KR100548117B1 - Multi-phase steel sheet excellent in hole expandability and method of producing the same - Google Patents

Abstract

The present invention provides a steel sheet having a good balance between strength and elongation and a balance between strength and hole expandability, that is, a composite tissue steel sheet having a good balance between strength and hole expandability. The present invention includes, as a chemical component, 0.5 to 4% by mass of C: 0.03 to 0.15%, P: 0.010% or less, S: 0.003% or less, and one or two of Si and Al in total amounts, Mn, A steel sheet containing 0.5 to 4% of Ni, Cr, Mo, and Cu in a total amount of 0.5 to 4% in total, and the remainder is Fe and an unavoidable impurity, and the microstructure of the cross section of the steel sheet is one of residual austenite and martensite, or Two types are 3 to 30% by the total area ratio, and the remaining part consists of one type or two types of ferrite and bainite, the maximum length of the microcrystalline grains is 10 microns or less, and the size is 20 microns in the cross section of the steel sheet. The composite steel sheet excellent in the hole expandability characterized by the above-mentioned number of inclusions being 0.3 or less per square millimeter.

Description

MULTI-PHASE STEEL SHEET EXCELLENT IN HOLE EXPANDABILITY AND METHOD OF PRODUCING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite tissue steel sheet having excellent hole expandability for the purpose of use in automobiles and industrial machines such as passenger cars and trucks, and a manufacturing method thereof.

In recent years, the demand for high strength steel sheet is increasing mainly due to the weight reduction of the automobile body and the increasing demand for securing the safety of the passenger in the event of a crash. In particular, the application of steel of TS 590 MPa class (60 kgf / mm 2 class) in tensile strength is rapidly expanding.

As the steel sheet used for this purpose, a composite tissue steel sheet containing residual austenite and / or martensite is widely known. For example, Japanese Patent Application Laid-open No. Hei 9-104947 discloses a steel sheet having a good balance between strength and elongation (33.8 to 40.5% of total elongation when the tensile strength is 60 to 69 kgf / mm 2). Doing. However, in this technique, the description of the balance between strength and hole expandability has not been fully considered, especially the control of the maximum length of very low P, inclusions and microstructures and the control of the hardness of microstructures. The requirement was not considered at all. Therefore, the properties of the steel sheet are inferior (the hole expansion rate d / d0 is 1.46 to 1.68, that is, 46 to 68% at the net hole expansion rate when the tensile strength is 60 to 69 kgf / mm 2), Its application is limited.

On the other hand, Japanese Patent Application Laid-Open No. 3-180426 discloses a bainite steel sheet having a good balance between strength and hole expandability (when the total elongation ratio d / d0 is 1.72 to 2.02, that is, the tensile strength is 60 to 67 kgf / mm2. Hole expansion rate is 72 to 102%). However, since this technique provides equalization of the structure rather than the complex tissue structure as a means to improve the net hole elongation, the balance between strength and elongation is largely insufficient (when tensile strength is 60 to 67 kgf / mm 2). , Total elongation is 27 to 30%), and the application is limited.

That is, the stretch flange formability represented by the punch stretch formability represented by the balance between the strength and the elongation and the balance between the strength and the hole expandability in the press working of the automotive parts are two important factors of molding. However, there is no such technology that satisfies both factors simultaneously, and excellence in both factors was the key to expanding the application.

In recent years, as long as the transition to high strength steel sheet is accelerated due to global environmental problems, their application to components having high molding difficulty is considered, so that both the balance of strength and elongation and the balance of strength and hole expandability are excellent. There is a need for a steel sheet, that is, a composite steel sheet having a good balance between strength and hole expandability.

The object of the present invention is to solve the problems of the conventional steel sheet, thereby achieving an excellent balance between strength and hole expandability (at least 35,000 MPa, preferably 46,000 MPa, obtained by multiplying tensile strength by net hole expansion rate) and strength and To provide a steel sheet having all excellent balance between elongation (18,500 MPa or more, preferably 20,000 MPa or more as a value obtained by multiplying the tensile strength by the total elongation), that is, a composite tissue steel sheet having excellent hole expandability and a method of manufacturing the same will be.

