Heat treated cold rolled steel sheet and method of manufacturing thereof

Abstract

To provide cold rolled steel sheets suitable for use in vehicles, the sheets having a tensile strength of 950 MPa or greater and a total elongation of more than 14%.SOLUTION: A cold rolled steel sheet has a composition including 0.05%≤C≤0.15%, 1.8%≤Mn≤2.7%, 0.1%≤Si≤1%, 0.01%≤Al≤0.8%, 0.1%≤Cr≤0.9%, 0%≤P≤0.09%, 0.0001%≤Ti≤0.1%, 0.0005%≤B≤0.003%, 0.01%≤Nb≤0.1%, 0%≤S≤0.09%, 0%≤N≤0.09%, 0%≤V≤0.2%, 0%≤Mo≤0.2%, 0%≤Ni≤2%, 0%≤Cu≤2%, 0%≤Ca≤0.005%, 0%≤Ce≤0.1%, 0%≤Mg≤0.05%, and 0%≤Zr≤0.05%, the remainder being composed of iron and unavoidable impurities caused by processing.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet that is suitable for use as a steel sheet for vehicles and has high strength and high formability, having a tensile strength of 980 MPa or more and a total elongation of more than 14%. [Background technology]

[0002] Automotive parts are required to satisfy two conflicting requirements, namely, ease of formability and high strength, but in recent years, a third requirement, improved fuel economy, has also been imposed on automobiles in light of global environmental issues. Therefore, automotive parts must now be made of materials with high formability to meet the standard of ease of fit in complex automotive assemblies, while at the same time improving strength for vehicle crashworthiness and durability while reducing vehicle weight to improve fuel economy.

[0003] Therefore, significant 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 there is a need to develop materials that combine high strength with 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] U.S. Patent No. 9,074,272 describes a steel having the following chemical composition: 0.1-0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn, and the balance consisting of iron and unavoidable impurities. The microstructure includes 5-20% retained austenite, 40-65% bainitic ferrite, 30-50% polygonal ferrite, and less than 5% martensite. While U.S. Patent No. 9,074,272 mentions a cold-rolled steel sheet with excellent elongation, the invention described therein fails to achieve the 900 MPa strength required to reduce weight while maintaining robustness of complex automotive parts.

[0006] Known prior art techniques for producing high strength and highly formable steel sheets suffer from one or more drawbacks, and therefore, there is a need for a cold rolled steel sheet having high strength and high formability and a method for producing the same. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 9,074,272 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to an ultimate tensile strength of at least 950 MPa, preferably greater than 980 MPa or greater than 1000 MPa; Total elongation of at least 14%, preferably at least 15% The object of the present invention is to solve these problems by making available a cold rolled steel sheet which simultaneously has the following properties:

[0009] In a preferred embodiment, the steel sheet according to the invention may have a yield strength value of at least 540 MPa, or even better above 550 MPa.

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

[0011] Another object of the invention is also to make available a method for manufacturing these plates that is compatible with conventional industrial applications and at the same time is robust to changes in manufacturing parameters.

[0012] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0013] Carbon is present in steel in an amount of 0.05% to 0.15%. Carbon is an element necessary for increasing the strength of steel sheets by forming low-temperature transformation phases such as martensite. Furthermore, carbon also plays a crucial role in austenite stabilization. At a content of less than 0.05%, the formation of martensite is not ensured, thereby reducing strength. On the other hand, at a carbon content exceeding 0.15%, the weld and heat-affected zones are significantly hardened, thus impairing the mechanical properties of the weld. Therefore, the preferred limits are 0.07% to 0.12%, and more preferably 0.08% to 0.11%.

