Method for manufacturing a high strength steel sheet having improved flatness

Abstract

The invention relates to a process for manufacturing a cold rolled and heated steel sheet, comprising the following steps: Providing a hot rolled steel sheet having a composition comprising by weight percent: C 0.05 – 0.3 %, Mn: 1.5 – 5.0%, Si: 0.1 - 2%, Al: 0.01 - 3%, Ti 0.001 – 0.050% the remainder being iron and unavoidable impurities resulting from the smelting, rolling, cooling and coiling the sheet to a temperature below 700°C, heating the steel sheet to a temperature Ta from 500°C to (Ae1+Ae3) / 2, for a holding time ta comprised from 0.01 and 100h, cold rolling the sheet, heating the sheet to a temperature TH comprised from (Ae3-10°C) to (Ae3+150°C) for a holding time comprised from 1s to 1000s, cooling to room temperature, heating the sheet to a temperature Ttemp comprised from 150°C to 350°C, for a holding time ttemp comprised from 100s to 3600s and cooling to room temperature.

Description

[0001] Method for manufacturing a high strength steel sheet having improved flatness

[0002]

[0001] The present invention relates to a method for manufacturing a high strength steel sheet having good flatness properties.

[0003]

[0002] To manufacture various items such as parts of body structural members and body panels for automotive vehicles, it is known to use sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels.

[0004]

[0003] One of the major challenges in the automotive industry is to decrease the weight of vehicles in order to improve their fuel efficiency in view of the global environmental conservation, without neglecting the safety requirements. To meet these requirements, new high strength steels are continuously developed by the steelmaking industry, to have sheets with improved yield and tensile strengths, and good ductility and formability.

[0005]

[0004] Martensitic products are always obtained after a high-speed cooling, which often produces important distortion of the strip. Even if the leveler could correct the produced flatness defects, it requires sometimes a too high elongation which affects the final mechanical properties. The objective of this study is to find solutions to improve the flatness of martensitic products.

[0006]

[0005] The aim of the present invention is therefore to find a method to improve the flatness of martensitic products, while preserving the mechanical properties, by providing a steel sheet having a tensile strength TS above or equal to the one of a fully martensitic steel sheet with the same chemical composition, calculated through the formula TStarg= 2900x> / (%C) + 70x%Mn, %C and %Mn being the carbon and manganese content being expressed in weight percent (wt.%), and having a difference between the mean value of the ten highest positions of the surface of the sheet hi and the mean value of the ten lowest positions of the surface of the sheet li in the direction perpendicular to the length of the sheet, over a length of the sheet of one meter, satisfying A = 2.3 x < 4mm.

[0007]

[0006] The object of the present invention is achieved by providing a method for manufacturing a cold rolled and heated steel sheet according to claim 1. The method can also comprise any of the characteristics of claims 2 to 6, taken alone or in combination.

[0008]

[0007] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0009]

[0008] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0010] Figure 1 , which illustrates the method to determine the flatness quality.

[0009] The composition of the steel sheet that can be used in the method according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0011]

[0010] The carbon content of the steel used in the method according to the invention can be from 0.05% to 0.3 % to ensure a satisfactory strength and good weldability properties to the steel sheet. Above 0.3% of carbon, weldability of the steel sheet may be reduced. If the carbon content is lower than 0.05%, the strength of the martensite is not sufficient to get the targeted tensile strength. Preferably, the carbon content is from 0.05% to 0.3%, more preferably from 0.08% to 0.3%, even more preferably from 0.08% to 0.25%.

[0012]

[0011] The manganese content is from 1 .5% to 5.0 % for ensuring a satisfactory strength. Above 5.0% of addition, weldability of the steel sheet may be reduced, and the risk of central segregation increases to the detriment of the mechanical properties. The minimum is defined to stabilize austenite, and to obtain the targeted microstructure. Moreover, below 1 .5% strength of the steel may be reduced. Preferably, the manganese content is from 2.6% to 5.0%. More preferably the manganese content is from 3.0% to 5.0%.

[0013]

[0012] The silicon content is from 0.1% to 2%. Above 2%, silicon oxides form at the surface, which impairs the coatability of the steel. A minimum of 0.1% of silicon content is added to increase the strength. Preferably a minimum of 0.15% of silicon is added. The maximum amount of silicon added is preferably 1 .5%.

[0014]

[0013] Aluminium is added in a content from 0.01% to 3% to decrease the manganese segregation during casting. Aluminium is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Above 3% of addition, the weldability of the steel sheet may be reduced, so as castability. Preferably, the maximum amount of aluminium added is 2.5%, more preferably 2.0%, even more preferably 1.5% or 1%.

