TWI464279B - High strength steel sheet and method for manufacturing the same - Google Patents

Description

High-strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel sheet for press forming which is used after being subjected to a press forming process in products such as automobiles and home appliances, and a method for producing the same.

In the past, for automobile exterior panels such as hoods, doors, trunk lids, tailgates, and fenders that required excellent dent resistance, most of them used BH steel sheets with a TS of 340 MPa (bake-hardened steel sheets). , hereinafter referred to as 340BH). 340BH is a very low carbon steel containing less than 0.01% by mass of C, and the amount of solid solution C is controlled by adding a carbonitride forming element such as Nb or Ti, and solid solution strengthening is performed by Si, Mn, and P. Fertilizer iron single phase steel. In recent years, as the demand for lightweight body has become higher, the 340BH exterior panels that were originally used are more targeted to: high strength to reduce the thickness of the steel plate, or even the same plate thickness. Reducing R/F (reinforcing parts on the inside) and reviewing all aspects of the baking process at low temperatures and in a short period of time.

However, when a large amount of Si, Mn, and P are added to the conventional 340BH to increase the strength, the deformation resistance of the press-formed product is remarkably deteriorated due to an increase in YP. The term "face deformation" as used herein refers to a pattern in which fine wrinkles and corrugations on the press-molded surface are easily generated on the outer periphery of the door handle portion of the door. "Face deformation" will obviously damage the appearance quality of the car. Therefore, it is suitable for the steel plate as an exterior panel. On the one hand, it is necessary to increase the strength of the press-formed product, and on the other hand, it is required to keep the stress under the stamping before the stamping. The lower YP of the existing 340BH. Similarly, if the TS of the material changes, the amount of rebound of the press-formed product also changes, which causes the surface deformation. Therefore, in order to reduce the surface deformation, it is necessary to suppress not only the YS to be low but also the variation of the TS in the steel strip roll.

Further, the steel sheet used for such use also needs to have excellent formability, and it is required to obtain stable high ductility (El) in the steel strip roll.

In addition, such a steel sheet used for an outer panel of an automobile body needs to have excellent corrosion resistance. That is, in the case where a car is hit by a stone and a dent wound is formed on the steel sheet, this portion is easily rusted and becomes a cause of forming a hole. In order to suppress such corrosion, the impact crack resistance required for the corrosion must be equal to or superior to the conventional 340BH.

Based on this technical background, for example, the method disclosed in Patent Document 1 has a cooling rate after annealing in which the composition contains C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, and Cr: 0.023 to 0.8%. Optimized, mainly to form a composite structure composed of ferrite iron and granulated iron, thereby producing alloyed galvanized with low drop stress (YP) and high ductility (El). Steel plate.

The method disclosed in Patent Document 2 is prepared by containing C: 0.005 to 0.04%, Mn: 1.0 to 2.0%, P: 0.10% or less, S: 0.03% or less, and Al: 0.01 to 0.1 by mass%. %, N: less than 0.008%, Cr: 0.2 to 1.0%, and Mn+1.29Cr is 2.1 to 2.8, which is formed by fertile iron and 3.0% or more and less than 10.0% of the volume of Ma Tian iron. Organize to obtain a steel sheet having excellent ductility and high BH.

Further, the method disclosed in Patent Document 3 contains C: 0.02 to 0.033%, Mn: 1.5 to 2.5%, Cr: 0.03 to 0.5%, and Mo: 0 to 0.5% by mass%. The total amount of Mn, Cr, and Mo is controlled in the range of 1.8 to 2.5% to obtain a steel sheet having YP of 300 MPa or less and excellent ductility (El) and flange stretch formability (hole expansion ratio λ).

The technique disclosed in Patent Document 4 contains C in a mass ratio of C: 0.03 to 0.09%, Mn: 1.0 to 2.0%, sol. Al: 0.005 to 0.1%, and B: 0.001 to 0.003%. The range of the composition of the component with Mn is controlled to be C + Mn / 20 ≧ 0.12%, whereby a high-strength cold-rolled steel sheet having excellent sinter hardenability and room temperature aging resistance is obtained.

Further, the method disclosed in Patent Document 5 is a composition range of N and B in a soft cold-rolled steel sheet containing C ≦ 0.05%, Mn ≦ 0.5%, N ≦ 0.005%, and B ≦ 0.005% by weight %. Controlled at N-14/11B≦10 (ppm) to reduce the amount of fine AlN deposited to reduce material variations in the coil.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Publication No. Sho 57-57945

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2007-211338

[Patent Document 3] Japanese Patent No. 3613129

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2009-174019

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2001-73074

However, the steel sheets described in the above Patent Documents 1 to 4 are a large amount of Mn and Cr added in comparison with the conventional 340BH, and therefore belong to the following: in the ferrite iron structure, the amount is mainly dispersed by the granulated iron. The 2-phase composite tissue steel plate has the following problems.

First, most of the steel sheets described in Patent Documents 1 to 3 are significantly inferior to impact rupture resistance when compared with the conventional 340 MPa steel (340BH). For example, a composite door plate containing 0.6% of Cr was used to make a part of a simulated door, and a broken stone was sprayed onto the surface of the steel plate, and as a result of evaluation of corrosion resistance, the maximum corrosion depth was conventional. The 340MPa steel (340BH) is approximately twice as large. In other words, although such a steel sheet has excellent press formability, the life of the etch hole is reduced to about half as compared with the conventional steel material, and thus it is difficult to apply it to an actual vehicle.

Further, some of the steels having the composition components described in Patent Documents 1 to 4 are considered to be steel sheets having a low BH and a low El or high YS even in a composite structure. In other words, the material must be further improved.

On the other hand, among the steels described in Patent Documents 1 to 4, some of them are steels to which B is added, and it is possible to reduce expensive elements such as Mo and Cr, and also excellent in impact crack resistance and chemical conversion treatability. It has also been shown to have lower YS and higher BH, but it has been found to have a problem of large material variations. For example, a steel strip containing C: 0.025%, Mn: 1.8%, Cr: 0.2%, P: 0.02%, sol. Al: 0.06%, B: 0.0025%, and N: 0.002% in mass% is In the hot rolling step, the steel was taken up at a temperature of 640 ° C, followed by cold rolling, and the steel was annealed at 770 ° C for 40 seconds by a continuous hot-dip galvanizing line (CGL). In the tape roll, the TS of the most edge portion in the width direction of the steel tape roll has a degree of 460 MPa, but the TS at the center portion in the width direction of the steel tape roll is reduced to only 430 MPa. In other words, the variation of the TS in the width direction of the steel strip roll is about 30 MPa. Similarly, in the longitudinal direction of the steel strip roll, a change in TS of about 30 MPa was also produced. Moreover, this steel strip roll, in response to the change of TS, El also causes a 3% change in the steel strip roll, and the stability of El in the steel strip roll is remarkably deteriorated, and the stability of press formability is not high. it is good. In addition, the TS and El of this steel coil have a large dependence on the annealing temperature. This phenomenon occurs in steels in which the amount of B added is 0.001% or more based on the purpose of lifting the material.

