KR20100029138A - High-strength steel sheet - Google Patents

Abstract

The invention provides a high-strength steel sheet excellent in stretch-flange characteristics after working and in corrosion resistance after coating. A steel sheet which has a composition containing by mass C: 0.02 to 0.20%, Si: 0.3% or below, Mn: 0.5 to 2.5%, P: 0.06% or below, S: 0.01% or below, Al: 0.1% or below, Ti: 0.05 to 0.25%, and V: 0.05 to 0.25% with the balance consisting of Fe and unavoidable impurities and a substantially ferrite single-phase structure wherein the contents of Ti, V, and solid-soluted V in precipitates of less than 20nm in size are 200 to 1750 mass ppm, 150 to 1750 mass ppm, and 200 to less than 1750 mass ppm respectively.

Description

High Strength Steel Sheets {HIGH-STRENGTH STEEL SHEET}

The present invention relates to a high strength steel sheet excellent in stretch flange characteristics after processing and corrosion resistance after coating.

Formability (mainly elongation and elongation flange characteristics) is required for parts such as suspension members of automobiles and frames for trucks, and conventionally steel having a tensile strength of 590 MPa has been used. In recent years, however, from the viewpoint of reducing environmental loads and improving impact characteristics of automobiles, increasing the strength of automobile steel sheets has been promoted, and the use of steel having a tensile strength of 780 MPa has begun.

In general, steel materials are deteriorated in workability as strength increases. For this reason, studies have been made on steel sheets having high strength and high workability. As a technique of improving elongation and an elongation flange characteristic, patent documents 1-6 are mentioned, for example.

Patent document 1 is a ferrite single-phase structure, and carbides containing Ti and Mo having an average particle diameter of less than 10 nm are dispersed and precipitated. A high tensile steel sheet having excellent workability with a tensile strength of 590 MPa or more. A technique is disclosed.

In Patent Literature 2, C: 0.08 to 0.20%, Si: 0.001% or more and less than 0.2%, Mn: more than 1.0% and 3.0% or less, Al: 0.001 to 0.5%, V: more than 0.1% and 0.5% or less, Ti : 0.05% or more and less than 0.2% and Nb: 0.005% to 0.5%, and further

4.5 × 10^-5 ,(Equation 1) (Ti / 48 + Nb / 93) × C / 12

4.5 × 10 ^-5 ,

(V/51 ＋ Ti/48 ＋ Nb/93)/(C/12)

1.5, (Formula 2) 0.5

(V / 51 + Ti / 48 + Nb / 93) / (C / 12)

1.5,

0.80 (Equation 3) V + Ti × 2 + Nb × 1.4 + C × 2 + Mn × 0.1

0.80

It satisfies Equation 3, and has a steel structure containing residual Fe and unavoidable impurities, containing 70 vol% or more of ferrite having an average particle diameter of 5 μm or less and a hardness of 250 Hv or more, and having a strength and yield ratio of 0.80 or more and 0.80 or more. The technique regarding the high strength hot rolled sheet steel which has the is disclosed.

In Patent Literature 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 0.2%, Al: 0.001 to 3%, V: 0.1% It contains up to 1.5%, the remainder consists of Fe and impurities, the structure is a ferrite having an average particle diameter of 1 to 5 μm as the main phase, and the carbonitride of V having an average particle diameter of 50 nm or less is present in the ferrite particles. A technique related to a hot rolled steel sheet is disclosed.

Patent document 4 discloses a high strength steel sheet excellent in thermal stability obtained by depositing carbide in a steel structure. This thin steel sheet has a NaCl-type crystal structure represented by MC when carbide is M as a metal element, the metal element M is composed of two or more metals, and these two or more metals are regular in the crystal lattice. Characterized in that the superlattice structure arranged in the form.

Patent Literature 5 discloses the following hot rolled steel sheets. A component composition contains C: 0.0002-0.25%, Si: 0.003-3.0%, Mn: 0.003-3.0%, and Al: 0.002-2.0% by mass%, and remainder consists of Fe and an unavoidable impurity, P in an impurity is 0.15% or less, S is 0.05% or less, and N is 0.01% or less. And 70% or more of metal structures are ferrite phase by area ratio, the average crystal grain diameter is 20 micrometers or less, and aspect ratio is three or less. In addition, 70% or more of the ferrite grain boundary is composed of diagonal grain boundaries, and among the ferrite phases formed by the diagonal grain boundaries, the area ratio of precipitates having a maximum diameter of 30 μm or less and a minimum diameter of 5 nm or more is 2% or less of the metal structure. In addition, the average crystal grain size of the second phase having the largest area ratio is 20 µm or less among the ferrite phase and the residual phase except for the precipitate, and a diagonal grain boundary of the ferrite phase exists between the nearest second phases.

