JP5659604B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet for press forming that is used through a press forming process in automobiles, home appliances, and the like and a method for manufacturing the same.

  Conventionally, automotive exterior panels that require excellent dent resistance such as hoods, doors, trunk lids, back doors, and fenders are TS: 340MPa class BH steel sheets (bake-hardened steel sheets, hereinafter simply referred to as 340BH). Has been applied. 340BH is a ferritic single-phase steel in which the amount of solid solution C is controlled by adding carbonitride-forming elements such as Nb and Ti, and is solid solution strengthened with Si, Mn and P in an ultra-low carbon steel with C: less than 0.01% by mass. is there. In recent years, the need for lighter vehicle bodies has further increased, and the outer panel to which these 340BH has been applied has been further strengthened to reduce the thickness of the steel sheet, or R / F (reinforcement: inner reinforcement parts with the same thickness) ), And the baking coating process is being conducted at a low temperature and in a short time.

  However, when a large amount of Si, Mn and P is further added to the conventional 340BH to increase the strength, the surface distortion resistance of the press-formed product is significantly deteriorated due to the increase in YP. Here, the surface distortion is a fine wrinkle or wavy pattern on the press-molded surface that is likely to occur on the outer periphery of the knob portion of the door. Since surface distortion significantly impairs the appearance quality of automobiles, the steel sheet applied to the outer panel may increase the strength of the pressed product, but the yield stress before press forming may have a low YP that is close to the current 340BH. Required. Similarly, when the TS of the material fluctuates, the amount of spring back of the pressed product changes, causing surface distortion. For this reason, in order to reduce surface distortion, it is necessary not only to keep YS low, but also to reduce fluctuations in TS in the coil.

  In addition, steel sheets used for such applications are also required to have excellent stretch formability, and high ductility (El) is required to be stably obtained in the coil.

  Furthermore, the steel plate used for such an automobile body outer plate part is also required to have excellent corrosion resistance. That is, when stones scatter and the steel plate is damaged when the automobile is running, rust is likely to be generated at that portion, which causes a hole. In order to suppress such corrosion, the chipping resistance needs to be equal to or better than the conventional 340BH.

  From such a background, for example, in Patent Document 1, the cooling rate after annealing of steel containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.023 to 0.8% is optimized, mainly ferrite. And a martensite composite structure is disclosed to obtain a galvannealed steel sheet having both low yield stress (YP) and high ductility (El).

  Patent Document 2 includes mass%, C: 0.005 to 0.04%, Mn: 1.0 to 2.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.01 to 0.1%, N: less than 0.008%, Cr : Steel plate with excellent ductility and high BH by containing 0.2-1.0% and Mn + 1.29Cr 2.1-2.8 to make a structure composed of ferrite and a martensite with a volume fraction of 3.0% or more and less than 10.0% Is disclosed.

  Patent Document 3 describes, in mass%, C: 0.02 to 0.033%, Mn: 1.5 to 2.5%, Cr: 0.03 to 0.5%, Mo: 0 to 0.5% of steel containing Mn, Cr, and Mo. It is disclosed that a steel plate having an excellent ductility (El) and stretch flange formability (hole expansion ratio, λ) can be obtained when YP is 300 MPa or less by making the total amount of 1.8 to 2.5%.

  In Patent Document 4, in steel containing C: 0.03 to 0.09%, Mn: 1.0 to 2.0%, sol.Al: 0.005 to 0.1%, B: 0.001 to 0.003% in mass%, C and Mn It is disclosed that a high-strength cold-rolled steel sheet excellent in bake hardenability and room temperature aging resistance can be obtained by controlling the composition range to C + Mn / 20 ≧ 0.12%.

  Further, in Patent Document 5, in a soft cold-rolled steel sheet containing C ≦ 0.05%, Mn ≦ 0.5%, N ≦ 0.005%, and B ≦ 0.005% by weight%, the composition range of N and B is N− A technique for reducing material fluctuation in the coil by reducing the amount of fine AlN deposited by controlling to 14 / 11B ≦ 10 (ppm) is disclosed.

Japanese Patent Publication No.57-57945 Japanese Unexamined Patent Publication No. 2007-211338 Japanese Patent No. 3613129 JP 2009-174019 JP 2001-73074 A

  However, the steel sheets described in Patent Documents 1 to 4 above are composite structure steel sheets in which Mn and Cr are added in a large amount compared to the conventional 340BH, and a suitable amount of the second phase mainly composed of martensite is dispersed in the ferrite structure. And had the following problems.

  First, many of the steel sheets described in Patent Documents 1 to 3 are significantly inferior in chipping resistance as compared with the conventional 340 MPa steel (340BH). For example, a part simulating a door using a composite steel sheet containing 0.6% Cr, spraying crushed stone on the steel sheet surface, and evaluating the corrosion resistance, the maximum perforated depth is It became clear that it was about twice that of 340MPa steel (340BH). In other words, even if such a steel plate is excellent in press formability, the perforated life is reduced to about half compared to conventional steel, so that it is difficult to apply to a real vehicle.

  Further, some of the component steels described in Patent Documents 1 to 4 were found to have steel sheets with low BH and El or high YS even in a composite structure. That is, further improvement of the material is necessary.

  On the other hand, among the steels described in Patent Documents 1 to 4, in some of the B-added steels, expensive elements such as Mo and Cr can be reduced, and chipping resistance and chemical conversion treatment are also good. In addition, although it shows relatively low YS and high BH, it has become clear that there is a problem that large material variations occur. For example, a coil containing, in mass%, C: 0.025%, Mn: 1.8%, Cr: 0.2%, P: 0.02%, sol.Al: 0.06%, B: 0.0025%, N: 0.002% In the coil that was wound at CT: 640 ° C, then cold-rolled and then annealed at 770 ° C x 40 sec with CGL, TS: 460MPa at the outermost edge in the coil width direction, but in the coil width direction In the central part, the strength decreases to TS: 430MPa. In other words, TS variation of 30 MPa occurs in the width direction of the coil. Similarly, TS fluctuation of about 30 MPa occurs in the longitudinal direction of the coil. In addition, in such a coil, El also changes by 3% in the coil corresponding to the change in TS, and the stability of El in the coil is extremely inferior, and the stability of press formability is also inferior. Furthermore, such a coil is highly dependent on the annealing temperature of TS and El. Such a phenomenon occurs in steel with 0.001% or more of B added for the purpose of improving the material.

  Further, although an attempt was made to utilize the technique for reducing material fluctuations described in Patent Document 5 for a composite structure steel plate to which B was added, the material fluctuations were not improved.

