JP4923982B2 - High-strength hot-rolled steel sheet with excellent stretch flange characteristics and stretch characteristics after processing - Google Patents

Description

The present invention is, stretch flangeability and elongation properties to the excellent tensile strength after optimal processed into suspension member or truck frame of a motor vehicle (TS) is concerned to the above high-strength hot-rolled steel sheet 780 MPa.

For automobile undercarriage members and truck frames, since excellent stretch flange characteristics and stretch characteristics are required, hot rolled steel sheets with a TS of 590 MPa have been used in the past. However, in recent years, from the viewpoint of reducing the environmental load of automobiles and improving impact characteristics, the strength of automobile steel sheets has been increased, and the use of hot-rolled steel sheets with a TS of 780 MPa class has begun to be studied. In steel materials, workability generally decreases as strength increases, so research on high-strength hot-rolled steel sheets with excellent workability has been conducted, but technology to improve stretch flange characteristics and stretch characteristics For example, Patent Document 1 includes a composite structure containing 5 to 70% by volume of bainite and the balance being substantially made of ferrite, and (Mo / 96) / {(Ti / 48 ) + (Mo / 96)} ≧ 0.25 A high-tensile hot-rolled steel sheet having excellent workability with TS of 690 MPa or more in which precipitates containing Ti and Mo are dispersed and precipitated is disclosed. Patent Document 2 discloses a high-tensile steel sheet having a workability of 590 MPa or more with a TS of 590 MPa or more, in which carbides containing Ti and Mo having an average particle diameter of less than 10 nm are dispersed and precipitated, which is substantially a ferrite single-phase structure. ing. Patent Document 3 contains C: 0.03% by mass or less, and in a high-strength steel sheet manufactured by hot rolling or heat treatment, the main phase of the microstructure is ferrite or bainite, and the occupation ratio of iron carbide at the grain boundaries No. 0.1 or less and the maximum particle size of this iron carbide is 1 μm or less, and a high workability high-strength steel sheet excellent in fatigue characteristics and local deformability is disclosed. Patent Document 4 includes, in mass%, C: 0.01 to 0.07%, N: 0.005% or less, S: 0.005% or less, Ti: 0.03 to 0.2%, B: 0.0002 to 0.002%, the balance being Fe and Punching workability with inevitable impurities, with ferrite or bainitic ferrite structure as the largest area ratio phase, hard second phase and cementite less than 3% area ratio, and TS of 690 MPa or more A high-strength hot-rolled steel sheet excellent in the above is disclosed. Patent Document 5 contains, in mass%, C: 0.02 to 0.20%, Mo: 0.1 to 0.8%, Ti: 0.02 to 0.40% and Zr: 0.0005 to 0.005%, and is substantially a ferrite single phase structure. A high-strength hot-rolled steel sheet having excellent workability with TS having a small anisotropy of r-value where carbides containing Ti and Mo having an average particle diameter of 10 nm or less are precipitated is disclosed. In Patent Document 6, in the hot dip galvanized steel sheet provided with a hot dip galvanized layer on the surface of the steel sheet, the chemical composition of the steel sheet is, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0%, Contains Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.0004-0.015%, Ti: 0.03-0.2%, the balance being Fe and inevitable While being an impurity, the metal structure of the steel sheet contains ferrite in an area ratio of 30 to 95%, and the remaining second phase is from one or more of martensite, bainite, pearlite, cementite, and retained austenite. When the martensite is contained, the area ratio of martensite is 0 to 50%, and the steel sheet contains Ti carbonitride precipitates having a particle size of 2 to 30 nm with an average interparticle distance of 30 to 300 nm. High-tensile hot-dip galvanizing containing crystallized TiN with a particle size of 3 μm or more with an average inter-particle distance of 50 to 500 μm The plate is disclosed. Patent Document 7 contains, in mass%, C: 0.0002 to 0.25%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0% and Al: 0.002 to 2.0%, and the balance consists of Fe and impurities, impurities Among them, P is 0.15% or less, S is 0.05% or less and N is 0.01% or less, and 70% or more of the metal structure in the area ratio is a ferrite phase, the average crystal grain size is 20 μm or less, and the aspect ratio is 3 or less. Furthermore, 70% or more of the ferrite grain boundaries are composed of large-angle grain boundaries, the maximum diameter of the ferrite phase formed at the large-angle grain boundaries is 30 μm or less, and the area ratio of precipitates with a minimum diameter of 5 nm or more is metal. The average crystal grain size of the second phase, which is 2% or less of the structure and has the largest area ratio among the remaining phases excluding the ferrite phase and precipitates, is 20 μm or less, and the large angle of the ferrite phase between the closest second phases A hot-rolled steel sheet having grain boundaries is disclosed. Patent Document 8 contains, in mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, and Ti- (48/12) C- (48/14 ) N- (48/32) S is a steel containing Ti that satisfies S ≧ 0%, the balance being Fe and unavoidable impurities, and the average size of precipitates containing Ti of 5 nm or more with particles in the steel A hot-rolled steel sheet having excellent burring workability and fatigue characteristics, which has a minimum interval of 10 nm to 10000 nm and a thickness of 10 to 1000 nm, is disclosed.
Japanese Patent Laid-Open No. 2003-321739 Japanese Patent No. 3595502 Japanese Patent Laid-Open No. 5-295485 JP 2004-315857 JP JP 2006-124789 A JP 2006-63360 A JP 2003-293083 JP JP 2002-161340 A

