Hot-rolled steel sheet and method for manufacturing same

Abstract

To provide a hot rolled steel sheet which simultaneously satisfies the easiness of forming and strength, and is suitable for use as a steel sheet for automobiles.SOLUTION: The hot-rolled steel sheet comprises the following elements, expressed in weight percent: 0.11% ≤ C ≤ 0.16%, 1% ≤ Mn ≤ 2%, 0.1% ≤ Si ≤ 0.7%, 0.02% ≤ Al ≤ 0.1%, 0.15% ≤ Mo ≤ 0.4%, 0.15% ≤ V ≤ 0.4%, 0.002% ≤ P ≤ 0.02%, 0% ≤ S ≤ 0.005%, 0% ≤ N ≤ 0.01%; One or more of the following optional elements can be included in given amounts: Cr, Nb, Ca, B, Mg, Ti, the balance of the composition being made up of iron and unavoidable impurities, wherein the microstructure of said steel sheet comprises, in area fraction, 70% to 90% of bainite, 10% to 25% of ferrite, wherein the cumulative amount of bainite and ferrite is at least 90%, and the cumulative amount of retained austenite and martensite is between 0% and 10%.SELECTED DRAWING: None

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 meet two contradictory 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 now must be made from materials with high formability to meet the criteria for ease of installation into complex automotive assemblies, while at the same time improving strength for vehicle crashworthiness and durability while reducing vehicle weight and improving 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 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] EP 1 138 796 claims a hot-rolled steel with very high elastic limit and mechanical resistance, particularly suitable for use in the manufacture of automotive parts, characterized by the following contents by weight: 0.08%<carbon<0.16%, 1%<manganese<2%, 0.02%<aluminium<0.1%, silicon<0.5%, phosphorus<0.03%, sulfur<0.01%, vanadium<0.3%, chromium<1%, nitrogen<0.015%, molybdenum<0.6%. However, the steel of EP 1 138 796 does not exhibit the hole expansion ratio essential for the manufacture of automotive parts.

[0006] EP 2171112 has a resistance higher than 800 MPa and an elongation at break higher than 10%, and has a composition consisting of, by weight, 0.050% ≤ C ≤ 0.090%, 1% < Mn ≤ 2%, 0.015% ≤ Al ≤ 0.050%, 0.1% ≤ Si ≤ 0.3%, 0.10% ≤ Mo ≤ 0.40%, S ≤ 0.010%, P ≤ 0.025%, 0.003% ≤ N ≤ 0.009%, 0.12% ≤ V ≤ 0.22%, Ti ≤ 0.005%, Nb ≤ 0.020% and optionally Cr ≤ 0.45%, with the balance being iron and unavoidable impurities resulting from production, and is a hot-rolled steel sheet, wherein the microstructure of the sheet or the part contains at least 80% upper bainite in surface fraction, and any remainder consists of lower bainite, martensite and retained austenite, and the sum of the martensite content and the retained austenite content is less than 5%. However, this invention cannot also show the hole expansion rate required for automotive parts.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

[0008] [[ID= 24]]An object of the present invention is to solve these problems by making available a hot-rolled steel sheet having the following simultaneously. - A tensile strength of 940 MPa or more, preferably exceeding 960 MPa, - An overall elongation of 8% or more, preferably exceeding 9%, - A hole expansion rate of 40% or more, preferably exceeding 45%.

[0009] In a preferred embodiment, the steel sheet according to the present invention can also exhibit a yield strength of 750 MPa or more.

[0010] In a preferred embodiment, the steel sheet according to the invention may also exhibit a yield strength to tensile strength ratio of 0.5 or greater.

[0011] Preferably, such steels also have good suitability for forming, in particular for rolling, with good weldability and coatability.

[0012] Another object of the invention is to make available a method for manufacturing these plates that is robust towards variations in manufacturing parameters, while being compatible with conventional industrial applications.

[0013] The hot rolled steel sheet of the present invention may optionally be coated with zinc or a zinc alloy to improve its corrosion resistance.

[0014] Carbon is present in the steel between 0.11 and 0.16%. Carbon is an essential element for controlling ferrite formation to increase the strength of the steel sheet, and carbon also provides precipitation strengthening strength to the steel sheet by forming vanadium carbide or niobium carbide. Therefore, carbon plays a crucial role in increasing strength. However, if the carbon content is less than 0.11%, the steel of the present invention cannot be provided with tensile strength. On the other hand, if the carbon content exceeds 0.16%, the steel exhibits poor spot weldability, limiting its application to automobiles. The preferred carbon content in the present invention can be maintained between 0.11% and 0.15%.

