JP2009191360A - High strength steel sheet, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having excellent elongation and stretch flange properties after working, and to provide a method for producing the same. <P>SOLUTION: The high strength steel sheet includes a componential composition comprising, by mass, 0.08 to 0.20% C, 0.2 to 1.0% Si, 0.5 to 2.5% Mn, ≤0.04% P, ≤0.005% S, ≤0.05% Al, 0.07 to 0.20% Ti and 0.05 to <0.20% V, and the balance Fe with inevitable impurities. Then, the steel sheet includes a structure composed of ferrite of 60 to 95% by a volume fraction and bainite of 5 to 35% as the second phase. Further, the content of Ti comprised in precipitates with a size of <20 nm is 450 to 1,800 mass ppm, and the content of V is 350 to <1,200 mass ppm. The difference (HV<SB>S</SB>-HV<SB>α</SB>) between the hardness (HV<SB>S</SB>) of the bainitic phase and the hardness (HV<SB>α</SB>) of the ferritic phase is ≤300. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet excellent in elongation and flange characteristics after processing, having a YR exceeding 85% and having a tensile strength (TS) of 980 MPa or more, and a method for producing the same.

  For automobile underbody members, or impact members such as bumpers and center pillars, formability (mainly stretch and stretch flange characteristics) is required, and thus, conventionally, steel with a tensile strength of 590 MPa has been used. . 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 steel having a tensile strength of 980 MPa class has begun to be studied. In general, as the strength increases, workability decreases. Therefore, research is currently being conducted on steel sheets having high strength and high workability. Examples of techniques for improving the stretch and stretch flange characteristics include the following.

  In Patent Document 1, carbides containing Ti, Mo, and V that are substantially ferrite single-phase structures and having an average of less than 10 nm are dispersed and precipitated, and the carbides containing Ti, Mo, and V are expressed in atomic%. A technique relating to a high-tensile steel sheet having an average composition satisfying V / (Ti + Mo + V) ≧ 0.3 and having a tensile strength of 980 MPa or more is disclosed.

In Patent Document 2, by mass, C: 0.08 to 0.20%, Si: 0.001% or more, less than 0.2%, Mn: more than 1.0%, 3.0% or less, Al: 0.001-0.5%, V: more than 0.1%, 0.5% or less, Ti: 0.05% or more, less than 0.2% and Nb: 0.005% to 0.5% (Formula 1) 9 (Ti / 48 + Nb / 93) × C / 12 ≦ 4.5 × 10 ⁻⁵ , (Formula 2) 0.5% ≦ (V / 51 + Ti / 48 + Nb / 93) / (C /12)≦1.5, (Formula 3) V + Ti × 2 + Nb × 1.4 + C × 2 + Mn × 0.1 ≧ 0.80, satisfying the three formulas, having a steel structure composed of the balance Fe and impurities, and having an average particle diameter High-strength hot-rolled steel having a steel structure containing 70% by volume or more of ferrite of 5 μm or less and hardness of 250 Hv or more, having a strength of 880 MPa or more and a yield ratio of 0.80 or more. Techniques relating to plates are disclosed.

  In Patent Document 3, by mass, C: 0.05 to 0.2%, Si: 0.001% to 3.0%, Mn: 0.5 to 3.0, P: 0.001 to 0.00. 2%, Al: 0.001 to 3%, V: Over 0.1% to 1.5%, the balance is composed of Fe and impurities, and the structure is mainly composed of ferrite with an average grain size of 1 to 5 μm And a technique related to a hot-rolled steel sheet characterized by the presence of V carbonitrides having an average grain size of 50 nm or less in ferrite grains.

  In Patent Document 4, in mass%, C: 0.04 to 0.17%, Si: 1.1% or less, Mn: 1.6 to 2.6%, P: 0.05% or less, S: 0.02% or less, Al: 0.001 to 0.05%, N: 0.02% or less, V: 0.11 to 0.3%, Ti: 0.07 to 0.25%, the balance Discloses a technology relating to a high-strength steel sheet having a steel composition of iron and inevitable impurities, a tensile strength of 880 MPa or more in the direction perpendicular to the rolling, and a yield ratio of 0.8 or more.

In Patent Document 5, in terms of mass%, C: 0.04 to 0.20%, Si: 0.001 to 1.1%, Mn: more than 0.8%, Ti: 0.05% or more, 0. Less than 15%, Nb: 0 to 0.05%, satisfying the following formulas (1) to (3), having a steel composition consisting of the balance Fe and inevitable impurities, a strength of 880 MPa or more and a yield ratio of 0 A high-strength hot-rolled steel sheet having.
(1) Formula: (Ti / 48 + Nb / 93) × C / 12 ≦ 3.5 × 10 ⁻⁵
(2) Formula: 0.4 ≦ (V / 51 + Ti / 48 + Nb / 93) / (C / 12) ≦ 2.0
(3) Formula: V + Ti × 2 + Nb × 1.4 + C × 2 + Si × 0.2 + Mn × 0.1 ≧ 0.7
Patent Document 6 is substantially a ferrite single-phase structure, and precipitates containing Ti, Mo, and C are precipitated in the ferrite structure, and the thickness of the cross section perpendicular to the vector parallel to the rolling direction. Technology for ultra-high-strength steel sheets with excellent stretch flangeability with a tensile strength of 950 MPa or more, with an area ratio of <110> orientation colonies of each adjacent crystal grain in the region of 1/4 to 3/4 being 50% or less Is disclosed.

  In Patent Document 7, C: 0.10 to 0.25% by mass, Si: 1.5% or less, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0 0.005% or less, Al: 0.01 to 0.5%, N: 0.010% or less and V: 0.10 to 1.0%, and (10Mn + V) / C ≧ 50 is satisfied, and the balance Has a composition of Fe and unavoidable impurities, and a technique relating to a thin steel sheet is disclosed in which the average particle diameter of carbide containing V obtained for precipitates having a particle diameter of 80 nm or less is 30 nm or less.