However, the above prior art has the following problems.
Patent Documents 1, 2, and 5: Mo, which has been increasing in price in recent years, is used, resulting in a significant increase in cost.
Patent Documents 3, 4, 6, 7, and 8: Due to recent advances in press technology, a process such as drawing (drawing and overhanging) → trimming (hole punching) → restriking (hole expanding) is performed at the stretched flange deformation site. The adoption of is increasing. Therefore, the stretch flange characteristics and stretch characteristics after draw trimming, that is, after the process, are important for the steel sheets to be formed through such processing steps, but in the steel sheets of Patent Documents 3, 4, 6, 7, and 8, When trying to obtain TS of 780 MPa or more, it is not always possible to obtain sufficient stretch flange characteristics and stretch characteristics after processing.

The present invention, without using an expensive Mo, stretch flangeability and elongation properties superior TS after processing and to provide a more high-strength hot-rolled steel sheet 780 MPa.

The target characteristics of the present invention are as follows.
TS ≧ 780MPa
Stretch flange characteristics after rolling at an elongation rate of 10% (λ ₁₀ ) ≧ 60%
Elongation characteristics after rolling at an elongation of 10% (El ₁₀ ) ≧ 17%

  The inventors of the present invention have studied the high strength hot-rolled steel sheet having a TS of 780 MPa or more, which is excellent in stretch flange characteristics and stretch characteristics after processing without using expensive Mo, and found the following.

  i) When Zr is added, precipitates containing Ti and / or V in the ferrite are finely dispersed, which is effective in increasing the strength.

  ii) When Zr is added, stretch flangeability and stretch characteristics after processing are improved.

  The present invention has been made based on such knowledge, in mass%, C: 0.06-0.15%, Si: 1.2% or less, Mn: 0.5-1.6%, P: 0.04% or less, S: 0.005% or less , Al: 0.05% or less, Ti: 0.03-0.20%, V: 0.03-0.15%, Zr: 0.0005-0.005%, the balance is composed of Fe and inevitable impurities, and the volume fraction of ferrite Is 50 to 95%, the volume fraction of cementite precipitated at the ferrite grain boundaries is 4% or less, the balance is substantially bainite, and precipitates containing Ti and / or V are precipitated in the ferrite, Provided is a high-strength hot-rolled steel sheet having a TS of 780 MPa or more excellent in stretch flange characteristics and elongation characteristics after processing, characterized by having a microstructure in which the average diameter of the precipitate is 20 nm or less.

  The high-strength hot-rolled steel sheet of the present invention can further contain at least one of Nb: 0.001 to 0.10% and Cr: 0.01 to 0.5% by mass%.

