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

Abstract

A cold rolled and coated steel sheet having a composition including of the following elements, 0.12%≤Carbon≤0.2%, 1.7%≤Manganese≤2.10%, 0.1%≤Silicon≤0.5%, 0.1%≤Aluminum≤0.8%, 0.1%≤Chromium≤0.5%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, Nickel≤3%, Niobium≤0.1%, Titanium≤0.1%, Calcium≤0.005%, Copper≤2%, Molybdenum≤0.5%, Vanadium≤0.1%, Boron≤0.003%, Cerium≤0.1%, Magnesium≤0.010%, Zirconium≤0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 10 to 60% Bainite, 25 to 55% Ferrite, 5% to 15% Residual Austenite wherein carbon content in residual austenite is between 0.7% and 1% and 5% to 18% Martensite, wherein the cumulated amount of Bainite and Ferrite is at least 70%.

Claims

1-27. (canceled)

28. A cold rolled and coated steel sheet comprising: a composition of the following elements, expressed in percentage by weight:
0.12%≤Carbon≤0.2%
1.7%≤Manganese≤2.10%
0.1%≤Silicon≤0.5%
0.1%≤Aluminum≤0.8%
0.1%≤Chromium≤0.5%
0%≤Phosphorus≤0.09%
0%≤Sulfur≤0.09%.
0%≤Nitrogen≤0.09% and optionally one or more of the following elements:
Nickel≤3%
Niobium≤0.1%
Titanium≤0.1%
Calcium≤0.005%
Copper≤2%
Molybdenum≤0.5%
Vanadium≤0.1%
Boron≤0.003%
Cerium≤0.1%
Magnesium≤0.010%
Zirconium≤0.010% a remainder of the composition being composed of iron and unavoidable impurities caused by processing, a microstructure of the steel sheet including in area fraction, 10 to 60% Bainite, 25 to 55% Ferrite, 5% to 15% Residual Austenite, a carbon content in the Residual Austenite being between 0.7% and 1%, and 5% to 18% Martensite, wherein a cumulated amount of Bainite and Ferrite is at least 70%.

29. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 0.1% to 0.4% of Silicon.

30. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 0.12% to 0.19% of Carbon.

31. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 0.2% to 0.8% of Aluminum.

32. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 1.7% to 2.08% of Manganese.

33. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 0.1% to 0.4% of Chromium.

34. The cold rolled and coated steel sheet as recited in claim 28 wherein the composition includes 1.8% to 2.08% of Manganese.

35. The cold rolled and coated steel sheet as recited in claim 30 wherein the composition includes 0.14% to 0.18% of Carbon.

36. The cold rolled and coated steel sheet as recited in claim 28 wherein a cumulated amount of Carbon and Manganese is between 2.1% and 2.25%.

37. The cold rolled and coated steel sheet as recited in claim 28 wherein a cumulated amount of Silicon and Aluminum is between 0.5% and 0.9%.

38. The cold rolled and coated steel sheet as recited in claim 28 wherein the cumulated amount of Ferrite and Bainite is more than or equal to 74% and the percentage of Ferrite is at least 30%.

39. The cold rolled and coated steel sheet as recited in claim 28 wherein the carbon content of Residual Austenite is between 0.7% and 0.9%.

40. The cold rolled and coated steel sheet as recited in claim 39 wherein the carbon content of Residual Austenite is between 0.7% and 0.8%

41. The cold rolled and coated steel sheet as recited in claim 28 wherein the bainite is between 20% and 60%.

42. The cold rolled and coated steel sheet as recited in claim 28 wherein the martensite is between 5% and 15%.

43. The cold rolled and coated steel sheet as recited in claim 28 wherein the steel sheet has an ultimate tensile strength of 780 MPa or more, and a total elongation of 18% or more.

44. The cold rolled and coated steel sheet as recited in claim 43 whereon the steel sheet has an ultimate tensile strength of 800 MPa or more and a total elongation of greater than equal to 20%.