0.03 %, N

0.005 %, Ti : 0.05 ∼ 0.5 % 를 함유하고, 추가로 Ti-48/12C-48/14N-48/32S

0 % 를 만족하는 범위에서 Ti 를 함유하고, 잔부가 Fe 및 불가피적 불순물로 이루어지고, 적어도 판 두께의 1/2 두께에 있어서의 판면의 {100}<011> ∼ {223}<110> 방위군의 X 선 랜덤 강도비의 평균값이 3 이상, 또한, {554}<225>, {111}<112> 및 {111}<110> 의 3 방위의 X 선 랜덤 강도비의 평균값이 3.5 이하이고, 적어도 일방의 강판 표면의 산술 평균 거칠기 Ra 가 1 ∼ 3.5 인 강판에 윤활 효과가 있는 조성물이 도포되어 있는 것을 특징으로 하는 형상 동결성이 우수한 조임 가능한 버링성 고강도 박강판에 대해 개시되어 있다. In patent document 6, by mass%, C: 0.01-0.1%, S

0.03%, N

0.005%, Ti: 0.05-0.5%, and further Ti-48 / 12C-48 / 14N-48 / 32S

{100} <011> to {223} <110> orientation group of the plate surface containing Ti in the range which satisfy | fills 0%, remainder consists of Fe and an unavoidable impurity, and is at least 1/2 thickness of plate | board thickness. The average value of the X-ray random intensity ratios is 3 or more, and the average value of the X-ray random intensity ratios of the three orientations of {554} <225>, {111} <112> and {111} <110> is 3.5 or less, A fastening burring-type high strength steel sheet excellent in shape freezing property is disclosed in which a composition having a lubricating effect is applied to a steel sheet having an arithmetic mean roughness Ra of at least one steel sheet surface of 1 to 3.5.

Japanese Patent No. 3591502 Japanese Laid-Open Patent Publication 2006-161112 Japanese Unexamined Patent Publication No. 2004-143518 Japanese Unexamined Patent Publication No. 2003-321740 Japanese Laid-open Patent Publication 2003-293083 Japanese Unexamined Patent Publication No. 2003-160836

However, the above-described prior art has the following problems.

In Patent Documents 1 and 4, since Mo is contained, there is a problem that causes a significant increase in cost in relation to the recent increase in the raw material price of Mo.

In addition, the globalization of the automobile industry is progressing, and steel sheets used in automobiles are used under severe corrosive environments, and thus higher post-painting corrosion resistance is required for the steel sheets. On the other hand, since the addition of Mo inhibits the formation or growth of chemical conversion crystals, the corrosion resistance after coating of the steel sheet is lowered, and the above requirements cannot be met. That is, in the steel of patent document 1 and patent document 4, the post-painting corrosion resistance which satisfy | fills the requirements of the recent automotive industry is not acquired.

On the other hand, due to recent advances in press technology, processing processes such as draw (tightening and elongation) → trim (hole drilling) + wrist like (hole widening) have begun to be adopted. The elongated flange portion of the steel sheet to be molded through requires the elongated flange characteristics after draw and trim, that is, after processing. However, in patent documents 2, 3, and 4, when obtaining TS of 780 Mpa or more, the elongation flange characteristic after sufficient processing is not necessarily obtained. Nb added to Patent Document 3 has a high function of suppressing recrystallization of austenite after hot rolling. Therefore, there exists a problem of remaining unrefined grain in a steel plate and reducing workability. Moreover, there exists a problem of increasing the rolling load at the time of hot rolling.

In patent document 5, the ferrite single phase steel plate (For example, Table 6 of Example, Test Nos. 1-5, and Table 8 of Example, Test No. 45) of tensile strength TS to 422 MPa, and tensile strength TS are 780 MPa or more. A composite steel sheet (eg, Example Table 6, Test Nos. 33 to 36 and Example Table 8, Test No. 49) composed of a ferrite phase and a second phase is disclosed. In the steel plate of these patent documents 5, the solid solution strengthening by Si or Mn and the transformation of the transformation structure using the hard 2nd phase are mainly utilized. Therefore, the steel sheet is cooled to a temperature range of 600 to 800 ° C at an average cooling rate of 30 ° C./s or more within 2 seconds after completion of the finish rolling, and then air-cooled for 3 to 15 seconds, after which the average cooling rate is 30 ° C. It needs to be cooled by water for more than a second. Thereby, two-phase separation at the time of ferrite transformation is accelerated | stimulated, and the structure of a steel plate becomes a composite structure by a ferrite phase and a 2nd phase. Moreover, finish rolling temperature is set to the temperature range lower than (Ae3 point + 100 degreeC)-Ae3 point and the temperature range considered suitable when manufacturing this invention mentioned later. For example, in the composite steel sheet (Table 6 of Examples, Test No. 33-36) whose tensile strength TS is 780 Mpa or more, finish rolling temperature was 871 degreeC-800 degreeC. When the finish rolling temperature is low, since the solid solution limit of carbide-forming elements such as Ti in the austenite phase is lowered, and precipitation sites are introduced by processing by rolling, precipitates of 20 nm or more are produced. This phenomenon is called strain induced precipitation. In the steel plate and the manufacturing method of patent document 5, since distortion induced precipitation generate | occur | produces, the production amount of the precipitate which is 20 nm or more in size increases.

In addition, Patent Literature 5 discloses a technique for realizing the production of a ferrite single phase structure by reducing the C content of the steel composition very much and reducing the content of Mn, which is an austenite stabilizing element (see Table 2 of Examples). Steel number AA to AE). However, in this case, since the content of Mn, which is also a solid solution strengthening element, decreases, the amount of solid solution strengthening decreases. In addition, by decreasing the C content, the amount of precipitation of carbides, such as Ti and Nb, which are effective in strengthening precipitation, is reduced, and the amount of precipitation strengthening is also reduced. As a result, even when the solid solution strengthening amount and the precipitation strengthening amount are combined, in the case of the ferritic single-phase tissue steel sheet, the strength of 780 MPa or more cannot be derived (see: Table 6 of Examples, Test Nos. 1 to 5, and Table 8 of Examples). , Test number 45).