Further, it is preferable that the following Ms is 0.5 or more and the following M ₀ is 0.4 or less.
Ms = (Ti ₂₀ / Ti _total + V ₂₀ / V _total ) / 2
M ₀ = (Ti _sol / Ti + V _sol / V) / 2
In the above formula, Ti ₂₀ and V ₂₀ are the contents of each element in the precipitate having a diameter of 20 nm or less (mass% relative to steel), Ti _total and V _total are the contents of each element in the total precipitate. (Mass% relative to steel), Ti _sol and V _sol represent the solid solution amount (mass%) of each element in the steel, and Ti and V represent the content (mass%) of each element in the steel. .

  The high-strength hot-rolled steel sheet of the present invention is a steel slab having the above component composition, heated to 1150-1300 ° C, hot-rolled at a finishing rolling temperature of 850-1100 ° C, and stopped at 650-800 ° C. The first forced cooling is performed at an average cooling rate of 30 ° C / s or higher until the temperature, and after air cooling for 3 to 15 seconds, the second forced cooling is performed at an average cooling rate of 20 ° C / s or higher, and the winding is performed at 300 to 600 ° C. It can manufacture by the manufacturing method wound up at the taking temperature.

In the above manufacturing method, it is preferable that the cooling stop temperature of the first forced cooling is 680 to 770 ° C.

  According to the present invention, a high-strength hot-rolled steel sheet having a TS of 780 MPa or more excellent in stretch flange characteristics and stretch characteristics after processing can be produced without using expensive Mo. By using the high-strength hot-rolled steel sheet of the present invention, it is possible to reduce the plate thickness of automobile underbody members, truck frames, etc., and it is expected that the environmental load of the automobile will be reduced and the impact characteristics will be greatly improved. .

  Hereinafter, the present invention will be specifically described. Unless otherwise specified, “%” in relation to ingredients means “% by mass”.

1) ingredients
C: 0.06-0.15%
C is an element that precipitates in the ferrite as a carbide containing Ti and V, and generates bainite to contribute to high strength. In order to increase TS to 780 MPa or more, the C amount needs to be 0.06% or more. However, if the C content exceeds 0.15%, weldability deteriorates, so the upper limit is made 0.15%. A more preferable range of C content is 0.07 to 0.12%.

Si: 1.2% or less
Since Si has a function of promoting ferrite transformation and a function of strengthening solid solution, the Si content is preferably 0.1% or more. However, if the amount exceeds 1.2%, the surface properties of the steel sheet are remarkably deteriorated and the corrosion resistance is also lowered. Therefore, the upper limit of the Si amount is set to 1.2%. A more preferable range of Si content is 0.2 to 1.0%.

Mn: 0.5-1.6%
Mn is an element effective for increasing the strength by solid solution strengthening. However, if the amount is less than 0.5%, a TS of 780 MPa or more cannot be obtained. On the other hand, if the amount exceeds 1.6%, the weldability is significantly reduced. Therefore, the amount of Mn needs to be 0.5 to 1.6%, preferably 0.8 to 1.2%.

P: 0.04% or less
P segregates at the prior γ grain boundaries to deteriorate the low temperature toughness and lower the workability. Therefore, the amount of P needs to be 0.04% or less, but it is preferable to reduce it as much as possible.

S: 0.005% or less
When S segregates at the old γ grain boundary or precipitates in a large amount as MnS, the low temperature toughness is lowered, and the stretch flangeability is remarkably lowered regardless of the presence or absence of processing. Therefore, the S amount needs to be 0.005% or less, but it is preferable to reduce it as much as possible.

Al: 0.05% or less
Al is added as a steel deoxidizer, and is an effective element for improving the cleanliness of steel. In order to acquire this effect, it is preferable to make it contain 0.001% or more. However, if the amount exceeds 0.05%, a large amount of inclusions are generated, causing surface flaws on the steel sheet. Therefore, the Al amount is 0.05% or less. A more preferable range of Al content is 0.01 to 0.04%.