  As described above, in the method disclosed in the above patent document, it was difficult to obtain a steel plate having excellent chipping resistance, low YP, high BH, high El, and small material fluctuation. .

  The present invention has been made to solve such problems, and has a high strength steel plate having excellent chipping resistance, low YP, high BH, high El, and further reduced material fluctuation in the coil. It aims at providing the manufacturing method cheaply.

  The present inventors conducted intensive studies on a method for simultaneously achieving low YP, high BH, high El, and material fluctuation reduction while improving chipping resistance, targeting a conventional composite steel sheet with low yield strength. The following conclusions were obtained.

  (I) Chipping resistance is greatly improved by reducing the Cr content to less than 0.35% and P to 0.05% or less. As a result, characteristics equivalent to or better than the conventional 340 MPa steel (340BH) can be obtained.

  (II) To obtain low YP, high BH, and high El properties, first, make a structure with ferrite and a small volume fraction of the second phase, suppress pearlite and bainite in the second phase, martensite and residual It is important to increase the ratio of γ. For this purpose, it is necessary to contain a predetermined amount of Mn, Mo, Cr, P, B, etc., which are hardenability improving elements. And in order to improve BH while obtaining even lower YP and higher El, it is necessary to leave a predetermined amount of solid solution C while uniformly coarsening the ferrite grains and the second phase. In particular, it is better to actively use Cr, P, and B while reducing Mn and Mo. However, in order to achieve compatibility with chipping resistance, it is necessary to avoid excessive addition of Cr and P, and among these elements, it is most desirable to make maximum use of B.

  (III) In steel using B, the material improves by containing 10 ppm or more of B, but on the other hand, the material variation becomes obvious. Such a material variation is because the solid solution B remains in the hot-rolled sheet to form a hardly solid-soluble carbide, and the material variation is reduced by suppressing the formation of such a carbide. Moreover, such carbides are reduced by selecting an appropriate winding temperature according to the N content, Ti content, sol.Al content, and B content in the steel.

That is, in the steel containing 10 ppm or more of B from the viewpoint of low YP and high BH, there is a slight amount of solute B that did not combine with N when wound by hot rolling. Alternatively, in steel containing a certain amount of sol.Al, precipitation of AlN occurs during the slow cooling of the coil after winding, resulting in solid solution B. When solute B is generated in this way, stable carbide is generated during coil cooling together with Fe, Mn, and C in steel, and C is consumed. In addition, the amount of such carbide generated varies significantly depending on hot rolling conditions such as the coiling temperature. Since such carbides are very stable compared to cementite (Fe ₃ C), they remain in the form of precipitates even at the end of annealing, and the amount of martensite generated is significantly higher in coil regions where the amount of carbide generated is large. Decrease. As a result, it has been clarified that the material change in the coil occurs remarkably in the composite steel with B added. Such a phenomenon is a phenomenon that has not been recognized in conventional soft cold-rolled steel sheets that do not contain martensite as a strengthened structure.

  Therefore, in order to avoid such a phenomenon, the amount of the solid solution B in the hot-rolled sheet may be reduced, or the generation of stable carbides may be suppressed even if the solid solution B is slightly generated. For that purpose, it discovered that the coiling temperature should just be controlled appropriately with respect to the solid solution B amount produced according to N, Ti, sol.Al, and B amount.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] Steel composition as mass%, C: more than 0.015% and less than 0.100%, Si: less than 0.50%, Mn: more than 1.0% and less than 2.0%, P: 0.05% or less, S: 0.03% or less, sol Al: 0.01% or more and 0.3% or less, N: 0.005% or less, Cr: less than 0.35%, B: 0.0010% or more and 0.0050% or less, Mo: less than 0.15%, Ti: less than 0.030%, and further 2.1 ≦ [ Mneq] ≦ 3.1, consisting of the balance iron and inevitable impurities, and as a steel structure, it has ferrite and the second phase, the volume fraction of the second phase is 2.0 to 12.0%, the martensite and residual in the second phase The ratio of volume fraction of γ is 60% or more, characterized in that it is present in ferrite grains and the number of carbide particles having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm is 10,000 / mm ² or less. High strength steel plate.
Where [Mneq] = [% Mn] +1.3 [% Cr] +3.3 [% Mo] +8 [% P] + 150B ^* , B ^* = [% B] + [% Ti] /48×10.8× 0.9 + [% Al] /27×10.8×0.025, [% Mn], [% Cr], [% Mo], [% P], [% B], [% Ti], [% Al] Represents the respective contents of Mn, Cr, Mo, P, B, Ti, sol.Al. When B ^* ≧ 0.0022, B ^* = 0.0022.

  [2] Further, in [1], characterized by containing at least one of Nb: less than 0.030%, V: 0.2% or less, W: 0.15% or less, Zr: 0.1% or less in mass%. High strength steel sheet as described.

  [3] Furthermore, by mass%, Sn: 0.2% or less, Sb: 0.2% or less, Cu: 0.5% or less, Ni: 0.5% or less, Ca: 0.01% or less, Ce: 0.01% or less, La: 0.01% or less The high-strength steel sheet according to [1] or [2], containing at least one of Mg: 0.01% or less.

  [4] In the step of hot-rolling the steel slab having the component composition according to any one of [1] to [3], the coiling temperature CT is set in the range shown in the formula (1) according to sol.B. After controlling and cold rolling at a cold rolling rate of 50 to 85%, hold it for 25 seconds or more at an annealing temperature of 740 ° C or higher and 830 ° C or lower in a continuous galvanizing line (CGL) or continuous annealing line (CAL). A method for producing a high-strength steel sheet characterized by annealing.

CT (° C) ≦ 670-50000 × sol.B (1)
sol.B = [% B]-{[% N] / 14-[% Ti] /48×0.8-[% Al] /27×0.0005× (CT-560)} × 10.8 ・ ・ ・ (A) formula
In the formula (A), [% B], [% N], [% Ti], [% Al] represents the respective contents of B, N, Ti, sol.Al, and CT represents the coiling temperature ( ° C). When CT-560 ≦ 0, CT-560 shall be 0 (zero). However, when sol.B ≦ 0, sol.B is calculated as 0 (zero).

  According to the present invention, a high-strength steel plate having excellent chipping resistance, low YP, high BH and El, and small material fluctuation in the coil can be produced at low cost. The high-strength steel sheet of the present invention has excellent corrosion resistance, excellent surface strain resistance, excellent overhang formability, excellent dent resistance, and excellent material stability. Make it possible.

The figure which shows the relationship of TS fluctuation | variation of sol.B and CT and the width direction. The figure which shows the change of TS of the coil width direction of the steel plate from which CT differs.

