Cold-rolled martensitic steel and method for producing the same

Abstract

Martensitic steel sheet containing the following elements expressed in weight percent: 0.07%≦C≦0.12%, 1.9%≦Mn≦2.5%, 0.2%≦Si≦0.6%, 0.01%≦Al≦0.1%, 0.1%≦Cr≦0.5%, 0.2%≦Mo≦0.6%, 0%≦S≦0.09%, 0%≦P≦0.09%, 0%≦N≦0.09%, 0.001%≦Ti≦0.1%, 0.0005%≦B≦0.005%, 0%≦Nb≦0. 0.1%, 0%≦V≦0.1%, 0%≦Ni≦1%, 0%≦Cu≦1%, 0%≦Sn≦0.1%, 0%≦Pb≦0.1%, 0%≦Sb≦0.1%, 0.001%≦Ca≦0.01%, with the remainder consisting of iron and unavoidable impurities resulting from processing, and the microstructure of the steel consisting of, by area percentage, 80% to 94% tempered martensite and 6% to 20% fresh martensite.

Description

[Technical Field]

[0001] The present invention relates to a method for producing cold-rolled martensitic steel suitable for the automotive industry, and in particular to a martensitic steel having a tensile strength of 960 MPa or more. [Background technology]

[0002] Automotive parts must satisfy two contradictory requirements: ease of formability and strength. However, in recent years, concerns about the global environment have placed a third demand on automobiles: improved fuel economy. Therefore, automotive parts must now be made from materials with high formability to meet standards for ease of fitting into complex automotive assemblies, while simultaneously reducing vehicle weight for improved fuel economy and improving strength for vehicle crashworthiness and durability.

[0003] Therefore, intensive research and development efforts are being made to reduce the amount of material used in vehicles by increasing the strength of materials. Conversely, increasing the strength of steel sheets reduces their formability, so it is necessary 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 led to 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] EP3901299 specifies a matrix alloy containing, by weight, 0.06 to 0.15% carbon (C), 1.2% or less (excluding 0) silicon (Si), 1.7 to 2.7% manganese (Mn), 0.15% or less (excluding 0) molybdenum (Mo), 1.0% or less (excluding 0) chromium (Cr), 0.1% or less phosphorus (P), 0.01% or less sulfur (S), 0.001 to 0.04% titanium (Ti), 0.001 to 0.04% niobium (Nb), 0.01% or less nitrogen (N), 0.01% or less (excluding 0) boron (B), the remainder being Fe and unavoidable impurities, at a thickness of 1 / 4t. The amounts of silicon (Si), carbon (C), manganese (Mn), molybdenum (Mo), and chromium (Cr) in the structure satisfy the following relationship 1; the microstructure contains, in area percentage, 10 to 70% ferrite, a total of 10 to 50% bainite and retained austenite, and the remainder is fresh martensite; and the ratio (Mb / Mt) of the proportion of all fresh martensite (Mt) to the proportion of fresh martensite adjacent to bainite (Mb) is 60% or more. The present invention provides a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, which have excellent workability, and methods for producing the same. [Relationship 1] ([Si] + [C] × 3) / ([Mn] + [Mo] + [Cr]) ≥ 0.18 ([Si], [C], [Mn], [Mo] and [Cr] are the weight percentages of silicon (Si), carbon (C), manganese (Mn), molybdenum (Mo) and chromium (Cr), respectively, present at the 1 / 4t thickness point of the cold-rolled steel plate. However, the steel of EP3901299 does not exhibit a hole expansion ratio of 40% or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent Application Publication No. 3901299 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to solve these problems by making available a cold-rolled martensitic steel sheet which simultaneously has: - an ultimate tensile strength of at least 960 MPa, preferably more than 1000 MPa, - a yield strength of at least 700 MPa, preferably greater than 720 MPa, - Hole expansion ratio of at least 40%, preferably at least 45%.

[0008] Preferably, such steels also have good suitability for forming and for rolling, as well as good weldability and coatability.

[0009] Another object of the invention is also to make available a method for manufacturing these plates that is stable to shifts in manufacturing parameters, while being compatible with conventional industrial applications. [Means for solving the problem]

[0010] The above objects and other advantages of the present invention will become more apparent from the detailed description of preferred embodiments of the present invention.

