Hot-rolled steel sheet and method for manufacturing the same

Abstract

A hot-rolled steel sheet having the following composition, and containing the following elements: 0.03% ≤ carbon ≤ 0.07%, 0.8% ≤ manganese ≤ 1.3%, 0.01% ≤ aluminum ≤ 0.07%, 0.01% ≤ niobium ≤ 0.07%, 0.05% ≤ titanium ≤ 0.15%, 0.0005% ≤ calcium ≤ 0.005%, 0% ≤ phosphorus ≤ 0.03%, 0% ≤ sulfur ≤ 0.015%, 0% ≤ nitrogen ≤ 0.02%, 0.001% ≤ silicon ≤ 0.09%, 0% ≤ chromium ≤ 0.2%, 0.03% ≤ copper ≤ 0.25%, 0% ≤ nickel ≤ 0.2%, 0% ≤ molybdenum ≤ 0.2%, 0% ≤ vanadium ≤ 0.1%, 0% ≤ boron The composition of the hot-rolled steel sheet is as follows: 0% ≤ 0.003%, 0% ≤ magnesium ≤ 0.010%, 0% ≤ cerium ≤ 0.1%, 0% ≤ zirconium ≤ 0.010%, the remaining composition consists of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet is as follows, by area percentage: 40% to 75% bainite, 25% to 60% ferrite, 0% to 2% pearlite, and 0% to 2% martensite-residual islands, the hot-rolled steel sheet has an inclusion density of 200 inclusions per square micrometer, and inclusions having a size of 2 microns or more account for 15% or less of the total number of inclusions.

Description

[Technical Field]

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

[0002] Automobile parts are required to satisfy two conflicting requirements, namely ease of forming and strength. In recent years, from the perspective of global environmental issues, a third requirement of improving fuel efficiency has also been imposed on automobiles. Therefore, at present, automobile parts must be made of materials with high formability to meet the criteria for ease of adaptation to complex automobile assemblies, and at the same time, must have improved strength for vehicle impact resistance and durability while reducing vehicle weight and improving fuel efficiency.

[0003] Accordingly, intensive research and development have been conducted to reduce the amount of materials used in automobiles by increasing the strength of materials. Conversely, increasing the strength of steel sheets reduces formability, so there is a demand for the development of materials that combine both high strength and high formability.

[0004] Previous research and development in the field of high-strength and high-formability steel sheets has resulted in several methods for producing high-strength and high-formability steel sheets, some of which are listed herein for a satisfactory understanding of the present invention.

[0005] International Publication No. 2022 / 180146 presents high-strength hot-rolled flat steel products and methods for manufacturing such flat steel products, thereby aiming to achieve a combination of high strength, high local cold formability, and high economic viability based on steel. This is achieved by high-strength hot-rolled flat steel products with high local cold formability, a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, a hole expansion ratio of at least 30%, preferably at least 40%, particularly favorably at least 50%, an elongation at break of at least 10%, preferably at least 16%, a cold formability measure of at least 0.12, favorably at least 0.17, and a local and global cold formability ratio of at least 5 and at most 13, as well as a bay of carbon-rich microstructural components such as martensite, retained austenite, pearlite, and retained precipitation-hardened ferrite. The steel has a microstructure consisting of more than 50 vol% of natriol, up to 10 vol%, and preferably up to 5 vol%, with the following chemical composition (in wt%) of steel: C: 0.04~0.08, Si: 0.1~0.6, Mn: 1.0~2.0, P: up to 0.06, S: up to 0.01, N: up to 0.012, Al: up to 0.06, Ti: up to 0.18 and / or Nb: up to 0.08, Mo: up to 0.35, and when Ti+Nb exceeds 0.06, the hyperstoichiometric ratio of carbon and nitrogen is given by the following formula: 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96), with the remainder being iron and unavoidable steel associated elements. However, International Publication No. 2022 / 146180 cannot demonstrate HER of more than 55%. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2022 / 180146 Brochure [Patent Document 2] International Publication No. 2022 / 146180 Brochure [Overview of the project] [Means for solving the problem]

[0007] The object of the present invention is to solve these problems by making available hot-rolled steel sheets that simultaneously have the following: - TS / YS ratio of 1.10 or higher, - Ultimate tensile strength of 600 MPa to 750 MPa, preferably 625 MPa to 725 MPa. - Yield strength of 550 MPa to 670 MPa, preferably 575 MPa to 650 MPa. - Total elongation of 12% or more, preferably 15% or more. - Hole expansion rate of 55% or more, preferably more than 60%.

