Hot-rolled steel sheet and method for manufacturing the same

Abstract

We provide hot-rolled steel sheets suitable for use as structural steel, or for the manufacture of industrial machinery, durable consumer goods, environmentally friendly products, and cryogenic applications. [Solution] A hot-rolled steel sheet having a composition comprising the following elements: 0.02% ≤ carbon ≤ 0.2%, 3% ≤ manganese ≤ 9%, 0.2% ≤ silicon ≤ 1.2%, 0.9% ≤ aluminum ≤ 2.5%, 0% ≤ phosphorus ≤ 0.03%, 0% ≤ sulfur ≤ 0.03%, 0% ≤ nitrogen ≤ 0.025%, 0% ≤ molybdenum ≤ 0.6%, 0% ≤ titanium ≤ 0.1%, 0.0001% ≤ boron ≤ 0.01%, 0% ≤ chromium ≤ 0.5%, 0% ≤ niobium ≤ 0.1%, 0% ≤ vanadium ≤ 0.2%, 0% ≤ nickel ≤ 1%, 0% ≤ copper ≤ 1%, 0% ≤ calcium ≤ 0.005%, and 0% ≤ magnesium ≤ 0.0010%, with the remainder being composed of iron and unavoidable impurities generated by processing.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet suitable for use as a structural steel or for the manufacture of industrial machines, durable consumer goods, environmentally friendly products, and cryogenic applications.

Background Art

[0002] In recent years, for the purpose of improving fuel efficiency and reducing the impact on the environment, it has been actively carried out to reduce the weight of equipment and structures by applying high-strength steel. However, when the strength of steel is increased, the toughness generally decreases. Therefore, in the development of high-strength steel, it is an important issue to increase the strength without sacrificing toughness.

[0003] Great research and development efforts have been made to reduce the amount of material used by increasing the strength of the material. Conversely, since the toughness decreases when the strength of steel is increased, there is a need to develop a material having both high strength and good toughness.

[0004] Previous research and development in the field of high-strength and good-toughness steel has led to several methods for manufacturing high-strength steel, some of which are listed herein for the final evaluation of the present invention.

[0005] European Patent No. 2392681 discloses a thick-walled, high-strength hot-rolled steel sheet having a composition in mass%, comprising 0.02-0.08% C, 1.0% or less Si, 0.50-1.85% Mn, 0.03% or less P, 0.005% or less S, 0.1% or less Al, 0.03-0.10% Nb, 0.001-0.05% Ti, 0.0005% or less B, optionally one or more elements selected from the group consisting of 0.010% or less Ca, 0.02% or less REM, 0.003% or less Mg, 0.5% or less V, 1.0% or less Mo, 1.0% or less Cr, 4.0% or less Ni, and 2.0% or less Cu, other unavoidable impurities, and the remainder Fe. This steel sheet has a structure formed of a bainite ferrite phase or bainite phase in which the solid solution carbon content in the ferrite particles is 10 ppm or more and the surface hardness is 230 HV or less on the Vickers hardness scale. However, the steel specified in European Patent No. 2392681 cannot reach a tensile strength of 700 MPa or more.

[0006] European Patent No. 2971211 specifies a composition containing manganese in the range of about 9 to about 20% by weight, carbon in the range of about 0.5 to about 2.0% by weight and the remainder being iron, and optionally, chromium in the range of 0.5 to 30% by weight; nickel or cobalt in the range of 0.5 to 20% by weight; aluminum in the range of 0.2 to 15% by weight; molybdenum, niobium, copper, titanium or vanadium in the range of 0.01 to 10% by weight; silicon in the range of 0.1 to 10% by weight; nitrogen in the range of 0.001 to 3.0% by weight; and chlorine in the range of 0.001 to 0.1% by weight. A method for producing a high-manganese steel component having a composition consisting of porcini or zirconium or hafnium in the range of 0.2 to 6% by weight of the total composition is disclosed, comprising the steps of: heating the composition to at least about 1000°C; cooling the composition at a rate of about 2°C / sec to about 60°C / sec; hot rolling the composition at a temperature in the range of about 700°C to about 1000°C; slowly cooling or isothermally holding the composition; and quenching, accelerated cooling or air-cooling the composition from a temperature in the range of 700°C to about 1000°C to a temperature in the range of 0°C to about 500°C at a rate of at least about 10°C / sec. However, European Patent No. 2971211 states that it is not possible to achieve an impact toughness of 60 J / cm2 or more when measured at -40°C. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] European Patent No. 2392681 [Patent Document 2] European Patent No. 2971211 [Overview of the project] [Problems that the invention aims to solve]

