Hot-rolled steel sheet and method for producing same

Abstract

A hot-rolled steel sheet having a composition containing 0.03% < = C < = 0.070%, 0.4% < = Mn < = 0.9%, 0.01% < = Al < = 0.08%, 0.01% < = Nb < = 0.08%, 0.05% < = Ti < = 0.15%, 0.0005% < = Ca < = 0.005%, 0% < = P < = 0.03%, 0% < = S < = 0.015%, 0% < = N < = 0.02%, 0.001% < = Si < = 0.09%, 0% < = Cr < = 0.2%, 0% < = Cu < = 0.25%, 0% < = Ni < = 0.2%, 0% < = Mo < = 0.2%, 0% < = V < = 0.1%, 0% < = B < = 0.003%, 0% < = Mg < = 0.010%, 0% < = Ce < = 0.1%, 0% A hot-rolled steel sheet having a microstructure comprising, in area fraction, 35% to 70% bainite, 30% to 60% ferrite, and 1% to 5% pearlite, in which the hot-rolled steel sheet has an inclusion density of 110 inclusions per square micron, and inclusions having a size of 2 microns or greater are 22% or less of the total number of inclusions.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets suitable for use as automotive steel sheets. Background Technology

[0002] Automotive components need to meet two inconsistent requirements: ease of forming and strength. However, in recent years, due to global environmental concerns, a third requirement has been placed on automobiles: improved fuel efficiency. Therefore, automotive components must now be made from highly formable materials to meet the standard of easy assembly in complex automotive assemblies, while simultaneously improving strength for vehicle crashworthiness and durability, and reducing vehicle weight to improve fuel efficiency.

[0003] Therefore, significant research and development efforts have been invested in reducing the amount of materials used in automobiles by increasing material strength. Conversely, increasing the strength of steel sheets reduces formability, thus necessitating the development of materials that combine high strength and high formability.

[0004] Early research and development in the field of high-strength and high-formability steel sheets have yielded several methods for producing them. Some of these methods are listed herein to facilitate a clear understanding of the present invention:

[0005] WO2022 / 180146 provides a high-strength hot-rolled flat steel product and a method for producing such a flat steel product, thereby achieving a combination of high strength with simultaneously high local cold formability and high economic feasibility on a steel basis. This is achieved by a high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8, a porosity of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, an elongation at break of at least 10%, preferably at least 16%, a cold formability measure of at least 0.12, advantageously at least 0.17, and a ratio of at least 5 and at most 13 for local and overall cold formability, and its microstructure comprising more than 50 vol% bainite and at most 10 vol%, advantageously at most 5 vol% carbon-rich microstructure components, such as martensite, residual... The steel, composed of austenite, pearlite, and residual precipitation-hardening ferrite, has the following chemical composition (in weight percent): C: 0.04 to 0.08; Si: 0.1 to 0.6; Mn: 1.0 to 2.0; P: maximum 0.06; S: ​​maximum 0.01; N: maximum 0.012; Al: maximum 0.06; Ti: maximum 0.18 and / or Nb: maximum 0.08; Mo: maximum 0.35; wherein Ti+Nb is greater than 0.06, and there exists an overstoichiometric ratio of C and N according to the following formula: 1.0 < (C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96), balance: iron, containing unavoidable steel-associated elements. However, WO2022 / 180146 does not confirm a porosity (HER) of 65% or greater. Summary of the Invention

[0006] The object of this invention is to solve these problems by providing a hot-rolled steel sheet that simultaneously possesses the following properties:

[0007] - The TS / YS ratio is greater than or equal to 1.10.

[0008] - Ultimate tensile strength greater than or equal to 560 MPa to 700 MPa, preferably 580 MPa to 680 MPa.

[0009] - Yield strength greater than or equal to 500 MPa to 620 MPa, preferably 500 MPa to 580 MPa.

[0010] - The total elongation is greater than or equal to 15%, preferably greater than or equal to 18%.

[0011] - The porosity is greater than or equal to 65%, preferably higher than 70%.

