Cold-rolled heat-treated steel sheet and its manufacturing method

Abstract

A cold-rolled heat-treated steel sheet, the steel having, by weight percent, 0.15%≦carbon≦0.25%, 2.2%≦manganese≦3%, 1.1%≦silicon≦2%, 0%≦aluminum≦0.09%, 0.05%≦molybdenum≦0.5%, 0.001%≦titanium≦0.06%, 0.001%≦boron≦0.010%, 0%≦phosphorus≦0.02%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.0 9%, 0%≦chromium≦1%, 0%≦copper≦2%, 0%≦niobium≦0.06%, 0%≦vanadium≦0.1%, 0%≦calcium≦0.005%, 0%≦magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦cerium≦0.1%, and the balance including iron and unavoidable impurities. The steel plate has an area fraction of 15% to 70% bainite and 15% to 70% separated martensite. a core microstructure comprising 10% to 30% ferrite, 9% to 22% retained austenite in bainite and separated martensite, and 0% to 5% fresh martensite; and a ferrite-rich layer extending up to 90 microns from each side of the steel sheet, the ferrite-rich layer having two sublayers, a first sublayer adjacent the surface of the steel sheet having an average ferrite fraction, by area fraction, of 40% to 80%, and a ferrite gradient (ΔF) of 20% to 70%, ΔF being the difference between the ferrite fraction at the top of the first sublayer and the ferrite fraction at the bottom of the second sublayer and at the top of the ferrite-rich layer, the second sublayer having a ferrite fraction, by area fraction, of 70% to 98%, and containing internal oxides of manganese and silicon.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled heat-treated steel sheet suitable for use as a steel sheet for vehicles. [Background technology]

[0002] Automotive parts must satisfy two contradictory requirements: ease of formability and strength. Recently, environmental concerns have placed a third demand on automobiles: improved fuel economy. Therefore, automotive parts must now be made from materials with high formability to meet the criteria for ease of incorporation into complex automotive assemblies, while simultaneously improving strength for vehicle crashworthiness and durability while reducing vehicle weight for improved fuel economy. Furthermore, steel parts must be weldable without the drawbacks of liquid metal embrittlement.

[0003] Therefore, vigorous research and development efforts are being made to reduce the amount of material used inside the vehicle by increasing the strength of the material. Conversely, increasing the strength of steel sheet reduces its formability, so there is a need to develop materials that combine high strength and high formability.

[0004] Previous research and development in the field of high strength and highly formable steel plates has resulted in several methods for producing high strength and highly formable steel plates, some of which are listed herein for a final understanding of the present invention.

[0005] EP3128027 discloses a steel sheet containing 0.15 to 0.25 mass% of C, 1.8 to 3.0 mass% of Mn, and 0.0003 to 0.0050 mass% of B, and having a composite structure with a ferrite volume fraction of 20% to 50%, a retained austenite volume fraction of 7% to 20%, a martensite volume fraction of 1% to 8%, and the balance including bainite and tempered martensite, in which the ferrite has an average grain size of 5 μm or less, and the retained austenite has a volume fraction of 20% to 50%, a volume fraction of 7% to 20%, a volume fraction of 1% to 8%, and a composite structure containing bainite and tempered martensite. The bainite has an average grain size of 0.3 to 2.0 μm and an aspect ratio of 4 or more, the martensite has an average grain size of 2 μm or less, the metallic phase including both bainite and tempered martensite has an average grain size of 7 μm or less, the ratio of the volume fraction of tempered martensite to the volume fraction of the metallic structure other than ferrite is 0.60 to 0.85, and the average C concentration in the retained austenite is 0.65 mass% or more. However, EP3128027 does not exhibit adequate LME resistance.

[0006] Known prior art techniques for producing high strength and highly formable steel sheets lack one or the other, and therefore there is a need for cold rolled steel sheets having strength greater than 1100 MPa and methods for producing the same. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent Application Publication No. 3128027 Summary of the Invention

[0008] The object of the present invention is to solve the above problems by making available a cold rolled, heat treated steel sheet which simultaneously has: - an ultimate tensile strength of at least 1150 MPa, preferably greater than 1180 MPa; - Hole expansion ratio of at least 22%, preferably more than 25% - Good resistance to liquid metal embrittlement - Total elongation of 13% or more

[0009] In a preferred embodiment, the cold rolled heat treated steel sheet exhibits a yield strength value of at least 750 MPa, preferably above 800 MPa.