Further, although the technique of reducing the material variation described in Patent Document 5 has been attempted to be applied to the composite structural steel sheet to which B is added, the variation in the material has not been improved.

Therefore, it is difficult to obtain a steel sheet having excellent impact crack resistance and having low YP, high BH, high El, and small material variation according to the method disclosed in the above patent documents.

The present invention has been developed in order to solve such a problem, and is aimed at providing excellent impact crack resistance, low YP, high BH, high El at a low price, and reducing material variation in a steel strip roll. High-strength steel sheet and its manufacturing method.

The present inventors have taken a conventional composite structure steel sheet having a low drop strength as a target, and it is possible to simultaneously improve the impact crack resistance while achieving low YP, high BH, high El, and reduced material variation. After reviewing the techniques, the following conclusions were obtained.

(I) Impact crack resistance can be greatly improved by controlling the Cr content to less than 0.35% and reducing the P content to 0.05% or less. In this way, characteristics equivalent to or higher than those of the conventional 340 MPa steel (340BH) can be obtained.

(II) In order to obtain low YP, high BH, and high El characteristics, first, a structure having a second phase with a ferrite iron and a small volume ratio is formed, and the wave iron and the toughness in the second phase are suppressed. Iron, the practice of increasing the ratio of granulated iron and residual gamma is very important. To this end, it is necessary to contain a predetermined amount of hardenability enhancing elements, that is, Mn, Mo, Cr, P, B, and the like. Moreover, if it is desired to obtain a lower YP and a higher El, and at the same time, the BH is also raised, the ferrite iron particles and the second phase must be uniformly coarsened, and a predetermined amount of solid solution C remains. It is possible to promote the use of Cr, P, and B by actively reducing the Mn and Mo among the elements by reducing the hardenability. However, if it is also desired to have impact crack resistance at the same time, it is necessary to avoid excessive addition of Cr and P. Among the above elements, it is preferable to use the content of B to the maximum extent.

(III) In the steel in which B is used, although the content of B is increased to 10 ppm or more, the material can be improved, but the reverse side causes the material variation to become apparent. This material change is caused by the fact that B after solid solution remains in the hot-rolled steel sheet to cause hard-melting carbides, and by suppressing the formation of such carbides, the variation of the material can be reduced. Further, such a carbide system can be reduced in accordance with the method of selecting an appropriate coiling temperature in accordance with the amount of N, the amount of Ti, the amount of sol. Al, and the amount of B contained in the steel.

That is, in the steel containing B amount of 10 ppm or more for the purpose of low YP and high BH, there is a very small amount of solid solution B which does not bond with N when coiling in the hot rolling step. Alternatively, in a steel containing a certain amount of sol. Al, during the slow cooling of the coil of steel after coiling, AlN precipitates to form a solid solution B. When the solid solution B is produced in this manner, stable carbides are formed together with Fe, Mn, and C in the steel during the cooling of the steel coil to consume C. Moreover, the amount of formation of such a carbide is significantly changed depending on the hot rolling conditions such as the coiling temperature. This kind of carbide is more stable than ferritic carbon iron (Fe ₃ C), so it will remain as a precipitate at the end of annealing, and the amount of such carbides generated in the steel coil is large. The amount of granulated iron produced in the field will be significantly reduced. As a result, it was found that in the steel to which the composite structure of B was added, the material variation in the steel strip roll tends to be remarkable. This phenomenon is not a phenomenon in the conventional soft cold-rolled steel sheet which does not contain the granulated iron as a reinforcing structure.

Therefore, in order to avoid such a phenomenon, the amount of solid-flux B in the hot-rolled steel sheet is reduced, or even if a very small amount of solid-solution B is generated, it is sufficient to suppress the formation of stable carbide. Thus, a concept has been discovered which is directed to the amount of solid solution B generated in response to the contents of N, Ti, sol. Al, and B, as long as the coiling temperature is properly controlled.

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

[1] A high strength steel sheet characterized by:

The composition of steel consists of: % by 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%, and Ti: less than 0.030%. And it conforms to 2.1≦[Mneq]≦3.1, and the rest is composed of iron and unavoidable impurities. The structure of the steel has ferrite iron and the second phase, and the volume ratio of the second phase is 2.0 to 12.0%. The ratio of the volume ratio of the granulated iron and the residual γ in the two phases is 60% or more, and the number of the carbide particles having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm in the particles of the ferrite iron is present. It is 10000/mm ² or less.

Here,

[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] are Mn, Cr, Mo, P, B, Ti, sol, respectively. The content of .Al, when B ^* ≧ 0.0022, is taken as B ^* = 0.0022.

[2] The high-strength steel sheet according to the above [1], further comprising, by mass%, Nb: less than 0.030%, V: 0.2% or less, W: 0.15% or less, and Zr: 0.1% or less. At least one of them.

[3] The high-strength steel sheet according to the above [1] or [2], further comprising: Sn: 0.2% or less, Sb: 0.2% or less, Cu: 0.5% or less, Ni: 0.5 by mass% % or less, Ca: 0.01% or less, Ce: 0.01% or less, La: 0.01% or less, and Mg: 0.01% or less.

[4] A method for producing a high-strength steel sheet, characterized in that the steel preform having the composition component according to any one of the above [1] to [3] is subjected to hot rolling, and the coiling temperature is set The CT is controlled in the range shown by the formula (1) in response to sol. B, and is subjected to cold rolling at a cold rolling ratio of 50 to 85%, followed by continuous hot-dip galvanizing line (CGL) or continuous annealing line (CAL). The annealing treatment is performed at an annealing temperature of 740 ° C. or higher and 830 ° C or lower for 25 seconds or longer;

CT(°C)≦670-50000×sol.B ‧‧‧(1)

Here,

sol.B=[%B]-{[%N]/14-[%Ti]/48×0.8-[%Al]/27×0.0005×(CT-560)}×10.8‧‧‧(A)

In the formula (A), [%B], [%N], [%Ti], [%Al] represent the contents of B, N, Ti, and sol.Al, respectively, and CT is the coiling temperature (°C). When CT-560≦0, take CT-560=0; however, when sol.B≦0, sol.B is treated as 0.

According to the present invention, a high-strength steel sheet having excellent impact crack resistance, low YP, high El, and high BH, and having little variation in material in the steel coil can be produced at low cost. The high-strength steel sheet of the present invention has excellent corrosion resistance, excellent surface deformation resistance, excellent protrusion formability, excellent dent resistance, and excellent material stability, so that automotive parts can be achieved. High strength and thinness.

Hereinafter, the present invention will be described in detail. Further, the % used to indicate the amount of the component, if not specifically limited, means: mass%.

