Hot-rolled steel sheet and method for manufacturing the same

Abstract

The composition is within the following elemental ranges: 0.03% ≤ Carbon ≤ 0.070%, 0.4% ≤ Manganese ≤ 0.9%, 0.01% ≤ Aluminum ≤ 0.08%, 0.01% ≤ Niobium ≤ 0.08%, 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% ≤ Copper ≤ 0.25%, 0% ≤ Nickel ≤ 0.2%, 0% ≤ Molybdenum ≤ 0.2%, 0% ≤ Vanadium ≤ 0.1%, 0% ≤ Boron ≤ 0. A hot-rolled steel sheet comprising 0.03%, 0%≦magnesium≦0.010%, 0%≦cerium≦0.1%, and 0%≦zirconium≦0.010%, with the remainder being iron and unavoidable impurities caused by the treatment, wherein the microstructure of the steel sheet comprises, by area fraction, 35% to 70% bainite, 30% to 60% ferrite, and 1% to 5% pearlite, the hot-rolled steel sheet having an inclusion density of 110 inclusions per square micrometer, and inclusions having a size of 2 microns or more accounting for 22% 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, automobiles have also been given a third requirement of improving fuel efficiency. Therefore, at present, automobile parts must be made of materials with high formability in order to meet the standards that allow easy installation into complex automobile assemblies. At the same time, it is necessary to improve strength for the impact resistance and durability of the vehicle while reducing vehicle weight and improving fuel efficiency.

[0003] Therefore, intensive research and development efforts have been made to reduce the amount of materials used in automobiles by increasing the strength of materials. As the strength of a steel sheet increases, its formability decreases conversely, so there is a demand for the development of a material that combines 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 the definitive evaluation of the present invention.

[0005] International Publication No. 2022 / 180146 provides high-strength hot-rolled flat steel products and methods for producing such flat steel products, thereby achieving a simultaneous combination of high strength, high local cold formability and high economic viability based on steel. This has 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 scale of at least 0.12, favorably at least 0.17, a local to overall cold formability ratio of at least 5, up to 13, and a microstructure of more than 50 volume% bainite and up to 10 volume%, favorably at least This is achieved by high-strength hot-rolled flat steel with high local cold formability, consisting of a 5 volume percent carbon-rich microstructure component, such as martensite, retained austenite, pearlite, and retained precipitation-hardened ferrite. The steel composition is as follows (weight %): C: 0.04~0.08; Si: 0.1~0.6; Mn: 1.0~2.0; P: max 0.06; S: ​​max 0.01; N: max 0.012; Al: max 0.06; Ti: max 0.18 and / or Nb: max 0.08; Mo: max 0.35; and Ti + Nb greater than 0.06 (if there is a hyperstoichiometric ratio of carbon and nitrogen according to the following formula: 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96)), remainder: iron including unavoidable steel-related elements. However, International Publication No. 2022 / 180146 does not demonstrate HER in more than 65% of cases. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2022 / 180146 [Overview of the project] [Problems that the invention aims to solve]

[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 560 MPa or more to 700 MPa, preferably 580 to 680 MPa. Yield strength of 500 MPa or more to 620 MPa, preferably 500 MPa to 580 MPa. Total elongation of 15% or more, preferably 18% or more. A hole expansion rate of 65% or more, preferably more than 70%.

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

[0009] Another objective of the present invention is to make available a method for manufacturing these plates that is compatible with conventional industrial applications and is robust against changes in manufacturing parameters. [Means for solving the problem]

[0010] The hot-rolled steel sheet of the present invention is coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, in order to improve its corrosion resistance. [Modes for carrying out the invention]