45. A method of production of a cold rolled and coated steel sheet comprising the following successive steps: providing a semi-finished product with a steel composition of the following elements, expressed in percentage by weight:
0.12%≤Carbon≤0.2%
1.7%≤Manganese≤2.10%
0.1%≤Silicon≤0.5%
0.1%≤Aluminum≤0.8%
0.1%≤Chromium≤0.5%
0%≤Phosphorus≤0.09%
0%≤Sulfur≤0.09%.
0%≤Nitrogen≤0.09% and optionally one or more of the following elements:
Nickel≤3%
Niobium≤0.1%
Titanium≤0.1%
Calcium≤0.005%
Copper≤2%
Molybdenum≤0.5%
Vanadium≤0.1%
Boron≤0.003%
Cerium≤0.1%
Magnesium≤0.010%
Zirconium≤0.010% a remainder of the steel composition being composed of iron and unavoidable impurities caused by processing reheating said semi-finished product to a temperature between 1000° C. and 1280° C.; rolling the semi-finished product in the temperature range between Ac3 and Ac3+100° C. wherein the hot rolling finishing temperature is above Ac3 to obtain a hot rolled steel; cooling the hot rolled steel at a cooling rate above 30° C./s to a coiling temperature between 475° C. and 650° C.; and coiling the hot rolled steel; cooling the hot rolled steel to room temperature; optionally performing a scale removal process on the hot rolled steel sheet; optionally annealing the hot rolled steel sheet between 400° C. and 750° C.; optionally performing a further 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; annealing the cold rolled steel sheet in two steps heating wherein: the first step starts from heating the steel sheet from room temperature to a temperature T1 between 600° C. and 750° C., with a heating rate HR1 of at least 3° C./s, the second step starts from heating further the steel sheet from T1 to a soaking temperature T2 between Ac1 and Ac3, with a heating rate HR2 of 15° C./s or less, HR2 being lower than HR1, annealing at T2 for 10 to 500 seconds; cooling the cold rolled steel sheet from T2 to an over aging temperature T over between 375° C. and 480° C. at an average cooling rate of at least 10° C./s, wherein the cooling optionally includes slow cooling sub-step between T2 and a temperature Tsc between 600° C. and 750° C. with a slow cooling rate of 2° C./s or less; over aging the cold rolled steel sheet at T over for 5 to 500 seconds and bringing to a temperature range between 420° C. and 680° C. to facilitate coating; and coating the cold rolled sheet to obtain a cold rolled coated steel sheet.

46. The method as recited in claim 45 wherein a coiling temperature is between 475° C. and 625° C.

47. The method as recited in claim 45 wherein the finishing rolling temperature is more than 850° C.

48. The method as recited in claim 45 wherein the average cooling rate after annealing is more than 15° C./s.

49. The method as recited in claim 45 wherein T2 is between Ac1+30° C. and Ac3 and T2 is selected so as to ensure the presence of at least 60% of austenite at the end of the annealing.

50. The method as recited in claim 49 wherein T2 is selected so as to ensure the presence of at least 70% of austenite at the end of the annealing.

51. The method as recited in claim 45 wherein the temperature for over aging T over is between 380° C. and 460° C.

52. The method as recited in claim 45 wherein the first step of heating the cold rolled steel sheet ends at a temperature T1 between 650° C. and 750° C. with a heating rate HR1 of at least 5° C./s.

53. A method for the manufacture of structural or safety parts of a vehicle comprising performing the method as recited in claim 45.

54. A vehicle comprising a part obtained by the method of claim 53.

55. A method for the manufacture of structural or safety parts of a vehicle comprising employing the cold rolled and coated steel sheet as recited in claim 28.

56. A vehicle comprising a part obtained by the method of claim 55.

Description

DETAILED DESCRIPTION

[0011] The cold rolled and heat treated steel sheet of the present invention is be coated with zinc or zinc alloys, or with aluminium or aluminium alloys to improve its corrosion resistance.

[0012] Carbon is present in the steel between 0.12% and 0.2%. Carbon is an element necessary for increasing the strength of the steel sheet by producing low-temperature transformation phases such as martensite and bainite, further Carbon also plays a pivotal role in Austenite stabilization and hence is 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 0.12% 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 0.2%, the steel exhibits poor spot weldability which limits its application for the automotive parts. The preferred range for carbon for the steel of the present invention is 0.12% to 0.19% and more preferably 0.14% to 0.18%.

[0013] Manganese content of the steel of the present invention is between 1.7% and 2.10%. 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 1.7% by weight of Manganese has been found to provide the strength and hardenability of the steel sheet as well as to stabilize Austenite. In addition the Manganese content of above 2.10% also reduces the ductility and also deteriorates the weldability of the present steel hence the elongation targets may not be achieved. A preferable content for the present invention may be kept between 1.7% and 2.08%, furthermore preferably 1.8% and 2.08%.

[0014] In a preferred embodiment, the cumulative amount of Carbon and Manganese is kept between 2.1% and 2.25% to secure an even increased amount of retained austenite.

