Cold-rolled martensitic steel and method for producing the same

Abstract

To provide a cold rolled martensitic steel having both high strength and high formability and suitable for the automotive industry, and to provide a method for producing the martensitic steel.SOLUTION: There is provided a cold rolled martensitic steel sheet comprising of the following elements, expressed in percentage by weight: 0.1%≤C≤0.2%; 1.5%≤Mn≤2.5%; 0.1%≤Si≤0.25%; 0.1%≤Cr≤1%; 0.01%≤Al≤0.1%; 0.001%≤Ti≤0.1%; S≤0.09%; P≤0.09%; N≤0.09%; and can contain one or more of the following optional elements Ni≤1%; Cu≤1%; Mo≤0.4%; Nb≤0.1%; V≤0.1%; B≤0.05%; Sn≤0.1%; Pb≤0.1%; Sb≤0.1%; 0.001%≤Ca≤0.01%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel comprising, by area percentage, at least 95% of martensite, a cumulated amount of ferrite and bainite between 1% and 5%, and an optional amount of residual austenite of 2% or less.SELECTED DRAWING: None

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 1280 MPa or more. [Background technology]

[0002] Automotive parts are required to satisfy two conflicting requirements: ease of formability and strength. In recent years, however, environmental considerations have placed a third requirement on automobiles: improved fuel economy. Thus, automotive parts must now be made from materials with high formability to meet the criteria for ease of fit into complex automotive assemblies, while simultaneously improving strength for the automobile's crashworthiness and durability while reducing the automobile's weight to improve fuel economy.

[0003] Therefore, vigorous research and development efforts are being made to reduce the amount of material used in automobiles by increasing the strength of the material. Conversely, increasing the strength of steel sheets reduces their formability, so it is necessary to develop materials that have both 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 thorough understanding of the present invention.

[0005] The steel sheet in WO2017 / 065371 is manufactured by the following steps: rapidly heating a raw steel sheet to above the Ac3 transformation point for 3 to 60 seconds and maintaining the temperature, where the raw steel sheet contains 0.08 to 0.30 wt% C, 0.01 to 2.0 wt% Si, 0.30 to 3.0 wt% Mn, 0.05 wt% or less P, and 0.05 wt% or less S, with the remainder being Fe and other unavoidable impurities; rapidly cooling the heated steel sheet in water or oil at 100°C / s or more; and rapidly tempering the steel sheet from 500°C to the A1 transformation point for 3 to 60 seconds, including the heating and maintaining times. However, the steel in WO2017 / 065371 cannot exceed a tensile strength of 1300 MPa, and does not mention the hole expansion ratio, even though it has a tempered martensitic single-phase structure.

[0006] WO2010 / 036028 relates to a hot-dip galvanized steel sheet and a manufacturing method thereof. The hot-dip galvanized steel sheet includes a steel sheet having a martensite structure as a matrix and a hot-dip galvanized layer formed on the steel sheet. The steel sheet contains 0.05% to 0.30% by weight of C, 0.5% to 3.5% by weight of Mn, 0.1% to 0.8% by weight of Si, 0.01% to 1.5% by weight of Al, 0.01% to 1.5% by weight of Cr, 0.01% to 1.5% by weight of Mo, 0.001% to 0.10% by weight of Ti, 5 ppm to 120 ppm of N, 3 ppm to 80 ppm of B, impurities, and the remainder of Fe. However, the steel in WO2010 / 036028 does not mention the hole expansion ratio. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 065371 [Patent Document 2] International Publication No. 2010 / 036028 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide the following: - an ultimate tensile strength of at least 1280 MPa, preferably greater than 1300 MPa; - a yield strength of at least 1100 MPa, preferably greater than 1150 MPa, - a hole expansion ratio of more than 40%, preferably more than 50%, The present invention aims to solve these problems by making available a cold-rolled martensitic steel sheet that simultaneously has the above properties.

[0009] Preferably, such steels have good weldability and coatability, while also having good suitability for forming and rolling.

[0010] Another object of the invention is to make available a method for manufacturing these plates that is stable to variations in manufacturing parameters, while being compatible with conventional industrial applications. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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.

