JP4181036B2 - Composite structure steel plate excellent in hole expansibility and its manufacturing method - Google Patents

Description

  The present invention relates to a composite structure steel plate excellent in hole expansibility intended for use in automobiles such as passenger cars and trucks, industrial machines, and the like, and a method for producing the same.

In recent years, the demand for high-strength steel sheets has increased mainly due to the light weight of automobile bodies and ensuring passenger safety in the event of a collision. In particular, the application of tensile strength TS590 MPa class (60 kgf / mm ² class) is rapidly expanding.

As steel plates used for such applications, composite steel plates having retained austenite and martensite are widely known. For example, as described in JP-A-9-104947, by containing an appropriate amount of retained austenite, an excellent strength-elongation balance (when the tensile strength is 60 to 69 kgf / mm ² , the total elongation is 33.8 to 40.40). 5%) is obtained. However, the technology for strength-hole expansion balance is inadequate, and in particular, technical requirements for ultra-low P, maximum length control of microstructures and inclusions, and microstructure hardness control are not considered at all. However, when the tensile strength was 60 to 69 kgf / mm ² , the hole expansion ratio d / d0 was 1.46 to 1.68, and the hole expansion ratio was 46 to 68%, and the application was limited.

On the other hand, a bainite steel sheet as described in JP-A-3-180426 as a steel sheet having an excellent strength-hole expansion balance (at a tensile strength of 60 to 67 kgf / mm ² , a hole expansion ratio d / d0 of 1.72 to 2.02 and 72 to 102% in terms of the hole expansion rate), but because the hole expansion rate is improved, the composite structure is avoided and the structure is homogenized (single organization of bainite). On the contrary, the strength-elongation balance is insufficient (the total elongation is 27-30% at a tensile strength of 60-67 kgf / mm ² ), and the application is similarly limited.

  In other words, in press molding of automotive parts, there are two major molding elements, namely, stretch molding represented by strength-elongation balance and stretch flange molding represented by strength-hole expansion balance. Being superior to both was the key to expanding application.

  In recent years, as the replacement of high-strength steel sheets has accelerated due to global environmental problems, steel sheets that are excellent in both strength-elongation balance and strength-hole expansion balance are being considered for application to parts with high forming difficulty. In other words, there has been a strong demand for a composite steel sheet having an excellent strength-hole expansion balance.

  The present invention eliminates the problems of the conventional steel sheet described above, and has an excellent strength-hole expansion balance (tensile strength x hole expansion ratio of 35,000 MPa ·% or more, preferably 46000 MPa ·% or more) and excellent strength-elongation. It is an object of the present invention to provide a steel sheet having a balance (tensile strength × total elongation of 18500 MPa ·% or more, preferably 20000 MPa ·% or more), that is, a composite structure steel plate excellent in hole expansibility and a method for producing the same.

  Strength-hole expansion balance (MPa ·%) and strength-elongation balance (MPa ·%) are indicators of press formability, and the larger the value, the better the characteristics. Strength-hole expansion balance is expressed as the product of tensile strength (MPa) by tensile test and hole expansion ratio (%) by hole expansion test, and strength-elongation balance is the same as tensile strength (MPa) by tensile test. It is expressed as the product of total elongation (%). In general steel plates, when the tensile strength increases, the hole expansion ratio deteriorates and the elongation also deteriorates, and the strength-hole expansion balance (MPa%) and the strength-elongation balance (MPa%) remain at low levels. In the invention, deterioration of the hole expansion ratio and elongation accompanying the increase in tensile strength is suppressed, and high values are obtained in the strength-hole expansion balance (MPa ·%) and strength-elongation balance (MPa ·%).

  The inventors of the present invention have intensively studied from the viewpoint of integrated manufacturing from steelmaking to hot rolling, and have come to invent a composite structure steel plate excellent in hole expansibility and its manufacturing method.

  The means is as follows.

