Hot-rolled steel sheet and its manufacturing method

Abstract

Hot-rolled steel sheet containing the following elements: 0.09%≦carbon≦0.15%, 1%≦manganese≦2%, 0.1%≦silicon≦0.5%, 0.01%≦aluminum≦0.1%, 0.3%≦chromium≦1%, 0.1%≦molybdenum≦0.5%, 0.05%≦vanadium≦0.14%, 0.002%≦phosphorus≦0.02%, 0%≦sulfur≦0.005%, 0%≦nitrogen≦0.01%, 0%≦niobium≦0.09%, 0%≦titanium≦0.09%, 0%≦calcium≦0.005%, 0% A hot-rolled steel sheet having a composition including: ≦copper≦1%, 0%≦nickel≦1%, 0%≦boron≦0.05%, 0%≦magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦cerium≦0.1%, with the remainder consisting of iron and unavoidable impurities resulting from processing, and having a microstructure comprising, by area fraction, 50% to 93% bainite, 5% to 50% ferrite, 1% to 15% martensite-austenite islands, and 1% to 15% pearlite.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet suitable for use as an automotive steel sheet. [Background technology]

[0002] Automotive parts are required to satisfy two conflicting requirements: ease of forming and strength. However, in recent years, a third requirement, improved fuel economy, has 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 being easily fitted into complex automotive assemblies, while at the same time improving strength for the vehicle's impact resistance and durability while reducing vehicle weight and improving fuel economy.

[0003] Therefore, research and development efforts are being actively conducted to reduce the amount of material used in automobiles by increasing the strength of materials.On the other hand, since increasing the strength of steel sheets reduces their formability, 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 highly formable steel plates has resulted in several methods for manufacturing high strength highly formable steel plates, some of which are listed here for a clear understanding of the present invention.

[0005] Chinese Patent Application Publication No. 113201691 discloses a 590 MPa-grade hot-rolled steel plate for hydraulic bulging and a manufacturing method thereof, which belongs to the technical field of steel material engineering. The 590 MPa-grade hot-rolled steel plate for hydraulic bulging contains the following chemical components in weight percent: 0.02-0.04% C, 0.05% or less Si, 1.05-1.15% Mn, 0.040-0.050% Nb, 0.050-0.060% Ti, 0.30-0.60% Cr, 0.02% or less P, 0.003% or less S, 0.0040% or less N, 0.010-0.050% Als, 0.0010-0.0050% Ca, and the balance being Fe and unavoidable impurities. This invention employs a micro / low C-Nb-Ti-Cr micro-alloying composition system and employs controlled rolling and controlled cooling processes to achieve a microstructure of ultrafine acicular ferrite and nano-precipitates. The mechanical properties of the steel sheet produced by this invention meet the following requirements: yield strength of 500 MPa or more, tensile strength of 590 MPa or more, elongation of 25% or more, n value of 0.12 or more, da pass in 180-degree cold bending tests, and grain size of grade 12 or more. This effectively solves the problem of poor formability of existing high-strength steels. However, the use of this steel in the automotive industry is limited due to the lack of uniform properties in all three directions, i.e., the transverse and longitudinal directions, of the steel in Chinese Patent Application Publication No. 113201691. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Chinese Patent Application Publication No. 113201691 Summary of the Invention

[0007] The object of the present invention is to solve these problems by making available a hot-rolled steel sheet which simultaneously has: an ultimate tensile strength of 770 MPa or more in both the transverse and longitudinal directions, preferably 800 MPa or more in both the transverse and longitudinal directions; A yield strength of 650 MPa or more in both the transverse and longitudinal directions, preferably a yield strength of 680 MPa or more in both the transverse and longitudinal directions; a total elongation of 14% or more in both the transverse and longitudinal directions, preferably a total elongation of 15% or more in both the transverse and longitudinal directions; A hole expansion ratio of 50% or more, preferably 60% or more. DETAILED DESCRIPTION OF THE INVENTION

[0008] In a preferred embodiment, the steel sheet according to the invention may also have a yield strength to tensile strength ratio of 0.5 or more.

[0009] Preferably, such steels are also well suited to forming, especially rolling, with good weldability and coatability.

[0010] Another object of the present invention is also to make available a method for manufacturing these sheets that is robust to changes in manufacturing parameters, yet compatible with conventional industrial applications.

[0011] The hot rolled steel sheet of the present invention may optionally be coated with zinc or a zinc alloy to improve its corrosion resistance.

[0012] Carbon is present in the steel in an amount of 0.09% to 0.15%. Carbon is a necessary element for imparting mechanical properties to the steel of the present invention by facilitating the formation of ferrite. Carbon also plays a central role in increasing strength, since it imparts strength to the steel through precipitation strengthening by forming vanadium carbides and niobium carbides. However, a carbon content of less than 0.09% does not impart tensile strength to the steel of the present invention. On the other hand, a carbon content greater than 0.15% results in poor spot weldability of the steel, thereby limiting its application to automotive parts. The preferred carbon content of the present invention can be maintained at 0.1% to 0.14%, more preferably 0.11% to 0.135%.

