COLD ROLLED AND HEAT-TREATED STEEL SHEET AND METHOD OF MANUFACTURING THEREOF

Abstract

A cold rolled and heat-treated steel sheet, the steel including, in weight percentage, 0.17%≤carbon≤0.25%, 2%≤manganese≤3%, 0.9%≤silicon≤2%, 0%≤aluminum≤0.09%, 0.01%≤molybdenum≤0.2%, 0%≤phosphorus≤0.02%, 0%≤sulfur≤0.03%, 0%≤nitrogen≤0.09%, and optionally one or more of the following elements 0%≤chromium≤0.3%, 0%≤niobium≤0.06%, 0%≤titanium≤0.06%, 0%≤vanadium≤0.1%, 0%≤calcium≤0.005%, 0%≤boron≤0.010%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05%, 0%≤Cerium≤0.1%, and the balance including iron and unavoidable impurities, the steel sheet having a microstructure of—50% to 80% of Bainite, 10% to 30% of residual austenite, 15% to 50% of Partitioned martensite, 0% to 10% of ferrite and 0% to 5% fresh martensite in area fractions, and a ferrite-enriched layer extending up to 50 microns from both surfaces of the steel sheet, such ferrite-enriched layer having a mean ferrite content from 55% to 80% in area fraction.

Claims

1-16. (canceled)

17: A cold-rolled and heat-treated steel sheet, the steel of the steel sheet having a composition comprising, in weight percentage: 0.17%≤carbon≤0.25%, 2%≤manganese≤3%, 0.9%≤silicon≤2%, 0%≤aluminum≤0.09%, 0.01%≤molybdenum≤0.2%, 0%≤phosphorus≤0.02%, 0%≤sulfur≤0.03%, 0%≤nitrogen≤0.09%, and optionally one or more of the following elements: 0%≤chromium≤0.3%, 0%≤niobium≤0.06%, 0%≤titanium≤0.06%, 0%≤vanadium≤0.1%, 0%≤calcium≤0.005%, 0%≤boron≤0.010%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05%, 0%≤Cerium≤0.1%, a balance including iron and unavoidable impurities, the steel sheet having a microstructure comprising 50% to 80% of Bainite, 10% to 30% of residual austenite, 15% to 50% of Partitioned martensite, 0% to 10% of ferrite and 0% to 5% fresh martensite in area fractions, and a ferrite-enriched layer extending up to 50 microns from both surfaces of the steel sheet, the ferrite-enriched layer having a mean ferrite content from 55% to 80% in area fraction.

18: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the composition includes 2.2% to 2.9% of manganese.

19: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the composition includes 0.18% to 0.23% of Carbon.

20: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the composition includes 1% to 1.9% of Silicon.

21: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the composition includes 0.05% to 0.15% of Molybdenum.

22: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the microstructure contains 55% to 75% of bainite.

23: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the microstructure contains residual 12% to 25% of residual austenite.

24: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the microstructure contains 15% to 45% of partitioned martensite.

25: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the steel sheet has a tensile strength greater than or equal to 1170 MPa, and a hole expansion ratio of 30% or more.

26: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the steels sheet has a yield strength greater than or equal to 780 MPa, and a total elongation of 12.0% or more.

27: The cold-rolled and heat-treated steel sheet as recited in claim 17 wherein the ferrite-enriched layer up to 50 microns from both surfaces contains 60% to 78% of ferrite in area fraction.