[0010] Preferably, such steels also have good weldability and coatability as well as good suitability for forming, especially rolling.

[0011] Another object of the invention is also to make available a method for manufacturing these plates that is robust to shifts in manufacturing parameters, while still being compatible with conventional industrial applications.

[0012] The cold-rolled heat-treated steel sheet of the present invention is coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, in order to improve corrosion resistance.

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0014] Carbon is present in steel in an amount of 0.15% to 0.25%. Carbon is an element necessary for delaying the formation of ferrite or bainite during cooling after annealing, thereby increasing the strength of the steel sheet. Carbon also plays an important role in stabilizing austenite. At a carbon content of less than 0.15%, austenite cannot be stabilized, resulting in reduced strength and ductility. On the other hand, at a carbon content exceeding 0.25%, the weld and heat-affected zones become significantly hardened, impairing the mechanical properties of the weld. The preferred carbon limit is 0.16% to 0.24%, and the more preferred limit is 0.17% to 0.22%.

[0015] The manganese content of the steel of the present invention is 2.2% to 3%. Manganese is an element that imparts strength and stabilizes austenite to obtain retained austenite. A manganese content of at least 2.2% is necessary to retard ferrite formation, providing strength and hardenability to the steel sheet, and to stabilize austenite. Therefore, a higher manganese content, such as 2.3 to 2.8%, is preferred. However, manganese content exceeding 3% has adverse effects, such as slowing the austenite-to-bainite transformation during isothermal holding of the bainite transformation, resulting in reduced ductility. Furthermore, manganese content exceeding 3% results in insufficient bainite formation, and martensite formation exceeds the target limit, thus reducing elongation. Furthermore, manganese content exceeding 3% also reduces the weldability of the steel.

[0016] The silicon content of the steel of the present invention is 1.1% to 2%. Silicon as a constituent delays the precipitation of carbon as carbides in bainite during soaking after cooling from high temperatures. Therefore, austenite is carbon-rich during the formation of carbide-free bainite. Therefore, the presence of 1.1% silicon stabilizes austenite at room temperature. Furthermore, silicon delays the precipitation of carbides in martensite. In both cases, carbides in bainite or martensite also cause a decrease in elongation. Preventing carbides through the presence of Si is very important. However, adding silicon in excess of 2% does not improve the above-mentioned effects and leads to problems such as liquid metal embrittlement. Silicon in excess of 2% in the steel of the present invention does not dissolve Zn into the grains. Therefore, during welding, liquid Zn travels along grain boundaries instead of entering the grains, causing liquid metal embrittlement. Therefore, its concentration is controlled to an upper limit of 2%. The preferred limit for silicon in the steel is 1.2% to 1.9%, more preferably 1.2% to 1.8%.

[0017] The aluminum content of the steel of the present invention is 0 to 0.09%. Aluminum is added during steelmaking to deoxidize the steel and capture oxygen. Above 0.09%, the Ac3 point increases, reducing productivity. Furthermore, within this range, aluminum combines with nitrogen in the steel to form aluminum nitride, reducing grain size. However, in the present invention, whenever the aluminum content exceeds 0.09%, the amount and size of aluminum nitride are detrimental to hole expanding and bending. The preferred limit for aluminum is 0 to 0.06%, more preferably 0 to 0.05%.

[0018] Molybdenum is an essential element present in the steel of the present invention at 0.05% to 0.5%. Molybdenum plays an important role in improving hardenability and hardness, and adding at least 0.05% of it retards the formation of ferrite and bainite during cooling after annealing. Mo also contributes to the toughness of hot-rolled products, making them easier to manufacture. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. Molybdenum also promotes the formation of a ferrite microstructure on the surface up to a depth of 50 microns measured from the outer surface, by slightly increasing Ac3 at the same soaking and dew point temperatures, thereby increasing the formation of ferrite on the surface of the steel of the present invention. The preferred limit of molybdenum is 0.06% to 0.3%, more preferably 0.07% to 0.2%.