  In Patent Document 8, C: 0.10 to 0.25% by mass, Si: 1.5% or less, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0 0.005% or less, Al: 0.01 to 0.5%, N: 0.010% or less and V: 0.10 to 1.0%, and (10Mn + V) / C ≧ 50 is satisfied, and the balance Has a composition of Fe and inevitable impurities, the volume occupancy of the tempered martensite phase is 80% or more, and the average particle size of carbide containing V having a particle size of 20 nm or less is 10 nm or less A technique related to a structural member is disclosed.

  In Patent Document 9, in a 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 0.2% or less, Si: 0.01 -2.0%, Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.0004 to 0.015%, Ti: 0.03 to 0.2%, the balance is Fe and impurities, and the metal structure of the steel sheet contains ferrite in an area ratio of 30 to 95%, When the remaining second phase contains martensite, bainite, pearlite, and cementite, the martensite area ratio is 0 to 50%, and the steel sheet is a Ti-based carbonitride precipitate having a particle size of 2 to 30 nm. Containing at an average interparticle distance of 30 to 300 nm and having a particle size of 3 μm or more Technology regarding high-strength galvanized steel sheet containing output system TiN with an average distance between particles 50~500μm is disclosed.

  In Patent Document 10, in mass%, C: 0.01 to 0.15%, Si: 2.0% or less, Mn 0.5 to 3.0%, P: 0.1% or less, S: 0.0. It has a composition containing 02% or less, Al: 0.1% or less, N: 0.02% or less, Cu: 0.5 to 3.0%, and the structure has a ferrite phase as a main phase, and the area ratio A thin steel plate, which is a composite structure having a phase containing a martensite phase of 2% or more as a second phase, is subjected to a strain aging treatment that generates fine precipitates having a particle size of 10 nm or less. A technique relating to a method for improving fatigue resistance is disclosed.

In Patent Document 11, in mass%, C: 0.18 to 0.3%, Si: 1.2% or less, Mn: 1 to 2.5%, P: 0.02% or less, S: 0.00. 00% or less, Sol.Al 0.01 to 0.1%, Nb: 0.005 to 0.030%, V: 0.01 to 0.10%, Ti: 0.01 to A steel containing either 0.10% or two or more of 0.10% in total in the range of 0.005 to 0.10%, the balance being Fe and unavoidable impurities is hot rolled at a finishing temperature of Ar3 or higher. After pickling at 500 to 650 ° C., pickling, cold rolling, heating to Ac3 to [Ac3 + 70 ° C.] by continuous annealing and soaking for 30 seconds or more, followed by primary cooling with ferrite in volume occupation ratio Precipitate 3 to 20%, then quench in jet water to room temperature and perform overaging at 120 to 300 ° C for 1 to 15 minutes Martensite volume occupancy is remainder at 80-97% having a fine two-phase structure made of ferrite, the tensile strength is excellent ultra-high strength cold rolled steel sheet formability and strip shape of 150~200kgf / mm ² Techniques relating to manufacturing methods are disclosed.

  In Patent Document 12, in mass%, C: 0.0005 to 0.3%, Si: 0.001 to 3.0% or less, Mn: 0.01 to 3.0%, Al: 0.0001 to Containing 0.3%, S: 0.0001-0.1%, N: 0.0010-0.05%, consisting of the balance Fe and inevitable impurities, with ferrite as the phase with the largest area ratio, Carbon: Sol. C and solute nitrogen: Sol. N is Sol. C / Sol. N: When satisfying 0.1 to 100 and adding 5 to 20% of pre-strain, the average or each value of the increase in yield strength and tensile strength after baking treatment at 110 to 200 ° C. for 1 to 60 minutes is A technique relating to a high-strength hot-rolled steel sheet having a high bake hardenability at the time of high pre-strain, which is characterized by being 50 MPa or more as compared with a steel sheet before baking treatment without adding pre-strain, is disclosed.

JP 2007-063668 A JP 2006-161112 A JP 2004-143518 A JP 2004-360046 A JP-A-2005-002406 JP 2005-232567 A JP 2006-183138 A JP 2006-183139 A JP 2007-16319 A JP 2003-105444 A JP-A-4-289120 JP 2003-96543 A

However, the above prior art has the following problems.
In Patent Documents 1 and 4, since Mo is contained, there is a problem that a significant cost increase is caused in connection with a recent increase in the raw material price of Mo. Furthermore, with the globalization of the automobile industry, steel plates used in automobiles are used in severe corrosive environments such as in foreign countries, and higher post-paint corrosion resistance is required for steel plates. On the other hand, since addition of Mo inhibits the formation or growth of chemical crystals, the corrosion resistance after painting of the steel sheet is lowered, and the above-mentioned demand cannot be met. That is, the steels described in Patent Document 1 and Patent Document 4 cannot sufficiently satisfy the demands of the automobile industry in recent years.

  On the other hand, due to the recent progress in press technology, processing processes such as drawing (drawing and overhanging) → trimming (hole punching) → restriking (hole expansion) have begun to be adopted, and molding is performed through such processing processes. The stretch flange portion of the steel plate is required to have stretch flange characteristics after draw trimming, that is, after processing. However, since the stretch flange characteristic after processing is a characteristic that has attracted attention in recent years, Patent Documents 1 to 12 are not necessarily sufficient.

