JP5488410B2 - Hot-dip galvanized steel sheet for heat treatment and method for producing the same - Google Patents

Description

  The present invention relates to a hot-dip galvanized steel sheet for heat treatment and a method for producing the same. The present invention relates to a hot-dip galvanized steel sheet suitable for heat-resistant parts such as machine structural parts such as automobile body structural parts and undercarriage parts, and a method for producing the same.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the weight used. In thin steel plates widely used in automobiles, press formability decreases with increasing steel plate strength, making it difficult to manufacture complex shapes. Specifically, there arises a problem that fracture occurs at a site where ductility is reduced and the degree of processing is high, springback and wall warp increase, and dimensional accuracy deteriorates. Therefore, it is not easy to produce parts by press molding using a steel plate having high strength, particularly a 780 MPa class or higher. According to roll forming instead of press forming, it is possible to process a high-strength steel sheet, but it can be applied only to parts having a uniform cross section in the longitudinal direction.

  However, as shown in Patent Document 1, in a method called hot press for press-forming a heated steel plate, the steel plate is soft and highly ductile at a high temperature, so that a complicated shape is formed with high dimensional accuracy. Is possible. Furthermore, the steel sheet is heated in the austenite region and rapidly cooled (quenched) in the mold, so that high strength of the steel sheet by martensitic transformation can be achieved at the same time. Patent Document 2 discloses a pre-press quench method that simultaneously achieves high strength and formability of a steel sheet by forming it into a predetermined shape at room temperature, heating it to an austenite region, and rapidly cooling it in a mold. Has been.

Such a hot press method and a pre-press quench method are excellent molding methods that can ensure the high strength and formability of the member at the same time.
By the way, at present, the need for applied parts of hot press-molded products has increased, and in automobiles and the like, they have come to be used for complex shaped members such as door beams, center pillar reinforcements, and bumper reinforcements. Therefore, there is a demand for a steel plate that can secure a uniform hardness distribution after quenching even for a complicated shape member, and a steel plate that can austenite the steel structure of the steel plate by heating in a short time in terms of operation efficiency.

  In addition, hot press-molded products are being applied to steel materials used in severe corrosive environments, and are required to have better corrosion resistance. Therefore, excellent corrosion resistance is also required for hot press-formed products, and application of hot-dip galvanized steel sheets is becoming mainstream.

From the viewpoint of providing corrosion resistance to such a hot press-formed product, the present applicant previously described in Patent Document 3 that the surface layer of zinc or a zinc-based alloy provided with a barrier layer that prevents evaporation of zinc during heating. In Patent Document 4, C: 0.08 to 0.45% (in this specification, “%” means “mass% unless otherwise specified”. ), A steel plate containing 0.5 to 3.0% of Mn and / or Cr in total, and made of an Fe—Zn alloy having an Fe content of 5 to 80% and a Zn deposition amount of 10 The invention relating to a steel sheet for hot pressing having a Zn plating layer of ˜90 g / m ² is further applied in Patent Document 5 to a hot press-formed product in which an iron-zinc solid solution phase exists in the plating layer provided on the surface. Each such invention has been disclosed.

  These are very excellent inventions, but the plating may be peeled off during hot pressing after heating, resulting in poor quality due to indentation of plating powder and loss of operation time for mold cleaning, etc. was there. Therefore, improvement of plating adhesion at the time of hot pressing is desired.

  Moreover, examination about making the steel structure of a steel plate into austenite by short time heating has not been made sufficiently, and long time heating or high temperature heating has been adopted. For this reason, a large amount of a zinc oxide layer is formed on the upper part of the zinc layer during heating, resulting in a problem that the corrosion resistance is deteriorated.

British Patent No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2003-73774 A JP 2003-147499 A JP 2003-126921 A

  The present invention has been made in view of the above-described situation, and even for short-time heating, the steel sheet member after heat treatment has a uniform hardness distribution and excellent toughness, and also has excellent plating adhesion. It is to provide a galvanized steel sheet.

The present inventors have intensively studied to solve the above problems.
As a result, by optimizing the chemical composition of the steel sheet, component segregation near the surface and the surface shape, the steel structure is optimized, and the thickness of the hot dip galvanized layer is optimized, so that the hardness distribution in the steel sheet member after heat treatment is improved. We obtained new knowledge that a steel sheet for heat treatment that is uniform and excellent in toughness and has excellent plating adhesion and corrosion resistance can be obtained.

The present invention completed based on the above findings is as follows.
(1) A hot-dip galvanized steel sheet for heat treatment provided with a hot-dip galvanized layer on the surface of the steel sheet, the steel sheet being in mass%, C: 0.07% to 0.50%, Si: 0.005% 2.0% or less, Mn: 0.3% to 4.0%, P: 0.0002% to 0.2%, S: 0.01% or less, sol. Al: 0.0002% or more and 2.0% or less, N: 0.010% or less and Sn: 0.0002% 0.01% or less, has a Ru chemical composition Na balance being Fe and impurities, the surface of the steel sheet The average interval between the concentrated portions, which is the average interval in the direction perpendicular to the rolling direction, of the concentrated portions of Mn, Si and P expanded in the rolling direction at a depth of 50 μm is 1000 μm or less, and the depth at the surface of the steel sheet is The number density of cracks of 3 μm or more and 10 μm or less is 3 / mm or more and 1000 / mm or less, and the latest of cementite, pearlite, bainite, martensite and austenite at the position of ¼ depth of the plate thickness from the surface of the steel plate It has a steel structure whose hard phase average interval which is an average value of the contact distance is 30 μm or less, and the hot dip galvanized layer has a thickness of 3 μm or more and 20 μm or less. Heat treatment for hot-dip galvanized steel sheet to be.

  (2) The hot dip galvanized steel sheet for heat treatment according to (1), wherein the chemical composition further contains Bi: 0.5% by mass or less, and the concentrated portion average interval is 500 μm or less. .

(3) The chemical composition further contains Ti: 0.5% by mass or less, the hard phase average interval is within 20 μm, and the number density of TiN having a particle diameter of 3 μm or more is 500 pieces / mm ² or less. The hot-dip galvanized steel sheet for heat treatment as described in (1) or (2) above, which has a steel structure.

  (4) The chemical composition is% by mass, Nb: 1.0% or less, V: 1.0% or less, W: 1.0%, Cr: 1.0% or less, Mo: 1.0% or less The above (1), further comprising one or more selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less -The hot-dip galvanized steel sheet for heat treatment according to any one of (3) above.

  (5) The chemical composition is selected from the group consisting of REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less, and Zr: 0.05% or less in mass%. The hot-dip galvanized steel sheet for heat treatment according to any one of (1) to (4) above, further comprising one or more kinds.

  (6) The value of the ratio of the maximum value and the minimum value of the total content of Mn, Si and P at a position of a depth of ¼ of the plate thickness from the surface of the steel plate is 1.30 or less. The hot-dip galvanized steel sheet for heat treatment according to any one of (1) to (5) above.

(7) The hot-dip galvanized layer is an alloyed hot-dip galvanized layer, wherein the Fe concentration in the alloyed hot-dip galvanized layer is 7% by mass or more and the Al concentration is 0.5% by mass or less. The hot-dip galvanized steel sheet for heat treatment according to any one of ( 1 ) to ( 6 ) above .

