WO2017183348A1 - Steel plate, plated steel plate, and production method therefor - Google Patents

Abstract

A high-strength steel plate that has a predetermined component composition, a tensile strength (TS) of 590 MPa or more, excellent molding properties at a yield ratio (YR) of 68% or more, a high yield ratio, and high hole expandability is obtained by configuring the steel plate in the following manner: including 20-65% of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 5-25% of martensite in terms of area ratio souch that the steel plate includes 8% or more of residual austenite by volume fraction and the average aspect ratios of the crystal grains in the individual phases (polygonal ferrite, martensite, and residual austenite) are each 2.0-15.0; and setting the average crystal grain size of the polygonal ferrite to 6 µm or less, the average crystal grain size of the martensite to 3 µm or less, and the average crystal grain size of the residual austenite to 3 µm or less such that the value obtained by dividing the Mn amount (mass%) within the residual austenite by the Mn amount (mass%) within the polygonal ferrite is 2.0 or more.

Description

Steel sheet, plated steel sheet, and manufacturing method thereof

The present invention relates to a steel sheet, a hot dip galvanized steel sheet, a hot dip galvanized steel sheet, an electrogalvanized steel sheet, and a method for producing them, and particularly suitable for use as a member used in industrial fields such as automobiles and electricity. The present invention relates to a steel sheet having excellent hole expandability and a high yield ratio.

In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of protecting the global environment. For this reason, efforts are being made to reduce the thickness of the steel sheet, which is the body material, and to reduce the weight of the vehicle body.

However, in general, increasing the strength of a steel sheet causes a decrease in formability. Therefore, when the strength is increased, the formability of the steel sheet is decreased, causing problems such as cracking during forming. Therefore, simply thinning the steel sheet cannot be achieved. Therefore, development of a material having both high strength and high formability is desired. Furthermore, steel sheets having a tensile strength (TS) of 590 MPa or more are particularly required to have a high impact absorption energy in addition to high formability. In order to improve the impact absorption energy characteristics, it is effective to increase the yield ratio (YR). This is because, when the yield ratio is high, the collision energy can be efficiently absorbed by the steel sheet with a low deformation amount.
Furthermore, when using a steel plate for an automobile body, stretch flange forming is performed according to the shape of the vehicle body, so that excellent hole expandability is also required.

For example, Patent Document 1 proposes a high-strength steel sheet having an extremely high ductility utilizing a work-induced transformation of retained austenite having a tensile strength of 1000 MPa or more and a total elongation (EL) of 30% or more.

Further, Patent Document 2 proposes a high-strength steel plate having a high balance between strength and ductility by performing heat treatment in a two-phase region of ferrite and austenite using high-Mn steel.

Further, in Patent Document 3, high Mn steel, the structure after hot rolling is a structure containing bainite and martensite, and further, after forming fine retained austenite by annealing and tempering, tempered bainite or A high-strength steel sheet that has improved local ductility by using a structure containing tempered martensite has been proposed.

JP 61-157625 A JP-A-1-259120 JP 2003-138345 A

Here, the steel sheet described in Patent Document 1 is manufactured by performing a so-called austempering process in which a steel sheet containing C, Si and Mn as basic components is austenitized, and then quenched into a bainite transformation temperature range and held isothermally. Is done. And when this austemper process is performed, a retained austenite is produced | generated by the concentration of C to austenite.

However, in order to obtain a large amount of retained austenite, a large amount of C exceeding 0.3% is required. However, when the C concentration exceeds 0.3%, the spot weldability is significantly reduced. It is difficult to put it into practical use as a steel plate.
In addition, the steel sheet described in Patent Document 1 is mainly intended to improve ductility, and no consideration is given to hole expandability, bendability, and yield ratio.

Similarly, in Patent Documents 2 and 3, from the viewpoint of formability, although improvement of the ductility of the steel sheet is described, consideration is not given to its bendability, yield ratio, and hole expandability.

The present invention has been made paying attention to the above-mentioned problems, and the object thereof is a steel sheet, hot-dip galvanized steel having a TS of 590 MPa or more, excellent YR of 68% or more, and excellent formability and hole expansibility. An object of the present invention is to provide a steel plate, a hot dip galvanized steel plate, an electrogalvanized steel plate, and a production method thereof.

In order to solve the above-mentioned problems, and to produce a high-strength steel sheet having excellent formability and hole expansibility, and having a high yield ratio and tensile strength, the inventors have earnestly studied from the viewpoint of the composition of the steel sheet and the production method. Repeated research. As a result, it was found that by appropriately adjusting the component composition and structure of steel, it is possible to produce a high yield ratio type high strength steel sheet having excellent formability such as ductility and hole expandability.

That is, the steel component is in a range of Mn: 2.60 mass% to 4.20 mass%, and after appropriately adjusting the addition amount of other alloy elements such as Ti, hot rolling is performed to perform hot rolling. And Next, after removing the scale from the hot-rolled sheet by pickling, the hot-rolled sheet is maintained in the temperature range of Ac ₁ transformation point + 20 ° C. or higher and Ac ₁ transformation point + 120 ° C. or lower in the range of 600 s to 21600 s, and further annealed after hot rolling. As it is, or cold rolling is performed at a reduction rate of less than 30% to obtain a cold-rolled sheet. Further, this hot-rolled sheet or cold-rolled sheet is cooled after being held for 20 to 900 s in a temperature range of Ac ₁ transformation point + 10 ° C. or higher and Ac ₁ transformation point + 100 ° C. or lower.

By passing through this process, the hot-rolled sheet or cold-rolled sheet has an area ratio of polygonal ferrite of 20% or more and 65% or less, non-recrystallized ferrite of 8% or more, and martensite of 5% or more and 25% or less. The volume ratio of the retained austenite is 8% or more, and the average aspect ratio of the crystal grains of each phase (polygonal ferrite, martensite, retained austenite) is 2.0 or more and 15.0 or less, respectively. In addition, the polygonal ferrite has an average crystal grain size of 6 μm or less, the martensite has an average crystal grain size of 3 μm or less, and the retained austenite has an average crystal grain size of 3 μm or less. Further, in the hot rolled sheet or cold rolled sheet, the value obtained by dividing the amount of Mn (mass%) in the retained austenite by the amount of Mn (mass%) in the polygonal ferrite is controlled to be 2.0 or more. 8% or more of retained austenite stabilized with can be secured.
The present invention has been made based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 2.60% to 4.20%, P: 0.001% or more 0.100% or less, S: 0.0200% or less, N: 0.0100% or less and Ti: 0.005% or more and 0.200% or less,
Furthermore, by mass%, Al: 0.01% to 2.00%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005 %: 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.010%, Ca : 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less arbitrarily selected from at least one element Contains, balance is Fe and inevitable It has a chemical composition of impurities,
In terms of area ratio, polygonal ferrite is 20% or more and 65% or less, non-recrystallized ferrite is 8% or more and martensite is 5% or more and 25% or less, and by volume ratio, residual austenite is 8% or more, The polygonal ferrite, the martensite, and the retained austenite have a steel structure having an average aspect ratio of 2.0 or more and 15.0 or less, respectively,
The average grain size of the polygonal ferrite is 6 μm or less,
An average crystal grain size of the martensite is 3 μm or less,
The average grain size of the retained austenite is 3 μm or less,
A steel sheet having a value obtained by dividing the amount of Mn (mass%) in the retained austenite by the amount of Mn (mass%) in the polygonal ferrite is 2.0 or more.