[0011] Carbon is present in steel at a concentration of 0.03% to 0.070%. Carbon is an essential element for increasing the strength of steel sheets by reinforcing the interstitial spaces and by forming fine alloying precipitates. If the carbon content is less than 0.03%, it becomes difficult to achieve the required tensile strength of 560 MPa or more, along with the required elongation of more than 15%. On the other hand, if the carbon content exceeds 0.070%, the steel has poor spot weldability, limiting its application to automotive parts. High carbon content lowers the Ac1 temperature, which can lead to the formation of a second phase such as pearlite, martensite, or cementite during cooling after hot rolling, resulting in a reduced hole expansion rate and increased undesirable treatment hardening during bending. 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 0.4% to 0.9%. 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 Bs and Ms temperatures, and therefore plays an important role in controlling bainite formation. If the Mn content is less than 0.4%, it becomes difficult to achieve the required strength of the steel according to the present invention, along with the required elongation of more than 15%. Furthermore, a manganese content exceeding 0.9% has adverse effects, for example, by delaying the transformation of austenite during cooling after hot rolling, thereby reducing the ductility of the steel of the present invention, worsening weldability, and potentially failing to achieve the elongation target. The preferred content of the present invention may be maintained at 0.5% to 0.9%.

[0013] Aluminum is an essential element and is present in the steel of the present invention at a concentration of 0.01% to 0.08%. Aluminum promotes ferrite formation, which allows the present invention to have a sufficient amount of ferrite to achieve the desired combination of strength and ductility of the steel. However, if the presence of aluminum exceeds 0.08%, the hot rolling finish temperature will be in the fully austenitic region. The aluminum content is preferably limited to 0.015% to 0.07%.

[0014] Niobium, in concentrations of 0.01% to 0.08%, is an essential element for the steel of the present invention 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 microstructural components through precipitation as carbides and by delaying recrystallization during the heating process. As a result, the finer microstructure formed in the final product allows the steel of the present invention to reach its target strength. However, niobium content exceeding 0.08% is not only economically uninteresting but also forms coarser precipitates that are detrimental to properties such as porosity and elongation of the steel. Furthermore, if the niobium content is above 0.08%, niobium is also detrimental to the hot ductility of the steel, causing problems during casting and rolling. The preferred limit for niobium content is 0.01% to 0.07%, and more preferably 0.01% to 0.05%.

[0015] Titanium is an essential element and can be added to the steel of the present invention in an amount of 0.01% to 0.1%. Like niobium, titanium is involved in the formation of carbonitrides and plays a role in the hardening of the steel of the present invention. In addition, titanium also forms titanium nitrides that appear during the solidification of the casting. The amount of titanium is limited to 0.1% or less to avoid the formation of rough titanium nitrides that are detrimental to formability.

[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 inclusion treatment. Calcium contributes to the refinement of the steel by suppressing harmful sulfur content in a spherical form, and furthermore, calcium also promotes 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, it is preferable that the phosphorus content be as low as possible, 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%, and more preferably less than 0.018%.

[0018] 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 cost, it should be 0.015% or less. Furthermore, if a larger amount of sulfur is present in the steel, it will form sulfides, especially in combination with manganese, which will reduce its beneficial effect on the steel of the present invention.

[0019] Nitrogen is limited to less than 0.02% to avoid material degradation over time and minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of 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 silicon content exceeds 0.09%, it 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. The silicon content is preferably 0.005% to 0.08%.

[0021] Copper can be present as an optional element, and can be present in amounts up to 0.25% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.03% copper is preferable to achieve these effects. However, exceeding 0.25% may degrade the surface appearance. The most preferable 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 strengthens and hardens 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 of up to 0.2% to increase the strength of steel and improve its toughness. A minimum of 0.01% is preferable to achieve such effects. However, when the content of nickel exceeds 0.2%, it causes deterioration of ductility and excessively increases the cost of adding alloying elements.

[0024] Molybdenum is an optional element that constitutes 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, the addition of molybdenum excessively increases the cost of adding alloying elements, so 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 actively 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 at the following weight proportions: cerium ≤ 0.1%, boron ≤ 0.003%, magnesium ≤ 0.010% and zirconium ≤ 0.010%. Up to the maximum content levels indicated, these elements enable grain refinement during solidification.

[0027] The remainder of the steel composition consists of iron and unavoidable impurities resulting 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, up to a cumulative amount of residual elements derived from such scrap, such as copper, nickel, molybdenum, zinc, antimony, arsenic, and lead, up to 1%.