The balance between strength and hole expandability (MPa ·%) and the balance between strength and elongation (MPa ·%) are all indexes of press workability. If this value is large, the resulting product exhibits excellent properties. The balance between strength and hole expandability is expressed as the product of the value of the strength (MPa) obtained by the tensile test and the value of the percentage of the hole expansion (%) obtained by the hole expandability test. Moreover, the balance between strength and elongation is expressed as the product of the value of the strength (MPa) obtained by the tensile test and the value of the total elongation obtained by the tensile test. In commonly used steels, if the tensile strength increases, both the hole expansion ratio and the elongation decrease, and as a result, the balance between strength and hole expandability (MPa ·%) and the balance between strength and elongation (MPa · %) All show low values. On the other hand, according to the present invention, lowering the values of both the hole expansion ratio and the elongation can be suppressed, and the balance between the high value of the strength and the hole expandability (MPa ·%) and the balance between the strength and the elongation (MPa · It is possible to obtain the value of%).

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched from the viewpoint of the integral manufacture of hot rolling from steelmaking, and invented the composite structure steel plate excellent in hole expansion property, and its manufacturing method.

The gist of the present invention is as follows.

(1) It is a composite tissue steel plate with excellent hole expandability,

As mass% of chemical component,

C: 0.03 to 0.15%,

P: 0.010% or less,

S: 0.003% or less,

It contains 0.5 to 4% of one or two types out of Si and Al in total amount,

0.5 to 4% of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of

The remainder is a steel plate composed of Fe and unavoidable impurities,

The microstructure of the cross section of the steel sheet is one to two kinds of retained austenite and martensite in a total area ratio of 3 to 30%, and the remainder is one or two of ferrite and bainite. Kind of

The maximum length of the crystal grains in the microstructure is 10 microns or less,

A composite steel sheet having excellent hole expandability, characterized in that the number of inclusions having a size of 20 microns or more in the cross section of the steel sheet is 0.3 or less per square millimeter.

(2) as mass% of a chemical component,

C: 0.03 to 0.15%,

P: 0.010% or less,

S: 0.003% or less,

It contains 0.5 to 4% of one or two types out of Si and Al in total amount,

0.5 to 4% of Mn, Ni, Cr, Mo, and Cu in a total amount of one or more,

The remainder is steel plate made of Fe and unavoidable impurities,

The microstructure of the cross section of the steel sheet is one to two kinds of the retained austenite and martensite, 3 to 30% in the total area ratio, more than 0% in the pearlite area ratio 3% or less, the remaining part 1 in ferrite and bainite Kind or two kinds,

The maximum length of the crystal grains of the microstructure is 10 microns or less,

A composite steel sheet having excellent hole expandability, characterized in that the number of inclusions having a size of 20 microns or more in the cross section of the steel sheet is 0.3 or less per square millimeter.

(3) The composite tissue steel sheet having excellent hole expandability according to item (1) or (2), wherein the micro-Vickers hardness of bainite is less than 240.

(4) The hole expansion according to any one of items (1) to (3), further comprising at least one kind of Nb, V, and Ti in a total amount of 0.3% or less as the mass% of the chemical component. Composite tissue steel plate with excellent properties.

(5) The composite tissue steel sheet having excellent hole expandability according to any one of items (1) to (4), further comprising B as 0.01% or less as the mass% of the chemical component.

(6) The item according to any one of items (1) to (5), further comprising one or two kinds of Ca of 0.01% or less and REM of 0.05% or less as the mass% of the chemical component. Composite tissue steel plate with excellent hole expandability.

(7) As mass% of chemical component,

C: 0.03 to 0.15%,

P: 0.010% or less,

S: 0.003% or less,

0.5 to 4% of one kind or two kinds of Si and Al are included in the total amount,

0.5 to 4% of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of

The remainder is a method for producing a steel sheet composed of Fe and unavoidable impurities,

When refining molten steel having the above components, the molten steel is refluxed at least 1.5 times after the deflux flux is added during the deflowing of the molten steel,

When the steel sheet is produced by hot rolling the slab obtained after casting the molten steel, finish rolling is performed by controlling the finish rolling inlet side temperature to be at least 950 ° C. and the finish rolling outlet side temperature in the range of 780 ° C. to 920 ° C.,

The steel sheet thus obtained is wound at a temperature of 500 ° C. or less, wherein the method for producing a composite structural steel sheet having excellent hole expandability.

