Hot-rolled steel sheet and method for producing same

Abstract

Provided are: a hot-rolled steel sheet having a yield strength (YS) of 680 MPa or more and having excellent bending workability and toughness; and a method for producing the hot-rolled steel sheet. The hot-rolled steel sheet has a component composition comprising C, Si, Mn, P, S, Al, N, Ti, and optional components, and has, in terms of the area ratio of the metallographic structure, 0-85% of ferrite, 3% or less in total of retained austenite, 5% or less of the area ratio of a structure having a lath form, 15% or more of the area ratio of a structure having a KAM value of 1.0 or more, and a Ti-containing carbide having an average particle diameter of 8 nm or less. And the yield strength is 680 MPa or more. The manufacturing method of the hot-rolled steel sheet comprises a rough rolling step, a finish rolling step, a cooling step and a coiling step, and is characterized in that in the rough rolling step, a steel billet with the above-mentioned component composition is heated or not heated; in the finish rolling step, the start temperature is 950 DEG C or higher, the total reduction from the first pass to the fifth pass is 75% or higher, and the end temperature is 860-910 DEG C.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet having a yield strength of 680 MPa or more and excellent bendability and toughness, and a method for manufacturing the same. Background Art

[0002] In recent years, from the viewpoint of protecting the global environment, in order to limit CO2 emissions, the entire automotive industry has aimed to improve the fuel consumption of automobiles. For improving the fuel consumption of automobiles, weight reduction of automobiles due to thinning of used components is the most effective. Therefore, in recent years, the usage amount of high-strength steel sheets as base materials for automotive components has been gradually increasing.

[0003] Generally, as the steel sheet becomes higher in strength, formability and toughness tend to deteriorate. Therefore, for further expanding the popularization of high-strength steel sheets, it is necessary to balance high strength, workability, and toughness.

[0004] Therefore, in order to solve these problems, various techniques for increasing the strength of steel sheets and improving workability have been proposed so far.

[0005] For example, Patent Document 1 discloses a hot-rolled steel sheet in which Ti carbides having an average particle size of less than 6 nm and TiS having an average particle size of 0.5 μm or less are dispersed in ferrite crystals having an area ratio of 95% or more. Thereby, a high-tensile hot-rolled steel sheet having good bendability and a tensile strength of 780 MPa to 900 MPa can be obtained.

[0006] Patent Document 2 discloses a technique in which a steel slab containing one or more of Ti and Nb is heated, hot rough-rolled to form a steel sheet, joined to the rear end of a previously rough-rolled steel sheet, and hot finish-rolled in the range of Ar3 to Ar3 + 50°C. Thereby, a hot-rolled steel sheet for working having good toughness can be obtained.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: WO 2013 / 099196

[0010] Patent Document 2: Japanese Patent Laid-Open No. 09-227949 Summary of the Invention

[0011] However, the prior arts disclosed in the above patent documents have the following problems.

[0012] In the technique described in Patent Document 1, for example, as shown in Steel Sheet No. 5 of the examples, the structure required in the present invention cannot be obtained. Therefore, it is impossible to balance a yield strength of 680 MPa or more and good bendability and toughness.

[0013] In addition, in the technology described in Patent Document 2, a high strength with a yield strength of 680 MPa or more cannot be obtained, and Patent Document 2 does not give any hint regarding the elements required to obtain good bendability. Furthermore, controlling the hot rolling temperature within a narrow range in order to obtain high toughness significantly hinders manufacturability, and in some cases, it cannot be implemented depending on the manufacturing dimensions.

[0014] The present invention has been developed in view of the above-mentioned problems of the prior art, and an object thereof is to provide a hot-rolled steel sheet having a yield strength (YS) of 680 MPa or more and excellent bendability and toughness, and a manufacturing method thereof.

[0015] In order to solve the above problems, the inventors have conducted in-depth research on the elements that achieve both bendability and toughness in a hot-rolled steel sheet with a yield strength of 680 MPa or more. In order to obtain good bendability, high ductility is required. Therefore, a process premised on a high coiling temperature was studied. Although this is a condition that is disadvantageous for high strength, a high total elongation rate can be obtained. And in order to obtain a yield strength of 680 MPa or more when the coiling temperature of the hot-rolled steel sheet is 600 °C or more, the hot-rolled steel sheet was strengthened by using carbide containing nano-sized extremely fine Ti.

[0016] However, it has been generally considered so far that when the coiling temperature of a hot-rolled steel sheet in which Ti-containing carbide precipitates is 600 °C or more, a ferrite phase without a large number of dislocations is formed in the steel sheet. The cleavage unit, which has a great influence on the toughness in this ferrite structure, is equivalent to the ferrite grain size. And the refinement of the ferrite phase was studied, and as a result, it was concluded that it is difficult to stably obtain the target toughness.

[0017] Therefore, in-depth research was conducted on the possibility of forming a crystal structure other than ferrite when the coiling temperature of the hot-rolled steel sheet is 600 °C or more. As a result, a new structure that is neither classified as ferrite nor as bainite was obtained, and the strength, bendability, and toughness of this structure are good.

[0018] It was found that this new structure recrystallizes during the finish rolling process of hot rolling and is formed from these fine recrystallized austenite grains.