(1) As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A steel plate composed of the remaining Fe and inevitable impurities,
As the microstructure of the steel sheet cross section,
One or two of the retained austenite and martensite in a total area ratio of 3 to 30%,
The balance microstructure consists of one or two of ferrite and bainite,
The maximum length of the microstructure grain is 10 microns or less,
Furthermore, the composite structure steel plate excellent in the hole expansibility characterized by the inclusion of 0.3 or less inclusions per 1 mm ^{2 in the} cross section of the steel plate.

(2) As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A steel plate composed of the remaining Fe and inevitable impurities,
As the microstructure of the steel sheet cross section,
One or two of the retained austenite and martensite in a total area ratio of 3 to 30%,
The pearlite area ratio is over 0% to 3%,
The balance microstructure consists of one or two of ferrite and bainite,
The maximum length of the microstructure grain is 10 microns or less,
Furthermore, the composite structure steel plate excellent in the hole expansibility characterized by the inclusion of 0.3 or less inclusions per 1 mm ^{2 in the} cross section of the steel plate.

  (3) The composite structure steel plate having excellent hole expansibility according to the above (1) or (2), wherein the bainite has a micro Vickers hardness of less than 240.

(4) As a chemical component in mass%,
One or more of Nb, V, and Ti are included in a total amount of 0.3% or less, and the hole expandability according to any one of the above items (1) to (3) is excellent. Composite steel sheet.

(5) As a chemical component, in mass%,
The composite structure steel plate excellent in hole expansibility according to any one of the above (1) to (4), wherein B is contained in an amount of 0.01% or less.

  (6) The chemical component is characterized by containing, in mass%, one or two of Ca and REM, 0.01% or less in Ca and 0.05% or less in REM. The composite structure steel plate excellent in hole expansibility as described in any one of said (1)-(5).

(7) As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
Including S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A method for producing a steel sheet comprising the balance Fe and inevitable impurities,
When melting the molten steel of the above components, the molten steel is refluxed 1.5 times or more after the desulfurization flux addition at the time of molten steel desulfurization,
Further, when the steel slab obtained after casting the molten steel is hot-rolled to produce a steel plate, finish rolling is performed at a finish entry temperature ≧ 950 ° C. and a finish exit temperature = 780 to 920 ° C., and 500 ° C. The manufacturing method of the composite structure steel plate excellent in the hole expansibility characterized by winding up the steel plate obtained below.

(8) As a chemical component, in mass%,
One or more of Nb, V, and Ti are included in a total amount of 0.3% or less. The method for producing a composite structure steel plate having excellent hole expansibility according to (7) above.

(9) As a chemical component, in mass%,
The method for producing a composite structure steel plate having excellent hole expansibility according to the above (7) or (8), wherein B is contained in an amount of 0.01% or less.

  (10) The chemical component as described above, wherein the chemical component further includes one or two of Ca and REM in 0.01% or less in Ca and 0.05% or less in REM. The manufacturing method of the composite structure steel plate excellent in the hole expansibility as described in any one of 7)-(9).

  According to the present invention, it has become possible to stably provide a low cost and stable composite steel sheet with excellent press formability having excellent strength-hole expansion ratio balance and excellent strength-elongation balance and its manufacturing method. The usage and conditions of use are greatly expanded, and the industrial and economic effects are very large.

  The present invention is described in detail below.

  First, chemical components will be described.

  C is an important element for stabilizing austenite and obtaining a composite structure. To stabilize austenite and obtain a total area ratio of one or two of retained austenite and martensite of 3% or more. Add 03% by mass or more. However, the upper limit is 0.15% by mass or less in order to avoid deterioration of weldability and to avoid adverse effects on the hole expansion rate. Preferably it is 0.11% or less.

  P is a very important element in the additive element of the present invention. The effect is shown in FIG. FIG. 1 shows the result of investigating the relationship between the P concentration and the hole expansion ratio of a steel plate using a steel plate having the component No. 1 in Table 1.