(8) The method for producing a hot-dip galvanized steel sheet for heat treatment according to any one of (1) to (6) above, comprising the following steps (A) to (C):
(A) a soluble steel casting step of casting under conditions that the average cooling rate of temperature range is 10 ° C. / sec or more from the slab surface to the solidus temperature of the liquidus temperature at the depth position of 10 mm;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 780 ° C. or higher, and cooled at an average cooling rate of 2 ° C./second or higher, and 750 ° C. or lower. A hot rolling process in which a hot rolled steel sheet is wound in the temperature range;
(C) a pickling step in which a hot-rolled steel sheet obtained by the hot rolling step is subjected to a pickling treatment under conditions satisfying the following formula (i) to form a pickled steel plate; and (D) the pickling step A hot dip galvanizing process in which a pickled steel sheet obtained by the above is subjected to a hot dip galvanizing process to obtain a hot dip galvanized steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (i)

(9) The method for producing a hot-dip galvanized steel sheet for heat treatment as described in (7) above, comprising the following steps (A) to (D) and (G):
(A) A casting process in which molten steel is cast under the condition that the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface is 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 780 ° C. or higher, and cooled at an average cooling rate of 2 ° C./second or higher, and 750 ° C. or lower. A hot rolling process in which a hot rolled steel sheet is wound in the temperature range;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment under the conditions satisfying the following formula (1) to form a pickled steel sheet.
(D) a hot dip galvanizing step in which the pickled steel plate obtained by the pickling step is subjected to a hot dip galvanizing treatment to obtain a hot dip galvanized steel plate; and
(G) The alloying process process which gives an alloying process to the hot dip galvanized steel plate obtained by the said hot dip galvanizing process, and makes it an alloyed hot dip galvanized steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 200000
(I)
( 10 ) The method for producing a hot-dip galvanized steel sheet for heat treatment as described in (8) or (9) above, which comprises the following steps (E) and (F) instead of the step (D):
(E) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 90% or less to obtain a cold rolled steel plate; and (F) obtained by the cold rolling step. A hot dip galvanizing process in which a hot dip galvanized steel sheet is subjected to hot dip galvanizing treatment.

  (11) The method for producing a hot-dip galvanized steel sheet for heat treatment according to any one of (8) to (10), wherein in the step (A), the molten steel is stirred by a moving magnetic field and cast. .

  (12) In the step (A), the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position of a depth of 1/4 of the slab thickness from the slab surface is 0.2 ° C / The method for producing a hot-dip galvanized steel sheet for heat treatment according to any one of (8) to (11) above, wherein the casting is performed under conditions of at least 2 seconds.

  (13) In the step (B), the cast slab held at a temperature range of 1200 ° C. or higher for 8 hours or more is subjected to hot rolling, and is described in any one of (8) to (12) above A method for producing a hot-dip galvanized steel sheet for heat treatment.

  Here, the surface of the steel sheet is an interface between the steel sheet that is the plating base and the hot-dip galvanized layer, and usually a cross-section of the hot-dip galvanized steel sheet for heat treatment is reflected electrons (BSE image) using a scanning electron microscope. It can be discriminated from the difference in contrast observed with the. If the interface is unclear even in the case of reflected electrons (BSE image), the cross section of the hot dip galvanized steel sheet is analyzed by EDX for elements contained in the hot dip galvanized layer, such as Fe, Al, Zn, etc. The interface between the hot dip galvanized layer and the steel sheet is determined by setting the portion having a mass% or more as a steel plate and the portion having an Fe concentration of less than 70 mass% as a hot dip galvanized layer.

  Since the hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion and toughness, it is suitable as a forming material for heat-pressed steel sheets, particularly hot press-formed products that require corrosion resistance. Among them, it is most suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts. Moreover, since it can be manufactured at a low cost, it has a remarkable industrial effect.

Hereinafter, the hot-dip galvanized steel sheet for heat treatment and the manufacturing method thereof according to the present invention will be described.
In the embodiment of the present invention according to the following description, the heat treatment applied to the hot-dip galvanized steel sheet for heat treatment is a heat treatment in which the austenite region is heated and then rapidly cooled below the Ms point in the mold.

1. The chemical composition of a steel plate The reason for limitation of the chemical composition of the steel plate which is the plating base material of the hot dip galvanized steel plate for heat treatment of the present invention will be described. “%” For chemical composition means “mass%”.

C: 0.07% or more and 0.50% or less C is an element necessary for ensuring the strength of the steel sheet after heat treatment. If the C content is less than 0.07%, it becomes difficult to ensure a tensile strength of 980 MPa or more after the heat treatment. Therefore, the C content is 0.07% or more. In order to ensure a tensile strength of 1480 MPa or more after heat treatment, the C content is preferably 0.18% or more, and in order to ensure a tensile strength of 1780 MPa or more after heat treatment, the C content is 0.28% or more. It is preferable that On the other hand, when the C content exceeds 0.50%, the weldability is significantly deteriorated. Therefore, the C content is 0.50% or less.

Si: 0.005% or more and 2.0% or less Si has an effect of improving hardenability. If the Si content is less than 0.005%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.005% or more. On the other hand, if the Si content exceeds 2.0%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Si content is set to 2.0% or less.

Mn: 0.3% or more and 4.0% or less Mn has an effect of improving hardenability. If the Mn content is less than 0.3%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.3% or more. Preferably it is 0.5% or more. On the other hand, if the Mn content exceeds 4.0%, the effect of the above action is saturated, which is disadvantageous in cost. Therefore, the Mn content is 4.0% or less. Preferably it is 3.0% or less.

P: 0.0002% or more and 0.2% or less P has an effect of improving hardenability. When the P content is less than 0.0002%, it is difficult to obtain the effect by the above action. Therefore, the P content is 0.0002% or more. Preferably it is 0.004% or more. On the other hand, if the P content exceeds 0.2%, the deterioration of toughness due to P segregation to the grain boundaries becomes significant. Therefore, the P content is 0.2% or less. Preferably it is 0.05% or less.

S: 0.01% or less S is contained as an impurity, and has the effect of forming sulfides in steel and degrading toughness. If the S content exceeds 0.01%, the toughness is significantly lowered. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less. The lower the S content, the better. Therefore, it is not necessary to define the lower limit of the S content, but it is preferably 0.0002% or more from the viewpoint of steelmaking cost. More preferably, it is 0.0004% or more.

sol. Al: 0.0002% or more and 2.0% or less Al has an action of deoxidizing steel and making the steel plate sound. sol. If the Al content is less than 0.0002%, it is difficult to obtain the effect by the above action. Therefore, sol. The Al content is 0.0002% or more. Preferably it is 0.0004% or more. On the other hand, sol. If the Al content exceeds 2.0%, coarse alumina inclusions increase and the toughness deteriorates significantly. Therefore, sol. The Al content is 2.0% or less. Preferably it is 1.5% or less.

N: 0.010% or less N is contained as an impurity and has a function of forming nitrides in steel and degrading toughness. When the N content exceeds 0.010%, the toughness is significantly lowered. Therefore, the N content is 0.010% or less. Preferably it is 0.008% or less. The lower the N content, the better. Therefore, it is not necessary to define the lower limit of the N content, but from the viewpoint of steelmaking cost, it is preferably 0.0002% or more. More preferably, it is 0.0004% or more.

Sn: 0.0002% or more and 0.01% or less Sn is an extremely important element in the present invention because it has an effect of improving the plating adhesion of the hot dip galvanized layer. Sn is an element that is more difficult to oxidize than Fe, and at the same time, it is an element that easily segregates on the steel sheet surface layer, so it concentrates on the steel sheet surface layer and improves the plating adhesion of the hot dip galvanized layer. If Sn is less than 0.0002%, it is difficult to obtain the effect by the above-described action. Therefore, the Sn content is 0.0002% or more. Preferably it is 0.0004% or more, More preferably, it is 0.0005% or more. On the other hand, if the Sn content exceeds 0.01%, the hot workability is significantly lowered, and cracks may occur during hot working. Therefore, the Sn content is 0.01% or less.