  Moreover, precipitation strengthening is mentioned as one of the general strengthening methods of steel. It is known that the precipitation strengthening amount is inversely proportional to the particle size of the precipitate and proportional to the square root of the precipitation amount. Thus, for example, in the steel sheets disclosed in Patent Documents 1 to 12, carbonitride-forming elements such as Ti, V, and Nb are added. In particular, Patent Documents 7, 9, and 10 relate to the size of precipitates. Research has been done. However, the amount of precipitates is not always sufficient, and the problem is that the cost is increased due to poor deposition efficiency.

  Nb added to Patent Documents 2, 5, and 11 has a high function of suppressing recrystallization of austenite after hot rolling. Therefore, there is a problem that unrecrystallized grains remain in the steel sheet and the workability is lowered. There is also a problem of increasing the rolling load during hot rolling.

  In view of such circumstances, an object of the present invention is to provide a high-strength steel sheet excellent in elongation and stretch flange characteristics after processing, and a method for producing the same.

The present inventors have studied to obtain a high-strength hot-rolled steel sheet that is excellent in elongation and stretch flange characteristics after processing and has a tensile strength of 980 MPa or more, and obtained the following knowledge.
i) In order to obtain a high-strength steel sheet, it is necessary to refine the precipitates (size less than 20 nm) and increase the proportion of fine precipitates (size less than 20 nm). In order to keep the precipitate fine, a precipitate containing Ti-Mo or a precipitate containing Ti-V can be mentioned. From the viewpoint of alloy cost, combined precipitation of Ti and V is useful. is there.
ii) When the hardness difference between the ferrite phase and the second phase bainite phase is 300 or less, the stretch flangeability after processing is improved. In addition, a structure having excellent stretch flange characteristics after processing can be obtained by controlling the first stage cooling stop temperature T1 and the winding temperature T2 to the optimum ranges. Furthermore, by setting the coiling temperature to 300 ° C. to 600 ° C. or less, the second phase structure mainly becomes a bainite phase, and YR exceeds 85%.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Component composition is mass%, C: 0.08% to 0.20%, Si: 0.2% to 1.0%, Mn: 0.5% to 2.5%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.07% or more and 0.20% or less, V: 0.05% or more and less than 0.20%, the balance is composed of Fe and inevitable impurities, and the metal structure is 60% or more and 95% by volume occupancy The following ferrite and the second phase have 5% or more and 35% or less bainite, and the amount of Ti contained in the precipitate with a size of less than 20 nm is 450 mass ppm or more and 1800 mass ppm or less, and the V amount is 350 mass ppm or more and 1200 mass ppm. High strength, characterized in that the difference between the hardness of the bainite phase (HV _S ) and the hardness of the ferrite phase (HV _α ) (HV _S −HV _α ) is 300 or less and the YR exceeds 85% steel sheet.
[2] In the above [1], in mass%, any one or two of Cr: 0.01% or more and 1.0% or less, W: 0.005% or more and 1.0% or less, Zr: 0.0005% or more and 0.05% or less A high-strength steel sheet characterized by containing the above.
[3] In mass%, C: 0.08% to 0.20%, Si: 0.2% to 1.0%, Mn: 0.5% to 2.5%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% Hereinafter, a steel slab containing Ti: 0.07% or more and 0.20% or less, V: 0.05% or more and less than 0.20%, and the balance composed of Fe and inevitable impurities is heated to a temperature of 1150 ° C to 1350 ° C. After that, hot rolling is performed at a finish rolling temperature of 850 ° C. or higher and 1000 ° C. or lower, and then the first stage cooling is performed at an average cooling rate of 30 ° C./s or higher to a temperature of 650 ° C. or higher and lower than 800 ° C. for 1 second or longer. Air-cooled in less than 10 seconds, then second-stage cooled at a cooling rate of 20 ° C / s or higher, wound at a temperature of 300 ° C to 600 ° C, and satisfying formula (1) A method of manufacturing a steel sheet.
T1 ≦ 0.06 × T2 + 764 (1)
However, T1: First stage cooling stop temperature, T2: Winding temperature [4] In the above [3], in the second stage cooling, at a temperature range of 500 ° C. or lower, a cooling rate of 120 ° C./s or higher. And the manufacturing method of the high strength steel plate characterized by cooling on the conditions used as nucleate boiling cooling.
[5] In the above [3] or [4], the component composition is mass%, Cr: 0.01% or more, 1.0% or less, W: 0.005% or more, 1.0% or less, Zr: 0.0005% or more, 0.05% or less A method for producing a high-strength steel sheet, comprising one or more of these.
In this specification, “%” and “ppm” indicating the components of steel are mass% and mass ppm, respectively. The high-strength steel plate in the present invention is a steel plate having a tensile strength (hereinafter sometimes referred to as TS) of 980 MPa or more, a hot-rolled steel plate, and further, a surface treatment such as plating treatment is applied to these steel plates. The surface-treated steel sheets that have been applied are also targeted.
Further, the target characteristics of the present invention are elongation (El) ≧ 13%, elongation ratio of 10%, stretched flange characteristic after rolling (λ ₁₀ ) ≧ 40%, and YR> 85%.

According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having excellent elongation and stretched flange characteristics after processing, YR exceeding 85%, and TS being 980 MPa or more. Furthermore, in the present invention, the above effect can be obtained without adding Mo, so that the cost can be reduced.
For example, by using the high-strength hot-rolled steel sheet of the present invention for an automobile undercarriage member, a truck frame, a collision-resistant member, etc., the plate thickness can be reduced, and the environmental load of the automobile is reduced. It is expected that the characteristics will be greatly improved.