[0019] The hot-rolled steel sheet of the present invention developed based on the above insights is configured as follows.

[0020] [1] A hot-rolled steel sheet having the following chemical composition:

[0021] Containing, by mass%, C: 0.035% or more and less than 0.110%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% - 0.080%, N: 0.0060% or less, Ti: 0.08% - 0.20%, optionally further containing one or both of the following Group A and Group B,

[0022] Group A: B: 0.0002% - 0.0050%,

[0023] Group B: Any one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total,

[0024] The balance consists of Fe and unavoidable impurities;

[0025] In the above steel plate, in terms of the area ratio of the metallographic structure, ferrite is 0% - 85%, retained austenite is 3% or less, lath-shaped structure is 5% or less, and the structure with a KAM value of 1.0 or more is 15% or more;

[0026] And having Ti-containing carbides with an average particle size of 8 nm or less,

[0027] The yield strength is 680 MPa or more.

[0028] [2] In the hot-rolled steel plate of the above [1], a coating is provided on the surface of the hot-rolled steel plate.

[0029] The manufacturing method of the hot-rolled steel plate of the present invention developed based on the above understanding is configured as follows:

[0030] [3] A manufacturing method of a hot-rolled steel plate, including the following steps:

[0031] Rough rolling step: Heating a steel billet having the composition described in the above [1] to a heating temperature of 1200 °C or more or not heating after casting, and performing rough rolling to form a strip blank;

[0032] Finish rolling step: Finish rolling the strip blank with a starting rolling temperature of 950 °C or more, a total reduction ratio from the first pass to the fifth pass of 75% or more, and an ending rolling temperature of 860 °C - 910 °C to form a hot-rolled steel plate;

[0033] Cooling step: Cooling the hot-rolled steel plate at an average cooling rate of 40 °C / s or more to 600 °C - 700 °C; and

[0034] Coiling step: Coiling the cooled hot-rolled steel plate at a coiling temperature of 600 °C - 700 °C.

[0035] [4] In the method for manufacturing a hot-rolled steel sheet described in [3] above, a casting process is included before the rough rolling process or the finish rolling process: casting a steel billet having the composition described in [1] and having a thickness of 35 mm to 200 mm;

[0036] And, a strip blank is produced with or without applying the above-mentioned rough rolling process.

[0037] [5] In the method for manufacturing a hot-rolled steel sheet described in [3] above, a joining process is included between the rough rolling process and the finish rolling process: joining the rough-rolled strip blank and a previous strip blank at 1050 °C or higher;

[0038] In the above-mentioned finish rolling process, the joined strip blank is finish-rolled.

[0039] [6] In the method for manufacturing a hot-rolled steel sheet according to any one of [3] to [5] above, the following processes are further included:

[0040] A hot-rolled sheet annealing process, annealing the above-mentioned hot-rolled steel sheet at an annealing temperature of 720 °C or lower; and

[0041] A plating process, performing a plating treatment on the annealed above-mentioned hot-rolled steel sheet.

[0042] [7] In the method for manufacturing a hot-rolled steel sheet described in [6] above, an alloying process is further included: performing an alloying treatment on the plated above-mentioned hot-rolled steel sheet at 480 °C to 600 °C.

[0043] According to the present invention, it is possible to manufacture a hot-rolled steel sheet having a high strength with a yield strength (YS) of 680 MPa or more, excellent bend formability, and toughness. The hot-rolled steel sheet according to the present invention is suitable as a blank for automotive suspension system components, and if applied to automotive components, further weight reduction of automotive components can be achieved. Detailed Description

[0044] Hereinafter, the hot-rolled steel sheet of the present embodiment will be described.

[0045] <Chemical Composition of Hot-Rolled Steel Sheet>

[0046] In the composition of the hot-rolled steel sheet, by mass%, C is contained in the following range: 0.035% or more and less than 0.110%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080%, N: 0.0060% or less, Ti: 0.08% to 0.20%. Each component will be described below. In the following description, "%" indicating the content of a component means "mass%".

[0047] C: not less than 0.035% and less than 0.110%

[0048] C contributes to the high strength of the steel plate by combining with Ti and forms a high dislocation structure during isothermal phase transformation. In order to obtain a steel plate with a yield strength of 680 MPa or more, the C content is set to 0.035% or more. On the other hand, if the C content is 0.110% or more, coarse cementite precipitates, increasing the risk of reduced bend formability and toughness. Therefore, the C content is set to not less than 0.035% and less than 0.110%. Preferably, it is 0.035% to 0.10%.

[0049] Si: 1.5% or less

[0050] Si increases the elongation of the steel plate and inhibits the precipitation of cementite, so it is an effective element for improving workability. On the other hand, if the Si content exceeds 1.5%, the improvement effect of bend formability decreases, the surface properties and weldability deteriorate, and the adverse effects caused by a large amount of added Si become significant. Therefore, the Si content is set to 1.5% or less. The Si content is preferably 1.2% or less. It should be noted that even if the Si content is 0%, the effects of the present embodiment are not impaired, but in order to stably generate a structure with a large crystal strain that does not have a lath structure, the Si content is preferably 0.15% or more.