Ti: 0.03-0.20%
Ti is a very important element that precipitates as carbides and strengthens ferrite. If the amount is less than 0.03%, it is difficult to obtain a TS of 780 MPa or more, and if it exceeds 0.20%, the effect of increasing the strength is saturated and the cost increases. Therefore, the Ti content is 0.03 to 0.20%, preferably 0.08 to 0.18%.

V: 0.03-0.15%
V, like Ti, is a very important element that precipitates as a carbide or strengthens ferrite in a solid solution state. If the amount is less than 0.03%, it is difficult to obtain a TS of 780 MPa or more, and if it exceeds 0.15%, the effect of increasing the strength is saturated and the cost is increased. Therefore, the V amount is 0.03 to 0.15%, preferably 0.05 to 0.10%.

Zr: 0.0005-0.005%
Zr is the most important element in the present invention, and precipitates containing Ti and / or V in the ferrite are finely dispersed to promote high strength, and the stretch flange characteristics and stretch characteristics after processing are remarkably improved. Improve. The reason for this is considered that when Zr is added, precipitation of cementite is suppressed at the ferrite grain boundary, and Zr carbide precipitates. In order to obtain such an effect, the Zr amount needs to be 0.0005% or more. Further, if the amount exceeds 0.005%, the Zr carbide coarsens, so that cracks are generated at the interface of the Zr carbide during processing, and the stretch flange characteristic and elongation characteristic after processing become insufficient, that is, λ ₁₀ is 60 %, El ₁₀ is less than 17%, so the Zr content is 0.005% or less. A more preferable range of the amount of Zr is 0.0007 to 0.003%.

  The balance needs to be Fe and the inevitable impurities balance, but at least one of Nb: 0.001 to 0.10% and Cr: 0.01 to 0.5% can be further contained for the following reason.

Nb: 0.001 to 0.10%, Cr: 0.01 to 0.5%
Nb and Cr, like V, have a function of forming a precipitate or strengthening ferrite in a solid solution state. If the Nb content is less than 0.001% or the Cr content is less than 0.01%, it will hardly contribute to the increase in strength, and if the Nb content exceeds 0.10% or the Cr content exceeds 0.5%, the workability deteriorates. Therefore, the Nb content is 0.001 to 0.10%, preferably 0.01 to 0.7%, and the Cr content is 0.01 to 0.5%, preferably 0.02 to 0.4%.

2) Microstructure
2-1) Volume ratio of ferrite: 50-95%
When the volume fraction of ferrite is less than 50%, the hard second phase is excessive, the stretch flange characteristic is deteriorated, and λ ₁₀ ≧ 60% cannot be satisfied. On the other hand, when the volume fraction of ferrite exceeds 95%, the second phase is too small and the elongation characteristics after processing are not improved, and TS ≧ 780 MPa and El ₁₀ ≧ 17% cannot be satisfied. Therefore, the volume ratio of ferrite is 50 to 95%, preferably 65 to 88%.

2-2) Volume ratio of cementite precipitated at ferrite grain boundaries: 4% or less Cementite precipitated at ferrite grain boundaries is a starting point for cracking during punching and reduces the stretch flange characteristics after machining. The lambda ₁₀ to 60% or more, the volume ratio of cementite 4% or less, it is preferably to be 3% or less. The cementite precipitated at the ferrite grain boundaries is granular or film-like cementite existing at the ferrite grain interface or triple point, and is not cementite produced in bainite described below.

  Here, the volume fraction of ferrite and the volume fraction of cementite are expressed in 3% nital as the microstructure of the plate thickness section parallel to the rolling direction, and the plate thickness is 1/500 times using a scanning electron microscope (SEM). The four positions were observed, and the area ratio of ferrite and the area ratio of cementite were measured using image processing software “Particle Analysis II” manufactured by Sumitomo Metal Technology Co., Ltd. The volume ratio of ferrite and the volume ratio of cementite were measured, respectively.

Further, the remaining microstructure other than cementite precipitated at the ferrite and ferrite grain boundaries needs to be substantially bainite in order to obtain TS of 780 MPa or more and λ _{10 of} 60% or more. Here, the fact that the remaining microstructure is substantially bainite means that bainite other than the phase inevitably mixed in the remainder, specifically, the proportion of the bainite in the remainder is the volume ratio Means 90% or more.