  Details of the present invention will be described below. Note that% representing the amount of a component means mass% unless otherwise specified.

1) Steel composition
Cr: Less than 0.35%
Cr improves the hardenability and generates a certain amount of martensite, and reduces YP and improves El by the action of dispersing martensite uniformly without refining the ferrite grains. Although it is an element desirable to do, chipping resistance deteriorates remarkably. Therefore, in order to ensure good chipping resistance, the Cr content needs to be less than 0.35%. In order to provide further excellent chipping resistance, Cr is desirably less than 0.30%. Cr is an element that can be arbitrarily added from the viewpoint of optimizing [Mneq] shown below, and the lower limit is not specified (including Cr: 0%), but from the viewpoint of low YP, Cr is 0.02% The above is preferably added, and more preferably 0.05% or more.

[Mneq]: 2.1 or more and 3.1 or less Low YP, high El, high BH, and in order to ensure excellent aging resistance, at least a steel structure has ferrite and mainly a second phase having martensite and residual γ. It needs to be a complex organization. In conventional steel, many steel plates with high YP and steel plates with low El were found, and as a result of investigating the cause, these steel plates produced pearlite and bainite as the second phase in addition to martensite and a small amount of residual γ. It became clear that

  Since this pearlite and bainite are as fine as 1 to 2 μm and are formed adjacent to martensite, for example, even when a sample corroded with a nital or repeller corrosion solution is observed with an optical microscope, it has a different structure from martensite. It is difficult to identify as an organization, and it is necessary to observe at a magnification of 3000 times or more by using SEM to strictly identify it as a different organization from martensite. For example, when the structure of the conventional 0.03% C-1.5% Mn-0.5% Cr steel is investigated in detail, only coarse pearlite is identified by observation with an optical microscope or SEM at a magnification of about 1000 times. The volume fraction of pearlite or bainite in the volume fraction of the second phase is measured to be about 10%, but if a detailed investigation is conducted with 3000 times SEM observation, the percentage of the volume fraction of pearlite or bainite in the second phase Account for 30-40%. By suppressing such pearlite or bainite, low YP and high El can be obtained simultaneously.

After annealing, primary cooling from annealing temperature to around 480 ° C (for example, cooling until immersion in plating bath for CGL) or secondary cooling from 480 ° C to around 350 ° C (for example, cooling after immersion in plating bath for CGL) In order to sufficiently reduce such fine pearlite or bainite in the thermal history of CGL and CAL with a cooling rate of approximately 1 to 200 ° C / sec (cooling to overaging zone in CAL) What is necessary is just to control the weighting equivalent formula of each following element to 2.1-3.1. However, when B is added in combination with Ti or Al, the effect of improving hardenability is remarkably increased, but the effect of improving hardenability is saturated even if it is added in a predetermined amount or more. It is expressed in
[Mneq] = [% Mn] +1.3 [% Cr] +3.3 [% Mo] +8 [% P] + 150B ^*
B ^* = [% B] + [% Ti] /48×10.8×0.9 + [% Al] /27×10.8×0.025
However, when B ^* ≧ 0.0022, B ^* = 0.0022.
Here, [% Mn], [% Cr], [% Mo], [% P], [% B], [% Ti], [% Al] are Mn, Cr, Mo, P, B, Ti, Each content of sol.Al is expressed.

When B ^* is 0.0022 or more, the effect of improving hardenability by B is saturated, so B ^* is 0.0022.

  By setting this [Mneq] to 2.1 or higher, low YP, high El, and high BH can be obtained. Furthermore, from the viewpoint of low YP and high El, [Mneq] is preferably 2.2 or more, and more preferably 2.3 or more. When [Mneq] exceeds 3.1, the amount of Mn, Cr, and P added is too large, and it becomes difficult to ensure a sufficiently low YP and excellent chipping resistance at the same time. Therefore, [Mneq] is 3.1 or less.

Mn: more than 1.0% and less than 2.0% In order to further reduce YP and ensure high El and high BH, it is better that the Mn content is small even in the steel composition of the same Mn equivalent. This is because if the amount of Mn is too large, the α → γ transformation temperature in the annealing process is lowered, and γ grains are formed before recrystallization is completed, resulting in a non-uniform structure in which the ferrite grains and the second phase are partially refined. This is because as YP increases, El decreases, and the amount of dissolved C after annealing decreases and BH decreases. From the viewpoint of low YP, high El, and high BH, the Mn content should be less than 2.0%. On the other hand, if the amount of Mn is too small, it becomes difficult to ensure sufficient hardenability even if a large amount of other elements are added. In addition, many MnS are finely dispersed, and the corrosion resistance and chipping resistance deteriorate. In order to ensure sufficient hardenability and corrosion resistance, Mn needs to be added at least over 1.0%.

  Therefore, the Mn content is more than 1.0% and less than 2.0%. Furthermore, from the viewpoint of improving corrosion resistance and chipping resistance, it is desirable that Mn is 1.2% or more.

Mo: Less than 0.15%
Mo can be added from the viewpoint of improving hardenability, suppressing the formation of pearlite, and ensuring a predetermined strength. However, Mo, like Mn, has a strong effect of refining the second phase and a strong effect of refining ferrite grains. Therefore, when Mo is added excessively, YP is remarkably increased. Moreover, when Mo is used as a cold-rolled steel sheet, the chemical conversion processability is remarkably deteriorated. Moreover, it is an extremely expensive element. Therefore, the amount of Mo is limited to less than 0.15% (including 0%) from the viewpoints of YP reduction, chemical conversion processability improvement, and cost reduction. From the viewpoint of further reducing the YP, it is desirable to make it 0.05% or less, and it is more preferable that Mo is not added (0.02% or less).

P: 0.05% or less
P can be used as a hardenable element in the present invention, and is an element that achieves low YP, high BH, and high El by using it as an alternative element for Mn, Cr, and Mo. In other words, P has a great effect of improving hardenability even with a small amount of addition, and also has the effect of uniformly dispersing the second phase at the triple point of the ferrite grain boundary, so Mn is used even with the same Mn equivalent. If P is used, YP will be lower and BH will be higher. El also gets higher. In order to obtain such effects of low YP, high BH, and high El by the addition of P, it is desirable to add 0.015% or more of P.

  However, if P is added in an amount exceeding 0.05%, the effect of improving hardenability, the homogenization of the structure, and the effect of coarsening are saturated, and the amount of solid solution strengthening becomes too large to obtain a low YP. In addition, the alloying reaction between the ground iron and the plating layer is significantly delayed to deteriorate the powdering resistance, and as a result, the chipping resistance is deteriorated. Therefore, the P content is 0.05% or less.