1) Composition of steel Cr: less than 0.35%

The Cr system has a function of improving the quenching property to generate a predetermined amount of granulated iron, and by reducing the ferrite iron particles to make the granulated iron uniformly dispersed, the YP can be lowered and the El can be improved. The material considerations are elements that are added as appropriate, but the impact crack resistance is significantly deteriorated. Therefore, in order to ensure good impact crack resistance, the content of Cr must be less than 0.35%. Further, in order to impart excellent impact crack resistance to the steel sheet, the Cr content is preferably less than 0.30%. From the viewpoint of optimizing [Mneq] shown below, Cr is an element which can be optionally added, and although the lower limit value (including Cr is 0%) is not specified, based on the viewpoint of low YP, The amount of Cr added is preferably 0.02% or more, and more preferably 0.05% or more.

[Mneq]: 2.1 or more and 3.1 or less

In order to ensure low YP, high El, high BH, and excellent aging resistance, at least the steel structure must be a composite structure having: ferrite iron, and a second phase having a main iron and a residual γ. In the conventional steel, many steel sheets with higher YP and steel sheets with lower El are found. As a result of investigation, it is found that in the second phase of these steel sheets, in addition to the granulated iron and a small amount of residual In addition to γ, Borne and toughened iron are also produced.

This kind of wave iron and toughened iron are very fine, only about 1 to 2 μm, and are formed next to the granulated iron. Therefore, it is difficult to observe the sample after etching with a light microscope, for example, using a nitric acid or a colored etching solution. It is determined to be a tissue other than the granulated iron, and it is necessary to accurately recognize it as a tissue other than the granulated iron. It must be observed at a magnification of 3000 times or more using a scanning electron microscope (SEM). For example, the results of examining the steel structure of the conventional 0.03% C-1.5% Mn-0.5% Cr in detail are observed by an optical microscope or by a scanning electron microscope (SEM) using a magnification of 1000 times. Observed, only the large wave of iron can be identified. The volume fraction of the iron or the toughened iron in the volume ratio of the second phase is only about 10%, but if it is 3000 times. When a detailed investigation is performed by electron microscopy (SEM) observation, the ratio of the volume ratio of the ferrite or the toughened iron to the second phase is 30 to 40%. By suppressing such a wave of iron or toughening iron, both low YP and high El can be obtained.

After annealing, the first cooling from the annealing temperature to the vicinity of 480 ° C (for example, the cooling process before immersion in the continuous hot-dip galvanizing line (CGL) to the galvanizing bath), or from 480 ° C up to 350 2 coolings around °C (for example: cooling process after immersion in galvanizing bath in continuous hot-dip galvanizing line (CGL), cooling process in over-aging domain in continuous annealing line (CAL) In the thermal history of a continuous hot-dip galvanizing line (CGL) or a continuous annealing line (CAL) with a cooling rate of about 1 to 200 ° C / sec, in order to make such fine waves of iron or toughened iron The reduction is as long as the weighted index equivalent formula of the following elements related to the hardenability is controlled to 2.1 to 3.1. However, when B is added together with Ti or Al, the effect of improving the hardenability is remarkably increased. However, if the addition is more than a predetermined amount, the effect of improving the hardenability tends to be saturated, so the effects are as follows. The formula is expressed.

[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, take B ^* = 0.0022.

Here, [%Mn], [%Cr], [%Mo], [%P], [%B], [%Ti], [%Al] represent Mn, Cr, Mo, P, B, respectively. Individual content of Ti, sol. Al.

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

By controlling the value of [Mneq] to 2.1 or more, low YP, high El, and high BH can be obtained. From the viewpoint of a further low YP and high El, it is preferable to control the value of [Mneq] to 2.2 or more, and it is more preferable to control it to 2.3 or more. However, when [Mneq] exceeds 3.1, the addition amount of Mn, Cr, and P is too large, and it is difficult to simultaneously ensure sufficiently low YP and excellent impact crack resistance. Therefore, [Mneq] is selected to be 3.1 or less.

Mn: more than 1.0% and less than 2.0%

If it is desired to further lower the YP and ensure high El and high BH, even in the steel composition of the same Mn equivalent, it is preferable that the amount of Mn is small. This is because if the Mn content is too much, the α→γ metamorphic temperature will be low during the annealing process, and γ grain formation will occur before the recrystallization ends, and the ferrite iron particles and the second phase will be localized. The uneven structure will cause YP to rise and El to decrease, and the amount of solid solution C after annealing to decrease and BH to decrease. From the viewpoint of low YP, high El, and high BH, the amount of Mn is selected to be less than 2.0%. On the other hand, if the amount of Mn is too small, it is difficult to ensure sufficient hardenability even if a large amount of other elements are added. Further, the MnS system is finely dispersed in a large amount, resulting in deterioration of corrosion resistance and impact crack resistance. In order to ensure sufficient hardenability and corrosion resistance, Mn must be added at least over 1.0%.

Therefore, the Mn content was selected to be more than 1.0% and less than 2.0%. From the viewpoint of further improving corrosion resistance and impact crack resistance, it is preferred to select the Mn content to be 1.2% or more.

Mo: less than 0.15%

The Mo system can be added based on the viewpoint of improving the hardenability, suppressing the formation of the ferrite, and ensuring the desired strength. However, in the same manner as Mn, the Mo system has a strong effect of making the second phase finer, and the effect of making the ferrite iron particles finer and finer. Therefore, if Mo is excessively added, YP will be significantly increased. Further, when used as a cold rolled steel sheet, Mo will significantly deteriorate the chemical conversion processability. In addition, Mo is a very expensive element. Therefore, the addition amount of Mo is limited to less than 0.15% (including 0%) from the viewpoint of reduction in YP, improvement in chemical conversion workability, and cost reduction. From the viewpoint of further lower YP, it is preferably limited to 0.05% or less, and Mo may be selected to be unadded (0.02% or less).

P: 0.05% or less

In the present invention, P can be used as a quenching element, and can be used as an alternative element of Mn, Cr, and Mo, and is an element for achieving low YP, high BH, and high El. In other words, even if P is added in a small amount, it has a large effect of improving the hardenability, and in addition, it has an effect of uniformly dispersing the three phases of the second phase in the iron grain boundary of the fat grain, and therefore, it is used under the same Mn equivalent. The P system can make YP lower and make BH higher than the use of Mn. In addition, El also becomes higher. In order to obtain such an effect of low YP, high BH, and high El due to the addition of P, the amount of P added is preferably 0.015% or more.

However, if P is added in excess of 0.05%, the effect of improving the hardenability, the homogenization effect of the structure, and the coarsening effect tend to be saturated, and the amount of solid solution strengthening becomes too large to obtain a low YP. Further, the alloying reaction between the steel sheet underlayer and the plating layer is remarkably delayed, and the pulverization resistance is deteriorated, and as a result, the impact crack resistance is deteriorated. Therefore, the amount of P contained is selected to be 0.05% or less.