For the above reason, in the technique of patent document 5, the steel plate aimed at by this invention is substantially a ferrite single phase, tensile strength is 780 Mpa or more, and cannot also manufacture the steel plate which has another characteristic.

In patent document 6, although the steel plate (for example, steel symbol A-4, A-8, A-10, C, E, H in Example Table 2) whose tensile strength (sigma) _B is 780 Mpa or more is disclosed, these Since YR (unit%, where YR is _Y , σ _Y / σ _B × 100) of the steel sheet is as low as 69% to 74%, these steel sheets are inferred to include a hard second phase such as a martensite phase. do.

From this, the design thought of the steel plate of 780 Mpa or more in patent document 6 thinks that it utilizes mainly solid solution strengthening by Si or Mn and transformation structure strengthening using a hard 2nd phase similarly to patent document 5 do. Therefore, similarly to patent document 5, it is necessary to perform rolling of 25% or more of the total compaction rate at the finishing rolling temperature (Ar3 point + 100 degrees C or less) lower than the temperature range considered suitable when manufacturing this invention mentioned later. There is. For example, according to the Example of patent document 6, the finish rolling temperature of the steel plate whose tensile strength (sigma) _B is 780 Mpa or more was 800 degreeC-890 degreeC. In the steel plate and the manufacturing method in Patent Document 6, similar to Patent Document 5, deformation-induced precipitation occurs and the amount of precipitates having a size of 20 nm or more increases, resulting in a substantially ferrite structure of the present invention. A steel sheet having a single phase and a tensile strength of 780 MPa or more and other properties cannot be produced.

In view of such circumstances, an object of the present invention is to provide a high-strength steel sheet excellent in elongated flange characteristics after processing and excellent in corrosion resistance after coating.

MEANS TO SOLVE THE PROBLEM The present inventors obtained the following knowledge, as a result of examining in order to obtain the high strength hot rolled sheet steel which is excellent in the elongation flange characteristic after processing and corrosion resistance after coating, and tensile strength is 780 Mpa or more.

i) In order to obtain a steel sheet excellent in high strength and corrosion resistance after coating, it is necessary to refine the precipitate (less than 20 nm in size) to increase the ratio of the fine precipitate (less than 20 nm in size). In order to keep the precipitate in a fine state, it is possible to include Ti-Mo or Ti-V as a precipitate, but a composite precipitation of Ti and V is useful from the viewpoint of improving corrosion resistance after coating. .

Ii) The solid solution of V is important for the improvement of the elongation flange property after processing, and the solid solution amount of V which is optimal for the characteristic improvement exists.

This invention is made | formed based on the above knowledge, The summary is as follows.

[1] In mass%, C: 0.02% or more, 0.20% or less, Si: 0.3% or less, Mn: 0.5% or more and 2.5% or less, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti : 0.05% or more and 0.25% or less, V: 0.05% or more and 0.25% or less, the remainder being a component composition consisting of Fe and an unavoidable impurity, and substantially a ferrite single phase structure. In the ferrite single phase structure, the size is 20 nm. A high strength steel sheet having a structure in which Ti contained in a precipitate of less than 200 mass ppm or more and 1750 mass ppm or less, V is 150 mass ppm or more and 1750 mass ppm or less and solid solution V is 200 mass ppm or more and less than 1750 mass ppm.

[2] The above-mentioned [1], in mass%, further one or two or more of Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less. A high strength steel sheet comprising a.

[3] The high strength steel sheet according to the above [1] or [2], wherein the tensile strength TS is 780 MPa or more.

[4] The high strength steel sheet according to the above [1] or [2], wherein the one-side maximum peeling width after the tape peeling test in the salt hot water immersion test is 3.0 mm or less.

[5] The high strength steel sheet according to the above [3], wherein the one-side maximum peeling width after the tape peeling test in the salt hot water immersion test is 3.0 mm or less.

[6] The high strength steel sheet according to the above [1] or [2], wherein the stretch flange characteristic λ ₁₀ after rolling at a stretch rate of 10% is 60% or more.

[7] The high strength steel sheet according to the above [3], wherein the elongation flange characteristic λ ₁₀ after rolling at an elongation of 10% is 60% or more.

In addition, in this specification,% and ppm which represent the component of steel are all mass% and mass ppm. The high strength steel sheet in the present invention is a steel sheet having a tensile strength (hereinafter sometimes referred to as TS) of 780 MPa or more, and the hot rolled steel sheet, and further, a surface treatment such as plating treatment is applied to these steel sheets, for example. Surface-treated steel sheet is also an object.

Moreover, the characteristic aimed at this invention is 10% of elongation rate, the elongation flange characteristic ((lambda) ₁₀ ) after rolling is 60% or more, and the one side maximum peeling width after the tape peeling test in the salt hot water immersion test (SDT) mentioned later is 3.0 mm or less.

According to the present invention, a high strength hot rolled steel sheet having excellent stretch flange characteristics after processing and corrosion resistance after coating and having a TS of 780 MPa or more is obtained. In addition, in the present invention, since the above effects are obtained without adding Mo, cost reduction can be achieved.

For example, by using the high strength hot rolled sheet steel of this invention for the suspension member of a motor vehicle, the frame for trucks, etc., plate | board thickness can be reduced, the environmental load of a motor vehicle is reduced, and the impact characteristic is anticipated to improve significantly.