B: 0.0010% or more and 0.0050% or less
B has the effect of uniformly and coarsening ferrite grains and martensite grains and the effect of suppressing pearlite by improving hardenability. In addition, B itself has the effect of increasing BH. For this reason, by replacing Mn with B while securing a predetermined amount of [Mneq], both low YP and high BH can be achieved. In order to obtain such an effect sufficiently, it is necessary to add 0.0010% or more of B. However, when B is added over 0.0050%, castability and rollability are remarkably lowered. Therefore, B is 0.0050% or less. In order to further exhibit the effects of lowering YP and increasing BH by adding B, B is preferably added in an amount of 0.0013% or more.

C: More than 0.015% and less than 0.100%
C is an element necessary to ensure a predetermined volume ratio of the second phase. If the amount of C is small, the second phase is not formed, YP increases remarkably and the balance between strength and ductility deteriorates. Moreover, high BH and excellent aging resistance cannot be obtained. In order to secure a predetermined volume ratio of the second phase and obtain a sufficiently low YP, C needs to be more than 0.015%. From the viewpoint of improving aging resistance and further reducing YP, C is preferably 0.02% or more. On the other hand, when the C content is 0.100% or more, the volume fraction of the second phase becomes too large, YP increases, and El and BH decrease. Moreover, weldability also deteriorates. Therefore, the C content is less than 0.100%. In order to secure high El and high BH while obtaining a lower YP, the C content is preferably less than 0.060%, and more preferably less than 0.040%.

Si: Less than 0.50%
Addition of a small amount of Si has the effect of improving the surface quality by delaying the scale formation in hot rolling, the effect of moderately delaying the alloying reaction between the iron and zinc in the plating bath or during alloying, From this point of view, it is possible to make the microstructure more uniform and coarse by reducing the YR and increasing the El. However, when 0.50% or more of Si is added, a scale pattern or non-plating occurs in the hot-dip galvanized steel sheet, and a scale pattern occurs in the cold-rolled steel sheet, making it difficult to apply to the outer panel. Moreover, chemical conversion processability is deteriorated and YP is increased. For this reason, the Si content is less than 0.50%. Further, from the viewpoint of improving the surface quality and reducing YP, Si is desirably less than 0.30%. Si is an element that can be arbitrarily added, and the lower limit is not specified (including Si: 0%), but from the above viewpoint, Si is preferably added in an amount of 0.01% or more, and more preferably 0.02% or more.

S: 0.03% or less
S can be contained because it has the effect of improving the primary scale peelability of the steel sheet and improving the appearance quality of the cold-rolled steel sheet and hot-dip galvanized steel sheet by containing an appropriate amount of S. However, if the content of S is large, the amount of MnS precipitated in the steel becomes too much and the elongation of the steel sheet and the stretch flangeability are deteriorated. Moreover, when hot-rolling a slab, hot ductility is reduced and surface defects are easily generated. Furthermore, corrosion resistance is reduced. For this reason, the amount of S shall be 0.03% or less. From the viewpoint of improving stretch flangeability and corrosion resistance, S is preferably 0.02% or less, and more preferably 0.01% or less.

sol.Al: 0.01% or more and 0.3% or less
Al is added for the purpose of fixing N and promoting the hardenability improvement effect of B, the purpose of improving aging resistance, and the purpose of improving the surface quality by reducing inclusions. From the viewpoint of improving the hardenability and aging resistance of B, the content of sol.Al is 0.01% or more. In order to exhibit such an effect more, it is desirable to contain sol.Al in an amount of 0.015% or more, and more preferably 0.04% or more. On the other hand, even if sol.Al is added in excess of 0.3%, the effect of leaving the solid solution B and the effect of improving the aging resistance are saturated and the cost is increased. In addition, the castability is deteriorated and the surface quality is deteriorated. For this reason, sol.Al is made 0.3% or less. From the viewpoint of ensuring excellent surface quality, sol.Al is preferably less than 0.2%.

N: 0.005% or less
N is an element that forms nitrides such as BN, AlN, and TiN in steel, and has the detrimental effect of eliminating the material improvement effect of B through the formation of BN. In addition, fine AlN is formed to lower the grain growth property and increase YP. If the N content exceeds 0.005%, the YP increases and the aging resistance deteriorates and the applicability to the outer panel becomes insufficient. Therefore, the N content is 0.005% or less. From the viewpoint of further reducing YP by reducing the amount of precipitated AlN, N is preferably 0.004% or less.

Ti: Less than 0.030%
Ti has the effect of fixing N and improving the hardenability of B, the effect of improving aging resistance and the effect of improving castability, and can be added arbitrarily to obtain such an effect as an auxiliary. It is an element. However, if the content increases, fine precipitates such as TiC and Ti (C, N) are formed in the steel to significantly increase YP, and TiC is generated during cooling after annealing to reduce BH. Since there exists an effect | action, when adding, it is necessary to control content of Ti to an appropriate range. When the Ti content exceeds 0.030%, YP increases remarkably. Therefore, the Ti content is less than 0.030%. Ti is an element that can be added arbitrarily, and the lower limit is not specified (including Ti: 0%), but in order to fix N by precipitation of TiN and to improve the hardenability of B, the content of Ti The amount is preferably 0.002% or more. In order to obtain a low YP by suppressing the precipitation of TiC, the Ti content is preferably less than 0.010%.

  The balance is iron and inevitable impurities, but may further contain a predetermined amount of the following elements.

At least one of the following V, Nb, W and Zr:
V: 0.2% or less
V can be added from the viewpoint of increasing the strength. From the viewpoint of increasing strength, 0.002% or more is preferably added, and 0.01% or more is more preferable. However, if added over 0.2%, the cost will increase significantly, so it is desirable to add V at 0.2% or less.

Nb: less than 0.030%
Nb has the effect of refining the structure and strengthening the steel sheet by precipitating NbC and Nb (C, N), so it can be added from the viewpoint of increasing the strength. In addition, the effect of delaying recrystallization in hot rolling and the effect of delaying subsequent transformation are large, so the texture is improved by adding a small amount of Nb, and the r value in the direction perpendicular to the rolling is reduced to the 45 ° direction. This has the effect of improving the r value. For this reason, the in-plane anisotropy of Δr and YP is reduced by adding 0.002 to 0.015% of Nb. From the above viewpoint, Nb is preferably added in an amount of 0.002% or more, and more preferably 0.005% or more. However, since YP increases remarkably when 0.030% or more is added, Nb is preferably added at less than 0.030%.