[0008] Preferably, such steel can also have good weldability and coating properties, and good suitability for forming and rolling.

[0009] Another objective of the present invention is to make available a method for manufacturing these sheets that is compatible with conventional industrial applications and is robust against changes in manufacturing parameters. [Modes for carrying out the invention]

[0010] The hot-rolled steel sheet of the present invention is coated with zinc or a zinc alloy to improve corrosion resistance.

[0011] Carbon is present in steel at a concentration of 0.03% to 0.07%. Carbon is an essential element for increasing the strength of steel sheets, not only by forming fine alloying precipitates but also by interstitial strengthening. If the carbon content is less than 0.03%, combined with the required elongation exceeding 12%, it becomes difficult to achieve the required tensile strength of 600 MPa or more. On the other hand, with a carbon content exceeding 0.07%, the steel exhibits poor spot weldability, limiting its application to automotive parts. High carbon content can promote the formation of second phases such as pearlite, martensite, or cementite during cooling after hot rolling, which reduces hole expansion. Therefore, the preferred range for carbon in the steel of this invention is 0.04% to 0.06%.

[0012] The manganese content of the steel of the present invention is 0.8% to 1.3%. The purpose of adding manganese is essentially to impart strength to the steel through solid solution strengthening. This element is gamma-active and also affects the temperatures of Bs and Ms, thus playing an important role in controlling the formation of bainite and martensite. If the Mn content is less than 0.8%, it is difficult to achieve the required strength of the steel according to the present invention, coupled with the required elongation of more than 12%. Furthermore, a manganese content exceeding 1.3% can have adverse effects, such as delaying the austenite transformation during cooling after hot rolling, thereby reducing ductility, and thus potentially failing to achieve the elongation target. The preferred content of the present invention can remain at 0.9% to 1.2%.

[0013] Aluminum is an essential element and is present in the steel of the present invention at a concentration of 0.01% to 0.07%. Aluminum promotes ferrite formation. This formation allows the present invention to have a sufficient amount of ferrite to achieve the desired combination of strength and ductility of the steel. However, when the presence of aluminum exceeds 0.07%, the finishing temperature of hot rolling reaches the full austenite region. The aluminum content is preferably limited to 0.015% to 0.06%.

[0014] Niobium is an essential element in the steel of the present invention, present in a concentration of 0.01% to 0.07%, and is suitable for forming carbides and carbonitrides to impart strength to the steel of the present invention through precipitation hardening. Niobium also influences the size of microstructure components by precipitation as carbides, by delaying recrystallization during hot rolling, and by delaying the expansion of austenite grain size. Therefore, the finer microstructure formed in the final product as a result of the steel of the present invention can reach the target strength. However, niobium content exceeding 0.07% is not economically interesting. Furthermore, niobium content above 0.07% is detrimental to the steel because it increases the rolling force of the finishing mill, thus increasing the difficulty during hot rolling of the steel. The preferred limit for niobium content is 0.01% to 0.06%, and more preferably 0.02% to 0.05%.

[0015] Titanium is an essential element and can be added to the steel of the present invention in an amount of 0.05% to 0.15%. Niobium participates in the formation of carbonitrides as niobium and therefore plays a role in the hardening of the steel of the present invention. Furthermore, titanium also forms titanium nitrides, titanium oxynitrides, and titanium carbonitrides that appear during the solidification of the casting. The amount of titanium is limited to 0.15% to avoid the formation of coarse titanium nitride inclusions that are detrimental to the formability of the steel. The preferred limit for titanium content is 0.06% to 0.12%.

[0016] Calcium is an essential element and is added to the steel of the present invention at a concentration of 0.0005% to 0.005%. Calcium is added to the steel of the present invention in a preferred minimum amount of 0.0005%, particularly during the processing of inclusions. Calcium contributes to the refining of the steel by blocking harmful sulfur content in a spherical form, and furthermore, calcium also facilitates casting by avoiding clogging during casting, thereby delaying the harmful effects of sulfur.

[0017] Although phosphorus is not an essential element, it may be present in steel as an impurity. From the viewpoint of the present invention, the phosphorus content is preferably kept as low as possible, to less than 0.03%. Phosphorus tends to segregate, especially at grain boundaries, or co-segregate with manganese, thus reducing spot weldability and hot ductility. For these reasons, its content is limited to less than 0.03%, preferably less than 0.02%.