[0051] Mn: 1.3% or less

[0052] Mn increases the hardenability and inhibits the formation of ferrite with a small crystal strain during the cooling process after hot rolling. In order to stably manufacture a hot-rolled steel plate, the Mn content is preferably 0.2% or more. In addition, in order to stably recrystallize austenite during hot rolling, it is preferable that hot working strain stably exists, and it is effective to control the contents of Si and Mn as substitutional solid solution elements within a narrow range. Therefore, it is preferable to satisfy the following formula (1).

[0053] 1.1 ≤ 0.8[%Si] + [%Mn] ≤ 1.5 ···(1)

[0054] Here, [%Si] and [%Mn] refer to the Si content and Mn content in mass%.

[0055] On the other hand, if the Mn content exceeds 1.3%, the driving force for the transformation from austenite to ferrite is excessively reduced, and a structure with a small crystal strain cannot be obtained. Therefore, the Mn content is set to 1.3% or less. The content of Mn is preferably 1.2% or less.

[0056] P: 0.05% or less

[0057] P is a harmful element that segregates at grain boundaries and reduces toughness. Therefore, it is preferably minimized as much as possible. In the present embodiment, the P content is allowed up to 0.05%. The P content is preferably 0.04% or less, but in order to be used in an environment where stricter toughness is required, it is more preferably 0.02% or less. On the other hand, 0.002% of P may be inevitably mixed in during manufacturing.

[0058] S: 0.010% or less

[0059] S forms coarse sulfides in steel, which extend during hot rolling and become wedge-shaped inclusions, thus having an adverse effect on toughness. Therefore, S is also a harmful element and is preferably reduced, and it is allowed up to 0.010%. The S content is preferably 0.003% or less, but in order to be used in an environment where stricter toughness is required, it is more preferably 0.001% or less. 0.0001% of S may be inevitably mixed in during manufacturing.

[0060] Al: 0.005% - 0.080%

[0061] When Al is added as a deoxidizer during the steelmaking stage, the Al content is 0.005% or more. Al reduces bendability and toughness due to the formation of oxides. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.010% - 0.070%.

[0062] N: 0.0060% or less

[0063] Since N combines with Ti to form coarse TiN, which is a harmful element that reduces strength, bendability, and toughness. Therefore, the N content is preferably minimized as much as possible, and it is allowed up to 0.0060%. The N content is preferably 0.0050% or less. Approximately 0.0005% of N may be inevitably mixed in during manufacturing.

[0064] Ti: 0.08% - 0.20%

[0065] Ti combines with C to form fine carbides containing Ti, which contributes to the high strength of the steel plate. In order to obtain a yield strength of 680 MPa or more, the Ti content is 0.08% or more. On the other hand, if the Ti content is higher than 0.20%, the coarse Ti-containing carbides cannot be dissolved in the heating process before hot rolling, not only saturating the effect of high strength but also having an adverse effect on bendability and toughness. Therefore, the Ti content is set to 0.08% - 0.20%. The Ti content is preferably 0.09% - 0.19%.

[0066] In addition, as described above, C helps to form a structure with large crystal strain. On the other hand, by combining with Ti, it is also used to form Ti-containing carbides. Therefore, in the hot-rolled steel sheet according to the present embodiment, in order to stably obtain the required metal structure, it is preferably to satisfy the following formula (2). In particular, if formula (2) is less than 1.4, the C concentration accumulated at the grain boundaries during isothermal phase transformation decreases, and a structure with large crystal strain cannot be stably obtained. Therefore, formula (2) is preferably 1.4 or more.

[0067] On the other hand, in order to obtain a steel sheet with a yield strength of 680 MPa or more, it is necessary to strengthen it with nano-sized fine Ti-containing carbides. However, if formula (2) exceeds 2.8, coarse TiC cannot be dissolved during reheating of the slab, resulting in a decrease in the strength of the steel sheet and a decrease in the bending workability of the steel sheet. Therefore, formula (2) is preferably 2.8 or less.

[0068] 1.4 ≤ ([%C] / 12) / ([%Ti * / 48) ≤ 2.8 ··· (2)

[0069] where, [%Ti * = [%Ti] - 48[%N] / 14.

[0070] Here, [%C], [%Ti], [%N] refer to the C content, Ti content, and N content in mass%.

[0071] The above is the basic composition of the component composition of the hot-rolled steel sheet according to the embodiment, and it may optionally further contain one or both of the following component groups A and B.

[0072] Group A: B: 0.0002% to 0.0050%

[0073] Group B: Any one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total

[0074] B: 0.0002% to 0.0050% B is an effective element for improving hardenability. In order to obtain a structure with large crystal strain, it is necessary to ensure hardenability. By setting the B content to 0.0002% or more, it helps to stably obtain the desired structure. On the other hand, if the B content exceeds 0.0050%, the effect on the hardenability of the steel is saturated, so it is set to 0.0050% or less. The B content is more preferably 0.0004% to 0.0030%.

[0075] Any one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total

[0076] If it contains any one or more within the range of 1% or less in total, the influence on the properties of the hot-rolled steel sheet according to this embodiment is small, so it is allowable. On the other hand, it is preferable to limit the content of each element to 0.03% or less.