W: 0.15% or less
W can be used as a hardenable element and a precipitation strengthening element. From the above viewpoint, W is preferably added in an amount of 0.002% or more, and more preferably 0.005% or more. However, if the amount added is too large, YP will increase, so it is desirable to add W at 0.15% or less.

Zr: 0.1% or less
Zr can also be used as a hardenable element and precipitation strengthening element. Zr is preferably added in an amount of 0.002% or more, more preferably 0.005% or more from the above viewpoint. However, if the amount added is too large, YP will increase, so it is desirable to add Zr at 0.1% or less.

At least one of the following Sn, Sb, Cu, Ni, Ca, Ce, La and Mg:
Sn: 0.2% or less
Sn is preferably added from the viewpoint of suppressing decarburization and de-B in the tens of microns region of the steel sheet surface layer caused by nitridation, oxidation, or oxidation of the steel sheet surface. This improves fatigue properties, aging resistance, surface quality, and the like. From the viewpoint of suppressing nitriding and oxidation, it is preferable to add Sn at 0.002% or more, and more preferably 0.005% or more, but if it exceeds 0.2%, it will cause YP increase and toughness deterioration, so Sn is 0.2% or less It is desirable to contain.

Sb: 0.2% or less
Sb is also preferably added from the viewpoint of suppressing decarburization and de-B in the tens of microns region of the steel sheet surface layer caused by nitriding, oxidation, or oxidation of the steel sheet surface, as with Sn. By suppressing such nitriding and oxidation, the amount of martensite generated in the steel sheet surface layer is prevented from decreasing, and the decrease in hardenability due to the decrease in B is prevented, resulting in fatigue properties and aging resistance. Improve. Moreover, the wettability of hot dip galvanization can be improved and plating external appearance quality can be improved. From the viewpoint of suppressing nitriding and oxidation, it is preferable to add 0.002% or more of Sb, and more preferably 0.005% or more, but if it exceeds 0.2%, Sb will be 0.2% or less because YP increases and toughness deteriorates. It is desirable to contain.

Cu: 0.5% or less
Since Cu slightly improves the chipping resistance, it is desirable to add it from the viewpoint of improving the chipping resistance. Moreover, it is an element mixed when scrap is used as a raw material. By allowing Cu to be mixed, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From the viewpoint of improving chipping resistance, Cu is preferably added in an amount of 0.01% or more, more preferably 0.03% or more. However, if the content is too large, it causes surface defects, so Cu is preferably 0.5% or less.

Ni: 0.5% or less
Ni is also an element that has the effect of improving chipping resistance. Ni also has the effect of reducing surface defects that are likely to occur when Cu is contained. Therefore, from the viewpoint of improving the surface quality while improving the corrosion resistance, Ni is preferably added in an amount of 0.01% or more, and more preferably 0.02% or more. However, if the amount of Ni added is too large, scale generation in the heating furnace becomes non-uniform, causing surface defects and a significant cost increase. Therefore, Ni is 0.5% or less.

Ca: 0.01% or less
Ca fixes S in the steel as CaS, and further increases the pH in the corrosion product and improves chipping resistance. Moreover, the production | generation of MnS which reduces stretch flange formability by the production | generation of CaS is suppressed, and there exists an effect | action which improves stretch flange formability. From such a viewpoint, it is desirable to add 0.0005% or more of Ca. However, Ca easily floats and separates as an oxide in molten steel, and it is difficult to leave a large amount in Ca. Therefore, the Ca content is 0.01% or less.

Ce: 0.01% or less
Ce can also be added for the purpose of fixing S in steel and improving stretch flangeability and chipping resistance. Ce is preferably added in an amount of 0.0005% or more from the above viewpoint. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add Ce at 0.01% or less.

La: 0.01% or less
La can also be added for the purpose of fixing S in steel and improving stretch flangeability and chipping resistance. From the above viewpoint, La is preferably added in an amount of 0.0005% or more. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add La at 0.01% or less.

Mg: 0.01% or less
Mg can be added from the viewpoint of finely dispersing the oxide and homogenizing the structure. From the above viewpoint, Mg is preferably added in an amount of 0.0005% or more. However, since the surface quality deteriorates if the content is large, it is desirable to add Mg at 0.01% or less.

2) Structure The steel sheet structure of the present invention mainly contains ferrite, martensite, residual γ, pearlite, bainite, and a small amount of carbide. First, a method for measuring these tissue forms will be described.

  The volume ratio of the second phase was determined by corroding the L cross-section of the steel sheet (vertical cross-section parallel to the rolling direction) with nital, observing 8 fields of view at a magnification of 3000 times with a SEM at the 1/4 thickness position of the steel sheet. The obtained tissue photograph was image-analyzed to determine the area ratio of the second phase. That is, the steel sheet of the present invention has a small difference in structure in the rolling direction and the direction perpendicular to the rolling direction, and the area ratio of the second phase measured in both directions showed almost the same value. The measured area ratio of the second phase was defined as the volume ratio of the second phase.

  In the tissue photograph, the area with a slightly black contrast was made ferrite. In the case where carbides are generated in a lamellar shape or in a sequence of dots, the area ratio was obtained by setting this region as pearlite or bainite and particles with white contrast as martensite or residual γ. However, based on the observation results by TEM, particles with a diameter of 0.90 μm or less dispersed in ferrite grains among white contrast particles are judged as carbide particles described later, and are excluded from the volume ratio of martensite or residual γ. did. The volume ratio of the second phase was the total amount of these structures, and the volume ratio of martensite and residual γ was the total amount of the area ratio of the white contrast region. Note that it is difficult to distinguish martensite and residual γ in the SEM photograph. Here, the structure is defined by the total area ratio of both phases, but from the analysis result by X-ray, the volume ratio of martensite and residual γ is determined. It has been confirmed that martensite is about 60% and residual γ is about 40%.

  In the case of passing through the aging zone of CAL in continuous annealing, after martensite is generated at about 350 ° C. or less, the martensite generated is slightly tempered because it is held for a long time in that temperature range. There is a case. This slightly tempered martensite was treated here as martensite. In addition, the discrimination between tempered martensite and bainite is as follows. That is, since the carbide in the tempered martensite is very fine compared to the carbide dispersed in the bainite, the average particle size of the carbide dispersed in each martensite grain and bainite grain is measured. These can be identified. Here, tempered martensite was used when the average particle size of the carbides in the grains was 0.15 μm or less, and bainite was used when it exceeded 0.15 μm.