[0019] Titanium is an essential element added to the steel of the present invention in an amount of 0.001% to 0.06%, preferably 0.001% to 0.03%. Like niobium, titanium plays a role in hardening because it participates in carbonitrides. However, titanium also participates in the formation of TiN, which appears during solidification of cast products. The amount of Ti is limited to 0.06% to avoid coarse TiN, which is detrimental to hole expansion. If the titanium content is less than 0.001%, titanium does not have any effect on the steel of the present invention.

[0020] Boron is an essential element and can be added in an amount of 0.001 to 0.010%, preferably 0.001 to 0.004%, to harden the steel. Boron imparts hardenability and strength to the steel of the present invention. However, it has been found that the addition of boron in excess of 0.010% significantly reduces the rollability of the steel sheet. Furthermore, boron segregation can occur at grain boundaries, which is detrimental to formability.

[0021] The phosphorus content of the steel of the present invention is limited to 0.02%. Phosphorus is a hardening element in solid solution. Therefore, small amounts of phosphorus, at least 0.002%, can be advantageous, but phosphorus also has its negative effects, such as reducing spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.015%.

[0022] Sulfur is not an essential element, but may be present in the steel as an impurity. The lower the sulfur content, the better, but from the viewpoint of production costs, it is 0.03% or less, preferably 0.005% at most. Furthermore, if more sulfur is present in the steel, sulfur will combine with Mn and Ti in particular to form sulfides that are detrimental to the bending, hole expanding, and elongation of the steel of the present invention.

[0023] Nitrogen is limited to 0.09% to avoid ageing of the material and to minimize the precipitation of nitrides during solidification, which are detrimental to the mechanical properties of the steel, such as bending and hole expansion ratio.

[0024] Chromium is an optional element in the steel of the present invention, between 0% and 1%. Chromium provides strength and hardening to the steel, but when used in excess of 1%, chromium detracts from the surface finish of the steel.

[0025] Copper may be added as an optional element in an amount of 0 to 2% to increase the strength and corrosion resistance of the steel of the present invention. A minimum of 0.01% is preferred to achieve this effect. However, if the copper content exceeds 2%, the surface morphology may deteriorate.

[0026] Niobium is an optional element that can be added to steel in an amount of 0% to 0.06%, preferably 0.0010% to 0.03%. Niobium is suitable for forming carbonitrides, which impart strength to the steel according to the present invention by precipitation hardening. Niobium retards recrystallization during heating, so that at the end of the holding temperature, the resulting microstructure formed after complete annealing is finer, which leads to hardening of the product. However, if the niobium content exceeds 0.06%, a large amount of carbonitrides tends to reduce the ductility of the steel, and therefore such an amount of carbonitrides is not preferred for the present invention.

[0027] Vanadium is an optional element that can be added to the steel of the present invention in an amount of 0% to 0.1%, preferably 0.001% to 0.1%. Like niobium, vanadium participates in carbonitrides and therefore plays a role in hardening. However, vanadium also participates in the formation of VN, which appears during solidification of cast products. Therefore, the amount of V is limited to 0.1% to avoid coarse VN, which is detrimental to hole expansion. If the vanadium content is less than 0.001%, vanadium does not provide any benefit to the steel of the present invention.

[0028] Calcium is an optional element that can be added to the steel of the present invention in an amount of 0% to 0.005%, preferably 0.001% to 0.005%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium contributes to the refining of the steel by preventing sulfur content, which is detrimental to the spheroidization of the steel.

[0029] Other elements such as cerium, magnesium or zirconium can be added individually or in combination in the following proportions: Ce≦0.1%, Mg≦0.05% and Zr≦0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the inclusion particles during solidification.

[0030] The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0031] The microstructure of the core of the steel plate according to the present invention contains, in area fractions, 15% to 70% bainite, 15% to 70% separated martensite, 10% to 30% ferrite, 9% to 22% retained austenite in the bainite and separated martensite, and 0% to 5% fresh martensite.

[0032] Bainite is the matrix of steel and is present in an amount of 15% to 70%. In the framework of the present invention, bainite includes upper bainite, lower bainite, granular bainite, carbide-free bainite, and / or lath bainite. The bainite of the present invention contains film-like austenite in the grains, and when controlled within the range of the present invention, this bainite provides the steel of the present invention with improved elongation and hole expansion. The preferred amount of bainite is 18% to 65%, more preferably 22% to 62%.