[0008] The object of the present invention is to solve these problems by making available hot-rolled steel that simultaneously has the following: - Yield strength of 715 MPa or higher, preferably 725 MPa or higher. - Tensile strength of 750 MPa or more, preferably 800 MPa or more. -Overall growth of 20% or more, - Impact toughness: 60 J / cm² or higher when measured at -40°C. [Means for solving the problem]

[0009] In a preferred embodiment, the steel sheet according to the present invention may also exhibit a yield strength-to-tensile strength ratio of 0.5 or higher.

[0010] Preferably, such steel can also have good suitability for forming, particularly for rolling with bending and good weldability.

[0011] Another objective of the present invention is to make available methods for manufacturing these steels that are compatible with conventional industrial processes while being robust to changes in manufacturing parameters. [Modes for carrying out the invention]

[0012] The hot-rolled steel sheet of the present invention may be optionally coated with zinc or a zinc alloy to improve its corrosion resistance.

[0013] Carbon is present in steel in concentrations between 0.02% and 0.2%. Carbon is an essential element for increasing the strength of steel by assisting in the stabilization of austenite at room temperature. However, a carbon content of less than 0.02% does not impart tensile strength to the steel of the present invention. On the other hand, a carbon content exceeding 0.2% results in poor weldability of the steel, which is detrimental to impact toughness and limits its application to structural components of durable consumer goods or environmentally friendly products. The preferred carbon content of the present invention should be maintained between 0.03% and 0.18%, more preferably between 0.04% and 0.15%.

[0014] The manganese content of the steel of this invention is between 3% and 9%.

[0015] This element is involved in gamma production and therefore plays a crucial role in controlling the residual austenite fraction and enriching the residual austenite with manganese to impart hardenability and impact toughness to the steel. It has been found that the amount of manganese should be at least 3% by weight to impart strength and toughness to the steel. However, if the manganese content exceeds 9%, it excessively stabilizes the austenite, leading to adverse effects such as the steel of the present invention lacking the TRIP effect. Furthermore, a manganese content exceeding 9% results in excessive central segregation, thus reducing formability and the weldability of the steel. The preferred content of the present invention should be maintained between 3.5% and 8.5%, more preferably between 4% and 8%.

[0016] The silicon content of the steel of the present invention is between 0.2% and 1.2%. Silicon is a solid solution strengthening agent in the steel of the present invention. Furthermore, silicon delays the precipitation of cementite and limits its formation, but in many cases it cannot completely eliminate cementite formation. Since silicon retains C in the austenite solid solution, the Ms temperature drops below room temperature. Therefore, silicon helps in the formation of retained austenite at room temperature. However, if the silicon content exceeds 1.2%, problems such as surface defects that adversely affect the steel of the present invention will occur. Therefore, the concentration is controlled with an upper limit of 1.2%. The preferred content of the present invention should be maintained between 0.3% and 1%, more preferably between 0.4% and 0.8%.

[0017] Aluminum is an essential element and is present in steel at concentrations between 0.9% and 2.5%. Aluminum is an alpha-forming element and has minimal ferrite, and a minimum of 0.9% aluminum is required to impart elongation and toughness to the steel of this invention. Aluminum is also used to purify the steel of this invention by removing oxygen from the molten state of the steel and preventing oxygen from forming a gas phase. However, whenever the aluminum content exceeds 2.5%, casting becomes difficult due to surface defects on the slab, such as breakouts. Therefore, the preferred range for the presence of aluminum is between 1% and 2.3%, and more preferably between 1% and 2%.