[0018] Although sulfur is not an essential element, it may be present in steel as an impurity. From the viewpoint of the present invention, the sulfur content is preferably as low as possible, but from the viewpoint of desulfurization manufacturing costs, it is 0.015% or less. Furthermore, if a higher amount of sulfur is present in the steel, it will combine with manganese in particular to form sulfides, reducing the beneficial effect of manganese on the steel in the present invention.

[0019] Nitrogen is limited to 0.02% to avoid material degradation over time and to control the precipitation of nitride inclusions during solidification, which are detrimental to the mechanical properties and formability of the steel.

[0020] Silicon is an optional element and can be present in concentrations of 0.001% to 0.09%. Silicon adds strength to ferrite through solid solution strengthening. However, if the amount exceeds 0.09%, silicon concentrates on the surface of the steel sheet in the form of an oxide during hot rolling. For this reason, the silicon content is limited to 0.09% or less. Preferably, the silicon content is 0.005% to 0.08%.

[0021] Copper may be present as an optional element, up to a maximum of 0.25% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.03% copper is preferred to achieve these effects. However, exceeding 0.25% may degrade the surface appearance. The most preferred limit is 0.05% to 0.2%.

[0022] Chromium is an optional element in the present invention. The chromium content may be 0% to 0.2% in the steel of the present invention. Chromium imparts strength and hardenability to steel, but when used in an amount exceeding 0.2%, it impairs the surface finish of the steel. The preferred range of chromium content in the present invention is 0% to 0.15%.

[0023] Nickel may be present as an optional element in an amount up to 0.2% to increase the strength of steel and improve its toughness. To achieve such effects, at least 0.01% is preferable. However, when the content exceeds 0.2%, nickel causes a decrease in ductility and excessively increases the cost of adding alloying elements.

[0024] Molybdenum is an optional element constituting 0% to 0.2% of the steel of the present invention. Molybdenum improves the hardenability of the steel of the present invention and affects the transformation of austenite to ferrite and bainite during cooling after hot rolling. However, since the addition of molybdenum excessively increases the cost of adding alloying elements, the content is limited to 0.2% for economic reasons. The preferred range of molybdenum content is 0% to 0.15%.

[0025] Vanadium is an optional element that may be present in trace amounts in the steel of the present invention. It is not intentionally added as an effective element for increasing the strength of steel by forming carbides, nitrides or carbonitrides, and the upper limit of its content is 0.1%.

[0026] Other elements such as cerium, boron, magnesium or zirconium may be added individually or in combination in the following weight proportions: cerium ≤ 0.1%, boron ≤ 0.003%, magnesium ≤ 0.010% and zirconium ≤ 0.010%. Up to the indicated maximum content levels, these elements enable grain refinement during solidification.

[0027] The remainder of the steel composition consists of iron and unavoidable impurities arising from the smelting process and depending on the process route. In the case of a manufacturing route using a blast furnace, the level of unavoidable impurities is very low. In the case of a manufacturing route using an electric arc furnace loaded with scrap, the steel sheet may contain, in addition to the amount obtained by the blast furnace route, residual elements derived from such scrap, such as copper, nickel, molybdenum, zinc, antimony, arsenic, and lead, up to a cumulative amount of 1%.

[0028] Next, we will explain the microstructure of steel sheets.

[0029] Ferrite constitutes 25% to 60% of the microstructure of the steel of the present invention by area percentage. The ferrite cumulatively includes polygonal ferrite and acicular ferrite. Ferrite imparts elongation and formability to the steel of the present invention. To ensure an elongation of 12%, preferably 15% or more, it is necessary to have 25% 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 60%, the tensile strength is not achieved. Therefore, the preferred limit for the presence of ferrite in the present invention is 30% to 55% by area percentage.

[0030] Bainite constitutes 40% to 75% of the microstructure in the steel of the present invention in terms of area percentage. Bainite forms 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 600 MPa, preferably 625 MPa or higher, it is necessary to include 40% bainite. Bainite begins to form during the coiling step and during cooling after hot rolling, particularly below the Bs temperature. Therefore, the preferred limit of the presence of bainite in the present invention is 45% to 75% in terms of area percentage.

[0031] Pearlite is an optional microstructure of the steel of the present invention, present in amounts from 0% to 2%. Pearlite can impart strength and toughness to the steel. Pearlite is formed during the cooling period from the hot rolling temperature to the coiling temperature. Whenever pearlite is present in amounts exceeding 2%, the steel of the present invention cannot achieve a hole expansion ratio of 55%.