[0033] Separated martensite is present in an amount of 15% to 70% to achieve strength levels of 1150 MPa or greater. A martensite content greater than 70% would adversely affect ductility. The separated martensite in this steel can be in the form of laths with lath thicknesses greater than 0.1 microns and membrane-like austenite present between these laths. The martensite formed during cooling after annealing is converted to separated martensite during heating to the overaging temperature. The preferred amount of separated martensite for the steel of this invention is 18% to 65%, more preferably 18% to 60%.

[0034] The retained austenite is present in an amount of 9 to 22% and provides ductility to the steel. In the context of the present invention, the retained austenite may include membrane-like austenite and / or block-like austenite. The membrane-like austenite of the present invention may exist in bainite and separated martensite and exhibit an aspect ratio of greater than 3. The block-like austenite may exist in the form of islands in bainite, exhibiting an aspect ratio of less than 2, and may act as an effective carbon trap, thereby assisting the formation of bainite. The block-like austenite has a maximum grain size of less than 5 microns, preferably less than 3 microns, and may occur during overaging.

[0035] The retained austenite of the present invention preferably contains 0.4 to 0.7% carbon. It is preferable to have 9% to 20%, more preferably 9% to 16%, of retained austenite. It is preferable to have blocky austenite having the above-mentioned properties.

[0036] Ferrite is present in the steel at 10% to 30%, excluding the ferrite-rich surface layer. Such ferrite may include polygonal ferrite, lath ferrite, acicular ferrite, tabular ferrite, or epitaxial ferrite. The presence of ferrite in the present invention imparts formability and elongation to the steel. The presence of ferrite also has adverse effects due to the fact that ferrite increases the hardness gap with hard phases such as martensite and bainite, reducing local ductility. If the presence of ferrite exceeds 30%, the target tensile strength may not be achieved, and the hole expansion ratio may decrease due to the increased amount of interface between the ferrite phase and the hard phase. Therefore, its preferred presence is 12% to 28%, more preferably 14% to 25%.

[0037] Fresh martensite may be present as an isolated phase in the steel according to the invention, and may also be present in an amount of 0% to 5%, preferably 0% to 3%.

[0038] Additionally, within this core microstructure of the steel sheet, the steel sheet also includes a ferrite-rich layer on each side of the steel sheet to a thickness of up to 90 microns. This ferrite-rich layer is further divided into two sublayers, with the first sublayer formed adjacent to the surface of the steel sheet, exhibiting an average ferrite fraction of 40% to 80%, preferably 42% to 78%, and more preferably 43% to 70%, by area. This first sublayer can have a thickness of 30 microns to 70 microns. Furthermore, this first sublayer has a ferrite presence gradient, referred to as ΔF, in which the ferrite present at the top of the first sublayer and the ferrite present at the bottom of the first sublayer have a difference in ferrite presence of 20 to 70%. ΔF is determined as follows: ΔF = (Ferrite content at the top of the first sublayer - Ferrite content at the bottom of the first sublayer) 20%≦ΔF≦70%

[0039] If ΔF is less than 20%, the LME properties are not good, and if ΔF is greater than 70%, the amount of ferrite is not sufficient to have a thick enough second sublayer that is detrimental to the LME properties due to the reduced presence of ferrite at the bottom of the ferrite-rich layer. The preferred ΔF is 22% to 60%, more preferably 24% to 55%.

[0040] The second sublayer forms the top layer of the ferrite-rich layer and is adjacent to the first sublayer. The second sublayer has a ferrite area fraction of 70% to 98%, preferably 72% to 96%, more preferably 74% to 94%. In addition to ferrite, the second sublayer also comprises inner oxides of manganese and silicon. The second sublayer preferably has a thickness of 1 micron to 15 microns, preferably 3 to 10 microns.

[0041] The ferrite-rich layer formed on the surface preferably comprises any or all possible ferrite types, in particular polygonal ferrite, lath ferrite, acicular ferrite, tabular ferrite or epitaxial ferrite, and this ferrite layer confers resistance to liquid metal embrittlement (LME) to the steel sheet of the present invention.