[0018] The phosphorus content of the steel of the present invention is between 0% and 0.03%. Phosphorus tends to segregate, especially at grain boundaries, or co-segregate with manganese, thus reducing hot ductility and toughness. For these reasons, its content is limited to 0.03%, preferably less than 0.015%.

[0019] 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 manufacturing costs, it should be 0.03% or less. Furthermore, if a higher amount of sulfur is present in the steel, it will combine with manganese, which is particularly harmful to the steel of the present invention, to form sulfides, so it is preferable that the sulfur content be less than 0.01%.

[0020] To avoid material degradation over time and minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of steel, nitrogen is limited to 0.025%. Therefore, the preferred upper limit for nitrogen is 0.02%, and more preferably 0.005%.

[0021] Molybdenum is an optional element that constitutes 0% to 0.6% of the steel of the present invention. Molybdenum enhances hardenability and enables the steel of the present invention to achieve the properties targeted for a thicker gauge. To be beneficial for enhancing hardenability, a minimum of 0.1% molybdenum is required. However, since the addition of molybdenum excessively increases the addition cost of alloying elements, its content is limited to 0.6% for economic reasons. The preferred limit of molybdenum is between 0% and 0.4%, more preferably between 0% and 0.3%.

[0022] Titanium is an optional element and is present between 0% and 0.1% of the steel of the present invention. Titanium imparts strength to the steel of the present invention by forming carbides and controlling the particle size. However, whenever titanium is present in excess of 0.1%, titanium imparts excessive strength and hardness to the steel of the present invention and reduces toughness beyond the target limit. The preferred limit of titanium is between 0% and 0.09%, more preferably between 0% and 0.08%.

[0023] Boron is an optional element for the steel of the present invention and can be present between 0.0001% and 0.01%. Boron imparts toughness to the steel of the present invention when added together with titanium.

[0024] Chromium is an optional element of 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 an element that gives hardenability to steel, but a chromium content exceeding 0.5% results in center co-segregation with manganese.

[0025] Vanadium is an optional element that can be present between 0% and 0.2% of the steel of the present invention. Vanadium is effective in increasing the strength of steel by forming carbides, nitrides or carbonitrides, and the upper limit is 0.2% for economic reasons. Even if vanadium is present in excess of 0.2%, it does not bring great benefits to the steel of the present invention.

[0026] Niobium is an optional element of the present invention. The niobium content may be between 0% and 0.1% in the steel of the present invention and is added to the steel of the present invention to impart strength to the steel of the present invention by precipitation strengthening by forming carbides or carbonitrides. The preferred limit is between 0% and 0.05%.

[0027] Nickel may be added in an amount of 0% to 1% as an optional element to increase the strength and improve the toughness of the steel of the present invention. A minimum of 0.01% is preferred to achieve such effects. However, the nickel content is limited to 1% for economic viability.

[0028] To increase the strength and improve the corrosion resistance of the steel of the present invention, copper may be added in an optional amount of 0% to 1%. To obtain such effects, a minimum of 0.01% is preferable. However, if the content exceeds 1%, problems such as copper red-hot embrittlement during the casting process may occur.

[0029] The calcium content in the steel of the present invention is less than 0.005%. In the steel of the present invention, calcium is added in a preferred amount of 0.0001 to 0.005% as an optional element, particularly during inclusion treatment, thereby suppressing the harmful effects of sulfur.

[0030] Other elements, such as magnesium, can be added in the following weight ratios, with magnesium ≤ 0.0010%. Up to the indicated maximum content levels, these elements allow for grain refinement during solidification.

[0031] The remainder of the composition of steel consists of iron and impurities that inevitably arise from processing.