[0012] The chemical composition of cold rolled martensitic steel consists of the following elements:

[0013] The steel of the present invention contains carbon between 0.1% and 0.2%. Carbon is an element necessary for the formation of low-temperature transformation phases such as martensite, thereby increasing the strength of the steel of the present invention. Therefore, carbon plays two important roles, one of which is to increase strength. However, a carbon content of less than 0.1% does not impart tensile strength to the steel of the present invention. On the other hand, a carbon content of more than 0.2% results in poor spot weldability of the steel, limiting its use in automotive parts. The preferred carbon content for the present invention can be maintained between 0.11% and 0.19%, more preferably between 0.12% and 0.18%.

[0014] The manganese content of the steel of the present invention is between 1.5% and 2.5%. This element is gamma-geneous. Manganese provides solid solution strengthening, suppresses the ferrite transformation temperature, and slows the ferrite transformation rate, thus aiding in the formation of martensite. Amounts of at least 1.5% are necessary to aid in the formation of martensite and provide strength. However, manganese contents exceeding 2.5% have adverse effects, such as delaying the transformation of austenite to martensite during cooling after annealing. Manganese contents exceeding 2.5% can excessively segregate in the steel upon solidification, impairing the internal homogeneity of the material, which can lead to surface cracking during high-temperature processing. The preferred limit for manganese presence is between 1.6% and 2.4%, more preferably between 1.6% and 2.2%.

[0015] The silicon content of the steel of the present invention is between 0.1% and 0.25%. Silicon is an element that contributes to increasing strength through solid solution strengthening. Silicon is an element that can delay the precipitation of carbides 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, which is why the silicon content is kept to a maximum of 0.25%. A silicon content above 0.25% can also control embrittlement, and silicon also impairs coatability. The preferred limit for silicon presence is between 0.16 and 0.24%, more preferably between 0.18 and 0.23%.

[0016] The chromium content of the steel composite coil of the present invention is between 0.1% and 1%. Chromium is an essential element that provides strength to steel through solid solution strengthening, and a minimum of 0.1% is required to provide strength, but use of more than 1% will impair the surface finish of the steel. The preferred limit for the presence of chromium is between 0.1% and 0.5%.

[0017] In the present invention, the aluminum content is between 0.01% and 1%. Aluminum removes oxygen present in the molten steel, preventing 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 higher aluminum content than 1% will raise the Ac3 point to a high temperature, reducing productivity. The preferred limit of aluminum presence is between 0.01% and 0.05%.

[0018] Titanium is added to the steel of the present invention in an amount between 0.001% and 0.1%. Titanium forms titanium nitrides that appear during solidification of the cast product. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides, which would adversely affect formability. In this case, titanium contents of less than 0.001% have no effect on the steel of the present invention.

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

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

[0021] Nitrogen is limited to 0.09% to avoid deterioration of the material over time and to minimize the precipitation of aluminum nitride during solidification, which adversely affects the mechanical properties of the steel.

[0022] Molybdenum is an optional element constituting 0% to 0.4% of the steel of the present invention. Molybdenum plays an effective role in improving hardenability and hardness, and delays the appearance of bainite and therefore promotes the formation of martensite, especially when added in an amount of at least 0.001%, or even at least 0.002%. However, the addition of molybdenum excessively increases the cost of adding metallic elements, so its content is limited to 0.4% for economic reasons.

[0023] Niobium, present in the steel of the present invention between 0% and 0.1%, is suitable for forming carbonitrides that impart strength to the steel of the present invention by precipitation hardening. Niobium also influences the size of the constituents of the microstructure through its precipitation as carbonitrides and by retarding recrystallization during heat treatment. Thus, the finer microstructure formed at the end of the holding temperature, and consequently after full annealing, leads to hardening of the product. However, a saturation effect of its influence is observed (meaning that additional amounts of niobium do not result in any strength improvement in the product), so niobium contents above 0.1% are not economically attractive.

[0024] Vanadium is effective in increasing the strength of steel by forming carbides or carbonitrides, and from an economical viewpoint, the upper limit is 0.1%.

[0025] Nickel can be added as an optional element in amounts of 0% to 1% to increase the strength of the steel of the present invention and improve its toughness. A minimum of 0.01% is preferred to achieve this effect. However, if its content exceeds 1%, nickel causes a deterioration in ductility.

[0026] Copper can be added as an optional element in amounts of 0% to 1% to increase the strength of the steel of the present invention and improve its corrosion resistance. A minimum of 0.01% is preferred to achieve this effect. However, if the copper content exceeds 1%, it may deteriorate the surface morphology.