  The hole expansion rate was obtained from the Japan Iron and Steel Federation standard JFS T1001-1996. As shown in FIG. 1, when P is 0.010% or less, the hole expansion rate is remarkably improved exponentially, and an effect on the hole expansion rate that cannot be assumed in the conventional extension is recognized. This makes it possible to avoid press cracks. The reason for this is still unclear, but the reduction of P improves the punch hole end face properties (such as minimizing the fracture surface size of the fracture surface, reducing roughness, reducing microcracks, etc. It is thought that this led to an improvement in the hole expansion rate.

  The content of S is 0.003% or less (preferably ≦ 0.001%) from the viewpoint of the hole expansion rate due to sulfide inclusions and the prevention of weldability deterioration.

  Si and Al are useful elements for obtaining a composite structure. By promoting the formation of ferrite and suppressing the formation of carbides, and strengthening ferrite, ferrite and hard structures (bainite, martensite, etc.) By contributing to the uniformity of the structure by reducing the difference in hardness, the total area ratio of one or two of the retained austenite and martensite can be 3% or more, and the hole expansion ratio can be improved. It also acts as a deoxidizing element. From the above viewpoint, the total addition lower limit amount of one or two of Si and Al needs to be 0.5% by mass or more. From the balance of cost and effect, the total addition upper limit amount is 4% by mass or less.

  About the individual addition amount of Si and Al, the following may be considered.

  When particularly excellent surface properties are required, Si <0.1% by mass (preferably 0.01%) is avoided to avoid Si scale, or conversely, Si> 1.0% by mass (preferably 1.2%), the Si scale may be rendered harmless (generated on the entire surface and made inconspicuous).

  From the viewpoint of the material, such as when it is desired to keep the tensile strength low by utilizing the difference in ferrite strengthening action between Si and Al, it is also possible to increase the Al addition amount and decrease the Si addition amount.

  Al ≦ 0.2% (preferably 0.1%) may be used in relation to steelmaking disadvantages and materials such as refractory melting and nozzle clogging.

  Mn, Ni, Cr, Mo, and Cu are useful elements for obtaining a composite structure, and are also ferrite reinforcing elements. From the above viewpoint, the total lower limit amount of one or more of them needs to be 0.5% by mass or more. However, from the balance of cost and effect, the total addition upper limit amount is 4% by mass or less.

  Furthermore, you may add 1 type (s) or 2 or more types of Nb, V, Ti, B, Ca, and REM as a selection element.

  Nb, V, and Ti are effective elements for increasing the strength. However, from the balance between the effect and the cost, the amount added is one or two or more and the total amount is 0.3% or less.

  B acts as a strengthening element and may be added in an amount of 0.01% or less. It also has the effect of reducing the adverse effects of P.

  Ca may be added in an amount of 0.01% or less in order to further improve the hole expansion rate by controlling the form (spheroidization) of sulfide inclusions.

  Further, REM may be added at 0.05% or less for the same reason.

  If necessary, N may be added in an amount of 0.02% or less for the purpose of stabilizing or increasing the strength of austenite.

  Next, the microstructure will be described.

  In order to obtain an excellent hole expansion rate, the maximum length control and inclusions of the crystal grains of the microstructure are not made from the viewpoint of not impairing the uniformity of the fracture surface size of the punched hole end face properties improved by extremely low P. The control of the amount and size is an especially important point, which will be described first.

  Since the size of the microstructure affects the fracture surface size of the end face of the punched hole, it greatly affects the hole expansion rate. Even if the average particle size of the microstructure is fine, if the maximum particle size is large, the hole expansion rate is adversely affected. Although the microstructure is composed of a large number of crystal grains, the average grain size cannot regulate the hole expansion rate, and if there are large crystal grains in a large number of crystal grains, the average grain size is fine. It adversely affects the hole expansion rate. Furthermore, the size of one crystal grain is not the equivalent circle diameter, but its maximum length affects the hole expansion rate.