[0013] The manganese content of the steel of the present invention is 1% to 2%. This element is a γ-phase stabilizer and also influences the Bs and Ms temperatures, thus playing an important role in controlling bainite formation. The purpose of adding manganese is essentially to impart hardenability to the steel. It has been found that manganese in an amount of at least 1% by weight imparts strength and hardenability to steel sheets. However, if the manganese content exceeds 2%, manganese has adverse effects, such as delaying the austenite transformation during cooling after hot rolling. Furthermore, a manganese content exceeding 2% promotes center segregation, thus reducing formability and also reducing the weldability of the steel. The preferred manganese content of the present invention can be maintained at 1.3% to 1.9%, more preferably 1.4% to 1.7%.

[0014] The silicon content of the steel of the present invention is 0.1% to 0.5%. Silicon is a solid solution strengthener, particularly for the bainite microstructure. Furthermore, a high silicon content can delay the precipitation of cementite. However, an imbalance in silicon content can lead to problems such as surface defects, e.g., tiger stripes, which adversely affect the coatability of the steel of the present invention. Therefore, its concentration is controlled to an upper limit of 0.5%. The preferred content of the present invention can be kept between 0.15% and 0.4%, more preferably between 0.15% and 0.35%.

[0015] Aluminum is an element present in the steel of the present invention in an amount of 0.01% to 0.1%. Aluminum is an α-phase stabilizing element and provides ductility to the steel of the present invention. Since aluminum in steel has a tendency to combine with nitrogen to form aluminum nitride, from the standpoint of the present invention, the aluminum content must be kept as low as possible, preferably between 0.02% and 0.06%.

[0016] Chromium is an essential element in the present invention. The chromium content is present in the steel of the present invention in an amount of 0.3% to 1%. Chromium provides strength and hardenability to the steel, but if used in an amount greater than 1%, it will impair the surface finish of the steel. The preferred limit for chromium in the present invention is 0.35% to 0.9%, more preferably 0.35% to 0.7%.

[0017] Molybdenum is an essential element constituting 0.1% to 0.5% of the steel of the present invention. Molybdenum improves the hardenability of the steel of the present invention and influences the transformation of austenite to ferrite and bainite during cooling after hot rolling. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit for molybdenum is 0.15% to 0.4%, more preferably 0.2% to 0.35%.

[0018] Vanadium is an essential element constituting 0.05% to 0.14% of the steel of the present invention. Vanadium is effective in increasing the strength of the steel by forming carbides, nitrides, or carbonitrides, and for economic reasons, the upper limit is 0.14%. These carbides, nitrides, or carbonitrides are formed during the second and third cooling steps. The preferred limit for vanadium is 0.05% to 0.12%.

[0019] The phosphorus content of the steel of the present invention is 0.002% to 0.02%. Phosphorus tends to segregate, particularly at grain boundaries, or co-segregate with manganese, thereby reducing spot weldability and hot ductility. For these reasons, its content is limited to 0.02%, preferably less than 0.015%.

[0020] Sulfur is not an essential element, but may be present in steel as an impurity. From the viewpoint of the present invention, the lower the sulfur content, the better. However, from the viewpoint of production costs, the sulfur content is 0.005% or less. Furthermore, if sulfur is present in steel in a larger amount, sulfur will combine with manganese in particular to form sulfides, reducing its beneficial effect on the steel of the present invention. Therefore, a sulfur content of less than 0.003% is preferred.

[0021] Nitrogen is limited to 0.01% to avoid deterioration of the material over time. Nitrogen provides strength to the steel of the present invention through precipitation strengthening by forming nitrides with vanadium and niobium. However, whenever nitrogen is present in excess of 0.01%, nitrogen can form large amounts of aluminum nitride, which is detrimental to the present invention, so the preferred upper limit for nitrogen is 0.005%.

[0022] Niobium is an optional element of the present invention. The niobium content may be present in the steel of the present invention in a range of 0% to 0.09%, but is added to the steel of the present invention to form carbides or carbonitrides, which provide strength to the steel of the present invention by precipitation strengthening. The preferred content of the present invention may be kept between 0.02% and 0.07%, more preferably between 0.02% and 0.05%.

[0023] Titanium is an optional element added to the steel of the present invention in an amount of 0% to 0.09%, preferably 0.01% to 0.06%. Like niobium, titanium is involved in the formation of carbides and carbonitrides, thereby playing a role in hardening and strengthening the steel. However, titanium also participates in the formation of TiN, which appears during solidification of the casting. Therefore, the amount of Ti is limited to 0.09% to avoid coarse TiN, which is unfavorable for hole expansion. If the titanium content is less than 0.01%, it will not have any effect on the steel of the present invention.