B: 0.0010% or more and 0.0050% or less

The B system has a function of homogenizing and coarsening the ferrite iron particles and the granulated iron particles, and can improve the hardenability and suppress the action of the iron. Also, B itself has the effect of increasing BH. Therefore, it is possible to simultaneously achieve low YP and high BH by ensuring a predetermined amount of [Mneq] and replacing Mn with B. In order to sufficiently obtain such an effect, it is necessary to add B to 0.0010% or more. However, if B is added in excess of 0.0050%, castability and rollability will be remarkably lowered. Therefore, B is selected to be 0.0050% or less. In order to further exert the effect of low YP and high BH by the addition of B, it is preferable to add 0.0013% or more of the B system.

C: more than 0.015% and less than 0.100%

C is an essential element for ensuring a predetermined amount of the volume fraction of the second phase. If the C content is too small, the second phase cannot be formed, resulting in a significant increase in YP and a deterioration in the balance between strength and ductility. In addition, high BH and excellent aging resistance can no longer be obtained. In order to obtain a predetermined amount of the second phase volume ratio and a sufficiently low YP, the C content must exceed 0.015%. From the viewpoint of improving the aging resistance and lowering the YP, it is preferred to select the C content to be 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. Also, the weldability is also deteriorated. Therefore, the content of C was selected to be less than 0.100%. In order to obtain a lower YP and a high El and a high BH, it is preferred to select the C content to be less than 0.060%, preferably less than 0.040%.

Si: less than 0.50%

When Si is added in a small amount, it has the effect of delaying the formation of scale and improving the surface quality during hot rolling, and moderately delaying the alloy between the base layer of the steel sheet and zinc in the galvanizing bath or alloying treatment. The effect of the chemical reaction can make the fine structure of the steel sheet more uniform and coarse, and the effect of low YR and high El can be increased. Therefore, it can be added based on such a viewpoint. However, when Si is added in an amount of 0.50% or more, a phenomenon such as a rust pattern and a partial unplating phenomenon may occur on the hot-dip galvanized steel sheet, and a rust pattern may be generated on the cold-rolled steel sheet to be difficult to apply. Install the panel. In addition, the processing property is deteriorated and the YP rises. Therefore, the Si content was selected to be less than 0.50%. Based on the viewpoint of improving the surface quality and lowering the YP, the Si content is preferably selected to be less than 0.30%. The Si system is an element which can be added arbitrarily. Although the lower limit value (including Si is 0%) is not specified, the amount of Si added is preferably 0.01% or more, more preferably 0.02% or more.

S: 0.03% or less

When the S is contained in an appropriate amount, it can have the effect of improving the peeling property of the primary skin of the steel sheet and improving the appearance quality of the cold-rolled steel sheet and the hot-dip galvanized steel sheet, and therefore S can be contained. However, when the content of S is large, the amount of MnS precipitated in the steel is too large, and the stretchability of the steel sheet and the stretchability of the flange are lowered. Further, when the steel blank is subjected to hot rolling, the thermal ductility is lowered, and surface defects are easily caused. In addition, it will reduce corrosion resistance. Therefore, the S content is selected to be 0.03% or less. From the viewpoint of improving the stretchability of the flange and the corrosion resistance, the S content is preferably 0.02% or less, more preferably 0.01% or less.

sol.Al: 0.01% or more and 0.3% or less

Al is based on the purpose of fixing N to promote the hardenability-improving effect of B, improving the aging resistance, and reducing the amount of the medium to improve the surface quality. The content of sol. Al is selected to be 0.01% or more from the viewpoint of the improvement of the hardenability and the aging resistance of B: In order to further exert this effect, the content of sol. Al is preferably selected to be 0.015% or more, more preferably 0.04% or more. On the other hand, if the content of sol.Al exceeds 0.3%, the effect of improving the solid solution B residual effect and the improvement of the aging resistance are saturated, which only leads to an increase in cost. In addition, the castability is deteriorated and the surface quality is deteriorated. Therefore, sol. Al is selected to be 0.3% or less. From the viewpoint of ensuring excellent surface quality, it is preferred to select the content of sol. Al to be less than 0.2%.

N: 0.005% or less

N is an element used in steel to form nitrides such as BN, AlN, TiN, etc., and has the disadvantage that the material enhancement effect by B is offset by the formation of BN. In addition, fine AlN is formed to lower grain growth, resulting in an increase in YP. When the content of N is more than 0.005%, YP will rise and the aging resistance will deteriorate, and it is not suitable for use as an exterior panel. Therefore, the content of N is selected to be 0.005% or less. The viewpoint of reducing the precipitation amount of AlN can further reduce the viewpoint of YP, and the content of N is preferably 0.004% or less.

Ti: less than 0.030%

The Ti system has an effect of fixing N to enhance the hardenability of B, an effect of improving the aging resistance, and an effect of improving the castability, and is an element which can be arbitrarily added in order to obtain such an effect in an auxiliary manner. However, if the content is too large, fine precipitates such as TiC and Ti(C, N) are formed in the steel, which causes the YP to rise remarkably, and TiC will be generated during the cooling after annealing to cause a decrease in BH. Therefore, if you want to add Ti, the content of Ti must be controlled within a proper range. When the content of Ti is 0.030% or more, YP is remarkably increased. Therefore, the content of Ti is limited to less than 0.030%. Ti is an element that can be added arbitrarily. Although the lower limit (including Ti is 0%) is not specified, in order to enhance the hardenability of B by fixing N due to precipitation of TiN, It is preferable to select the content of Ti to be 0.002% or more, and it is preferable to limit the content of Ti to less than 0.010% in order to suppress precipitation of TiC to obtain low YP.

Although the rest is iron and unavoidable impurities, it may contain the following elements to a predetermined amount.

At least one of the following V, Nb, W, and Zr:

V: 0.2% or less

The V system can be added based on the viewpoint of high strength. From the viewpoint of the increase in strength, it is preferably added in an amount of 0.002% or more, more preferably 0.01% or more. However, if the addition exceeds 0.2%, the cost will be significantly increased, so the addition amount of V is preferably 0.2% or less.

Nb: less than 0.030%

The Nb system has a function of refining the structure and precipitating NbC and Nb (C, N) to strengthen the steel sheet. Therefore, it can be added based on the viewpoint of high strength. Further, the effect of retarding recrystallization during hot rolling and the effect of delaying subsequent deformation are also large, so that a small amount of Nb can be added to improve the aggregate structure, and it is possible to reduce the direction at the right angle of rolling. The upper r value increases the effect of the r value in the 45 degree direction. Therefore, by adding Nb of 0.002 to 0.015%, the in-plane directionality of Δr and YP can be made small. Based on the above viewpoint, it is preferable to add Nb to 0.002% or more, more preferably 0.005% or more. However, if it is added to 0.030% or more, YP will rise remarkably, so the amount of Nb added is preferably less than 0.030%.

W: 0.15% or less

The W system can be used as a hardenable element or a precipitation strengthening element. Based on the above viewpoint, it is preferable to add W to 0.002% or more, more preferably 0.005% or more. However, if the amount of addition is too large, the YP will increase, so it is preferable to add W to 0.15% or less.