Hereinafter, the present invention will be described in detail.
1) First, the reasons for limiting the chemical composition (component composition) of steel in the present invention will be described.
C: 0.08% or more and 0.20% or less C is an element that contributes to strengthening of the steel sheet by forming carbides with Ti and V and precipitating in ferrite. In order to increase TS to 980 MPa or more, the C content needs to be 0.08% or more. On the other hand, if the amount of C exceeds 0.20%, the stretch flange characteristics deteriorate due to coarsening of precipitates. From the above, the C content is 0.08% or more and 0.20% or less, preferably 0.09% or more and 0.18 or less.

Si: 0.2% or more, 1.0% or less
Si is an element that contributes to the promotion of ferrite transformation and solid solution strengthening. Therefore, Si is 0.2% or more. However, if the amount exceeds 1.0%, the surface properties of the steel sheet deteriorate significantly and the corrosion resistance decreases, so the upper limit of Si is 1.0%. Accordingly, the Si content is 0.2% to 1.0%, preferably 0.3% to 0.9%.

Mn: 0.5% or more, 2.5% or less Mn is an element contributing to solid solution strengthening. However, if the amount is less than 0.5%, TS of 980 MPa or more cannot be obtained. On the other hand, if the amount exceeds 2.5%, the weldability is significantly reduced. Therefore, the amount of Mn is 0.5% to 2.5%, preferably 0.5% to 2.0%. More preferably, it is 0.8% or more and 2.0% or less.

P: 0.04% or less P segregates at the prior austenite grain boundaries, causing low temperature toughness deterioration and workability reduction. Therefore, the P content is preferably reduced as much as possible, and is 0.04% or less.

S: 0.005% or less S segregates at the prior austenite grain boundaries or precipitates in a large amount as MnS, which lowers the low temperature toughness and remarkably lowers the stretch flangeability regardless of the presence or absence of processing. Therefore, the amount of S is preferably reduced as much as possible, and is 0.005% or less.

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, which causes the steel sheet to become wrinkled. Therefore, the Al amount is 0.05% or less. A more preferable amount of Al is 0.01% or more and 0.04% or less.

Ti: 0.07% or more, 0.20% or less Ti is an extremely important element for precipitation strengthening of ferrite. If it is less than 0.07%, it is difficult to ensure the required strength, and if it exceeds 0.20%, the effect is saturated and only the cost is increased. Therefore, the Ti content is 0.07% or more and 0.2% or less, preferably 0.08% or more and 0.18% or less.

V: 0.05% or more and less than 0.20% V is an element that contributes to improvement in strength as precipitation strengthening or solid solution strengthening, and is an important requirement for obtaining the effects of the present invention along with the above Ti. By adding an appropriate amount together with Ti, there is a tendency to precipitate as fine Ti-V carbide having a particle size of less than 20 nm, and the corrosion resistance after coating does not decrease like Mo. Further, the cost can be reduced compared to Mo. If the amount of V is less than 0.05%, the effect of addition is poor. On the other hand, when the amount of V is 0.20% or more, the effect is saturated and only the cost is increased. Therefore, the V content is 0.05% or more and less than 0.20%, preferably 0.06% or more and less than 0.20%.

  With the above essential additive elements, the steel of the present invention can achieve the desired properties, but in addition to the above essential additive elements, Cr: 0.01% or more, 1.0% or less, W: 0.005% or more for the following reasons Any one or more of 1.0% or less and Zr: 0.0005% or more and 0.05% or less may be added.

Cr: 0.01% to 1.0%, W: 0.005% to 1.0%, Zr: 0.0005% to 0.05%
Cr, W and Zr, like V, have the function of strengthening ferrite in the form of precipitates or in a solid solution state. If the Cr content is less than 0.01%, the W content is less than 0.005%, or the Zr content is less than 0.0005%, it hardly contributes to the increase in strength. On the other hand, if the Cr content exceeds 1.0%, the W content exceeds 1.0%, or the Zr content exceeds 0.05%, the workability deteriorates. Therefore, when adding one or more of Cr, W, and Zr, the addition amount is Cr: 0.01% to 1.0%, W: 0.005% to 1.0%, Zr: 0.0005% to 0.05% The following. Preferably, Cr: 0.1% to 0.8%, W: 0.01 to 0.8, Zr: 0.001% to 0.04%.

  The remainder other than the above consists of Fe and inevitable impurities. As an inevitable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. In the present invention, Cu, Ni, Sn, and Sb may be contained in a range of 0.1% or less as trace elements that do not impair the effects of the invention.

2) Next, the structure of the high-strength steel sheet of the present invention will be described.
Ferrite having a low dislocation density is effective in improving the stretch flangeability after bainite processing of 60% to 95% and 5% to 35% as the second phase. When the volume occupancy of the ferrite is less than 60%, the hard second phase becomes excessive and the second phase is connected, so that the stretch flangeability (λ) and the elongation (El) after processing are lowered. On the other hand, when the volume occupancy of ferrite exceeds 95%, the elongation does not improve because the second phase is small. Accordingly, the volume occupancy of ferrite is 60% or more and 95% or less, preferably 70% or more and 90% or less.
When the volume fraction of bainite is less than 5%, the elongation is not improved because the second phase is small. On the other hand, when it exceeds 35%, the hard second phase becomes excessive, and when the steel sheet is deformed, the second phase is connected, so the stretch flangeability (λ) and the elongation (El) after processing are low. descend. If the volume ratio is 2% or less, some martensite may be included. In the steel sheet in which 5% or more and 35% or less of bainite is generated as the second phase, YR exceeds 85%. That is, it is possible to obtain a steel sheet that is very advantageous for a member that requires impact resistance.
Here, the volume occupancy ratio of ferrite and bainite is expressed by 3% nital in the thickness cross section parallel to the rolling direction, and is 1500 times using the scanning electron microscope (SEM). The area ratio of ferrite and bainite is measured using image processing software “Particle Analysis II” manufactured by Sumitomo Metal Technology Co., Ltd. to obtain the volume occupation ratio.