(8) In the item (7), the steel sheet further contains one or more of Nb, V, and Ti in a total amount of 0.3% or less as a mass% of the chemical component, and has a good hole expandability. Method of manufacturing steel sheet.

(9) The method according to item (7) or (8), wherein the steel sheet further contains B as 0.01% or less as the mass% of the chemical component.

(10) The item according to any one of items (7) to (9), wherein the steel sheet further includes one or two types of 0.01% or less of Ca and 0.05% or less of REM as the mass% of the chemical component. A method for producing a composite structural steel sheet having excellent hole expandability.

1 is a graph showing the effect of chemical component P on the net pore expansion rate.

FIG. 2 is a graph showing the effect of the maximum length of microstructure on net hole expansion rate.

3 is a graph showing the influence of the number of inclusions on the net hole expansion rate.

4 is a schematic diagram showing the refining of molten steel when RH is used.

FIG. 5 is a graph showing the effect of recovery of reflux of molten steel after addition of flux for desulfurization on the number of inclusions.

6 is a graph showing the influence of finish inlet and outlet temperatures in a finish mill of hot rolled rolling on the maximum length of the microstructure.

The invention is described in detail below.

First, the chemical composition is explained.

C is an important element for stabilizing austenite and obtaining a composite tissue structure, and C is 0.03 mass in order to obtain austenite and to obtain at least 3% of one or two of the retained austenite and martensite by the total area ratio. % Or more is added. However, the upper limit of the C content is set at 0.15 mass% or less, preferably 0.11 mass% or less in order to avoid deterioration of weldability and adverse effects of the net hole expansion ratio.

P is an important element among the additional elements of the present invention. The effect of P is shown in FIG. FIG. 1 shows the results of a study of the relationship between the net expansion rate and the P content of a steel sheet using the steel sheet having the chemical composition of steel number 1 of Table 1. FIG.

The net hole expansion rate is calculated based on Japanese Steel Federation Standard JFS T 1001-1996. From Fig. 1, the net hole expansion rate is significantly improved exponentially by controlling the P content to 0.010 mass% or less, and its effect on the net hole expansion rate, which was not expected within the scope of the conventional concept, was recognized. . By doing this, press cracking can be avoided. The reason is not completely clear, but the decrease in the P content is characterized by the characteristics of the edges of the punched holes (e.g. minimization of the face size of the fracture surface, reduction of roughness, reduction of microcracks; microstructure of the shear surface). Suppression of deterioration of workability, etc.), and the net hole expansion ratio.

S content is set to 0.003 mass% or less, Preferably it is 0.001 mass% or less from a viewpoint of the prevention of the weldability and deterioration of a net hole expansion rate which are caused by sulfide type interference | inclusion.                 

Si and Al are useful elements for obtaining a composite structure. They promote the formation of ferrite and inhibit the formation of carbides, and also by strengthening ferrite, thereby reducing the hardness difference between ferrite and hard tissues (such as bainite and martensite) and contributing to increasing the uniformity of the structure, It functions to improve the net hole expandability by making one or two kinds of residual austenite and martensite corresponding to 3% or more in the total area ratio. In addition, they function as deoxidation elements. From the above point of view, the total amount of one or two of Si and Al should be 0.5% by mass or more. In consideration of the balance between cost and effect, the upper limit of the total amount of addition is set at 4 mass% or less.

For each addition amount of Si and Al, the following may be considered.

When particularly good surface quality is required, the method of avoiding the Si scale by controlling the Si content to 0.1 mass% or less, preferably 0.01 mass% or less, and the Si content to 1.0 mass% or more, preferably 1.2 One of the methods of making the Si scale harmless by controlling to mass% or more (the method of making the scale inconspicuous by forming the scale over the entire surface) may be adopted.

