JP6589903B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot dip galvanized steel sheet and a method for producing the same. The present invention is particularly used for underbody members such as lower arms and frames of automobiles, skeleton members such as pillars and members and their reinforcing members, door impact beams, seat members, vending machines, desks, home appliances / OA devices, building materials, etc. The present invention relates to a high-strength hot-dip galvanized hot-rolled steel sheet excellent in warm formability, which is optimal for structural members and the like, and a method for producing the same.

In recent years, in response to increasing interest in the global environment, there is an increasing demand for reducing the amount of steel sheets that have a large CO ₂ emission during production. In addition, in the automobile field and the like, there is an increasing need for reducing the exhaust gas while improving the fuel efficiency by reducing the body. Therefore, the thinning of the steel plate is progressing by application of a high strength steel plate. On the other hand, since the formability and shape freezing property of a steel sheet are deteriorated due to an increase in strength of the steel sheet, application studies on warm forming in which the steel sheet is heated and pressed are also progressing. Here, the bare material generates scale due to heating, and the scale breaks and scatters during warm forming, so the working environment is remarkably deteriorated. There are problems such as deterioration, drawability and the like rather than cold working using lubricating oil. On the other hand, the hot-dip galvanized material is highly demanded for hot-dip galvanized steel sheets with excellent warm formability because scale formation during heating is suppressed and the plated layer has a lubricating action.

  Conventionally, as a hot dip galvanized steel sheet excellent in warm formability, for example, in Patent Document 1, in mass%, C: 0.1 to 0.3%, Si: 0.5 to 2.5%, Mn: 1.5-3.5%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005-0. The steel structure has an area ratio, the polygonal ferrite phase is 40% or more, the bainite phase is 5% or more, and the residual austenite phase is 3%. A thin steel sheet for warm forming, which is excellent in formability and strength increasing capability, and a method for producing the same is disclosed. Further, in Patent Document 2, in mass%, C: 0.05 to 0.3%, Si: 1 to 3%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, with the balance being a component composition composed of iron and impurities, Nitic ferrite: 50 to 90%, retained austenite: 3% or more, martensite + the above retained austenite: 10 to 50%, ferrite: 40% or less, the retained austenite has a C concentration of 0 A high-strength steel sheet having excellent formability, in which 0.3% or more of the retained austenite is surrounded by martensite, and its manufacturing technology are disclosed. Furthermore, Patent Document 3 includes mass%, C: 0.02 to 0.20%, Si: 1.5% or less, Mn: 0.50 to 3.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.01 to 0.5%, N: 0.005% or less, and one or two of Ti and Nb in total 0.10 to 0.50% A hot-rolled steel sheet suitable for warm forming and containing the remainder, which consists of Fe and inevitable impurities, and in which 60% or more of the total content of Ti and Nb is in a solid solution state, and a method for producing the same are disclosed. Further, in Patent Document 4, in mass%, C: 0.01 to 0.2%, Si: 0.5% or less, Mn: 2% or less, P: 0.03% or less, S: 0.01 %: Al: 0.07% or less, N: 0.01% or less, Ti: 0.005-0.3%, Nb: 0.005-0.6%, V: 0.005 -1.0%, Mo: 0.005-0.5%, W: 0.01-1.0%, B: One or more selected from 0.0005-0.0040% And the composition comprising the balance Fe and inevitable impurities, and the test temperature: obtained by a tensile test conducted at 400 ° C. or higher. The maximum load from the start of tension obtained in a tensile test that was larger than the deformation amount before the test and obtained at a test temperature of less than 400 ° C. A tensile property in which the amount of deformation before slag is 40% or more in a ratio to the total amount of deformation from the start of tension to fracture, and a substantially single phase matrix of ferrite phase in which the area ratio of the ferrite phase is 95% or more A high-strength steel sheet excellent in warm workability having a structure in which an alloy carbide having a size of less than 10 nm is dispersed and precipitated in a state in which no variant is selected in the matrix and a method for producing the same are disclosed. Further, in Patent Document 5, C: 0.03% or more and 0.14% or less, Si: 0.3% or less, Mn: more than 0.60%, 1.8% or less, and P: 0.03% by mass. 03% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.005% or less, Ti: 0.25% or less, W: 0.01% or more and 1.0% or less The balance consists of Fe and inevitable impurities, ([Ti] / 48 + [V] / 51 + [Mo] / 96 + [W] / 184)> 0.0031 and 0.8 ≦ ([C] / 12 ) / ([Ti] / 48 + [V] / 51 + [Mo] / 96 + [W] / 184) ≦ 1.20, the ferrite grain size is 1 μm or more, and the ferrite phase area ratio is Having a matrix of 95% or more, and carbides having an average particle size of 10 nm or less precipitated in the matrix A steel sheet having a weave, a tensile strength at room temperature of 780 MPa or more, a yield stress in a heating temperature range of 400 ° C. or more and 700 ° C. or less of 80% or less of a yield stress at room temperature, The elongation is 1.1 times or more of the total elongation at room temperature, and the yield stress after cooling from the heating temperature to room temperature after heating the heating temperature range to give a strain of 20% or less is the room temperature before the heating. 70% or more of the yield stress at 70 ° C., and after applying a strain of 20% or less by heating to the heating temperature range, the total elongation after cooling from the heating temperature to room temperature is 70% of the total elongation at room temperature before the heating. % Or more of a high-strength steel sheet for warm forming and a method for producing the same.