[0011] The chemical composition of cold rolled martensitic steel includes the following elements:

[0012] The carbon content of the steel of the present invention is 0.07% to 0.12%. Carbon is an element necessary for increasing the strength of the steel of the present invention by forming low-temperature transformation phases such as martensite. A carbon content of less than 0.07% does not impart tensile strength to the steel of the present invention. On the other hand, if the carbon content exceeds 0.12%, the steel exhibits poor spot weldability, limiting its application to automotive parts. A preferred carbon content for the present invention can be maintained at 0.08 to 0.11%, more preferably 0.09 to 0.11%.

[0013] The manganese content of the steel of the present invention is 1.9% to 2.5%. This element promotes austenite formation. Manganese provides solid solution strengthening and slows the ferrite transformation rate, thus aiding in the formation of martensite. A content of at least 1.9% is required to provide strength and aid in the formation of martensite. However, manganese content exceeding 2.5% can have adverse effects, such as delaying the transformation from austenite to martensite during cooling after annealing. Manganese content exceeding 2.5% can cause excessive segregation in the steel during solidification, impairing the internal homogeneity of the material and potentially causing surface cracks during the hot working process. The preferred limit for manganese presence is 2% to 2.4%, more preferably 2.1% to 2.35%.

[0014] The silicon content of the steel of the present invention is 0.2% to 0.6%. Silicon is an element that contributes to improving strength through solid solution strengthening. Silicon is a component that can delay carbide precipitation during cooling after annealing, thus promoting the formation of martensite. However, silicon is also a ferrite former and increases the Ac3 transformation point, which pushes the annealing temperature into a higher temperature range. This is why the silicon content is kept at a maximum of 0.6%. A silicon content above 0.6% can also increase embrittlement, and in addition, silicon also impairs coatability. The preferred limit for silicon presence is 0.25% to 0.5%, more preferably 0.25% to 0.4%.

[0015] The chromium content of the steel of the present invention is 0.1% to 0.5%. Chromium is an essential element that imparts strength to steel through solid solution strengthening, and a minimum of 0.1% is required to impart strength, but anything above 0.5% will impair the surface finish of the steel. The preferred limit for chromium presence is 0.15% to 0.4%, more preferably 0.15% to 0.35%.

[0016] In the present invention, the aluminum content is 0.01-0.1%. Aluminum removes oxygen present in the molten steel and prevents it from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing the grain size. A high aluminum content above 0.1% raises the Ac3 point to a high temperature, thereby reducing productivity. The preferred limit for the presence of aluminum is 0.01-0.09%.

[0017] Molybdenum is an essential element constituting 0.2% to 0.6% of the steel of the present invention. Molybdenum plays an important role in improving hardenability and hardness, delaying the appearance of bainite and thus promoting the formation of martensite, especially when added in an amount of at least 0.2%. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons its content is limited to 0.6%. The preferred limit for the presence of molybdenum is 0.25% to 0.5%, more preferably 0.25% to 0.4%.

[0018] Niobium, present in the steel of the present invention at 0.001% to 0.1%, is suitable for forming carbonitrides to impart additional strength to the steel of the present invention through precipitation hardening. Niobium also influences the size of microstructural constituents through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Therefore, the resulting finer microstructure formed at the end of the holding temperature after full annealing leads to hardening of the product. However, a saturation effect of niobium content above 0.1% is observed, which means that additional amounts of niobium do not improve the strength of the product and are therefore not economically attractive.

[0019] Although sulfur is not an essential element, it may be contained in the steel as an impurity, and from the viewpoint of the present invention, the sulfur content is preferably as low as possible, but from the viewpoint of production costs, it is 0.09% or less. Furthermore, if a large amount of sulfur is present in the steel, sulfur will combine with manganese in particular to form sulfides, reducing its beneficial effect on the present invention.

[0020] The phosphorus constituent of the steel of the present invention is between 0% and 0.09%, and phosphorus reduces spot weldability and hot ductility due to its tendency to segregate, especially at grain boundaries, or to co-segregate with manganese, and for these reasons its content is limited to 0.09%, preferably less than 0.06%.