[0014] The manganese content of the steel of the present invention is 1.8% to 2.7%. Manganese is an element that imparts strength to steel through solid solution strengthening. A manganese content of at least about 1.8% by weight is required to impart strength and hardenability to steel sheets and to form ferrite. Therefore, a higher manganese content, such as 1.9% to 2.5%, is preferred, and more preferably 2.1% to 2.5%. However, manganese in excess of 2.7% can have adverse effects, such as slowing the transformation of austenite during cooling after annealing, leading to reduced ductility. Furthermore, a manganese content greater than 2.7% also reduces the weldability of the steel.

[0015] The silicon content of the steel of the present invention is between 0.1% and 1%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. Silicon promotes the ferrite transformation. However, adding silicon in excess of 1% does not improve the above effects and can lead to problems such as hot rolling embrittlement. Therefore, the concentration is controlled within an upper limit of 1%. The preferred limits for the presence of silicon are kept between 0.2% and 0.9%, more preferably between 0.3% and 0.7%.

[0016] The aluminum content of the steel of the present invention is 0.01 to 0.8%. Within this range, aluminum combines with nitrogen in the steel to form aluminum nitride, reducing the grain size. However, in the present invention, whenever the aluminum content exceeds 0.8%, aluminum increases the Ac3 point, thereby reducing productivity. Therefore, the preferred range of aluminum is maintained at 0.01% to 0.7%, more preferably 0.01% to 0.6%.

[0017] In a preferred embodiment, the cumulative amount of silicon and aluminum is at least 0.6% since both elements are ferrite phase formers and thereby participate in the formation of ferrite, which is advantageous in terms of both elongation and ductility.

[0018] The chromium content of the steel of the present invention is between 0.1% and 0.9%. Chromium is an essential element that imparts strength and hardenability to the steel, but if used in amounts greater than 0.9%, it will impair the surface finish of the steel. Therefore, to achieve the optimum effect of chromium, the preferred limits are between 0.2% and 0.8%, more preferably between 0.2% and 0.7%.

[0019] Titanium is an essential element that can be added to the steel of the present invention in amounts of 0.0001% to 0.1%, preferably 0.01% to 0.08%. Like niobium, titanium plays a role in hardening by participating in carbonitrides. However, titanium also participates in the formation of TiN, which appears during solidification of the casting. To avoid coarse TiN that is detrimental to hole expansion, the amount of Ti is limited to 0.1%. If the titanium content is below 0.0001%, titanium has no effect on the steel of the present invention.

[0020] Boron is an essential element in the present invention and is added in very small amounts, between 0.0005% and 0.003%. Boron imparts hardenability and strength to the steel of the present invention. However, it has been found that adding boron in excess of 0.003% significantly reduces the rollability of the steel sheet. Additional boron can cause segregation at grain boundaries, which is detrimental to formability.

[0021] Niobium is an essential element that can be added to steel in an amount of 0.01% to 0.1%, preferably 0.01% to 0.06%. Niobium is suitable for forming carbonitrides to impart strength to the steel according to the present invention by precipitation hardening. Niobium delays recrystallization during heating, so that they are formed at the end of the holding temperature. As a result, the microstructure formed after full annealing is finer, which leads to hardening of the product. However, if the niobium content exceeds 0.1%, a large amount of carbonitrides tends to reduce the ductility of the steel, so that the amount of carbonitrides is not preferred for the present invention.

[0022] Vanadium is an optional element that may be added to the steel of the present invention in amounts up to 0.2%, preferably 0.001% to 0.01%. 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 the casting. The amount of V is limited to 0.2% to avoid coarse VN, which is detrimental to hole expansion. If the vanadium content is below 0.001%, vanadium has no effect on the steel of the present invention.

[0023] The phosphorus content of the steel of the present invention is limited to 0.09%. Phosphorus is an element that hardens in solid solution and also prevents the formation of carbides. Therefore, small amounts of phosphorus, at least 0.002%, can be advantageous, but due to its tendency to segregate, especially at grain boundaries, or co-segregate with manganese, phosphorus also has its adverse effects, such as reducing spot weldability and hot ductility. For these reasons, the phosphorus content is preferably limited to a maximum of 0.02%.