[0032] The microstructure of this steel includes the following: Martensite is present in the steel of the present invention at a concentration of at least 60%, and the martensite of the present invention is tempered martensite and fresh martensite, with tempered martensite being the matrix phase of the steel of the present invention. The tempered martensite of the steel of the present invention preferably has an aspect ratio between 4 and 12, more preferably between 5 and 11. The aspect ratio is the ratio of the longest dimension to the shortest dimension within a single particle. The tempered martensite is formed from martensite formed during cooling after hot rolling. Such martensite is then tempered during the annealing process. The tempered martensite of the steel of the present invention imparts ductility and strength. The tempered martensite content is preferably between 65% and 84%, more preferably between 70% and 80%, in terms of the area fraction of the total microstructure. Fresh martensite can also be optionally present in the steel of the present invention. Fresh martensite can be formed from remaining unstable retained austenite during cooling after annealing. Fresh martensite can be present in amounts between 0% and 15%, preferably between 0% and 10%, but it is even better if fresh martensite is not present at all.

[0033] Retained austenite is an essential microstructural component of the steel of the present invention, present in a concentration between 15% and 40%. The retained austenite of the present invention imparts toughness to the steel. When enriched with manganese and carbon, the retained austenite of the present invention can only be stabilized at room temperature. The percentage of carbon in the retained austenite is higher than 0.8% and lower than 1.1%. The percentage of manganese in the retained austenite is preferably greater than 5%, more preferably greater than 5.5%. However, if the retained austenite of the present invention is not enriched with carbon and manganese, it is not stable at room temperature and results in the formation of excess fresh martensite instead of a sufficient amount of retained austenite. This effect provides the steel with excessive strength and is also detrimental to elongation and toughness. The preferred limit for the presence of austenite is between 18% and 35%, more preferably between 18% and 30%, and the preferred limit for the carbon content in the austenite is between 0.9% and 1.1%, more preferably between 0.95% and 1.05%.

[0034] Polygonal ferrite constitutes 0% to 10% of the microstructure in the area fraction of the steel of the present invention. In the present invention, polygonal ferrite imparts high strength and elongation to the steel of the present invention. Polygonal ferrite can be formed in the steel of the present invention during soaking and cooling after annealing. However, strength is not achieved whenever the polygonal ferrite content in the steel of the present invention exceeds 10%.

[0035] Bainite and cementite may be present in the steel of the present invention in amounts between 0% and 5%. Up to 5%, bainite does not affect the target properties of the steel of the present invention.

[0036] In addition to the microstructure described above, the microstructure of hot-rolled steel does not contain microstructure components such as pearlite. Alloying elements, such as niobium, titanium, vanadium, iron carbides, and other carbides, may be present in the steel of the present invention at concentrations between 0% and 5%. These carbides impart strength to the steel of the present invention through precipitation strengthening, but whenever the presence of carbides exceeds 5%, the carbides partially consume the amount of carbon, hindering the stabilization of retained austenite.

[0037] The hot-rolled steel according to the present invention can be manufactured by any suitable method. A preferred method consists of providing a semi-finished casting of steel having the chemical composition according to the present invention. Casting can be carried out in the form of an ingot or continuously in the form of thick slabs, thin slabs or thin strips, i.e., in thicknesses ranging from about 220 mm to 350 mm in the case of slabs to several tens of millimeters in the case of thin strips.

[0038] For example, slabs having the above-mentioned chemical composition were manufactured by continuous casting, and the slabs were optionally subjected to direct light compression during the continuous casting process to avoid central segregation. Slabs provided by the continuous casting process can be used directly at high temperatures after continuous casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0039] The slab is reheated to a temperature between Ac3+50°C and 1300°C. If the slab temperature is below Ac3+50°C, an excessive load is placed on the rolling mill. Therefore, the slab temperature must be high enough to allow hot rolling to be completely completed in the austenite region. Reheating at temperatures above 1300°C should be avoided because it causes a loss of productivity, is industrially expensive, and can lead to the melting of some segregated portions, resulting in slab breakage or cracking. Therefore, the preferred reheating temperature is between Ac3+100°C and 1280°C.

[0040] The hot rolling finishing temperature of the present invention is at least Ac3, preferably between Ac3 and Ac3+100°C, more preferably between 840°C and 1000°C, and even more preferably between 850°C and 990°C.