[0015] Silicon content of the steel of the present invention is between 0.1% and 0.5%. Silicon is a constituent that can retard the precipitation of carbides during over ageing, therefore, due to the presence of Silicon, carbon rich Austenite is stabilized at room temperature However, a 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 0.5%. A preferable content for the present invention may be kept between 0.1% and 0.4%

[0016] Aluminum is an essential element and is present in the steel of the present invention between 0.1% and 0.8%. Aluminum promotes ferrite formation and increases the Ms temperature which allows the present invention to have both Martensite and Ferrite in adequate amount as required by the steel of the present invention to impart steel of the present invention with ductility as well as strength. However, the presence of Aluminum more than 0.8% increases the Ac3 temperature which makes the annealing and hot rolling finishing temperature in complete Austenitic region economically unreasonable. The Aluminum content is preferably limited between 0.2% and 0.8% and more preferably 0.3% and 0.6%.

[0017] The cumulative amount of Silicon and Aluminium is preferably between 0.5% and 0.9% and more preferably between 0.6% and 0.9%, to increase further the amount of retained austenite.

[0018] Chromium is an essential element for the present invention. Chromium content is present in the steel of the present invention between 0.1% and 0.5%. Chromium provides strength and hardening to the steel but when used above 0.5% it impairs the surface finish of steel. The preferred limit for Chromium for the present invention is between 0.1% and 0.4% and more preferably 0.2% and 0.4%.

[0019] Phosphorus is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the phosphorus content is preferably as low as possible, and below 0.09%. 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 less than 0.09, preferably less than 0.3% and more preferably less than 0.014%.

[0020] Sulfur is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the Sulfur content is preferably as low as possible, but is 0.09% 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.

[0021] Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel.

[0022] Nickel may be added as an optional element in an amount up to 3% to increase the strength of the steel and to improve its toughness. A minimum of 0.01% is preferred to produce such effects. However, when its content is above 3%, Nickel causes ductility deterioration.

[0023] Niobium is an optional element for the present invention. Niobium content may be present in the steel of the present invention up to 0.1% and is added in the Steel of the present invention for forming carbo-nitrides to impart strength of the Steel of the 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 the 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 and this means that additional amount of Niobium does not result in any strength improvement of the product.

[0024] Titanium is an optional element and may be added to the Steel of the present invention up to 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.

[0025] Calcium content in the steel of the present invention is up to 0.005%. Calcium is added to steel of the present invention as an optional element especially during the inclusion treatment with a preferred minimum amount of 0.0001%. Calcium contributes towards the refining of Steel by arresting the detrimental Sulfur content in globular form, thereby, retarding the harmful effects of Sulfur.

[0026] Copper may be added as an optional element in an amount up to 2% to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01% of Copper is preferred to get such effect. However, when its content is above 2%, it can degrade the surface aspects.

[0027] Molybdenum is an optional element that constitutes up 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%.

[0028] 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≤0.1%, Boron≤0.003%, Magnesium≤0.010% and Zirconium≤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.

[0029] The microstructure of the Steel sheet will now be described.

[0030] Bainite constitutes from 10% to 60% of microstructure by area fraction for the steel of the invention. Bainite can be under the form of upper bainite and/or lower bainite. Bainite may be formed during over-aging holding. Bainite impart strength to the steel of the present invention. To achieve the tensile strength of 780 MPa or more it is necessary to have 10% bainite. The preferred range for the presence of bainite according to the present invention is between 20% and 60% and more preferably between 30% and 55%.

[0031] Ferrite constitutes from 25% to 55% of microstructure by area fraction for the Steel of the present invention. Ferrite imparts high strength as well as elongation to the steel of the present invention. To ensure an elongation of 18% and preferably 20% or more it is necessary to have 25% of Ferrite. Ferrite of the present invention is formed during annealing and cooling done after annealing. But whenever ferrite content is present above 55% in steel of the present invention it is not possible to have both the desired tensile strength and the total elongation at the same time. The preferred limit for presence of ferrite for the present invention is between 30% and 55% and more preferably 30% and 50%.

[0032] The cumulated amount of ferrite and bainite is at least 70%, this cumulative amount of Ferrite and Bainite ensures that the steel of the present invention always has a total elongation above 18%. This cumulative presence also ensures that the presence of ferrite above 30% provides enough soft phase in the steel of the present invention to impart formability to the steel of the present invention.

[0033] Residual Austenite constitutes from 5% 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 higher than 0.7% and lower than 1%. Residual Austenite of the Steel according to the invention imparts an enhanced ductility. However, when the Carbon content of Residual Austenite is below 0.7%, it will not able to trap enough carbon and will lead to formation of excess martensite instead of adequate amount of bainite, this effect provides excess strength to the steel and is also detrimental to elongation. The preferable limit of for the presence of Austenite is between 6% and 15% wherein the preferable Carbon content limit in austenite is preferred between 0.7% and 0.9% and more preferably between 0.7% and 0.8%.