[0015] The manganese content of the steel of the present invention is between 1% and 2%. This element is gamma-generating and also affects the Bs and Ms temperatures, so it plays an important role in controlling ferrite formation. The purpose of adding manganese is essentially to impart hardenability to the steel. Manganese in an amount of at least 1% by weight has been found to provide strength and hardenability to steel sheets. However, if the manganese content exceeds 2%, manganese has adverse effects, such as delaying the transformation of austenite during cooling after hot rolling. In addition, a manganese content exceeding 1.8% promotes central segregation, thus reducing formability and degrading the weldability of the steel. The preferred manganese content in the present invention can be kept between 1.3% and 1.8%.

[0016] The silicon content of the steel of the present invention is between 0.1% and 0.7%. Silicon is a solid solution strengthener, especially for ferrite and bainite in the microstructure. Furthermore, a high silicon content can delay the precipitation of cementite. However, an unbalanced silicon content can lead to problems such as surface defects like tiger stripes, which adversely affect the coatability of the steel of the present invention. Therefore, the concentration is controlled to an upper limit of 0.7%. The preferred content in the present invention can be kept between 0.2% and 0.6%.

[0017] Aluminum is an element present in the steel of the present invention at between 0.02% and 0.1%. Aluminum is alpha-generating and imparts ductility to the steel of the present invention. Since aluminum in steel has a tendency to combine with nitrogen to form aluminum nitride, in the view of the present invention, the aluminum content should be kept as low as possible, preferably between 0.02% and 0.06%.

[0018] Molybdenum is an essential element comprising 0.15% to 0.4% 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 0.4%. The preferred limit for molybdenum is between 0.15% and 0.3%.

[0019] Vanadium is an essential element that constitutes between 0.15% and 0.4% 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 0.4%. These carbides, nitrides, or carbonitrides are formed during the second and third cooling steps. The preferred limit for vanadium is between 0.15% and 0.3%.

[0020] The phosphorus content of the steel of the present invention is between 0.002% and 0.02%. 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 0.02%, preferably less than 0.015%.

[0021] Although sulfur is not an essential element, it may be present in steel as an impurity. From the viewpoint of the present invention, it is preferable that the sulfur content be as low as possible, but from the viewpoint of production costs, it is not more than 0.005%. Furthermore, if a higher sulfur content is present in the steel, sulfur will combine, especially with manganese, to form sulfides, thereby reducing its beneficial effect on the steel of the present invention. Therefore, it is preferably less than 0.003%.

[0022] Nitrogen is limited to 0.01% to avoid ageing of the material, and nitrogen forms nitrides that impart strength to the steel of the present invention by precipitation strengthening with vanadium and niobium, but whenever nitrogen is present in excess of 0.01%, nitrogen can form large amounts of aluminum nitride that are detrimental to the present invention, so the preferred upper limit for nitrogen is 0.005%.

[0023] Chromium is an optional element for the present invention. The chromium content that can be present in the steel of the present invention is between 0% and 0.5%. Chromium is the element that provides hardenability to the steel, but high chromium contents above 0.5% lead to central co-segregation, similar to manganese.

[0024] Niobium is an optional element for the present invention, and the niobium content may be between 0% and 0.05% and is added to the steel of the present invention to form carbides or carbonitrides in order to impart strength to the steel of the present invention by precipitation strengthening.

[0025] The calcium content in the steel of the present invention is between 0.0001% and 0.005%. Calcium is added to the steel of the present invention as an optional element, especially during the treatment of inclusions, thereby delaying the deleterious effects of sulfur.

[0026] 0.3≦Mo+V+Nb≦0.6 The cumulative presence of molybdenum, vanadium and niobium is kept between 0.3% and 0.6% to impart strength and hole expansion ratio to the steel of the present invention since both niobium and vanadium form nitrides, carbonitrides or carbides whereas molybdenum ensures the formation of adequate ferrite, therefore this formula supports the present invention to strike a balance between tensile strength by ensuring the formation of precipitates and imparting hole expansion ratio by ensuring adequate ferrite.

[0027] Other elements such as boron or magnesium can be added individually or in combination in the following weight ratios: boron≦0.001%, magnesium≦0.0010%. These elements, up to the maximum content levels indicated, allow for grain refinement during solidification.

[0028] Titanium is a residual element and can be present up to 0.01%.

[0029] The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0030] The microstructure of the steel plate includes:

[0031] Bainite constitutes 70% to 90% of the microstructure in terms of area fraction in the steel of the present invention. Bainite constitutes the main phase of the steel as the matrix, and is cumulatively composed of upper bainite and lower bainite. To ensure a tensile strength of 940 MPa or more, preferably 960 MPa or more, it is necessary to have 70% bainite. Bainite begins to form during the third cooling step and continues until coiling.