[0024] Sulfur is not an essential element, but may be contained in steel as an impurity. The sulfur content is preferably as low as possible, but from the viewpoint of production costs, it is 0.09% or less, preferably less than 0.03%. Furthermore, if a higher sulfur content is present in steel, sulfur will combine to form sulfides, particularly with Mn and Ti, reducing the beneficial effects of Mn and Ti on the present invention.

[0025] 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.

[0026] Molybdenum is an optional element constituting 0% to 0.2% of the steel of the present invention; it improves hardenability and hardness, delays the appearance of bainite, and therefore promotes the formation of martensite when added in an amount of at least 0.01%. Molybdenum also promotes the formation of ferrite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.2%. The preferred limit for molybdenum is 0.01% to 0.2%.

[0027] Nickel may be added as an optional element in an amount of 0% to 2% to increase the strength of the steel of the present invention and to improve its toughness. A minimum of 0.01% is preferred to achieve this effect. However, above 2%, nickel causes a decrease in ductility.

[0028] Copper may be added as an optional element in amounts of 0% to 2% to increase the strength of the steel of the present invention and to improve its corrosion resistance. A minimum of 0.01% is preferred to achieve this effect. However, if its content exceeds 2%, copper may deteriorate the surface morphology.

[0029] Calcium is an optional element that may be added to the steel of the present invention in amounts up to 0.005%, preferably 0.0001% 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 binding harmful sulfur during spheroidizing of the steel.

[0030] 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 allow for grain refinement during solidification.

[0031] The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0032] The microstructure of the steel sheet according to the present invention comprises, by area fraction, 40% to 60% martensite, 5% to 40% inter-critical ferrite, 10% to 35% cumulative amounts of transformed ferrite and bainite, and 0% to 5% retained austenite.

[0033] Martensite constitutes 40% to 60% of the microstructure by area fraction. Martensite may form during cooling after annealing, especially past the Ms temperature, especially between Ms-10°C and Ms-20°C, or during cooling after overaging. Martensite provides strength to the present invention. The preferred limits for martensite are 42% to 58%, more preferably 43% to 56%.

[0034] Intercritical ferrite constitutes 15% to 40% of the microstructure by area fraction in the steel of the present invention. This intercritical ferrite imparts a total elongation of at least 14% to the steel of the present invention. Intercritical ferrite results from annealing at temperatures below Ac3. Intercritical ferrite is hereinafter referred to as "transformed ferrite" and differs from the ferrite that may be produced after annealing, as described below. In contrast to transformed ferrite, intercritical ferrite is polygonal. Furthermore, transformed ferrite is enriched in carbon and manganese, i.e., has a higher carbon and manganese content than that of intercritical ferrite. Therefore, intercritical ferrite and transformed ferrite can be distinguished by observing micrographs with a SEM microscope using secondary electrons after etching with a 2% Nital etchant. In such micrographs, intercritical ferrite appears medium gray, while transformed ferrite appears dark gray due to its higher carbon and manganese content. It is preferable that the alloy has 20% to 40%, more preferably 25% to 38% intercritical ferrite.

[0035] The total amount of transformed ferrite and bainite constitutes 10% to 35% of the microstructure by area fraction for the steel of the present invention. The transformed ferrite of the present invention is composed of ferrite formed during cooling after annealing, and the steel of the present invention always contains transformed ferrite, i.e., the presence of transformed ferrite is always greater than 0%. The transformed ferrite imparts high strength and elongation to the steel of the present invention. The transformed ferrite of the present steel is richer in carbon and manganese than intercritical ferrite, and having transformed ferrite in the steel is essential. During averaging, especially when held at 400°C to 480°C, bainite forms. To ensure 14% elongation, it is necessary to have 10% transformed ferrite and bainite. However, whenever the total amount is greater than 35% in the steel of the present invention, it is impossible to simultaneously obtain both tensile strength and total elongation. The preferred limits for transformed ferrite and bainite for the present invention are 15% to 30%.