In order to meet the requirements of material properties, it is also possible to increase the addition amount of Al and reduce the addition amount of Si, for example, if it is desirable to lower the tensile strength by using the difference in the wastelite reinforcement function between Si and Al. Do.

Al may be limited to 0.2% by mass or less, preferably 0.1% by mass or less, considering the disadvantages of steelmaking and material properties such as corrosion of the refractory material, clogging of the nozzle, and the like.

Mn, Ni, Cr, Mo, and Cu are elements useful for obtaining a composite structure, and also elements for strengthening ferrite. In view of the foregoing, the lower limit of the total amount of one or more of these additions should be 0.5% by mass or more. However, considering the balance between cost and effect, the upper limit of the total amount of addition is set to 4 mass% or less.

In addition, one or more of Nb, V, Ti, B, Ca and REM may be added as the selection atom.

Nb, V and Ti are effective elements in obtaining higher strength. However, considering the balance between cost and effect, the total amount of one or more of these elements added is set to 0.3% by mass or less.

B has a function as a reinforcing element, and may be added at 0.01% by mass or less. In addition, B also has the effect of alleviating the adverse effects of P.

Ca may be added at 0.01% by mass or less because Ca further improves the net pore expansion ratio by controlling the shape of the sulfide inclusions.

(Ellipsification)

In addition, REM may also be added at 0.05 mass% or less for the same reason.

In addition, N may be added in an amount of 0.02% by mass or less for the purpose of strengthening the steel sheet and stabilizing austenite, if necessary.

Next, the microstructure is described below.

Control of the maximum length of the crystal grains in the microstructure in terms of not deteriorating the uniformity of the fracture surface size, which is one of the edge characteristics of the punched holes, improved by the extremely low P, in order to obtain excellent hole expandability. Of particular importance is the control of the amount and size of the and inclusions. Therefore, it is explained first.

Since the grain size of the microstructure affects the size of the fracture surface at the edge of the punching hole, this greatly affects the net hole expansion rate. Even if the average size of the crystal grains in the microstructure is fine, if the maximum particle size is large, this adversely affects net pore expandability. Since the microstructure contains many crystal grains, the net pore expansion rate is not dominated by the average particle size. If large crystal grains are present between many crystal grains, this adversely affects the net pore expansion ratio even when the average particle size is fine. Here, with respect to the size of the crystal grains, it is not the diameter of the reduced circle but its maximum length that affects the net expansion rate.

FIG. 2 shows the results of a study on the relationship between the net hole expansion rate of the steel sheet and the maximum length of the microstructure of the steel sheet using the steel sheet having the chemical composition of steel number 2 of Table 1. FIG. As shown in Figure 2, the net hole expansion rate is significantly improved exponentially when the maximum length of the microstructure is 10 microns or less, and its effect on the net hole expansion rate that was not expected within the scope of the conventional concept. Is recognized. By doing this, press cracking can be avoided.

Here, the maximum length of the microstructure is taken from an optical micrograph taken 400 times after etching a cross section perpendicular to the rolling direction of the steel sheet with a nitral reagent and the reagent disclosed in Japanese Patent Application Laid-open No. 59-219473. It is calculated by averaging the whole cross section in the thickness direction.

Moreover, for inclusion control, the net hole expansion rate can be improved by reducing the number of coarse inclusions. The number of coarse inclusions can be obtained by integrating the number of coarse inclusions having a maximum length of 20 microns or more by observing a cross section polished along the rolling direction of the steel plate surface under a microscope (400 times). FIG. 3 shows the results of an investigation of the relationship between the net hole expansion rate and the number of coarse inclusions (maximum length of 20 microns or more) using the steel sheet having the chemical composition of steel number 2 of Table 1. FIG. It should be noted that when the number of coarse inclusions (maximum length 20 microns or more) is below a certain number (0.3 per square millimeter), the net hole expansion rate can be greatly improved and press cracks can be avoided.