2. The amount of C in the retained austenite is related to the amount of Mn in the retained austenite by the following formula: 0.09 × [Mn amount] −0.026−0.150 ≦ [C amount] ≦ 0.09 × [ Amount of Mn] −0.026 + 0.150
[C amount]: C amount (% by mass) in retained austenite
[Mn amount]: Mn amount (% by mass) in retained austenite
2. The steel sheet according to 1 above, which satisfies

3. 3. A plated steel sheet, wherein the steel sheet according to 1 or 2 further comprises one selected from a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plated layer, and an electrogalvanized layer.

4). It is a manufacturing method of the steel plate according to 1 or 2,
The steel slab having the component composition described in 1 above is heated, hot rolled at a finish rolling exit temperature of 750 ° C. to 1000 ° C., wound up at 300 ° C. to 750 ° C., and then scaled by pickling. Is maintained at 600 s to 21600 s in a temperature range of Ac ₁ transformation point + 20 ° C. or higher and Ac ₁ transformation point + 120 ° C., optionally cold-rolled at a reduction rate of less than 30%, and then Ac ₁ transformation point +10 ° C. or higher, the production method of the steel sheet to cool and kept at Ac ₁ transformation point + 100 ° C. below the temperature range 20s or 900s or less.

5. 5. The method for producing a steel sheet according to 4 above, wherein a value obtained by dividing a volume ratio of retained austenite after applying a tensile process of 10% by an elongation value by a residual austenite volume ratio before the tensile process is 0.3 or more.

6). It is a manufacturing method of the plated steel plate of said 3, Comprising:
After the cooling, one type selected from hot dip galvanizing, hot dip galvanizing, and electrogalvanizing is applied, or after the hot dip galvanizing is performed, alloying is performed at 450 ° C. or higher and 600 ° C. or lower. The manufacturing method of the steel plate of said 4 which performs a process.

According to the present invention, a high-yield ratio type high-strength steel sheet excellent in formability and hole-expandability with TS of 590 MPa or more and YR of 68% or more can be obtained. By applying the steel plate of the present invention to, for example, an automobile structural member, fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is extremely high.

It is a figure which shows the relationship between the workability of a tension process, and a retained austenite volume fraction. It is a figure which shows the relationship between the value which remove | divided the volume fraction which remains austenite when 10% of tensile processing is given by elongation value, and the residual austenite volume fraction before a process, and the elongation of a steel plate.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the scope of the present invention in the present invention will be described. In addition, the% display concerning the component composition of the following steel or slab means the mass%. Moreover, the balance of the component composition of steel or slab is Fe and inevitable impurities.

C: 0.030% or more and 0.250% or less C is an element necessary for generating a low-temperature transformation phase such as martensite and increasing the strength. It is also an element effective in improving the stability of retained austenite and improving the ductility of steel. If the amount of C is less than 0.030%, it is difficult to secure a desired martensite area ratio, and a desired strength cannot be obtained. Moreover, it is difficult to ensure a sufficient volume ratio of retained austenite, and good ductility cannot be obtained. On the other hand, if C is added in excess of 0.250%, the area ratio of hard martensite becomes excessive, and the microvoids at the grain boundaries of martensite increase, and the bending test and the hole expansion test. Sometimes the propagation of cracks tends to progress, and the bendability and stretch flangeability deteriorate. Further, excessive addition of C makes the welded part and the heat-affected zone harder and lowers the mechanical properties of the welded part, so that spot weldability, arc weldability, and the like are deteriorated. From these viewpoints, the C amount is set to be 0.030% or more and 0.250% or less. Preferably, it is 0.080% or more. Preferably, it is 0.200% or less.

Si: 0.01% or more and 3.00% or less Si is an element effective for ensuring good ductility in order to improve the work hardening ability of ferrite. If the amount of Si is less than 0.01%, the effect of addition becomes poor, so the lower limit is made 0.01%. On the other hand, excessive addition of Si exceeding 3.00% not only causes embrittlement of steel but also causes deterioration of surface properties due to the occurrence of red scale and the like. For this reason, Si amount is taken as 0.01% or more and 3.00% or less of range. Preferably, it is 0.20% or more. Preferably, it is 2.00% or less.

Mn: 2.60% or more and 4.20% or less Mn is an extremely important element in the present invention. Mn is an element that stabilizes retained austenite and is effective in securing good ductility. Further, Mn is an element that can raise TS of steel by solid solution strengthening. Such an effect is recognized when the Mn content of the steel is 2.60% or more. On the other hand, an excessive addition of Mn exceeding 4.20% causes an increase in cost. From such a viewpoint, the amount of Mn is set in the range of 2.60% to 4.20%. Preferably, it is the range of 3.00% or more and 4.20% or less.

P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening action and can be added according to a desired TS. It is also an element that promotes ferrite transformation and is effective in the formation of a composite structure of steel sheets. In order to acquire such an effect, it is necessary to make P amount in a steel plate 0.001% or more. On the other hand, if the amount of P exceeds 0.100%, weldability is deteriorated, and when galvanizing is alloyed, the alloying speed is lowered and the quality of galvanizing is impaired. Therefore, the P amount is in the range of 0.001% to 0.100%. Preferably it is 0.005% or more. Preferably it is 0.050% or less.

S: 0.0200% or less S segregates at the grain boundary and embrittles the steel during hot working, and also exists as a sulfide, thereby reducing the local deformability of the steel sheet. Therefore, the S content is 0.0200% or less, preferably 0.0100% or less, more preferably 0.0050% or less. However, the amount of S is preferably 0.0001% or more because of restrictions on production technology. Therefore, the S amount is preferably in the range of 0.0001% to 0.0200%. More preferably, it is 0.0001% or more and 0.0100% or less, and still more preferably 0.0001% or more and 0.0050% or less.

N: 0.0100% or less N is an element that deteriorates the aging resistance of steel. In particular, when the N content exceeds 0.0100%, the deterioration of aging resistance becomes significant. Therefore, the smaller the amount of N, the better. However, the amount of N is preferably set to 0.0005% or more because of restrictions on production technology. For this reason, the N amount is preferably in the range of 0.0005% to 0.0100%. More preferably, it is 0.0010% or more. More preferably, it is 0.0070% or less.

