1 634 386 0.61 34.7 I2 672 401 0.60 33.2 I3 633 411 0.65 36.1 R1 677 389 0.57 27.7 R2 602 365 0.61 28.6 R3 622 343 0.55 22.5

I=according to the invention; R=reference; underlined values: not according to the invention.

Claims

1-20. (canceled)

21. A cold rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight: 0.13%.ltoreq.Carbon.ltoreq.0.18% 1.1%.ltoreq.Manganese.ltoreq.1.8% 0.5%.ltoreq.Silicon.ltoreq.0.9% 0.6%.ltoreq.Aluminum.ltoreq.1% 0.002%.ltoreq.Phosphorus.ltoreq.0.02% 0%.ltoreq.Sulfur.ltoreq.0.003%. 0%.ltoreq.Nitrogen.ltoreq.0.007% and optionally one or more of the following elements: 0.05%.ltoreq.Chromium.ltoreq.1% 0.001%.ltoreq.Molybdenum.ltoreq.0.5% 0.001%.ltoreq.Niobium.ltoreq.0.1% 0.001%.ltoreq.Titanium.ltoreq.0.1% 0.01%.ltoreq.Copper.ltoreq.2% 0.01%.ltoreq.Nickel.ltoreq.3% 0.0001%.ltoreq.Calcium.ltoreq.0.005% 0%.ltoreq.Vanadium.ltoreq.0.1% 0%.ltoreq.Boron.ltoreq.0.003% 0%.ltoreq.Cerium.ltoreq.0.1% 0%.ltoreq.Magnesium.ltoreq.0.010% 0%.ltoreq.Zirconium.ltoreq.0.010%; a remainder of the composition being composed of iron and unavoidable impurities caused by processing, a microstructure of the steel sheet comprising in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein cumulated amounts of the Residual Austenite and the Ferrite is between 70% and 80%.

22. The cold rolled steel sheet as recited in claim 21 wherein the composition includes 0.6% to 0.8% of Silicon.

23. The cold rolled steel sheet as recited in claim 21 wherein the composition includes 0.14% to 0.18% of Carbon.

24. The cold rolled steel sheet as recited in claim 23 wherein the composition includes 0.6% to 0.8% of Aluminum.

25. The cold rolled steel sheet as recited in claim 21 wherein the composition includes 1.2% to 1.8% of Manganese.

26. The cold rolled steel sheet as recited in claim 25 wherein the composition includes 1.3% to 1.7% of Manganese.

27. The cold rolled steel sheet as recited in claim 21 wherein the cumulated amount of the Ferrite and the Residual Austenite is between 73% and 80% and the percentage of Residual Austenite is less than 13%.

28. The cold rolled steel sheet as recited in claim 21 wherein the amount of Martensite is between 0% and 3%.

29. The cold rolled steel sheet as recited in claim 21 wherein the Carbon content of Residual Austenite is between 0.9 to 1.1%.

30. The cold rolled steel sheet as recited in claim 21 wherein the steel sheet has an ultimate tensile strength of 600 MPa or more, and a total elongation of 31% or more.

31. The cold rolled steel sheet as recited in claim 30 wherein said steel sheet has yield strength of 320 MPa or more and a total elongation of 33% or more.

32. The cold rolled steel sheet as recited in claim 21 wherein the steel sheet is coated.

33. A method of production of a cold rolled steel sheet comprising the following successive steps: providing a semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight: 0.13%.ltoreq.Carbon.ltoreq.0.18% 1.1%.ltoreq.Manganese.ltoreq.1.8% 0.5%.ltoreq.Silicon.ltoreq.0.9% 0.6%.ltoreq.Aluminum.ltoreq.1% 0.002%.ltoreq.Phosphorus.ltoreq.0.02% 0%.ltoreq.Sulfur.ltoreq.0.003%. 0%.ltoreq.Nitrogen.ltoreq.0.007% and optionally one or more of the following elements: 0.05%.ltoreq.Chromium.ltoreq.1% 0.001%.ltoreq.Molybdenum.ltoreq.0.5% 0.001%.ltoreq.Niobium.ltoreq.0.1% 0.001%.ltoreq.Titanium.ltoreq.0.1% 0.01%.ltoreq.Copper.ltoreq.2% 0.01%.ltoreq.Nickel.ltoreq.3% 0.0001%.ltoreq.Calcium.ltoreq.0.005% 0%.ltoreq.Vanadium.ltoreq.0.1% 0%.ltoreq.Boron.ltoreq.0.003% 0%.ltoreq.Cerium.ltoreq.0.1% 0%.ltoreq.Magnesium.ltoreq.0.010% 0%.ltoreq.Zirconium.ltoreq.0.010%; a remainder of the composition being composed of iron and unavoidable impurities caused by processing; reheating the semi-finished product to a temperature between 1150.degree. C. and 1280.degree. C.; rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between Ac1+50.degree. C. and Ac1+250.degree. C. to obtain a hot rolled steel sheet; cooling the sheet at a cooling rate above 30.degree. C./s to a coiling temperature below 625.degree. C.; and coiling the hot rolled sheet; cooling the hot rolled sheet to room temperature; optionally performing scale removal process on said hot rolled steel sheet; optionally annealing the hot rolled steel sheet at temperature between 400.degree. C. and 750.degree. C.; optionally performing scale removal process on the hot rolled steel sheet; cold rolling the hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet; then performing an annealing at a soaking temperature between Ac1+30.degree. C. and Ac3 for a duration between 10 and 500 seconds by heating the cold rolled steel sheet by a two step heating wherein: in step one of the two step heating, the cold rolled steel sheet is heated at a heating rate between 10.degree. C./s and 40.degree. C./s to a temperature range between 550.degree. C. and 650.degree. C.; then in step two of the two step heating, the cold rolled steel sheet is heated at a heating rate between 1.degree. C./s and 5.degree. C./s from a temperature range between 550.degree. C. and 650.degree. C. to the annealing soaking temperature and maintained at the annealing soaking temperature, then cooling the cold rolled steel sheet in a two step cooling wherein: in step one of the two step cooling, the cold rolled steel sheet is cooled at a cooling rate of less 5.degree. C./s to temperature range between 600.degree. C. and 720.degree. C.; then in step two of the two step cooling, the sheet is cooled at a cooling rate between 10.degree. C./s to 100.degree. C./s from a temperature range between 600.degree. C. and 720.degree. C. to an overaging temperature; then the cold rolled steel sheet is overaged at a temperature range between 250.degree. C. and 470.degree. C. for 5 to 500 seconds and then cooling to room temperature to obtain the produced cold rolled steel sheet.

34. The method as recited in claim 33 wherein the coiling temperature is below 600.degree. C.

35. The method as recited in claim 33 wherein the finishing rolling temperature is between Ac1+50.degree. C. and Ac1+200.degree. C.

36. The method as recited in claim 33 wherein the cooling rate after annealing is less than 3.degree. C./s in the temperature range between 625.degree. C. and 720.degree. C.

37. The method as recited in claim 33 wherein the cold rolled steel sheet is annealed between Ac1+30.degree. C. and Ac3 and the temperature of annealing is selected so as to ensure the presence of at least 30% of austenite at the end of the soaking.

38. The method as recited in claim 33 wherein the cold rolled steel sheet can be coated a temperature range between 400.degree. C. and 480.degree. C.

39. A structural or safety part of a vehicle comprising the steel sheet as recited in claim 21.

40. A vehicle comprising the structural of safety part recited in claim 39.

41. A method for manufacturing a structural or safety part of a vehicle comprising the method as recited in claim 33.

42. A vehicle comprising the structural or safety part obtained according to claim 41.