Bi: 0.5% or less Bi serves as an inoculation nucleus for coagulation, has a function of reducing the interval between dendritic arms during coagulation and making the coagulated tissue fine. As a result, segregation of elements such as Mn, Si, and P that are likely to segregate is suppressed, the local hardness difference of the steel plate member after heat treatment is reduced, and the toughness of the steel plate member after heat treatment is improved. Therefore, it is preferable to contain Bi. However, since Bi forms an oxide that becomes a starting point of cracking in steel, if the Bi content exceeds 0.5%, the toughness may be deteriorated. Therefore, the Bi content is 0.5% or less. Preferably it is 0.03% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably set to 0.0002% or more. By doing so, a position having a depth of 50 μm from the surface of the steel sheet (hereinafter referred to as “depth position”). A ”)) in the direction perpendicular to the rolling direction of the enriched portion of Mn, Si and P expanded in the rolling direction, that is, in the width direction of the steel sheet (referred to as“ the perpendicular direction of rolling ”in the present invention). It can be more stably achieved that the average interval (also referred to as “concentrated portion average interval” in the present invention) is 500 μm or less. More preferably, it is 0.0003% or more. By doing in this way, it is possible to more stably achieve the concentration portion average interval of 300 μm or less.

Ti: 0.5% or less Ti has an action of finely dispersing hard phases such as cementite, pearlite, bainite, martensite, and austenite, so it suppresses hardness variation in the steel sheet member after heat treatment, and after heat treatment Even better toughness can be obtained in the steel plate member. Therefore, Ti may be included. However, if the Ti content exceeds 0.5%, a large amount of coarse crystallized TiN particles are formed, which may deteriorate the toughness of the steel sheet member after quenching. Therefore, the Ti content is 0.5% or less. By doing so, it is possible to more stably achieve the number density of TiN having a particle diameter of 3 μm or more (hereinafter referred to as “coarse TiN number density”) of 500 pieces / mm ² or less. Preferably it is 0.2% or less. In order to more reliably obtain the effect of the above action, the Ti content is preferably set to 0.003% or more. By doing in this way, it can achieve more stably that the below-mentioned hard phase average space | interval shall be 20 micrometers or less. More preferably, it is 0.01% or more.

Nb: 1.0% or less, V: 1.0% or less, W: 1.0%, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1 0.0% or less and B: one or more selected from the group consisting of 0.01% or less Nb, V, W, Cr, Mo, Cu, Ni, and B are hardened steel like Mn Has the effect of enhancing the sex. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content of Nb, V, W, Cr, Mo, Cu and Ni exceeds 1.0%, and the content of B exceeds 0.01%, the effect of the above action is saturated. This is disadvantageous in terms of cost. Accordingly, the contents of Nb, V, W, Cr, Mo, Cu, Ni, and B are as described above. In order to more surely obtain the effect of the above action, any element of Nb, V, W, Cr, Mo, Cu and Ni is made 0.005% or more, or the content of B is made 0.0002% or more. It is preferable that More preferably, the content of B is 0.0004% or more.

REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less and Zr: one or more selected from the group consisting of 0.05% or less REM (rare earth element), Mg, Ca and Zr have the effect of improving the toughness by finely spheroidizing oxides and sulfides formed in the steel. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content exceeds 0.1% for REM, and when the content exceeds 0.05% for Mg, Ca and Zr, the number of oxides and sulfides formed in the steel becomes excessive. Degradation of toughness. Therefore, the contents of REM (rare earth element), Mg, Ca and Zr are as described above. In order to more reliably obtain the effect of the above action, the content of any one of REM, Mg, Ca and Zr is preferably set to 0.0002% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

2. The concentrated part of a steel plate, surface shape, and steel structure The reason for limitation of the concentrated part of the steel plate which is a plating base material of the hot-dip galvanized steel sheet for heat treatment of this invention, and a steel structure is demonstrated.

(1) Average interval in the direction perpendicular to the rolling direction of the concentrated portions of Mn, Si, and P expanded in the rolling direction at a depth of 50 μm from the surface of the steel sheet (concentrated portion average interval): 1000 μm or less Concentrated portion average By setting the interval to 1000 μm or less, the hardness distribution of the steel plate member after the heat treatment is made uniform, and good toughness can be obtained in the steel plate member after the heat treatment. Here, the concentration part of Mn, Si and P is defined as a site where the concentration of at least one element of Mn, Si and P is 1.1 times or more of the bulk concentration.

  The method for obtaining the thickened part average interval is as follows. That is, the surface of the hot dip galvanized steel sheet is ground to expose the surface at the depth position A. EPMA line analysis is performed on the exposed surface in the direction perpendicular to the rolling direction. The measurement distance by one line analysis is preferably set to 3 mm or more so as to be able to cope with the case where the average interval of the concentrated portion is 1000 μm. For each of the line profiles of Si concentration, Mn concentration, and P concentration obtained by line analysis, the average concentration is obtained, and these concentrations are used as the bulk content. Regions in which the Si concentration, Mn concentration, or P concentration in the line profile is 1.1 times the average concentration are obtained, and these regions are defined as the thickening portions. A portion showing the maximum density in each region constituting the obtained thickened portion is set as a center point of the region. The distance between the center points of adjacent regions is obtained, and these are averaged within the line profile, and the obtained average value is taken as the thickened portion average interval.

  When the average interval between the thickened parts exceeds 1000 μm, the concentration of Mn, Si and / or P is unevenly generated, resulting in local variation in hardenability due to the concentration of components in the steel sheet. Variations occur in the hardness of the steel plate members. When a load is applied to such a steel plate member, cracking is likely to occur from a location where a hardness difference is caused by the hardness variation. For this reason, toughness deteriorates. Therefore, the thickened portion average interval is set to 1000 μm or less. Preferably it is 500 micrometers or less, More preferably, it is 300 micrometers or less. These concentrated part average intervals can be achieved more reliably by containing Bi as described above. The lower the concentrated portion average interval, the better, so the lower limit is not particularly specified. However, considering the case of casting a slab of about 30 mm to 350 mm, which is usually a slab thickness, it is practically preferable to be 3 μm or more from the viewpoint of the cooling rate. .

(2) Number density of cracks having a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet: 3 / mm or more and 1000 pieces / mm or less Number of cracks having a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet (hereinafter referred to as “ When the number of cracks is abbreviated as “number of cracks” is 3 / mm or more and 1000 / mm or less, good oxide scale adhesion can be achieved in the steel plate member during and after heat treatment.

  In addition to increasing the contact area between the hot dip galvanized layer and the steel sheet by forming appropriate cracks on the surface of the steel sheet, the anchoring effect of the hot dip galvanized layer entering the steel sheet is coupled with the plating adhesion. Improve dramatically.

  If the depth of the crack is less than 3 μm, the effect of improving the plating adhesion is small. On the other hand, if the crack depth exceeds 10 μm, when a load is applied to the steel plate member after heat treatment, the crack itself tends to develop into a crack, and the toughness deteriorates. Further, if the crack number density is less than 3 / mm, the effect of improving the plating adhesion may not be sufficiently obtained. On the other hand, when the crack number density exceeds 1000 / mm, when a load is applied to the steel plate member after the heat treatment, the cracks are easily connected to each other, and the possibility of developing into a large crack is increased, and the toughness is deteriorated. There is a case. Therefore, the crack number density with a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3 / mm or more and 1000 / mm or less.