[0036] Retained austenite is an optional microstructure and can be present in steel at 0% to 5%.

[0037] In addition to the above microstructure, the microstructure of the cold rolled and heat treated steel sheet does not contain microstructural components such as pearlite, tempered martensite and cementite without impairing the mechanical properties of the steel sheet.

[0038] 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 a chemical composition according to the invention. The casting can be ingot or in the form of continuous thin slabs or thin strip, i.e. thicknesses ranging from about 220 mm for slabs to up to several tens of millimeters for thin strip.

[0039] For example, slabs having the above-mentioned chemical compositions were produced by continuous casting, in which the slabs optionally underwent direct soft reduction during the continuous casting process to avoid center segregation and to ensure that the local carbon to nominal carbon ratio remained below 1.10. The slabs provided by the continuous casting process can be used directly at high temperature after continuous casting, or can be first cooled to room temperature and then reheated for hot rolling.

[0040] The temperature of the slab subjected to hot rolling must be at least 1000°C and below 1280°C. If the slab temperature is lower than 1000°C, excessive load is placed on the rolling mill, and the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, causing the steel to be rolled with transformed ferrite in its structure. Reheating at temperatures above 1280°C is industrially expensive and must be avoided. Therefore, the finish rolling temperature of the slab is above Ac3, preferably high enough to complete hot rolling in the temperature range of Ac3 + 150°C to Ac3 + 250°C.

[0041] To have a favorable structure for recrystallization and rolling, a final rolling temperature range of Ac3 to Ac3+200° C. is required. The final rolling pass is preferably carried out at a temperature above 850° C., more suitably at least 950° C.

[0042] The hot-rolled steel thus obtained is then cooled to the coiling temperature at a cooling rate of at least 30°C / s, preferably at a cooling rate of not more than 200°C / s.

[0043] The hot-rolled steel is then coiled at a temperature of 475°C to 650°C to avoid ovalization, preferably at a temperature of 475°C to 625°C to avoid scale formation. A more preferred range for such coiling temperatures is 500°C to 625°C. The coiled hot-rolled steel is then cooled to room temperature and optionally subjected to hot band annealing.

[0044] The hot-rolled steel may be subjected to an optional descaling step prior to optional hot band annealing to remove scale formed during hot rolling. The hot-rolled sheet may then be subjected to optional hot band annealing, for example, at a temperature of 400°C to 750°C, preferably for at least 12 hours and not more than 96 hours, with the temperature being kept below 750°C to avoid partially transforming the hot-rolled microstructure and thus losing microstructural homogeneity. This optional subsequent descaling step of the hot-rolled steel may be carried out, for example, through pickling of such sheet.

[0045] This hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction of 35-90%. The cold rolled steel sheet obtained from the cold rolling process is then subjected to annealing to impart the microstructure and mechanical properties to the steel of the present invention.

[0046] To anneal the cold-rolled steel sheet, the cold-rolled steel sheet is heated to a soaking temperature of Ac1+60°C to Ac3, preferably at a heating rate of at least 3°C / s, and then annealed at that temperature for 5 to 500 seconds, preferably 50 to 250 seconds. In a preferred embodiment, the heating is at least 10°C / s, more preferably at least 15°C / s. During this annealing, intercritical ferrite forms.

[0047] The preferred annealing soaking temperature is Ac1+70°C to Ac3, more preferably Ac1+80°C to Ac3-30°C.

[0048] In a preferred embodiment, the soak time and temperature are selected to ensure that the microstructure of the steel sheet at the end of the soak contains at least 50% austenite, more preferably at least 60% austenite.

