Hot-rolled steel sheet and method for manufacturing the same

Abstract

A hot-rolled steel sheet having a composition containing the following elements: 0.02% ≤ carbon ≤ 0.06% 1.1% ≤ manganese ≤ 1.60% 0.01% ≤ aluminum ≤ 0.08% 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 can contain one or more of the following optional elements: 0.001% ≤ silicon ≤ 0.09% 0% ≤ chromium ≤ 0.2% 0 ≤ copper ≤ 0.25% 0% ≤ nickel ≤ 0.2% 0% ≤ vanadium ≤ 0.1% 0% ≤ boron ≤ 0.003% 0% ≤ magnesium ≤ 0.010% 0% ≤ cerium ≤ 0.1% 0% ≤ zirconium ≤ 0.010%, The balance of the composition is composed of iron and inevitable impurities caused by the treatment. The microstructure of the steel sheet contains, in area fraction, 35% - 75% bainite, 25% - 65% ferrite, 0% - 2% pearlite and 0% - 2% martensite - retained island. 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 are 25% 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 meet two opposing requirements, namely ease of forming and strength. In recent years, from the perspective of global environmental problems, a third requirement of improving fuel consumption has also been imposed on automobiles. Therefore, currently, automobile parts must be made of materials having high formability in order to meet the criteria of ease of fitting in complex automobile assemblies, and at the same time, while reducing the weight of the vehicle and improving fuel consumption, the strength for impact resistance and durability of the vehicle must be improved.

[0003] Therefore, intensive research and development have been carried out to reduce the amount of materials used in automobiles by improving the strength of the materials. Conversely, since increasing the strength of the steel sheet reduces formability, the development of materials having both high strength and high formability is required.

[0004] Previous research and development in the field of high-strength and high-formability steel sheets have led to several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed herein for a better understanding of the present invention.

[0005] U.S. Patent No. 20140178712 provides a hot-rolled steel sheet and a method for manufacturing the same, having a maximum tensile strength of 600 MPa or more, and excellent low-temperature shock absorption energy and HAZ softening resistance, specifically containing, by mass%, C: 0.04~0.09%, Si: 0.4% or less, Mn: 1.2~2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02~0.09%, Ti: 0.02~0.07%, and N: 0.005% or less, where 2 The steel sheet has a molecular weight of 0.0 ≤ Mn + 8[%Ti] + 12[%Nb] ≤ 2.6, the remainder being Fe and unavoidable impurities, a pearlite area ratio of 5% or less, a total area ratio of martensite and retained austenite of 0.5% or less, the remainder being ferrite and / or bainite, an average grain size of ferrite and bainite of 10 μm or less, an average particle size of alloy carbonitrides with incompatible interfaces containing Ti and Nb of 20 nm or less, and a yield ratio of 0.85 or higher. However, the steel of U.S. Patent No. 20140178712 does not exhibit HER of 55% or more. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] U.S. Patent No. 20140178712 [Overview of the project]

[0007] The object of the present invention is to solve these problems by making available hot-rolled steel sheets that simultaneously possess the following properties. A TS / YS ratio of -1.10 or higher. -625MPa to 850MPa, preferably 650MPa to 825MPa maximum tensile strength. Yield strength of -575MPa to 750MPa, preferably 600MPa to 725MPa. -16% or more, preferably 18% or more in total elongation. A hole expansion ratio of -55% or more, preferably more than 65%.

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

[0009] Another objective of the present invention is to make available a method for manufacturing these steel sheets that is suitable for conventional industrial applications and is robust against variations in manufacturing parameters.

[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 0.02% to 0.06%. Carbon is an essential element for increasing the strength of steel sheets through interstitial solid solution strengthening and by forming fine alloy precipitates. If the carbon content is less than 0.02%, it becomes difficult to achieve the required tensile strength of 625 MPa or more while simultaneously meeting the required elongation of over 16%. On the other hand, if the carbon content exceeds 0.06%, the steel's spot weldability decreases, limiting its application to automotive parts. High carbon content can promote the formation of a second phase, such as pearlite, martensite, or cementite, during cooling after hot rolling, thereby reducing the hole expansion rate. Therefore, the preferred range for carbon in the steel of the present invention is 0.025% to 0.055%.