[0042] The remainder of this surface layer comprises bainite and / or retained austenite and / or martensite.

[0043] The 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 having the chemical composition according to the invention. Casting can be carried out in ingots or continuously in the form of thin slabs or thin strip, i.e., with thicknesses ranging from about 220 mm for slabs to several tens of millimeters for thin strip.

[0044] For example, slabs are considered semi-finished products. Slabs having the above chemical composition are produced by continuous casting, and the slabs are preferably subjected to soft reduction directly during casting to ensure the elimination of center segregation and reduced porosity. Slabs provided by the continuous casting process can be used directly at high temperature after continuous casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0045] The temperature of the slab to be subjected to hot rolling should preferably be at least 1000°C, preferably above 1200°C and below 1280°C. If the slab temperature is below 1000°C, excessive load is applied to the rolling mill, and furthermore, the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled in a state where the transformed ferrite is contained in the structure. Furthermore, the temperature should not exceed 1280°C because it is industrially expensive.

[0046] The temperature of the slab is preferably high enough to allow hot rolling to be completed entirely in the austenite field, with the finish hot rolling temperature remaining above 850°C, preferably above 900°C. Final rolling must be performed above 850°C, as below this temperature the steel sheet shows a significant decrease in rollability. A final rolling temperature of 900-950°C is preferred to provide a structure suitable for recrystallization and rolling.

[0047] The resulting sheet is then cooled to a temperature of 550°C or less at a cooling rate of more than 30°C / s. The cooling temperature is maintained at 550°C or less to prevent oxidation of alloying elements such as manganese, silicon, and chromium. Preferably, the cooling rate is 65°C / s or less and more than 35°C / s. The hot-rolled steel sheet is then coiled, and the coiling temperature must be below 550°C. The temperature of the coiled hot-rolled steel sheet must then be maintained below 550°C to avoid oxidation of silicon and manganese on the surface of the hot-rolled coil, as these oxides would form cracks on the surface of the hot-rolled steel sheet. The coiled hot-rolled steel sheet is then cooled to room temperature. The hot-rolled steel sheet may then be subjected to an optional descaling process, such as pickling, to remove scale formed during hot rolling and ensure that the surface of the hot-rolled steel sheet is free of scale before the hot-rolled steel sheet is subjected to optional hot band annealing.

[0048] The hot-rolled steel sheet may be optionally subjected to hot band annealing at a temperature of 350°C to 750°C for 1 to 96 hours. The temperature and time of such hot band annealing are selected to ensure softening of the hot-rolled steel sheet and to facilitate cold rolling of the hot-rolled steel sheet.

[0049] Next, this hot-rolled steel sheet is cooled to room temperature and then cold-rolled with a thickness reduction of 35 to 70% to obtain a cold-rolled steel sheet.

[0050] The cold rolled steel sheet is then annealed to impart the desired microstructure and mechanical properties to the steel of the present invention.

[0051] Annealing involves two heating stages: a soaking temperature TA of Ac3 -30°C to Ac3 +100°C, with the dew point maintained between -15°C and +15°C during the two heating stages to provide a ferrite-rich layer on the surface to provide the steel with adequate liquid metal embrittlement resistance. The preferred dew point is maintained between -10°C and +10°C. The Ac3 of the steel is determined by dilatometric testing according to the method described in the paper by M. Murat published in the journal "TECHNIQUES DE L'INGENIEUR, MESURES ET ANALYSE; FRA; PARIS: TECH.-ING.; DA. 1981; Vol. 20; No. 59; P1280."

[0052] In step 1, the cold-rolled steel sheet is heated from room temperature to a temperature HT1 in the range of 600°C to 800°C at a heating rate HR1 of 2°C / s to 70°C / s. It is preferable to have an HR1 rate of 5°C / s to 60°C / s, more preferably 10°C / s to 50°C / s. The preferred HT1 temperature is 625°C to 775°C, more preferably 640°C to 750°C.

[0053] Then, in a subsequent second heating step, the cold-rolled steel sheet is heated from temperature HT1 to a soaking temperature TA in the temperature range of Ac3-10°C to Ac3+100°C at a heating rate HR2 of 0.1°C / s to 10°C / s. The HR2 rate is preferably 0.1°C / s to 8°C / s, more preferably 0.1°C / s to 5°C / s. HR2 is always lower than HR1.