[0041] Next, the hot-rolled strip obtained in this manner is cooled at a cooling rate of 1°C / sec to 50°C / sec to a temperature range of Ms~20°C from the hot-rolling finish temperature. In a preferred embodiment, the cooling rate in this cooling step is between 1°C / sec and 20°C / sec, more preferably between 5°C / sec and 20°C / sec. During this step, martensite is formed that is tempered during soaking under an annealing process, thus forming tempered martensite.

[0042] The hot-rolled strip may be optionally wound up, with a winding temperature between Ms and 20°C, and may be optionally cut into sheets.

[0043] Hot-rolled steel strips, sheet metal, or steel plates are heated from a temperature between Ms and 20°C to an annealing temperature Tsoak between 550°C and Ac3, preferably between 600°C and Ac3-40°C, and such heating is carried out at a heating rate HR1 of at least 1°C / second.

[0044] Hot-rolled steel strips, sheet metal, or steel plates are held in a Tsoak for 5 to 1000 seconds to ensure the target transformation from the initial structure to austenite.

[0045] Next, the hot-rolled steel is cooled starting from Tsoak and cooled to a cooling stop temperature T1 in the range of Ms-10°C to 20°C at a cooling rate CR1 between 0.1°C / sec and 150°C / sec. In a preferred embodiment, the cooling rate CR1 for such cooling is between 0.1°C / sec and 120°C / sec. During this cooling, fresh martensite may form from some residual unstable austenite.

[0046] The hot-rolled steel obtained in this way preferably has a thickness between 2 mm and 100 mm, more preferably between 2 mm and 80 mm, and even more preferably between 2 mm and 50 mm. [Examples]

[0047] The following tests, examples, graphic illustrations, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and illustrate advantageous features of the present invention.

[0048] Table 1 summarizes steel sheets made from different steel compositions, and each of these sheets is manufactured according to the process parameters specified in Table 2. Table 3 then summarizes the microstructure of the steel sheets obtained during testing, and Table 4 summarizes the evaluation results of the obtained properties. Ac3 and Ms temperatures are determined by thermodynamic calculations performed using software such as Thermo-Calc(R).

[0049] [Table 1]

[0050] [Table 2]

[0051] Table 3 Table 3 summarizes the results of tests performed according to standards using different microscopes such as SEM, EPMA, EBSD, XRD, or any other microscope to determine the microstructural composition of both the steel of the present invention and the reference test. The area fraction of carbides was measured on polished samples after etching in a 2% Nital etching solution for 10 seconds and observed by SEM. Polygonal ferrite and tempered martensite were measured using EBSD, which is an SEM-based technique for measuring crystal orientation with submicron resolution. The electron beam is focused onto a sample tilted at 70° in a scanning electron microscope (SEM). Electrons that satisfy the Bragg condition for a group of planes are channeled and induce Kikuchi bands. The electrons strike a phosphor screen, generating light which is detected and digitized by a camera. The resulting EBS pattern is analyzed and indexed. This process is carried out for each analyzed point. For a given steel sample, EBSD analysis of at least four images corresponding to a 1000x magnification can identify the polygonal ferrite and tempered martensite fine components, their locations, and area percentages. The retained austenite area fraction is measured using XRD as shown in Table 3.

[0052] The results are specified herein:

[0053] [Table 3]

[0054] Samples I1 and I2 contained niobium carbide, sample I3 contained titanium carbide, and sample R1 contained iron carbide (cementite). The samples did not contain any fresh martensite or bainite components.

[0055] Table 4 Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. Tensile tests are performed using tensile sample A25 according to the NBN EN ISO 6892-1 standard to determine tensile strength, yield strength, and total elongation. Toughness is tested by a Charpy test performed according to ISO 148-1. The results of various mechanical tests performed according to the standard are summarized.