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

[0029] Ferrite constitutes 30% to 60% of the microstructure of the steel of the present invention in terms of area fraction. 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 15%, preferably 18% or more, it is necessary to have 30% ferrite. In the steel of the present invention, ferrite is formed during cooling after hot rolling. However, if the ferrite content in the steel of the present invention exceeds 60%, the tensile strength will not necessarily be achieved. Therefore, the preferred limit for the presence of ferrite in the present invention is 35% to 60% in terms of area fraction, and more preferably 40% to 55%.

[0030] Bainite constitutes 35% to 70% of the microstructure of the steel of the present invention in terms of area fraction. Bainite forms the main phase of the steel as a matrix and is cumulatively composed of upper bainite and lower bainite. In order to ensure a tensile strength of 560 MPa, preferably 580 MPa or more, it is necessary to include 35% bainite. Bainite formation begins during the winding process and during cooling after hot rolling, especially after exceeding the Bs temperature. Therefore, the preferred range for the presence of bainite in the present invention is 40% to 65% in terms of area fraction, and more preferably 44% to 60%.

[0031] Pearlite is an essential microstructure of the steel of the present invention, present in a concentration of 1% to 5%. Pearlite imparts strength and toughness to the steel. The pearlite of the present invention has a lamellar structure. Pearlite is formed during cooling after the hot rolling temperature and up to the winding temperature. If the pearlite concentration exceeds 5%, the steel of the present invention will inevitably fail to achieve a 65% hole expansion ratio. Therefore, the preferred limit for the presence of pearlite in the present invention is 3% to 5% in terms of area fraction.

[0032] Conventionally, inclusions are known to reduce the ductility and flangebility of steel sheets and to cause defects such as internal defects. This occurs because inclusions form voids in the steel during deformation of the steel sheet, promoting ductile fracture and causing deterioration of the HER (Heat Value). However, the inventors are not bound by this phenomenon and have controlled the size and density of inclusions so that the HER of the steel of the present invention reaches more than 65%. The inclusions of the present invention are one or more 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 in an amount of at least 110 inclusions per square micrometer when measured on any surface of the steel. A preferred presence is 110 to 450 inclusions per square micrometer. The presence of the aforementioned inclusions having a size of 2 microns or more must be controlled to 22% or less of the total number of inclusions present, preferably less than 20% of the total number of inclusions, in order to enable the achievement of 65% HER of the steel of the present invention.

[0033] The steel plates according to the present invention can be manufactured by any suitable method. A preferred method is to provide a semi-finished casting of steel having the chemical composition according to the present invention. Casting is carried out in the form of slabs, i.e., with a thickness in the range of 40 mm to 120 mm for the slabs, continuously or in batches, and a minimum casting rate of 3.5 m / min, preferably 4 m / min or more, must be maintained during the casting process. The preferred range for slab thickness is 50 mm to 70 mm.

[0034] For example, slabs having the above-mentioned chemical composition are manufactured by continuous casting, and the slabs are optionally subjected to direct light reduction 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.

[0035] The slab is uniformly reheated to a reheating temperature of 1075°C to 1175°C, preferably 1100°C to 1150°C. Subsequently, the temperature of the slab subjected to hot rolling must be at least 1075°C and below 1175°C. If the slab temperature is lower than 1075°C, an excessive load is imposed on the rolling mill, and it is preferable that the slab temperature be sufficiently high so that hot rolling can be completed in the 100% austenite region.

[0036] The final rolling pass for hot rolling is carried out at a temperature exceeding 850°C because, at temperatures lower than this, 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 exceeding 30°C / second to an average coiling temperature that must be between 525°C and 675°C. Maintaining the average coiling temperature at 550°C to 650°C is preferable because it maximizes the precipitation of niobium and titanium during hot rolling and cooling after coiling. Preferably, the cooling rate is 150°C / second or less.

[0037] Hot-rolled steel sheets may, in some cases, be coated by an industrially known molten dipping coating process.

[0038] If necessary, skin pass rolling may be performed on hot-rolled steel sheets with a minimum skin pass reduction rate in the range of 0.3 to 1.5%.

[0039] Subsequently, the hot-rolled steel sheet of the present invention is obtained. [Examples]

[0040] 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.