[0012] Preferably, such steel also has good formability, good rolling suitability, and good weldability and coatability.

[0013] Another object of the present invention is to provide a method for manufacturing these sheets, which is compatible with conventional industrial applications and is robust to changes in manufacturing parameters.

[0014] The hot-rolled steel sheet of the present invention is coated with zinc or zinc alloy, or aluminum or aluminum alloy, to improve its corrosion resistance.

[0015] Carbon is present in steel at a concentration of 0.03% to 0.070%. Carbon is an essential element for improving the strength of steel sheets through interstitial strengthening and by forming microalloyed precipitates. If C is below 0.03%, it is difficult to achieve the desired combination of tensile strength of 560 MPa or higher and elongation above 15%. On the other hand, when the C content exceeds 0.070%, the steel exhibits poor spot weldability, which limits its application in automotive parts. High C content lowers the Ac1 temperature, and therefore second phases such as pearlite, martensite, or cementite may form during cooling after hot rolling, which reduces porosity and increases work hardening during bending, which is undesirable. Therefore, for the steel of the present invention, the preferred range of C is 0.04% to 0.06%.

[0016] The Mn content of the steel of the present invention is 0.4% to 0.9%. The purpose of adding Mn is primarily to impart strength to the steel through solid solution strengthening. This element is a γ-phase forming element and also affects the Bs and Ms temperatures, thus playing an important role in controlling bainite formation. If the Mn content is below 0.4%, it is difficult to achieve the desired combination of strength and elongation above 15% in the steel according to the present invention. Furthermore, a Mn content above 0.9% has adverse effects, such as delaying the austenite transformation during cooling after hot rolling, thereby reducing ductility and also deteriorating the weldability of the steel of the present invention, thus potentially failing to achieve the elongation target. The preferred content of the present invention can be maintained at 0.5% to 0.9%.

[0017] Al is an essential element and is present in the steel of the present invention at a concentration of 0.01% to 0.08%. Al promotes ferrite formation, which enables the present invention to have a sufficient amount of ferrite to achieve the desired combination of strength and ductility. However, when the presence of Al exceeds 0.08%, this results in the hot rolling finishing temperature being in the fully austenitic region. The Al content is preferably limited to 0.015% to 0.07%.

[0018] Nitrogen (Nb) is an essential element in the steel of the present invention, comprising 0.01% to 0.08%, and is suitable for forming carbides and carbonitrides to impart strength to the steel of the present invention through precipitation hardening. Nb also influences the size of the microstructure components through its precipitation as a carbide and through recrystallization during the delayed heating process. Therefore, a finer microstructure is formed in the final product, thereby enabling the steel of the present invention to achieve the target strength. However, Nb content above 0.08% is economically unattractive and forms coarser precipitates that are detrimental to properties such as porosity and elongation of the steel. Furthermore, when the Nb content is 0.08% or greater, Nb is also detrimental to the hot ductility of the steel, leading to difficulties during casting and rolling. The preferred limit for Nb content is 0.01% to 0.07%, more preferably 0.01% to 0.05%.

[0019] Ti is an essential element and can be added to the steel of the present invention at a rate of 0.01% to 0.1%. Like Nb, it participates in the formation of carbonitrides and thus plays a role in the hardening of the steel of the present invention. Furthermore, Ti also forms titanium nitrides that appear during the solidification of the cast product. The amount of Ti is therefore limited to 0.1% to avoid the formation of coarse titanium nitrides that are detrimental to formability.

[0020] Ca is a mandatory element and is added to the steel of the present invention at a rate of 0.0005% to 0.005%. Ca is added to the steel of the present invention, especially during inclusion treatment, preferably in a minimum amount of 0.0005%. Ca promotes steel refining by capturing harmful sulfur content in a spherical form, and furthermore, Ca promotes casting by preventing blockages during casting, thereby delaying the harmful effects of sulfur.