From the TEM observation results, spherical or elliptical particles with a diameter of around 0.5 μm dispersed in ferrite grains are Fe, Mn, C, and B series carbides, and these precipitates cause material variations in B-added steel. It has become clear that this is the cause of the failure, so on the SEM photograph, particles with an aspect ratio of 3.0 or less and an average particle diameter of 0.25 to 0.90 μm distributed in the ferrite grains are Fe, Mn, C The number of B-based carbides was measured. In the case of elliptical particles on the SEM photograph, the major axis a and the uniaxial axis b perpendicular to the major axis were measured, and (a × b) ^0.5 was determined as the corresponding particle diameter.

Volume ratio of the second phase: 2.0-12.0%
In order to obtain a low YP, the volume fraction of the second phase needs to be 2.0% or more. However, if the volume fraction of the second phase exceeds 12.0%, YP increases and EL and BH deteriorate. Therefore, the volume ratio of the second phase is in the range of 2.0 to 12.0%. In order to obtain even lower YP and higher BH, the volume fraction of the second phase is preferably 10.0% or less, more preferably 8.0% or less, and even more preferably 6.0% or less.

Ratio of volume ratio of martensite and residual γ to volume ratio in the second phase: 60% or more In order to sufficiently suppress pearlite and bainite in the second phase and secure low YP and high El at the same time, The ratio of the volume ratio of martensite and residual γ to the volume ratio in the phase needs to be 60% or more.

The number of carbide particles present in the ferrite grains with an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm: 10000 / mm ² or less The steel composition and the coiling temperature, annealing temperature, and holding time in hot rolling are appropriate. In the steel sheet that has not been made, material fluctuations occur significantly in the coil width direction and longitudinal direction. In such a portion, spherical or elliptical carbides having an aspect ratio of about 3.0 or less and a diameter of 0.25 μm or more and 0.90 μm or less are dispersed in the ferrite grains in excess of 10,000 / mm ² . By reducing this carbide to 10,000 pieces / mm ² or less, material fluctuations in the coil are almost eliminated. Accordingly, the number of carbide particles present in the ferrite grains and having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm is set to 10,000 / mm ² or less. In the steel of the present invention, the average ferrite particle size and the average diameter of the second phase are not specified, but the average ferrite particle size is in the range of 7 to 12 μm, and the average diameter of the second phase is in the range of 0.8 to 1.3 μm. It was.

  Such a structural form optimizes the composition range of Mn, Cr, P, B, sol.Al, Ti, N, etc., and also optimizes the annealing temperature and holding time in CT, CAL and CGL during hot rolling. Can be obtained.

3) Manufacturing conditions As described above, the steel sheet of the present invention has a coiling temperature CT of B, sol.Al, Ti, N in the step of hot rolling the steel slab having the component composition limited as described above. Is controlled to an appropriate range according to the content of the steel, and after cold rolling at a cold rolling rate of 50 to 85%, in a continuous hot dip galvanizing line (CGL) or a continuous annealing line (CAL), 740 ° C. or more and 830 ° C. It can be manufactured by a method of annealing by holding for 25 seconds or more at an annealing temperature of ℃ or less.

Hot rolling:
In order to hot-roll steel slabs, a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating, a method of rolling the slab after continuous casting by performing a short heat treatment, etc. You can do it. In the hot rolling, for example, the slab heating temperature may be 1100 to 1300 ° C., and the finish rolling temperature may be Ar ₃ transformation point to Ar ₃ transformation point + 150 ° C. From the viewpoint of reducing the r value in-plane anisotropy and improving the BH, the average cooling rate after hot rolling is preferably 20 ° C./sec or more.

  In order to reduce material fluctuation in the coil in B-added steel containing 0.0010% or more of B, it is necessary to control the coiling temperature to an appropriate range according to the contents of B, sol.Al, Ti, N . The results of investigations on the relationship between each element and appropriate CT are described below.

  Five types of steels with different amounts of B, sol.Al, Ti and N shown in Table 1 were produced. The obtained slab was hot-rolled to obtain a 3.2 mm hot-rolled coil. At this time, the slab heating temperature was 1220 ° C., and the finish rolling temperature was 850 ° C. Immediately after rolling, it was rapidly cooled to 690 ° C., and then laminar cooled on a run-out table, and wound in the range of 500 to 675 ° C. The obtained hot-rolled sheet was pickled, cold-rolled to a thickness of 0.70 mm, and then annealed at 780 ° C. × 40 sec with CGL. It was immersed in a galvanizing bath in the middle of cooling, plated, then alloyed, then cooled to room temperature and subjected to temper rolling with an elongation of 0.4%. A JIS No. 5 tensile specimen was taken from the obtained coil parallel to the rolling direction, and the mechanical properties in the coil width direction were investigated. Moreover, the metal structure was investigated by the method described above.

Fig. 1 shows the results of investigating the presence or absence of TS variation in the coil width direction of each component steel with various changes in CT. Here, sol.B is a value calculated by the equation (A), and is a value obtained by estimating the amount of B in a solid solution state in the hot-rolled sheet.
sol.B = [% B]-{[% N] / 14-[% Ti] /48×0.8-[% Al] /27×0.0005× (CT-560)} × 10.8 ... (A)
[% B], [% N], [% Ti], and [% Al] represent the respective contents of B, N, Ti, and sol.Al, and CT represents the coiling temperature (° C.). When CT-560 ≦ 0, CT-560 is 0 (zero), and when sol.B ≦ 0, sol.B is 0 (zero).

  In other words, the amount of solute B is expected to occur when B is added in excess of the amount of N, but when Ti or Al is added, the amount of precipitation of these in relation to the amount of N Need to be considered. When Ti is heated, 80% of the added amount is precipitated as TiN, and the remainder is precipitated as TiC. Al precipitates when CT exceeds 560 ° C, and the amount of precipitation increases as CT increases. In consideration of such behavior, the amount of remaining N obtained by subtracting the amount of N deposited as TiN or AlN from the amount of N contained is obtained, and the amount of B added is subtracted from the amount of remaining N (A) It is a formula.

  FIG. 1 is a plot of the relationship between sol.B, CT obtained from equation (A) and the presence or absence of material fluctuation in the coil width direction. The TS in the width direction was evaluated by collecting a JIS No. 5 tensile test piece in parallel with the rolling direction and conducting a tensile test. The sampling position of the test piece in the coil longitudinal direction is the center position of the coil length, and the sampling position in the width direction is started from the position where the center line of the test piece is 18 mm inside the edge in the width direction. The samples were collected at intervals of 30 mm to 600 mm in the width direction so that the change in strength could be sufficiently confirmed. A coil having a difference between the maximum value and the minimum value of TS in the width direction thus obtained of less than 20 MPa is indicated by ◯, and a coil having a difference of 20 MPa or more is indicated by ●. In the coil under the conditions indicated by ●, material fluctuations in the coil width direction and the longitudinal direction are conspicuous. In the figure, the boundary line obtained by the relational expression between CT and sol.B represented by the equation (1) is also shown.