Implement the invention  Best form for

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

(1) First, the reason for limitation of the chemical component (component composition) of the steel in this invention is demonstrated.

C: 0.02% or more and 0.20% or less

C is an element which forms carbide with Ti and V and precipitates in ferrite, thereby contributing to the strengthening of the steel sheet. In order to make TS 780 MPa or more, it is necessary to make C amount 0.02% or more. On the other hand, when the amount of C exceeds 0.20%, the elongation flange characteristic after processing will fall by coarsening of a precipitate and formation of a 2nd phase structure. From the above, the amount of C is made into 0.02% or more and 0.20% or less, Preferably, you may be 0.03% or more and 0.15% or less.

Si: 0.3% or less

Si is an element contributing to solid solution strengthening, but when added in excess of 0.3%, cementite is formed at grain boundaries, and the elongated flange characteristics after processing decrease. Therefore, the amount of Si is made into 0.3% or less. Preferably, you may be 0.001% or more and 0.2% or less.

Mn: 0.5% or more and 2.5% or less

Mn is an element contributing to strengthening employment. However, if the amount is less than 0.5%, TS of 780 MPa or more is not obtained. On the other hand, when Mn amount exceeds 2.5%, weldability will fall remarkably. As mentioned above, Mn amount is 0.5% or more and 2.5% or less, Preferably it is 0.6% or more and 2.0% or less.

P: 0.06% or less

P segregates at the former austenite grain boundary, resulting in low temperature toughness degradation and workability deterioration. Therefore, it is preferable to reduce P amount as much as possible, and you may be 0.06% or less. Preferably, you may be 0.001% or more and 0.055% or less.

S: 0.01% or less

S segregates at the former austenite grain boundary or precipitates in a large amount as MnS, thereby degrading low-temperature toughness. Moreover, extension flange property is remarkably reduced regardless of the presence or absence of processing. Therefore, it is preferable to reduce S amount as much as possible, and you may be 0.01% or less. Preferably, it is at least 0.001% and at most 0.005%.

Al: 0.1% or less

Al is added as a deoxidizer of steel and is an element effective in improving the cleanliness of steel. In order to acquire this effect, it is preferable to contain 0.001% or more. However, when it exceeds 0.1%, inclusions generate | occur | produce abundantly and it will cause a steel plate damage. Therefore, Al amount is made into 0.1% or less. Preferably, they are 0.01% or more and 0.04% or less.

Ti: 0.05% or more and 0.25% or less

Ti is a very important element in the precipitation strengthening of ferrite, and is an important requirement in obtaining the effect of the present invention. If the amount of Ti is less than 0.05%, it is difficult to secure the required strength. On the other hand, when it exceeds 0.25%, the effect will be saturated and only a cost will increase. Therefore, Ti amount is 0.05% or more and 0.25% or less, Preferably it is 0.08% or more and 0.20% or less.

V: 0.05% or more and 0.25% or less

V is an element contributing to the improvement of strength as precipitation strengthening or solid solution strengthening, and like Ti, it is an important requirement in obtaining the effect of the present invention. When the appropriate amount is added in combination with Ti, the particle size (hereinafter sometimes referred to as "size") tends to precipitate as fine Ti-V carbides of less than 20 nm, and also decreases the corrosion resistance after coating such as Mo. There is no. If the amount of V is less than 0.05%, the above-mentioned addition effect is insufficient. On the other hand, when the amount of V exceeds 0.25%, the effect is saturated and only the cost increases. Therefore, the amount of V is 0.05% or more and 0.25% or less, Preferably, it is 0.06% or more and 0.20% or less.

As the above essential addition elements, the steel of the present invention obtains the desired properties, but, in addition to the above essential addition elements, Cr: 0.01% or more and 0.5% or less, W: 0.005% or more 0.2 for the following reasons You may add any 1 type or 2 types or more of% or less and Zr: 0.0005% or more and 0.05% or less.

Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less

Cr, W and Zr, like V, have a function of forming a precipitate or strengthening ferrite in a solid solution state. If the amount of Cr is less than 0.01%, the amount of W is less than 0.005%, or the amount of Zr is less than 0.0005%, it hardly contributes to high strength. On the other hand, when Cr amount exceeds 0.5%, W amount exceeds 0.2%, or Zr amount exceeds 0.05%, workability deteriorates. Therefore, when any one or two or more of Cr, W, and Zr are added, the amount of the additive is Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less. do. Preferably, they are Cr: 0.03% or more and 0.3% or less, W: 0.01% or more and 0.18% or less, Zr: 0.001% or more and 0.04% or less.

In addition, remainder other than the above consists of Fe and an unavoidable impurity. As an unavoidable impurity, for example, since O forms a nonmetallic inclusion and adversely affects quality, it is preferable to reduce it to 0.003% or less. Moreover, in this invention, you may contain Cu, Ni, Sn, and Sb in 0.1% or less of range as a trace element which does not inhibit the effect of this invention.

(2) Next, the structure of the high strength steel plate of this invention is demonstrated.

Substantially ferrite single phase tissue

TS is 780 MPa or more, and ferrite having a low dislocation density is effective for improving the elongation flangeability after processing, and it is effective to form a single phase structure. In particular, by using a ferrite single-phase structure rich in ductility, the effect of improving the elongation flange properties after processing becomes remarkable. However, it does not necessarily need to be a ferrite single phase structure completely, and the said effect is fully acquired as it is a ferrite single phase structure substantially. Herein, the ferrite single phase structure substantially allows a small amount of other phases or precipitates in addition to the carbide of the present invention, and preferably the volume ratio of the ferrite is 95% or more. If the volume fraction is in the range of up to 5%, even if the structure of cementite, pearlite or bainite is included, the characteristics of the present invention are not affected.