[0032] Martensite-austenite islands and austenite may also be optionally present as trace amounts of microstructure, ranging from 0% to 2%.

[0033] Conventionally, inclusions are known to reduce the ductility and flangability of steel sheets and to cause defects such as internal defects. This occurs because, while the steel sheet is deforming, the inclusions form voids in the steel, promoting ductile fracture and causing degradation of the HER (Heat-Efficacy). However, the inventors are not bound by this phenomenon and have controlled the size and density of inclusions in the steel of the present invention to achieve an HER of more than 55%. The inclusions of the present invention are derived from one or more inclusions selected from the group consisting of oxides, sulfides, oxynitrides, oxysulfides, nitrides and / or carbonitrides. The inclusions of the present invention are formed during cooling after the casting process and are present at a rate of at least 200 inclusions per square micrometer when measured on any surface of the steel. A preferred presence of inclusions is 200 inclusions / square micrometer to 600 inclusions / square micrometer. The proportion of the aforementioned inclusions having a size of 2 microns or more must be controlled to be 15% or less, preferably less than 12%, of the total number of inclusions present. The steel of the present invention can achieve a 55% HER (Heat Recovery Rating).

[0034] The steel plate according to the present invention can be manufactured by any suitable method. A preferred method is to produce a semi-finished casting of steel having the chemical composition according to the present invention. The casting is carried out continuously or in batches in the form of slabs, i.e., with a thickness in the range of 40 mm to 120 mm relative to the slab, and a minimum casting speed of 3.5 m / min must be maintained during the casting process, preferably 4 m / min or more. The preferred range for the slab thickness is 50 mm to 70 mm.

[0035] For example, slabs having the above-mentioned chemical composition were manufactured by continuous casting, and the slabs were optionally subjected to direct soft reduction during the continuous casting process to avoid central segregation. Slabs obtained 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.

[0036] The slab is reheated to a uniform reheating temperature of 1075°C to 1175°C, preferably 1100°C to 1150°C. The temperature of the slab then subjected to hot rolling must be at least 1075°C and below 1175°C. If the slab temperature is below 1075°C, the rolling mill will be overloaded, and the steel temperature may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled with ferrite transformation incorporated into its structure. Therefore, it is preferable that the slab temperature be sufficiently high to complete hot rolling within the 100% austenite range.

[0037] The final rolling pass is preferably carried out at a temperature above 850°C, because below this temperature the steel sheet exhibits a significant decrease in rollability. The preferred hot-rolling finish temperature is 850°C to 975°C. The hot-rolled steel thus obtained is then cooled at a cooling rate of more than 30°C / second to an average coiling temperature, which must be between 525°C and 675°C. Preferably, the average coiling temperature is maintained between 550°C and 650°C to maximize the precipitation of niobium and titanium during hot-rolling and cooling after coiling. Preferably, the cooling rate is 150°C / second or less.

[0038] Hot-rolled steel sheets can optionally be coated using an industrially known molten dipping coating process.

[0039] If necessary, skin pass rolling may be optionally performed on hot-rolled steel sheets with a minimum skin pass reduction in the range of 0.3 to 1.5%.

[0040] Subsequently, the hot-rolled steel sheet of the present invention is obtained. [Examples]

[0041] The following tests, examples, graphic illustrations, and tables presented herein are not limiting in nature and should be considered for illustrative purposes only, and illustrate advantageous features of the present invention.

[0042] Table 1 summarizes steel sheets made from different steel compositions, each 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.

[0043] [Table 1] Underlined values: Not according to the present invention.

[0044] Table 2 Table 2 summarizes the annealing process parameters performed on the steels shown in Table 1. Steel compositions I1-I3 and R1-R3 are used in the production of the sheets according to the present invention.

[0045] The following process parameters are the same for all steels in Table 1. Table 2 is as follows:

[0046] [Table 2] I = According to the present invention; R = Reference; Underlined value: Not according to the present invention.

[0047] Table 3 Table 3 illustrates the results of tests performed according to standards using different microscopes, such as scanning electron microscopes, to determine the microstructure of both the steel of the present invention and the reference steel.

[0048] The results are specified herein.

[0049] [Table 3] I = According to the present invention; R = Reference; Underlined value: Not according to the present invention.

[0050] The steel according to the present invention contains inclusions exceeding 350 particles / square micrometer, and even more so, inclusions exceeding 400 particles / square micrometer.