[0024] Nickel may be added as an optional element in amounts of 0% to 1% to increase the strength of steel and improve its toughness. A minimum of 0.01% is required to achieve this effect. However, at a content greater than 1%, nickel causes a decrease in ductility.

[0025] Copper may be added as an optional element in amounts of 0% to 1% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to achieve this effect. However, if its content exceeds 1%, copper will cause a decrease in hot ductility during hot rolling.

[0026] Calcium is an optional element that may be added to the steel of the present invention in amounts up 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 refinement of the steel by capturing unwanted sulfur during spheroidization.

[0027] Other elements, such as cerium, boron, magnesium or zirconium, can be added individually or in combination in the following proportions: Ce≦0.1%, B≦0.05%, Mg≦0.05% and Zr≦0.05%. Up to the maximum content levels indicated, these elements allow for grain refinement during solidification.

[0028] The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of the steel plate includes:

[0030] Bainite constitutes 50% to 93% of the microstructure of the steel of the present invention in terms of area fraction. Bainite constitutes the main phase of the steel as a matrix, and is cumulatively composed of upper bainite and lower bainite. To ensure a tensile strength of 770 MPa, preferably 800 MPa or more, it is necessary to have 50% bainite. Bainite begins to form during the cooling process and continues to form until coiling. Preferably, the bainite content is 60% to 92%, more preferably 70% to 90%.

[0031] Ferrite constitutes 5% to 50% of the microstructure of the steel of the present invention by area fraction. Cumulatively, ferrite includes polygonal ferrite and acicular ferrite. Ferrite provides the steel of the present invention with formability as well as elongation. To ensure an elongation of 14% or more, it is necessary to have 5% ferrite. In the steel of the present invention, ferrite is formed during cooling after hot rolling. However, whenever the ferrite content in the steel of the present invention exceeds 50%, tensile strength is not obtained. Preferably, the ferrite content is 5% to 30%, more preferably 5% to 25%.

[0032] The martensite-austenite islands are a component of the steel of the present invention, with an area fraction of 1% to 15%. The martensite-austenite islands provide strength to the steel of the present invention. If the martensite-austenite islands exceed 15%, the strength will be excessive, and the yield strength will exceed the allowable upper limit. In a preferred embodiment, the martensite-austenite islands are 1% to 12%, more preferably 1% to 6%.

[0033] Pearlite is an essential component of the microstructure of the steel of the present invention and is present in an amount of 1% to 15%. Pearlite provides strength to the steel. Pearlite forms during cooling after hot rolling temperature and continues to form up to the coiling temperature. In a preferred embodiment, pearlite is present in an amount of 1% to 12%, more preferably 1% to 8%.

[0034] In addition to the microstructure described above, the microstructure of the hot rolled steel sheet may contain, but is free of, other microstructural components such as cementite.

[0035] The steel sheet according to the invention can be produced by any suitable method, the preferred method being to provide a semi-finished product cast of steel having the chemical composition according to the invention, which can be cast in the form of an ingot or continuously thin slab or flake, i.e. in thicknesses ranging from about 220 mm for slabs to several tens of millimeters for flakes.

[0036] For example, slabs having the above-mentioned chemical composition are produced by continuous casting, but the slabs are optionally subjected to a soft reduction directly during the continuous casting process to avoid center segregation and maintain a local carbon to nominal carbon ratio of less than 1.10. The slabs resulting from 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.

[0037] The temperature of the slab subjected to hot rolling should preferably be at least 1200°C and less than 1300°C. If the slab temperature is less than 1200°C, excessive load is placed on the rolling mill. Therefore, the slab temperature is preferably high enough to complete hot rolling in the 100% austenite region. Reheating at temperatures above 1275°C must be avoided because it reduces productivity and is industrially expensive. Therefore, the preferred reheating temperature is 1200°C to 1275°C.

[0038] The hot rolling finishing temperature in the present invention is 850°C to 975°C, and preferably 880°C to 930°C.

[0039] The hot-rolled strip thus obtained is then cooled, with cooling commencing immediately after the finish of hot rolling, and the hot-rolled strip is cooled from the finish of hot rolling to a cooling stop temperature range of 480°C to 600°C at a cooling rate of more than 20°C / sec, preferably 40°C / sec to 150°C / sec, more preferably 40°C / sec to 120°C / sec. The cooling stop temperature range is preferably 490°C to 590°C, more preferably 500°C to 580°C.

[0040] Thereafter, the hot-rolled strip is coiled at a coiling temperature range of 480° C. and 550° C., preferably 490° C. to 540° C. The coiled hot-rolled strip is then cooled to room temperature to obtain a hot-rolled steel sheet.