By doing in this way, it can combine with the prescription | regulation of the thickening part average space | interval mentioned above, and can obtain the outstanding plating adhesiveness, after ensuring the outstanding toughness.
The crack number density may be measured as follows. That is, a cross section of the hot dip galvanized steel sheet is observed to identify a crack having a depth of 3 μm or more and 10 μm or less. Count the number of these cracks identified in the field of view. The surface of the steel sheet observed linearly in the observation image is linearly approximated, and the crack number density is obtained by dividing the number of cracks determined by the length of the observation field of the straight line.

(3) Average interval of hard phase in steel structure of steel sheet: 30 μm or less Average value of closest distance of cementite, pearlite, bainite, martensite and austenite at a position of ¼ depth of the plate thickness from the surface of the steel sheet (below) In the present invention, the hardness distribution of the steel plate member after the heat treatment is made uniform, and good toughness can be obtained in the steel plate member after the heat treatment. .

  What is necessary is just to obtain | require a hard phase average space | interval as follows. That is, the cross section parallel to the rolling direction of the steel sheet is subjected to night etching to obtain a cross section observation sample. About the obtained sample, a steel structure is observed using a scanning electron microscope. The measurement magnification is 1000 times, and 10 visual fields and 20 visual fields are observed at a position at a depth of 1/4 of the plate thickness from the surface of the steel plate (hereinafter referred to as “plate thickness 1/4 position”). Cementite, pearlite, bainite, martensite, and austenite (hereinafter collectively referred to as “hard phase” in the explanation of the hard phase average interval) are specified for all of the obtained 20-view steel structure images. For each of the identified hard phases, the distance from the closest other hard phase (hereinafter referred to as “closest distance”) is measured. By selecting the longest and shortest closest measured distances for each field of view, the 20 longest closest distances and the 20 shortest closest distances are determined. The arithmetic average value in the data of these 40 nearest neighbor distances is defined as the hard phase average interval.

  Phases and structures having a higher C content than ferrite, such as cementite, pearlite, bainite, and martensite, are transformed into austenite early in the heating step of the heat treatment. Therefore, a steel sheet having a narrow hard phase average interval has a dense region that transforms to austenite at an early stage, and does not have a wide region composed of a phase that is difficult to transform to austenite. Therefore, the austenitization of the entire steel sheet in the heating process of the heat treatment is completed early. And thereby, homogenization of C concentration in austenite is promoted, hardness variation in the steel plate member after heat treatment is suppressed, and good toughness can be obtained in the steel plate member after heat treatment.

When the average interval between the hard phases exceeds 30 μm, it is difficult to sufficiently suppress the hardness variation in the steel plate member after the heat treatment, and the toughness may deteriorate in the steel plate member after the heat treatment. Therefore, the hard phase average interval is 30 μm or less. Preferably it is 20 micrometers or less. This can be achieved more reliably by containing Ti as described above.
In addition, if the hard phase average interval is as described above, the area ratio and average particle diameter of ferrite need not be specified.

(4) Number density of TiN having a particle size of 3 μm or more (coarse TiN number density): 500 pieces / mm ² or less The coarse TiN number density is preferably 500 pieces / mm ² or less.
As described above, by containing Ti, it becomes possible to finely disperse hard phases such as cementite, pearlite, bainite, martensite, and austenite, and to obtain better toughness in the steel plate member after heat treatment. It becomes possible. However, when Ti is excessively contained, a large amount of coarse TiN particles are formed, and the coarse TiN and other steel structures are easily connected to voids generated at the interface, and the toughness may be deteriorated. Therefore, it is preferable to suppress the formation of coarse TiN particles by optimizing the Ti content. The reason why the threshold value of the particle size of TiN is 3 μm is that the particle size of fine Ti-based compounds that contribute to the refinement and strengthening of ferrite is generally several tens of nanometers, so the particle size greatly exceeds 1 μm. TiN of 3 μm or more does not contribute to strengthening of the steel sheet. On the other hand, when the particle size of TiN is 3 μm or more, voids generated at the interface between TiN and other steel structures have an effect on mechanical properties. This is because it becomes prominent.

When the coarse TiN number density exceeds 500 / mm ² , the toughness may be significantly deteriorated. Therefore, the coarse TiN number density is preferably 500 pieces / mm ² or less, more preferably 100 pieces / mm ² or less, and particularly preferably 50 pieces / mm ² or less.

(5) Ratio value of maximum value and minimum value of Mn + Si + P content at a position of 1/4 depth of the plate thickness from the surface of the steel plate: 1.30 or less 1/4 depth of the plate thickness from the surface of the steel plate The ratio of the maximum value and the minimum value of Mn + Si + P content (maximum value / minimum value, hereinafter also referred to as “segregation ratio”) at the position (plate thickness 1/4 position) is 1.30 or less. It is preferable.

  In order to obtain even better toughness for the steel sheet member after heat treatment, in addition to suppressing segregation of Mn, Si and P near the surface of the steel sheet, suppressing segregation of Mn, Si and P inside the steel sheet. Is preferred. By setting the value of the segregation ratio, which is an index of segregation of Mn, Si and P inside the steel plate, to 1.30 or less, the hardness variation inside the steel plate is also reduced for the steel plate member after heat treatment. Propagation of cracks in the steel plate is suppressed. For this reason, the toughness of the steel plate member after the heat treatment is further improved.

3. Hot-dip galvanized layer The reason for limitation of the hot-dip galvanized layer that is the plated layer of the hot-dip galvanized steel sheet for heat treatment of the present invention will be described.

(1) Thickness of hot dip galvanized layer: 3 μm or more and 20 μm or less If the thickness of the hot dip galvanized layer is less than 3 μm, it is difficult to ensure sufficient corrosion resistance. Accordingly, the thickness of the hot dip galvanized layer is 3 μm or more. Preferably it is 4 micrometers or more. On the other hand, when the thickness of the hot dip galvanized layer exceeds 20 μm, the adhesiveness of the hot dip galvanized layer is significantly reduced. Therefore, the thickness of the hot dip galvanized layer is set to 20 μm or less. Preferably it is 15 micrometers or less.

(2) Fe concentration of alloyed hot dip galvanized layer: 7% by mass or more and Al concentration: 0.5% by weight or less Alloyed hot dip galvanized layer by alloying after hot dip galvanizing treatment This is preferable because the plating adhesion is further improved. When the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the Fe concentration in the hot-dip galvanized layer is preferably 7% by mass or more and the Al concentration is preferably 0.5% by mass or less.

  By setting the Fe concentration in the alloyed hot-dip galvanized layer to 7% by mass or more, it is possible to more reliably obtain an effect of improving plating adhesion by alloying. More preferably, it is 8 mass% or more. Although the upper limit of the Fe concentration is not particularly defined, it is preferable to set the Fe concentration to 25% by mass or less because powdering during flattening or blanking of a steel sheet can be suppressed. More preferably, it is 20 mass% or less.

  Also, by setting the Al concentration in the alloyed hot dip galvanized layer to 0.5 mass% or less, Al concentration at the interface between the steel sheet and the alloyed hot dip galvanized layer can be suppressed, and plating peeling is likely to occur. This is preferable because the interface area of the Al concentrated portion can be reduced. Although the lower limit of the Al concentration is not particularly specified, Al in the alloyed hot-dip galvanized layer exhibits an action of suppressing exfoliation of the alloyed hot-dip galvanized layer if the amount is very small. It is preferable to do.