[0027] Boron is an optional element in the steel of the present invention and can be present between 0% and 0.05%. Boron forms boron nitrides and imparts additional strength to the steel of the present invention when added in an amount of at least 0.0001%.

[0028] Calcium can be added to the steel of the present invention in amounts between 0.001% and 0.01%. Calcium is added to the steel of the present invention as an optional element, especially during inclusion treatment. Calcium contributes to the refinement of the steel by binding with the harmful spheroidal sulfur content and counteracting the harmful effects of sulfur.

[0029] Other elements such as Sn, Pb, or Sb can be added individually or in combination in proportions of 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 unavoidable impurities resulting from the steel and processing.

[0030] The microstructure of martensitic steel sheet is described in detail below, and all percentages are area fractions.

[0031] Martensite constitutes at least 95% of the microstructure by area fraction. The martensite of the present invention can include both fresh and tempered martensite. However, fresh martensite is an optional microscopic component and is limited in the steel in an amount between 0% and 4%, preferably between 0 and 2%, and even better, equal to 0%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from martensite formed during the second stage of cooling after annealing, particularly below the Ms temperature, more specifically between Ms-10°C and 20°C. Such martensite is tempered during holding at a tempering temperature Ttemper between 150°C and 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the martensite content is between 96% and 99%, more preferably between 97% and 99%.

[0032] The cumulative amount of ferrite and bainite represents between 1% and 5% of the microstructure. The cumulative presence of bainite and ferrite up to 5% does not adversely affect the present invention, but above 5% may adversely affect mechanical properties. Therefore, the preferred limits for the cumulative presence of ferrite and bainite are kept between 1% and 4%, more preferably between 1% and 3%.

[0033] Bainite forms during reheating before tempering. In a preferred embodiment, the steel of the present invention contains 1-3% bainite. Bainite can impart formability to the steel, but if present in too much amount, it can adversely affect the tensile strength of the steel.

[0034] Ferrite may form during the first stage of cooling after annealing, but it is not a necessary microstructural component and should be kept as low as possible, preferably below 2%, or even below 1%.

[0035] Retained austenite is an optional microstructure that can be present in steel at between 0% and 2%.

[0036] In addition to the above microstructure, the microstructure of cold rolled martensitic steel sheet does not contain microstructural constituents such as pearlite or cementite.

[0037] The steel of 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 detailed below.

[0038] Such a preferred method comprises providing a semi-finished casting of steel having the chemical composition of the prime steel according to the invention, which can be carried out continuously in the form of ingots or thin slabs or thin strip (i.e., thicknesses ranging from about 220 mm for slabs to a few tens of millimeters for thin strip).

[0039] For example, slabs having the chemical composition of the present invention are produced by continuous casting, where the slabs are optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and ensure that the local carbon to nominal carbon ratio is kept 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.

[0040] The temperature of the slab undergoing hot rolling must be at least 1000°C and less than 1280°C. If the slab temperature is lower than 1280°C, excessive loads will be applied to 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 the structure. Therefore, the slab temperature must be high enough to complete hot rolling in the temperature range of Ac3 to Ac3 + 100°C. Reheating at temperatures above 1280°C is industrially expensive and should be avoided.

[0041] The sheet thus obtained is then cooled at a cooling rate of at least 20° C. / s to the coiling temperature, which must be less than 650° C. Preferably, the cooling rate is no greater than 200° C. / s.

[0042] The hot-rolled steel sheet is then coiled at a coiling temperature of less than 650°C to avoid ovalization, preferably between 475°C and 625°C to avoid scale formation, with a more preferred range for such a coiling temperature being between 500°C and 625°C. The coiled hot-rolled steel sheet is then cooled to room temperature and then subjected to optional hot band annealing.

[0043] The hot-rolled steel sheet can be subjected to an optional descaling step to remove scale formed during hot rolling prior to the optional hot band annealing. The hot-rolled sheet may then be subjected to an optional hot band annealing. In a preferred embodiment, such hot band annealing is carried out at a temperature between 400°C and 750°C for a time period of preferably at least 12 hours and not more than 96 hours, with the temperature preferably being less than 750°C, to avoid partial transformation of the hot-rolled microstructure and thus potentially losing microstructural homogeneity. The hot-rolled steel sheet can then be subjected to an optional descaling step, for example by pickling such sheet.