28: A method of manufacturing of a cold-rolled and heat-treated steel sheet comprising the following successive steps: providing a semi-finished product with a steel composition comprising, in weight percentage: 0.17%≤carbon≤0.25%, 2%≤manganese≤3%, 0.9%≤silicon≤2%, 0%≤aluminum≤0.09%, 0.01%≤molybdenum≤0.2%, 0%≤phosphorus≤0.02%, 0%≤sulfur≤0.03%, 0%≤nitrogen≤0.09%, and optionally one or more of the following elements: 0%≤chromium≤0.3%, 0%≤niobium≤0.06%, 0%≤titanium≤0.06%, 0%≤vanadium≤0.1%, 0%≤calcium≤0.005%, 0%≤boron≤0.010%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05%, 0%≤Cerium≤0.1%, a balance including iron and unavoidable impurities, reheating the semi-finished product to a temperature from 1000° C. to 1280° C.; rolling the semi-finished product completely in the austenitic range wherein the hot rolling finishing temperature is greater than or equal to 850° C. to obtain a hot rolled steel sheet; cooling the hot rolled steel sheet at a cooling rate above 30° C./s to a temperature below or equal to 550° C.; and coiling the hot rolled steel sheet and keeping the temperature of coiled sheet below 500° C.; cooling the hot rolled steel sheet; cold rolling the hot rolled steel sheet with a reduction rate from 35 to 70% to obtain a cold rolled steel sheet; annealing the cold rolled steel sheet in two steps heating during which the dew point is controlled from −15° C. to +15° C. and wherein: the first step starts from heating the steel sheet from room temperature to a temperature HT1 from 600° C. to 800° C., with a heating rate HR1 from 2° C./s to 70° C./s, the second step starts from heating further the steel sheet from HT1 to a soaking temperature TA from Ac3−10° C. and Ac3+100° C., with a heating rate HR2 from 0.1° C./s and 10° C./s or less, HR2 being lower than HR1, then annealing at TA during 10 to 500 seconds, time being selected to obtain a minimum percentage of 90% austenite, the dew point being controlled from −10° C. to +10° C. during the annealing; then cooling the cold rolled steel sheet from TA to cooling stop temperature CS1 from Ms−5° C. to Ms−100° C. with a cooling rate CR1 greater than 30° C./s; then heating the cold rolled steel sheet from CS1 temperature to an overaging temperature TOA from 250° C. to 580° C. at an average heating rate HR3 from 1° C./s to 100° C./s; then overaging the cold rolled steel sheet at TOA during 5 to 500 seconds.

29: The method as recited in claim 28 wherein the HT1 temperature is from 625° C. to 775° C.

30: The method as recited in claim 28 further comprising coating the cold rolled steel sheet with zinc or a zinc-based alloy.

31: The method as recited in claim 28 further comprising performing a scale removal process on the hot rolled steel sheet.

32: The method as recited in claim 28 further comprising subjecting the hot rolled steel sheet to an annealing at a temperature from 350° C. to 750° C. during 1 h to 96 h.

33: The method as recited in claim 32 further comprising performing a scale removal process on the hot rolled annealed steel sheet.

34: A method comprising employing the cold rolled steel sheet produced according to the method of claim 28 for manufacture of a structural or safety part of a vehicle.

35: A vehicle comprising a part obtained according to the method of claim 34.

36: A method comprising employing the cold-rolled and heat-treated steel sheet as recited in claim 17 for manufacture of a structural or safety part of a vehicle.

37: A vehicle comprising a part obtained according to the method of claim 36.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 is a schematic demonstration of the cold rolled steel sheet which is in accordance of the present invention and corresponds to trial I1, the cold rolled steel sheet having a layer enriched in ferrite, wherein the mean ferrite percentage in the layer extending up to 50 microns from the surface is 70%. Ferrite layer designated as 10 shows the ferrite layer having ferrite presence as 70%.

[0046] FIG. 2 is a schematic demonstration of the cold rolled steel sheet which is not in accordance of the present invention, the cold rolled steel sheet having a layer enriched in ferrite, wherein the mean ferrite percentage in the layer extending up to 50 microns from the surface is 43%. Ferrite layer designated as 20 shows the ferrite layer having ferrite presence as 43%.

DETAILED DESCRIPTION

[0047] 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.

[0048] For example, a slab will be considered as a semi-finished product. A slab having the above-described chemical composition is manufactured by continuous casting wherein the slab preferably underwent a direct soft reduction during casting to ensure the elimination of central segregation and porosity reduction. 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.