  FIG. 2 shows the result of investigating the relationship between the maximum length of the microstructure in the steel sheet and the hole expansion rate of the steel sheet, using the steel sheet having the component No. 2 in Table 1. As shown in FIG. 2, when the maximum length of the microstructure is 10 microns or less, the hole expansion rate is remarkably improved exponentially, and an effect on the hole expansion rate that cannot be assumed in the conventional extension is recognized. This makes it possible to avoid press cracks.

  The maximum length of the microstructure is determined by taking into account the entire cross section in the plate thickness direction from the optical micrograph at a magnification of 400 times that corrodes the cross section in the steel plate rolling direction with the Nital reagent and the reagent disclosed in JP-A-59-219473. Calculated.

  In inclusion control, the hole expansion rate can be improved by reducing the number of coarse inclusions. The inclusions were polished and polished, and the cross section in the rolling direction of the steel sheet was observed with a microscope (400 times magnification), and the number of coarse inclusions having a maximum length of 20 microns or more was integrated. FIG. 3 shows the results of investigating the relationship between the number of coarse inclusions (maximum length of 20 microns or more) in the steel sheet and the hole expansion rate using the steel sheet of component No. 2 in Table 1. It can be seen that when the number of coarse inclusions (maximum length of 20 microns or more) is less than a certain number (0.3 or less per square mm), the hole expansion rate is greatly improved and press cracks can be avoided.

  In addition, making the bainite micro Vickers hardness less than 240 preferably acts to improve the hole expansion characteristics. The reduction of bainite hardness reduces the hardness difference between ferrite and bainite and contributes to the improvement of the uniformity of the structure. However, when the bainite micro Vickers hardness exceeds 240, the hardness difference between ferrite and bainite is preferable for the hole expanding property. It deviates from the range, and further deterioration of workability of bainite itself causes deterioration of hole expansion characteristics. Although the reduction of P (0.01% or less) greatly contributes to the manifestation of the effect, details are not clear.

  The micro Vickers hardness of bainite was measured by applying a load of 1 to 10 g by identifying bainite by corroding the cross section in the rolling direction of the steel sheet with the reagent disclosed in JP-A-59-219473. The average value (measured 7 points, averaged excluding maximum and minimum).

  Furthermore, in order to obtain an excellent strength-elongation balance in addition to an excellent strength-hole expansion balance, it is necessary to control the type and area ratio of the composite structure.

  Excellent strength-elongation balance (tensile strength x total elongation of 18500 MPa ·% or more) and excellent strength by adjusting the total area ratio of one or two of retained austenite and martensite to 3-30%- A hole expansion balance (tensile strength x hole expansion ratio of 35000 MPa ·% or more) is obtained.

  If the total area ratio of one or two of retained austenite and martensite is less than 3%, the effect of improving the strength-elongation balance by retained austenite or martensite cannot be obtained stably, so 3% is the lower limit. And

  If the total area ratio of one or two of the retained austenite and martensite exceeds 30%, the effect of improving the strength-elongation balance is saturated, and conversely, the deterioration of the hole expansion ratio is caused. From the viewpoint, the total is limited to 30%.

  Although pearlite is preferably not included because it impairs the strength-elongation balance and the strength-hole expansion balance, the area ratio is at most 3% (more preferably 1% or less).

  More preferably, in addition to the above, it is desirable to add the following restrictions.

  In order to obtain a particularly excellent strength-elongation balance (20,000 MPa ·% or more), it is desired that the retained austenite area ratio is 3% or more.

  In order to obtain a particularly excellent strength-hole expansion balance (tensile strength × hole expansion ratio of 46000 MPa ·% or more), it is desired that the martensite area ratio is 3% or less.