[0021] Nitrogen is limited to 0.09% to avoid ageing of the material and to minimize the precipitation of aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel.

[0022] Titanium is an alloying element and can be added to the steel of the present invention in an amount of 0.001% to 0.1%. Titanium forms titanium nitrides, which protect B in solution during steelmaking. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides, which are detrimental to formability. Titanium contents below 0.001% do not have any effect on the steel of the present invention.

[0023] Boron is an alloying element of the steel of the present invention and can be present in an amount of between 0.0005% and 0.005%. Boron can increase the hardenability of the steel during cooling, which, when added in an amount of at least 0.0005%, can avoid the formation of soft phases and promote the formation of hard martensite in the final microstructure.

[0024] Vanadium is an optional element in the steel of the present invention. Vanadium is effective in improving the strength of the steel by forming carbides or carbonitrides, and the upper limit is set to 0.1% from an economical standpoint.

[0025] To increase the strength and toughness of the steel of the present invention, nickel can be added as an optional element in an amount of 0 to 1%. To achieve this effect, a minimum of 0.01% is preferred. However, if the nickel content exceeds 1%, nickel will cause a decrease in ductility.

[0026] Copper can be added as an optional element in an amount of 0 to 1% 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 1%, the surface morphology may deteriorate.

[0027] Calcium can be added to the steel of the present invention in an amount between 0.001% and 0.01%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium contributes to the refinement of the steel by binding to harmful sulfur content in spheroidal form, thereby delaying the harmful effects of sulfur.

[0028] Other elements such as Sn, Pb or Sb can be added individually or in combination in the following proportions: Sn≦0.1%, Pb≦0.1% and Sb≦0.1%. Up to the maximum content levels indicated, these elements allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of martensitic steel sheets will be described in detail below, with all percentages expressed as area fractions.

[0030] Tempered martensite constitutes 80% to 94% of the microstructure by area. Tempered martensite is formed from martensite that occurs during cooling after annealing, particularly at temperatures between Ms-100°C and Ms, more specifically between Ms-70°C and Ms. Such martensite is then tempered during holding at a tempering temperature T of 400°C to 550°C. The tempered martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of tempered martensite is 84% to 94%, more preferably 85% to 94%.

[0031] Fresh martensite constitutes 6% to 20% of the microstructure by area. In the steel of the present invention, fresh martensite is formed by cooling the cold-rolled steel sheet after tempering. Martensite imparts ductility and strength to the steel of the present invention. However, if the presence of fresh martensite exceeds 20%, it imparts excessive strength but reduces the hole expansion ratio beyond the allowable limit of the steel of the present invention. Because fresh martensite contains a large amount of carbon, it is brittle and hard; therefore, the preferred limit of fresh martensite for the steel of the present invention is 6% to 18%, more preferably 6% to 15%.

[0032] In addition to the above-mentioned microstructure, the structure of the cold-rolled martensitic steel sheet does not contain microstructural components such as ferrite, bainite, retained austenite, pearlite, cementite, etc.

[0033] The steel according to the invention can be produced by any suitable method, but it is preferred to use, by way of non-limiting example, the method according to the invention, which is detailed below.

[0034] Such a preferred method consists in providing a semi-finished casting of steel having the chemical composition of the prime steel according to the invention, which can be cast in ingots or continuously in the form of thin slabs or thin strip, i.e., in thicknesses ranging from about 220 mm for slabs to several tens of millimeters for thin strip.

[0035] For example, slabs having a chemical composition according to the present invention are produced by continuous casting, and the slabs are optionally subjected to a soft reduction directly during the continuous casting process to avoid center segregation and ensure that the ratio of local carbon to nominal carbon is maintained below 1.10. The 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.

[0036] The temperature of the slab used for hot rolling must be at least 1000°C and less than 1280°C. If the slab temperature is less than 1000°C, excessive loads will be placed on the rolling mill, and the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled with transformed ferrite in its structure. Therefore, the slab temperature must be high enough so that hot rolling is completed in the temperature range of Ac3 to Ac3 + 100°C. Reheating at temperatures above 1280°C is industrially expensive and must be avoided.