The amount of Ti contained in precipitates with a size of less than 20 nm is 450 mass ppm or more and 1800 mass ppm or less, and the V amount is 350 mass ppm or more and less than 1200 mass ppm (where Ti amount and V amount are 100 mass% of the total composition of the steel) The concentration when
In the high-strength steel sheet of the present invention, precipitates containing Ti and / or V are mainly precipitated in the ferrite as carbides. 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 a TS of 980 MPa or more is obtained.

In order to obtain a high-strength steel plate, as described above, it is important to refine the precipitates (less than 20 nm in size) and increase the proportion of the fine precipitates (less than 20 nm in size). If the size of the precipitate is 20 nm or more, the effect of suppressing the movement of dislocations is small, and the ferrite cannot be hardened sufficiently, so that the strength may decrease. Therefore, the size of the precipitate is preferably less than 20 nm. This fine precipitate of less than 20 nm is achieved by adding both Ti and V. V mainly forms composite carbide with Ti. Although the reason is not clear, it has been found that these precipitates exist stably and finely under high temperature and long time within the coiling temperature within the range of the present invention.
Furthermore, in the present invention, it is important to control the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm. When the amount of Ti contained in precipitates of less than 20 nm is less than 450 mass ppm and the amount of V is less than 350 mass ppm, the number density of the precipitates is reduced and the distance between the precipitates is increased, thereby suppressing dislocation movement. It turned out that the effect to do becomes small. For this reason, ferrite cannot be hardened sufficiently, so that a strength of TS of 980 MPa or more cannot be obtained. Further, when the amount of Ti contained in the precipitate of less than 20 nm is 450 mass ppm or more and the amount of V contained in the precipitate of less than 20 nm is less than 350 mass ppm, the precipitate tends to be coarsened, so TS is 980 MPa. The above strength may not be obtained. In addition, when the amount of Ti contained in the precipitate of less than 20 nm is less than 450 mass ppm and the amount of V contained in the precipitate of less than 20 nm is 350 mass ppm or more, the precipitation efficiency of V deteriorates, so TS is 980 MPa or more. Strength may not be obtained. On the other hand, if the Ti content in precipitates less than 20 nm exceeds 1800 mass ppm, or if the V content precipitates 1200 mass ppm or more, the reason is not clear, but the steel sheet breaks brittlely and the desired characteristics are obtained. Disappear. Therefore, both the amount of precipitated Ti and the amount of precipitated V contained in the precipitate having a size of less than 20 nm must be satisfied.
From the above, the amount of Ti contained in the precipitate having a size of less than 20 nm is 450 mass ppm or more and 1800 mass ppm or less, and the amount of V is 350 mass ppm or more and less than 1200 mass ppm.
In addition, precipitates and / or inclusions are collectively referred to as precipitates.
Moreover, the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm can be confirmed by the following method.
After the sample is electrolyzed in a predetermined amount in the electrolytic solution, the sample piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Next, the precipitate contained in the solution is filtered using a filter having a pore diameter of 20 nm. The precipitate that has passed through the filter having a pore diameter of 20 nm together with the filtrate has a size of less than 20 nm. Next, the filtrate after filtration is analyzed by appropriately selecting from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc. Find the amount.

Hardness difference between second phase (bainite phase) and ferrite phase: HV _S -HV _α ≦ 300
This is an important configuration requirement in the present invention. If the hardness difference between the second phase (bainite) and the ferrite phase is 300 or less, the required stretched flange characteristics after processing can be obtained. If the hardness difference between the second phase (bainite phase) and the ferrite phase exceeds 300, the difference in deformation between the ferrite phase and the second phase (bainite) increases when the steel sheet is processed, which increases cracks and is necessary. The stretched flange characteristics after processing cannot be obtained. The hardness difference should be as small as possible, preferably 250 or less.

3) Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention, for example, after heating a steel slab adjusted to the above chemical composition range to a temperature of 1150 ° C. or higher and 1350 ° C. or lower, and then hot rolling with a finish rolling temperature of 850 ° C. or higher and 1000 ° C. or lower 1st stage cooling at an average cooling rate of 30 ° C / s or higher until a temperature of 650 ° C or higher and lower than 800 ° C, air-cooled in a time of 1 second or longer and less than 10 seconds, and then a cooling rate of 20 ° C / s or higher In the second stage, and wound at a temperature exceeding 300 ° C. and not more than 600 ° C., and satisfying the formula (1).
T1 ≦ 0.06 × T2 + 764 (1)
However, T1: First stage cooling stop temperature, T2: Winding temperature These conditions will be described in detail below.

Slab heating temperature: 1150 ° C. or higher and 1350 ° C. or lower Most carbide-forming elements such as Ti or 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 this purpose, it is necessary to heat at 1150 ° C or higher. On the other hand, if the temperature exceeds 1350 ° C., the crystal grain size becomes too coarse and the stretch flange characteristics and elongation characteristics after processing deteriorate, so the temperature is set to 1350 ° C. or less. Therefore, the slab heating temperature is 1150 ° C. or higher and 1350 ° C. or lower. More preferably, it is 1170 ° C. or higher and 1260 ° C. or lower.

Finishing rolling temperature in hot rolling: The steel slab after processing at 850 ° C. or more and 1000 ° C. or less is hot rolled at a finishing rolling temperature of 850 ° C. 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 in the ferrite + austenite region and becomes an expanded ferrite structure, so that the stretch flange characteristics and the stretch characteristics deteriorate. When the finish rolling temperature exceeds 1000 ° C., the ferrite grains become coarse, so that a TS of 980 MPa cannot be obtained. Therefore, finish rolling is performed at a finish rolling temperature of 850 ° C. or higher and 1000 ° C. or lower. More preferably, it is 870 ° C. or higher and 960 ° C. or lower.