  On the other hand, when a low yield ratio (yield ratio YR = yield stress / tensile strength × 100, 70% or less) is desired from the viewpoint of shape freezing property, the martensite area ratio is set to 3% or more.

  Preferably, the effect is further enhanced by setting the maximum length of the retained austenite and / or martensite microstructure to 2 microns or less.

  The remaining structure is composed of one or two of ferrite and bainite. By making the total area ratio 80% or more, hard structures other than ferrite and bainite are connected in a network form. It is possible to suppress press formability deterioration.

  Due to the effects described above, an excellent strength-hole expansion balance (tensile strength × hole expansion ratio of 35,000 MPa ·% or more, preferably 46000 MPa ·% or more) and an excellent strength-elongation balance (tensile strength × total elongation of 18500 MPa) % Or more, preferably 20000 MPa ·% or more), and the press formability is greatly improved.

  Incidentally, the structural identification of the microstructure and the measurement of the area ratio, the measurement of the maximum length of retained austenite and / or martensite are performed by the Nital reagent, the reagent disclosed in Japanese Patent Laid-Open No. 59-219473, and Japanese Patent Laid-Open No. 5-163590. It was performed by an optical micrograph at a magnification of 1000 times and X-ray analysis in which a steel plate rolling direction cross section was corroded by the disclosed reagent.

  Next, the manufacturing method will be described.

First, in the steel making process, when melting molten steel, it is important to circulate the molten steel at least 1.5 times after adding desulfurization flux during molten steel desulfurization using a secondary smelting apparatus such as RH. The reflux of molten steel here indicates the amount of molten steel circulated in the secondary smelting apparatus such as RH per unit time, and there are various calculation formulas. For example, “refining limit of impurity elements in mass production scale” As shown in (Japan Steel Association, High Temperature Scouring Process Group Scouring Forum, Japan Society for the Promotion of Science, Steel Manufacturing 19th Committee Reaction Process Study Group, March 1996, pages 184 to 187) The molten steel reflux amount Q represented by 1 is defined as one time.
Amount of reflux Q = 11.4 × V ^1/3 × D ^4/3 × {ln (P1 / P0)} ^1/3 × k Equation 1
Q: Flow rate of molten steel ring (t / min), V: Flow rate of circulating gas (Nl / min)
D: inner diameter of dip tube (m), P0: pressure inside vacuum chamber (Pa)
P1: Circulating gas injection position pressure (Pa)
k: Constant (constant by the secondary scouring device, this time set to 4)
Here, a schematic diagram of molten steel melting using RH is shown in FIG. 4, and two dip tubes 3 of the degassing tank 2 are immersed in the molten steel pan 1, and gas is blown from below one of them. (Ar is blown from the injection lance 4 from below the dip tube), the molten steel in the molten steel pan 1 rises and enters the degassing tank 2, and descends from the other dip tube 3 to the molten steel pan after degassing treatment. It is a return. In addition, although the example using the secondary scouring apparatus by RH was shown here, it cannot be overemphasized that another secondary scouring apparatus (for example, DH) may be used.

  FIG. 5 shows the number of times of molten steel recirculation after addition of desulfurization flux when molten steel having the composition of steel No. 2 in Table 1 is melted, and a steel plate cross section of 1 mm 2 after hot rolling from a cast slab after the obtained molten steel is cast. The result of investigating the relationship with the number of inclusions of 20 microns or more per hit is shown. As shown in FIG. 5, this significantly promotes the rise of desulfurization flux inclusions, and allows the inclusion of coarse inclusions (20 microns or more) to a certain number or less (0.3 pieces or less per square mm). Thus, the hole expansion rate can be improved and press cracks can be avoided.

  Next, when the steel of the present invention was obtained from a hot-rolled steel sheet, the temperature conditions for finish rolling were examined in the hot rolling process. FIG. 6 shows the results of arranging the finishing side and finishing side temperatures and the maximum length of the crystal grain of the microstructure of the obtained steel plate cross section when the slab of the steel No. 2 component in Table 1 is hot-rolled. Show.