[0049] The cold-rolled steel is then cooled in a two-stage cooling process, the first stage starting from the soaking temperature to a temperature T1 of 550°C to 650°C at a cooling rate CR1 of at least 3°C / s, preferably at least 5°C / s, more preferably at least 10°C / s. During this stage, transformed ferrite forms. The cold-rolled steel is then held at T1 for a time of 1 to 20 seconds, preferably 2 to 15 seconds, more preferably 5 to 12 seconds.

[0050] Thereafter, the second step begins with further cooling the cold-rolled steel sheet from T1 to an overaging temperature T2 of 400°C-480°C at a cooling rate CR2 of at least 3°C / s, preferably at least 5°C / s, more preferably at least 7°C / s.

[0051] Then, overaging is carried out at T2 for 5 to 100 seconds. During overaging, some bainite is formed. The preferred temperature for overaging, T2, is 420°C to 475°C. The preferred time at the overaging temperature is 15 to 75 seconds, more preferably 20 and 75 seconds.

[0052] The cold-rolled steel sheet can then be cooled to room temperature or brought to a hot-dip bath temperature of 420°C to 680°C, depending on the nature of the coating, to facilitate hot-dip coating of the cold-rolled steel sheet.

[0053] In any case, the final cooling to room temperature is carried out at a cooling rate of at least 5°C / s, preferably at least 9°C / s, to ensure the formation of fresh martensite in the steel of the invention.

[0054] Cold rolled steel sheet can also be coated by any of the known industrial processes such as electrogalvanizing, JVD, PVD, etc., which may not require the above temperature ranges before coating. [Example]

[0055] 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 explain in detail the importance of the parameters selected by the inventors after extensive experimentation, and to further establish the properties that can be achieved with the steel according to the present invention.

[0056] 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.

[0057] Table 1: Test composition Table 1 shows the steels with their compositions expressed as percentages by weight.

[0058] [Table 1]

[0059] Table 1 also shows the Ac1 and Ac3 temperature points calculated by dilatometry.

[0060] Table 2: Process parameters Table 2 summarizes the annealing process parameters and two-stage cooling scheme, including overaging, performed on the steel samples in Table 1, which were all reheated at 1230°C, hot rolled at a finish rolling temperature of 875°C, coiled at 550°C, and cold rolled with a 50% reduction before undergoing annealing.

[0061] [Table 2]

[0062] Table 3 summarizes the results of tests carried out according to the standard with different microscopes, such as scanning electron microscopes, to determine the microstructural composition of both the steel of the invention and the reference test.

[0063] [Table 3]

[0064] Table 4 summarizes the mechanical properties of both the steel of the invention and the reference steel. Tensile strength, yield strength and total elongation tests are carried out according to the JIS Z2241 standard.

[0065] [Table 4]

[0066] The examples show that the steel sheets according to the invention are unique in that they exhibit all the targeted properties thanks to their specific composition and microstructure.

Claims

1. 1. A cold rolled steel sheet having a composition, expressed in weight percent, comprising the following elements: 0.05%≦Carbon≦0.15% 1.8%≦Manganese≦2.7% 0.1%≦Silicon≦1% 0.01%≦Aluminum≦0.8% 0.1%≦chromium≦0.9% 0%≦phosphorus≦0.09% 0.0001%≦Titanium≦0.1% 0.0005%≦Boron≦0.003% 0.01%≦niobium≦0.1% 0%≦sulfur≦0.09% 0%≦nitrogen≦0.09% and may contain one or more of the following optional elements: 0%≦vanadium≦0.2% 0%≦molybdenum≦0.2% 0%≦Nickel≦2% 0%≦Copper≦2% 0%≦Calcium≦0.005% 0%≦Cerium≦0.1% 0%≦Magnesium≦0.05% 0%≦zirconium≦0.05% A cold-rolled steel sheet, the remainder of which is composed of iron and unavoidable impurities caused by processing, and the microstructure of the steel sheet comprises, by area fraction, 40% to 60% martensite, 15 to 40% intercritical ferrite, 10 to 35% cumulative amounts of transformed ferrite and bainite, and 0% to 5% retained austenite.