[0012] The manganese content of the steel of the present invention is 1.1% to 1.60%. The purpose of adding manganese is essentially to impart strength to the steel through solid solution strengthening. This element is an austenite-forming element and also affects the Bs and Ms temperatures, thus playing an important role in controlling the formation of bainite and martensite. If the Mn content is less than 1.1%, it is difficult to achieve the required strength of the steel according to the present invention in combination with the required elongation of more than 16%. Furthermore, if the manganese content exceeds 1.60%, it delays the austenite transformation during cooling after hot rolling, resulting in adverse effects such as a decrease in ductility, and as a result, the elongation target may not be achieved. The preferred content of the present invention may be maintained at 1.15% to 1.55%.

[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, thereby allowing the steel of the present invention to have a sufficient amount of ferrite to achieve the desired combination of strength and ductility. However, if the presence of aluminum exceeds 0.08%, the finishing temperature of hot rolling will be in the full austenite region. The aluminum content is preferably limited to 0.015% to 0.07%.

[0014] Niobium is an essential element present in the steel of the present invention at 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 the microstructural components by precipitation as carbides and by delaying recrystallization during hot rolling, thereby suppressing austenite grain size. Consequently, a finer microstructure is formed in the final product, and the steel of the present invention can reach its target strength. However, niobium content exceeding 0.07% is not economically appealing. Furthermore, a niobium content of 0.07% or more is detrimental to the steel because it increases the rolling force in the finishing mill, making hot rolling of the steel difficult. 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 may be added to the steel of the present invention in an amount of 0.05% to 0.15%. Similar to niobium, titanium is involved in the formation of carbonitrides 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. A preferred limit for the 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 inclusion treatment. Calcium contributes to the refining of the steel by fixing harmful sulfur content in a spherical form, and furthermore, calcium facilitates casting by avoiding clogging during casting, thereby suppressing 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, preferably 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, it is preferable that the sulfur content be as low as possible, but from the viewpoint of desulfurization manufacturing costs, it should be 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 of the present invention.

[0019] Nitrogen is limited to 0.02% to avoid aging of the material and to control the precipitation of nitride inclusions during solidification, which are detrimental to the mechanical and formable properties 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 silicon content exceeds 0.09%, it concentrates on the steel sheet surface 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 may be present as an optional element and may be present up to a maximum of 0.25% to increase the strength of the steel and improve its corrosion resistance. To obtain such an effect, copper is preferably at least 0.03%. However, if its content exceeds 0.25%, it may deteriorate the surface appearance. The most preferred limit is 0.05% - 0.2%.

[0022] Chromium is an optional element of the present invention. The chromium content may be present in the steel of the present invention from 0% to 0.2%. Chromium imparts strength and hardening to the steel, but if used in excess of 0.2%, it impairs the surface finish of the steel. The preferred limit of chromium in the present invention is 0% - 0.15%.

[0023] Nickel may be present as an optional element in an amount of 0.2% or less to increase the strength of the steel and improve its toughness. To achieve such an effect, a minimum of 0.01% is preferred. However, if its content exceeds 0.2%, nickel causes a decrease in ductility and excessively increases the addition cost of alloying elements.

[0024] Molybdenum is an optional element that constitutes 0% - 0.2% of the steel of the present invention. Molybdenum increases 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 addition cost of alloying elements, so its content is limited to 0.2% for economic reasons. The preferred limit of molybdenum is 0% - 0.15%.

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

[0026] Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following weight ratios: cerium ≤ 0.1%, boron ≤ 0.003%, magnesium ≤ 0.010% and zirconium ≤ 0.010%. Up to the indicated maximum contents, these elements enable the refinement of the crystal grains during solidification.

[0027] The remainder of the steel composition consists of iron and unavoidable impurities resulting from the steelmaking 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 charged with scrap, the steel sheet can further contain residual elements derived from such scrap, such as copper, nickel, molybdenum, zinc, antimony, arsenic and lead, up to a cumulative amount of 1% in addition to the amount obtained by the blast furnace route.

[0028] Next, the microstructure of the steel sheet will be described.

[0029] Ferrite constitutes 25% - 65% of the area fraction of the microstructure of the steel of the present invention. Ferrite cumulatively contains polygonal ferrite and acicular ferrite. Ferrite imparts elongation and formability to the steel of the present invention. In order to ensure an elongation of 16%, preferably 18% or more, it is necessary to have 25% ferrite. Ferrite is formed during cooling after hot rolling in the steel of the present invention. However, whenever the ferrite content in the steel of the present invention exceeds 65% by area fraction, the tensile strength is not achieved. Therefore, the preferred limit of the presence of ferrite in the present invention is 30% - 60% by area fraction.