JP 2012-92358 A JP2012-122130A JP 2006-161139 A Japanese Patent No. 5609223 Japanese Patent No. 5754279

  However, the techniques described in Patent Document 1 and Patent Document 2 have a problem that the heating temperature at the time of warm pressing can be increased only to about 400 to 450 ° C., and the shape freezing property is insufficient. In addition, the technique described in Patent Document 3 has a problem that the structure is mainly bainite and the moldability is insufficient, although there is a load reduction during pressing due to warm forming. Furthermore, the techniques described in Patent Document 4 and Patent Document 5 have a problem that minute cracks are generated on the surface of the steel sheet during pressing.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-dip galvanized steel sheet that is superior in warm formability and a manufacturing method thereof.

The present invention has been made as a result of intensive studies to solve the above problems, and has the following configuration.
[1] By mass%, C: 0.08 to 0.20%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.10% or less, S: 0.030 %: Al: 0.10% or less, N: 0.010% or less, Ti: 0.01-0.3%, Nb: 0.01-0.1%, V: 0.01- Containing 1.0% of one or two or more of C ^* determined by the following formula (1) to be 0.07 or more, the balance Fe and inevitable impurities;
The total of the ferrite phase and the tempered bainite phase is 95% or more by area ratio, and the average grain size of the structure of the surface layer is 5.0 μm or less, and further Ti deposited as a precipitate having a grain size of less than 20 nm, A structure in which the precipitation amount of Nb and V is 0.025 mass% or more as the precipitation C equivalent amount obtained by the following formula (2), and the solid solution C amount of the surface layer is 0.005 mass% or more. Hot-dip galvanized steel sheet characterized by
C ^* = (Ti / 48 + Nb / 93 + V / 51) × 12 (1)
However, each element symbol in the formula (1) represents the content (% by mass) of each element.
([Ti] / 48 + [Nb] / 93 + [V] / 51) × 12 (2)
However, [Ti], [Nb], and [V] in the formula (2) represent the respective precipitation amounts (mass%) of Ti, Nb, and V deposited as precipitates having a particle size of less than 20 nm.
[2] In addition to the above composition, in addition, by mass%, Mo: 0.005 to 0.50%, Ta: 0.005 to 0.50%, W: 0.005 to 0.50% or The hot-dip galvanized steel sheet according to [1], containing two or more kinds.
[3] In addition to the above-mentioned composition, further, by mass%, Cr: 0.01-1.0%, Ni: 0.01-1.0%, Cu: 0.01-1.0% The hot-dip galvanized steel sheet according to [1] or [2], containing two or more kinds.
[4] In addition to the above composition, the composition further contains one or two of Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.01% by mass%. The hot-dip galvanized steel sheet according to any one of [1] to [3].
[5] The hot-dip galvanized steel sheet according to any one of [1] to [4], further containing, in addition to the composition, Sb: 0.005 to 0.050% by mass.
[6] The hot dip galvanized steel sheet according to any one of [1] to [5], further containing B: 0.0005 to 0.0030% by mass% in addition to the composition.
[7] A steel having the composition described in any one of [1] to [6] is cast into a slab, and the slab is reheated to 1200 ° C. or higher after being cast or after being cooled once. , Rough rolling is performed, and after finishing the rough rolling, descaling is performed so that the collision pressure of the high-pressure water is (1 + Si) MPa or higher before finishing rolling, and then the finish rolling exit temperature is set to 950 ° C. or lower and 850 ° C. or higher. After rolling and finish rolling, the temperature range from the finish rolling exit temperature to 650 ° C. is cooled at an average cooling rate of 30 ° C./s or more, the winding temperature is set to 350 ° C. or more and 600 ° C. or less, and pickling is performed. After that, annealing is performed with the dew point of the atmosphere at 550 ° C. or higher being −25 ° C. or lower, a soaking temperature of 650 to 770 ° C., and a soaking time of 10 to 300 s, and after annealing, zinc plating at 420 to 500 ° C. After immersed in were melt galvanizing method for producing a molten zinc plated steel sheet characterized by cooling at a temperature range of 400-200 ° C. The average cooling rate of 1 ° C. / s or higher. However, Si in the said collision pressure is content (mass%) of Si.
[8] After dip galvanizing by immersing in the galvanizing bath at 420 to 500 ° C., after reheating to 460 to 600 ° C. and holding for 1 s or more, the temperature range from 400 to 200 ° C. is average cooling rate 1 The method for producing a hot-dip galvanized steel sheet according to [7], wherein cooling is performed at a temperature of ° C / s or more.
[9] The temperature range of 400 to 200 ° C. is cooled at an average cooling rate of 1 ° C./s or more, and then the thickness is reduced to 0.1 to 3.0%. 7] or the method for producing a hot-dip galvanized steel sheet according to [8].

  Although the warm formability is improved by the present invention, and the mechanism that can prevent the occurrence of microcracks particularly during warm forming is not necessarily clear, it can be considered as follows. In other words, microcracks are generated when the steel sheet before warm forming is heated and the galvanizing melts and penetrates into the grain boundaries, and then the alloying of the plating proceeds to become a Γ solid phase, which occurs from the grain boundary plating phase during warm forming. However, the generation of microcracks during warm forming can be suppressed by suppressing grain boundary penetration of the galvanizing during heating of the steel sheet by refining the structure of the surface layer and strengthening the grain boundaries by solid solution C.