[0054] The preferred TA temperature is Ac3-25°C to Ac3+75°C, more preferably Ac3-20°C to Ac3+50°C. The dew point is maintained at the soaking temperature between -10°C and +10°C, preferably between -5°C and +5°C, to provide a ferrite-enriched layer at the surface with a target depth in the steel.

[0055] As described above, the ferrite-rich layer according to the present invention is formed during annealing. Carbon reacts with oxygen to form carbon monoxide, which escapes from the steel, resulting in a decarburized surface layer. Such a layer has a ferrite-rich microstructure and extends from the surface of the sheet to a depth of 90 microns. This ferrite-rich layer is created during pre-annealing heating and soaking by controlling the dew point. The dew point is controlled between -15°C and +15°C during pre-annealing heating and between -10°C and +10°C during soaking, using conventional means known to those skilled in the art, such as water injection.

[0056] The cold rolled steel sheet is then held at the annealing soak temperature TA for 10 to 1000 seconds to ensure proper transformation of the heavily work-hardened initial structure to an austenitic microstructure.

[0057] The cold-rolled steel sheet is then cooled in a cooling process, the first step of cooling being optional, starting from TA, where the cold-rolled steel sheet is cooled to a temperature CT1 in the range of 580°C to 860°C at a cooling rate CR1 of 0.01°C / s to 15°C / s. In a preferred embodiment, the cooling rate CR1 of such a first cooling step is 0.02°C / s to 5°C / s. The preferred T1 temperature of such a first step is 590°C to 850°C.

[0058] In the second step of cooling, the cold-rolled steel sheet is cooled from CT1 or TA to a temperature CT2 that is Ms-30°C to 20°C at a cooling rate CR2 of at least 10°C / s. In a preferred embodiment, the cooling rate CR2 of the second step of cooling is at least 20°C / s, more preferably at least 25°C / s. A preferred CT2 temperature for such a second step is Ms-50°C to 100°C. Whenever the first step of cooling is not performed, CT1 is equal to TA.

[0059] In a subsequent step, the cold-rolled steel sheet is heated from the CT2 temperature to the overaging temperature range TOA of 250°C to 580°C at a heating rate HR3 of 1°C / s to 100°C / s. During this step, the martensite formed during the cooling after annealing is converted to separated martensite, thereby supporting the formation of bainite during the hold at the TOA temperature. The cold-rolled steel sheet is then held at the TOA temperature and overaged for 5 to 500 seconds to form the bainite of the present invention.

[0060] The cold-rolled steel sheet can then be brought to the temperature of the hot-dip galvanizing bath, which, depending on the nature of the coating, can be between 420° C. and 680° C. The coating can be made of zinc or a zinc-based alloy, or of aluminum or an aluminum-based alloy.

[0061] Alternatively, the cold rolled steel sheet may also be coated by any known industrial process that does not require the steel sheet to be brought to the above temperature range after overaging, such as electrogalvanizing, JVD, PVD, hot dip (GI), GA or ZM, in which case the steel sheet can be cooled to room temperature before being coated in a later step.

[0062] To ensure degassing of the coated product, an optional post-batch anneal can be performed after annealing the coated product, preferably at 170-210°C for 12-30 hours. [Example]

[0063] The following tests and examples presented herein are non-limiting in nature and must be considered for illustrative purposes only, to illustrate the advantageous features of the present invention, to detail the importance of the parameters selected by the inventors after extensive experimentation, and to further establish the properties that can be achieved with steels according to the present invention.

[0064] Steel sheets according to the invention and some comparative grade samples were prepared using the compositions summarized in Table 1 and the processing parameters summarized in Table 2. The corresponding microstructures of these steel sheets are summarized in Table 3 and the properties are summarized in Table 4.

[0065] Table 1 shows the steels with compositions expressed in weight percent.

[0066] Table 1 also shows the martensitic transformation Ms and Ac3 temperatures of the steels according to the invention and of the reference steels. Ms and Ac3 for all the invention and reference steels are determined by dilatometric tests according to the method described in the article by M. Murat published in the journal "TECHNIQUES DE L'INGENIEUR, MESURES ET ANALYSE; FRA; PARIS: TECH.-ING.; DA. 1981; Vol. 20; No. 59; P1280".