[0037] The sheet thus obtained is then cooled at a cooling rate of at least 10°C / s to the coiling temperature, which must be less than 675°C. Preferably, the cooling rate is not more than 200°C / s.

[0038] The hot-rolled steel sheet is then coiled at a coiling temperature of less than 675°C to avoid ovalization, preferably between 475°C and 675°C to avoid scale formation, with a more preferred range for such a coiling temperature being between 500°C and 660°C. The coiled hot-rolled steel sheet is then cooled to room temperature and then subjected to optional hot band annealing.

[0039] The hot-rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling prior to optional hot band annealing. The hot-rolled sheet may then be subjected to optional hot band annealing. In a preferred embodiment, such hot band annealing is carried out at a temperature between 400°C and 750°C, preferably for at least 12 hours and not more than 96 hours, with the temperature preferably remaining below 750°C to avoid partial deformation of the hot-rolled microstructure and thus potentially losing microstructural homogeneity. The hot-rolled steel sheet may then be subjected to an optional descaling step, such as by pickling such sheet.

[0040] Next, this hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet with a thickness reduction ratio of 35 to 90%.

[0041] The cold rolled steel sheet is then heat treated to impart the mechanical properties and microstructure required for the steel of the present invention.

[0042] The cold-rolled steel sheet is heated at a heating rate HR1 of at least 1°C / sec, preferably 2°C / sec or more, to a soaking temperature Tsoak between Ac3 and Ac3+100°C, preferably between Ac3°C and Ac3+50°C, and Ac3 of the steel sheet is calculated by a dilatometry test performed in accordance with the ASTM A1033-18 standard.

[0043] The cold rolled steel sheet is held at Tsoak for 10 seconds to 500 seconds to ensure complete recrystallization and transformation of the initial severe work hardening structure to austenite.

[0044] Next, the cold-rolled steel sheet is cooled from Tsoak to a temperature T1 in the range of Ms-100°C to Ms at a cooling rate CR1 of 1 degree / sec to 150°C / sec. In a preferred embodiment, the cooling rate CR1 in such a cooling step is 20°C / sec to 120°C / sec. A preferred T1 temperature in such a first step is Ms-70°C to Ms. Next, the cold-rolled steel sheet is held at T1 for 5 to 100 seconds, preferably 5 to 50 seconds, and more preferably 5 to 30 seconds.

[0045] The Ms of the steel plate is calculated by using a dilatometry test according to the ASTM A1033-18 standard.

[0046] The cold-rolled steel sheet is then reheated to a tempering temperature T of 400°C to 550°C in 10 to 600 seconds at a heating rate of at least 1°C / s, preferably at least 2°C / s, more preferably at least 10°C / s. The preferred tempering temperature range is 420°C to 500°C, and the preferred duration of the tempering hold is 20 to 300 seconds.

[0047] The cold-rolled martensitic steel sheet of the present invention may be coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve its corrosion resistance. Next, the cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled martensitic steel. [Example]

[0048] The following tests, examples, figurative examples, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0049] Steel plates with different compositions are summarized in Table 1, and the steel plates are manufactured according to the process parameters specified in Table 2. The structure of the steel plates obtained during the tests is then summarized in Table 3, and the results of evaluating the obtained properties are summarized in Table 4.

[0050] [Table 1]

[0051] Table 2 summarizes the process parameters for hot rolling and annealing that were performed on the cold rolled steel sheets in order to impart to the steels of Table 1 the mechanical properties required to become cold rolled martensitic steels.

[0052] Table 2 is as follows:

[0053] [Table 2] TIFF2025527100000003.tif197169

[0054] Table 3 illustrates the results of tests carried out according to the standard on different microscopes, such as scanning electron microscopes, to determine the microstructure of both the steel of the invention and the reference steel in terms of area fraction. The results are defined herein.

[0055] [Table 3]

[0056] The results of various mechanical tests carried out in accordance with the standards are summarized. Regarding the tests, the ultimate tensile strength and yield strength are tested in accordance with JIS-Z2241. To estimate the hole expansion, a test called hole expansion is applied, in which a 10 mm hole is drilled in the test specimen, deformed, and the diameter of the hole is measured after deformation, and HER%=100*(Df-Di) / Di (where Df is the diameter measured in millimeters after deformation, and Di is the diameter measured in millimeters before deformation) is calculated.