Ti: 0.005% or more and 0.200% or less Ti is an extremely important additive element in the present invention. Ti is effective for the precipitation strengthening of steel, can secure the desired area ratio of non-recrystallized ferrite, and contributes to a high yield ratio of the steel sheet. In addition, by utilizing relatively hard non-recrystallized ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, which contributes to the improvement of stretch flangeability. And these effects are acquired by addition of Ti amount 0.005% or more. On the other hand, if the Ti content in the steel sheet exceeds 0.200%, the area ratio of hard martensite becomes excessive, increasing the number of microvoids at the martensite grain boundaries, and during the bending test and the hole expansion test. Propagation of cracks easily progresses, and the bendability and stretch flangeability of the steel sheet are reduced.
Therefore, the amount of Ti added is in the range of 0.005% to 0.200%. Preferably it is 0.010% or more. Preferably it is 0.100% or less.

The basic components of the present invention have been described above. The balance other than the above components is Fe and inevitable impurities, but in addition, the following elements can be appropriately contained as required.

Al: 0.01% to 2.00%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000% Hereinafter, Cr: 0.005% to 1.000%, V: 0.005% to 0.500%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1. 000% or less, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% or more Contains at least one element selected from 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less. Al is composed of ferrite and austenite Expanded biphasic area This is an element effective in reducing the annealing temperature dependency, that is, the material stability. Further, Al acts as a deoxidizer and is also an effective element for maintaining the cleanliness of steel. However, if the Al content is less than 0.01%, the effect of addition is poor, so the lower limit is made 0.01%. On the other hand, a large amount of addition exceeding 2.00% increases the risk of steel piece cracking during continuous casting, and decreases productivity. From such a viewpoint, the Al amount when added is in the range of 0.01% to 2.00%. Preferably, it is 0.20% or more. Preferably, it is 1.20% or less.

Nb is effective for precipitation strengthening of steel, and the effect of addition is obtained at 0.005% or more. Further, similarly to the effect of adding Ti, it is possible to ensure the desired area ratio of non-recrystallized ferrite, which contributes to a high yield ratio of the steel sheet. In addition, by utilizing relatively hard non-recrystallized ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, which contributes to the improvement of stretch flangeability. On the other hand, if the amount of Nb exceeds 0.200%, the area ratio of hard martensite becomes excessive and the number of microvoids at the grain boundaries of martensite increases, and crack propagation occurs during bending and hole expansion tests. Becomes easier to progress. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. In addition, the cost increases. Therefore, when adding Nb, it is set as 0.005% or more and 0.200% or less. Preferably it is 0.010% or more. Preferably it is 0.100% or less.

B has the effect of suppressing the formation and growth of ferrite from the austenite grain boundaries, and can be flexibly controlled in the structure, so it can be added as necessary. The effect of addition is obtained at 0.0003% or more. On the other hand, if the amount of B exceeds 0.0050%, the formability of the steel sheet is lowered. Therefore, when adding B, it is set as 0.0003% or more and 0.0050% or less of range. Preferably it is 0.0005% or more. Preferably it is 0.0030% or less.

Ni is an element that stabilizes retained austenite, and is effective in ensuring good ductility. Further, Ni is an element that raises steel TS by solid solution strengthening. The effect of addition is obtained at 0.005% or more. On the other hand, if added over 1.000%, the area ratio of hard martensite becomes excessive, the number of microvoids at the grain boundary of martensite increases, and crack propagation propagates during the bending test and the hole expansion test. Easy to progress. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. In addition, the cost increases. Therefore, when adding Ni, it is set as 0.005% or more and 1.000% or less.

Cr, V, and Mo are elements that can be added as necessary because they have an effect of improving the balance between TS and ductility. The addition effect is obtained when Cr: 0.005% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, when Cr is added in excess of 1.000%, V: 0.500%, and Mo: 1.000%, the area ratio of hard martensite becomes excessive, and martensite crystal grains. The number of microvoids at the boundary increases, and crack propagation easily progresses during a bending test and a hole expansion test. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. In addition, the cost increases. Therefore, when these elements are added, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, and Mo: 0.005% or more and 1.000%, respectively. The following range.

Cu is an element effective for strengthening steel, and may be used for strengthening steel as long as it is within the range specified in the present invention. The effect of addition is obtained at 0.005% or more. On the other hand, if added over 1.000%, the area ratio of hard martensite becomes excessive, the number of microvoids at the grain boundary of martensite increases, and crack propagation propagates during the bending test and the hole expansion test. Easy to progress. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. Therefore, when adding Cu, it is set as 0.005% or more and 1.000% or less.

Sn and Sb are added as necessary from the viewpoint of suppressing decarburization in a thickness region of about several tens of μm of the steel sheet surface layer caused by nitriding and oxidation of the steel sheet surface. Thus, by suppressing nitriding and oxidation, it is effective to prevent the martensite area ratio on the steel sheet surface from decreasing and to secure TS and material stability. On the other hand, excessive addition over 0.200% causes a reduction in toughness. Therefore, when adding Sn and Sb, the range is from 0.002% to 0.200%.

Ta, like Ti and Nb, generates alloy carbides and alloy carbonitrides and contributes to increasing the strength of steel. In addition, by partially dissolving in Nb carbide or Nb carbonitride and generating a composite precipitate such as (Nb, Ta) (C, N), the coarsening of the precipitate is effectively suppressed, It is considered that there is an effect of stabilizing the contribution to the strength improvement of the steel sheet by precipitation strengthening. For this reason, in this invention, it is preferable to contain Ta. Here, the effect of adding Ta is obtained by setting the content of Ta to 0.001% or more. On the other hand, even if Ta is added excessively, the addition effect is saturated and the alloy cost is also increased. Therefore, when Ta is added, the range is 0.001% or more and 0.010% or less.

Ca, Mg, and REM are effective elements for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on the hole expandability (stretch flangeability). In order to obtain this effect, 0.0005% or more must be added. On the other hand, excessive addition exceeding 0.0050% causes an increase in inclusions and the like, and causes the surface and internal defects of the steel sheet. Therefore, when adding Ca, Mg, and REM, it is set as 0.0005% or more and 0.0050% or less, respectively.

Next, the microstructure will be described.
In order to ensure sufficient ductility in the steel sheet, it is only necessary to promote the formation of polygonal ferrite in the structure, which causes a decrease in tensile strength and yield strength. It also changes depending on the martensite area ratio, and the ductility is greatly influenced by the amount of retained austenite. Therefore, controlling the amount (area ratio, volume ratio) of these phases (structures) is effective in building the mechanical properties of high-strength steel sheets. The inventors have studied from such a viewpoint and have newly found that the area ratio of polygonal ferrite and non-recrystallized ferrite can be controlled by the rolling reduction ratio of cold rolling. Furthermore, it was found that the area ratio of martensite and the volume ratio of retained austenite are largely determined by the amount of Mn added. Moreover, the area ratio of polygonal ferrite is reduced (relative to the entire structure) by not performing cold rolling or preventing the rolling reduction of cold rolling from exceeding 30% (can be controlled within an appropriate range). In addition, it has been found that the final product has a large change in the shape of the structure, resulting in a steel sheet having crystal grains with a large aspect ratio. As a result, it became clear that the value of the hole expansibility λ was improved. That is, the microstructure of a steel sheet having high ductility and good hole expansibility is as follows.