[0021] P is not an essential element, but may be included in steel as an impurity, and from the perspective of this invention, the P content is preferably as low as possible, and below 0.03%. P reduces spot weldability and hot ductility, particularly due to its tendency to segregate at grain boundaries or co-segregate with Mn. For these reasons, its content is limited to less than 0.03%, preferably less than 0.02%, and more preferably less than 0.018%.

[0022] S is not an essential element, but may be contained in steel as an impurity. From the perspective of this invention, the S content is preferably as low as possible, but from the perspective of manufacturing cost, the S content is 0.015% or less. Furthermore, if there is a high level of S in the steel, it will combine to form sulfides, especially with Mn, and reduce its beneficial effects on the steel of this invention.

[0023] N is limited to 0.02% to avoid material aging and to minimize the precipitation of nitrides that are detrimental to the mechanical properties of steel during solidification.

[0024] Si is an optional element and can be present from 0.001% to 0.09%. Si increases the strength of ferrite through solid solution strengthening. However, when included in amounts greater than 0.09%, Si accumulates as oxides on the surface of the steel sheet during hot rolling. For this reason, the Si content is limited to 0.09% or less. The Si content is preferably from 0.005% to 0.08%.

[0025] Cu can be present as an optional element, and can be present at a maximum of 0.25% to improve the strength of steel and enhance its corrosion resistance. For this effect, a minimum of 0.03% Cu is preferred. However, when its content exceeds 0.25%, it deteriorates the surface appearance. The most preferred limit is 0.05% to 0.2%.

[0026] Cr is an optional element in this invention. The Cr content can be present in the steel of this invention from 0% to 0.2%. Cr provides strength and hardness to the steel, but when used at a concentration higher than 0.2%, it impairs the surface finish of the steel. For the purposes of this invention, the preferred limit for Cr is 0% to 0.15%.

[0027] Ni can be present as an optional element in amounts up to 0.2% to increase the strength and improve the toughness of steel. A minimum of 0.01% is preferred to achieve this effect. However, when its content exceeds 0.2%, Ni leads to deterioration of ductility and excessively increases the cost of alloying element addition.

[0028] Mo is an optional element comprising 0% to 0.2% of the steel of the present invention; Mo 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, excessive addition of Mo increases the cost of alloying element addition, therefore, for economic reasons, its content is limited to 0.2%. The preferred limit for Mo is 0% to 0.15%.

[0029] V is an optional element that can be found in trace amounts in the steel of the present invention, and is not voluntarily added as an element that effectively enhances the strength of the steel by forming carbides, nitrides or carbonitrides, with a maximum content of 0.1%.

[0030] Other elements such as Ce, B, Mg, or Zr can be added alone or in combination in the following weight ratios: Ce≦0.1%, B≦0.003%, Mg≦0.010%, and Zr≦0.010%. At the indicated maximum content levels, these elements enable grain refinement during solidification.

[0031] The remaining components of steel are iron and unavoidable impurities produced during the smelting process and depending on the process route. In the case of a production route using a blast furnace, the level of unavoidable impurities is very low. In the case of a production route using an electric arc furnace loaded with scrap steel, in addition to the amount obtained through the blast furnace route, the steel plate may also contain residual elements such as Cu, Ni, Mo, Zn, Sb, As, and Pb from such scrap steel, with a cumulative amount of up to 1%.

[0032] The microstructure of the steel plate will now be described.

[0033] For the steel of the present invention, ferrite constitutes 30% to 60% of the microstructure by area fraction, and 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 higher, 30% ferrite is required. Ferrite is formed during cooling after hot rolling in the steel of the present invention. However, tensile strength cannot be achieved whenever the ferrite content is present in the steel of the present invention at a level higher than 60%. Therefore, the preferred limit for the presence of ferrite in the present invention is 35% to 60% by area fraction, more preferably 40% to 55%.