[0032] Ferrite constitutes 10% to 25% of the microstructure of the steel of the present invention by area fraction. The ferrite cumulatively includes polygonal ferrite and acicular ferrite. Ferrite imparts elongation as well as formability to the steel of the present invention. To ensure an elongation of 8% or more, preferably 9% or more, it is necessary to have 10% ferrite. Ferrite forms in the steel of the present invention during cooling after hot rolling. However, whenever a ferrite content greater than 25% is present in the steel of the present invention, this tensile strength is not achieved.

[0033] To ensure a balance between strength and formability, the cumulative amount of bainite and ferrite is greater than 90%. The cumulative presence of bainite and ferrite imparts a tensile strength of 940 MPa and ensures formability.

[0034] Martensite and retained austenite are optional components of the steel of the present invention and may be present in a cumulative area fraction of 0% to 10%, and are found in trace amounts. In the present invention, martensite includes both fresh martensite and tempered martensite. Martensite imparts strength to the steel of the present invention. If the martensite content exceeds 10%, the martensite provides excessive strength, and the yield strength exceeds the allowable upper limit. In a preferred embodiment, the cumulative amount of martensite and retained austenite is between 2% and 10%.

[0035] In addition to the above microstructure, the microstructure of hot rolled steel sheet does not contain microstructural components such as pearlite and cementite, although they may be found in trace amounts.

[0036] The steel sheet of the present invention can be produced by any suitable method. A preferred method comprises providing a semi-finished casting of steel having the chemical composition according to the present invention. Casting can be carried out continuously, either in ingots or in the form of thin slabs or thin strip, i.e., with thicknesses ranging from about 220 mm for slabs to several tens of mm for thin strip.

[0037] For example, slabs having the above chemical composition 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 to maintain a local carbon to nominal carbon ratio of less than 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.

[0038] The temperature of the slab undergoing hot rolling should preferably be at least 1200°C and less than 1300°C. If the slab temperature is lower than 1200°C, excessive load will be placed on the rolling mill. Therefore, the slab temperature is preferably high enough to complete hot rolling in the 100% austenite range. Reheating at temperatures above 1275°C should be avoided because it causes productivity losses and is industrially expensive. Therefore, the preferred reheating temperature is between 1200°C and 1275°C.

[0039] The hot rolling finishing temperature in the present invention is between 850°C and 975°C, preferably between 880°C and 930°C.

[0040] The hot rolled strip thus obtained is then cooled in a three-stage cooling process, where Stage 1 cooling begins immediately after the finish of hot rolling, and in Stage 1 the hot rolled strip is cooled from the finish of hot rolling to a temperature range of between 650°C and 720°C at a cooling rate of between 40°C / s and 150°C / s. In a preferred embodiment, the cooling rate for Stage 1 cooling is between 40°C / s and 120°C / s.

[0041] Thereafter, cooling in stage 2 starts from a temperature range between 650°C and 725°C and lasts for a time between 1 and 10 seconds, preferably 2 to 9 seconds, with stage 2 being stopped between 620°C and 690°C. During this stage, cooling is carried out by air cooling, and the time limit is determined depending on the expected ferritic microstructure of the steel that will be further produced during this stage, where the ferritic microstructure is formed and the microalloying elements such as vanadium and / or niobium form nitrides, carbides and carbonitrides that impart strength to the steel.

[0042] Stage 3 cooling then begins from a temperature range between 620°C and 690°C at a cooling rate greater than 20°C / s to a coiling temperature range between 450°C and 550°C. During this cooling stage, bainite transformation begins and continues until the coiled hot rolled strip passes the Ms temperature during cooling, after which the bainite transformation stops. In a preferred embodiment, the coiling temperature range is between 470°C and 530°C.

[0043] Thereafter, the hot-rolled strip is coiled at a temperature in the range of 450° C. to 550° C., preferably 470° C. to 530° C. The coiled hot-rolled strip is then cooled to room temperature to obtain a hot-rolled steel sheet. [Example]

[0044] The following tests, examples, figurative examples and tables presented herein are not limiting in nature and should be considered merely for illustrative purposes, illustrating advantageous features of the present invention.

[0045] The steel plates made of steels with different compositions are summarized in Table 1, and the steel plates are 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.

[0046] [Table 1]

[0047] Table 2 summarizes the processing parameters performed on the steels in Table 1.

[0048] [Table 2]

[0049] 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 inventive steel and the reference steel.