  In the alloyed hot-dip galvanized layer, alloying elements such as Si, Mn, P, and S may be taken in from the steel sheet as the plating base material during the alloying treatment, but under normal conditions. As long as it is within a range that can be incorporated into the alloyed hot-dip galvanized layer during hot-dip plating and alloying, the plating quality is not adversely affected.

4). Manufacturing method Next, a preferable manufacturing method of the hot-dip galvanized steel sheet for heat treatment of the present invention will be described.
(1) Casting process The molten steel having the above chemical composition is subjected to an average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface (hereinafter referred to as “10 mm depth cooling”). It is preferable to perform casting under the condition of “speed”) of 10 ° C./second or more.

The above 10 mm depth cooling rate greatly affects the segregation of Mn, Si and P.
When the 10 mm depth cooling rate is less than 10 ° C./second, the cooling rate is too slow, so that the dendrite arm interval in the slab is widened, and it becomes difficult to set the above-described concentration portion average interval to 1000 μm or less. Therefore, the average cooling rate is preferably 10 ° C./second or more.

  As described above, when Bi is contained, it is possible to more stably achieve the concentrated portion average interval of 500 μm or less in combination with the effect of making the solidified structure fine by Bi. Further, when the molten steel is stirred by the moving magnetic field in the casting process, the distance between the dendrite arms can be further narrowed, so that it is possible to more stably achieve the concentrated part average distance of 300 μm or less.

When Ti is contained in an amount of 0.5% or less, the average cooling rate within the temperature range from the liquidus temperature to the solidus temperature at the position of the slab thickness 1/4 depth from the slab surface (below) , “1/4 depth cooling rate”) is preferably cast under the condition of 0.2 ° C./second or more. TiN is segregated between dendrid trees in the same manner as MnS and P, and TiN crystallizes in the dendrid trees. Therefore, coarse TiN crystallization can be suppressed by increasing the overall cooling rate of the slab. As an index of the overall cooling rate of the slab, the number density of TiN having a particle size of 3 μm or more (coarse TiN) is obtained by casting under strong cooling conditions with a 1/4 depth cooling rate of 0.2 ° C./second or more. The number density) can be more reliably realized to be 500 pieces / mm ² or less.

The 10 mm depth cooling rate and the 1/4 depth cooling rate are specifically determined by the following method. That is, the cross section of the obtained slab is etched with picric acid, and the dendrite 2 is placed at a pitch of 5 mm in the casting direction at a position of a depth of 10 mm from the surface of the slab and a position at a depth of 1/4 of the thickness of the slab. The next arm interval λ (μm) is measured at 100 points. Then, the cooling rate A (° C./second) in the temperature range from the liquidus temperature to the solidus temperature of the slab is calculated from the measured λ value based on the following equation. The obtained cooling rate A is arithmetically averaged, and these average values are set as a 10 mm depth cooling rate and a 1/4 depth cooling rate, respectively.
λ = 710 × A ^−0.39

(2) Hot rolling step The slab obtained by the above casting step is subjected to hot rolling, and hot rolling is completed at a temperature range of 780 ° C or higher, and cooled at an average cooling rate of 2 ° C / second or higher. It is preferable to wind in a temperature range of 750 ° C. or lower to obtain a hot rolled steel sheet.

  When the hot rolling completion temperature is less than 780 ° C., there is a concern that coarse ferrite is generated due to ferrite region rolling. Therefore, it becomes difficult to make the above-mentioned hard phase average interval stably 30 μm or less. Therefore, the hot rolling completion temperature is preferably 780 ° C. or higher. The upper limit of the hot rolling completion temperature is not particularly defined, but it is preferably set to 1050 ° C. or higher in order to prevent the formation of scale and the occurrence of surface flaws due to scale biting.

  When the average cooling rate after completion of hot rolling is less than 2 ° C./second or the coiling temperature exceeds 750 ° C., ferrite is excessively generated, and it becomes difficult to stably set the average hard phase interval to 30 μm or less. Therefore, the average cooling rate from completion of hot rolling to winding is preferably 2 ° C./second or more, and the winding temperature is preferably 750 ° C. or less. The upper limit of the average cooling rate from completion of hot rolling to winding is not particularly required, but is preferably 200 ° C./second or less from the viewpoint of securing a good flat shape. Further, the lower limit of the coiling temperature does not need to be specified in particular and may be room temperature.

  The slab to be subjected to hot rolling is preferably held at a temperature range of 1200 ° C. or higher for 8 hours or longer. By doing in this way, elements, such as Mn, Si, and P which segregated between the dendrite trees inside the slab in the casting process, can be diffused again, and segregation of these elements can be reduced. By holding the slab in the above temperature range for 8 hours or more, it is possible to more stably realize the segregation ratio value of 1.30 or less.

  There is no particular upper limit on the temperature at which the slab for hot rolling is held. It is preferable to set it as 1300 degrees C or less from a viewpoint of refractory life, such as a heating furnace. Moreover, the upper limit of the time for holding the slab for hot rolling is not particularly specified. From the viewpoint of productivity, it is preferably 60 hours or less.

(3) Pickling process It is preferable that the hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling process under the conditions satisfying the following formula (1) to obtain a pickled steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1)

Cracks on the surface of the steel sheet are formed by selective oxidation of grain boundaries of the steel structure by pickling. When the value of acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) is less than 5000, selective oxidation of the grain boundary portion of the steel structure becomes insufficient, and the above crack number density is stably 3 It becomes difficult to make the number of pieces / mm or more. On the other hand, when the value of acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) exceeds 2000000, the selective oxidation of the grain boundary portion of the steel structure proceeds excessively, and the crack number density is reduced. It becomes difficult to stably set it to 1000 pieces / mm or less.
In addition, the kind of acid is not specifically limited, Hydrochloric acid and a sulfuric acid are illustrated.

(4) Cold rolling process The above pickled steel sheet may be subjected to hot dip galvanizing as it is to obtain a hot dip galvanized steel sheet for heat treatment. It may be a hot dip galvanized steel sheet. When cold rolling is performed, the pickled steel sheet obtained by the pickling step is subjected to cold rolling at a reduction rate of 90% or less to obtain a cold rolled steel sheet.

  If the rolling reduction of cold rolling exceeds 90%, the rolling reduction is too high, and there is a concern that the cracks formed by pickling will disappear. Therefore, when cold rolling is performed, the rolling reduction is preferably 90% or less.

(5) Hot-dip galvanizing step The hot-dip galvanized steel plate for heat treatment is applied to the pickled steel plate obtained by the pickling step or the cold-rolled steel plate obtained by the cold rolling step. . An alloying treatment after the hot dip galvanizing treatment may be performed. The temperature of the hot dip galvanizing bath is not limited. From the viewpoint of productivity, the melting point of zinc or zinc alloy in the hot dip galvanizing bath is preferably set to the melting point or more (the melting point + 200 ° C.) or less.

When the alloying treatment is performed, it is preferable that after the hot dip galvanizing treatment is performed, the surface temperature of the hot dip galvanized steel sheet is maintained at 470 ° C. or higher and 680 ° C. or lower for 1 second or longer and 40 seconds or shorter. By setting the surface temperature of the hot-dip galvanized steel sheet to 470 ° C. or higher, the alloying reaction can be promoted efficiently, which is preferable from the viewpoint of productivity. More preferably, it is 500 degreeC or more. Further, it is preferable that the surface temperature of the hot dip galvanized steel sheet is 680 ° C. or lower because the alloying reaction can be easily controlled and the Fe concentration can be adjusted with high accuracy. By setting the alloying treatment temperature to 470 ° C. or more and 680 ° C. or less and the alloying treatment time to be 1 second or more and 40 seconds or less, the Fe concentration of the alloying hot-dip galvanized layer may be 7% by mass or more and 25% by mass or less. it can. Further, the Al concentration of the galvannealed layer can be adjusted to 0.5 mass% or less by adjusting the Al concentration in the hot dip galvanizing bath.
Note that the hot dip galvanized steel sheet after the hot dip galvanizing treatment or the alloying treatment may be subjected to temper rolling with a temper rolling roll.