[0049] The temperature of the slab which is subjected to hot rolling is preferably at least 1000° C., preferably above 1200° 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 contained in the structure. Further, the temperature must not be above 1280° C. because industrially expensive.

[0050] The temperature of the slab is preferably sufficiently high so that hot rolling can be completed entirely in the austenitic range, the finishing hot rolling temperature remaining above 850° C. and preferably above 900° C. It is necessary that the final rolling be performed above 850° C., because below this temperature the steel sheet exhibits a significant drop in rollability. A final rolling temperature from 900 to 950° C. is preferred to have a structure that is favorable to recrystallization and rolling.

[0051] The sheet obtained in this manner is then cooled at a cooling rate above 30° C./s to a temperature which is below 550° C. The cooling temperature is kept below 550° C. to avoid oxidation of alloying elements such as manganese, silicon and chromium. Preferably, the cooling rate will be less than or equal to 65° C./s and above 35° C./s. Thereafter the hot rolled steel sheet is coiled and the temperature of the coiled hot rolled steel sheet must be kept below 500° C. to avoid oxidation of Silicon, Manganese, Aluminum and Chromium on the surface of hot rolled coil as these oxides forms cracks on the surface of the hot rolled steel sheet. Thereafter the coiled hot rolled steel sheet is allowed to cool down to room temperature. Then the hot rolled sheet is subjected to on optional scale removal process such as pickling to remove scale formed during hot rolling and ensure that there is no scale on the surface of hot rolled steel sheet before subjecting it to an optional hot band annealing.

[0052] The hot rolled sheet may be subjected to an optional hot band annealing at a temperature from 350° C. to 750° C. during 1 to 96 hours. The temperature and time of such hot band annealing is selected to ensure softening of the hot rolled sheet to facilitate the cold rolling of the hot rolled steel sheet.

[0053] The Hot rolled steel sheet is then cooled down to room temperature, thereafter, the hot rolled sheet is then cold rolled with a thickness reduction from 35 to 70% to obtain a cold rolled steel sheet.

[0054] The cold rolled steel sheet is then subjected to annealing to impart the steel of the present invention with targeted microstructure and mechanical properties.

[0055] In the annealing, the cold rolled steel sheet is subjected to two steps of heating to reach the soaking temperature TA from Ac3−10° C. to Ac3+100° C., during the two step heating the dewpoint is maintained from −15° C. to +15° C. to provide the steel of the present invention with a ferrite rich layer on surface to have adequate Liquid metal embrittlement resistance, the preferred dew point is maintained from −10° C. to +10° C. and more preferably from −10° C. to +5° C. The Ac3 for the present steel is determined by a dilatometry test as per the method described in article published in journal “TECHNIQUES DE L'INGENIEUR, MESURES ET ANALYSE; FRA; PARIS: TECH.-ING.; DA. 1981; VOL. 20; NO 59; P1280” by M. Murat.

[0056] In step one cold rolled steel sheet is heated from room temperature to temperature HT1 which is in a range from 600° C. to 800° C. at a heating rate HR1 from 2° C./s to 70° C./s. It is preferred to have HR1 rate from 5° C./s to 60° C./s and more preferably from 10° C./s to 50° C./s. The preferred HT1 temperature is from 625° C. to 775° C., more preferably from 640° C. to 750° C.

[0057] Thereafter in subsequent second step of heating, the cold rolled steel sheet is heated from temperature HT1 to the soaking temperature TA which is in temperature range from Ac3−10° C. to Ac3+100° C. at a heating rate HR2 from 0.1° C./s to 10° C./s. It is preferred to have HR2 rate from 0.1° C./s to 8° C./s and more preferably from 0.1° C./s to 5° C./s.

[0058] The preferred TA temperature is from Ac3 to Ac3+75° C., more preferably from Ac3 to Ac3+50° C. Dew point is maintained from −10° C. to +10° C. at the soaking temperature and preferably from −5° C. to +5° C. to provide the present steel with the ferrite-enriched layer at the surface with the targeted depth.