Zr: 0.1% or less

Zr is also used as a quenching element and a precipitation strengthening element in the same manner. Based on the above viewpoint, it is preferable to add Zr to 0.002% or more, more preferably 0.005% or more. However, if the amount of addition is too large, the YP will increase, so it is preferable to add Zr to 0.1% or less.

At least one of the following Sn, Sb, Cu, Ni, Ca, Ce, La, and Mg:

Sn: 0.2% or less

The Sn system is preferably added in such a manner as to suppress nitriding, oxidation, or decarburization or de-B in the tens of micrometers of the surface layer of the steel sheet due to oxidation. Thereby, fatigue resistance, aging resistance, surface quality, and the like can be improved. From the viewpoint of suppressing nitriding and oxidation, Sn is preferably added in an amount of 0.002% or more, more preferably 0.005% or more. However, if it exceeds 0.2%, YP is increased and toughness is deteriorated, so the amount of Sn is 0.2. % below is appropriate.

Sb: 0.2% or less

Similarly to Sn, Sb is preferably added in such a manner as to suppress nitriding, oxidation, or decarburization or de-B in the tens of micrometers of the surface layer of the steel sheet due to oxidation. By suppressing such nitriding and oxidation, the amount of generation of granulated iron in the surface of the steel sheet is prevented from being reduced, and the deterioration of the hardenability due to the decrease in B is prevented, and the fatigue resistance and the aging resistance can be improved. Moreover, the wettability at the time of hot-dip galvanizing can be improved, and the appearance quality of galvanization can be improved. From the viewpoint of suppressing nitriding and oxidation, it is preferable to add Sb to 0.002% or more, more preferably 0.005% or more, but if it exceeds 0.2%, the YP is increased and the enthalpy is deteriorated, so the Sb-containing amount is 0.2. % below is appropriate.

Cu: 0.5% or less

Since the Cu system can slightly improve the impact crack resistance, it is preferable to add it based on the viewpoint of improving the impact crack resistance. In addition, the Cu system is an element that is mixed when the waste material is used as a raw material. Therefore, by mixing the Cu, the recycled material can be used as a raw material, and the manufacturing cost can be reduced. The addition amount of Cu is preferably 0.01% or more, and more preferably 0.03% or more, from the viewpoint of improving the impact crack resistance. However, if the content is too large, it will cause surface defects, so the Cu content is preferably 0.5% or less.

Ni: 0.5% or less

Ni is also an element that has an effect of improving impact crack resistance. Further, the Ni system has an effect of reducing the incidence of surface defects which are likely to occur when Cu is contained. Therefore, based on the viewpoint of improving corrosion resistance and improving surface quality, the addition amount of Ni is preferably 0.01% or more, more preferably 0.02% or more. However, if the amount of Ni added is too large, the generation of scale in the heating furnace becomes uneven, which causes surface defects, and the cost is remarkably increased. Therefore, Ni is limited to 0.5% or less.

Ca: 0.01% or less

The Ca system has a function of fixing S in the steel as CaS, increasing the pH of the corrosion product, and improving the impact crack resistance. Further, it is also possible to suppress the generation of MnS which can deteriorate the flange stretch formability due to the formation of CaS, thereby enhancing the flange stretch formability. It is preferable to add Ca to 0.0005% or more based on this viewpoint. However, Ca is easily separated as an oxide in the molten steel, and it is difficult to remain in the steel in a large amount. Therefore, the content of Ca is limited to 0.01% or less.

Ce: 0.01% or less

Ce is also based on the fact that S can be fixed in steel to improve the formability of the flange and the impact crack resistance. It is preferable to add Ce to 0.0005% or more based on the above viewpoint. However, because it is a high-priced element, adding more will lead to higher costs. Therefore, it is preferable to add Ce to 0.01% or less.

La: 0.01% or less

La is also based on the fact that S can be fixed in steel to improve the formability of the flange and the impact crack resistance. It is preferable to add La to 0.0005% or more based on the above viewpoint. However, because it is a high-priced element, adding more will lead to higher costs. Therefore, it is preferable to add La to 0.01% or less.

Mg: 0.01% or less

The Mg system can be added from the viewpoint of finely dispersing the oxide and homogenizing the structure. It is preferable to add Mg to 0.0005% or more based on the above viewpoint. However, if the content is too large, the surface quality is deteriorated, so that Mg is preferably added in an amount of 0.01% or less.

2) Organization

The steel sheet structure of the present invention is mainly composed of ferrite iron, granulated iron, residual gamma, ferritic iron, and toughened iron, and others contain a trace amount of carbide. First, the measurement method of these tissue forms will be explained.

The method for measuring the volume fraction of the second phase is to polish the L section (a vertical section parallel to the rolling direction) of the steel sheet, and then etch it with nitric acid to use the scanning electron at a position of 1/4 of the thickness of the steel sheet. The microscope (SEM) was observed at eight magnifications at a magnification of 3000 times, and the photographed tissue photograph was subjected to image analysis to measure the area ratio of the second phase, thereby obtaining the volume fraction of the second phase. That is, the difference in the microstructure of the steel sheet of the present invention in the rolling direction and the direction perpendicular to the rolling is small, and the area ratio of the second phase after measurement in any direction shows almost the same value. The area ratio of the second phase measured on the L section is taken as the volume ratio of the second phase.

In the tissue photo, the area with a slight black contrast is the ferrite iron. If the carbide is formed in a lamellar or dot-like shape, this region is treated as a ferritic or toughened iron, and the particles having a white contrast are granulated iron or residual γ, and the area ratio thereof is calculated. However, the particles having a diameter of 0.90 μm or less dispersed in the ferrite particles in the white contrast particles are judged to be carbide particles to be described later, and are taken from the field by the observation by a transmission electron microscope (TEM). The volume fraction of loose iron or residual γ is removed. The volume ratio of the second phase is the total amount of these tissues, and the volume ratio of the granulated iron and the residual γ is the total amount of the area ratio of the region in which the white contrast is obtained. In addition, it is difficult to distinguish between the granulated iron and the residual γ according to the photograph of the scanning electron microscope (SEM). Although the area is determined as the total area ratio of the two phases, the analysis of the X-ray results confirms that the field is confirmed. Among the volume ratios of scattered iron and residual γ, the granulated iron accounts for about 60% and the residual γ accounts for about 40%.

In addition, in the continuous annealing process, when the overaging range of the continuous annealing treatment line (CAL) is about 350 ° C or less, after the generation of the granulated iron is maintained, the temperature range is maintained for a long time, and therefore, sometimes it is generated. The Ma Tian loose iron was only slightly tempered to handle a little. This kind of 麻田散铁, which was only slightly tempered and treated a little, is used here as a granulated iron. In addition, the method of identifying the granulated iron and the toughened iron after tempering is as follows. That is, the carbides in the tempered granulated iron are very fine compared with the carbides dispersed in the toughened iron, so the internal dispersion of the granulated iron particles and the toughened iron particles is determined by measurement. The average particle diameter of the carbides can be used to distinguish between the tempered iron and the toughened iron. Here, when the average particle diameter of the carbide in the granule is 0.15 μm or less, it is regarded as tempered granule iron, and if it exceeds 0.15 μm, it is regarded as toughened iron.