2-3) Average diameter of precipitates containing Ti and / or V precipitated in ferrite: 20 nm or less In the steel sheet of the present invention, precipitates containing Ti and / or V are mainly precipitated in ferrite as carbides. is doing. This is presumably because the solid solubility limit of C in ferrite is smaller than the solid solubility limit of austenite, and supersaturated C is likely to precipitate as carbide in the ferrite. And, such a precipitate hardens (strengthens) soft ferrite, and the hardness difference from hard bainite is reduced. Therefore, it is considered that stretch flangeability after processing is improved. However, if the average diameter of precipitates containing Ti and / or V exceeds 20 nm, the effect of suppressing the movement of dislocations is small, and ferrite cannot be hardened sufficiently, so excellent stretch flangeability after processing cannot be obtained. . Therefore, the average diameter of the precipitate containing Ti and / or V needs to be 20 nm or less.

  Here, the average diameter of the precipitate containing Ti and / or V is observed by a transmission electron microscope (TEM), and analyzed by an energy dispersive X-ray spectrometer (EDX) equipped in the TEM. Alternatively, after confirming that the precipitate contains V, image analysis was performed using the above-described image processing software “Particle Analysis II”, and the diameter of each precipitate was obtained by spherical approximation. At this time, the thickness of the sample for TEM observation was about 50 nm, the observation magnification was 100,000 times, and the number of observation fields was five. The average diameters of the precipitates containing Ti and / or V thus obtained were averaged to determine the average diameter of the precipitates.

In order to achieve high strength by dispersing the precipitates, the amount and size of the precipitates and the degree of the dispersion are extremely important. Precipitates in steel have various size distributions depending on the precipitation temperature range, but in general, those with a diameter exceeding 20 nm that precipitate in the high temperature range of the production process are considered to have little strengthening ability. The inventors of the present invention have said Ms and M ₀ , that is, the ratio of the amount of Ti (Ti ₂₀ ) in the precipitate having a diameter of 20 nm or less to the amount of Ti in the total precipitate (Ti _total ) and the amount of V in the total precipitate. The average value Ms of the ratio of the amount of V (V ₂₀ ) in the precipitate having a diameter of 20 nm or less with respect to (V _total ) is 0.5 or more, preferably 0.6 or more, and the amount of solid solution Ti with respect to the Ti content in the steel (Ti _sol ) and the average value M ₀ of the ratio of the solid solution V amount (V _sol ) to the V content in the steel, when TS is 0.4 or less, preferably 0.3 or less, a TS of 800 MPa or more is surely obtained. I found. When Ms is less than 0.5 and M ₀ exceeds 0.4, TS of 800 MPa or more cannot be obtained because there are few fine precipitates having a diameter of 20 nm or less.

Here, Ti _total , V _total, Ti ₂₀ , V _20, Ti _sol , and V _sol were determined as follows.
Ti _total , V _total : It was determined by an extraction residue method using 10% AA (acetylacetone electrolytic solution) which is a conventional extraction residue method. That is, Ti _total is a value determined as the amount of precipitated Ti in steel (mass%) and V _total is the amount of precipitated V in steel (mass%).
Ti ₂₀ , V ₂₀ : Of the precipitates observed in the above TEM image, precipitates with a diameter of 20 nm or less were extracted by image analysis, and the volume fraction of these precipitates was obtained from the diameters and totaled, The volume fraction of precipitates of 20 nm or less was calculated. As a result of EDX analysis, most of the precipitates of 20 nm or less were Ti-V composite carbide, and the composition ratio of Ti and V was 3: 1. From this result, it is assumed that all precipitates of 20 nm or less are Ti-V composite carbides (Ti ₃ V ₁ C ₄ ), and the lattice constants of Ti-V composite carbides under the assumptions of i) to iii) below. It was calculated to 4.265Å determined density (5.19g / cm ³⁾ and is contained 20nm in the following precipitates in steel by using the density of the volume ratio and Fe obtained (7.86g / cm ³⁾ above Ti ₂₀ and V ₂₀ which were Ti amount and V amount were determined.
i) Ti-VC is NaCl type crystal structure
ii) Lattice constant is distributed by the ratio of Ti and V based on VC (4.130mm) and TiC (4.310mm)
iii) Composition ratio Ti: V = 3: 1
Ti _sol, V _sol: Ti _sol is dissolved Ti content in steel was determined as the difference between Ti _total obtained as Ti content and the in the steel (= Ti-Ti _total). V _sol was determined in the same manner.