[0034] Martensite constitutes between 5% and 18% of microstructure by area fraction. Martensite for present invention includes both fresh martensite and tempered martensite. The present invention forms martensite due to the cooling after annealing and get tempered during over aging holding. Fresh Martensite also forms during cooling after the coating of cold rolled steel sheet. Martensite imparts ductility and strength to the Steel of the present invention. However, when martensite presence is above 18% it imparts excess strength but diminishes the elongation beyond the acceptable limit for the steel of the present invention. The preferred limit for martensite for the steel of the present invention is between 5% and 15%.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] The temperature of the slab, which is subjected to hot rolling, is at least 1000° C. and must be below 1280° C. In case the temperature of the slab is lower than 1000° C., excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite is contained in the structure. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 to Ac3+100° C. and final rolling temperature remains above Ac3. Reheating at temperatures above 1280° C. must be avoided because they are industrially expensive.

[0039] A final rolling temperature range between Ac3 to Ac3+100° C. is necessary to have a structure that is favorable to recrystallization and rolling. It is preferred that the final rolling pass to be performed at a temperature greater than 850° C., because below this temperature the steel sheet exhibits a significant drop in rollability. The hot rolled steel obtained in this manner is then cooled at a cooling rate above 30° C./s to the coiling temperature which must be between 475° C. and 650° C. Preferably, the cooling rate will be less than or equal to 200° C./s.

[0040] The hot rolled steel is then coiled at a coiling temperature between 475° C. and 650° C. to avoid ovalization and preferably between 475° C. and 625° C. to avoid scale formation. A more preferred range for such coiling temperature is between 500° C. and 625° C. The coiled hot rolled steel is cooled down to room temperature before subjecting it to optional hot band annealing.

[0041] The hot rolled steel 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, for example, temperatures between 400° C. and 750° C. for at least 12 hours and not more than 96 hours, the temperature remaining below 750° 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 may performed through, for example, pickling of such sheet. This hot rolled steel 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 the present invention with microstructure and mechanical properties.

[0042] Annealing the said cold rolled steel sheet occurs in a two step heating wherein the first step starts from heating the steel sheet from room temperature to a temperature T1 between 600° C. and 750° C., with a heating rate HR1 of at least 3° C./s, thereafter the second step starts from heating further the steel sheet from T1 to a soaking temperature T2 between Ac1 and Ac3, with a heating rate HR2 of 15° C./s or less, HR2 being lower than HR1, then perform annealing at T2 during 10 to 500 seconds. In a preferred embodiment, the heating rate for the second step the heating rate is less than 10° C./s and more preferably less than 5° C./s. The preferred temperature T2 for soaking is between Ac1+30° C. and Ac3.

[0043] Then the cold rolled steel sheet is annealed at soaking temperature T2 between Ac1 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=955−350 C−25Mn+51Si+106Nb+100Ti+68Al−11Cr−33Ni−16Cu+67Mo

wherein the elements contents are expressed in weight percent.

[0044] Then the cold rolled steel sheet is held at the soaking temperature T2 during 10 to 500 seconds. In a preferred embodiment, the time and temperature of soaking are selected so as to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 60% of Austenite and more preferably at least 70% of austenite.

[0045] Then the cold rolled steel is cooled from T2 to an over aging holding temperature T over between 375° C. and 480° C., preferably between 380° C. and 460° C., at an average cooling rate of at least 10° C./s and preferably at least 15° C./s, wherein the cooling step may include an optional slow cooling sub-step between T2 and a temperature Tsc between 600° C. and 750° C., with a cooling rate of 2° C./s or less and preferably of 1° C./s or less.

[0046] The cold rolled steel sheet is then held at T over during 5 to 500 seconds.

[0047] The cold rolled steel sheet can then be brought to the temperature of the coating bath between 420° C. and 460° C., depending on the nature of the coating, to facilitate hot dip coating of the cold rolled steel sheet.

[0048] The cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc, which may not require bringing it to the above mentioned temperature range before coating.

[0049] Then an optional post batch annealing may be done at a temperature between 150° C. and 300° C. during 30 minutes to 120 hours.

EXAMPLES

[0050] 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.