In addition, controlling the micro-Vickers hardness of bainite to less than 240 preferably serves to improve hole expandability. The reduction in the hardness of the bare knight lowers the hardness difference between the ferrite and the bare knight, contributing to the improvement of the uniformity of the structure. However, if the micro-Vickers hardness exceeds 240, the hardness difference between ferrite and bainite is beyond the range required for hole expandability, and the degradation of hole expandability is further caused by the degradation of workability of bare knight. The decrease of P (0.01% or less) greatly contributes to improving the effect, and details are unknown.

Here, the micro Vickers hardness of bainite is a reagent disclosed in Japanese Patent Application Laid-open No. 59-219473, etching the cross section perpendicular to the rolling direction of the steel sheet to check bainite, and loading of 1 to 10 gr. This is obtained by averaging the values measured at point 5 (averaging the values excluding the maximum and minimum values measured at point 7).

Moreover, in order to obtain a good balance between strength and elongation as well as a good balance between strength and elongation, it is essential to control the type and area ratio of the composite tissue.

Excellent balance between strength and elongation (value obtained by multiplying tensile strength by total elongation at least 18,500 MPa%) and excellent balance between strength and hole expandability (35,000 MPa by value obtained by multiplying tensile strength by net hole expansion ratio % Or more) is obtained by controlling one or two total area ratios of residual austenite and martensite to 3 to 30%.

When one or two total area ratios of residual austenite and martensite are less than 3%, it becomes impossible to obtain a stable effect of improving the balance between the strength and elongation obtained by the residual austenite and martensite. Therefore, its lower limit is set to 3%.

When the ratio of one or two total areas of retained austenite and martensite exceeds 30%, the effect of improving the balance between strength and elongation is saturated and deterioration such as net pore expansion ratio is caused. Therefore, from the viewpoint of press formability, the upper limit of the total area ratio is set to 30%.

Here, the pearlite is preferably not contained in the steel sheet because it inhibits the balance between strength and elongation and the balance between strength and hole expandability. Therefore, the area ratio of pearlite is at most 3% or less, preferably 1% or less.

In addition to the above limitation, it is better to add the following limitation.

When a particularly good balance (20,000 MPa or more) between strength and elongation is required, the area ratio of retained austenite is preferably set to 3% or more.

Moreover, when a particularly good balance between strength and hole expandability (46,000 MPa or more by the value obtained by multiplying the tensile strength by the net hole expansion rate) is desired, the area ratio of martensite is preferably set to 3% or less. Do.

On the other hand, when a low yield rate (yield stress is divided by tensile strength and the yield value (YR), which is obtained by multiplying the divided value by 100) is required in terms of shape fixability, the area ratio of martensite is 3%. The above is set.

Preferably, the effect is further increased by controlling the maximum length of the microstructure of residual austenite and / or martensite to 2 microns or less.

The remaining tissue of the microstructure is composed of one or two types of ferrite and bainite, and by controlling the total area ratio of ferrite and bainite to 80% or more, hard tissues other than ferrite and bainite are joined together in a reticular form. Deterioration of the press formability caused by the use can be suppressed.

Due to the above-mentioned effects, a good balance between strength and hole expandability (35,000 MPa or more, preferably 46,000 MPa or more by the value obtained by multiplying the tensile strength by the net hole expansion ratio) and a good balance between the strength and the elongation (18,500 MPa or more, preferably 20,000 MPa or more by a value obtained by multiplying the tensile strength by the total elongation), all can be obtained simultaneously, and press formability can be greatly improved.

Here, confirmation of the structure of the microstructure, the measurement of the area ratio and the measurement of the maximum length of the retained austenite and / or martensite have a cross section perpendicular to the rolling direction of the steel sheet. It was carried out via optical micrographs and X-ray analysis at a magnification of 1,000 taken after etching with the reagents disclosed in 59-219473.

Next, the manufacturing method will be described below.