[0077] The chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, and the remainder is Fe and inevitable impurities.

[0078] <Metallographic structure of hot-rolled steel sheet>

[0079] Next, the metallographic structure of the hot-rolled steel sheet will be described.

[0080] In the metallographic structure of the hot-rolled steel sheet according to this embodiment, the area ratio of ferrite is 0% to 85%, the area ratio of retained austenite is 3% or less, the area ratio of the structure with a lath morphology is 5% or less, the area ratio of the structure with a KAM value of 1.0 or more is 15% or more, and it has Ti-containing carbides with an average particle size of 8 nm or less.

[0081] In the following description, "%" indicating the metallographic structure means "area ratio".

[0082] The area ratio of ferrite is 0% to 85%

[0083] Compared with the new structure with large crystal strain in this embodiment, ferrite has a large cleavage unit during brittle fracture, so it is a structure with poor toughness. Since the crystal strain of ferrite in the grain is small, the KAM value is less than 1.0. In order to obtain the desired toughness, it is necessary to limit the area ratio of ferrite to 85% or less. The area ratio of ferrite is preferably 80% or less, more preferably 70% or less.

[0084] Retained austenite is 3% or less (including 0%)

[0085] In this embodiment, bainite and tempered martensite specified are observed as lath structures within grains. Martensite is a structure observed as white contrast on SEM. Since there is a possibility of cementite, it can be separated by crystal structure through Electron BackScatter Diffraction pattern (EBSD) analysis. For example, for bainite, martensite, and tempered martensite that satisfy the relationship between the parent phase and Kurdjumov-Sachs, it can be determined whether they correspond by obtaining the (001)α pole figure of a single original γ grain region. Retained austenite can be obtained by grinding the steel plate surface from the surface to 1 / 4 of the plate thickness, and then performing XRD analysis using a sample chemically polished by 0.1 mm or more. These structures reduce the strength, workability, and toughness of the hot-rolled steel plate of this embodiment. These structures are preferably minimized as much as possible, and the retained austenite is set to 3% or less. The total of bainite, martensite, tempered martensite, martensite, and retained austenite is preferably 5% or less, and more preferably 3% or less.

[0086] The most significant technical feature of this embodiment is that the area ratio of the structure with a lath morphology is 5% or less, the area ratio of the structure with a KAM value of 1.0 or more is 15% or more, and the crystal strain large structure without a lath structure is strengthened by Ti-containing carbides of 8 nm or less.

[0087] The crystal strain of ferrite is small, i.e., the KAM value is less than 1. Low-temperature phase transformations such as bainite, martensite, and tempered martensite have a lath structure. Tissues with large crystal strains that do not have a lath structure cannot be classified as ferrite or bainite. Laths are tissues observed in the form of plates in grains through transmission electron microscopy (TEM) and EBSD analysis. Tissues with this lath structure are hard but lack workability and cannot obtain the desired bendability. The tissue with a large crystal strain in the present invention refers to a tissue with a KAM value of 1.0 or more obtained through EBSD analysis. The KAM value represents the disorder of the crystal structure. Due to the disorder of this crystal, the effective cleavage unit is refined, making it stronger and tougher than ferrite-structured steel. Based on the above, through this tissue, a steel plate with good workability and toughness can be obtained. Therefore, a tissue without a lath structure means that the area ratio of the tissue with a lath morphology is 5% or less, and a tissue with a large crystal strain means that the area of the tissue with a KAM value of 1.0 or more is 15% or more. More preferably, the area of the tissue with a KAM value of 1.0 or more is 20% or more. It should be noted that the KAM value of crystal grain boundaries is mostly 1.0 or more. Even in a single-phase ferrite tissue, the area ratio of the tissue with a KAM value of 1.0 or more is not 0%, but inevitably contains about 3%. Since the measurement of the ferrite area ratio is judged based on the morphology of the grains and the grain boundaries are excluded, the sum of the area ratio with a KAM value of 1.0 or more and the ferrite area ratio sometimes exceeds 100%.

[0088] Ti-containing carbide with an average particle size of 8 nm or less

[0089] In the present embodiment, the steel plate is strengthened using Ti-containing carbide. In order to obtain a high-strength hot-rolled steel plate with a yield strength of 680 MPa or more, the average particle size of the Ti-containing carbide dispersed in the steel needs to be set to 8 nm or less. In order to stably obtain a strength of 680 MPa or more in yield strength, the average particle size of the Ti-containing carbide is preferably 5 nm or less.

[0090] In addition, if the coiling temperature is set to 600 °C or more, even substitutional elements can have Ti sufficiently dispersed in the steel. Utilizing this property of Ti, Ti diffuses and precipitates, so that even if the tissues of bainite, martensite, and tempered martensite, which are often used in high-strength steel plates, are in small amounts, a steel plate with a yield strength of 680 MPa or more can be obtained. In order to obtain a steel plate with a yield strength of 680 MPa or more, 80% or more of the contained Ti is used for precipitation. Preferably, 85% or more of the contained Ti is used.