[0030] Bainite constitutes 35% to 75% of the area fraction of the steel microstructure of the present invention. 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 625 MPa, preferably 650 MPa or more, it is necessary to have 35% bainite. Bainite formation begins during the winding process and during cooling after hot rolling, particularly below the Bs temperature. Therefore, the preferred limit of bainite presence in the present invention is 40% to 70% in area fraction.

[0031] Pearlite is an optional microstructure of the steel of the present invention, present in amounts of 0% to 2%. Pearlite can impart strength and toughness to the steel. Pearlite is formed during cooling from the hot rolling temperature to the winding temperature. If 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 optionally be present as trace microstructures at concentrations of 0% to 2%.

[0033] Conventionally, inclusions are known to reduce the ductility and flangability of steel sheets and cause defects such as internal defects. This is because inclusions form voids in the steel during deformation of the steel sheet, promoting ductile fracture and consequently causing degradation of the HER (Heat Recovery Rating). However, the inventors of the present invention are not bound by this phenomenon and have controlled the size and density of inclusions so that the steel of the present invention achieves an HER of more than 55%. The inclusions of the present invention are derived from 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, when measured on any surface of the steel, are present in an amount of at least 200 per square micrometer. A preferred amount of inclusions is 200 to 680 per square micrometer. The proportion of the aforementioned inclusions having a size of 2 microns or more must be controlled to 25% or less, preferably less than 20%, of the total number of inclusions present, so that the steel of the present invention can achieve an HER of 55%.

[0034] The steel plates according to the present invention can be manufactured by any suitable method. A preferred method is to provide a cast semi-finished product of steel having the chemical composition according to the present invention. Casting is carried out continuously or in batches in the form of slabs, with a thickness in the range of 40 mm to 120 mm, 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 slab thickness is 50 mm to 70 mm.

[0035] For example, a slab having the above-described chemical composition may be manufactured by continuous casting, during which the slab may be optionally subjected to direct light reduction during the continuous casting process to avoid central segregation. The slab provided by the continuous casting process may be used directly while still hot after continuous casting, or it may be cooled to room temperature first 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, it is preferable that the slab temperature be sufficiently high so that an excessive load is applied to the rolling mill, allowing the hot rolling to be completed in the 100% austenite region.

[0037] The final rolling pass must be performed at a temperature above 850°C to complete the hot rolling process, as 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 to an average coiling temperature at a cooling rate exceeding 30°C / second, which should be between 525°C and 675°C. Maintaining the average coiling temperature between 550°C and 650°C is preferable 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] The hot-rolled steel sheet may optionally be coated by an industrially known molten dipping coating method.

[0039] If necessary, skin pass rolling may be optionally performed on hot-rolled steel sheets with a minimum skin pass reduction ratio 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 steel of different compositions, and each of these steel sheets is manufactured according to the method parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the steel sheets obtained during the test, and Table 4 summarizes the results of the evaluation of the obtained properties.

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

[0044] Table 2 Table 2 summarizes the annealing method parameters applied to the steels in Table 1. The steel compositions I1, I2 and R1, R2 are useful for manufacturing steel sheets according to the present invention.

[0045] The following processing parameters are the same for all steels listed in Table 1.

[0046] Table 2 is as follows: [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 and the reference steel of the present invention.

[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 more than 350 inclusions per square micrometer, and moreover, more than 400 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 tensile strength, yield strength, and total elongation. Hole expansion tests are performed according to ISO 16630.

[0052] The results of various mechanical tests conducted according to the standards are compiled.

[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: 0.02% ≤ Carbon ≤ 0.06% 1.1% ≤ Manganese ≤ 1.60% 0.01% ≤ Aluminum ≤ 0.08% 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 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% ≤ 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 75% bainite, 25% to 65% 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 inclusions having a size of 2 microns or more account for 25% or less of the total number of inclusions.

2. The hot-rolled steel sheet according to claim 1, wherein the composition contains 0.025% to 0.055% 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, wherein the composition contains 1.15% to 1.55% manganese.

5. A hot-rolled steel sheet according to any one of claims 1 to 4, wherein the composition contains 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 70%.

7. The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a tensile strength of 625 MPa to 850 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 strength of 650 MPa to 825 MPa and a total elongation of 16% 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 rate of 3.5 m / min must be maintained during casting. - The slab provided by the casting process may optionally be used directly while still hot 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 process 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 steel 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 during 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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