Polygonal ferrite area ratio: 20% or more and 65% or less In the present invention, in order to ensure sufficient ductility, the area ratio of polygonal ferrite needs to be 20% or more. On the other hand, in order to ensure TS of 590 MPa or more, it is necessary to suppress the area ratio of soft polygonal ferrite to 65% or less. Preferably, the area ratio is 30% or more. Preferably, the area ratio is 55% or less. The polygonal ferrite in the present invention is a ferrite that is relatively soft and rich in ductility.

Area ratio of non-recrystallized ferrite: 8% or more It is extremely important in the present invention that the area ratio of non-recrystallized ferrite is 8% or more. Here, although non-recrystallized ferrite is effective in increasing the strength of the steel sheet, it causes a significant decrease in the ductility of the steel sheet, and is therefore generally reduced.
However, in the present invention, by using polygonal ferrite and retained austenite, it is possible to ensure good ductility and to actively utilize relatively hard non-recrystallized ferrite, such as an area ratio exceeding 25%. It is possible to secure the TS of the intended steel sheet without requiring a large amount of martensite.

Furthermore, in the present invention, since the amount of heterogeneous interface between polygonal ferrite and martensite is reduced, the yield strength (YP) and YR of the steel sheet can be increased.
In order to obtain the above effects, the area ratio of non-recrystallized ferrite needs to be 8% or more. Preferably, it is 10% or more.
The non-recrystallized ferrite in the present invention is a ferrite having a crystal orientation difference of less than 15 ° in the grains, and is harder than the above-described polygonal ferrite rich in ductility.
In the present invention, the upper limit of the area ratio of non-recrystallized ferrite is not particularly limited, but is preferably about 45% because material anisotropy in the plane of the steel sheet may be increased.

Martensite area ratio: 5% or more and 25% or less In order to achieve a TS of 590 MPa or more, the martensite area ratio needs to be 5% or more. On the other hand, in order to ensure good ductility, it is necessary to limit the area ratio of martensite to 25% or less.
Here, in the present invention, the area ratio of ferrite (polygonal ferrite and non-recrystallized ferrite) and martensite can be determined as follows.
That is, after polishing a plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate, it corrodes with 3 vol% nital and corresponds to a plate thickness 1/4 position (1/4 of the plate thickness in the depth direction from the steel plate surface). Position) is observed using a SEM (scanning electron microscope) at a magnification of 2000 times for about 10 fields of view to obtain a tissue image. Using the obtained tissue image, the area ratio of each structure (ferrite, martensite) can be calculated for 10 visual fields using Image-Pro of Media Cybernetics, and the area ratio can be obtained by averaging. it can. In the above-mentioned structure image, polygonal ferrite and non-recrystallized ferrite are identified by showing a gray structure (underground structure) and martensite by showing a white structure.

In the present invention, the area ratio of polygonal ferrite and non-recrystallized ferrite can be obtained as follows.
That is, by using EBSD (Electron Backscatter Diffraction), a low-angle grain boundary having a crystal orientation difference of 2 ° to less than 15 ° and a large-angle grain boundary having a crystal orientation difference of 15 ° or more are identified. . Then, IQ Map is created by using ferrite containing low-angle grain boundaries in the grains as non-recrystallized ferrite. Next, after extracting 10 fields of view from the prepared IQ Map, the areas of polygonal ferrite and unrecrystallized ferrite are calculated by obtaining the areas of the low-angle and large-angle grain boundaries in the 10 fields of view respectively. Then, the area ratio of polygonal ferrite and unrecrystallized ferrite for 10 fields of view is obtained. Then, the area ratios of the polygonal ferrite and the non-recrystallized ferrite are obtained by averaging those area ratios.

Volume ratio of retained austenite: 8% or more In the present invention, the volume ratio of retained austenite needs to be 8% or more in order to ensure sufficient ductility. Preferably it is 10% or more.
In the present invention, the upper limit of the volume fraction of retained austenite is not particularly limited. However, since the concentration of the retained austenite is small and unstable, such as C or Mn, which has a small effect on improving ductility, the amount of retained austenite increases. % Is preferable.

The volume ratio of retained austenite is determined by polishing the steel sheet to a ¼ surface in the plate thickness direction (a surface corresponding to ¼ of the plate thickness in the depth direction from the steel plate surface). It is determined by measuring the diffracted X-ray intensity. MoKα rays are used as incident X-rays, and {111}, {200}, {220}, {311} planes of the retained austenite have peak integrated intensities of ferrite {110}, {200}, {211}. The intensity ratios of all 12 combinations with respect to the integrated intensity of the peak of the surface are obtained, and the average value thereof is taken as the volume ratio of retained austenite.

Polygonal ferrite average crystal grain size: 6 μm or less Refinement of polygonal ferrite crystal grains contributes to improvement of YP and TS. Therefore, in order to ensure high YP and high YR and the desired TS, the average crystal grain size of polygonal ferrite needs to be 6 μm or less. Preferably, it is 5 μm or less.
In the present invention, the lower limit of the average crystal grain size of polygonal ferrite is not particularly limited, but is preferably about 0.3 μm industrially.

Martensite average crystal grain size: 3 μm or less Refinement of martensite crystal grains contributes to the improvement of bendability and stretch flangeability (hole expandability). Therefore, in order to ensure high bendability and high stretch flangeability (high hole expansibility), it is necessary to suppress the average crystal grain size of martensite to 3 μm or less. Preferably, it is 2.5 μm or less.
In the present invention, the lower limit of the average grain size of martensite is not particularly limited, but industrially, it is preferably about 0.1 μm.

Average crystal grain size of retained austenite: 3 μm or less The refinement of crystal grains of retained austenite contributes to the improvement of ductility and the improvement of bendability and stretch flangeability (hole expandability). Therefore, in order to ensure good ductility, bendability, stretch flangeability (hole expandability), the average crystal grain size of retained austenite needs to be 3 μm or less. Preferably, it is 2.5 μm or less.
In the present invention, the lower limit of the average crystal grain size of retained austenite is not particularly limited, but industrially, it is preferably about 0.1 μm.

The average crystal grain size of polygonal ferrite, martensite and retained austenite was determined by calculating the area of each of the polygonal ferrite grains, martensite grains and retained austenite grains using the above-mentioned Image-Pro. Calculate and average the values. Polygonal ferrite, non-recrystallized ferrite, martensite, and retained austenite are separated by EBSD, and martensite and retained austenite are identified by Phase Map of EBSD. In the present invention, when the average crystal grain size is determined, those having a grain size of 0.01 μm or more are measured. This is because a thickness of less than 0.01 μm does not affect the present invention.