[0034] For the steel of the present invention, bainite constitutes 35% to 70% of the microstructure by area fraction. Bainite, as the matrix, constitutes the main phase of the steel and is cumulatively composed of upper and lower bainite. To ensure a tensile strength of 560 MPa, preferably 580 MPa or higher, a 35% bainite content is necessary. Bainite begins to form during the coiling step and during cooling after hot rolling, especially after passing the Bs temperature. Therefore, the preferred limit for the presence of bainite in the present invention is 40% to 65% by area fraction, more preferably 44% to 60%.

[0035] Pearlite is an essential microstructure of the steel of the present invention and is present in an amount 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 the cooling period after hot rolling until the coiling temperature. Whenever pearlite is present in an amount greater than 5%, the steel of the present invention cannot achieve a porosity of 65%. Therefore, for the purposes of the present invention, the preferred limit for the presence of pearlite is 3% to 5% in area fraction.

[0036] It is conventionally known that inclusions degrade the ductility and flanging properties of steel sheets and can also lead to defects such as internal defects. This is because inclusions create voids in the steel during deformation and promote ductile fracture, thus degrading the HER (Heat). However, the inventors were not bound by this phenomenon and controlled the size and density of inclusions to achieve an HER of over 65% in the steel of the present invention. The inclusions of the present invention are derived from one or more of 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 contained in an amount of at least 110 inclusions per square micrometer when measured on any surface of the steel. The preferred presence is from 110 to 450 inclusions per square micrometer. For the aforementioned inclusions with a size of 2 micrometers or larger, the inclusion presence must be controlled to be 22% or less of the total number of inclusions, and preferably less than 20%, so that the steel of the present invention can achieve a total inclusion content of 65% HER.

[0037] The steel plate according to the invention can be produced by any suitable method. A preferred method involves providing a semi-finished casting of steel having the chemical composition according to the invention. Casting is carried out continuously or in batches in slab form, i.e., the thickness of the slab ranges from 40 mm to 120 mm, wherein a minimum casting speed of 3.5 m / min must be maintained during the casting process, and preferably greater than or equal to 4 m / min. The preferred range for slab thickness is 50 mm to 70 mm.

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

[0039] The slab is uniformly reheated to a temperature of 1075°C to 1175°C, preferably 1100°C to 1150°C. Thereafter, the temperature of the slab undergoing hot rolling is at least 1075°C and must be below 1175°C. If the slab temperature is below 1075°C, it would place an excessive load on the rolling mill, and the slab temperature is preferably high enough to allow hot rolling to be completed within the 100% austenitic range.

[0040] The final rolling pass is performed at a temperature above 850°C to complete the hot rolling, as the steel sheet exhibits a significant decrease in rollability below this temperature. A preferred hot-rolling finishing temperature is 850°C to 975°C. The hot-rolled steel obtained in this manner is then cooled to an average coiling temperature of 525°C to 675°C at a cooling rate above 30°C / s. Preferably, the average coiling temperature is maintained at 550°C to 650°C to maximize the precipitation of Nb and Ti during the cooling period after hot rolling and coiling. Preferably, the cooling rate is less than or equal to 150°C / s.

[0041] Hot-rolled steel sheets can be coated using industrially known hot-dip galvanizing processes.

[0042] If necessary, the hot-rolled steel sheet may be optionally leveled, with a minimum leveling reduction rate ranging from 0.3% to 1.5%.

[0043] Subsequently, the hot-rolled steel sheet of the present invention was obtained. Detailed Implementation

[0044] Example

[0045] The tests, embodiments, illustrative examples, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and will present advantageous features of the invention.

[0046] Steel plates made from steels with different compositions are summarized in Table 1, wherein the steel plates were produced according to the process parameters listed in Table 2. Subsequently, Table 3 summarizes the microstructure of the steel plates obtained during the experiment, and Table 4 summarizes the evaluation results of the obtained properties.

[0047] Table 1

[0048]

[0049] Underlined values: Not in accordance with the present invention.

[0050] Table 2

[0051] Table 2 summarizes the annealing process parameters applied to the steels in Table 1. Steel compositions I1 to I3 and R1 to R3 are used to manufacture plates according to the present invention.