3) Manufacturing method
3-1) Heating temperature of steel slab: 1150 ~ 1300 ℃
Most carbide-forming elements such as Ti and V are present as carbides in steel slabs. In order to cause precipitation in ferrite as desired after hot rolling, it is necessary to once dissolve precipitates that have precipitated as carbides before hot rolling. For that purpose, it is necessary to heat the steel slab at 1150 ° C or higher, but if it is heated above 1300 ° C, the crystal grain size after hot rolling becomes too coarse, and both the stretch flange characteristics and elongation characteristics after processing Since it deteriorates, the heating temperature of steel slab shall be 1150-1300 degreeC, Preferably it shall be 1170-1260 degreeC.

3-2) Finishing rolling temperature in hot rolling: 850-1100 ° C
The steel slab after heating is hot-rolled at a finish rolling temperature of 850 to 1100 ° C., which is the end temperature of hot rolling. If the finish rolling temperature is less than 850 ° C., the ferrite structure is rolled and expanded in the ferrite + austenite region, so that the stretch flange characteristics and elongation characteristics after processing deteriorate. When the finish rolling temperature exceeds 1100 ° C., the strain introduced by rolling recovers and the nucleation sites of ferrite transformation decrease, so the ferrite volume fraction does not exceed 50%. Therefore, the finish rolling temperature is 850 to 1100 ° C, preferably 870 to 960 ° C.

3-3) First forced cooling condition: cooling at an average cooling rate of 30 ° C./s or more to a cooling stop temperature of 650 to 800 ° C. After hot rolling, from the finish rolling temperature to a cooling stop temperature of 650 to 800 ° C., The first forced cooling must be performed at an average cooling rate of 30 ° C / s or higher. When the cooling stop temperature exceeds 800 ° C., nucleation hardly occurs, so the volume fraction of ferrite does not exceed 50%, and a predetermined precipitation state of precipitates containing Ti and / or V cannot be obtained. When the cooling stop temperature is less than 650 ° C, the diffusion rate of C and Ti decreases, so the ferrite volume fraction does not exceed 50%, and the predetermined precipitation state of precipitates containing Ti and / or V cannot be obtained. . Therefore, the cooling stop temperature is set to 650 to 800 ° C. In particular, when the temperature is 680 to 770 ° C., Ms is 0.5 or more and M ₀ is 0.4 or less, and a TS of 800 MPa or more can be reliably obtained. In addition, when the average cooling rate from the finish rolling temperature to the cooling stop temperature is less than 30 ° C./s, pearlite is generated, so that the stretch flange characteristics and elongation characteristics after processing deteriorate. Therefore, the average cooling rate is 30 ° C./s or higher, preferably 70 ° C./s or higher. The upper limit of the cooling rate is not particularly limited, but is preferably about 300 ° C./s in order to accurately stop the cooling rate within the above cooling stop temperature range.

3-4) Air cooling after the first forced cooling: 3-15s
After the first forced cooling, forced cooling is stopped and air cooled for 3 to 15 seconds. If the air cooling time is less than 3 s, the volume fraction of ferrite does not exceed 50%, and if it exceeds 15 s, pearlite is generated, and the stretch flange characteristics and elongation characteristics after processing deteriorate. The cooling rate during air cooling is approximately 15 ° C./s or less.