Average aspect ratio of polygonal ferrite, martensite and retained austenite crystal grains: 2.0 to 15.0 or less The average aspect ratio of polygonal ferrite, martensite and retained austenite crystal grains is 2.0 or more. This is extremely important in the present invention.
First, the fact that the aspect ratio of the crystal grains is small means that during the holding in the heat treatment after cold rolling (cold rolled sheet annealing), the grains grow after the ferrite and austenite have recovered and recrystallized, resulting in equiaxed grains. This means that near crystal grains were formed. The ferrite produced here is soft. On the other hand, when cold rolling is not performed or when the reduction ratio of cold rolling is less than 30%, the amount of added strain is reduced, so that the formation of polygonal ferrite is suppressed, and crystal grains having a large aspect ratio are mainly used. Become an organization. Such a structure composed of crystal grains having a large aspect ratio becomes harder because there are more strains than those described above, or there are places where the distance between the grain boundaries and the grain boundaries is small. Therefore, in addition to improving TS, the hardness difference from the hard phase such as retained austenite and martensite is reduced, and the hole expandability is improved without impairing the ductility. On the other hand, when the aspect ratio exceeds 15.0, the TS rises remarkably and good ductility cannot be obtained.
Therefore, the average aspect ratios of the crystal grains of polygonal ferrite, martensite and retained austenite are 2.0 or more and 15.0 or less, respectively. For ductility, it is more preferably 2.2 or more, and more preferably 2.4 or more.

In addition, the aspect ratio of the crystal grain here is a value obtained by dividing the major axis length of the crystal grain by the minor axis length, and the average aspect ratio of each crystal grain can be obtained as follows. it can.
That is, using the above-mentioned Image-Pro, the major axis length and minor axis length of 30 crystal grains are calculated for each of the polygonal ferrite grains, martensite grains, and retained austenite grains, and the major axis lengths are calculated. Can be obtained by dividing the value by the minor axis length and averaging the values.

Value obtained by dividing the amount of Mn (mass%) in retained austenite by the amount of Mn (mass%) in polygonal ferrite: 2.0 or more The amount of Mn (mass%) in retained austenite is the amount of Mn in polygonal ferrite ( It is extremely important in the present invention that the value divided by (mass%) is 2.0 or more. This is because in order to ensure good ductility, it is necessary to increase stable retained austenite enriched in Mn.
In the present invention, the upper limit of the value obtained by dividing the amount of Mn (mass%) in retained austenite by the amount of Mn (mass%) in polygonal ferrite is not limited, but from the viewpoint of securing stretch flangeability, 16 About 0.0 is preferable.

Further, the amount of Mn (mass%) in the retained austenite and the amount of Mn (mass%) in the polygonal ferrite can be obtained as follows.
That is, using an EPMA (Electron Probe Micro Analyzer; electronic probe microanalyzer), the distribution state of Mn to each phase of the cross section in the rolling direction at the ¼ thickness position is quantified. Subsequently, the amount of Mn of 30 retained austenite grains and 30 ferrite grains is analyzed. And the amount of Mn calculated | required from the result of the analysis can be calculated | required by averaging.

Here, in the microstructure of the present invention, in addition to the above-described polygonal ferrite and martensite, etc., it is usually found in steel sheets such as granular ferrite, acicular ferrite, bainitic ferrite, tempered martensite, pearlite and cementite. Carbides (except for cementite in pearlite) may be included. If these structures are within a range of 10% or less in terms of area ratio, the effects of the present invention are not impaired even if they are included.

In addition, the inventors diligently investigated the steel sheet structure when press forming and processing were performed on the steel sheet.
As a result, when press forming or processing is applied, martensite transformation occurs immediately, and it remains as retained austenite until the amount of processing increases, and finally martensite transformation occurs to cause a TRIP phenomenon (processing induced transformation phenomenon). It was found that there is something that produces. It was found that good elongation can be obtained particularly effectively when the amount of retained austenite that undergoes martensitic transformation after the amount of processing increases.

That is, various elongations with good elongation and low elongation were selected, and the amount of retained austenite was measured while changing the degree of work of tensile processing from 0 to 20%. A tendency as shown in 1 was observed. Here, the degree of work means a tensile test using a JIS No. 5 test piece obtained by taking a sample so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and means the elongation at that time.
As shown in FIG. 1, it can be seen that a sample with good elongation has a gradual way of reducing retained austenite when the degree of processing is increased.

Therefore, the residual austenite amount of a sample having a TS of 780 MPa class and a tensile processing of 10% in elongation value was measured, and the effect of the ratio of this value and the residual austenite amount before processing on the total elongation of the steel sheet. investigated. The result is shown in FIG.
As shown in FIG. 2, the value obtained by dividing the residual volume ratio of retained austenite when tensile processing of 10% is given by the elongation value by the residual austenite volume ratio before processing is 0.3 or more. It can be seen that a high elongation is obtained, and those that fall outside this range have a low elongation.

Therefore, in the present invention, the volume ratio of the retained austenite remaining in the steel after the tensile processing of 10% in terms of the elongation value is set to 0.3 or more by the value divided by the residual austenite volume ratio before the tensile processing. It is preferable. This is because retained austenite that undergoes martensitic transformation after the processing amount becomes large can be secured.

The TRIP phenomenon requires that residual austenite be present before press molding or processing. However, in order for residual austenite to exist before press molding or processing, depending on the component elements contained in the steel structure, The determined Ms point (martensitic transformation start point) needs to be as low as about 15 ° C. or less.

Further, the step of applying a tensile process of 10% with an elongation value according to the present invention will be specifically described. Using a JIS No. 5 test piece in which a sample was taken so that the tensile direction was a direction perpendicular to the rolling direction of the steel sheet. A tensile test is performed, and when the elongation rate is 10%, the test is interrupted to give the test piece a tensile process of 10% in terms of elongation value.
The volume ratio of retained austenite can be determined by the method described above.

Furthermore, when the sample satisfying the above conditions was examined in detail, the relationship between the amount of C and the amount of Mn in the retained austenite,
0.09 × [Mn amount] −0.026−0.150 ≦ [C amount] ≦ 0.09 × [Mn amount] −0.026 + 0.150
[C amount]: C amount (% by mass) in retained austenite
[Mn amount]: Mn amount (% by mass) in retained austenite
When satisfying the above, it was found that the TRIP phenomenon exhibiting a high work-hardening ability was produced when processing was performed, and a better elongation was exhibited.

By satisfying the above requirements, the TRIP phenomenon, which is the main factor for improving ductility, can be intermittently expressed until the end of the processing of the steel sheet, and so-called stable retained austenite can be achieved.

The amount of C (% by mass) in the retained austenite can be determined by the following procedure.
That is, using the above-mentioned EPMA, the distribution state of C to each phase of the cross section in the rolling direction at the ¼ thickness position is quantified. Next, the amount of C in 30 residual austenite grains is analyzed. And the C amount calculated | required from the result of the analysis is calculated | required and averaged.
The amount of Mn (mass%) in the retained austenite can be determined by the same procedure as the amount of C in the retained austenite.