[0051] 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 Trials C Mn Si Al Cr Nb P S A 0.161 2.00 0.25 0.53 0.21 0 0.003 0.0012 B 0.168 2.00 0.25 0.50 0.22 0 0.008 0.0014 C 0.173 2.06 0.25 0.59 0.25 0 0.010 0.0011 D 0.173 2.06 0.25 0.59 0.25 0 0.010 0.0012 E 0.173 2.06 0.25 0.59 0.25 0 0.010 0.0013 F 0.178 2.00 0.24 0.49 0.22 0 0.012 0.0012 G 0.172 1.99 0.25 0.52 0.21 0 0.012 0.0011 H 0.170 2.00 0.25 0.55 0.21 0 0.011 0.0010 I 0.161 2.15 0.25 0.72 0.20 0.0019 0.003 0.0012 J 0.161 2.15 0.25 0.53 0.20 0.0019 0.003 0.0013 K 0.168 2.21 0.27 0.62 0.23 0 0.009 0.0011 underlined values: not according to the invention.
underlined values: not according to the invention.

Table 2

[0052] Table 2 gathers the annealing process parameters implemented on steels of Table 1. The Steel compositions A to G serve for the manufacture of sheets according to the invention. 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=955−350 C−25Mn+51Si+106Nb+100Ti+68Al−11Cr−33Ni−16Cu+67Mo

wherein the elements contents are expressed in weight percent.

[0053] Following processing parameters are same for all the steels of Table 1. All steels of table 1 are heated to a temperature of 1200° C. before hot rolling. The cold rolling reduction for all the steels is 60% and they were finally brought at a temperature of 460° C. before zinc hot dip coating.

The table 2 is as follows:

TABLE-US-00002 TABLE 2 Hot Rolling Annealing Cooling rate Slow cooling after Hot rate after Finish Rolling Coiling HR1 T1 HR2 Soaking annealing Trials Steel T(° C.) (° C./s) T(° C.) (° C./s) (° C.) (° C./s) T2 time (° C./s) I1 A 920 110 570 6 720 1.9 840 60 NO I2 B 920 110 570 4.7 705 1.5 830 73 NO I3 C 920 110 570 18.9 720 0.8 840 390 NO I4 D 920 110 570 28.3 720 1.3 820 260 NO I5 E 920 110 570 18.9 720 0.8 840 238 0.6 I6 F 920 110 570 20.3 690 1.2 840 334 NO I7 G 920 110 570 3.9 690 1.2 820 88 NO R1 H 890 120 425 3.9 705 1.2 825 90 NO R2 I 920 110 570 3.8 680 1.2 800 90 NO R3 J 920 110 570 3.8 680 1.2 800 90 NO R4 K 920 110 570 5.9 705 1.9 825 60 NO Annealing Average cooling cooling stop Overaging Tsc rate temperature Tover time Ac3 Ac1 Trials (° C.) (° C./s) (° C.) (° C.) (s) (° C.) (° C.) I1 NO 27 470 470 27 895 712 I2 NO 21 470 470 33 891 712 I3 NO 31 440 440 150 893 712 I4 NO 48 420 420 95 893 712 I5 700 25 420 420 150 893 712 I6 NO 35.4 440 440 125 886 712 I7 NO 17.3 470 470 40 891 713 R1 NO 17 460 460 40 893 712 R2 NO 16 460 460 40 904 712 R3 NO 16 460 460 40 892 712 R4 NO 26 470 470 27 894 712 I = according to the invention; R = reference; underlined values: not according to the invention.

Table 3

[0054] 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 Residual C content Ferrite + Trials Ferrite Bainite Austenite Martensite RA (%) Bainite I1 45.1 32.9 12.0 10.0 0.76 78.0 I2 36.8 41.2 9.3 12.7 0.70 78.0 I3 35.7 41.3 11.8 11.2 0.75 77.0 I4 45.9 29.1 12.2 12.8 0.74 75.0 I5 36.0 38.0 12.1 13.9 0.71 74.0 I6 32.3 43.7 11.7 12.3 0.72 76.0 I7 41.0 36.0 9.4 13.6 0.73 77.0 R1 23.0 43.0 8.6 24.4 0.60 66.0 R2 57.0  5.0 13.7 24.3 0.63 62.0 R3 58.0  7.0 14.1 20.9 0.63 65.0 R4 21.6 44.2 8.3 25.9 0.62 65.8 I = according to the invention; R = reference; underlined values: not according to the invention.

Table 4

[0055] 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.

[0056] The results of the various mechanical tests conducted in accordance to the standards are gathered

TABLE-US-00004 TABLE 4 Tensile Total Trials Strength(MPa) Elongation(%) I1 812 19.0 I2 803 21.6 I3 819 20.3 I4 793 20.5 I5 827 19.2 I6 823 19.5 I7 814 21.1 R1 916 16.4 R2 899 12.6 R3 868 14.7 R4 918 16.5 I = according to the invention; R = reference; underlined values: not according to the invention.