[0057] [Table 4]

Claims

1. Martensitic steel sheet containing the following elements expressed in weight percent: 0.07%≦C≦0.12%, 1.9%≦Mn≦2.5%, 0.2%≦Si≦0.6%, 0.01%≦Al≦0.1%, 0.1%≦Cr≦0.5%, 0.2%≦Mo≦0.6%, 0.001%≦Nb≦0.1%, 0%≦S≦0.09%, 0%≦P≦0.09%, 0%≦N≦0.09% and any of the following elements: 0.001%≦Ti≦0.1%, 0.0005%≦B≦0.005%, 0%≦V≦0.1%, 0%≦Ni≦1%, 0%≦Cu≦1%, 0%≦Sn≦0.1%, 0%≦Pb≦0.1%, 0%≦Sb≦0.1%, 0.001%≦Ca≦0.01% The remainder of the composition is composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel is, by area percentage, 80% to 94% tempered martensite and 6% to 20% fresh martensite.

2. 2. The martensitic steel sheet according to claim 1, wherein said composition comprises 0.25% to 0.5% silicon.

3. The martensitic steel sheet according to claim 1 or 2, wherein the composition contains 0.08% to 0.11% carbon.

4. The martensitic steel sheet according to any one of claims 1 to 3, wherein the composition comprises 2.0% to 2.4% manganese.

5. The martensitic steel sheet according to any one of claims 1 to 4, wherein the amount of fresh martensite is 6% to 18%.

6. The martensitic steel sheet according to any one of claims 1 to 5, having a hole expansion ratio of 40% or more.

7. A method for manufacturing a martensitic steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 4, - reheating said semi-finished product to a temperature between 1000°C and 1280°C, - rolling said semi-finished product in the austenite range with a hot rolling finishing temperature of Ac3 to Ac3+100°C, to obtain a hot rolled steel sheet; - cooling the sheet at a cooling rate of at least 10°C / s to a coiling temperature of less than 675°C and coiling the hot-rolled sheet; - cooling the hot-rolled sheet to room temperature; - optionally subjecting said hot rolled steel sheet to a descaling process; - optionally subjecting the hot-rolled steel sheet to annealing; - optionally subjecting said hot rolled steel sheet to a descaling process; - cold rolling the hot-rolled steel sheet at a reduction ratio CR of 35 to 90% to obtain a cold-rolled steel sheet; - then heating said cold-rolled steel sheet, starting from room temperature, at a heating rate HR1 of at least 1°C / s to a temperature Tsoak of Ac3 to Ac3+100°C, and holding it there for 10 to 500 seconds; - then cooling the cold-rolled steel sheet starting from Tsoak at a cooling rate CR1 of 1 ° C. / s to 150 ° C. / s to a temperature T1 of Ms to Ms-100 ° C., - then holding the cold-rolled steel sheet at temperature T1 for 5 to 100 seconds; - then reheating said cold-rolled steel sheet at a rate of at least 1°C / s to a tempering temperature Ttemper of between 400°C and 550°C, which is held for 10 to 600 seconds; - then cooling to room temperature at a cooling rate of at least 1°C / s to obtain a martensitic steel sheet.

8. The method of claim 7, wherein the coiling temperature is between 475°C and 675°C.

9. 9. The method according to claim 7 or 8, wherein Tsoak is Ac3 to Ac3+50°C.

10. The method according to any one of claims 7 to 9, wherein CR1 is from 20°C / s to 120°C / s.

11. The method according to any one of claims 7 to 10, wherein T1 is Ms to Ms-70°C.

12. The method according to any one of claims 7 to 11, wherein the cold-rolled steel sheet is held at the T1 temperature for 5 to 50 seconds.

13. 13. The method according to any one of claims 7 to 12, wherein the heating rate to temperature Ttemper is at least 2°C / s.

14. The method of any one of claims 7 to 13, wherein Ttemper is between 420°C and 500°C.

15. Use of a steel sheet according to any one of claims 1 to 6 or produced by the method according to any one of claims 7 to 14 for the production of structural parts of a vehicle.

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