(6) Annealing The hot dip galvanized steel sheet for heat treatment obtained by the above method may be annealed after hot rolling or cold rolling and before hot dip galvanizing. However, if annealing is performed under a condition where the steel structure is held for 60 hours or more in a temperature range where the austenite and ferrite are in a two-phase region (hereinafter referred to as “two-phase region long-term holding condition”), ferrite is excessively generated. , It may be difficult to make the average interval between the hard phases 30 μm or less. Therefore, when annealing is performed after hot rolling or after cold rolling, it is preferable that the heating and holding conditions in annealing are not the above two-phase region long-term holding conditions. Other than these conditions, for example, even if the cooling rate is changed, the above-mentioned hard phase average interval is not affected. Therefore, it is not necessary to specify other annealing conditions.

Specific examples of the present invention will be described below.
1. Preparation of test materials Steels having the chemical components shown in Table 1 were melted in a converter and subjected to continuous casting using a continuous casting tester to obtain a slab having a width of 1000 mm and a thickness of 250 mm. The 10 mm depth cooling rate was changed by changing the cooling water amount of the mold. The quarter depth cooling rate was changed by changing the amount of water in the secondary spray of the continuous casting tester. Some slabs were subjected to electromagnetic stirring by a moving magnetic field in the mold.

  The slab thus obtained was heated and hot-rolled with a hot rolling tester to obtain a hot-rolled steel sheet, and then pickled with hydrochloric acid to obtain a pickled steel sheet. Some slabs were subjected to heat treatment (hereinafter referred to as “homogenization treatment”) held at 1250 ° C. for 24 hours before being subjected to hot rolling. Further, some pickled steel sheets were cold-rolled to obtain cold-rolled steel sheets. The pickled steel plate and cold-rolled steel plate thus obtained were subjected to annealing and hot dip galvanizing using a continuous hot dip galvanizing tester. The conditions for annealing and hot dip galvanizing treatment are as follows: hold at 780 ° C. for 60 seconds, cool to 520 ° C. at a cooling rate of 7 ° C./second, hold for 60 seconds, and immerse in a hot dip galvanizing bath at 470 ° C. did. The thickness of the plating was 2 to 25 μm, and the Al concentration of the hot dip galvanized layer was changed by changing the Al concentration of the hot dip galvanizing bath. Moreover, about the one part hot-dip galvanized steel plate, the alloying process hold | maintained for 30 second at 600 degreeC was performed after the hot-dip galvanization process.

  These production conditions are shown in Tables 2 and 3.

  The steel plate thus obtained was subjected to hot pressing as a heat treatment using a hot press test apparatus. In hot pressing, a steel sheet is brought to a steel sheet surface temperature of 900 ° C. in a heating furnace, held at that temperature for 150 seconds, removed from the heating furnace, and immediately pressed with a mold with a cooling device, and molded. At the same time, quenching was performed. The shape of the steel plate member after hot pressing (hereinafter referred to as “hot pressed steel plate member”) was a flat plate. The size of the test piece for hot pressing is 2.6 mm for a hot-rolled steel plate as a plating base, 1.4 mm for a cold-rolled steel plate as a plating base, and a width of 300 mm × The length was 80 mm.

2. Evaluation method (1) Average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the slab The position of the slab (slab) at a depth of 10 mm from the slab surface and a quarter depth of the slab thickness The average cooling rate (10 mm depth cooling rate, 1/4 depth cooling rate) in the temperature range from the liquidus temperature to the solidus temperature is obtained by etching the cross section of the obtained slab with picric acid, and the slab surface From the position of the depth of 10 mm and the position of the quarter depth of the slab thickness, 100 intervals of the dendrite secondary arm interval were measured at a pitch of 5 mm in the casting direction. Based on these values, the cooling rate A (° C./second) in the temperature range from the liquidus temperature to the solidus temperature is calculated based on the following equation, and the average value is obtained by arithmetic averaging, The average cooling rate was determined.
λ = 710 × A ^−0.39

(2) Thickened part average interval The concentrated part average interval was measured by EPMA line analysis. That is, it was ground from the surface of the steel plate (interface between the steel plate and the hot-dip galvanized layer) to a depth of 50 μm, and EPMA line analysis was performed. The average spacing in the direction perpendicular to the rolling direction of the concentrated portions of Mn, Si and P expanded in the rolling direction is 1.1 times the average concentration value by reading the waveform of the Mn, Si and P concentrations obtained from the line analysis. It calculated | required from the space | interval of the concentration maximum value which is the above.

  Specifically, the method for measuring the thickened portion average interval was as follows. That is, the surface of the steel plate was ground to expose the surface at the depth position A. The exposed surface was subjected to EPMA line analysis in the direction perpendicular to the rolling direction. The measurement distance by one line analysis was set to 3 mm or more so that the average thickness of the thickened portion was 1000 μm. For each of the line profiles of Si concentration, Mn concentration, and P concentration obtained by line analysis, an average concentration was obtained, and this concentration was used as a bulk content. Regions in which the Si concentration, Mn concentration, or P concentration in the line profile is 1.1 times the average concentration were obtained, and these regions were designated as the thickening portions. The portion showing the maximum density in each of the regions constituting the obtained thickened portion was defined as the center point of that region. The distance between the center points of adjacent regions was determined, averaged within the line profile, and the average value obtained was defined as the thickened portion average interval.

(3) Segregation ratio The total content of Mn, Si and P necessary for obtaining the segregation ratio was measured by EPMA line analysis. That is, in the same manner as the above-described method for obtaining the concentrated portion average interval, the 1/4 position of the plate thickness is exposed, and EPMA line analysis is performed on the exposed surface, and each of the Mn concentration, Si concentration, and P concentration is measured. A line profile was determined. By summing up the obtained line profiles, a line profile of the total content of Mn, Si and P was obtained. The maximum value and the minimum value in the line profile of this total content were determined, and the segregation ratio (maximum value / minimum value) was determined from these numerical values.

(4) Evaluation of steel structure About the cross section parallel to the rolling direction of a steel plate, the night etching was given and the steel structure of the obtained cross section was observed using the scanning electron microscope. The measurement magnification was 1000 times, and 10 visual fields (20 visual fields in total) were observed at the 1/4 position of the plate thickness. The average interval between the hard phases was determined from the obtained steel structure image. That is, cementite, pearlite, bainite, martensite, and austenite (hereinafter collectively referred to as “hard phase etc.” in the description of the steel structure evaluation method) were specified for all of the obtained 20-view steel structure images. . The closest distance was measured for each of the identified hard phases. By selecting the longest and shortest closest distances measured for each steel structure image, 20 longest closest distances and 20 shortest closest distances were obtained. The arithmetic average value in the data of the 40 closest distances in total was defined as the hard phase average interval.