[0041] The hot rolled steel thus obtained preferably has a thickness of 0.1 mm to 12 mm, more preferably 0.1 mm to 8 mm, and even more preferably 0.1 mm to 5 mm. [Example]

[0042] The following tests, examples, symbolic 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.

[0043] Table 1 summarizes the steel plates made of various steel compositions, each manufactured according to the processing 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.

[0044] Table 1

[0045] [Table 1]

[0046] Table 2: Table 2 summarizes the processing parameters carried out on the steels in Table 1.

[0047] [Table 2]

[0048] Table 3: Table 3 illustrates the microstructural observations, where the microstructure was observed by SEM after 2% nital etching. The phase volume fractions were measured by the systematic manual point counting method according to ASTM E562 for both the inventive and reference steels.

[0049] The results are set forth herein.

[0050] [Table 3]

[0051] Table 4 Table 4 illustrates the mechanical properties of both the steel of the invention and the reference steel. Tensile tests are carried out according to the JIS Z2241 standard to determine the tensile strength, yield strength and total elongation, and the hole expansion ratio is measured according to the ISO 16630 standard.

[0052] The results of various mechanical tests carried out in accordance with these standards are summarized below.

[0053] Table 4

Claims

1. 1. A hot rolled steel sheet comprising the following elements, expressed in weight percent: 0.09%≦Carbon≦0.15% 1%≦Manganese≦2% 0.1%≦Silicon≦0.5% 0.01%≦Aluminum≦0.1% 0.3%≦chromium≦1% 0.1%≦molybdenum≦0.5% 0.05%≦vanadium≦0.14% 0.002%≦phosphorus≦0.02% 0%≦sulfur≦0.005% 0%≦Nitrogen≦0.01% having a composition comprising Any of the following elements: 0%≦niobium≦0.09% 0%≦Titanium≦0.09% 0%≦Calcium≦0.005% 0%≦Copper≦1% 0%≦Nickel≦1% 0%≦Boron≦0.05% 0%≦Magnesium≦0.05% 0%≦zirconium≦0.05% 0%≦Cerium≦0.1% and A hot-rolled steel plate, the remainder of whose composition is composed of iron and inevitable impurities resulting from processing, and the microstructure of the steel plate comprises, in area fractions, 50% to 93% bainite, 5% to 50% ferrite, 1% to 15% martensite-austenite islands, and 1% to 15% pearlite.

2. 2. The hot rolled steel sheet according to claim 1, wherein the composition comprises 0.15% to 0.4% silicon.

3. 3. The hot rolled steel sheet according to claim 1, wherein the composition contains 0.1% to 0.14% carbon.

4. 4. The hot rolled steel sheet according to claim 3, wherein the composition comprises 0.05% to 0.12% vanadium.

5. 5. The hot rolled steel sheet according to claim 1, wherein the composition comprises 1.3% to 1.9% manganese.

6. 6. The hot rolled steel sheet according to claim 1, wherein the composition comprises 0.35% to 0.9% chromium.

7. The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the amount of pearlite is 1% to 12% in terms of area fraction.

8. The hot rolled steel sheet according to any one of claims 1 to 7, wherein the steel sheet has a yield strength of 650 MPa or more in both the transverse and longitudinal directions.

9. The hot rolled steel sheet according to any one of claims 1 to 8, wherein the steel sheet has a tensile strength of 770 MPa or more in both the transverse and longitudinal directions.

10. 1. A method for producing a hot rolled steel sheet, comprising the following successive steps: - preparing a steel composition according to any one of claims 1 to 7, - Reheating the semi-finished product to a temperature of 1200°C to 1300°C; A step of rolling the semi-finished product in the austenite region at a hot rolling finishing temperature of 850 ° C to 975 ° C to obtain a hot rolled steel strip; - then cooling the hot rolled strip, the cooling starting immediately after finishing the hot rolling; - then cooling the hot rolled strip from the finish of hot rolling to a cooling stop temperature range of 480°C to 600°C at a cooling rate of more than 20°C / s; - Thereafter, the hot rolled strip is coiled at a coiling temperature range of 480°C to 600°C; Next, the coiled hot-rolled strip is cooled to room temperature to obtain a hot-rolled steel sheet.

11. The method of claim 10, wherein the reheating temperature of the semi-finished product is between 1200°C and 1275°C.

12. The method according to claim 10 or 11, wherein the hot rolling finishing temperature is 880°C to 930°C.

13. The method according to any one of claims 10 to 12, wherein the cooling rate from the finish of hot rolling to the cooling stop temperature is from 40°C / s to 120°C / s.

14. Use of a steel sheet according to any one of claims 1 to 9 or produced according to the method of claims 10 to 13 for the manufacture of structural or safety parts of a vehicle.

15. A vehicle comprising a part obtained according to claim 14.

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