First, when molten steel is refined in the steelmaking process, it is important to reflux (circulate) the molten steel after the addition of the deflux flux when the molten steel is degassed using a second refining apparatus such as RH. Here, reflux of the molten steel is represented by the amount of molten steel circulating inside the second refining apparatus, such as RH per unit time, and there are various formulas for calculation. For example, "Refining Limits of Impurity Elements on Mass Production Scale" (Japan Iron and Steel Association, Forum of the High Temperature Refining Process Section, Japan Academic Society, 19th Steelmaking Committee, Reaction Process Research Group, March 1996, pages 184-187) As disclosed in, the amount of refluxed molten steel (Q) represented by Equation 1 below is defined as one refluxed amount.

Reflux Q = 11.4 × V ^1/3 × D ^4/3 × {ln (P1 / P0)} ^1/3 × k ... (Equation 1)

here,

Q: reflux of molten steel (t / min.),                 

V: flow rate of reflux gas (N1 / min.),

D: inner diameter of snorkel (m),

P0: pressure in the vacuum chamber,

P1: pressure at the inlet of the reflux gas,

k: constant (constant determined on the basis of the second refining apparatus, in this case 4).

A schematic illustration of the refining of molten steel using RH is shown in FIG. The two snorkels 3 of the degassing chamber 2 are immersed inside the molten steel ladle 1, the gas is blown underneath one of these snorkels (in this case Ar is snorkeled through the blown lance 4). Blown from the bottom of one of them), and then the molten steel in the molten steel ladle 1 rises and enters the degassing chamber 2, and after the degassing process, the molten steel descends to the molten steel ladle from the other snorkel 3. To return. Here, although an example in which a second refining apparatus employing RH is used is shown, it is obvious that other apparatus (for example, DH) may be used.

5 is the size per square mm in the cross section of the steel sheet obtained by recovering the reflux of the molten steel after the addition of the deflux flux and hot rolling the slab cast from the molten steel when the molten steel having the steel number 2 component of Table 1 is refined. The result is a study of the relationship between the number of inclusions greater than 20 microns. As shown in Figure 5, by increasing the recovery of reflux of the molten steel, the surfacing of the deflux flux system inclusions is greatly facilitated, and the number of coarse inclusions (20 microns or more) is increased to the above-mentioned number (0.3 per square mm). Up to), and the net hole expansion rate is improved, so that press cracking is avoided.

Next, when the hot rolled steel sheet according to the present invention is produced, the conditions of the temperature in finish rolling in the hot rolled rolling process are examined. FIG. 6 shows the result summarizing the relationship between the finish rolling inlet and outlet temperatures and the maximum length of the microstructured crystal grains of the cross section of the obtained steel sheet when the slab having the steel number 2 component of Table 1 is hot rolled; do.

As shown in Fig. 6, by controlling the finish rolling inlet side temperature to 960 ° C or higher and the finish rolling exit side temperature to 780 ° C or higher, the maximum length of the microstructure is reliably controlled to 10 microns or less, and thus the net The hole expansion rate can be improved and press cracks can be avoided. Preferably, it is preferable to adjust the finish rolling inlet side temperature, the finish rolling speed, and the finish rolling outlet side temperature according to a chemical component.

Here, if the finish rolling exit temperature exceeds 920 ° C., the whole microstructure becomes rough, and disadvantages such as deterioration of press formability and occurrence of scale defects appear remarkably, and thus this temperature is set as an upper limit.

Although not particularly defined, the conditions for the cooling table after finish rolling, multistage control of the cooling rate (combination of rapid cooling, slow cooling and isothermal holding) or commonly known as immediate quenching at the finish rolling exit side are microstructures. May be employed for the purpose of controlling the area ratio of and promoting micronization of the microstructure and formation of the composite tissue.

The upper limit of the coiling temperature is set to 500 ° C so that the total area ratio of one or two of the retained austenite and martensite is 3% or more. If the coiling temperature exceeds 500 ° C., a total area ratio of 3% or more cannot be secured and thus a good balance between strength and elongation (tensile strength multiplied by total elongation) cannot be obtained.

Here, air cooling or forced cooling may be employed for cooling the steel sheet after it is wound up.

The slab may also be rolled once it has been cooled and reheated, or may be rolled by HCR or HDR. Slabs may also be produced by so-called thin slab continuous casting.

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

Yes

The chemical components other than Fe of the steel to be used for the test are shown in Table 2.