[0059] As mentioned above, the ferrite-enriched layer according to the invention is formed during annealing. Carbon reacts with oxygen to form carbon monoxide that escapes from the steel, resulting in a decarburization of the surface layer, such layer having a microstructure enriched in ferrite and extending from the surface of the sheet up to the depth of 50 microns. This ferrite-enriched layer forms during the heating before annealing and during soaking thanks to the control of dew point. The dew point is controlled from −15° C. to +15° C. during the heating before annealing and from −10° C. to +10° C. during the soaking by using conventional means known by the man skilled in the art, like water injection for example.

[0060] Then the cold rolled steel sheet is held at the annealing soaking temperature TA during 10 to 1000 seconds to ensure adequate transformation to Austenite microstructure of the strongly work-hardened initial structure. It is Then the cold rolled steel sheet is cooled in a single step cooling, at a cooling rate CR1 which is more than 30° C./s and preferably more than 40° C./s and more preferably more than 50° C./s to a cooling stop temperature range CS1 from Ms−5° C. to Ms−100° C. and preferably from Ms−5° C. to Ms−75° C. and more preferably from Ms−10° C. to Ms−50° C. During this step of cooling, martensite of the present invention is formed.

[0061] In a subsequent step the cold rolled steel sheet is heated to an overaging temperature range TOA from 250° C. to 580° C. from CS1 temperature at a heating rate HR3 from 1° C./s to 100° C./s. During this step, martensite formed during cooling after annealing is transformed into partitioned martensite, thereby assisting in formation of bainite during the holding at TOA temperature. Then the cold rolled steel sheet is held at TOA temperature for over-aging during 5 to 500 seconds allowing the bainite of the present invention to be formed.

[0062] Then the cold rolled steel sheet can be brought to the temperature of a hot dip coating bath, which can be from 420° C. to 680° C., depending on the nature of the coating. The coating can be made with zinc or a zinc-based alloy or with aluminium or with an aluminum-based alloy.

[0063] Alternatively, the cold rolled steel sheet may also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, Hot dip (GI), GA or ZM etc., which do not require the steel sheet to be brought to the above described range of temperature after averaging. In that case, the steel sheet can be cooled down to room temperature before being coated in a subsequent step.

[0064] An optional post batch annealing, preferably done at 170 to 210° C. during 12 h to 30 h can be performed after annealing on a coated product in order to ensure degassing for coated products.

EXAMPLES

[0065] The following tests and examples 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 and expound the significance of the parameters chosen by inventors after extensive experiments and further establish the properties that can be achieved by the steel according to the invention.

[0066] Samples of the steel sheets according to the invention and to some comparative grades were prepared with the compositions gathered in table 1 and the processing parameters gathered in table 2. The corresponding microstructures of those steel sheets were gathered in table 3 and the properties in table 4.

[0067] Table 1 depicts the steels with the compositions expressed in percentages by weight.

TABLE-US-00001 TABLE 1 composition of the trials Trials C Mn Si Al Mo P S N Cr Nb Ti B Ac3 1 0.199 2.620 1.270 0.030 0.097 0.0150 0.002 0.0050 0.017 0.002 0.002 0.0004 850 2 0.197 2.630 1.780 0.024 0.093 0.0120 0.001 0.0037 0.015 0.002 0.002 0.0004 880 3 0.198 2.720 1.740 0.025 0.096 0.0120 0.001 0.0041 0.023 0.002 0.002 0.0004 875 4 0.196 2.710 1.780 0.024 0.095 0.0110 0.001 0.0043 0.015 0.021 0.002 0.0004 875 5 0.190 2.720 1.770 0.024 0.096 0.0110 0.001 0.0044 0.015 0.022 0.022 0.0022 880 6 0.193 2.750 1.780 0.024 0.002 0.0110 0.001 0.0038 0.017 0.001 0.002 0.0004 870 7 0.188 2.750 1.680 0.021 0.002 0.0120 0.001 0.0050 0.019 0.020 0.024 0.0026 870 underlined values: not according to the invention

[0068] Table 2 gathers the annealing process parameters implemented on steels of Table 1.