  The steel sheets targeted by the present invention are hot-dip galvanized hot-rolled steel sheets and galvannealed hot-rolled steel sheets. Furthermore, the steel plate which formed the film | membrane by chemical conversion etc. on it is also included. Moreover, the heating temperature at the time of target warm forming is about 500-800 degreeC.

The hot dip galvanized steel sheet of the present invention is superior in warm formability.
The hot-dip galvanized steel sheet according to the present invention is excellent in suppressing the occurrence of microcracks during warm forming.
According to the present invention, when hot rolling a steel slab with controlled amounts of C, Si, Mn, P, S, Al, N, and Ti, Nb, V, the descaling impact pressure, rolling temperature, and Control the cooling rate and coiling temperature after rolling, further anneal and plate, and control the dew point and soaking temperature, soaking time, and cooling rate of the atmosphere when cooling, the particle size Precipitates less than 20 nm can be deposited, the structure of the surface layer and solid solution C can be controlled, high strength, and excellent resistance to cracking during warm forming, suitable for warm forming A galvanized hot-rolled steel sheet can be obtained, and an industrially effective effect is brought about.

FIG. 1 is a diagram showing a hat part warm-pressed in an example. FIG. 2 is a photograph showing a crack generated in the surface layer of a hat part by warm press molding. FIG. 3 is a diagram showing the relationship between the descaling collision pressure and the amount of solid solution C in the surface layer. FIG. 4 is a view showing the relationship between the dew point and the amount of dissolved C in the surface layer. FIG. 5 is a diagram showing the relationship between the amount of dissolved C in the surface layer and the crack depth. FIG. 6 is a diagram showing the relationship between the average grain size of the surface layer structure and the crack depth.

Hereinafter, the present invention will be specifically described.
First, the component composition of the hot dip galvanized steel sheet according to the present invention will be described. In the following, the unit “%” of content means “% by mass” unless otherwise specified.

[Ingredient composition]
C: 0.08 to 0.20%
C forms fine carbides with Ti, Nb, and V, contributes to the improvement of strength, and does not decrease the strength by heating during warm forming, and therefore can provide an effect as a steel sheet for warm forming. In addition, the solid solution C at the steel grain surface layer boundary can prevent the occurrence of microcracks during warm forming. Therefore, the C content needs to be 0.08% or more. On the other hand, a large amount of C promotes martensitic transformation and suppresses the formation of fine carbides with Ti, Nb, and V. Further, excessive C reduces weldability and greatly reduces toughness and moldability. Therefore, the C content needs to be 0.20% or less. The content of C is preferably 0.15% or less, more preferably 0.12% or less.

Si: 0.5% or less Si forms an oxide on the steel sheet surface and causes non-plating. Furthermore, by promoting ferrite transformation, the crystal grain size of the structure is also increased. Therefore, the Si content needs to be 0.5% or less. The Si content is preferably 0.2% or less, more preferably 0.1% or less, and even more preferably 0.05% or less. Although the lower limit of the Si content is not particularly specified, there is no problem even if 0.005% is contained as an inevitable impurity.

Mn: 0.8 to 1.8%
Mn delays ferrite transformation, reduces the crystal grain size, and contributes to higher strength through solid solution strengthening. In order to obtain such an effect, the Mn content needs to be 0.8% or more. The Mn content is preferably 1.0% or more. On the other hand, a large amount of Mn causes slab cracking and promotes martensitic transformation. Therefore, the Mn content needs to be 1.8% or less. The Mn content is preferably 1.5% or less.

P: 0.10% or less P decreases weldability and segregates at grain boundaries to deteriorate ductility, bendability and toughness. When added in a large amount, the ferrite grain transformation is promoted to increase the crystal grain size. Therefore, the P content needs to be 0.10% or less. The content of P is preferably 0.05% or less, more preferably 0.03% or less, and still more preferably 0.01% or less. The lower limit of the content of P is not particularly specified, but there is no problem even if 0.005% is contained as an inevitable impurity.

S: 0.030% or less S lowers the weldability and remarkably lowers the hot ductility, thereby inducing hot cracking and significantly deteriorating the surface properties. Further, S hardly contributes to the strength, but also reduces the ductility, bendability and stretch flangeability by forming coarse sulfides as impurity elements. These problems become significant when the S content exceeds 0.030%, and it is desirable to reduce the S content as much as possible. Therefore, the S content needs to be 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.003% or less, and still more preferably 0.001% or less. Although the lower limit of the S content is not particularly specified, there is no problem even if 0.0001% is contained as an inevitable impurity.

Al: 0.10% or less When a large amount of Al is added, the crystal grain size is also increased by promoting ferrite transformation. Furthermore, an Al oxide is generated on the surface to cause non-plating. Therefore, the Al content needs to be 0.10% or less. The Al content is preferably 0.06% or less. Although the lower limit of the Al content is not particularly specified, there is no problem even if 0.01% is contained as Al killed steel.