3-5) Second forced cooling condition: Cooling at an average cooling rate of 20 ° C / s or higher up to a winding temperature of 300 to 600 ° C After air cooling, at an average cooling rate of 20 ° C / s or higher from a winding temperature of 300 to 600 ° C Perform a second forced cooling. At this time, if the average cooling rate is less than 20 ° C./s, pearlite precipitates during cooling, so the average cooling rate is 20 ° C./s or more, preferably 80 ° C./s or more. The upper limit of the cooling rate is not particularly limited, but is preferably about 300 ° C./s in order to accurately stop the cooling rate within the above winding temperature range. On the other hand, when the coiling temperature is less than 300 ° C., the remaining portion becomes martensite because of quenching, and the stretch flange characteristics and stretch characteristics after processing deteriorate. When the coiling temperature exceeds 600 ° C, pearlite is generated, and the stretch flange characteristics and stretch characteristics after processing deteriorate. Therefore, the coiling temperature is 300 to 600 ° C, preferably 350 to 550 ° C.

Steels A1 to 2, B1 to 5, C1 to 8, D1 to 2, E1 to 2, and F1 having compositions shown in Table 1 were melted in a converter and formed into a steel slab by continuous casting. Thereafter, these steel slabs were heated, hot-rolled and cooled under the conditions shown in Table 2, and wound to produce hot-rolled steel sheets 1 to 28 having a thickness of 2.0 mm. Then, the microstructure was analyzed by the method described above to determine M ₀ and Ms, and TS, λ ₁₀ , and El ₁₀ were determined by the following method.
TS: A tensile test was performed by a method based on JIS Z 2241 using a JIS No. 5 test piece with the rolling direction as the tensile direction, and TS was obtained.
λ ₁₀ : After rolling at an elongation rate of 10%, a hole expansion test was performed according to the iron standard JFST 1001, and λ ₁₀ was obtained.
El ₁₀ : After rolling at an elongation rate of 10%, a tensile test was performed by a method based on JIS Z 2241 using a JIS No. 5 test piece with the rolling direction as the tensile direction, and El ₁₀ was obtained.

The results are shown in Table 3. Steel sheets 1 to 3, 6, 8, 11, 16, 21 to 24, 26, and 28 of the present invention have TS of 780 MPa or more, λ ₁₀ of 60% or more, El ₁₀ of 17% or more, and elongation after processing. It can be seen that this is a high-strength hot-rolled steel sheet having excellent flange characteristics and elongation characteristics.

Claims (3)

  In mass%, C: 0.06 to 0.15%, Si: 1.2% or less, Mn: 0.5 to 1.6%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.03 to 0.20%, V : 0.03 to 0.15%, Zr: 0.0005 to 0.005%, with the balance being a component composition consisting of Fe and inevitable impurities, volume fraction of ferrite being 50 to 95%, cementite precipitated at the ferrite grain boundaries 4% or less, the balance is substantially bainite, and precipitates containing Ti and / or V are precipitated in the ferrite, and the average diameter of the precipitates is 20 nm or less. A high-strength hot-rolled steel sheet with a tensile strength of 780 MPa or more, excellent in stretch flange characteristics and elongation characteristics after processing.   The stretch flange characteristic and elongation after processing according to claim 1, further comprising a component composition containing at least one of Nb: 0.001 to 0.10% and Cr: 0.01 to 0.5% in mass%. A high-strength hot-rolled steel sheet with excellent tensile strength of 780 MPa or more. The following Ms is 0.5 or more, and the following M ₀ is 0.4 or less.The tensile strength excellent in stretch flange characteristics and elongation characteristics after processing according to claim 1 or 2, wherein the tensile strength is 780 MPa or more. High strength hot rolled steel sheet;
Ms = (Ti ₂₀ / Ti _total + V ₂₀ / V _total ) / 2
M ₀ = (Ti _sol / Ti + V _sol / V) / 2
Where
Ti ₂₀ , V ₂₀ is the content of each element in the precipitate having a diameter of 20 nm or less (mass% relative to steel),
Ti _total , V _total is the content of each element in the total precipitate (mass% relative to steel),
Ti _sol and V _sol are the solid solution amount (% by mass) of each element in steel, and
Ti and V are the contents (mass%) of each element in steel.
To express.