[0015]

[0014] Titanium is added from 0.001 % to 0.050 % to provide precipitation strengthening. Moreover, titanium is added when boron is present, to protect boron against the formation of BN. Preferably, the minimum content of titanium is 0.005%, more preferably 0.01%.

[0016]

[0015] Optionally some elements can be added to the composition of the steel according to the invention.

[0017]

[0016] Chromium can be added up to 0.5% to improve the hardenability of the steel sheet. Above 0.5% a saturation effect is noted, and adding chromium is both useless and expensive. Higher chromium causes surface cleaning issues during pickling process and as a result, affects coatability of the steel.

[0018]

[0017] Molybdenum can optionally be added up to 0.3 % in order to decrease the phosphorus segregation. Above 0.3%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably the minimum amount of molybdenum is 0.05%.

[0019]

[0018] Niobium can optionally be added up to 0.05 % to refine the austenite grains during hot-rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.

[0020]

[0019] Boron can be added up to 0.005% to improve the toughness of the hot rolled steel sheet and the spot weldability of the cold rolled steel sheet. Above 0.005%, the formation of boro-carbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Preferably, the minimum amount of boron is 0.0005%, more preferably 0.001 %. Preferably the maximum amount of boron is 0.004%.

[0021]

[0020] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route.

[0022]

[0021] In the case of a production route without the use of scraps, as it is generally the case in the Blast Furnace-Basic Oxygen Furnace (BF-BOF) route, the level of unavoidable impurities is very low.

[0023]

[0022] In the case of a production route using scraps, as in an Electric Arc Furnace (EAF) or loaded in a converter in a BF BOF, the steel sheet can further comprise residual elements coming from such scraps such as copper up to 0.4%, nickel up to 0.25%, tin up to 0.05%, arsenic up to 0.03%, antimony up to 0.03%, or lead up to 0.03% which are considered as unavoidable impurities.

[0024]

[0023] P, S and N are also part of the unavoidable impurities whatever the production route. Their content is below or equal to 0.025 % for P, below or equal to 0.010 % for S, and below or equal to 0.02 % for N.

[0025]

[0024] The microstructure of the steel sheet manufacturing by the method according to the invention will now be described. It contains, in surface fraction:

[0026] - 45% or less of bainite,

[0027] - 5% or less of ferrite,

[0028] - the rest being tempered martensite.

[0029]

[0025] The microstructure of the steel sheet contains from 45% or less of bainite in order to obtain a high tensile strength level. Above 45% of bainite, the tensile strength can be reduced. Bainite can be formed during the cooling of the steel sheet after being heated to TH. Preferably, the microstructure of the steel sheet contains 30% or less of bainite, more preferably 15% or less of bainite, even more preferably 5% or less of bainite.

[0026] The microstructure of the steel sheet can contain 5% of less of ferrite. The ferrite can be formed during the heating of the hot rolled steel sheet at temperature Ta, and also during the heating of the cold rolled steel sheet at a temperature TH, when TH is below Ae3. Preferably, the microstructure of the steel sheet contains no ferrite.

[0030]

[0027] The rest of the microstructure of the steel sheet according to the invention is tempered martensite. Martensite is formed during the cooling after the soaking of the steel sheet, once Ms, the martensite start temperature, is reached. This martensite is then tempered during the tempering of the steel sheet.

[0031]

[0028] The manganese distribution in the microstructure is heterogeneous. There are areas with high amount of manganese, and areas with low amount of manganese. The variation of the manganese content in the microstructure A%Mn is calculated through the standard deviation o of the distribution of manganese in the microstructure, such that A%Mn = 4 x o > 1 , the standard deviation being calculated through a method described below

[0032]

[0029] The cold rolled and heated steel sheet is produced by the method according to the invention, comprising the following steps:

[0033]

[0030] A semi-product able to be further hot-rolled, is provided with the steel composition described above. Such semi-product can for example be a slab.

[0034]

[0031] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process with or without the use of scraps.

[0035]

[0032] The semi product is heated to a temperature Theat from 1100°C to 1350°C, so to make it possible to ease hot rolling, with a final hot rolling temperature FRT from 800°C to 1000°C. Preferably, the FRT is from 800°C to 950°C, more preferably from 800°C to 930°C, even more preferably from 830°C to 930°C.