First stage cooling: After cooling hot rolling at an average cooling rate of 30 ° C / s or higher to a cooling stop temperature of 650 ° C or higher and lower than 800 ° C, the average cooling rate of 30 to 650 ° C to 800 ° C from the finish rolling temperature It is necessary to cool at a temperature of at least ° C / s. When the cooling stop temperature is 800 ° C. or higher, nucleation is difficult to occur, so the ferrite volume fraction does not exceed 60%, 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 60%, and a predetermined precipitation state of precipitates containing Ti and / or V cannot be obtained. . Therefore, the cooling stop temperature is set to 650 ° C. or higher and lower than 800 ° C. 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 stretch flange characteristics and stretch characteristics after processing deteriorate. The upper limit of the cooling rate is not particularly limited, but is preferably about 300 ° C./s in order to stop the cooling within the cooling stop temperature range.

Air cooling after the first stage cooling: 1 second or more and less than 10 seconds After the first cooling, the cooling is stopped and air cooling is performed for 1 second or more and 10 seconds or less. If this air cooling time is less than 1 second, the ferrite volume occupancy does not exceed 60%, and if it exceeds 10 seconds, pearlite is generated, and the stretch flange characteristics and stretch characteristics deteriorate. Note that the cooling rate during air cooling is approximately 15 ° C./s or less.

Second-stage cooling: Cooling to an average cooling rate of 20 ° C / s or higher to a coiling temperature of 300 ° C to 600 ° C or less, then air cooling to a winding temperature of 300 ° C to 600 ° C or less to an average cooling rate of 20 ° C / s or higher. Cool down. At this time, if the average cooling rate is less than 20 ° C./s, pearlite is generated during cooling, so the average cooling rate is 20 ° C./s or more, preferably 50 ° 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.
When the coiling temperature is 300 ° C. or lower, the main component of the second phase is martensite, and YR is 85% or lower. On the other hand, when the temperature exceeds 600 ° C., pearlite is generated and the elongation characteristics deteriorate. Preferably, they are 400 degreeC or more and 550 degrees C or less.

T1 ≦ 0.06 × T2 + 764 However, T1: First stage cooling stop temperature, T2: Winding temperature Fine precipitation to ferrite occurs during air cooling after the first stage cooling. As a result, most of the ferrite phase is strengthened by precipitation. The hardness of the precipitation strengthened ferrite phase is affected by the temperature at which precipitates are formed, that is, the first stage cooling stop temperature. On the other hand, the hardness of the second phase (bainite) is influenced by the transformation temperature, that is, the coiling temperature. The results of various studies revealed that the hardness difference is 300 or less when T1 ≦ 0.06 × T2 + 764 is satisfied, where T1 is the first stage cooling stop temperature and T2 is the coiling temperature. When T1> 0.06 × T2 + 764, the hardness of the ferrite phase is low and the hardness of the second phase is high, so the hardness difference exceeds 300.

Furthermore, as a result of the examination, when the second stage cooling described above is performed, cooling at a cooling rate of 120 ° C./s or more in the temperature range of 500 ° C. or less and cooling under the condition of nucleate boiling cooling, the material fluctuations in the steel plate It was found that there is an effect that can be reduced stably. This second stage cooling step will be described in detail below.
In the second stage cooling, if water cooling is performed in a temperature range of 500 ° C. or lower, transition boiling from film boiling to nucleate boiling is likely to occur, and a problem of temperature unevenness is likely to occur in the steel surface. Therefore, a temperature range of 500 ° C. or less, which is transition boiling cooling in which film boiling cooling and nucleate boiling cooling coexist, is a cooling rate of 120 ° C./s or more, preferably 250 ° C./s or more, and conditions for nucleate boiling cooling. If it cools by, a temperature nonuniformity can be eliminated reliably and the material variation in a steel plate can be reduced stably. The reason for this is not clear, but it is considered that when the cooling rate is slow, a portion where the water film on the steel sheet surface is not sufficiently broken is generated.
When performing nucleate boiling cooling in the second stage cooling step, first, cooling is performed at an average cooling rate of 20 ° C./s or more from the start of the second stage cooling after the air cooling until nucleate boiling cooling is performed. Subsequently, nucleate boiling cooling is started. At this time, the cooling rate is 120 ° C./s. In addition, the cooling rate in the cooling under the above nucleate boiling cooling condition is an average cooling rate during nucleate boiling cooling. In addition, as a result of investigations by the inventors, nucleate boiling cooling may be performed at least in a temperature range of 500 ° C. or less, and after cooling is stopped, cooling is started under a condition of nucleate boiling cooling from a temperature of 500 ° C. or more. Also good. When the coiling temperature is 600 ° C. or lower to 500 ° C. or higher, water cooling at 500 ° C. or lower is not necessary, so there is no need to consider nucleate boiling cooling.
In order to ensure cooling under the conditions of cooling rate of 120 ° C./s or more and nucleate boiling cooling, for example, the water density is preferably 2000 l / (min.m ² ) or more. Furthermore, in order to perform nucleate boiling cooling, it is preferable to use a conventional method, that is, laminar or jet cooling excellent in straightness with respect to the upper surface of the steel sheet. As the shape of the nozzle, a generally used circular tube or slit nozzle can be adopted. The laminar or jet cooling flow rate is preferably 4 m / s or more. This is because it is necessary to obtain a momentum for stably breaking through the liquid film formed on the steel plate during cooling by laminar or jet cooling. In addition, since cooling water falls with respect to the steel plate lower surface by gravity, since a liquid film cannot be formed on the steel plate surface, there is no problem even if spray cooling is used. Of course, the same laminar and jet cooling as in the case of the upper surface of the steel plate can be employed.