Next, manufacturing conditions will be described.
Heating temperature of steel slab: 1100 ° C or higher and 1300 ° C or lower Precipitates present in the heating stage of steel slabs (or simply slabs) exist as coarse precipitates in the finally obtained steel sheet, contributing to strength do not do. For this reason, it is necessary to redissolve the Ti and Nb-based precipitates precipitated during casting.
Here, when the heating temperature of the steel slab is less than 1100 ° C., it is difficult to sufficiently dissolve the carbide, and problems such as an increased risk of trouble during hot rolling due to an increase in rolling load occur. Therefore, the heating temperature of the steel slab is preferably 1100 ° C. or higher.
In addition, the heating temperature of the steel slab should be 1100 ° C. or higher from the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab and reducing the cracks and irregularities on the steel plate surface to achieve a smooth steel plate surface. preferable.
On the other hand, when the heating temperature of the steel slab exceeds 1300 ° C., the scale loss increases as the oxidation amount increases. Therefore, the heating temperature of the steel slab is preferably 1300 ° C. or lower. Therefore, the heating temperature of the slab is preferably 1100 ° C. or higher and 1300 ° C. or lower. More preferably, it is 1150 degreeC or more. More preferably, it is 1250 degrees C or less.

The steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but can also be manufactured by an ingot-making method or a thin slab casting method. Moreover, in this invention, after manufacturing a steel slab, after cooling to room temperature once, the conventional method of heating again can be used. Furthermore, in the present invention, energy-saving processes such as direct feed rolling and direct rolling that are not cooled to room temperature, are charged in a heating furnace as they are, but are heated immediately after performing slight heat retention, can be applied without any problem. be able to. Steel slabs are made into sheet bars by rough rolling under normal conditions. However, if the heating temperature is low, a bar heater or the like is used before finish rolling from the viewpoint of preventing problems during hot rolling. It is preferred to use and further heat the sheet bar.

Finishing rolling exit temperature of hot rolling: 750 ° C. or higher and 1000 ° C. or lower The heated steel slab is hot rolled by rough rolling and finish rolling to become a hot rolled sheet. At this time, when the finish rolling exit temperature exceeds 1000 ° C., the amount of oxide (scale) generated increases rapidly, the interface between the base iron and the oxide becomes rough, and pickling and cold rolling are performed. The surface quality of the steel sheet tends to deteriorate. In addition, if a part of the hot-rolled scale remains after pickling, it adversely affects the ductility and stretch flangeability of the steel sheet. Furthermore, the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during processing. On the other hand, when the finish rolling exit temperature is less than 750 ° C., the rolling load increases, and the reduction ratio of the austenite in an unrecrystallized state increases. As a result, an abnormal texture develops in the steel sheet, the in-plane anisotropy in the final product becomes remarkable, and not only the material uniformity (material stability) is impaired, but also the ductility of the steel sheet itself decreases. . Further, when the finish rolling outlet temperature of hot rolling is less than 750 ° C. or more than 1000 ° C., a structure having a volume ratio of 8% or more of retained austenite cannot be obtained.
Therefore, in the present invention, it is necessary to set the finish rolling exit temperature of hot rolling to 750 ° C. or more and 1000 ° C. or less. Preferably it is 800 degreeC or more. Preferably it is 950 degrees C or less.

Average coiling temperature after hot rolling: 300 ° C. or more and 750 ° C. or less When the average coiling temperature after hot rolling exceeds 750 ° C., the ferrite crystal grain size of the hot rolled sheet structure becomes large, and the final annealed sheet It is difficult to secure the desired strength. When the average coiling temperature after hot rolling exceeds 750 ° C., the average crystal grain size of polygonal ferrite is 6 μm or less, the average crystal grain size of martensite is 3 μm or less, and the average crystal grain size of retained austenite is 3 μm. The following organization cannot be obtained. On the other hand, if the average coiling temperature after hot rolling is less than 300 ° C., the hot-rolled sheet strength is increased, the rolling load in cold rolling is increased, and a defective plate shape is generated. Decreases. Therefore, the average winding temperature after hot rolling needs to be 300 ° C. or higher and 750 ° C. or lower. Preferably it is 400 degreeC or more. Preferably it is 650 degrees C or less.

In the present invention, at the time of hot rolling, rough rolling sheets may be joined together to perform finish rolling continuously. Moreover, you may wind up a rough rolling board once. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable that the friction coefficient at the time of lubrication rolling shall be 0.10 or more and 0.25 or less.

The pickling is performed on the hot-rolled sheet manufactured through this process. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion properties and plating quality of the high-strength steel sheet as the final product. The pickling may be performed once or may be performed in a plurality of times.

Hot-rolled sheet annealing (first heat treatment): Ac ₁ transformation point + 20 ° C. or higher, Ac ₁ transformation point + 120 ° C. or lower, maintained at 600 s or more and 21600 s or lower Ac ₁ transformation point + 20 ° C. or higher, Ac ₁ transformation point + 120 ° C. or lower It is very important in the present invention to maintain the temperature in the temperature range of 600 s to 21600 s.
When the annealing temperature of hot-rolled sheet annealing is less than Ac ₁ transformation point + 20 ° C., more than Ac ₁ transformation point + 120 ° C., and when the holding time is less than 600 s, all of Mn concentrates in austenite. Without proceeding, it becomes difficult to secure a sufficient volume ratio of retained austenite after final annealing, and ductility is lowered. Moreover, the structure | tissue which the value which remove | divided the amount of Mn (mass%) in retained austenite by the amount of Mn (polygonal ferrite) in polygonal ferrite is 2.0 or more cannot be obtained. On the other hand, if it is maintained for more than 21600 s, the concentration of Mn in austenite is saturated, and not only the effect on ductility after final annealing is reduced, but also the cost is increased.
Therefore, the hot-rolled sheet annealing (first heat treatment) of the present invention is held for a period of 600 s to 21600 s in a temperature range of Ac ₁ transformation point + 20 ° C. or higher and Ac ₁ transformation point + 120 ° C. or lower.

In addition, the annealing method may be any annealing method such as continuous annealing or batch annealing. In addition, after the above heat treatment, it is cooled to room temperature, but the cooling method and cooling rate are not particularly specified, and any cooling such as furnace cooling in batch annealing, gas jet cooling in air annealing and continuous annealing, mist cooling, water cooling, etc. I do not care. The pickling may be performed according to a conventional method.

Annealing (second heat treatment): Ac ₁ transformation point + 10 ° C. or higher, and Ac ₁ transformation point + 100 ° C. or lower and held for 20 to 900 s Ac ₁ transformation point + 10 ° C. or higher, Ac ₁ transformation point + 100 ° C. or lower Holding for 20 to 900 s is extremely important in the present invention. In the case where the annealing temperature is less than Ac ₁ transformation point + 10 ° C., or more than Ac ₁ transformation point + 100 ° C., and when the holding time is less than 20 s, none of the Mn concentration in the austenite proceeds sufficiently. It is difficult to ensure a sufficient volume ratio of retained austenite, and ductility is reduced. Moreover, the structure | tissue which the value which remove | divided the amount of Mn (mass%) in retained austenite by the amount of Mn (polygonal ferrite) in polygonal ferrite is 2.0 or more cannot be obtained. On the other hand, in the case of holding over 900 s, the area ratio of unrecrystallized ferrite decreases, the amount of heterophase interface between ferrite and hard second phase (martensite and retained austenite) increases, and YP decreases. , YR also decreases. Further, a structure in which the average crystal grain size of martensite is 3 μm or less and the average crystal grain size of retained austenite is 3 μm or less cannot be obtained.