In addition, the volume ratio of the ferrite exhibits the microstructure of the plate thickness cross section parallel to the rolling direction at 3% nital, and observes the plate thickness 1/4 position at 1500 times using a scanning electron microscope (SEM), For example, it is calculated | required by measuring the ferrite area ratio using the image processing software "particle analysis II" by Sumitomo Metal Technology Co., Ltd. make.

In the ferrite single-phase structure, 200 ppm or more and 1750 ppm or less of Ti and 150 ppm or more and 1750 ppm or less of Ti contained in precipitates having a size of less than 20 nm are included.

In the high strength steel sheet of the present invention, the precipitate containing Ti and / or V is mainly precipitated in ferrite as a carbide. This is considered to be because the solid solution limit of C in ferrite is small and supersaturated C tends to precipitate as a carbide in ferrite. And by such a precipitate, soft ferrite becomes hard (high intensity | strength) and TS of 780 Mpa or more is obtained. Moreover, YS becomes high and 83% or more of YR (= YS / YR) is obtained.

In order to obtain a high strength steel sheet, as described above, it is important to refine the precipitate (less than 20 nm in size) and to increase the ratio of the fine precipitate (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 sufficiently hardened, so the strength may be lowered.

Moreover, as a result of examination, it became clear that the fine particle size is important about corrosion resistance after coating. In conventional Ti-based (Ti alone) HSLA steel, as the amount of Ti added increases, the precipitate tends to coarsen. Therefore, in such a steel plate, corrosion resistance after coating also tended to fall with the strength fall. Although the reason for the fall of corrosion resistance after coating by coarsening of a precipitate is not clear, it is thought that coarse precipitate inhibits formation or growth of chemical conversion crystals.

As mentioned above, it is preferable that the size of a precipitate shall be less than 20 nm. This fine precipitate of less than 20 nm is achieved by adding both Ti and V. V mainly forms complex carbides with Ti. Although the reason is not clear, it turned out that these precipitates exist in the stable fine state under high temperature long time in the winding temperature of the range of this invention.

In addition, control of the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm becomes important. If the amount of Ti contained in the precipitates less than 20 nm is less than 200 ppm and the amount of V is less than 150 ppm, the number density of the precipitates is decreased, and the interval between each precipitate is widened, so that the effect of suppressing the shift of dislocation becomes small. I could see that. Therefore, since ferrite cannot be hardened sufficiently, the strength whose TS is 780 Mpa or more cannot be obtained. When the amount of Ti contained in the precipitates less than 20 nm is 200 ppm or more and the amount of V contained in the precipitates less than 20 nm is less than 150 ppm, the precipitates tend to be coarse, so that an intensity of TS of 780 MPa or more is obtained. You may not lose. When the amount of Ti contained in the precipitates less than 20 nm is less than 200 ppm and the amount of V contained in the precipitates less than 20 nm is 150 ppm or more, the precipitation efficiency of V deteriorates, so that the strength of TS of 780 MPa or more is not obtained. There may be a case. On the other hand, when the amount of Ti contained in the precipitate of less than 20 nm exceeds 1750 ppm or the amount of V exceeds 1750 ppm, the corrosion resistance after coating decreases, so that the target characteristics cannot be obtained. This is considered to be because a large amount of fine precipitates inhibit the formation or growth of chemical conversion crystals on the surface of the steel sheet. 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 need to be satisfied.

(Ti/48)/(V/51)

2.5 일 때, 785 MPa 이상의 TS 가 얻어져, 보다 바람직한 상태가 되는 것을 알 수 있었다. 이유는 분명하지 않지만, Ti 와 V 의 조성비가 최적화됨으로써, 열적인 안정성이 향상되었기 때문인 것으로 생각된다. In addition, the ratio of Ti to V contained in precipitates smaller than 20 nm in size was 0.4

(Ti / 48) / (V / 51)

When it was 2.5, TS of 785 MPa or more was obtained, indicating that a more preferable state was obtained. Although the reason is not clear, it is thought that the thermal stability improved because the composition ratio of Ti and V was optimized.

(Ti/48)/(V/51)

2.5 가 바람직하다. As mentioned above, Ti amount contained in the precipitate whose size is less than 20 nm is 200 ppm or more and 1750 ppm or less, and V amount is made into 150 ppm or more and 1750 ppm or less. Furthermore, the ratio of Ti amount and V amount contained in the precipitates smaller than 20 nm is 0.4

(Ti / 48) / (V / 51)

2.5 is preferred.

In addition, precipitates and / or inclusions may be referred to collectively as precipitates and the like.

In addition, the said Ti amount and V amount can be controlled by winding temperature. As for the winding temperature at this time, 500 degreeC or more and 700 degrees C or less are preferable. When the winding temperature exceeds 700 ° C, coarsening of precipitates occurs, and the amount of Ti and V contained in the precipitates less than 20 nm is respectively less than 200 ppm and less than 150 ppm, and TS of 780 MPa or more is not obtained. Moreover, even if a coiling temperature is less than 500 degreeC, the precipitation amount of Ti and V contained in less than 20 nm of precipitate becomes 200 ppm and less than 150 ppm, respectively. This is considered to be because diffusion of Ti and V is insufficient because the coiling temperature is low.