[0044] Next, this hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet with a reduction ratio of 35 to 90%.

[0045] The cold rolled steel sheet is then heat treated to impart the required mechanical properties and microstructure to the steel of the present invention.

[0046] The cold-rolled steel sheet is heated to a soaking temperature Tsoak between Ac3 and Ac3+100°C, preferably between Ac3+10°C and Ac3+100°C, at a heating rate of at least 2°C / s, preferably greater than 3°C / s, where Ac3 of the steel sheet is calculated using the following formula: Ac3=910-203[C]^(1 / 2)-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]-30[Mn]-11[Cr]-20[Cu]+700[P]+400[Al]+120[As]+400[Ti] In the formula, the element contents are expressed as weight percentages of the cold-rolled steel sheet.

[0047] The cold-rolled steel sheet is held at Tsoak for 10 to 500 seconds to ensure complete recrystallization of the initial structure of severe work hardening and complete transformation to austenite.

[0048] The cold rolled steel sheet is then cooled in a two-stage cooling process, where the first stage of cooling starts at Tsoak and the cold rolled steel sheet is cooled at a cooling rate CR1 between 15°C / s and 150°C / s to a temperature T1 in the range between 650°C and 750°C. In a preferred embodiment, the cooling rate CR1 of the first stage of such cooling is between 20°C / s and 120°C / s. The preferred T1 temperature of such first stage is between 660°C and 725°C.

[0049] In the second stage of cooling, the cold-rolled steel sheet is cooled from T1 to a temperature T2 between Ms-10°C and 20°C at a cooling rate CR2 of at least 50°C / s. In a preferred embodiment, the cooling rate CR2 in the second stage of cooling is at least 100°C / s, more preferably at least 150°C / s. A preferred T2 temperature for such a second stage is between Ms-50°C and 20°C.

[0050] Calculate Ms of the steel plate using the following formula. Ms=545-601.2*(1-EXP(-0.868[C]))-34.4[Mn]-13.7[Si]-9.2[Cr]-17.3[Ni] -15.4[Mo]+10.8[V]+4.7[Co]-1.4[Al]-16.3[Cu]-361[Nb]-2.44[Ti]-3448[B]

[0051] The cold rolled steel sheet is then reheated to a tempering temperature T between 150 and 300°C at a heating rate of at least 1°C / s, preferably at least 2°C / s, more preferably 10°C / s, between 100 and 600 seconds. The preferred temperature range for tempering is between 200 and 300°C, and the preferred duration for holding at T is between 200 and 500 seconds.

[0052] The cold-rolled steel sheet is then cooled to room temperature to obtain cold-rolled martensitic steel.

[0053] The cold rolled martensitic steel sheet of the present invention can optionally be coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve its corrosion resistance. [Example]

[0054] The following tests, examples, figurative illustrations and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0055] Steel plates made of steels with different compositions are summarized in Table 1, where the steel plates are manufactured according to the process parameters specified in Table 2. Table 3 then summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0056] [Table 1]

[0057] Table 2 Table 2 summarizes the hot rolling and annealing treatment parameters 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.

[0058] Table 2 is as follows:

[0059] [Table 2]

[0060] Table 3 illustrates the results of tests carried out according to different microscopic standards, such as scanning electron microscopy, to determine the microstructure of both the steel of the invention and the reference steel in terms of area fraction, and the results are specified herein.

[0061] [Table 3]

[0062] Table 4 The results of various mechanical tests conducted in accordance with the standard are summarized below. The tests are based on JIS-Z2241 and test the ultimate tensile strength and yield strength. A test called hole expansion is applied to evaluate hole expansion. In this test, a 10 mm hole is drilled in the sample, deformed, and the diameter of the hole is measured after deformation, and HER% = 100 * (Df - Di) / Di is calculated.

[0063] [Table 4]

Claims

1. A cold rolled martensitic steel sheet containing the following elements expressed in weight percent: 0.1%≦C≦0.2%, 1.5%≦Mn≦2.5%, 0.1%≦Si≦0.25%, 0.1%≦Cr≦1%, 0.01%≦Al≦0.1%, 0.001%≦Ti≦0.1%, 0%≦S≦0.09%, 0%≦P≦0.09%, 0%≦N≦0.09%, and one or more of the following optional elements: 0%≦Ni≦1%, 0%≦Cu≦1%, 0%≦Mo≦0.4%, 0%≦Nb≦0.1%, 0%≦V≦0.1%, 0%≦B≦0.05%, 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 contains, by area percentage, at least 95% martensite, a cumulative amount of ferrite and bainite between 1 and 5%, and an optional amount of retained austenite between 0 and 2%; The steel plate has an ultimate tensile strength of 1280 MPa or more, a yield strength of 1100 MPa or more, and a hole expansion ratio of more than 40%; Cold rolled martensitic steel plate.