Cold rolling reduction: less than 30% Cold rolling may be performed after the hot-rolled sheet annealing and before annealing (second heat treatment). In that case, it is essential that the rolling reduction is less than 30%. By not performing cold rolling or performing cold rolling at a reduction rate of less than 30%, polygonal ferrite formed by recrystallization after heat treatment does not form, and a structure stretched in the rolling direction remains, and finally This is because polygonal ferrite, retained austenite and martensite having a high aspect ratio are obtained, and not only the strength-ductility balance is improved but also stretch flangeability (hole expandability) is improved. On the other hand, when the rolling reduction is 30% or more, the average aspect ratio of the crystal structure of polygonal ferrite, martensite, and retained austenite is 2. A structure of 0 or more and 15.0 or less cannot be obtained.

Hot dip galvanizing treatment In the present invention, when the hot dip galvanizing treatment is performed, the steel sheet subjected to the annealing (second heat treatment) is immersed in a galvanizing bath at 440 ° C. or higher and 500 ° C. or lower to obtain hot dip galvanizing. Apply processing. Thereafter, the plating adhesion amount on the steel sheet surface is adjusted by gas wiping or the like. In addition, it is preferable to use the zinc plating bath whose amount of Al is 0.10 mass% or more and 0.22 mass% or less for hot dip galvanization.

Furthermore, when performing the alloying process of hot dip galvanization, the alloying process of galvanization can be performed in the temperature range of 450 degreeC or more and 600 degrees C or less after the said hot dip galvanization process. Here, when the alloying treatment is performed at a temperature exceeding 600 ° C., untransformed austenite is transformed into pearlite, and a desired volume ratio of retained austenite cannot be ensured, and ductility is lowered. On the other hand, if the alloying treatment temperature is less than 450 ° C., alloying does not proceed and it is difficult to produce an alloy layer.
Therefore, when the alloying treatment of galvanization is performed, the treatment is performed in a temperature range of 450 ° C. or more and 600 ° C. or less.

The conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, the series of treatments such as annealing, hot dip galvanization, galvanizing alloying treatment, etc. are performed by CGL (Continuous Galvanizing) which is a hot dip galvanizing line. Line).

Further, when the hot dip aluminum plating treatment is performed, the steel plate subjected to the annealing treatment is immersed in an aluminum plating bath at 660 to 730 ° C. to perform the hot dip aluminum plating treatment. Thereafter, the plating adhesion amount is adjusted by gas wiping or the like. In addition, a steel plate having a composition suitable for an aluminum plating bath temperature range of Ac ₁ transformation point + 10 ° C. or higher and Ac ₁ transformation point + 100 ° C. or lower produces finer and more stable retained austenite by hot-dip aluminum plating treatment. Therefore, it is preferable because the ductility can be further improved.

Electrogalvanizing treatment Further, in the present invention, the steel plate after the heat treatment may be subjected to an electrogalvanizing treatment. The electrogalvanizing treatment at that time is not particularly limited, but it is preferable to adjust the electrogalvanizing treatment conditions so that the film thickness is in the range of 5 μm to 15 μm.

Here, in the present invention, skin pass rolling can be performed on the above steel plate, hot dip galvanized steel plate, hot dip galvanized steel plate and electrogalvanized steel plate for the purpose of shape correction, adjustment of surface roughness, and the like. The rolling reduction of the skin pass rolling is preferably in the range of 0.1% to 2.0%.
If the rolling reduction of skin pass rolling is less than 0.1%, the effect of skin pass rolling is small and control is difficult, so 0.1% is the lower limit of the preferred range. On the other hand, if the rolling reduction ratio of the skin pass rolling exceeds 2.0%, the productivity of the steel sheet is remarkably lowered, so 2.0% is made the upper limit of the preferred range.

Note that the skin pass rolling may be performed online or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
Furthermore, the steel sheet according to the present invention, the hot dip galvanized steel sheet, the hot dip galvanized steel sheet, and the electrogalvanized steel sheet can be subjected to various coating treatments such as coating using resin or oil.

A steel having the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method. The obtained slab was made into the following various steel plates under the conditions shown in Table 2.
That is, after hot rolling, annealing is performed at Ac ₁ transformation point + 20 ° C. or more, Ac ₁ transformation point + 120 ° C. or less, and after cold rolling (in some cases, cold rolling is not performed), Ac ₁ transformation point + 10 ° C. or more. And Ac ₁ transformation point + 100 ° C. or lower. Thereafter, a cold-rolled steel sheet (CR) is obtained, and further subjected to a plating treatment, and a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), a hot-dip aluminum-plated steel sheet (Al), and an electrogalvanized steel sheet ( EG).

In addition, as a hot dip galvanizing bath, a zinc bath containing Al: 0.19% by mass in a hot dip galvanized steel plate (GI), and a zinc containing Al: 0.14% by mass in a galvannealed steel plate (GA). A bath was used. In both cases, the bath temperature was 465 ° C., and the amount of plating adhered was 45 g / m ² per side (double-sided plating). Furthermore, with GA, the Fe concentration in the plating layer was adjusted to 9 mass% or more and 12 mass% or less. The bath temperature of the hot-dip aluminum plating bath for hot-dip aluminum-plated steel sheets was 700 ° C.
The steel sheet thus obtained was examined for the cross-sectional microstructure, tensile properties, hole expansibility, bendability, etc., and the results are shown in Tables 3 to 5.

The Ac ₁ transformation point was determined using the following equation.
Ac ₁ transformation point (° C)
= 751-16 × (% C) + 11 × (% Si) −28 × (% Mn) −5.5 × (% Cu) −16 × (% Ni) + 13 × (% Cr) + 3.4 × (% Mo)
Here, (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr) and (% Mo) are the contents of each element in steel (mass% ).

The tensile test is performed in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece sampled so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and YP, YR, TS and EL was measured. YR is a value expressed by percentage by dividing YP by TS. In the present invention, the case where YR ≧ 68% and TS × EL ≧ 24000 MPa ·% was judged as good. Moreover, it was judged that EL ≧ 34% for TS: 590 MPa class, EL ≧ 30% for TS: 780 MPa class, and EL ≧ 24% for TS: 980 MPa class, respectively. In this example, TS: 590 MPa class is a steel sheet having a TS of 590 MPa or more and less than 780 MPa, TS: 780 MPa class is a steel sheet having a TS of 780 MPa or more and less than 980 MPa, and TS: 980 MPa class is a TS of 980 MPa. The steel sheet is less than 1180 MPa.