[0036]

[0033] The hot-rolled steel sheet is then cooled and coiled at a temperature TCOii below or equal to 700°C, and preferably from 400°C to 650°C. At this stage, the manganese distribution in the microstructure of the hot rolled steel sheet is heterogeneous due to the formation of Mn-rich pearlite or carbides.

[0037]

[0034] The coiled steel sheet is then annealed to an annealing temperature Tacomprised from 500°C and (Ae1 +Ae3) / 2. The steel sheet is maintained at said temperature Tafor a holding time tacomprised from 0.01 and 100h in order to decrease the hardness while maintaining the toughness of the hot-rolled steel sheet. This heating step allows the manganese to further diffuse in the carbides or austenite with the formation of highly Mn- enriched carbides and austenite areas, leading to a higher heterogeneity of manganese distribution in the microstructure.

[0035] The hot rolled steel sheet is then cooled and cold rolled. The reduction ratio can be comprised from 5% to 80%, depending on the targeted thickness. The aim of this cold rolling would be to reduce the thickness of the steel sheet.

[0038]

[0036] The steel sheet is then heated to a temperature TH comprised from (Ae3-10°C) to (Ae3+150°C) and maintaining at said TH temperature for a holding time tn comprised from 1 s to 1000s, in order for the steel sheet to have a microstructure mainly austenitic. Such heating can be performed by continuous annealing. Preferably the TH temperature is above or equal to Ae3. Preferably the TH temperature is below or equal to (Ae3+100°C).

[0039]

[0037] The steel sheet is cooled to room temperature. The cooling can be done in four steps:

[0040] - In a first step cooling, the cold rolled steel sheet is cooled from TH to a temperature Ti comprised from 600°C to 700°C, with a cooling rate Vi comprised from 10°C / s to 1000°C / s. Preferably Ti is comprised from 620°C to 680°C, more preferably from 620°C to 670°C. Preferably Vi is comprised from 10°C / s to 500°C / s, more preferably from 10°C / s to 250°C / s, Vi being chosen to limit the ferrite formation to less than 5%.

[0041] - In a second step, the cold rolled steel sheet is cooled from Ti to a temperature T2comprised from ((Bs+Ms) / 2)-20°C to ( (Bs+Ms) / 2)+20°C with a cooling rate v2below or equal to Vi.

[0042] - In a third step, the steel sheet is cooled from T2to a temperature T3comprised from (Ms+10°C) to (Ms-10°C) with a cooling rate v3below or equal to v2, v3being chosen to avoid a formation of more than 45% of bainite, which would reduce the tensile strength.

[0043] - The steel sheet is then cooled from T3to room temperature with a cooling rate v4comprised from 10°C / s to 500°C / s. Below T3, the martensite begins to be formed. The martensite phase transformation does not depend on the cooling rates.

[0044]

[0038] The cooling rates Vi, v2, v3and v4can be equal, the cooling being thus performed in one step from TH to room temperature.

[0045]

[0039] The manganese heterogeneity created during coiling and annealing at Taof the hot rolled steel sheet, is kept during the soaking and cooling to room temperature, and allows to slow down the martensite transformation rate, which allow to obtain a good flatness quality. Moreover, the successive cooling steps can also be done to decrease the cooling rate, in order to slow down the phase transformation kinetics .

[0046]

[0040] The steel sheet is reheated to a temperature Ttempcomprised from 150°C to 350°C, and maintained at said temperature for a holding time ttempcomprised from 100s to 3600s. The fresh martensite is totally transformed into tempered martensite at the end of this tempering step.

[0041] Preferably Ttempis from 150°C to 300°C. Preferably the holding time ttemp is from 50s to 3600s, more preferably from 80s to 3600s, even more preferably from 100s to 3600s, or from 100s to 1800s.

[0047]

[0042] The steel sheet is then cooled to room temperature.

[0048] 5

[0043] The obtained cold rolled and heated steel sheet can then be coated by any suitable process including electrodeposition or vacuum coating of zinc or zinc-based alloys or of aluminium or aluminium-based alloys.

[0049]

[0044] The cold rolled and heated steel sheet produced by the method according to the 10 invention has a tensile strength TS above or equal to the one TStarg= 290C (%C) +

[0050] 70x%Mn, %C and %Mn being the carbon and manganese content being expressed in weight percent (wt.%).

[0051]

[0045] The cold rolled and heated steel sheet produced by the method according to the invention has good flatness quality, with a difference between the mean value of the ten 15 highest positions of the surface of the sheet hi and the mean value of the ten lowest positions of the surface of the sheet in the direction perpendicular to the length of the sheet, over a length of the sheet of one meter, as illustrated in Figure 1 , satisfying the following condition:

[0052] 20

[0046] The invention will be now illustrated by the following examples, which are by no way limitative.