N: 0.010% or less N forms coarse nitrides at high temperatures with Ti, Nb, V and does not contribute much to the strength, so the effect of increasing the strength by adding Ti, Nb, V is reduced. In addition, the toughness is also reduced. If it is further contained in a large amount, surface cracks may occur due to slab cracking during hot rolling. Therefore, the N content needs to be 0.010% or less. The N content is preferably 0.005% or less, more preferably 0.003% or less, and still more preferably 0.002% or less. Although the lower limit of the N content is not particularly specified, there is no problem even if 0.0005% is included as an inevitable impurity.

One or two or more of Ti: 0.01 to 0.3%, Nb: 0.01 to 0.1%, V: 0.01 to 1.0%, C ^* = (Ti / 48 + Nb / 93 + V / 51) × 12 ≧ 0.07
Ti, Nb, and V form fine carbides with C, contributing to finer structure and higher strength. In order to obtain such an effect, the content of at least one of Ti, Nb, and V is set to 0.01% or more, and the content of Ti, Nb, and V is calculated by the following formula (1) C ^* Needs to be 0.07 or more. On the other hand, even if Ti, Nb, and V are added in large amounts exceeding 0.3%, 0.1%, and 1.0%, respectively, the effect of increasing the strength is not so great, but precipitation with a particle size of less than 20 nm Since the product (fine precipitates) precipitates in a large amount and the toughness decreases, the upper limit of the content of Ti, Nb, and V must be 0.3%, 0.1%, and 1.0%, respectively. .
C ^* = (Ti / 48 + Nb / 93 + V / 51) × 12 (1)
However, each element symbol in the formula (1) represents the content (% by mass) of each element. The element not contained is 0.

  The balance is Fe and inevitable impurities. In the present invention, the following elements can be added for the purpose of further improving the strength and warm formability.

One or more of Mo: 0.005-0.50%, Ta: 0.005-0.50%, W: 0.005-0.50% Mo, Ta, W are finely precipitated with C It contributes to finer structure and higher strength by forming the structure. In order to acquire such an effect, when adding Mo, Ta, and W, it is preferable to add 0.005% or more of at least one of Mo, Ta, and W. On the other hand, even if a large amount of Mo, Ta, W is added, the effect of increasing the strength is not so great, but a large amount of fine precipitates are precipitated and the toughness is lowered. The content of Mo, Ta, W is preferably 0.50% or less.

One or more of Cr: 0.01-1.0%, Ni: 0.01-1.0%, Cu: 0.01-1.0% Cr, Ni, Cu is fine-grained It contributes to high strength by acting as a solid solution strengthening element. In order to acquire such an effect, when adding Cr, Ni, and Cu, it is preferable to add 0.01% or more of at least one of Cr, Ni, and Cu. On the other hand, the addition of a large amount of Cr, Ni, Cu not only saturates the effect, but also inhibits the plating properties. Therefore, when adding Cr, Ni, Cu, the content of Cr, Ni, Cu should be It is preferable to make each 1.0% or less.

1 type or 2 types of Ca: 0.0005-0.01%, REM: 0.0005-0.01% Ca, REM can improve ductility and toughness by controlling the form of sulfide. . In order to obtain such an effect, when adding Ca and REM, it is preferable to add at least one of Ca and REM in an amount of 0.0005% or more. On the other hand, since ductility may be adversely affected by the addition of a large amount of Ca and REM, when Ca and REM are added, the content of Ca and REM is preferably 0.01% or less, respectively. .

Sb: 0.005 to 0.050%
Since Sb segregates on the surface during hot rolling, the formation of coarse nitrides can be suppressed by preventing the slab from nitriding. In order to obtain such an effect, when adding Sb, it is preferable to add 0.005% or more of Sb. On the other hand, the addition of a large amount of Sb not only saturates the effect but also degrades the workability. Therefore, when Sb is added, the Sb content is preferably 0.050% or less.

B: 0.0005 to 0.0030%
Since B contributes to the refinement of the structure, when B is contained, the content of B is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, since a large amount of B may increase the rolling load during hot rolling, when B is contained, the content of B is preferably 0.0030% or less. % Or less is more preferable.
In addition, there is no problem in characteristics even if impurities such as Sn, Mg, Co, As, Pb, Zn, and O are included in total of 0.5% or less.

  Next, the structure of the hot dip galvanized steel sheet of the present invention will be described.

The total of ferrite phase and tempered bainite phase is 95% or more in area ratio. Since the ferrite phase and tempered bainite phase are excellent in cold and warm ductility, the total of ferrite phase and tempered bainite phase is expressed in area ratio. It is necessary to be 95% or more. The total of the ferrite phase and the tempered bainite phase is preferably 98% or more, more preferably 100% in terms of area ratio.

Average particle size of surface layer structure: 5.0 μm or less If the average particle size of the surface layer is large, the galvanization melts when the steel plate is heated before warm forming and the penetration depth penetrates into the grain boundary, which occurs during warm forming. Therefore, the average grain size of the surface layer structure (the average crystal grain size of the entire surface layer structure) needs to be 5.0 μm or less. The average particle size of the surface layer structure is preferably 3.0 μm or less.