(5) Crack Number Density The crack number density on the surface of the steel sheet was measured by observing 100 fields at a magnification of 2000 using a scanning electron microscope for the cross section parallel to the rolling direction of the steel sheet, and converted to the number per unit length. And asked. Specifically, the cross section of the steel sheet was observed to identify a crack having a depth of 3 μm or more and 10 μm or less. The number of these cracks identified in the observation field was counted. The surface of the steel plate observed linearly in the observation image was linearly approximated, and the crack number density was obtained by dividing the number of cracks counted by the length in the observation field of the straight line.

(6) Coarse TiN
The average particle size of TiN and the number density of TiN of 3 μm or more (coarse TiN number density) based on the measurement of the average particle size are 200 times the cross section of the obtained steel sheet with a scanning electron microscope at a magnification of 2000 times. The field of view was photographed and calculated by image processing. The particle diameter was obtained as a converted diameter (circle converted diameter) when the cross sectional area of TiN obtained by image processing was obtained and the cross sectional shape was assumed to be a circle.

(7) Mechanical properties A) Tensile test A tensile test was performed on the obtained hot-dip galvanized steel sheet.
A JIS No. 5 tensile test was taken from the direction perpendicular to the rolling of each steel plate. The test method conformed to JIS Z2241. Tensile strength (TS) was measured.

B) Evaluation of hot-pressed steel sheet member The hardness variation, toughness, and plating adhesion of the hot-pressed steel sheet member were evaluated by the following methods.

<Hardness variation of hot-pressed steel sheet member>
The hot pressed steel sheet member was ground from the surface to a depth of 50 μm, and the hardness of the ground surface was measured with a Vickers hardness meter. The measurement load was 98 kN. The measuring method was based on JIS Z2244. This hardness measurement was carried out 50 times in total while moving at a pitch of 200 μm in the width direction of the test piece. A difference (ΔHv) in Vickers hardness was determined from the maximum value and the minimum value of the 50 Vickers hardness values (Hv) thus obtained for each member.

<Toughness of hot pressed steel sheet member>
A hot pressed steel plate member is attached to a three-point bending jig, cooled with a cooling medium, and a falling weight test is performed at 9.80 × 10 ² Nm (100 kgf · m) for a cold-rolled steel plate as a plating base. And what used the hot-rolled steel plate as the plating base was carried out at 1.96 × 10 ³ Nm (200 kgf · m). The upper limit of the temperature at which brittle cracking occurs (hereinafter referred to as “brittle cracking temperature”) was measured.

<Plating adhesion of hot pressed steel plate member>
The plating adhesion of hot pressed steel sheet members was investigated. An adhesive tape (cello tape (registered trademark) manufactured by Nichiban Co., Ltd.) was cut to a length of 200 mm, applied onto a steel plate member, and then peeled off to measure the peel weight of the plating.

<Tensile properties of hot-pressed steel sheet members>
A JIS No. 13B tensile test was collected from the obtained hot-pressed steel sheet member. The test method conformed to JIS Z2241. Tensile strength (TS) was measured.

(8) Hot-dip galvanized layer The hot-dip galvanized steel sheet was cut in parallel to the rolling direction, and the thickness of the hot-dip galvanized layer was determined by observing the cross section obtained using a scanning electron microscope. The measurement magnification was 500 times, the thickness of N = 20 was measured for each sample, the average thickness was obtained by arithmetic calculation, and the value was used as the thickness of the hot-dip plating layer.
As for the chemical composition of the hot dip galvanized layer, the hot dip galvanized layer was dissolved in a solution obtained by dissolving inhibit in 10% concentration hydrochloric acid, and the Fe concentration and Al concentration were investigated by atomic absorption method.

(9) Corrosion resistance after coating The obtained hot-pressed steel sheet member was subjected to zinc phosphate treatment under normal chemical treatment conditions with PBL-3080 manufactured by Nihon Parkerizing Co., Ltd., and then electrodeposited paint GT manufactured by Kansai Paint Co., Ltd. -10 was electrodeposited by applying a slope of voltage 200V, and baked for 20 minutes at a baking temperature of 150 ° C. The coating thickness was 20 μm. After scratching scratches reaching the substrate with a cutter knife in the coating film of the test piece, a salt spray test specified in JIS Z2371 was conducted for 480 hours. The film swelling width or rust width from the scratch was measured, and the corrosion resistance after coating was evaluated. The evaluation standard is a larger value of the rust width and the coating film swelling width, and 0 mm or more to less than 4 mm is good: evaluation symbol ○, 4 mm or more is bad: evaluation symbol x.

3. Evaluation Results Tables 4 and 5 show the results of the above evaluation tests. In Tables 1 to 5, numerical values indicating the chemical composition, manufacturing conditions, structure characteristics, and plating layer thickness are underlined, indicating that they are out of the scope of the present invention.

(1) Present invention Test material No. which is the present invention. In Nos. 1 to 30, since the average concentrated portion interval was 1000 μm or less, the hardness variation of the hot-pressed steel sheet member was 25 Hv or less, the brittle crack initiation temperature was −40 ° C. or less, and the toughness was excellent. Moreover, the plating peeling weight was 240 mg or less, and the plating adhesion was excellent.

  Among them, test material No. 1 containing Ti. 2, 3, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 22, 23, 24, 25, 26, and 27 have a hard phase average interval of 20 μm or less. Therefore, the variation in hardness of the hot-pressed steel sheet member was 20 Hv or less, the three-point bending brittle crack generation temperature was −50 ° C. or less, and the toughness was further excellent.

  Sample No. containing Bi Nos. 19 to 30 had a thick portion average interval of 500 μm or less, so that the hardness variation of the hot pressed steel sheet member was 15 Hv or less, the brittle crack initiation temperature was −60 ° C. or less, and the toughness was further excellent. .

  No. containing both Bi and Ti. Nos. 22 to 27 have a hard phase average interval of 20 μm or less and a concentrated portion average interval of 500 μm or less, so that the hardness variation of the hot-pressed steel sheet member is 10 Hv or less and the brittle crack occurrence temperature is −70 ° C. The toughness was particularly excellent.

  Among the test materials containing Bi and subjected to electromagnetic stirring in the mold, No. Nos. 24, 27 and 29 have an average interval of the thickened portion of 300 μm or less, so that the hardness variation of the hot-pressed steel sheet member is 5 Hv or less, the brittle cracking temperature is −80 ° C. or less, and the toughness is outstanding. It was excellent.

  No. which performed the homogenization process. Since 2, 4, 19, 25 and 26 have an uneven distribution ratio of 1.30 or less, the hardness variation of the hot-pressed steel sheet member is 3 Hv or less, the brittle crack initiation temperature is −100 ° C. or less, and the toughness is It was the best.

(2) Comparative Example Sample No. Since No. 31 had a 10 mm cooling rate of 8 ° C./second, the average interval between the thickened parts was 1020 μm. Therefore, the hardness variation of the hot-pressed steel sheet member was 80 Hv, and the brittle crack initiation temperature was inferior to -10 ° C. and toughness.

Test material No. containing Ti No. 32 had a 1/4 depth cooling rate of 0.1 ° C./second, so the coarse TiN number density was 550 / mm ² . Therefore, the brittle crack initiation temperature was inferior to -10 ° C. and toughness.

  Specimen No. Since No. 33 had a hot rolling completion temperature of 760 ° C., ferrite formation was promoted in the steel structure of the steel sheet, and as a result, the average interval between the hard phases became 35 μm. Therefore, the hardness variation of the hot-pressed steel sheet member was 70 Hv, and the brittle crack initiation temperature was -10 ° C., which was inferior in toughness.