[0067] [Table 1]

[0068] Table 2 summarizes the annealing process parameters that were applied to the steels in Table 1.

[0069] Furthermore, before the annealing treatment of the steels of the present invention and the references, the samples were heated to a temperature of 1000°C to 1280°C and then hot rolled at a finishing temperature above 850°C. The cooling rate after hot rolling was above 30°C / s until the samples cooled to below 550°C. After the overaging hold, all cold-rolled steel sheets were coated in a zinc bath at a temperature of 460°C.

[0070] [Table 2] TIFF2026500289000003.tif241169

[0071] Table 3 summarizes the results of tests carried out according to standards with different microscopes, such as scanning electron microscopes, to determine the microstructural composition of both the steel of the invention and the reference samples. Ferrite and fresh martensite are determined by image analysis carried out with a scanning electron microscope at 3000x magnification. Bainite is determined by symmetry. Retained austenite is determined by image analysis carried out through X-ray diffraction.

[0072] [Table 3] TIFF2026500289000005.tif134169

[0073] Table 4 summarizes the mechanical and surface properties of both the inventive and reference steels. Tensile strength, yield strength and total elongation tests are performed according to the ISO 6892-1 standard, and hole expansion ratio tests are performed according to the ISO 16630 standard.

[0074] Table 4: Mechanical and surface properties of the trials The susceptibility of the trials to LME was assessed by resistance spot welding. For this purpose, for each trial, one steel plate, corresponding to trials I1 to I7 and trials R1 to R4, respectively, was spot welded to two additional steel plates to build a three-plate stack-up containing, in succession: - one steel plate corresponding to trials I1 to I7 and trials R1 to R4, - 1.5 mm interstitial-free galvanized steel sheet containing 0.003% carbon and 0.11% manganese, - 1.5mm interstitial-free galvanized steel sheet containing 0.003% carbon and 0.11% manganese.

[0075] The welding conditions were in accordance with the standard ISO-18278-2. The type of welding electrode was F1 with a face diameter of 6 mm. The electrode clamping force was set to 450 daN. The welding cycle was as follows:

[0076] [Table 4]

[0077] Each trial was reproduced 10 times to produce 10 spot welds at current levels defined as the upper welding limit in the current range from Imax to Imax+10%, where Imax was included between 0.9 and 1.1*Iexp, and Iexp was the intensity above which emissions appeared during welding, determined according to ISO standard 18278-2.

[0078] The crack lengths in 10 spot welded joints were then assessed using an optical microscope after traversing through the surface crack. A grade was considered to offer sufficient LME resistance if less than 60% of the spots had cracks longer than 200 μm.

[0079] Yield strength YS, tensile strength TS and total elongation TE are measured according to ISO standard ISO 6892-1 published in September 2009. Hole expansion ratio is measured according to ISO standard 16630:2009.

[0080] [Table 5]

[0081] From the above table, it can be seen that all trials according to the present invention meet the performance targets.

Claims

1. 1. A cold rolled heat treated steel sheet, the steel comprising, in weight percent: 0.15%≦carbon≦0.25%; 2.2%≦Manganese≦3%, 1.1%≦silicon≦2%; 0%≦aluminum≦0.09%, 0.05%≦molybdenum≦0.5%, 0.001%≦Titanium≦0.06%, 0.001%≦Boron≦0.010%, 0%≦phosphorus≦0.02%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.09%, and optionally the following elements: 0%≦chromium≦1%, 0%≦copper≦2%, 0%≦niobium≦0.06%, 0%≦vanadium≦0.1%, 0%≦Calcium≦0.005%, 0%≦Magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦Cerium≦0.1% and the remainder comprises iron and unavoidable impurities, the steel plate having a core microstructure comprising, by area fraction, 15% to 70% bainite, 15% to 70% separated martensite, 10% to 30% ferrite, 9% to 22% retained austenite in the bainite and in the separated martensite, and 0% to 5% fresh martensite, and a ferrite-rich layer extending up to 90 microns from both sides of the steel plate, the ferrite-rich layer being divided into two sub-layers:

1. A cold rolled, heat treated steel sheet comprising a first sub-layer adjacent to a surface of the steel sheet and a second sub-layer that is an uppermost layer of a ferrite-rich layer, the first sub-layer having an average ferrite fraction, by area fraction, of 40% to 80%, a ferrite gradient (ΔF) of 20% to 70%, ΔF being the difference between the ferrite content at the top of the first sub-layer and the ferrite content at the bottom of the second sub-layer, the second sub-layer having a ferrite fraction, by area fraction, of 70% to 98%, and including inner oxides of manganese and silicon.