Precipitation C equivalent amount of Ti, Nb, and V deposited as a precipitate having a particle size of less than 20 nm: A precipitate having a particle size of 0.025 mass% or more and less than 20 nm contributes to strength. Furthermore, it contributes to strength even after heating in warm forming. Thereby, it contributes also to the effect which suppresses the strength reduction after the said heating. In order to obtain such an effect, it is necessary that the precipitation amount of Ti, Nb, and V deposited as a precipitate having a particle size of less than 20 nm be 0.025% by mass or more in terms of the precipitation C equivalent obtained by the following equation (2). is there. The amount of precipitation C is preferably 0.035% by mass or more. On the other hand, the upper limit of the amount of precipitation C is not particularly specified, but the toughness is lowered when the number of precipitates having a particle size of less than 20 nm increases, so the amount of precipitation C is preferably 0.10% by mass or less. 08 mass% or less is more preferable, and 0.05 mass% or less is further more preferable.
([Ti] / 48 + [Nb] / 93 + [V] / 51) × 12 (2)
However, [Ti], [Nb], and [V] in the formula (2) represent the respective precipitation amounts (mass%) of Ti, Nb, and V deposited as precipitates having a particle size of less than 20 nm.

Surface solid solution C amount: 0.005 mass% or more If the surface layer solid solution C amount is small, the galvanization melts when the steel plate is heated before warm forming and enters the grain boundary, and microcracks occur during warm forming. Resulting in. Therefore, the amount of C in the surface layer needs to be 0.005% by mass or more. The amount of C in the surface layer is preferably 0.010% by mass or more. The upper limit of the amount of solid solution C in the surface layer is not particularly defined, but the effect is saturated even if the amount of solid solution C in the surface layer is excessively large, so about 0.030% by mass is sufficient.

  The TS of the hot dip galvanized steel sheet of the present invention is not particularly defined, but is preferably 980 MPa or more. Although the plate thickness is not particularly defined, it is preferably 4.0 mm or less, more preferably 3.0 mm or less, still more preferably 2.0 mm or less, and even more preferably 1.5 mm or less. The lower limit of the plate thickness may be about 1.0 mm that can be manufactured by hot rolling.

  Next, production conditions for the hot dip galvanized steel sheet of the present invention will be described. In the following description, the temperature is the surface temperature of a steel plate or the like.

In the present invention, the starting material is a steel material (slab) obtained by casting steel having the above-described composition.
The method for producing the starting material is not particularly limited. For example, the molten steel having the above composition is melted by a conventional melting method such as a converter, and the steel material (slab) is obtained by a normal casting method such as a continuous casting method. And the like.

Slab: In order to precipitate Ti, Nb, V finely as it is after casting or after cooling once to 1200 ° C. or higher, it is necessary to make solid precipitates precipitated in the slab before starting rolling. There is. Therefore, the slab after casting is transferred as it is (high temperature) to the entry side of the hot rolling mill and rough rolling is started, or once cooled, it becomes a hot piece or a cold piece, and Ti, Nb and V are precipitated. It is necessary to start rough rolling after reheating the slab deposited as a product to 1200 ° C. or higher. The holding time at 1200 ° C. or higher is not particularly defined, but is preferably 10 minutes or longer, more preferably 30 minutes or longer. The reheating temperature is preferably 1220 ° C. or higher, more preferably 1250 ° C. or higher.

Descaling with high-pressure water is performed before finishing rolling (on the finishing mill entry side) after rough rolling with high-pressure water having a collision pressure (1 + Si) MPa or higher before finishing rolling. At this time, if the descaling collision pressure due to the high-pressure water is small, the surface roughness increases, and the decarburization of the surface layer proceeds by promoting the scale generation during hot rolling. Since the progress of decarburization due to such an increase in surface roughness becomes more significant as the amount of Si increases, the collision pressure needs to be (1 + Si) MPa or more. The collision pressure is preferably (2 + Si) MPa or more, more preferably (3 + Si) MPa or more. The upper limit of the collision pressure is not particularly defined, but about 5 MPa is sufficient because the steel sheet is excessively cooled when the collision pressure increases. In addition, Si in the said collision pressure is content (mass%) of Si.

Finishing rolling delivery temperature: 950 ° C. or lower 850 ° C. or higher Next, finish rolling is performed. At this time, if the exit temperature of finish rolling is lowered, carbides of Ti, Nb, and V are coarsely precipitated due to strain-induced precipitation. Therefore, the finish rolling exit temperature (temperature at the finish final rolling exit) needs to be 850 ° C. or higher. The finish rolling exit temperature is preferably 880 ° C. or higher. On the other hand, when the finish rolling exit temperature increases, the crystal grains become coarse. For this reason, the finish rolling delivery temperature needs to be 950 ° C. or lower. The finish rolling exit temperature is preferably 930 ° C. or lower.

Average cooling rate in the temperature range from the finish rolling exit temperature to 650 ° C .: 30 ° C./s or more After the finish rolling, if the cooling rate in the temperature range from the finish rolling exit temperature to 650 ° C. is small, the ferrite transformation is high As the crystal grain size increases, Ti, Nb, and V carbides precipitate coarsely. Therefore, the average cooling rate in the temperature range from the finish rolling exit temperature to 650 ° C. needs to be 30 ° C./s or more. The average cooling rate is preferably 50 ° C./s or more, more preferably 80 ° C./s or more. The upper limit of the average cooling rate is not particularly defined, but about 200 ° C./s is sufficient from the viewpoint of temperature control.