According to the observation results of the transmission electron microscope (TEM), the spherical or elliptical particles having a diameter of 0.5 μm dispersed in the ferrite grains are Fe, Mn, C, and B-based carbides, and this precipitate (Cleavage) is the cause of the material change in the B-added steel. Therefore, the particles having an average particle diameter of 0.25 to 0.90 μm in the aspect ratio of 3.0 or less in the SEM photograph are distributed. The number of carbides of Fe, Mn, C, and B systems was measured. When the particles are elliptical in the SEM photograph, the major axis a and the uniaxial b in the direction perpendicular to the long axis are measured, and (a × b) ^{0.5 is} used as the corresponding particle diameter.

Volume ratio of the second phase: 2.0 to 12.0%

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

The ratio of the volume ratio of the granulated iron and the residual γ to the volume ratio of the second phase: 60% or more

In order to sufficiently suppress the ferrite and the toughening iron in the second phase to ensure low YP and high El, the ratio of the volume ratio of the granulated iron and the residual γ to the volume ratio of the second phase must be 60. %the above.

The number of carbide particles present in the ferrite grains having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm: 10,000 pieces/mm ² or less in the steel composition and the coiling temperature and annealing temperature during hot rolling In the steel sheet in which the holding time is not properly normalized (optimized), significant material variation occurs in the width direction and the length direction of the steel strip roll. In such a portion, spherical or elliptical carbide 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 is dispersed in the ferrite iron particles, and is more than 10,000 pieces/mm ² . By reducing the number of such carbides to 10,000 pieces/mm ² or less, the material variation in the steel strip roll can be almost solved. Therefore, the number of the carbide particles present in the ferrite iron particles having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm is limited to 10,000 pieces/mm ² or less. Further, in the steel of the present invention, although the average ferrite iron particle diameter and the average diameter of the second phase are not specified, the average ferrite iron particle diameter is in the range of 7 to 12 μm, and the average diameter of the second phase is It is in the range of 0.8 to 1.3 μm.

This histological morphology can be moderated by the composition range of Mn, Cr, P, B, sol. Al, Ti, N, etc., and the annealing temperature at the time of hot rolling, CT, and CAL and CGL Keep the time and get it right.

3) Manufacturing conditions

The steel sheet according to the present invention is a steel preform having the above-defined composition in the above-described manner, and the coiling temperature is set in accordance with the contents of B, sol. Al, Ti, and N in the hot rolling step. The CT is controlled in a proper range, followed by cold rolling at a cold rolling ratio of 50 to 85%, and then in a continuous hot-dip galvanizing line (CGL) or continuous annealing line (CAL), 740 It can be produced by an annealing treatment at a temperature of 830 ° C or higher and a temperature of 830 ° C or lower for 25 seconds or more.

Hot rolling:

If it is desired to carry out hot rolling for a steel blank, a method of rolling the steel embryo and then rolling it, and after continuous casting, a method of directly rolling the steel without heating the steel embryo, after continuous casting, After the steel slab is subjected to a heat treatment for a short period of time, a method of rolling or the like is performed. For hot rolling, for example, the steel billet heating temperature is 1100 to 1300 ° C, and the final refining rolling temperature may be an Ar ₃ metamorphic point to an Ar ₃ metamorphic point + 150 ° C. From the viewpoint of reducing the in-plane anisotropy of the r value and the viewpoint of improving BH, the average cooling rate after hot rolling is preferably 20 ° C /sec or more.

In the B-added steel containing B of 0.0010% or more, in order to reduce the material variation in the steel coil, it is necessary to control the coiling temperature to a proper range in accordance with the contents of B, sol. Al, Ti, and N. The results of investigating the relationship between each element and a suitable CT are as follows.

First, five kinds of steels containing different amounts of B, sol. Al, Ti, and N as shown in Table 1 were melted. For the obtained steel blank, hot rolling was performed to obtain a hot rolled steel coil having a thickness of 3.2 mm. At this time, the steel embryo heating temperature was 1220 ° C, and the final refining rolling temperature was 850 ° C. Immediately after the rolling, rapid cooling was carried out to 690 ° C, and then laminar cooling was carried out on a steel belt conveying platform, and coiling was carried out in the range of 500 to 675 ° C. The obtained hot-rolled sheet was pickled, and further subjected to cold rolling until a thickness of 0.70 mm, followed by annealing at 780 ° C for 40 seconds in CGL. During the cooling, the galvanization treatment was performed by immersing in a galvanizing bath, and then, after alloying treatment, after cooling to room temperature, temper rolling was performed at an elongation of 0.4%. A JIS No. 5 tensile test piece parallel to the rolling direction was taken from the obtained steel tape roll, and the mechanical properties in the width direction of the steel tape roll were investigated. In addition, metal structures were investigated in the manner previously described.

[Table 1]

Fig. 1 shows the results of investigations on the presence or absence of changes in the width of the TS in the width direction of the steel strips of the respective component steels after various CTs. Here, sol. B is a value calculated based on the formula (A), and is a value of the B content in the solid solution state estimated 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], [%Al] indicate the contents of B, N, Ti, and sol. Al, respectively, and CT indicates the coiling temperature (°C). When CT-560≦0, take CT-560 to 0. When sol.B≦0, take sol.B to 0.

In other words, the amount of solid solution B is considered to be: when B is added excessively with respect to the amount of N, so when Ti and Al are added, the amount relative to N must also be considered. The amount of precipitation. When the Ti-based steel is heated, 80% of the added amount is precipitated in the form of TiN, and the rest is precipitated in the form of TiC. Al is precipitated when CT exceeds 560 ° C, and the amount of precipitation increases as CT increases. In consideration of such a behavior, the amount of N precipitated in the form of TiN and AlN is subtracted from the amount of N contained, and the amount of N in the remaining portion is obtained, and after the amount of N in the remaining portion is subtracted from the amount of added B, It becomes (A).

Fig. 1 is a view showing the relationship between the sol. B, CT obtained in the equation (A) and the presence or absence of material variation in the width direction of the steel strip. Further, the TS in the width direction was subjected to a JIS No. 5 tensile test piece in a direction parallel to the rolling direction, and subjected to a tensile test for evaluation. Regarding the position of the test piece in the longitudinal direction of the steel strip roll, it is selected at the center position of the length of the steel strip roll, and the position taken in the width direction is from the center line of the test piece just falling from the width direction of the steel strip roll. The upper edge is taken at a position on the inner side of 18 mm, and the start is taken, and it is taken in the width direction at intervals of 30 mm to 600 mm in order to sufficiently confirm the change in strength. The steel strip coil having a difference between the maximum value and the minimum value of the TS in the width direction obtained in this manner is less than 20 MPa, and the symbol "○" is indicated, and the steel strip coil having a difference of 20 MPa or more is marked. The symbol of "●". The steel strip roll with the symbol "●" produces significant material variations in the width direction of the strip roll and in the direction of the long hand. Further, in the figure, the boundary line obtained by the CT expressed by the equation (1) and the relational expression according to sol. B is also indicated.