The bending test was performed based on the V-block method of JIS Z2248 (1996). With respect to the outside of the bent portion, the presence or absence of a crack was determined with a stereomicroscope, and the minimum bending radius at which no crack was generated was defined as a limit bending radius R. In the present invention, when the limit bending R / t ≦ 1.5 (t: plate thickness of the steel plate) at 90 ° V bending is satisfied, the bendability of the steel plate is determined to be good.

The hole expandability was performed in accordance with JIS Z 2256 (2010). Each obtained steel plate was cut into 100 mm × 100 mm, and a hole with a diameter of 10 mm was punched out with a clearance of 12% ± 1%. Next, a 60 ° conical punch was pushed into the hole with a crease holding force of 9 ton (88.26 kN) using a die having an inner diameter of 75 mm, and the hole diameter at the crack initiation limit was measured. Furthermore, the critical hole expansion rate λ (%) was obtained from the following formula, and the hole expansion property was evaluated from the value of the critical hole expansion rate.
Limit hole expansion ratio λ (%) = {(D _f −D ₀ ) / D ₀ } × 100
However, _{D f} hole diameter at crack initiation (mm), _{D 0} is the initial hole diameter (mm). In the present invention, the case of λ ≧ 34% for the TS: 590 MPa class, λ ≧ 30% for the TS: 780 MPa class, and λ ≧ 25% for the TS: 980 MPa class was determined to be good.

Regarding the determination of the plateability of hot rolling, when the finishing temperature of hot rolling is low, the reduction rate of austenite is high in the non-recrystallized state, or rolling is performed in a two-phase region of austenite and ferrite For example, it was assumed that the risk of troubles such as defective plate shape during hot rolling due to an increase in rolling load increased, and this case was judged as defective.
In addition, regarding the determination of the plateability of the cold rolling, when the coiling temperature of the hot rolling is low and the steel structure of the hot rolled sheet is mainly composed of a low-temperature transformation phase such as bainite or martensite, rolling is performed. This case was judged to be defective, assuming that the risk of troubles such as defective plate shape during cold rolling due to an increase in load would increase.

Regarding the determination of the surface properties of the final annealed plate, it is determined that the surface of the steel plate cannot be scaled off and defects such as bubbles and segregation on the surface of the slab cannot be scaled off. did. In addition, the surface properties of the final annealed plate include a case where the amount of oxide (scale) generated increases sharply, the interface between the base iron and the oxide becomes rough, and the surface quality after pickling and cold rolling deteriorates. It was also judged as defective when there was a part of the hot rolled scale remaining after washing.

Regarding the determination of productivity, (1) when a hot-rolled sheet shape defect occurs, (2) when hot-rolled sheet shape correction is necessary to proceed to the next step, or (3) annealing treatment holding time The lead time cost was evaluated when the time was long. A case that does not correspond to any of (1) to (3) is determined as “good”, and a case that corresponds to any of (1) to (3) is determined to be “bad”.

Using a JIS No. 5 test piece that was sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, the tensile process was performed in accordance with JIS Z 2241 (2011), and the elongation value was 10%. A value obtained by dividing the volume ratio of residual austenite when processing was applied by the volume ratio of residual austenite before processing (10% application) was obtained. The retained austenite volume fraction was measured according to the method described above.
The measurement results are also shown in Table 4.

The amount of C (mass%) in the retained austenite and the amount of Mn (mass%) in the retained austenite were measured according to the method described above.
The measurement results are also shown in Table 4.

From the above results, according to the present invention, a high-strength steel sheet having a TS of 590 MPa or more, excellent formability with a YR of 68% or more, and having a high yield ratio and hole expansibility is obtained. I understand that. On the other hand, in the comparative example, at least one characteristic among YR, TS, EL, λ, and R / t is inferior.

According to the present invention, all have TS of 590 MPa or more, YR of 68% or more, excellent formability of TS × EL ≧ 24000 MPa ·%, high yield ratio and hole expandability Steel plate can be manufactured. By applying the steel plate of the present invention to, for example, an automobile structural member, fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is extremely large.

Claims (6)

In mass%, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 2.60% to 4.20%, P: 0.001% or more 0.100% or less, S: 0.0200% or less, N: 0.0100% or less and Ti: 0.005% or more and 0.200% or less,
Furthermore, by mass%, Al: 0.01% to 2.00%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005 %: 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.010%, Ca : 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less arbitrarily selected from at least one element Contains, balance is Fe and inevitable It has a chemical composition of impurities,
In terms of area ratio, polygonal ferrite is 20% or more and 65% or less, non-recrystallized ferrite is 8% or more and martensite is 5% or more and 25% or less, and by volume ratio, residual austenite is 8% or more, The polygonal ferrite, the martensite, and the retained austenite have a steel structure having an average aspect ratio of 2.0 or more and 15.0 or less, respectively,
The average grain size of the polygonal ferrite is 6 μm or less,
An average crystal grain size of the martensite is 3 μm or less,
The average grain size of the retained austenite is 3 μm or less,
A steel sheet having a value obtained by dividing the amount of Mn (mass%) in the retained austenite by the amount of Mn (mass%) in the polygonal ferrite is 2.0 or more. The amount of C in the retained austenite is related to the amount of Mn in the retained austenite by the following formula: 0.09 × [Mn amount] −0.026−0.150 ≦ [C amount] ≦ 0.09 × [ Amount of Mn] −0.026 + 0.150
[C amount]: C amount (% by mass) in retained austenite
[Mn amount]: Mn amount (% by mass) in retained austenite
The steel plate according to claim 1 satisfying A plated steel sheet, wherein the steel sheet according to claim 1 or 2 further comprises one selected from a hot dip galvanized layer, an alloyed hot dip galvanized layer, a hot dip galvanized layer, and an electrogalvanized layer. It is a manufacturing method of the steel plate according to claim 1 or 2,
The steel slab having the composition according to claim 1 is heated, hot rolled at a finish rolling exit temperature of 750 ° C. to 1000 ° C., wound up at 300 ° C. to 750 ° C., and then pickled. The scale is removed, and it is maintained at 600 s to 21600 s in the temperature range of Ac ₁ transformation point + 20 ° C. or higher and Ac ₁ transformation point + 120 ° C., and optionally cold-rolled at a reduction rate of less than 30%, and then the Ac ₁ transformation point. + 10 ° C. or higher, the production method of the steel sheet to cool and kept at Ac ₁ transformation point + 100 ° C. below the temperature range 20s or 900s or less. The method for producing a steel sheet according to claim 4, wherein a value obtained by dividing a volume ratio of retained austenite after applying a tensile process of 10% by an elongation value by a residual austenite volume ratio before the tensile process is 0.3 or more. . It is a manufacturing method of the plated steel plate according to claim 3,
After the cooling, one type selected from hot dip galvanizing, hot dip galvanizing, and electrogalvanizing is applied, or after the hot dip galvanizing is performed, alloying is performed at 450 ° C. or higher and 600 ° C. or lower. The manufacturing method of the steel plate of Claim 4 which performs a process.

Images