Winding temperature: 350 ° C. or higher and 600 ° C. or lower When the winding temperature is high, the crystal grain size becomes coarse. Therefore, the winding temperature needs to be 600 ° C. or less. The winding temperature is preferably 550 ° C. or lower. On the other hand, when the coiling temperature is low, bainite transformation is suppressed and martensitic transformation is promoted. Therefore, the winding temperature needs to be 350 ° C. or higher. The winding temperature is preferably 400 ° C. or higher.

  Next, the hot-rolled coil after winding is pickled and then annealed.

Dew point of atmosphere at 550 ° C. or higher: −25 ° C. or lower When the atmospheric dew point is high during steel plate annealing, decarburization at the steel sheet surface layer proceeds. Since such an effect becomes significant when the steel plate temperature is 550 ° C. or higher, the dew point of the atmosphere when the steel plate temperature is 550 ° C. or higher needs to be −25 ° C. or lower. The dew point is preferably −30 ° C. or lower, more preferably −35 ° C. or lower. Although the lower limit of the dew point is not specified, the effect is saturated even if the dew point is lowered too much, so about -50 ° C is sufficient. The atmosphere at the dew point is maintained until the pickled steel sheet is heated to 550 ° C. or higher, soaked at the following soaking temperature, and then cooled to a temperature of less than 550 ° C. The average heating rate from the steel plate temperature 550 ° C. to the soaking temperature is preferably 0.5 to 20 ° C./s.

Soaking temperature: If the soaking temperature during annealing in the temperature range of 650 to 770 ° C. is low, fine carbides of Ti, Nb, and V do not precipitate, so the soaking temperature needs to be 650 ° C. or higher. The soaking temperature is preferably 700 ° C. or higher, more preferably 730 ° C. or higher. On the other hand, if the soaking temperature becomes too high, the carbides of Ti, Nb, and V are coarsened, and austenite transformation occurs during soaking, and bainite and martensite transformation proceeds by subsequent cooling. Furthermore, since the decarburization at the surface layer of the steel sheet also proceeds, the soaking temperature needs to be 770 ° C. or lower.

Soaking time (residence time in soaking temperature range): 10 to 300 s
When the soaking time at the time of soaking is short, Ti, Nb, and V carbides are not sufficiently precipitated. Therefore, the soaking time at the time of soaking needs to be 10 s or more. The soaking time is preferably 30 s or more. On the other hand, when the soaking time becomes longer, the carbides of Ti, Nb, and V become coarser and the crystal grain size also becomes larger. Therefore, the soaking time needs to be 300 s or less. The soaking time is preferably 150 s or less.

  After annealing, it is immersed in a galvanizing bath at 420 to 500 ° C. to perform hot dip galvanization, and then cooled.

When the cooling rate after immersion in the cooling galvanizing bath is small at an average cooling rate of 1 ° C./s or more in a temperature range of 400 to 200 ° C., precipitation of cementite is promoted and the amount of solid solution C in the surface layer is reduced. Since precipitation of such cementite becomes significant at 400 to 200 ° C., the region of 400 to 200 ° C. needs to be cooled at an average cooling rate of 1 ° C./s or more. The average cooling rate is preferably 5 ° C./s or more. The upper limit of the average cooling rate is not particularly defined, but about 30 ° C./s is sufficient because the effect is saturated even if the cooling rate is increased.

  In addition, it is good also as an galvannealed steel plate by reheating to 460-600 degreeC after a galvanization bath immersion, and hold | maintaining for 1 second or more. The holding time is preferably 1 to 10 s.

  Further, the dislocation may be increased by adding light processing to the plated steel sheet to improve the formability. Examples of such light processing include processing for reducing the plate thickness reduction rate to 0.1% or more. The plate thickness reduction rate is preferably 0.3% or more. On the other hand, when the plate thickness reduction rate is large, dislocations are less likely to move due to the interaction of dislocations, and formability is reduced. Therefore, when applying such processing, the plate thickness reduction rate is 3.0% or less. Preferably, it is 2.0% or less, more preferably 1.0% or less. Here, when performing the said process, the reduction by a rolling roll may be added and the process by the tension | tensile_strength which added the tension | tensile_strength to the steel plate may be performed. Furthermore, both rolling and tensioning may be performed.

Examples of the present invention will be described.
The steel having the composition shown in Table 1 is continuously cast into a slab, reheated to 1220 ° C., then subjected to rough rolling, and then descaling with high-pressure water under the conditions shown in Table 2, followed by finishing. Rolling, cooling, and winding are performed to form a hot rolled coil, pickled, annealed, dipped in a 470 ° C. zinc plating bath, plated, and then subjected to specimen no. 1-18 hot-dip galvanized steel sheets were obtained. Further, some of the specimens were subjected to reheating treatment and plate thickness reduction rate shown in Table 2 after plating. In Table 2, “-” in the columns of reheating temperature, holding time, and plate thickness reduction rate indicates that the treatment is not performed.

  A specimen was collected from the specimen and subjected to precipitate measurement, structure observation, tensile test, and warm formability evaluation test. The test method was as follows.