[0041] Table 1 summarizes steel sheets made from different steel compositions, and each sheet is manufactured according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the steel sheets obtained during testing, and Table 4 summarizes the evaluation results of the obtained properties.

[0042] [Table 1] Underlined values: Not according to the present invention.

[0043] Table 2 Table 2 summarizes the annealing process parameters performed on the steels shown in Table 1. The steel compositions I1-I3 and R1-R3 are useful for manufacturing plates according to the present invention.

[0044] The following process parameters are the same for all steels in Table 1.

[0045] 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 clearly stated 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 more than 110 inclusions per square micrometer, and more than 120 inclusions per 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 are performed according to NBN EN ISO 6892-1 Method B to determine the tensile strength, yield strength, and total elongation. Hole expansion tests are performed according to ISO 16630, and the bendability in terms of the ratio of the critical bending radius to the thickness of the crack-free substrate is determined according to ISO 7438.

[0052] The results of various mechanical tests performed according to the standards are collected.

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

Claims

1. A hot-rolled steel sheet having a composition containing the following elements, expressed in weight percent, 0.03% ≤ Carbon ≤ 0.070% 0.4% ≤ Manganese ≤ 0.9% 0.01% ≤ Aluminum ≤ 0.08% 0.01% ≤ Niobium ≤ 0.08% 0.05% ≤ Titanium ≤ 0.15% 0.0005% ≤ Calcium ≤ 0.005% 0% ≤ Phosphorus ≤ 0.03% 0% ≤ Sulfur ≤ 0.015% 0% ≤ Nitrogen ≤ 0.02%, It may contain one or more of the following elements: 0.001% ≤ Silicon ≤ 0.09% 0% ≤ Chromium ≤ 0.2% 0%≦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%, A hot-rolled steel sheet wherein the remainder of the composition consists of iron and unavoidable impurities caused by the process, the microstructure of the steel sheet contains, by area fraction, 35% to 70% bainite, 30% to 60% ferrite, and 1% to 5% pearlite, the hot-rolled steel sheet has an inclusion density of at least 110 inclusions per square micrometer, and inclusions having a size of 2 microns or more account for 22% or less of the total number of inclusions.

2. A hot-rolled steel sheet according to claim 1, comprising a carbon content of 0.04% to 0.06%.

3. A hot-rolled steel sheet according to claim 1 or 2, comprising 0.06% to 0.12% titanium.

4. A hot-rolled steel sheet according to any one of claims 1 to 3, comprising 0.5% to 0.9% manganese.

5. A hot-rolled steel sheet according to any one of claims 1 to 4, comprising 0.015% to 0.07% aluminum.

6. A hot-rolled steel sheet according to any one of claims 1 to 5, wherein the amount of bainite is 40% to 65%.

7. The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a tensile strength of 560 MPa to 700 MPa and a hole expansion ratio of 65% or more.

8. The hot-rolled steel sheet according to claim 7, wherein the steel sheet has a tensile strength of 580 MPa to 650 MPa and a total elongation of 15% or more.

9. A method for manufacturing hot-rolled steel sheets, comprising the following series of steps: - A step of providing the steel composition according to any one of claims 1 to 5, - The steel composition is provided in the form of a cast semi-finished product having a thickness in the range of 40 mm to 120 mm for the slab, and a minimum casting speed of 3.5 m / min must be maintained during casting. - The slab provided by the casting process may optionally be used directly at a high temperature after casting, or it may be cooled to room temperature first and then reheated for hot rolling. - A step of reheating the slab to a temperature of 1075°C to 1175°C, - A step of rolling the aforementioned semi-finished product in the 100% austenite region, wherein the hot rolling finishing temperature is raised to 850°C or higher 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 exceeding 30°C / second, - Subsequently, the hot-rolled steel strip is wound up in a temperature range of 525°C to 675°C, A method comprising the step of cooling a wound hot-rolled steel strip to room temperature to obtain a hot-rolled steel sheet.

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 winding 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 for cooling after hot rolling is 30°C / sec to 150°C / sec.

14. Use of a steel plate according to any one of claims 1 to 8, or use of 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.

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