[0053] Examples

[0054]

[0047] Two grades, whose compositions are gathered in table 1 , were cast in semi-products 25 and processed into steel sheets.

[0055] Table 1 - Compositions

[0056]

[0048] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0057] 30

[0049] Ae3, and Ae1 temperatures of the steel sheet has been determined through thermodynamic calculations with software as Thermo-calc®.

[0058]

[0050] Ms is measured through dilatometry.

[0059]

[0051] Bs is the bainite start temperature, close to equilibrium conditions and is measured according to the following formula, the chemical elements being expressed in weight percent (%wt.):

[0060]

[0052] Steel semi-products, as cast, were reheated at Theat, hot rolled with a final rolling temperature FRT of 900°C, coiled at a TCOii temperature. The hot rolled steel sheet is then batch annealed at a temperature Ta, and maintained at said temperature Tafor a holding time ta, before being cold rolled with a reduction rate of 50%. The steel sheets are then heated to TH and maintained at said temperature for a holding time tn. The steel sheet is then cooled to room temperature directly (vi=v2=v3=v4), or in four steps:

[0061] - from TH to a temperature Ti of 640°C with a cooling rate Vi,

[0062] - from Ti to T2= (BS+MS) / 2 with a cooling rate v2,

[0063] - from T2to T3= Ms, with a cooling rate v3,

[0064] - and cooled to room temperature with a cooling rate v4.

[0065] The steel sheets are then reheated to a temperature Ttempand maintained at said Ttemptemperature for a holding time ttemp, before being cooled to room temperature.

[0066]

[0053] The following specific conditions to obtain the steel sheets were applied:

[0067] Table 2 - Process parameters of the hot rolled steel sheets.

[0068] Table 3 - Process of the cold rolled steel sheets.

[0069]

[0054] The steel sheets were analyzed, and the corresponding phase percentages of the microstructure of the steel sheets were determined and gathered in table 4.

[0070] Table 4 - Microstructure of the steel sheets

[0071]

[0055] The surface fractions of phases in the microstructure are determined through the following method: a specimen is cut from the steel sheet, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, in secondary electron mode.

[0072]

[0056] The determination of phase fractions of martensite is performed by coupling SEM observations and dilatometry curve analyses using lever rule.

[0073]

[0057] The manganese heterogeneity can be measured through the variation of the manganese content in the microstructure A%Mn. A section of the specimen is observed through Electron probe micro-analysis (EPMA), with a Field Emission Gun (“FEG”) at a magnification greater than 10000x. Three maps of 12pm*9pm were acquired. These maps are constructed with pixels of 0.01 pm2. Manganese amount M is measured in each pixel, and a distribution of manganese is obtained by plotting the values of the manganese M as a function of the number of pixels having the said values. An average value Mnaverageis then calculated from all M values. The standard deviation o is calculated from the values of the manganese M in the pixels, the mean value of manganese Mnaverage and the number of pixels N, here 32400, such as : o = -|-^e varjatjon of themanganese content in the microstructure is calculated through A%Mn = 4c.

[0074] Underlined values: out of the invention

[0075]

[0058] Mechanical properties of the steel sheets were determined and gathered in table 5.

[0076] Table 5 - Mechanical of the steel sheet

[0077]

[0059] The targeted tensile strength TStargis calculated through the carbon and manganese content: 2900 x - (%C) + 70x%Mn.

[0078]

[0060] The tensile strength TS of trials 1 and 2 are measured according to ISO standard ISO 6892-1 , published in October 2009. The tensile strength TS of trials 3 and 4 are calculated through the Vickers hardness value HV: TS=(HV / 3)x10, the hardness being measured according to ISO standard ISO 6507-1 :2018, the steel sheet of the trial 3 having a hardness value of 455HV, the steel sheet of the trial 4 of 457HV.

[0079]

[0061] A specimen is cut from the steel sheet to determine the flatness quality. The flatness quality of the sheet is determined through the difference between the mean value of the ten highest positions hi and the mean value of the ten lowest positions of the surface of the sheet in the direction perpendicular to the length of the sheet, over a length of the sheet of 1 meter, as illustrated in Figure 1 , according to the following formula: these highest and lowest values being measured with a measuring arm, such as a Faro® Arm for example. The flatness quality is considered as OK if this difference A is below or equal to 4mm, otherwise, the flatness quality is not ok.