CT (° C.) ≦ 670-50000 × sol.B Equation (1) However, when sol.B ≦ 0, sol.B is calculated as 0 (zero).

  From FIG. 1, it can be seen that there is a CT range in which TS is stable according to sol.B estimated from equation (A), and the appropriate range is lower in temperature as sol.B increases. The boundary is given by the equation (1), and it can be seen that the material fluctuation is suppressed even in the B-added steel by winding the various component steels at a temperature lower than the CT of this equation. As described above, CT is limited to the range represented by the equation (1).

Fig. 2 shows the change in TS in the width direction when CT is set to 620 ° C and 530 ° C for steel 1. It can be seen that even if the steel has the same steel composition, significant material fluctuations occur if CT is not optimized. Here, in the coil in which the strength fluctuation of 20 MPa or more occurred in the width direction, carbide particles having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm are recognized in excess of 10,000 particles / mm ² .

  In order to obtain a beautiful plating surface quality for the outer plate, the slab heating temperature should be 1250 ° C or less, and sufficient descaling should be performed to remove the primary and secondary scales generated on the steel plate surface. It is desirable that the temperature is 1080 ° C. or lower and the finish rolling temperature is 900 ° C. or lower. For example, in the conventional steel with a Cr content of 0.40% or more, the primary scale during slab heating tends to remain after rolling, which has been a factor that deteriorates the appearance quality after annealing with CAL and CGL. By reducing it to less than 0.35%, further reducing the slab heating temperature to 1250 ° C or lower, sufficiently performing descaling with high-pressure spray, and controlling the rough rolling end temperature to 1080 ° C or lower and the finish rolling temperature to 900 ° C or lower. The beautiful appearance quality required for automobile outer panel can be obtained.

Cold rolling:
In cold rolling, the rolling rate may be 50 to 85%. From the viewpoint of improving the r value to improve deep drawability, the rolling rate is preferably 65 to 73%, and from the viewpoint of reducing the r value and the in-plane anisotropy of YP, the rolling rate is 70 to 73%. 85% is preferable.

Annealing:
The steel sheet after cold rolling is annealed with CGL or CAL and, if necessary, hot dip galvanized or, after hot dip galvanized, further alloyed. The annealing temperature is 740 to 830 ° C. If it is less than 740 ° C., the solid solution of the carbide becomes insufficient, and the volume fraction of the second phase cannot be secured stably. If it exceeds 830 ° C, pearlite and bainite are easily generated, and a sufficiently low YP cannot be obtained. From the viewpoint of dissolving the carbide, the holding time during soaking is preferably 25 seconds or more, more preferably 40 seconds or more, and from the viewpoint of ensuring productivity, it is preferably 300 seconds or less.

  After soaking, the temperature range from the annealing temperature to 480 ° C. may be cooled at a cooling rate of 2 to 200 ° C./sec, and 3 to 50 ° C./sec is preferable from the viewpoint of low YP.

  Thereafter, in CGL, galvanization is performed by dipping in a galvanizing bath, and if necessary, alloying treatment can also be performed by maintaining within a temperature range of 470 to 650 ° C. within 40 seconds. When galvanizing or alloying is performed, cooling to a temperature range up to 100 ° C at an average cooling rate of 5 to 200 ° C / sec after alloying suppresses the formation of bainite and reduces YP. It is preferable from the viewpoint of.

  In CAL, cooling is performed at an average cooling rate of 2 to 200 ° C / sec from 480 ° C to room temperature, or in the case of a furnace with an overaging zone, cooling is performed at an average cooling rate of 5 to 200 ° C / sec. Then, it may be cooled to 100 ° C. or lower at an average cooling rate of 0.1 to 200 ° C./sec.

  The obtained galvanized steel sheet or cold-rolled steel sheet can be subjected to skin pass rolling from the viewpoint of stabilizing the press formability such as adjusting the surface roughness and flattening the plate shape. In that case, the skin pass elongation rate is preferably 0.1 to 0.6% from the viewpoint of low YP and high El.

  Steels of steel numbers A to U shown in Table 2 were melted and then continuously cast into a 230 mm thick slab.

  The slab was heated to 1180-1250 ° C. and then hot-rolled at a finish rolling temperature in the range of 820-900 ° C. Then, it cooled at the average cooling rate of 15-35 degreeC / sec, and wound up in the temperature range of 450-670 degreeC. The obtained hot-rolled sheet was cold-rolled at a rolling rate of 70 to 77% to obtain a cold-rolled sheet having a thickness of 0.8 mm.

As shown in Table 3, the obtained cold-rolled sheet was annealed at an annealing temperature AT in CGL or CAL. At this time, annealing was performed so that the holding time in the temperature range of 740 ° C. or higher was 15 to 150 seconds, and the average cooling rate from the annealing temperature AT to 480 ° C. was cooled at 10 ° C./sec. After that, CGL is immersed in hot dip galvanizing bath and galvanized, and after alloying treatment or after galvanizing, after galvanization, average cooling from plating bath temperature to 100 ° C It cooled to 100 degrees C or less so that it might become a speed | rate 25 degrees C / sec. Zinc plating is performed at a bath temperature of 460 ° C and Al in the bath: 0.13%. Alloying is performed after immersion in the plating bath and heated to 480-540 ° C at an average heating rate of 15 ° C / sec. The amount was kept for 10 to 25 seconds so that the amount was in the range of 9.5 to 11.5%. The amount of plating adhered was 45 g / m ² per side and adhered on both sides. In CAL, cooling is performed so that the temperature range from 480 ° C to 370 ° C has an average cooling rate of 10 ° C / sec, then cooling to 100 ° C at an average cooling rate of 1 ° C / sec in the overaging zone, and further to room temperature Cooling was performed at an average cooling rate of 10 ° C / sec. The obtained hot-dip galvanized steel sheet and cold-rolled steel sheet were subjected to temper rolling with an elongation of 0.4%, and samples were collected.

  About the obtained sample, the volume ratio of the second phase by the method described above, the ratio of the volume ratio of martensite and residual γ in the second phase (ratio of martensite and residual γ in the second phase), The number of carbide particles present in the ferrite grains and having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm (in-grain carbide density) was investigated. Moreover, the type of steel structure was separated by SEM observation. Furthermore, JIS No. 5 test specimens were taken from the direction perpendicular to the rolling direction and subjected to a tensile test (based on JIS Z2241). YP (yield strength), TS (tensile strength), YR (yield ratio), El (total elongation) ) Was evaluated. In addition, a JIS No. 5 tensile specimen was taken in the width direction of the coil in parallel with the rolling direction, and the amount of change in TS (ΔTS) in the width direction was investigated.