2. 2. The cold rolled heat treated steel sheet according to claim 1, wherein the composition comprises 2.3% to 2.8% manganese.

3. The cold rolled heat treated steel sheet according to claim 1 or 2, wherein the composition comprises a composition containing 0.16% to 0.24% carbon.

4. The cold rolled heat treated steel sheet according to any one of claims 1 to 3, wherein the composition comprises a composition containing 1.2% to 1.9% silicon.

5. The cold rolled heat treated steel sheet according to any one of claims 1 to 4, wherein the composition includes a composition containing 0.06% to 0.3% molybdenum.

6. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 5, wherein the microstructure contains 18% to 65% bainite.

7. The cold rolled heat treated steel sheet according to any one of claims 1 to 6, wherein the microstructure contains 9% to 20% of retained austenite in bainite and separated martensite.

8. The cold rolled heat treated steel sheet according to any one of claims 1 to 7, wherein the microstructure contains 18% to 65% separated martensite.

9. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 8, having a tensile strength of 1150 MPa or more and a hole expansion ratio of 22% or more.

10. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 9, having a total elongation of 13% or more.

11. The cold rolled heat treated steel sheet according to any one of claims 1 to 10, having a first sub-layer of the ferrite enriched layer having a thickness of 30 microns to 70 microns.

12. A method for producing a cold rolled heat treated steel sheet comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 5 and obtaining a semi-finished product, - reheating the semi-finished product to a temperature between 1000°C and 1280°C, - rolling the semi-finished product completely in the austenitic range with a hot rolling finishing temperature of 850°C or higher to obtain a hot rolled steel sheet; - cooling the sheet at a cooling rate of more than 30°C / s to a temperature of not more than 550°C; coiling the hot-rolled sheet and maintaining the temperature of the coiled sheet below 500°C; - cooling the hot-rolled sheet; - subjecting the hot rolled steel plate to an optional descaling process; - subjecting the hot-rolled steel sheet to an optional annealing at a temperature of 350°C to 750°C for 1 hour to 96 hours, and cold-rolling the hot-rolled steel sheet at a reduction ratio of 35 to 70% to obtain a cold-rolled steel sheet; - annealing the cold-rolled steel sheet in a two-stage heating process with a controlled dew point between -15°C and +15°C, a first step begins with heating the steel plate from room temperature to a temperature HT1 between 600°C and 800°C at a heating rate HR1 between 2°C / s and 70°C / s; a second step starting with further heating the steel plate from HT1 to a soaking temperature TA of Ac3-10°C to Ac3+100°C at a heating rate HR2 of 0.1°C / s to 10°C / s, HR2 being lower than HR1; - followed by annealing at TA for 10-1000 seconds, during which the dew point is controlled between -10°C and +10°C; - then cooling the cold-rolled steel sheet, an optional first step of cooling the cold rolled steel sheet starting from TA to a temperature CT1 of between 580°C and 860°C at a cooling rate CR1 of between 0.01°C / s and 15°C / s; a cooling step starting from CT1 or TA to a temperature CT2 of Ms-30°C to 20°C at a cooling rate CR2 of at least 10°C / s; - then heating the cold-rolled steel sheet from the CT2 temperature to an overaging temperature TOA of 250°C to 580°C at an average heating rate HR3 of 1°C / s to 100°C / s, - then overaging the cold rolled steel sheet at TOA for 5 to 500 seconds; - Then, the cold rolled steel sheet is cooled to room temperature to obtain a cold rolled heat treated steel sheet.

13. The method of claim 12, wherein the HT1 temperature is between 625°C and 775°C.

14. Use of a steel sheet according to any one of claims 1 to 11 or produced according to the method according to claim 12 or 13 for the manufacture of structural or safety parts of a vehicle.

15. A vehicle including the component of claim 14.

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