As described above, a high-strength steel sheet excellent in elongation and stretch flange characteristics after processing can be obtained.
In addition, the steel plate of this invention contains what gave the surface treatment and surface coating process to the surface. In particular, the steel sheet of the present invention can be suitably applied to a steel sheet obtained by forming a hot-dip galvanized coating film on the hot-dip galvanized steel sheet. That is, since the steel sheet of the present invention has good workability, good workability can be maintained even when a hot dip galvanized film is formed. Here, the hot dip galvanizing is hot dip plating mainly composed of zinc and zinc (that is, containing about 90% or more), including those containing alloy elements such as Al and Cr in addition to zinc. Moreover, even if hot dip galvanizing is performed, alloying treatment may be performed after plating.

  Moreover, the melting method of steel is not particularly limited, and all known melting methods can be applied. For example, as the melting method, a method of melting in a converter, electric furnace or the like and performing secondary refining in a vacuum degassing furnace is suitable. The casting method is preferably a continuous casting method from the viewpoint of productivity and quality. Further, the effect of the present invention is not affected even if the direct feed rolling, in which the hot rolling is performed as it is, immediately after casting or after heating for the purpose of supplementary heating is performed. Furthermore, after the rough rolling, before the finish rolling, the hot rolled material may be heated, or even if the continuous hot rolling is performed by joining the rolled material after the rough rolling, and further, the heating material of the rolled material is heated. Even if continuous rolling is performed simultaneously, the effect of the present invention is not impaired.

  Steels having the compositions shown in Table 1 were melted in a converter and steel slabs were obtained by continuous casting. Next, these steel slabs were heated, hot-rolled, cooled and wound under the conditions shown in Table 2 to produce hot-rolled steel sheets having a thickness of 2.0 mm.

With respect to the obtained hot-rolled steel sheet, the amount of Ti and the amount of V contained in precipitates of less than 20 nm were determined by the following method.

Measurement of Ti content and V content in precipitates with a size of less than 20 nm The hot-rolled steel sheet obtained as described above was cut into a suitable size, and 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethyl chloride) was obtained. (Ammonium-methanol) About 0.2 g was subjected to constant current electrolysis at a current density of 20 mA / cm ² .
After the electrolysis, remove the sample piece with deposits on the surface from the electrolyte and immerse it in an aqueous solution of sodium hexametaphosphate (500 mg / l) (hereinafter referred to as the SHMP aqueous solution) to apply ultrasonic vibration. The precipitate was peeled off from the sample piece and extracted into an aqueous SHMP solution. Next, the SHMP aqueous solution containing the precipitate is filtered using a filter with a pore size of 20 nm, and the filtrate after filtration is analyzed using an ICP emission spectrophotometer, and the absolute amounts of Ti and V in the filtrate are determined. It was measured. Next, the absolute amounts of Ti and V were divided by the electrolytic weight to obtain the Ti amount and the V amount contained in the precipitate having a size of less than 20 nm. In addition, the electrolysis weight was calculated | required by measuring a weight with respect to the sample after deposit peeling, and subtracting from the sample weight before electrolysis.

In addition, from the center of the width direction at a position 30 m from the coil tip, a JIS No. 5 tensile test piece (parallel to the rolling direction), a hole expanding test piece and a sample for structure observation were collected, and the tensile strength was obtained by the following method: TS, YR, elongation: El, stretch flange characteristics after processing: λ ₁₀ and hardness difference were determined and evaluated.

Tensile strength: TS
Three JIS No. 5 test pieces were collected with the rolling direction as the tensile direction, and a tensile test was performed by a method based on JIS Z 2241 to determine tensile strength (TS), yield stress (YS), and elongation (El). YR was a value obtained by dividing the falling yield stress by TS.

Stretch flange characteristics after processing: λ ₁₀
Three test pieces for hole expansion test were collected, rolled at a stretch rate of 10%, and subjected to a hole expansion test in accordance with the Iron Federation Standard JFST 1001, and λ ₁₀ was obtained from the average of the three sheets.

Hardness difference: HV _S −HV _α
The tester used for the Vickers hardness test was one conforming to JISB7725. Take one sample for structure observation, reveal the structure with a 3% nital solution on the cross section parallel to the rolling direction, and indent each ferrite grain and bainite grain at a test load of 3g at 1/4 thickness position. It was. The hardness was calculated from the diagonal length of the indentation using the Vickers hardness calculation formula in JISZ2244. Measure the hardness of each of 30 ferrite grains and bainite grains, and calculate the hardness difference (HV _S −HV _α ) by taking the average value of each as the hardness of the ferrite phase (HV _α ) and the hardness of the bainite phase (HV _S ). It was.
The results obtained as described above are shown in Table 2 together with the production conditions.

From Table 2, in the present invention example, TS (strength) is 980 MPa or more, λ ₁₀ is 40% or more, YR is more than 85%, El (elongation) is 13% or more, and hot rolling with excellent stretch flange characteristics after processing A steel plate is obtained.

On the other hand, the comparative example, TS, YR, El, any one or more of the lambda ₁₀ is inferior.

  Steel having the composition shown in Table 3 was melted in a converter, and a steel slab was formed by continuous casting. Subsequently, these steel slabs were heated, hot-rolled, cooled and wound under the conditions shown in Table 4 to produce hot-rolled steel sheets having a thickness of 2.0 mm.

With respect to the obtained hot-rolled steel sheet, the amount of Ti and the amount of V contained in precipitates of less than 20 nm were determined in the same manner as in Example 1. Further, tensile strength: TS, YR, elongation: El, stretched flange characteristic after processing: λ ₁₀ and hardness difference were obtained and evaluated in the same manner as in Example 1.
Table 4 shows the results obtained as described above.