The Ti amount and V amount contained in the precipitate whose size is less than 20 nm can be confirmed by the following method.

After electrolyzing a sample in electrolyte solution a predetermined amount, a sample piece is taken out of electrolyte solution and immersed in the solution which has dispersibility. Next, the precipitate contained in this solution is filtered using the filter of 20 nm of pore diameters. The precipitate which passed this filter of 20 nm of diameters with a filtrate is less than 20 nm in size. Subsequently, the filtrate after filtration was appropriately selected from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, and the like, and analyzed to determine the amount of Ti and V contained in precipitates smaller than 20 nm in size. Obtain

Organizations with a solid-solution V content of 200 ppm or more and less than 1750 ppm

In the present invention, employment V is the most important requirement. The solid solution of V is important for the improvement of the extension flange characteristic after processing. If the solid solution V is less than 200 ppm, the effect is insufficient, and in order to obtain the effect, the solid solution V amount is required to be 200 ppm or more. On the other hand, when the amount of solid solution V was 1750 ppm or more, since the effect was saturated, it was set as an upper limit.

As mentioned above, the solid-solution V amount shall be 200 ppm or more and less than 1750 ppm. In addition, although the workability | liquidity falls slightly with the increase of intensity | strength of this invention, within the range whose Ti amount contained in the precipitate whose size is less than 20 nm is 1750 ppm or less and V amount is 1750 ppm or less, the solid solution V amount is 200 By setting it as ppm or more, the extension flange characteristic after the target process is fully ensured.

In addition, about the amount of solid solution V is 200 ppm or more and less than 1750 ppm, for example, it can confirm by the following method.

After electrolyzing a sample by predetermined amount in a nonaqueous solvent-type electrolyte solution, elemental analysis is performed using an electrolyte solution as an analysis solution. As an analysis method, inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, or atomic absorption spectrometry, etc. are mentioned.

(3) Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.

The high strength steel sheet of this invention heats a steel slab adjusted to the said chemical composition range, for example to 1150 degreeC or more and 1350 degrees C or less, and then hot-rolls by setting finish rolling temperature to 850 degreeC or more and 1100 degrees C or less. Then, it is obtained by winding up at 500 to 700 degreeC. These suitable conditions are demonstrated in detail below.

Steel slab heating temperature: more than 1150 ℃ 1350 ℃

Carbide-forming elements such as Ti or V exist mostly as precipitates in steel slabs. In order to precipitate as a target in a ferrite structure after hot rolling, it is necessary to dissolve the precipitate which precipitated as carbide before hot rolling once. For that purpose, it is necessary to heat above 1150 degreeC.

Below 1150 ° C, since carbides having a size of 20 nm or more that do not contribute to precipitation strengthening or corrosion resistance after coating remain, the amount of Ti and V related to the production of fine precipitates having a size of less than 20 nm necessary for obtaining the effect of the present invention. The amount is reduced, and the amount of precipitates having a size of less than 20 nm at the time of winding to be described later cannot be obtained as a target. In the method for producing a steel sheet of the present invention, it is most preferable to precipitate as fine carbide containing Ti or V at the time of winding after finishing rolling, while carbides containing Ti or V are dissolved at the time of slab heating and finish rolling. It is a preferred form. Therefore, as a temperature at which the said carbide dissolves almost completely, it is more preferable to make heating temperature into 1170 degreeC or more.

On the other hand, when it heats more than 1350 degreeC, a grain size will become coarse, and both the extension flange characteristic and extension characteristic after processing will deteriorate. In consideration of such heat treatment conditions afterwards, coarsening of the crystal grain size can be almost completely prevented when the temperature is 1300 ° C or lower.

Therefore, as for slab heating temperature, 1150 degreeC or more and 1350 degrees C or less are preferable. More preferably, they are 1170 degreeC or more and 1300 degrees C or less.

Finish rolling temperature in hot rolling: 850 ° C or more and 1100 ° C or less

In order to obtain the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm in the present invention, control of the finish rolling temperature becomes important. It is preferable to hot-roll the steel slab after a process at the finishing rolling temperature of 850 degreeC-1100 degreeC which is the finishing temperature of hot rolling. If the finish rolling temperature is less than 850 ° C., the sheet is rolled in the region of ferrite + austenite to form a whole ferrite structure, and thus the stretching flange characteristic and the stretching characteristic after processing may deteriorate. Moreover, even if the precipitate which is precipitated as a carbide before rolling by performing steel slab heating temperature at 1150 degreeC or more once melt | dissolved, when the finishing rolling temperature is less than 850 degreeC, carbide containing Ti or V will be formed by strain induced precipitation. Precipitates. Therefore, the amount of Ti and the amount of V associated with the production of fine precipitates having a size of less than 20 nm required for the effect of the present invention are reduced, and the amount of precipitates having a size of less than 20 nm at the time of winding to be described later cannot be obtained as desired. That is, it becomes important to advance to the next winding process in the state which the carbide containing Ti or V once melted at the time of slab heating mentioned above melt | dissolved as much as possible also in this finishing rolling. Therefore, in order to maintain the state in which carbide was melt | dissolved, it is more preferable that finish rolling temperature shall be 935 degreeC or more.

On the other hand, when the finish rolling temperature exceeds 1100 ° C, ferrite particles are coarsened, so TS of 780 MPa may not be obtained. In order to prevent the coarsening of ferrite particles, it is more preferable to set it as 990 degreeC or less.