(Equivalent amount of deposited C of Ti, Nb, and V deposited as a precipitate having a particle size of less than 20 nm)
As shown in Japanese Patent No. 4737278, the amounts of Ti, Nb, and V having a particle diameter of less than 20 nm are 10% AA electrolyte (10 Constant current electrolysis in a volume% acetylacetone-1 mass% tetramethylammonium chloride-methanol electrolyte solution) After dissolving a certain amount of this test piece for electrolysis, deposits adhered to the surface of the test piece for electrolysis are dispersed. The dispersion liquid ultrasonically peeled therein was filtered using a filter having a pore diameter of 20 nm, and then the amounts of Ti, Nb, and V in the obtained filtrate were determined by ICP emission spectroscopic analysis. Since all the deposits of Ti, Nb, and V adhere to the surface of the electrolysis test piece, all the precipitates of Ti, Nb, and V are dispersed in the dispersion. Then, assuming that all the precipitates of Ti, Nb, and V are carbides, the amounts of precipitation (mass%) of Ti, Nb, and V precipitated as precipitates having a particle diameter of less than 20 nm are expressed as [Ti], [Nb ], [V], the value calculated from ([Ti] / 48 + [Nb] / 93 + [V] / 51) × 12 is the value of Ti, Nb, and V deposited as precipitates having a particle size of less than 20 nm. The amount corresponding to precipitation C was used.

(Solution amount of C on the surface layer)
The solid solution C amount of the surface layer was determined as a value obtained by subtracting the precipitated C amount of the sample surface layer from the total C amount of the surface layer determined by emission spectroscopic analysis (countback) from the sample surface from which the plating of the test piece was peeled with hydrochloric acid. . Here, the amount of precipitated C on the surface layer was determined as follows. First, 0.2 g of a sample having a thickness of 20 × 20 mm obtained by peeling the plating of the test piece with hydrochloric acid as an anode was dissolved by constant current electrolysis in a 10% AA-based electrolytic solution, and then the extraction residue obtained by electrolysis Is filtered using a filter having a pore diameter of 0.2 μm to collect Fe precipitates, and then the collected Fe precipitates are dissolved with a mixed acid, and then Fe is quantified by ICP emission spectroscopic analysis. The amount of Fe in the precipitate was calculated. Since Fe precipitates are aggregated, it is possible to collect Fe precipitates having a particle size of less than 0.2 μm by performing filtration using a filter having a pore size of 0.2 μm. Furthermore, constant current electrolysis was performed in a 10% AA-based electrolyte using a sample of plate thickness × 20 × 20 mm obtained by peeling the test piece plating with hydrochloric acid as an anode, and 0.2 g of this sample was dissolved, The amount of Ti, Nb, and V in the dispersion liquid obtained by ultrasonically peeling the deposit adhering to the dispersion liquid was determined by ICP emission spectroscopic analysis. Then, from the precipitation amounts of Fe, Ti, Nb, and V thus obtained, it is assumed that Fe is precipitated as cementite, Ti, Nb, and V as carbides ((Fe) / 167 + [Ti] / 48 + [Nb ] / 93 + [V] / 51) × 12 (wherein [Fe], [Ti], [Nb] and [V] are the precipitation amounts (mass%) of Fe, Ti, Nb and V, respectively) Was the amount of precipitated C on the surface layer. And the value which subtracted the said precipitation C amount from the total C amount of the said surface layer was made into the solid solution C amount of a surface layer.

(Tissue observation)
The area ratio of the ferrite phase and the tempered bainite phase is determined by embedding and polishing the cross-section in the rolling direction-thickness direction of the specimen for structure observation taken from the test piece, and after the nital corrosion, the thickness is measured with a scanning electron microscope (SEM). Three photographs of a 100 × 100 μm region with a magnification of 1000 × centered on the ¼ part were taken, and the SEM photographs were obtained by image processing. Furthermore, the average particle size of the structure of the surface layer (surface layer 10 μm position excluding plating) is measured by embedding and polishing the rolling direction-thickness direction cross section of the structure observation specimen taken from the specimen, and measuring step 0.1 μm after nital corrosion. Then, EBSD (Electron Back Scatter Diffraction) measurement was performed, and an orientation difference of 15 ° or more was determined as a grain boundary. The measurement length at the 10 μm position of the surface layer excluding plating is 500 μm, and for all the crystal grains at the 10 μm position of the surface layer, the respective areas are converted into circles to obtain the diameter, and the average value of the diameters is the average of the structure of the surface layer The particle size was taken.

(Tensile test)
In the tensile test, a JIS No. 5 tensile test piece is cut out from the specimen with the direction perpendicular to the rolling direction as the longitudinal direction, a tensile test is performed according to JIS Z2241, and the yield strength (YP), tensile strength (TS), and total elongation (El) are measured. evaluated.

(Warm formability)
Evaluation of warm formability is performed by collecting a 100 × 240 mm plate from the specimen, warm-pressing the hat part shown in FIG. 1, and embedding a sample cut out in the direction of the arrow from the vertical wall A in FIG. After polishing, the crack depth of the vertical wall was measured by SEM. The crack depth was the maximum value of the crack depth from the plating / base metal interface as shown in FIG. A crack depth of less than 10 μm was considered acceptable. In the pressing, the steel plate was taken out after being held in an atmospheric furnace at 700 ° C. for 5 minutes, and it was confirmed that the steel plate temperature was 600 ° C. The pressing pressure at the time of pressing was 20 tons, and the die shoulder R was 5 mm.