[0091] The hot-rolled steel plate according to the present embodiment preferably has a coating on the surface. Even if a coating is formed, the function of the hot-rolled steel plate is not impaired. The composition of the coating is preferably selected from one or more of Zn, Si, Al, Ni, and Mg.

[0092] It should be noted that the coated steel sheet in this embodiment may be any one of a steel sheet (GI) obtained by hot-dip galvanizing treatment, a steel sheet (GA) obtained by further alloying treatment after hot-dip galvanizing treatment, and a steel sheet (EG) obtained by electro-galvanizing treatment.

[0093] Next, a first method for manufacturing the hot-rolled steel sheet according to this embodiment will be described.

[0094] Generally, in the manufacture of hot-rolled steel sheets, after casting, a slab (billet) whose temperature has been reduced to below 1000 °C is charged into a heating furnace, heated for a short time, and then reduced to a specified thickness on a hot-rolling production line and coiled into a steel coil. Or after casting, a slab (billet) temporarily cooled to room temperature is heated in a heating furnace for a long time, and then reduced to a specified thickness on a hot-rolling production line and coiled into a steel coil. There is also the following manufacturing method: a cast slab (billet) is directly sent to a hot-rolling production line without being heated in a heating furnace, reduced to a specified thickness, and coiled into a steel coil.

[0095] In the manufacturing method of the hot-rolled steel sheet of this embodiment, it is applicable not only to the process of heating the billet after casting, but also to the process of directly sending the billet to the hot-rolling production line without heating the billet after casting.

[0096] <Steel billet of the first method>

[0097] The melting method for manufacturing the billet in this embodiment is not particularly limited, and known melting methods such as a converter and an electric furnace can be used. In addition, secondary refining can be carried out in a vacuum degassing furnace. Then, considering productivity and quality, it is preferable to make the molten steel adjusted to the above composition into a slab (billet) by a continuous casting method. In addition, a slab can be formed by an ingot - blooming rolling method or other known casting methods.

[0098] <Rough rolling process of the first method>

[0099] In this embodiment, the billet is heated to a heating temperature of 1200 °C or higher, or the billet is rough-rolled without heating after casting to form a strip blank.

[0100] <Finish rolling process of the first method>

[0101] Next, hot rolling is carried out with the start temperature of finish rolling being 950 °C or higher, the total reduction ratio from the first pass to the fifth pass being 75% or higher, and the end temperature of finish rolling being 860 °C to 910 °C to produce a hot-rolled steel sheet.

[0102] <Cooling process of the first method>

[0103] Next, the hot-rolled steel sheet is cooled at an average cooling rate of 40°C / s or more to a cooling stop temperature of 600°C to 700°C.

[0104] <Coiling process of the first method>

[0105] Then, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C to 700°C.

[0106] Heating of the steel billet: Heating to 1200°C or more or not heating

[0107] Dissolve the coarse Ti-containing carbides precipitated in the slab (steel billet) in the heating process before hot rolling, thereby precipitating fine Ti-containing carbides after hot rolling. Therefore, in order to obtain Ti-containing carbides with an average particle size of 8 nm or less of the carbides, the slab (steel billet) is heated to 1200°C or more. Preferably 1220°C or more, and when the Ti content is 0.12% or more, it is more preferably to heat the slab (steel billet) to 1240°C or more. There is no particular upper limit set, but in order to avoid thermal damage to the heating furnace, it is limited to 1300°C in manufacturing.

[0108] When the steel billet maintained at 1200°C or more after casting is directly sent to the hot rolling production line, the cast steel billet is not heated.

[0109] Start temperature of finish rolling: 950°C or more, total reduction ratio from the first pass to the fifth pass: 75% or more, finish temperature of finish rolling: 860°C to 910°C

[0110] In the hot-rolled steel sheet according to this embodiment, in order to generate a structure with a large crystal strain as a feature, it is necessary to precisely control the hot rolling conditions. Specifically, fine austenite is formed by austenite recrystallization during finish rolling. Therefore, the start temperature of finish rolling is set to 950°C or more, and the total reduction ratio from the first pass to the fifth pass is set to 75% or more. If it is within the range of the chemical composition of the hot-rolled steel sheet according to this embodiment, austenite recrystallization occurs in the finish rolling stands after the fifth pass.

[0111] Therefore, finish rolling is carried out 5 passes or more. If the start temperature of finish rolling is lower than 950°C, austenite recrystallizes early during finish rolling, and the recrystallized austenite is rolled again. As a result, ferrite is generated and a structure with a large crystal strain cannot be obtained.

[0112] If the start temperature of finish rolling is higher than 1100°C, the possibility of no austenite recrystallization occurring in the finish rolling stands increases, so the start temperature of finish rolling is preferably 1100°C or less.

[0113] If the finish temperature of finish rolling is lower than 860°C, the risk of ferrite generation during rolling increases.

[0114] On the other hand, if the finish rolling temperature is higher than 910 °C, austenite cannot recrystallize during finish rolling. Therefore, the finish rolling temperature is set to 860 °C to 910 °C. In order to stably obtain recrystallization of austenite, the finish rolling temperature is preferably 890 °C or lower.