[0069] Table 1 also shows Bainite transformation Bs and Martensite transformation Ms temperatures of inventive steel and reference steel. The calculation of Bs is done by using Van Bohemen formula published in Materials Science and Technology (2012) vol 28, no. 4, pp 487-495, which is as follows:

Bs=839−(86*[Mn]+23*[Si]+67*[Cr]+33*[Ni]+75*[Mo])−270*(1−EXP(−1.33*[C]))

[0070] Ms was determined through dilatometry tests in a similar way as Ac3.

[0071] Further, before performing the annealing treatment on the steels of invention as well as reference, the samples were heated to a temperature from 1000° C. to 1280° C. and then subjected to hot rolling with finishing temperature above 850° C. The cooling rate after hot rolling was above 30° C./s until cooling down below 550° C. The HT1 temperature is 650° C. for all trials and the HR2 heating rate is at 0.5° C./s for all trials. All cold rolled steel sheets were coated in a zinc bath at temperature 460° C. after the over aging holding.

TABLE-US-00002 TABLE 2 process parameters of the trials Coiling HBA Trials steel sample Temp (° C.) Pickling (° C.) t (h) Pickling CR reduction (%) I1 1 450 Y 550 10 Y 53 I2 2 450 Y 580 10 Y 53 I3 3 450 Y 580 10 Y 50 I4 4 450 Y 620 10 Y 52 I5 5 450 Y 620 10 Y 38 R1 6 450 Y 580 10 Y 52 R2 7 450 Y 550 10 Y 43 R3 3 450 Y 580 10 Y 50 R4 3 450 Y 580 10 Y 50 Annealing Dew point Dew point Holding HR1 TA Soaking during during CR1 CS1 HR3 TOA Time Trials (° C./s) (° C.) time (s) heating (° C.) soaking (° C.) (° C./s) (° C.) (° C./s) (° C.) (s) Bs Ms I1 10 880 290   0   2 100 301 45 460 110 513 335 I2 20 890 335   3   2 90 300 45 460 130 502 325 I3 15 890 335 −5 −2 90 300 45 460 130 494 325 I4 25 890 335   2 −1 90 279 51 460 130 495 320 I5 10 890 600 −5 −5 50 279 51 460 225 496 320 R1 21 870 260 −10    0 100 309 42 460 100 499 325 R2 13 870 260 −2   0 100 320 40 460 100 503 320 R3 15 890 335   0 −45  90 289 48 460 130 494 325 R4 15 890 335 −20  −45  90 289 48 460 130 494 320 HBA: hot band annealing of steel sheet I = according to the invention; R = reference; underlined values: not according to the invention.

[0072] Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials.

TABLE-US-00003 TABLE 3 microstructures of the trials and the presence of Ferrite in Ferrite layer Steel Sheet core Ferrite Austenite Layer Mean % total ferrite from Martensite residual 0 to 50 μm ferrite bainite partitioned austenite from the Trials (%) (%) (%) fresh (%) blocky (%) film like (%) (%) surface I1 0 65 30 0 5 10 15 70 I2 1 69 25 0 5 8 13 70 I3 0 70 25 0 5 7 12 65 I4 0 55 40 0 5 10 15 70 I5 0 55 40 0 5 10 15 65 R1 5 70 20 0 5 9 14 50 R2 2 86  0 5 7 0  7 70 R3 0 65 30 0 5 9 14 35 R4 0 65 30 0 5 10 15  5 I = according to the invention; R = reference; underlined values: not according to the invention.

[0073] It can be seen from the table above that the trials according to the invention all meet the microstructure targets.