  Further, BH, which is the amount of increase in YP after 20 minutes of heat treatment at 170 ° C. with respect to the stress when 2% pre-strain was applied to the same test piece (JIS No. 5 tensile test piece) as described above, was obtained.

Furthermore, the chipping resistance of each steel plate was evaluated. That is, after chemical conversion treatment and electrodeposition coating were performed on the obtained steel plate, 500 g of crushed stone of JIS-A5001 S-13 (No. 6) was sprayed on the steel plate under the condition of an injection pressure of 490 kPa (5 kgf / cm ² ). Corrosion test was conducted in JASO-CCT corrosion cycle test. The electrodeposition coating film thickness was 20 μm. For hot-dip galvanized steel sheets (manufactured by CGL), corrosion samples are removed after 90 cycles.For cold-rolled steel sheets (manufactured by CAL), corrosion products are removed from the corroded samples after 30 cycles. The maximum value of the reduction of the plate thickness from the original plate thickness was determined and used as the maximum corrosion depth.

  The results are shown in Table 3.

  The steel sheet of the present invention has a significantly reduced corrosion weight loss compared to conventional steels whose Cr content is not optimized, and has a low B content, a low Mn equivalent steel, and a steel with a large amount of Mn added. Compared with steel with Mo added, steel with the same TS level has low YP, that is, low YR and high El and high BH.

  That is, the conventional steels O and P to which a large amount of Cr is added have a significantly large corrosion weight loss (maximum corrosion depth) of 0.59 to 0.75 mm. Such steel is difficult to use as an outer panel because the drilling life of actual parts is reduced by 30-50%. On the other hand, the maximum corrosion weight loss of the steel of the present invention is 0.29 to 0.38 mm, which is greatly reduced. Although not shown in the table, corrosion resistance of the conventional 340BH was also evaluated, and the corrosion weight loss was 0.36 mm. The chemical composition of this steel (conventional 340BH) is mass%, C: 0.002%, Si: 0.01%, Mn: 0.4%, P: 0.05%, S: 0.008%, Cr: 0.04%, sol. Al: 0.06%, Nb: 0.01%, N: 0.0018%, B: 0.0008%. Thus, it can be seen that the steel of the present invention has almost the same chipping resistance as that of the conventional steel. Among them, the component steel with a Cr content of less than 0.30% and the steel with a composite addition of Ce, Ca, La, Cu, and Ni have even better chipping resistance.

  Even in steels with reduced Cr in this way, steel with controlled Mn equivalent, Mn, B addition, coiling temperature, annealing temperature, and soaking (holding) time suppresses the formation of pearlite and bainite. At the same time, the generation of carbides in the ferrite grains is reduced, and the material variation in the coil is suppressed. In other words, among steels A to L, CT is a condition equal to or less than the value of equation (1), and the annealing temperature and the soaking time are within a predetermined range are lower than the comparative steel of the same TS level. Has YP, high BH, high El, small ΔTS.

  According to the present invention, a high-strength steel sheet having excellent chipping resistance, low YP, high El and BH, and small material fluctuation in the coil can be manufactured at low cost. The high-strength steel sheet according to the present invention has excellent chipping resistance, excellent surface strain resistance, excellent stretch formability, and excellent material stability, enabling high strength and thinning of automotive parts. To do.

Claims (4)

As a component composition of steel, in mass%, C: more than 0.015% and less than 0.100%, Si: less than 0.50%, Mn: more than 1.0% and less than 2.0%, P: 0.05% or less, S: 0.03% or less, sol.Al: Contains 0.01% or more and 0.3% or less, N: 0.005% or less, Cr: less than 0.35%, B: 0.0010% or more and 0.0050% or less, Mo: less than 0.15%, Ti: less than 0.030%, and further 2.1 ≦ [Mneq] ≦ 3.1, balance iron and inevitable impurities, the structure of the steel has ferrite and second phase, the volume ratio of the second phase is 2.0-12.0%, the volume of martensite and residual γ in the second phase A high-strength steel sheet characterized by having a rate ratio of 60% or more, existing in ferrite grains, having an aspect ratio of 3.0 or less, and a number of carbide particles having a diameter of 0.25 to 0.90 μm of 10,000 particles / mm ² or less .
Where [Mneq] = [% Mn] +1.3 [% Cr] +3.3 [% Mo] +8 [% P] + 150B ^* , B ^* = [% B] + [% Ti] /48×10.8× 0.9 + [% Al] /27×10.8×0.025, [% Mn], [% Cr], [% Mo], [% P], [% B], [% Ti], [% Al] Represents the respective contents of Mn, Cr, Mo, P, B, Ti, sol.Al. When B ^* ≧ 0.0022, B ^* = 0.0022. The high content according to claim 1, further comprising at least one of Nb: less than 0.030%, V: 0.2% or less, W: 0.15% or less, Zr: 0.1% or less in mass%. Strength steel plate. Further, in mass%, Sn: 0.2% or less, Sb: 0.2% or less, Cu: 0.5% or less, Ni: 0.5% or less, Ca: 0.01% or less, Ce: 0.01% or less, La: 0.01% or less, Mg: The high-strength steel sheet according to claim 1 or 2, which contains at least one of 0.01% or less. In the step of hot rolling the steel slab having the component composition according to any one of claims 1 to 3, the coiling temperature CT is controlled within the range shown in the formula (1) according to sol. After cold rolling at a cold rolling rate of 85%, in a continuous hot-dip galvanizing line (CGL) or continuous annealing line (CAL), hold at an annealing temperature of 740 ° C or higher and 830 ° C or lower for 25 seconds or more and anneal. Characteristic steel structure has ferrite and second phase, volume ratio of second phase is 2.0-12.0%, ratio of volume ratio of martensite and residual γ in second phase is 60% or more, ferrite A method for producing a high-strength steel sheet, wherein the number of carbide particles present in the grains and having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm is 10,000 particles / mm ² or less .
CT (° C) ≦ 670-50000 × sol.B (1)
sol.B = [% B]-{[% N] / 14-[% Ti] /48×0.8-[% Al] /27×0.0005× (CT-560)} × 10.8 ・ ・ ・ (A) formula
In the formula (A), [% B], [% N], [% Ti], [% Al] represents the respective contents of B, N, Ti, sol.Al, and CT represents the coiling temperature ( ° C). When CT-560 ≦ 0, CT-560 shall be 0 (zero).
However, when sol.B ≦ 0, sol.B is calculated as 0 (zero).

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