From Table 4, in the present invention example, a hot-rolled steel sheet having excellent stretch flange characteristics after processing with TS of 980 MPa or more, λ ₁₀ of 40% or more, YR of over 85%, and El (elongation) of 13% or more is obtained. It has been. Furthermore, it can be seen that TS is improved in steel added with Cr, W and Zr compared to steel No. 1 (Table 2).

Steels having the compositions shown in Tables 1 and 3 were melted in a converter and steel slabs were formed by continuous casting. Subsequently, these steel slabs were heated, hot-rolled, cooled and wound under the conditions shown in Table 5 to produce hot-rolled steel sheets having a thickness of 2.0 mm.
Here, the coiling temperature is a value obtained by measuring the coiling temperature at the center in the width direction of the steel strip in the longitudinal direction of the steel strip and averaging them. In addition, a radiation thermometer (NEC Sanei Co., Ltd. Model TH7800) capable of two-dimensional measurement of the steel sheet surface temperature was installed immediately before the winding device, and the temperature unevenness S on the steel sheet surface was determined and evaluated as follows. did.

Temperature irregularity S on the steel surface
The area of the low temperature part where the coiling temperature was locally measured with a radiation thermometer was less than 350 ° C. was determined, and the ratio of the area to the total area of the steel sheet was defined as temperature unevenness S (see the following formula).
S = (area of low temperature part) / (total area of steel plate) × 100 (%)
If S was less than 5%, there was no temperature unevenness.

Moreover, Ti amount and V amount contained in precipitates of less than 20 nm were determined for the obtained hot-rolled steel sheet by the same method as in Example 1. Further, tensile strength: TS, YR, elongation: El, stretched flange characteristic after processing: λ ₁₀ and hardness difference were obtained and evaluated in the same manner as in Example 1.

Furthermore, in order to investigate the material variation in the steel plate surface, the standard deviation of TS was determined as follows.
Standard deviation of TS: σ
At each position 100, 200, 400, 600, 700 m in the longitudinal direction from the coil tip, the direction parallel to the rolling direction is the longitudinal direction of the test piece, and the width of the steel sheet is 25 from the inside of both ends 25 m in the width direction. A total of 125 JIS5 tensile test pieces were collected at regular intervals, TS was obtained by the same method as described above, and the standard deviation σ was calculated.
The results obtained as described above are shown in Table 5 together with the production conditions.

According to Table 5, in the present invention example, TS (strength) is 980 MPa or more, λ ₁₀ is 40% or more, YR is more than 85%, and El (elongation) is 13% or more. A steel plate is obtained. In addition, by performing nucleate boiling cooling, the temperature unevenness in the coil is almost eliminated, and the standard deviation σ of TS is as small as 30 MPa or less, showing that the material variation in the steel sheet is small.
On the other hand, the comparative example, TS, YR, El, any one or more of the lambda ₁₀ is inferior.

  Since the steel sheet of the present invention has high strength and excellent stretch flange characteristics after processing, it is optimal as a part that requires stretch and stretch flange characteristics, such as a frame for automobiles and trucks.

Claims (5)

Ingredient composition is mass%, C: 0.08% to 0.20%, Si: 0.2% to 1.0%, Mn: 0.5% to 2.5%, P: 0.04% or less, S: 0.005% or less, Al: 0.05 % Or less, Ti: 0.07% or more and 0.20% or less, V: 0.05% or more and less than 0.20%, the balance is Fe and inevitable impurities, and the metal structure is ferrite with volume occupancy of 60% or more and 95% or less And the second phase has a bainite of 5% to 35% as the second phase, the Ti amount contained in the precipitate having a size of less than 20 nm is 450 mass ppm to 1800 mass ppm, and the V amount is 350 mass ppm to 1200 mass ppm. A high-strength steel sheet characterized in that the difference between the hardness of the bainite phase (HV _S ) and the hardness of the ferrite phase (HV _α ) (HV _S −HV _α ) is 300 or less and the YR exceeds 85%.   It is mass%, and further contains Cr: 0.01% or more and 1.0% or less, W: 0.005% or more and 1.0% or less, Zr: 0.0005% or more and 0.05% or less The high-strength steel plate according to claim 1. In mass%, C: 0.08% to 0.20%, Si: 0.2% to 1.0%, Mn: 0.5% to 2.5%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, Ti : Steel slab containing 0.07% or more and 0.20% or less, V: 0.05% or more and less than 0.20%, with the balance being composed of Fe and inevitable impurities, after heating to a temperature of 1150 ° C or more and 1350 ° C or less, Hot rolling is performed at a finish rolling temperature of 850 ° C or higher and 1000 ° C or lower, and then the first stage cooling is performed at an average cooling rate of 30 ° C / s or higher to a temperature of 650 ° C or higher and lower than 800 ° C. Air-cooled for a period of time, followed by second-stage cooling at a cooling rate of 20 ° C / s or more, winding at a temperature exceeding 300 ° C and 600 ° C or less, and satisfying formula (1). Method.
T1 ≦ 0.06 × T2 + 764 (1)
However, T1: First stage cooling stop temperature, T2: Winding temperature   The high-strength steel sheet manufacturing method according to claim 3, wherein in the second stage cooling, cooling is performed at a cooling rate of 120 ° C / s or more and nucleate boiling cooling in a temperature range of 500 ° C or lower. Method.   As a component composition, in mass%, Cr: 0.01% or more, 1.0% or less, W: 0.005% or more, 1.0% or less, Zr: 0.0005% or more, 0.05% or less, containing one or more kinds The manufacturing method of the high strength steel plate of Claim 3 or 4 characterized by these.