2. 10. The cold rolled and coated steel sheet of claim 1, wherein the composition comprises 0.2% to 0.9% silicon.

3. Cold rolled and coated steel sheet according to claim 1 or 2, wherein the composition comprises 0.07% to 0.12% carbon.

4. Cold rolled and coated steel sheet according to any one of claims 1 to 3, wherein the composition comprises 0.01% to 0.7% aluminium.

5. Cold rolled and coated steel sheet according to any one of claims 1 to 4, wherein the composition comprises 1.9% to 2.5% manganese.

6. Cold rolled and coated steel sheet according to any one of claims 1 to 5, wherein the composition comprises 0.2% to 0.8% chromium.

7. The cold rolled and coated steel sheet according to any one of claims 1 to 6, wherein the cumulative amount of silicon and aluminum is greater than 0.6%.

8. The cold rolled and coated steel sheet according to claims 1 to 7, wherein the cumulative amount of transformed ferrite and bainite is 15% to 30%.

9. Cold rolled and coated steel sheet according to any one of claims 1 to 8, wherein the amount of martensite is between 42% and 58%.

10. The cold rolled and coated steel sheet according to any one of claims 1 to 9, wherein the steel sheet has an ultimate tensile strength of at least 950 MPa and a total elongation of at least 14%.

11. 11. The cold rolled and coated steel sheet of claim 10, wherein the steel sheet has a yield strength of 540 MPa or greater.

12. 1. A method for producing cold rolled and coated steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 7; - reheating said semi-finished product to a temperature between 1000°C and 1280°C; - rolling said semi-finished product, the hot rolling finishing temperature being above Ac3, to obtain hot rolled steel; - cooling the hot rolled steel at a cooling rate of at least 30°C / s to a coiling temperature of 475°C to 650°C; and coiling the hot rolled steel; - cooling the hot rolled steel to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling process; - optionally annealing the hot rolled steel sheet; - optionally subjecting the hot rolled steel sheet to a descaling process; a step of cold rolling the hot-rolled steel sheet at a rolling reduction of -35 to 90% to obtain a cold-rolled steel sheet; - heating the cold-rolled steel sheet from room temperature to a soaking temperature of Ac1+60°C to Ac3; - then carrying out an annealing at a soaking temperature for 5 to 500 seconds; The cold-rolled steel is then cooled in a two-stage cooling process: a first step starting from said soaking temperature and cooling at a cooling rate CR1 of at least 3°C / s to a temperature T1 between 550°C and 650°C; -Then, the cold rolled steel is held at T1 for 1 second to 20 seconds, The second step then begins by further cooling the cold-rolled steel sheet from T1 to an overaging temperature T2 of 400°C to 480°C at a cooling rate CR2 of at least 3°C / s; - then overaging at T2 for 5 to 100 seconds; - then optionally subjecting said cold rolled steel sheet to a temperature in the range of 420°C to 680°C to facilitate coating, and optionally coating said cold rolled sheet; - thereafter cooling said cold rolled steel sheet to room temperature at a cooling rate of at least 5°C / s to obtain a cold rolled coated steel sheet; A method comprising:

13. The method of claim 12, wherein the coiling temperature is from 475°C to 625°C.

14. 14. A method according to claim 12 or 13, wherein the soaking temperature is selected to ensure the presence of at least 50% austenite at the end of the soaking.

15. The method according to any one of claims 12 to 14, wherein the temperature for overaging is between 420°C and 475°C.

16. A method according to any one of claims 12 to 15, wherein the cooling rate after coating is at least 9°C / s.

17. Use of a steel sheet according to any one of claims 1 to 11 or produced according to the method of claims 12 to 16 for the production of structural or safety parts of a vehicle.

18. A vehicle comprising a component obtained according to claim 17.

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