[0050] The results are set forth herein.

[0051] [Table 3]

[0052] Table 4 illustrates the mechanical properties of both the steels of the invention and the reference steels. Tensile tests according to the JIS Z2241 standard were carried out to determine the tensile strength, yield strength and total elongation.

[0053] The results of the various mechanical properties carried out according to the standard are summarized.

[0054] [Table 4]

Claims

1. Hot rolled steel sheet containing the following elements, expressed in weight percent: 0.11%≦carbon≦0.16% 1%≦Manganese≦2% 0.1%≦Silicon≦0.7% 0.02%≦Aluminum≦0.1% 0.15%≦molybdenum≦0.4% 0.15%≦vanadium≦0.4% 0.002%≦phosphorus≦0.02% 0%≦sulfur≦0.005% 0%≦Nitrogen≦0.01% and any one or more of the following elements: 0%≦chromium≦0.5% 0%≦niobium≦0.05% 0.0001%≦Calcium≦0.005% 0%≦Boron≦0.001% 0%≦Magnesium≦0.0010% 0%≦Titanium≦0.01% and a composition comprising: Here, 0.3%≦Mo+V+Nb≦0.6% and the remainder of the composition is composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel plate contains, in area fractions, 70% to 90% bainite and 10% to 25% ferrite, wherein the cumulative amount of bainite and ferrite is at least 90%, and the cumulative amount of retained austenite and martensite is between 0% and 10%.

2. 2. The hot rolled steel sheet according to claim 1, wherein the composition comprises 0.2% to 0.6% silicon.

3. The hot rolled steel sheet according to claim 1 or 2, wherein the composition contains 0.11% to 0.15% carbon.

4. 4. The hot rolled steel sheet according to claim 3, wherein the composition comprises 0.15% to 0.3% vanadium.

5. 5. The hot rolled steel sheet according to claim 1, wherein the composition comprises 1.3% to 1.8% manganese.

6. 6. The hot rolled steel sheet according to claim 1, wherein the composition comprises 0.15% to 0.3% molybdenum.

7. The hot rolled steel sheet according to any one of claims 1 to 6, wherein the composition comprises 0.02 to 0.06% aluminum.

8. The hot-rolled steel sheet according to any one of claims 1 to 7, wherein the cumulative amount of the retained austenite and martensite is between 2 and 10%.

9. The hot-rolled steel sheet according to any one of claims 1 to 8, having a tensile strength of 950 MPa or more and a hole expansion ratio of 40% or more.

10. 10. The hot rolled steel sheet according to claim 9, having a tensile strength of 960 MPa or more and a total elongation of 8% or more.

11. 1. A method for producing a hot-rolled heat-treated steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 7, - reheating the semi-finished product to a temperature between 1200°C and 1300°C, - rolling said semi-finished product in the austenitic range so that the hot rolling finishing temperature is between 850°C and 975°C to obtain a hot rolled steel strip; - then cooling said hot rolled strip in a three-stage cooling, the step 1 of cooling the hot-rolled steel sheet starts from a temperature range of between 850°C and 975°C and reaches a temperature range of between 650°C and 725°C at a cooling rate of between 40°C / s and 150°C / s; a cooling step in which stage 2 of cooling the hot rolled steel sheet starts from a temperature range between 650°C and 725°C to a temperature range between 620°C and 690°C, said stage 2 having a duration between 1 second and 10 seconds and being air cooling, and a cooling step 3 of cooling the hot rolled steel sheet starts from a temperature range between 620°C and 690°C to a temperature range between 450°C and 550°C at a cooling rate greater than 20°C / second; - thereafter coiling said hot rolled steel strip at a temperature ranging between 450 and 550°C, - cooling the coiled hot rolled steel strip to room temperature.

12. 12. The method of claim 11, wherein the reheat temperature of the blank is between 1200°C and 1275°C.

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

14. 14. The method according to any one of claims 11 to 13, wherein the coiling temperature range is between 470°C and 530°C.

15. The method according to any one of claims 11 to 14, wherein the cooling rate of the stage 1 cooling is between 40°C / s and 120°C / s.

16. The method according to any one of claims 11 to 15, wherein the cooling rate of the cooling in step 3 is 25°C / sec or more.

17. 17. The method according to any one of claims 11 to 16, wherein the duration of the cooling in stage 2 is between 2 seconds and 9 seconds.

18. Use of a steel sheet according to any one of claims 1 to 10 or produced by the method of claims 11 to 17 for the manufacture of structural or safety parts of a vehicle.

19. A vehicle comprising a part obtained according to claim 18.

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