  In Table 3, the specimen No. Characteristic values of 1-18 are shown.

  FIG. 3 shows a comparison between the steel of the present invention and a comparative steel manufactured under the condition that only the collision pressure of high-pressure water in descaling (descaling collision pressure) is out of the scope of the present invention. The relationship of C amount is shown. It can be seen that by setting the descaling collision pressure within the range of the present invention, the solid solution C amount of the surface layer can be made 0.005% or more. FIG. 4 shows the relationship between the dew point and the amount of solid solution C in the surface layer of the steel according to the present invention and a comparative steel manufactured under the condition that only the dew point of the annealing atmosphere is outside the scope of the present invention. It turns out that the amount of solid solution C of a surface layer can be 0.005% or more by making the dew point of an annealing atmosphere into the range of this invention. FIG. 5 shows the relationship between the amount of solute C in the surface layer and the crack depth in relation to the steel of the present invention and the comparative steel in which only the amount of solute C in the surface layer is outside the scope of the present invention. It turns out that the crack depth can be made less than 10 micrometers by making the amount of solid solution C of a surface layer into the range of this invention. FIG. 6 shows the relationship between the average grain size of the surface layer structure and the crack depth with respect to the steel of the present invention and the comparative steel in which only the average grain size of the surface layer structure falls outside the scope of the present invention. It turns out that the crack depth can be made less than 10 micrometers by making the average particle diameter of the structure | tissue of a surface layer in the range of this invention.

Claims (9)

In mass%, C: 0.08 to 0.20%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less , V: 0.01 to 1.0% , Ti: 0.01 to 0.3%, Nb: 0.01 to 0.1 1 type or 2 types of %, so that the C ^* calculated by the following formula (1) is 0.07 or more, the composition comprising the balance Fe and inevitable impurities,
The total of the ferrite phase and the tempered bainite phase is 95% or more by area ratio, and the average grain size of the structure of the surface layer is 5.0 μm or less, and further Ti deposited as a precipitate having a grain size of less than 20 nm, A structure in which the precipitation amount of Nb and V is 0.025 mass% or more as the precipitation C equivalent amount obtained by the following formula (2), and the solid solution C amount of the surface layer is 0.005 mass% or more. Hot-dip galvanized steel sheet characterized by
C ^* = (Ti / 48 + Nb / 93 + V / 51) × 12 (1)
However, each element symbol in the formula (1) represents the content (% by mass) of each element.
([Ti] / 48 + [Nb] / 93 + [V] / 51) × 12 (2)
However, [Ti], [Nb], and [V] in the formula (2) represent the respective precipitation amounts (mass%) of Ti, Nb, and V deposited as precipitates having a particle size of less than 20 nm.   In addition to the above-described composition, one or more of Mo: 0.005-0.50%, Ta: 0.005-0.50%, W: 0.005-0.50% in mass% The hot-dip galvanized steel sheet according to claim 1, comprising:   In addition to the above composition, further, by mass, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, one or more The hot-dip galvanized steel sheet according to claim 1 or 2, characterized by comprising:   In addition to the above composition, the composition further contains one or two of Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.01% by mass%. 3. The hot dip galvanized steel sheet according to any one of 3 above.   The hot-dip galvanized steel sheet according to any one of claims 1 to 4, further comprising, in addition to the composition, Sb: 0.005 to 0.050% by mass.   The hot-dip galvanized steel sheet according to any one of claims 1 to 5, further comprising B: 0.0005 to 0.0030% by mass% in addition to the composition. A method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 6 ,
The steel having the above composition is cast into a slab, and the slab is subjected to rough rolling as it is after casting, or after reheating to 1200 ° C. or higher after being cooled,
After the rough rolling, before the final rolling, the high pressure water is subjected to descaling with a collision pressure of (1 + Si) MPa or higher, and then the final rolling outlet temperature is set to 950 ° C. or lower and 850 ° C. or higher.
After finishing rolling, the temperature range from the finish rolling exit temperature to 650 ° C. is cooled at an average cooling rate of 30 ° C./s or more, the winding temperature is 350 ° C. to 600 ° C., and after pickling,
An annealing at 550 ° C. or higher with an atmospheric dew point of −25 ° C. or lower, a soaking temperature of 650 to 770 ° C., and a soaking time of 10 to 300 s is performed,
After annealing, after dip galvanizing by immersing in a galvanizing bath at 420 to 500 ° C., the temperature range of 400 to 200 ° C. is cooled at an average cooling rate of 1 ° C./s or more. A method of manufacturing a steel sheet.
However, Si in the said collision pressure is content (mass%) of Si.   After dip galvanizing by immersing in the galvanizing bath at 420 to 500 ° C., reheating to 460 to 600 ° C. and holding for 1 s or more, the temperature range of 400 to 200 ° C. is average cooling rate 1 ° C./s. The method for producing a hot-dip galvanized steel sheet according to claim 7, wherein cooling is performed as described above.   8. The process according to claim 7, wherein after the temperature range of 400 to 200 [deg.] C. is cooled at an average cooling rate of 1 [deg.] C./s or more, the thickness is further reduced to 0.1 to 3.0%. The manufacturing method of the hot dip galvanized steel plate of 8.