[0051] Table 4 Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. Tensile tests were performed according to NBN EN ISO 6892-1 Method B to determine tensile strength, yield strength, and total elongation. Hole expansion tests were performed according to ISO 16630.

[0052] The results of various mechanical tests conducted in accordance with the standards are summarized here.

[0053] [Table 4] I = According to the present invention; R = Reference; Underlined value: Not according to the present invention.

Claims

1. Hot-rolled steel sheet, expressed in weight percentage, containing the following elements: 0.03% ≤ Carbon ≤ 0.07% 0.8% ≤ Manganese ≤ 1.3% 0.01% ≤ Aluminum ≤ 0.07% 0.01% ≤ Niobium ≤ 0.07% 0.05% ≤ Titanium ≤ 0.15% 0.0005% ≤ Calcium ≤ 0.005% 0% ≤ Phosphorus ≤ 0.03% 0% ≤ Sulfur ≤ 0.015% 0% ≤ Nitrogen ≤ 0.02% It includes and may contain one or more of the following optional elements: 0.001% ≤ Silicon ≤ 0.09% 0% ≤ Chromium ≤ 0.2% 0.03%≦Copper≦0.25% 0% ≤ Nickel ≤ 0.2% 0% ≤ Molybdenum ≤ 0.2% 0% ≤ Vanadium ≤ 0.1% 0% ≤ Boron ≤ 0.003% 0% ≤ Magnesium ≤ 0.010% 0% ≤ Cerium ≤ 0.1% 0% ≤ Zirconium ≤ 0.010% The composition of the steel sheet is comprised of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet comprises, by area percentage, 40% to 75% bainite, 25% to 60% ferrite, 0% to 2% pearlite, and 0% to 2% martensite-residual islands, the hot-rolled steel sheet has an inclusion density of at least 200 inclusions per square micrometer, and the inclusions having a size of 2 microns or more account for 15% or less of the total number of inclusions.

2. The hot-rolled steel sheet according to claim 1, wherein the composition contains 0.04% to 0.06% carbon.

3. A hot-rolled steel sheet according to claim 1 or 2, wherein the composition contains 0.06% to 0.12% titanium.

4. A hot-rolled steel sheet according to any one of claims 1 to 3, comprising 0.9% to 1.2% manganese.

5. A hot-rolled steel sheet according to any one of claims 1 to 4, comprising 0.015% to 0.06% aluminum.

6. A hot-rolled steel sheet according to any one of claims 1 to 5, wherein the amount of bainite is 45% to 75%.

7. The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a tensile strength of 600 MPa to 750 MPa and a hole expansion ratio of 55% or more.

8. The hot-rolled steel sheet according to claim 7, wherein the steel sheet has a tensile strength of 625 MPa to 725 MPa and a total elongation of 12% or more.

9. A method for manufacturing hot-rolled steel sheets, comprising the following series of steps: - A step of obtaining the steel composition according to any one of claims 1 to 5, - The steel composition is provided in the form of cast semi-finished products having a thickness ranging from 40 mm to 120 mm for slabs, but a minimum casting rate of 3.5 m / min must be maintained during the casting process. - The slab produced by the casting process may optionally be used directly at high temperatures after casting, or it may be first cooled to room temperature and then reheated for hot rolling. - Reheating the slab to a temperature of 1075°C to 1175°C; - A step of rolling the semi-finished product in the 100% austenite range where the hot rolling finishing temperature exceeds 850°C to obtain a hot-rolled steel strip. - Next, the hot-rolled strip is cooled to a temperature range of 525°C to 675°C at an average cooling rate of more than 30°C / second; - The next step is to coil the hot-rolled steel strip at a temperature range of 525°C to 675°C; - The step of cooling the coiled hot-rolled steel strip to room temperature to obtain a hot-rolled steel sheet. Methods that include...

10. The method according to claim 9, wherein the reheating temperature of the semi-finished product is 1100°C to 1150°C.

11. The method according to claim 9 or 10, wherein the hot rolling finishing temperature is 850°C to 975°C.

12. The method according to any one of claims 9 to 11, wherein the coiling temperature range is 550°C to 650°C.

13. The method according to any one of claims 9 to 12, wherein the average cooling rate after hot rolling for cooling is 30°C / sec to 150°C / sec.

14. Use of a steel plate according to any one of claims 1 to 8 or a steel plate manufactured according to the method of claims 9 to 13 for the manufacture of a vehicle structural component or safety component.

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

Images