Therefore, as for finish rolling temperature, 850 degreeC or more and 1100 degrees C or less are preferable. More preferably, they are 935 degreeC or more and 990 degrees C or less.

Winding temperature: above 500 ℃ 700 ℃

In order to obtain the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm in the present invention, control of the winding temperature becomes important. As described above, in this winding process, it is the most preferable production form that a large number of precipitation sites are formed, carbides are precipitated at this precipitation site, and further, the growth of the carbides is suppressed so as not to be 20 nm or more in size. to be. In order to make a structure substantially a ferrite single phase structure, and to acquire the characteristic of this invention, as for winding temperature, 500 degreeC or more and 700 degrees C or less are preferable.

In the present invention, when the coiling temperature is less than 500 ° C, the amount of precipitation of carbides containing Ti and / or V may become insufficient, resulting in a decrease in strength. In addition, a bainite phase may be produced and a ferrite single phase structure may not be obtained.

In order to form many precipitation sites and to produce carbide in this precipitation site, it is preferable that temperature is higher and it is more preferable that it is 550 degreeC or more.

On the other hand, when the coiling temperature exceeds 700 ° C., coarsening of the precipitated carbide occurs, which may cause a decrease in strength. Moreover, a pearlite phase tends to generate | occur | produce, and may cause the fall of the elongation flange property after a process. If it is 650 degrees C or less, since coarsening of the carbide which precipitated reliably can be prevented, it is more preferable.

Therefore, the winding temperature is preferably 500 ° C or more and 700 ° C or less, and more preferably 550 ° C or more and 650 ° C or less.

The steel plate of this invention includes what surface-treated and surface-coated to the surface. In particular, the steel sheet of the present invention can be preferably applied to a hot dip galvanized steel sheet formed by forming a hot dip galvanized coating film. That is, since the steel plate of this invention has favorable workability, even if it forms a hot-dip galvanized coating film, favorable workability can be maintained. Here, the hot dip galvanizing is a hot dip galvanizing mainly zinc and zinc (that is, containing about 90% or more), and also includes an alloy element such as Al, Cr, in addition to zinc, Even in the state of performing interlocking plating, you may perform an alloying process after plating.

In addition, the solvent method of steel is not specifically limited, All the well-known solvent methods can be adapted. For example, as a solvent method, the method of solvent-processing with a converter, an electric furnace, etc., and performing secondary refining in a vacuum degassing furnace is preferable. As for the casting method, continuous casting is preferable from the viewpoint of productivity and quality. Moreover, even if it carries out the direct transfer rolling which hot-rolls as it is immediately after casting or after heating for the purpose of heat retention, it does not affect the effect of this invention. Moreover, after rough rolling, you may heat a hot rolled material before finishing rolling, and it carries out continuous hot rolling which joins and rolls a rolled material after rough rolling, Furthermore, even if heating and continuous rolling of the heating material of a rolled material are performed simultaneously, The effect of is not inhibited.

Example

Example 1

The steel of the composition shown in Table 1 was melted in the converter, and it was set as the steel slab by continuous casting. Next, these steel slabs were heated, hot rolled, and wound up under the conditions shown in Table 2 to produce a hot rolled steel sheet having a sheet thickness of 2.0 mm.

About the obtained hot rolled sheet steel, a microstructure was analyzed by the method shown below, and the Ti amount, V amount, and solid solution V contained in the precipitate below 20 nm were calculated | required. In addition, the tensile strength: TS, the elongated flange characteristics after processing (lambda) _10, and the corrosion resistance after coating: SDT one side maximum peeling width were calculated | required and evaluated by the method shown below.

Microstructure Interpretation

The hot rolled steel sheet obtained by the above was cut | disconnected to the suitable magnitude | size, and constant current electrolysis was carried out about 10 g in 10% AA type | system | group electrolyte solution (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) by a current density of 20 mA / cm <2>. It was.

Determination of Ti content and V content in precipitates less than 20 nm in size

After electrolysis, a sample piece having a precipitate adhered to the surface thereof is taken out of the electrolyte solution, immersed in an aqueous solution of sodium hexametaphosphate (500 mg / L) (hereinafter referred to as SHMP aqueous solution), an ultrasonic vibration is applied, and the precipitate is sampled. It peeled from and extracted in SHMP aqueous solution. Next, the SHMP aqueous solution containing a precipitate was filtered using the filter of 20 nm of pore diameters, and the filtrate after filtration was analyzed using the ICP emission spectrophotometer, and the absolute amount of Ti and V in a filtrate was measured. Subsequently, the absolute amounts of Ti and V were divided by the electrolytic weight to obtain Ti amounts and V amounts contained in precipitates having a size of less than 20 nm. In addition, the electrolytic weight was calculated | required by measuring the weight with respect to the sample after precipitate peeling, and subtracting from the sample weight before electrolysis.

Determination of the amount of solid solution V

The electrolytic solution after electrolysis was used as the analysis solution, and Fe concentration was measured as V and a comparative element using ICP mass spectrometry. Based on the obtained concentration, the concentration ratio of V to Fe was calculated, and the amount of V in the solid solution state was determined by further multiplying the content of Fe in the sample. In addition, the content rate of Fe in a sample can be calculated | required by subtracting the sum total of composition values other than Fe from 100%.