  As shown in FIG. 6, by setting the finishing side temperature ≧ 950 ° C. and the finishing side temperature ≧ 780 ° C., the maximum length of the microstructure can be controlled to 10 microns or less, thereby improving the hole expansion rate. , Press cracks can be avoided. Preferably, it is desirable to regulate the finish entry temperature according to the chemical composition, finish rolling speed, finish finish temperature.

  If the finishing temperature exceeds 920 ° C., the microstructure becomes coarse as a whole, and negative aspects such as deterioration of press formability and generation of scale wrinkles appear strongly, so this temperature is set as the upper limit.

  Conditions for the cooling table after finish rolling are not specified, but generally known multi-stage control of the cooling rate with the aim of controlling the microstructure area ratio, refining the microstructure, and promoting complex formation (Combination of rapid cooling, slow cooling, and isothermal holding) or rapid cooling immediately after finishing rolling may be performed.

  The upper limit of the coiling temperature is 500 ° C. in order to obtain a total area ratio of one or two of retained austenite and martensite of 3% or more. When the coiling temperature exceeds 500 ° C., the total area ratio cannot be 3% or more, and an excellent strength-elongation balance (tensile strength × total elongation) cannot be obtained.

  Furthermore, the steel sheet after winding may be cooled by cooling or forced cooling.

  In addition, the steel piece to be used for rolling may be so-called cold piece reheating, HCR, or HDR. Further, a steel piece by so-called thin-wall continuous casting may be used.

  Further, the steel sheet according to the present invention may be plated with Zn or the like to improve corrosion resistance, or may be coated with a lubricant or the like to further improve press formability.

  Table 2 shows chemical components other than Fe of the test steel.

  Table 3 shows the production conditions in steelmaking and hot rolling of the test steel. Tables 4 and 5 show the microstructure and material of the obtained hot-rolled steel sheet.

  In addition, characteristic evaluation and microstructure evaluation were implemented with the following method.

  The tensile test was carried out in accordance with JIS No. 5, and tensile strength (TS), yield strength (YS), yield ratio (YR = YS / TS × 100), total elongation (T.EL), strength-elongation balance (TS × T) .EL).

  The hole expansion rate was determined according to the Japan Iron and Steel Federation standard JFS T1001-1996.

  The maximum length of the crystal grains of the microstructure was calculated from an optical micrograph at a magnification of 400 times in which the steel plate rolling direction cross section was corroded by the Nital reagent and the reagent disclosed in JP-A-59-219473.

  The inclusions in the steel sheet were polished and polished, and the cross section in the rolling direction of the steel sheet was observed with a microscope (magnification 400 times), and the number of coarse inclusions having a maximum length of 20 microns or more was integrated.

  Microstructure identification and area ratio measurement, maximum retained austenite and / or martensite length measurement are disclosed in Nital reagent, reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473 and Japanese Patent Application Laid-Open No. 5-163590. An X-ray analysis and an optical micrograph at a magnification of 1000 times in which a cross section in the rolling direction of the steel plate was corroded with the above reagent.

When calculating the retained austenite area ratio (Fγ: unit is%) by X-ray analysis, it was calculated according to the following formula using Mo-Kα rays.

Fγ (%) = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
However, α (211), γ (220), α (211), and γ (311) indicate surface strength.

  In the present invention examples (Nos. 1, 2, 6, 8, 10, 14, 15, 20), as shown in Table 5, an excellent strength-hole expansion balance (tensile strength × hole expansion ratio of 35000 MPa ·%) As described above, a hot-rolled high-strength steel sheet having excellent press formability and an excellent strength-elongation balance (tensile strength × 18500 MPa ·% or more in total elongation) is obtained.