[0115] In order to recrystallize austenite during finish rolling, as described above, it is necessary to accumulate strain from the first pass to the fifth pass of finish rolling. Therefore, if the rolling interval from the first pass to the fifth pass becomes longer, the strain imparted by rolling recovers, and austenite cannot stably recrystallize during finish rolling. Therefore, from the viewpoint of avoiding the adverse effects of this austenite recovery, the rolling interval time from the first pass to the fifth pass is preferably at least 1.5 seconds or less.

[0116] Until the cooling stop temperature after finish rolling reaches 600 °C to 700 °C, the average cooling rate is 40 °C / s or more

[0117] If the cooling rate after hot rolling to below 700 °C is slow, coarse polygonal ferrite (ferrite) with small crystal strain within the grains is formed at high temperatures. In order to suppress the formation of this ferrite, it is necessary to cool at an average cooling rate of 40 °C / s or more after hot rolling, and it is preferably cooled to below 700 °C at an average cooling rate of 50 °C / s within 2 s after hot rolling.

[0118] On the other hand, if the cooling stop temperature is lower than 600 °C, it is difficult to obtain Ti-containing carbides, and a steel sheet with a yield strength of 680 MPa or more cannot be obtained.

[0119] Therefore, the range of the cooling stop temperature is set to 600 °C to 700 °C. The range of the cooling stop temperature is preferably 610 °C to 690 °C. Here, the average cooling rate can be calculated by {(cooling start temperature) - (cooling end temperature)} / (forced cooling time other than air cooling) during forced cooling other than air cooling after hot rolling. As a method of forced cooling, for example, water cooling can be cited.

[0120] Coiling temperature: 600 °C to 700 °C

[0121] For the same reason as the cooling stop temperature, the coiling temperature is set to 600 °C to 700 °C. It is preferably 610 °C to 690 °C. If coiling is performed in this temperature range, the formation of ferrite, bainite, martensite, and retained austenite can be extremely suppressed.

[0122] Next, a second method of the manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described. In the present embodiment, differences from the first method will be described.

[0123] <Casting process of the second method>

[0124] The hot-rolled steel sheet according to this embodiment can also be manufactured by the thin slab continuous casting method. In the case of manufacturing by the thin slab continuous casting method, a steel slab with a casting thickness of 35 mm to 200 mm is cast.

[0125] <Rough rolling process of the second method>

[0126] The above-mentioned cast steel slab is heated to a heating temperature of 1200 °C or higher, or is not heated after casting and is rough rolled as needed to form a strip blank.

[0127] The finishing rolling process and subsequent processes are the same as those in the first method.

[0128] Here, the thickness of the slab (steel slab) unique to the thin slab continuous casting method will be described.

[0129] Slab (steel slab) thickness: 35 mm to 200 mm

[0130] The thin slab continuous casting method is different from the continuous casting method. Since the slab before hot rolling is thin, the degree of working of austenite during hot rolling is low. If the slab thickness is less than 35 mm, the desired total reduction ratio from the first pass to the fifth pass cannot be obtained. On the other hand, if the slab thickness is greater than 200 mm, the casting speed becomes slow, and compared with the continuous casting method, the productivity advantage of the thin slab continuous casting method is lost. From the above viewpoints, the slab thickness in the thin slab continuous casting method is set to 35 mm to 200 mm.

[0131] Next, a third method for manufacturing the hot-rolled steel sheet according to this embodiment will be described. In this embodiment, the differences from the first method and the second method will be described. The third method can apply hot continuous rolling technology.

[0132] <Joining process of the third method>

[0133] The strip blank obtained in the first method or the second method is joined with a previous strip blank at 1050 °C or higher before finishing rolling. If it is lower than 1050 °C, it is difficult to perform rolling at a finishing rolling start temperature of 950 °C or higher. The heating temperature of the strip blank during joining is preferably 1070 °C or higher.

[0134] The cooling process and subsequent processes are the same as those in the first method.

[0135] In the method for manufacturing a hot-rolled steel sheet according to this embodiment, an annealing process of annealing on a continuous annealing production line at an annealing temperature of 720 °C or lower and a plating process of plating on a continuous plating production line can be applied. In addition, an alloying process of heating the hot-rolled steel sheet obtained by the plating treatment to 480 °C to 600 °C and performing an alloying treatment can be further provided. Even if this annealing treatment or this plating treatment is performed, it will not affect the material of the hot-rolled steel sheet according to this embodiment. Therefore, by further performing a plating treatment on the surface of the hot-rolled steel sheet, a coating layer can be formed on the steel sheet surface.

[0136] In addition, as described above, since the plating treatment and the composition of the plating bath do not affect the material of the hot-rolled steel sheet according to this embodiment, any one of hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, and electro-galvanizing treatment can be applied as the plating treatment. The composition of the plating bath can contain one or more of Zn, Al, Mg, Si, and Ni. That is, in the plating treatment, the composition of the coating layer formed on the surface of the hot-rolled steel sheet can contain one or more of Zn, Si, Al, Ni, and Mg.

[0137] Examples

[0138] The embodiments of the present invention will be further described through examples. It should be noted that the present invention is not limited to the manufacturing conditions and product performances shown in the following examples. An embodiment is a mode in which desired performances can be achieved within the scope of the present invention.