  Specimen No. No. 34 had an average cooling rate of 1 ° C./second from completion of hot rolling to winding, so that ferrite formation was promoted and the average interval of hard phases became 32 μm. Therefore, the hardness variation of the hot-pressed steel sheet member was 70 Hv, and the brittle crack initiation temperature was -10 ° C., which was inferior in toughness.

  Specimen No. Since the coiling temperature of No. 35 was 760 ° C., the formation of ferrite was promoted, and the average interval between the hard phases was 32 μm. Therefore, the hardness variation of the hot-pressed steel sheet member was 70 Hv, and the brittle crack initiation temperature was -10 ° C., which was inferior in toughness.

  Specimen No. Since 36 had a value of acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) of 4800, the crack number density was 2 / mm. Therefore, the plating peeling amount was 1000 mg and the scale adhesion was inferior.

  Specimen No. No. 37 had an acid concentration (mass%) × acid temperature (° C.) × acid immersion time (seconds) of 2002000, and thus the crack number density was 1020 / mm. Therefore, the brittle crack initiation temperature was inferior to -10 ° C. and toughness.

  Specimen No. No. 38 had a cold rolling reduction of 92%, so the crack number density was 2 / mm. Therefore, the plating peel weight was inferior to 1100 mg and scale adhesion.

Specimen No. In No. 39, since the thickness of the hot dip galvanized layer was 2 μm, the swollen width of the coating film from the scratched part was 4 mm or more, and the corrosion resistance after coating was poor.
Specimen No. In No. 40, since the thickness of the hot dip galvanized layer was 22 μm, the plating peeling amount was 1100 mg, which was inferior to the plating adhesion.

  Specimen No. No. 41 had an Sn content of 0.0001%, so the plating peel-off amount was 1000 mg, which was poor in plating adhesion.

Claims (13)

A hot-dip galvanized steel sheet for heat treatment comprising a hot-dip galvanized layer on the surface of the steel sheet,
The steel plate
In mass%, C: 0.07% to 0.50%, Si: 0.005% to 2.0%, Mn: 0.3% to 4.0%, P: 0.0002% or more 0.2% or less, S: 0.01% or less, sol. Al: 0.0002% or more and 2.0% or less, N: 0.010% or less and Sn: 0.0002% 0.01% or less, has a Ru chemical composition Na balance being Fe and impurities,
The concentrated portion average interval which is the average interval in the direction perpendicular to the rolling direction of the concentrated portion of Mn, Si and P expanded in the rolling direction at a depth of 50 μm from the surface of the steel sheet is 1000 μm or less,
The number density of cracks with a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3 pieces / mm or more and 1000 pieces / mm or less,
The steel structure has a hard phase average interval of 30 μm or less, which is an average value of closest distances of cementite, pearlite, bainite, martensite, and austenite at a position of ¼ depth of the plate thickness from the surface of the steel plate,
The hot-dip galvanized steel sheet for heat treatment, wherein the hot-dip galvanized layer has a thickness of 3 μm or more and 20 μm or less.   2. The hot-dip galvanized steel sheet for heat treatment according to claim 1, wherein the chemical composition further contains Bi: 0.5% by mass or less, and the concentrated portion average interval is 500 μm or less. A steel structure in which the chemical composition further contains Ti: 0.5 mass% or less, the hard phase average interval is within 20 μm, and the number density of TiN having a particle diameter of 3 μm or more is 500 pieces / mm ² or less. The hot-dip galvanized steel sheet for heat treatment according to claim 1 or 2, characterized by comprising:   The chemical composition is mass%, Nb: 1.0% or less, V: 1.0% or less, W: 1.0%, Cr: 1.0% or less, Mo: 1.0% or less, Cu: It further contains 1 type (s) or 2 or more types selected from the group which consists of 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less. The hot-dip galvanized steel sheet for heat treatment as described in any of the above.   The chemical composition is one type selected from the group consisting of REM: 0.1% or less, Mg: 0.05% or less, Ca: 0.05% or less, and Zr: 0.05% or less in terms of mass%. The hot-dip galvanized steel sheet for heat treatment according to any one of claims 1 to 4, further comprising two or more kinds.   The ratio value of the maximum value and the minimum value of the total content of Mn, Si and P at a position of a depth of ¼ of the plate thickness from the surface of the steel plate is 1.30 or less. The hot-dip galvanized steel sheet for heat treatment according to any one of claims 1 to 5.   The hot-dip galvanized layer is an alloyed hot-dip galvanized layer, and the Fe concentration in the alloyed hot-dip galvanized layer is 7 mass% or more and the Al concentration is 0.5 mass% or less. The hot-dip galvanized steel sheet for heat treatment according to any one of claims 1 to 6. It has the following process (A)-( D ), The manufacturing method of the hot-dip galvanized steel plate for heat processing in any one of Claims 1-6 characterized by the following:
(A) a soluble steel casting step of casting under conditions that the average cooling rate of temperature range is 10 ° C. / sec or more from the slab surface to the solidus temperature of the liquidus temperature at the depth position of 10 mm;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 780 ° C. or higher, and cooled at an average cooling rate of 2 ° C./second or higher, and 750 ° C. or lower. A hot rolling process in which a hot rolled steel sheet is wound in the temperature range;
(C) a pickling step in which the hot-rolled steel sheet obtained by the hot rolling step is subjected to a pickling treatment under conditions satisfying the following formula (1) to form a pickled steel plate; and (D) the pickling step A hot dip galvanizing process in which a pickled steel sheet obtained by the above is subjected to a hot dip galvanizing process to obtain a hot dip galvanized steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 2000000 (1) Under Symbol Step (A) ~ (D) and manufacturing method of the heat treatment for hot-dip galvanized steel sheet according to claim 7, characterized in that it has a (G):
(A) A casting process in which molten steel is cast under the condition that the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface is 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to hot rolling, and hot rolling is completed in a temperature range of 780 ° C. or higher, and cooled at an average cooling rate of 2 ° C./second or higher, and 750 ° C. or lower. A hot rolling process in which a hot rolled steel sheet is wound in the temperature range;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment under conditions satisfying the following formula (1) to form a pickled steel sheet;
(D) a hot-dip galvanized steel sheet obtained by subjecting the pickled steel sheet obtained by the pickling process to a hot-dip galvanized steel sheet; and (G) a hot-dip galvanized steel sheet obtained by the hot-dip galvanized steel sheet. An alloying process for forming an alloyed hot-dip galvanized steel sheet.
5000 ≦ acid concentration (% by mass) × acid temperature (° C.) × acid immersion time (seconds) ≦ 200000
(1) It replaces with the said process (D), and has the following process (E) and (F), The manufacturing method of the hot dip galvanized steel sheet for heat processing of Claim 8 or Claim 9 characterized by the above-mentioned:
(E) a cold rolling step in which the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 90% or less to obtain a cold rolled steel plate; and (F) obtained by the cold rolling step. A hot dip galvanizing process in which a hot dip galvanized steel sheet is subjected to hot dip galvanizing treatment.   The method for producing a hot-dip galvanized steel sheet for heat treatment according to any one of claims 8 to 10, wherein in the step (A), the molten steel is stirred by a moving magnetic field and cast.   In the step (A), the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position at a depth of 1/4 of the slab thickness from the slab surface is 0.2 ° C./second or more. The method for producing a hot-dip galvanized steel sheet for heat treatment according to any one of claims 8 to 11, wherein the casting is performed under the following conditions.   The hot-dip galvanizing for heat treatment according to any one of claims 8 to 12, wherein in the step (B), the slab held at a temperature range of 1200 ° C or higher for 8 hours or more is subjected to hot rolling. A method of manufacturing a steel sheet.