[0052] The following process parameters are the same for all steels listed in Table 1. Table 2 is as follows:

[0053] Table 2

[0054]

[0055] I = according to the invention; R = reference; underlined value: not according to the invention.

[0056] Table 3

[0057] Table 3 illustrates the test results, performed according to standards on different microscopes, such as scanning electron microscopes, for determining the microstructure of both the steel of the present invention and the reference steel.

[0058] The results are listed here:

[0059]

[0060] I = according to the invention; R = reference; underlined value: not according to the invention.

[0061] The steel according to the invention comprises more than 110 inclusions per square µm, or even more than 120 inclusions per square µm.

[0062] Table 4

[0063] Table 4 illustrates the mechanical properties of the steel of the present invention and the reference steel. Tensile strength, yield strength, and total elongation were determined according to NBN EN ISO 6892-1 Method B. Hole enlargement tests were performed according to ISO 16630, and the bending properties, expressed as the ratio of the critical bending radius to the substrate thickness without cracking, were determined according to ISO 7438.

[0064] The results of various mechanical tests conducted according to the standards are summarized below:

[0065] Table 4

[0066]

[0067] I = according to the invention; R = reference; underlined value: not according to the invention.

Claims

1. A hot-rolled steel sheet, said hot-rolled steel sheet having a composition comprising the following elements: expressed as a percentage by weight, 0.03%≤C≤0.070% 0.4%≤Mn≤0.9% 0.01%≤Al≤0.08% 0.01%≤Nb≤0.08% 0.05%≤Ti≤0.15% 0.0005%≤Ca≤0.005% 0%≤P≤0.03% 0%≤S≤0.015% 0%≤N≤0.02% And it can contain one or more of the following optional elements: 0.001%≤Si≤0.09% 0%≤Cr≤0.2% 0%≤Cu≤0.25% 0%≤Ni≤0.2% 0%≤Mo≤0.2% 0%≤V≤0.1% 0%≤B≤0.003% 0%≤Mg≤0.010% 0%≤Ce≤0.1% 0%≤Zr≤0.010% The remaining portion consists of iron and unavoidable impurities caused by processing. The microstructure of the steel plate, by area fraction, comprises: 35% to 70% bainite, 30% to 60% ferrite, and 1% to 5% pearlite, wherein... The hot-rolled steel sheet has an inclusion density of at least 110 inclusions per square micrometer, and inclusions with a size of 2 micrometers or larger account for 22% or less of the total number of inclusions.

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

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.06% to 0.12% Ti.

4. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the composition comprises 0.5% to 0.9% Mn.

5. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the composition comprises 0.015% to 0.07% Al.

6. The 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 tensile strength of the steel sheet is from 560 MPa to 700 MPa, and the hole expansion ratio is 65% or greater.

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

9. A method for producing hot-rolled steel plate, comprising the following sequential steps: - Provides a steel composition according to any one of claims 1 to 5; - in, The steel composition is provided in the form of cast semi-finished products, with a thickness ranging from 40 mm to 120 mm for slabs, while a minimum casting speed of 3.5 m / min must be maintained during casting. - The slab provided by the casting process can optionally be used directly at high temperature after casting, or can be cooled to room temperature first and then heated for hot rolling; - Reheat the slab to a temperature between 1075°C and 1175°C; - The semi-finished product is rolled in 100% austenitic temperature, wherein the hot rolling final rolling temperature should be higher than 850°C, to obtain hot-rolled steel strip; - Then begin cooling the hot-rolled steel strip to a temperature range of 525°C to 675°C at an average cooling rate of greater than 30°C / second; - The hot-rolled steel strip is then wound up in a temperature range between 525°C and 675°C; - The coiled hot-rolled steel strip is cooled 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 between 1100°C and 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 used for cooling after hot rolling is from 30°C / second to 150°C / second.

14. Use of the steel sheet according to any one of claims 1 to 8 or the steel sheet produced by the method according to claims 9 to 13 for manufacturing structural or safety components of a vehicle.

15. A vehicle comprising the component obtained according to claim 14.

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