[0062] The steel sheets of trials 1 to 2 are in accordance with the invention, with a good flatness quality and a tensile strength TS above or equal to TStarg. Thanks to the annealing of the hot rolled steel sheet, the manganese distribution becomes more heterogeneous, with a formation of highly manganese-enriched carbides and austenite areas, kept all along the heating and cooling steps, as shown in table 4 with the variation A%Mn. This heterogeneity allows to slow down the martensite transformation rate and so, to obtain an improved flatness quality in comparison to trials 3 and 4 for which the quality of flatness is not according to the invention. Indeed, in the case of trials 3 and 4, the heterogeneity of manganese distribution is not created, as seen in Table 4 with a variation of manganese amount A%Mn lower than 1 .

Claims

CLAIMS1 . A process for manufacturing a cold rolled and heated steel sheet, comprising the following successive steps: casting a steel to obtain a semi product having a composition comprising by weight percent:C: 0.05 - 0.3 %Mn: 1.5 - 5.0%Si: 0.1 - 2%Al: 0.01 - 3%Ti 0.001 - 0.050% and comprising optionally one or more of the following elements, in weight percentage:Cr < 0.5%Mo < 0.3%Nb < 0.05 %B <0.005% the remainder of the composition being iron and unavoidable impurities resulting from the smelting, heating the semi product at a temperature Theat from 1100°C to 1350°C, hot rolling the heated semi product at a finish hot rolling temperature FRT from 800°C to 1000°C, coiling the hot rolled steel sheet at a coiling temperature TCOii below or equal to 700°C, heating the steel sheet to a temperature Tacomprised from 500°C to (Ae1 +Ae3) / 2, and maintaining at said temperature for a holding time tacomprised between 0.01 and 10Oh, cold rolling the steel sheet, heating the cold rolled steel sheet to a temperature TH comprised from (Ae3-10°C) to (Ae3+150°C) and maintaining at said TH temperature for a holding time tn comprised from 1 s to 1000s, cooling the cold rolled steel sheet to room temperature in four steps:- in a first step, cooling the cold rolled steel sheet from TH to a temperature Ti comprised from 600°C to 700°C, with a cooling rate Vi comprised from 10°C / s to 1000°C / s,- in a second step, cooling the cold rolled steel sheet from Ti to a temperature T2comprised from ((Bs+Ms) / 2)-20°C to ((Bs+Ms) / 2)+20°C with a cooling rate v2below or equal to Vi,- in a third step, cooling the heated steel sheet from T2to a temperature T3comprised from (Ms+10°C) to (Ms-10°C) with a cooling rate v3below or equal to V2,- cooling the heated steel sheet from T4to room temperature with a cooling rate v4comprised from 10°C / s to 500°C / s, heating the steel sheet to a temperature Ttempcomprised from 150°C to 350°C, and maintaining at said temperature for a holding time ttempcomprised from 100s to 3600s, cooling the steel sheet to room temperature in order to obtain a cold rolled and heated steel sheet having a microstructure comprising 5% or less of ferrite, 45% or less of bainite, the rest being tempered martensite, and a variation of manganese content in the microstructure A%Mn above or equal to 1 , A%Mn being calculated from the standard deviation G of the distribution of manganese in the microstructure, through A%Mn = 4 x o.

2. A process for manufacturing a cold rolled and heated steel sheet according to claim 1 , wherein the steel sheet has a composition comprising manganese content Mn from 2.6% to 5.0%.

3. A process for manufacturing a cold rolled and heated steel sheet according to anyone of claims 1 and 2, wherein the cold rolled and heated steel sheet has a microstructure comprising of 15% or less of bainite, the rest being tempered martensite.

4. A process for manufacturing a cold rolled and heated steel sheet according to any one of claims 1 to 3, wherein the temperature TH is above or equal to Ae3.

5. A process for manufacturing a cold rolled and heated steel sheet according to any one of claims 1 to 4, wherein the cold rolled and heated steel sheet has a difference between the mean value of the ten highest positions of the surface of the sheet hi and the mean value of the ten lowest positions of the surface of the sheet I in the direction perpendicular to the length of the sheet, over a length of the sheet of one meter, satisfying the formula6. A process for manufacturing a cold rolled and heated steel sheet according to any one of claims 1 and 5, wherein the cold rolled and heated steel sheet has a tensile strength TS above or equal to the targeted tensile strength TStarg= 2900x> / (%C) + 70x%Mn, %C and %Mn being the carbon and manganese content expressed in weight percent.

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