CT(°C)≦670-50000×sol.B ...(1)

However, when sol.B ≦ 0, sol.B is calculated as 0.

It can be seen from Fig. 1 that, in view of the sol. B estimated from the formula (A), there is a stable CT range in which the TS is present, and the proper range is increasingly low as the sol.B increases. . And it can be known that the boundary is obtained from the formula (1), and for the steel of various composition points, if the coil is taken at a low temperature below the CT of the formula (1), even in the B-added steel, Suppress material changes. Therefore, CT is limited to the range expressed by the formula (1).

Fig. 2 shows the change in TS in the width direction when CT is selected to be 620 ° C and 530 ° C for steel 1. It can be known that even steels of the same steel composition will produce significant material changes if CT is not properly normalized. Here, it has been confirmed that the body of the steel strip having a strength variation of 20 MPa or more in the width direction has an aspect ratio of 3.0 or less and a carbide particle having a diameter of 0.25 to 0.90 μm in excess of 10,000 pieces/mm ² .

In order to obtain the galvanized surface quality of the outer panel, the steel embryo heating temperature is limited to 1250 ° C or less. In order to remove the primary and secondary scales formed on the surface of the steel sheet, the rust removal treatment is fully performed. It is preferable that the rough rolling end temperature is selected to be 1080 ° C or lower and the final refining rolling temperature is selected to be 900 ° C or lower. For example, in a conventional steel material having a Cr content of 0.40% or more, the primary scale of the steel embryo during heating tends to remain after rolling, which is a cause of deterioration in appearance quality after annealing of CAL and CGL. If the Cr content is reduced to less than 0.35%, and the steel embryo heating temperature is selected to be below 1250 ° C, and the scale is sufficiently removed by a high pressure spray gun, the rough rolling end temperature is selected below 1080 ° C, and finally The refining roll rolling temperature is controlled below 900 ° C, so that the beautiful appearance quality required for the exterior panel of the automobile can be obtained.

Cold rolling:

When performing cold rolling, the rolling ratio may be set to 50 to 85%. From the viewpoint of improving the r-value to improve the deep-drawing formability, it is preferable to select the rolling ratio at 65 to 73%, and to select the rolling ratio based on the reduction of the r-value and the in-plane anisotropy of YP. 70 to 85% is appropriate.

annealing:

The cold-rolled steel sheet is subjected to an annealing treatment in CGL or CAL, a hot-dip galvanizing treatment as necessary, or an alloying treatment after the hot-dip galvanizing treatment. The annealing temperature is selected to be 740 ° C or higher and 830 ° C or lower. When the temperature is less than 740 ° C, the solid solution of the carbide is insufficient, and the volume fraction of the second phase cannot be stably ensured. When it exceeds 830 ° C, it is easy to generate ferrite and toughened iron, and it is not possible to obtain a sufficiently low YP. The holding time at the time of the soaking is preferably 25 seconds or more, more preferably 40 seconds or more, and is preferably selected to be 300 seconds or less from the viewpoint of ensuring productivity.

After the 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 the cooling rate is selected to be 3 to 50 ° C from the viewpoint of low YP. Seconds is better.

Then, it is immersed in a galvanization bath in the CGL to carry out galvanization, and if necessary, it is further kept within a temperature range of 470 to 650 ° C for 40 seconds to carry out alloying treatment. From the viewpoint of low YP, after the galvanization treatment or when the alloying treatment is performed, the alloy is cooled at an average cooling rate of 5 to 200 ° C / sec in a temperature range up to 100 ° C or less after the alloying treatment. This method is suitable for suppressing the formation of toughened iron.

In CAL, it is cooled at an average cooling rate of 2 to 200 ° C / sec from 480 ° C to room temperature, and in the case of a furnace having an overaged range, it is 5 before 370 ° C or lower. The cooling is performed at an average cooling rate of -200 ° C / sec, and then cooling may be performed at an average cooling rate of 0.1 to 200 ° C / sec before 100 ° C or lower.

The galvanized steel sheet or the cold-rolled steel sheet to be obtained may be subjected to mild temper rolling by the viewpoint of stabilizing the surface roughness and improving the press formability such as flattening the shape of the sheet. In this case, from the viewpoint of low YP and high El, it is preferred to set the elongation of the mild temper rolling to 0.1 to 0.6%.

[Examples]

After the steels of the steel numbers A to U shown in Table 2 were melted, continuous casting was performed to prepare a steel blank having a thickness of 230 mm.

[Table 2]

After heating the steel bristles to 1,180 to 1,250 ° C, hot rolling is performed by a final refining rolling temperature in the range of 820 to 900 °C. Then, the mixture was cooled at an average cooling rate of 15 to 35 ° C / sec, and coiled at a temperature of 450 to 670 ° C. The obtained hot-rolled steel sheet was subjected to cold rolling at a rolling ratio of 70 to 77%, thereby producing a cold-rolled steel sheet having a sheet thickness of 0.8 mm.

The obtained cold-rolled steel sheets were annealed in CGL or CAL at an annealing temperature AT in the manner shown in Table 3. At this time, annealing is performed so that the holding time in the temperature range of 740 ° C or higher is 15 to 150 seconds, and the average cooling rate until the annealing temperature AT reaches 480 ° C is cooled at 10 ° C / sec. Then, in CGL, the steel sheet is immersed in a galvanizing bath for galvanization, and then subjected to alloying treatment, or a steel sheet which is not alloyed after galvanizing, is a liquid temperature from a galvanizing bath after galvanizing. The average cooling rate up to 100 ° C was 25 ° C / sec, and it was cooled to 100 ° C or less. The galvanizing treatment is carried out in a galvanizing bath containing 0.13% of Al at a liquid temperature of 460 ° C, and the alloying treatment is performed after immersing in a galvanizing bath and heating to 480 ～ at an average heating rate of 15 ° C / sec. 540 ° C is maintained for 10 to 25 seconds in such a manner that the Fe content in the galvanized film is in the range of 9.5 to 11.5%. The amount of galvanization adhered was 45 g/m ² per one side, and a galvanized film was attached to the side of the enamel of the steel sheet. In CAL, the temperature range from 480 ° C to 370 ° C is cooled at an average cooling rate of 10 ° C / sec, and then, in the overaging range, at an average cooling rate of 1 ° C / sec. The mixture was cooled to 100 ° C, and then cooled to room temperature at an average cooling rate of 10 ° C / sec. The prepared hot-dip galvanized steel sheets and cold-rolled steel sheets were subjected to temper rolling at an elongation of 0.4%, and samples were taken.