[0139] <First mode based on the continuous casting method>

[0140] A steel billet with a thickness of 250 mm having the composition shown in Table 1 is hot-rolled under the rough rolling and finish rolling conditions shown in Table 2, and then after performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, a steel sheet for evaluation is manufactured.

[0141] <Second mode based on the thin slab continuous casting method>

[0142] The steel having the composition shown in Table 1 is subjected to thin slab hot rolling under the conditions shown in Table 3, and after performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, a steel sheet for evaluation is manufactured.

[0143] <Third mode based on the hot continuous rolling method>

[0144] The steel having the composition shown in Table 1 is subjected to strip bonding under the conditions shown in Table 4, the strip obtained by this bonding is hot-rolled, and after performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, it is used for manufacturing an evaluation steel sheet.

[0145] <Manufacturing method for applying a coating layer to a hot-rolled steel sheet>

[0146] The hot-rolled steel coils manufactured under the conditions of Table 2 are pickled, and then Zn plating treatment is performed on the hot-rolled steel sheets on a continuous hot-dip coating production line (CGL) according to the conditions shown in Table 5. Thus, continuously hot-dip coated steel sheets (GI) and alloyed hot-dip coated steel sheets (GA) are manufactured.

[0147]

[0148]

[0149] [Table 3]

[0150]

[0151] [Table 4]

[0152]

[0153] [Table 5]

[0154]

[0155]

[0156] From the viewpoints of metallographic structure, tensile properties, bend formability, and toughness, the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 are evaluated by the following method. The results are shown in Table 6.

[0157] (i) Area ratio of metallographic structure

[0158] A test piece is cut from the hot-rolled steel sheet in such a way that the cross-section parallel to the rolling direction is used as the observation surface, and the center part of the plate thickness is etched with 1% nitric acid ethanol to reveal the structure. It is photographed at 10 fields of view at a plate thickness of 1 / 4t at an acceleration voltage of 15 kV with a scanning electron microscope (SEM) magnified 2000 times.

[0159] Ferrite is a grain in which no corrosion marks are found inside the grain and the brightness is lower than that of martensite (gray in SEM). Bainite and tempered martensite are grains in which three or more lath-shaped corrosion marks with a width of 500 nm or less are adjacent inside the grain. Martensite is a grain in which no corrosion marks are found inside the grain, but the brightness is higher than that of ferrite (white in SEM). The area ratio of the metallographic structure separated in the above manner is obtained using image analysis software (Photoshop elements and Image J).

[0160] For retained austenite, the surface of the test piece was ground to 3 / 4 of the total thickness, and then chemically polished by more than 0.1 mm. The polished surface was measured by X-ray diffraction method. For the volume fraction of retained austenite, using MoKα ray as the incident ray source, the measurement was carried out according to the peaks of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ. Thus, the volume fraction of the obtained retained austenite phase was used as the area fraction of retained austenite.

[0161] The area fraction of the structure with large crystal strain without lath structure was measured by SEM and EBSD methods. Before observation, the test piece was marked using a Vickers hardness tester or the like to obtain the same field of view in SEM and EBSD methods before observation. When observing by SEM, the structure with large crystal strain without lath structure has corrosion marks in the grain. At this time, depending on the shape of the corrosion marks, although it is not a lath, the corrosion marks may look like a lath. In this case, in order to distinguish the structure that looks like a lath from the lath structure, a rectangular structure with a width exceeding 500 nm on the short side of the grain and generated in two or fewer adjacent grains was not regarded as a lath structure. A structure with a width of 500 nm or less on the short side of the grain and three or more adjacent structures was regarded as the lath structure observed in bainite and tempered martensite. If observed by transmission electron microscope (TEM), the lath structure can be more clearly distinguished. And, EBSD analysis was carried out using OIM Analysis software (TSL Corporation). The analysis of the KAM value was carried out under the condition of 1st nearest neighbor.

[0162] Through EBSD analysis, in the grains surrounded by large-angle grain boundaries with an angular difference of 15° or more, the structure with a KAM value exceeding 1.0 and without lath structure was regarded as the structure with large crystal strain without lath structure, and its area fraction was obtained in a field of view of 1 mm 2 or more.

[0163] (ii) Average particle size of Ti-containing carbides

[0164] Observation films were taken from a position equivalent to 1 / 4 of the plate thickness of the hot-rolled steel plate, and more than 300 Ti-containing carbides were photographed at a magnification of 600,000 times or more by transmission electron microscope. The equivalent circle diameter of the photographed Ti-containing carbides was obtained, and its average value was used as the average particle size. The determination of Ti-containing carbides can be confirmed by the presence or absence of peaks from Ti using EDX attached to TEM.

[0165] (iii) Precipitation amount analysis of Ti-containing carbides

[0166] Grind both the front and back sides of the test piece by 25% of each plate thickness. Next, dissolve it in a 10% AA electrolytic solution, filter the dissolved solution with a membrane having a mesh diameter of 0.2 μm, and analyze the Ti concentration contained in the filtered electrolytic solution using ICP-MS. Furthermore, calculate the amount of Ti precipitated as TiN according to [the amount of Ti contained] × 48 / 14. In addition, calculate the amount of Ti precipitated as TiS according to [the amount of Ti contained] × 48 / 32. And subtract the Ti concentration contained in the electrolytic solution, the amount of Ti precipitated as TiN, and the amount of Ti precipitated as TiS from the amount of Ti contained, thereby obtaining the precipitation amount of Ti-containing carbide.