[0074] On the contrary, trial R1, which involves a composition out of the scope of the invention as it lacks the minimum value of molybdenum, shows a surface layer that is not sufficiently high in ferrite content, as molybdenum has a direct impact on the ferrite enrichment at the surface of the steel.

[0075] Trial R2, which involves a composition out of the scope of the invention as it lacks the minimum value of molybdenum was submitted to a CS1 temperature above Ms−5° C., which, in combination, triggered too much bainite formation. The ferrite layer is in target thanks to the optimal value of the dew point during heating.

[0076] Trials R3 and R4, where the required dew points control was not performed, show a ferrite surface layer that is clearly not sufficiently high in ferrite content.

[0077] Table 4 gathers the mechanical and surface properties of both the inventive steel and reference steel. The tensile strength yield strength and total elongation tests are conducted in accordance with ISO 6892-1 standards and the test for Hole expansion ratio is conducted accordance with ISO 16630 standards.

[0078] Table 4: mechanical and surface properties of the trials

[0079] The susceptibility of LME of the trials was evaluated by resistance spot welding method. To this end, for each Trial, one steel sheet corresponding respectively to trials I1 to I5 and to trials R1 to R4 was spot welded with two additional steel sheets to build a three-sheet stack-up including successively: [0080] one steel sheet corresponding to trials I1 to I5 and to trials R1 to R4, [0081] a sheet of 1.5 mm of an Interstitial free galvanized steel comprising 0.003% of carbon and 0.11% of manganese, [0082] a sheet of 1.5 mm of an Interstitial free galvanized steel comprising 0.003% of carbon and 0.11% of manganese.

[0083] Welding conditions were according to standard ISO-18278-2. The type of the welding electrode was F1 with a face diameter of 6 mm; the clamping force of the electrode was set at 450 daN. The welding cycle is as follows:

TABLE-US-00004 Welding time Weld time Current (Hz) (ms) Cool time (ms) Cycle 50 380 260

[0084] Each trial was reproduced 10 times to produce 10 spot welds at a current level defined as the upper welding limit of the current range from Imax to Imax+10%, max being comprised between 0.9 and 1.1*Iexp, Iexp being the intensity beyond which expulsion appears during welding, determined according to ISO standard 18278-2.

[0085] The cracks length in the 10 spot-welded joints was then evaluated after cross-sectioning through the surface crack and using an optical microscope. A grade was considered as providing enough LME resistance if less than 60% of the spots had a crack longer than 200 μm.

[0086] The yield strength YS, the tensile strength TS and the total elongation TE are measured according to ISO standard ISO 6892-1, published in October 2009. The hole expansion ratio is measured according to ISO standard 16630:2009.

TABLE-US-00005 Total elongation Hole expansion LME Trials TS (MPa) (%) ratio (%) resistance YS (MPa) I1 1181 12.0 37 OK 846 I2 1181 13.3 40 OK 852 I3 1191 15.0 39 OK 868 I4 1205 14.1 47 OK 965 I5 1250 14.1 35 OK 969 R1 1165 13.5 37 NOT OK 845 R2 1218 11.5 25 OK 745 R3 1243 13.9 45 NOT OK 981 R4 1250 13.8 45 NOT OK 992 I = according to the invention; R = reference; underlined values: not according to the invention.

[0087] It can be seen from the table above that the trials according to the invention all meet the properties targets.

[0088] On the contrary, trial R1 shows a tensile strength value that is not enough, which is linked to the low content in molybdenum in the grade. Moreover, the LME resistance is not good, due to the low enrichment in ferrite in the surface layer, which is also linked to the low molybdenum content.

[0089] Trial R2 shows a TS value that is satisfactory, despite a low level in molybdenum. This is due to the content of niobium that can compensate for low molybdenum in terms of strength. However, the hole expansion ratio is below target notably because of an excessive amount of bainite and a too low amount of austenite.

[0090] Trials 3 and 4 do not show enough LME resistance, which is explained by the low ferrite amount in the surface layer.