With respect to the obtained sample, the ratio of the volume ratio of the second phase, the granulated iron in the second phase, and the volume ratio of the residual γ in the second phase was investigated by the method described above (the maiden iron in the second phase) The ratio of the residual γ), the number of carbide particles present in the ferrite iron particles having an aspect ratio of 3.0 or less and a diameter of 0.25 to 0.90 μm (intragranular carbide density). Further, the type of the steel structure was separated by observation by a scanning electron microscope. Further, a JIS No. 5 test piece was taken from a direction perpendicular to the rolling direction to carry out a tensile test (according to JIS Z2241), and its YP (falling strength), TS (tensile strength), and YR (ratio of falling ratio) were evaluated. ), El (full elongation). Further, in the width direction of the steel strip roll, a JIS No. 5 tensile test piece was taken in parallel with the rolling direction, and the amount of change (ΔTS) of TS in the width direction of the steel strip roll was examined.

In addition, BH was obtained for the test piece (JIS No. 5 tensile test piece) similar to the above, that is, YP after heat treatment at 170 ° C for 20 minutes with respect to stress at the time of imparting 2% of deformation in advance. The amount of increase.

Further, the impact crack resistance of each steel sheet was evaluated. That is, after the chemical conversion treatment and electrostatic coating of the obtained steel sheet, 500 g of crushed stone is sprayed at a pressure of 490 kPa (5 kgf/cm ² according to Japanese Industrial Standard JIS-A5001 S-13 (No. 6). The conditions were sprayed onto the steel sheet, and then the JASO-CCT corrosion cycle test was used to conduct the corrosion test. The thickness of the electrostatic coating film was 20 μm. For molten galvanized steel sheets (manufactured by CGL), the samples are etched after 90 cycles, and for cold-rolled steel sheets (made of CAL), the samples are corroded after 30 cycles. After the corrosion product was removed, the maximum value of the plate thickness reduction amount was determined from the previously measured plate thickness to obtain the maximum corrosion depth.

The results are shown in Table 3.

[table 3]

In the steel sheet of the present invention, the corrosion loss is remarkably reduced as compared with the conventional steel in which the Cr content is not properly normalized, and the steel containing a small amount of B, the steel having a low Mn equivalent, and a large amount of Mn are added. Steel, steel with Mo added, compared to steel of the same TS grade, has a low YP, that is, both low YR and high El and high BH.

That is, the corrosion reduction (maximum corrosion depth) of the conventional steels O and P in which a large amount of Cr is added is remarkably large, and is 0.59 to 0.75 mm. When the steel is used on actual parts, the life of the etched hole is also reduced by 30 to 50%, so it is difficult to use it as an exterior panel. On the other hand, the maximum corrosion loss of the steel of the present invention is 0.29 to 0.38 mm, which is greatly reduced. Further, although not shown in the table, the corrosion resistance was also evaluated for the conventional 340BH, and the corrosion loss was 0.36 mm. In addition, the chemical composition of such steel (formerly 340BH) contains C: 0.002%, Si: 0.01%, Mn: 0.4%, P: 0.05%, S: 0.008%, and Cr: 0.04% by mass%. Sol. Al: 0.06%, Nb: 0.01%, N: 0.0018%, and B: 0.0008%. Therefore, it can be understood that the steel system of the present invention has impact fracture resistance which is almost equivalent to that of the conventional steel (340BH). Among them, steel having a composition having a Cr content of less than 0.30%, and a steel having Ce, Ca, La, Cu, or Ni compositely added thereto have better impact fracture resistance.

Even in the steel in which the Cr content is reduced in this manner, if the steel containing the Mn equivalent, the addition amount of Mn, B, the coiling temperature, the annealing temperature, and the soaking (holding) time is further controlled, It can suppress the formation of ferrite and toughened iron, and can reduce the formation of carbides in the ferrite grains, and can suppress the material variation in the steel coil. In other words, among the steels A to L, CT is a condition below the value of the formula (1), and the steel having the annealing temperature and the soaking holding time falling within the predetermined range is compared with the comparative steel of the same TS grade. It has: low YP, high BH, high El, small ΔTS.

[Industrial availability]

According to the present invention, a high-strength steel sheet having excellent impact crack resistance, low YP, high El, and high BH, and having little variation in material in the steel coil can be produced at low cost. The high-strength steel sheet of the present invention has excellent impact fracture resistance, excellent surface deformation resistance, excellent protrusion formability, and excellent material stability, so that high strength and thickness of automobile parts can be achieved. .

Fig. 1 shows the relationship between sol. B, CT and TS variation in the width direction.

Fig. 2 is a graph showing changes in TS in the width direction of the steel strip in different steel sheets of CT.

Claims (4)

A high-strength steel sheet characterized in that the composition of the steel contains: 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: 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 conforms to 2.1≦[Mneq]≦3.1, the rest is composed of iron and unavoidable impurities. The structure of steel has fermented iron and phase 2, phase 2 The volume ratio is 2.0 to 12.0%, and the ratio of the volume ratio of the granulated iron and the residual γ in the second phase is 60% or more, and the aspect ratio in the particles of the ferrite iron is 3.0 or less and the diameter is 0.25 to 0.90. The number of the carbide particles of μm is 10000/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] represent the contents is Mn, Cr, Mo, P, B, Ti, sol.Al when B ^* ≧ 0.0022 when the take-based B ^* = 0.0022 . The high-strength steel sheet according to the first aspect of the invention, further comprising: Nb: less than 0.030%, V: 0.2% or less, W: 0.15% or less, and Zr: 0.1% or less in mass% At least one of them. The high-strength steel sheet according to claim 1 or 2, further comprising: Sn: 0.2% or less, Sb: 0.2% or less, Cu: 0.5% or less, Ni: 0.5% by mass% Hereinafter, at least one of Ca: 0.01% or less, Ce: 0.01% or less, La: 0.01% or less, and Mg: 0.01% or less. A method for producing a high-strength steel sheet, characterized in that, in a step of performing hot rolling, a steel blank having the composition component according to any one of claims 1 to 3, a coiling temperature CT Controlled in the range indicated by the formula (1) in response to sol. B, after cold rolling at a cold rolling ratio of 50 to 85%, in a continuous hot-dip galvanizing line (CGL) or in a continuous annealing line (CAL) In the middle, the annealing temperature is maintained at an annealing temperature of 740 ° C. or higher and 830 ° C or lower for 25 seconds or more; CT (° C) ≦ 670 - 50000 × sol. B (1) where sol. B = [% B] -{[%N]/14-[%Ti]/48×0.8-[%Al]/27×0.0005×(CT-560)}×10.8 (A) In the formula (A), [%B ], [%N], [%Ti], [%Al] are the contents of B, N, Ti, and sol.Al, respectively, and CT is the coiling temperature (°C). When CT-560≦0, Take CT-560=0; however, when sol.B≦0, calculate sol.B as 0.