[0167] (iv) Tensile test

[0168] Make JIS No. 5 tensile test pieces from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 in the direction perpendicular to the rolling direction, and conduct 5 tensile tests according to the provisions of JIS Z 2241 (2011) to obtain the average yield strength (YS) and tensile strength (TS). The crosshead speed of the tensile test is 10 mm / min.

[0169] In Table 6, those with a yield strength of 680 MPa or more are regarded as inventive examples.

[0170] (v) Bending test

[0171] Take test pieces with a width of 35 mm and a length of 100 mm whose end faces have been ground from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, and conduct 5 bending tests using the V-block method described in JIS Z 2248.

[0172] Test pieces with R / t of 0.5 or less have the characteristics required by the present invention, denoted as "〇", and under the condition of R / t of 0.5 or less, test pieces with cracks observed on the surface of the test piece one or more times do not have the characteristics required by the present invention, denoted as "×".

[0173] (vi) Charpy impact test

[0174] Take V-notch test pieces described in JIS Z 2242 from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 in such a way that the long side direction is the normal direction with respect to the rolling direction. When the thickness of the hot-rolled steel sheet is less than 10 mm, stack multiple test pieces, open holes at the ends of the test pieces, and adjust the thickness to 10 ± 1 mm by bolt connection. Immerse the test pieces in a bath adjusted to -40 °C for more than 10 minutes, and then conduct the test according to the method of JIS Z 2242. Show the test results in Table 6. At this time, the absorbed energy is 30 J / cm 2 The above is the characteristic required by the present invention, denoted as "〇", less than 30 J / cm 2The level is not a characteristic required by the present invention and is denoted as "×".

[0175] In the examples of the present invention, the yield strength (YS) is all 680 MPa or more, and good bendability and toughness can be obtained. On the other hand, in the comparative examples outside the scope of the present invention, the yield strength does not reach 680 MPa or the bendability or toughness required by the present invention is not obtained.

Claims

1. A hot-rolled steel sheet having the following composition: By mass%, it contains C: not less than 0.035% and less than 0.110%, Si: not more than 1.5%, Mn: not more than 1.3%, P: not more than 0.05%, S: not more than 0.010%, Al: 0.005% - 0.080%, N: not more than 0.0060%, Ti: 0.08% - 0.20%, and optionally further contains one or both of the following components in Group A and Group B, Group A: B: 0.0002% - 0.0050%, Group B: Any one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total, The balance consists of Fe and unavoidable impurities; In the steel sheet, based on the area ratio of the metallographic structure, ferrite is 0% - 85%, retained austenite is 3% or less, lath-shaped structure is 5% or less, and the structure with a KAM value of 1.0 or more is 15% or more; It has Ti-containing carbides with an average particle size of 8 nm or less, The yield strength is 680 MPa or more.

2. The hot-rolled steel sheet according to claim 1, characterized in that, The surface of the hot-rolled steel sheet has a coating.

3. A method for manufacturing a hot-rolled steel sheet, characterized in that, It includes the following processes: Rough rolling process, heating the steel billet having the composition described in claim 1 to a heating temperature of 1200 °C or higher or performing rough rolling without heating after casting to form a strip blank; Finish rolling process, finish rolling the strip blank with a starting rolling temperature of 950 °C or higher, a total reduction ratio from the first pass to the fifth pass of 75% or higher, and a finishing rolling temperature of 860 °C - 910 °C to form a hot-rolled steel sheet; Cooling process, cooling the hot-rolled steel sheet to a cooling stop temperature of 600 °C - 700 °C at an average cooling rate of 40 °C / s or higher; And Coiling process, coiling the cooled hot-rolled steel sheet at a coiling temperature of 600 °C - 700 °C.

4. The manufacturing method of the hot-rolled steel sheet according to claim 3, characterized in that, Before the rough rolling process or the finish rolling process, it includes a casting process: casting a steel billet having the composition described in claim 1 and a thickness of 35 mm - 200 mm; And, forming a strip blank with or without applying the rough rolling process.

5. The manufacturing method of the hot-rolled steel sheet according to claim 3, characterized in that, Between the rough rolling process and the finish rolling process, it includes a joining process: joining the rough-rolled strip blank with a previous strip blank at 1050 °C or higher; In the finish rolling process, finish rolling the joined strip blank.

6. The manufacturing method of the hot-rolled steel sheet according to any one of claims 3 to 5, characterized in that, It further includes the following processes: Hot-rolled sheet annealing process, annealing the hot-rolled steel sheet at an annealing temperature of 720 °C or lower; and Coating process, performing a coating treatment on the annealed hot-rolled steel sheet.

7. The manufacturing method of the hot-rolled steel sheet according to claim 6, characterized in that, It further includes an alloying process: performing an alloying treatment on the coated hot-rolled steel sheet at 480 °C - 600 °C.

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