[0056] [Table 4]

Claims

1. Hot-rolled steel sheet, containing the following elements expressed in weight percentage: 0.02% ≤ Carbon ≤ 0.2% 3% ≤ Manganese ≤ 9% 0.2% ≤ Silicon ≤ 1.2% 0.9% ≤ Aluminum ≤ 2.5% 0% ≤ Phosphorus ≤ 0.03% 0% ≤ Sulfur ≤ 0.03% 0% ≤ Nitrogen ≤ 0.025% It includes, and one or more of the following optional elements 0% ≤ Molybdenum ≤ 0.6% 0% ≤ Titanium ≤ 0.1% 0.0001% ≤ Boron ≤ 0.01% 0% ≤ Chromium ≤ 0.5% 0% ≤ Niobium ≤ 0.1% 0% ≤ Vanadium ≤ 0.2% 0% ≤ Nickel ≤ 1% 0%≦Copper≦1% 0% ≤ Calcium ≤ 0.005% 0% ≤ Magnesium ≤ 0.0010% It can contain, A hot-rolled steel sheet having a composition in which the remainder consists of iron and unavoidable impurities generated by processing, wherein the microstructure of the steel sheet contains, by area fraction, at least 60% tempered martensite, 15% to 40% retained austenite, 0% to 10% polygonal ferrite, 0% to 5% bainite, 0% to 15% fresh martensite, and 0% to 5% niobium, titanium, vanadium, or iron carbides.

2. A hot-rolled steel sheet according to claim 1, wherein the composition contains 0.3% to 1% silicon.

3. A hot-rolled steel sheet according to claim 1 or 2, wherein the composition contains 0.03% to 0.18% carbon.

4. A hot-rolled steel sheet according to any one of claims 1 to 3, comprising 3.5% to 8.5% manganese.

5. A hot-rolled steel sheet according to any one of claims 1 to 4, comprising 1% to 2.3% aluminum.

6. A hot-rolled steel sheet according to any one of claims 1 to 5, wherein the amount of martensite is between 65% and 84%.

7. A hot-rolled steel sheet according to any one of claims 1 to 6, wherein the amount of retained austenite is between 18% and 35%.

8. A hot-rolled steel sheet according to any one of claims 1 to 7, having a tensile strength of 750 MPa or more and a total elongation of 20% or more.

9. A hot-rolled steel sheet according to any one of claims 1 to 8, wherein the shape factor of the tempered martensite is between 4 and 12.

10. A method for manufacturing hot-rolled steel sheets, comprising the following series of steps: - A step of providing a steel composition according to any one of claims 1 to 5; - Reheating the semi-finished product to a temperature between Ac3 + 50°C and 1300°C; - The semi-finished product is rolled in the austenite range where the hot rolling finishing temperature is at least Ac3 to obtain a hot-rolled steel strip; - A step in which hot-rolled steel is optionally wound up in a winding temperature range of 20°C to Ms, - Next, the hot-rolled steel is cooled from the hot-rolling finishing temperature to a temperature range of Ms to 20°C at a cooling rate of 1°C / second to 50°C / second; - Next, the hot-rolled steel is heated from a temperature range of Ms to 20°C to a temperature Tsoak between 550°C and Ac3 at a heating rate HR1 of at least 1°C / second, and held for 5 to 1000 seconds; Next, the hot-rolled strip is cooled starting from Tsoak and cooled to a cooling stop temperature T1 between Ms -10°C and 20°C at a cooling rate CR1 between 0.1°C / sec and 150°C / sec; -Then, the hot-rolled steel strip is cooled to room temperature at a cooling rate CR2 between 0.1°C / sec and 150°C / sec to obtain a hot-rolled steel sheet. A method that includes this.

11. The method according to claim 10, wherein the Tsoak temperature is between 600°C and Ac3-40°C.

12. The method according to claim 13 or 14, wherein the T1 temperature is between Ms -20°C and 25°C.

13. Use of a steel sheet according to any one of claims 1 to 9 or a steel sheet manufactured according to the method of claims 10 to 12 for the manufacture of industrial machinery or components of environmentally friendly products or durable consumer goods.

14. An industrial machine comprising a component obtained according to claim 13.

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