  On the other hand, Comparative Examples (Nos. 3 to 5, 7, 9, 11 to 13, 16 to 19) are out of the scope of the present invention as described in the remarks column of Tables 1 to 3, respectively. Only those with low strength-hole expansion balance and strength-elongation balance) were obtained.

It is a figure which shows the influence of the chemical component P which acts on a hole expansion rate. It is a figure which shows the influence of the maximum length of a microstructure on a hole expansion rate. It is a figure which shows the influence of the number of inclusions which acts on a hole expansion rate. It is a schematic diagram of molten steel melting at the time of using RH. It is a figure which shows the influence of the molten steel recirculation frequency after the flux for desulfurization having added on the number of inclusions. It is a figure which shows the influence of the finishing entry side temperature of a finishing mill of a hot rolling, and the finishing delivery side temperature on the maximum length of a microstructure.

Claims (10)

As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A steel plate composed of the remaining Fe and inevitable impurities,
As the microstructure of the steel sheet cross section,
One or two of the retained austenite and martensite in a total area ratio of 3 to 30%,
The balance microstructure consists of one or two of ferrite and bainite,
The maximum length of the microstructure grain is 10 microns or less,
Furthermore, the composite structure steel plate excellent in the hole expansibility characterized by the inclusion of 0.3 or less inclusions per 1 mm ^{2 in the} cross section of the steel plate. As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A steel plate composed of the remaining Fe and inevitable impurities,
As the microstructure of the steel sheet cross section,
One or two of the retained austenite and martensite in a total area ratio of 3 to 30%,
The pearlite area ratio is over 0% to 3%,
The balance microstructure consists of one or two of ferrite and bainite,
The maximum length of the microstructure grain is 10 microns or less,
Furthermore, the composite structure steel plate excellent in the hole expansibility characterized by the inclusion of 0.3 or less inclusions per 1 mm ^{2 in the} cross section of the steel plate.   The composite steel sheet having excellent hole expandability according to claim 1 or 2, wherein the bainite has a micro Vickers hardness of less than 240. As a chemical component, in mass%,
The composite structure excellent in hole expansibility according to any one of claims 1 to 3, characterized by containing one or more of Nb, V, and Ti in a total amount of 0.3% or less. steel sheet. As a chemical component, in mass%,
The composite structure steel plate excellent in hole expansibility according to any one of claims 1 to 4, wherein B is contained in an amount of 0.01% or less.   2. The chemical component according to claim 1, further comprising one or two of Ca and REM in 0.01% by mass or less and 0.05% or less in REM by mass%. The composite structure steel plate excellent in the hole expansibility as described in any one of -5. As a chemical component,
C: 0.03-0.15%,
P ≦ 0.010%,
Including S ≦ 0.003%,
Including one or two of Si and Al in a total amount of 0.5 to 4%,
Including one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%,
A method for producing a steel sheet comprising the balance Fe and inevitable impurities,
When melting the molten steel of the above components, the molten steel is refluxed 1.5 times or more after the desulfurization flux addition at the time of molten steel desulfurization,
Furthermore, when the steel slab obtained after casting of the molten steel is hot-rolled to produce a steel plate, finish rolling is performed at a finish entry temperature ≧ 950 ° C. and a finish exit temperature: 780 to 920 ° C., and 500 ° C. The manufacturing method of the composite structure steel plate excellent in the hole expansibility characterized by winding up the steel plate obtained below. As a chemical component, in mass%,
The method for producing a composite steel sheet having excellent hole expansibility according to claim 7, comprising one or more of Nb, V, and Ti in a total amount of 0.3% or less. The method for producing a composite steel sheet having excellent hole expansibility according to claim 7 or 8, further comprising 0.01% or less B by mass% as a chemical component.   The chemical component further includes one or two of Ca and REM in terms of mass%, 0.01% or less in Ca and 0.05% or less in REM. The manufacturing method of the composite structure steel plate excellent in the hole expansibility as described in any one of the items.

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