TS

Using the JIS No. 5 test piece as the rolling direction as the tensile direction, a tensile test was conducted by a method in accordance with JIS Z 2241, and TS was obtained.

Stretch Flange Characteristics After Machining: λ ₁₀

After rolling at an elongation rate of 10%, a hole expansion test was performed in accordance with the steel sheet standard JFST 1001 to obtain λ ₁₀ .

Corrosion resistance after painting: SDT one side maximum peeling width

Chemical conversion treatment is a degreasing agent of Nippon Paint Co., Ltd. product; Surf cleaner ECO90, surface conditioner; Surf pine 5N-10, chemical conversion agent; Surf Dyne SD2800 was used, and each temperature or concentration condition was carried out under conditions worse than standard conditions. As an example of the standard condition, the degreasing step has a concentration of 16 g / L, a treatment temperature of 42 to 44 ° C., a treatment time of 120 s, a spray degreasing, and a surface adjustment step of a total alkalinity of 1.5 to 2.5 points, free acidity of 0.7 to 0.9 points, Accelerator concentration was set to 2.8 to 3.5 points, process temperature 44 degreeC, and processing time 120s. As poor conditions, the process temperature in the chemical conversion treatment process was reduced to 38 degreeC. Then, the electrodeposition coating agent by Nippon Paint Co., Ltd .; Electrodeposition coating was performed using V-50. The adhesion amount of the chemical conversion treatment film was 2 to 2.5 g / m 2, and the electrodeposition coating aimed at a film thickness of 25 μm.

Evaluation of corrosion resistance after coating was performed by the salt hot water immersion test (SDT). Crosscut scratches were made with a cutter on a sample subjected to chemical treatment and electrodeposition coating, and then immersed in salted hot water (5% NaCl: 55 ° C) for 10 days, washed with water, dried, and tape peeled to the cut scratches. The maximum peeling widths on the left and right sides of the cut scratches were measured. If one side maximum peeling width is 3.0 mm or less, it can be said that corrosion resistance after coating is favorable.

The result obtained by the above is shown in Table 2 with a manufacturing condition.

From Table 2, in the example of this invention, TS is 780 Mpa or more, (lambda) ₁₀ is 60% or more, SDT single side maximum peeling width is 3.0 mm or less, and it is a hot rolled steel plate excellent in the extension flange characteristic after processing and corrosion resistance after coating. It can be seen that.

On the other hand, the comparative example is inferior to any one or more of TS (strength), (lambda ₁₀ ) (extension flange property after processing), and SDT single side maximum peeling width (corrosion resistance after painting).

Example 2

The steel of the composition shown in Table 3 was melted in the converter, and it was set as the steel slab by continuous casting. Next, these steel slabs were heated, hot rolled and rolled up under the conditions shown in Table 4 to produce a hot rolled steel sheet having a sheet thickness of 2.0 mm.

About the obtained hot rolled sheet steel, a microstructure was analyzed by the same method as Example 1, and the Ti amount, V amount, and solid solution V contained in the precipitate below 20 nm were calculated | required. In addition, tensile strength: TS, extension flange characteristics after processing: λ ₁₀ and corrosion resistance after coating: SDT one side maximum peeling width were determined and evaluated in the same manner as in Example 1.

Table 4 shows the results obtained by the above.

From Table 4, in the example of this invention, TS is 780 Mpa or more, (lambda) ₁₀ is 60% or more, and SDT single side maximum peeling width is 3.0 mm or less, and it is a hot rolled steel plate excellent in the extension flange characteristic after processing and corrosion resistance after coating. It can be seen that.

Moreover, compared with steel plate No. 1 (Table 2), it turns out that TS is more improved in steel plate No 25-28 and 35-37 which added Cr, W, or Zr.

The steel sheet of the present invention is high strength and has excellent stretch flange characteristics after processing and corrosion resistance after coating, so that it is most suitable as a component requiring stretch and stretch flange characteristics, such as a frame for automobiles or trucks, for example.

Claims (7)

By mass%, C: 0.02% or more, 0.20% or less, Si: 0.3% or less, Mn: 0.5% or more and 2.5% or less, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05% And a component composition containing not less than 0.25%, V: 0.05% or more and 0.25% or less, and the balance of Fe and inevitable impurities;
Substantially ferrite single-phase structure, among the ferrite single-phase structures, Ti contained in precipitates of less than 20 nm in size is 200 mass ppm or more and 1750 mass ppm, V is 150 mass ppm or more and 1750 mass ppm or less, and solid solution V is 200 mass. A high strength steel sheet having a structure of at least ppm and less than 1750 mass ppm. The method of claim 1,
The high strength steel sheet further contains any one or two or more of Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less. The method according to claim 1 or 2,
High strength steel sheet, characterized in that the tensile strength TS is 780 MPa or more. The method according to claim 1 or 2,
The high strength steel plate characterized by the one-side maximum peeling width after tape peeling test in the salt hot water immersion test being 3.0 mm or less. The method of claim 3, wherein
The high strength steel plate characterized by the one-side maximum peeling width after tape peeling test in the salt hot water immersion test being 3.0 mm or less. The method according to claim 1 or 2,
The high strength steel sheet characterized by the fact that the elongation flange characteristic λ ₁₀ after rolling at an elongation rate of 10% is 60% or more. The method of claim 3, wherein
The high strength steel sheet characterized by the fact that the elongation flange characteristic λ ₁₀ after rolling at an elongation rate of 10% is 60% or more.