2. 2. The cold rolled martensitic steel sheet according to claim 1, wherein said composition comprises 0.16% to 0.24% silicon.

3. 3. The cold rolled martensitic steel sheet according to claim 1, wherein the composition contains 0.11% to 0.19% carbon.

4. 4. The cold rolled martensitic steel sheet according to claim 1, wherein the composition comprises 0.01% to 0.05% aluminum.

5. 5. The cold rolled martensitic steel sheet according to claim 1, wherein the composition comprises 1.6% to 2.4% manganese.

6. The cold rolled martensitic steel sheet according to any one of claims 1 to 5, wherein the composition comprises 0.1% to 0.5% chromium.

7. Cold rolled martensitic steel sheet according to any one of claims 1 to 6, wherein the amount of martensite is between 96% and 99%.

8. The cold rolled martensitic steel sheet according to any one of claims 1 to 7, wherein a cumulative amount of ferrite and bainite is between 1% and 4%.

9. 1. A method for producing a cold rolled martensitic steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 6, - reheating the semi-finished product to a temperature between 1000°C and 1280°C, - rolling the semi-finished product in the austenitic range to obtain a hot-rolled steel sheet, the hot-rolling finishing temperature being between Ac3 and Ac3+100°C; - cooling the sheet at a cooling rate of at least 20°C / s to a coiling temperature of less than 650°C and coiling the hot-rolled sheet; - cooling the hot-rolled sheet to room temperature; - an optional step of carrying out a descaling treatment on the hot-rolled steel sheet; - an optional step in which the hot-rolled steel sheet can be annealed, - an optional step of carrying out a descaling treatment on the hot-rolled steel sheet; - cold rolling the hot-rolled steel sheet at a reduction ratio of between 35 and 90% to obtain a cold-rolled steel sheet; - then heating the cold-rolled steel sheet at a rate of at least 2°C / s to a soaking temperature Tsoak between Ac3 and Ac3+100°C, which is maintained for 10 to 500 seconds; - then cooling the cold-rolled steel sheet in two stages, the first stage of cooling of the cold-rolled steel sheet starts from Tsoak and is carried out to a temperature T1 between 650°C and 750°C, with a cooling rate CR1 between 15°C / s and 150°C / s, a second stage of cooling, starting from T1, to a temperature T2 between Ms-10°C and 20°C, with a cooling rate CR2 of at least 50°C / s; - then reheating the cold-rolled steel sheet at a rate of at least 1°C / s to a tempering temperature Ttemper between 150 and 300°C, which is held for 100 to 600 seconds; - Then, cooling to room temperature at a cooling rate of at least 1 ° C. / s to obtain a cold-rolled martensitic steel sheet. Including, The microstructure of the steel comprises, by area percentage, at least 95% martensite, a cumulative amount of ferrite and bainite between 1 and 5%, and an optional amount of retained austenite between 0 and 2%; The steel plate has an ultimate tensile strength of 1280 MPa or more, a yield strength of 1100 MPa or more, and a hole expansion ratio of more than 40%; Manufacturing method.

10. 10. The method of claim 9, wherein the coiling temperature is between 475°C and 625°C.

11. 11. The method according to claim 9 or 10, wherein Tsoak is between Ac3+10°C and Ac3+100°C.

12. The method of any one of claims 9 to 11, wherein CR1 is between 20°C / s and 120°C / s.

13. 13. The method of any one of claims 9 to 12, wherein T1 is between 660°C and 725°C.

14. The method of any one of claims 9 to 13, wherein CR2 is greater than 100°C / s.

15. 15. The method according to any one of claims 9 to 14, wherein T2 is between Ms-50°C and 20°C.

16. 16. The method of any one of claims 9 to 15, wherein Ttemper is between 200°C and 300°C.

17. Use of a steel sheet obtained according to any one of claims 1 to 8 or produced according to the method according to any one of claims 9 to 16 for manufacturing structural parts for vehicles.

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