Table 3 shows the conditions for making steel and hot rolled steel for the test. The microstructure and material properties of the obtained hot rolled steel sheets are shown in Tables 4 and 5.                 

Here, evaluation of characteristics and microstructures was performed by the following method.

Tensile test is JIS No. 5 specimens were used to provide tensile strength (TS), yield strength (YS), yield ratio (YR = YS / TS x 100), total elongation (T.EL) and balance between strength and elongation (TS x T.EL ) Was saved.

Net hole expansion rate was calculated based on the Japan Steel Federation standard JFS T1001-1996.

The maximum length of the crystal grains of the microstructure was calculated from an optical micrograph of 400 magnification taken after etching a cross section perpendicular to the rolling direction of the steel sheet with a nitral reagent and the reagent disclosed in Japanese Patent Application Laid-open No. 59-219473.

The number of coarse inclusions of a steel plate was calculated | required by observing the polished cross section orthogonal to the rolling direction of a steel plate with a microscope (400 magnification), and integrating the number of coarse grains whose maximum length is 20 microns or more.

Confirmation of the structure of the microstructure, the measurement of the area ratio and the measurement of the maximum length of the retained austenite and / or martensite are disclosed in Nitral reagent, Japanese Patent Application Laid-Open No. 59-219473, with a cross section perpendicular to the rolling direction of the steel sheet. It was performed by optical micrograph and X-ray analysis of 1,000 magnification taken after etching with the reagent and the reagent disclosed in Japanese Patent Application Laid-open No. Hei 5-163590.

The area ratio of retained austenite (Fγ:%) was calculated according to the following equation based on Mo-Kγ in the X-ray analysis.

Fγ (%) = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)},

Here, α 211, γ 220, α 211 and γ 311 represent the intensity in the individual planes.

As shown in Table 5, in the examples of the present invention (numbers 1, 2, 6, 8, 10, 14, 15, 20), there is a good balance between strength and hole expandability (net hole expansion ratio in tensile strength). A high-strength hot rolled steel sheet having excellent press formability is obtained, having both a multiplied value of 35,000 MPa or more) and an excellent balance between strength and elongation (18,500 MPa or more by a value obtained by multiplying the tensile strength by total elongation).

On the other hand, in Comparative Examples (No. 3 to 5, 7, 9, 11 to 13, 16 to 19), since some conditions are outside the scope of the present invention described in Tables 1 to 3, poor mechanical properties (strength Steel sheet having a balance between the hole expansion property and the balance between the strength and the elongation).

The present invention makes it possible to provide a composite tissue steel sheet excellent in press formability and a method of manufacturing the same, which have both an excellent balance between strength and hole expandability and an excellent balance between strength and elongation, and resultantly, and consequently, This significantly extends the scope of application and service conditions and the industrial and economic effects of the present invention are remarkable.

Claims (10)

As mass% of chemical component, C: 0.03 to 0.15%, P: 0.010% or less, S: 0.003% or less, It contains 0.5 to 4% of one or two types out of Si and Al in total amount, 0.5 to 4% of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of The remainder is a steel plate composed of Fe and unavoidable impurities, The microstructure of the cross section of the steel sheet is one to two kinds of the retained austenite and martensite in a total area ratio of 3 to 30%, the remaining part is made of one or two of ferrite and bainite, The maximum length of the crystal grains of the microstructure is 10 microns or less, A composite steel sheet having excellent hole expandability, characterized in that the number of inclusions having a size of 20 microns or more in the cross section of the steel sheet is 0.3 or less per square millimeter. As mass% of chemical component, C: 0.03 to 0.15%, P: 0.010% or less, S: 0.003% or less, It contains 0.5 to 4% of one or two types out of Si and Al in total amount, 0.5 to 4% of the total amount of one or more of Mn, Ni, Cr, Mo, and Cu, The remainder is steel plate made of Fe and unavoidable impurities, The microstructure of the cross section of the steel sheet is one to two kinds of the retained austenite and martensite, 3 to 30% in the total area ratio, more than 0% in the pearlite area ratio 3% or less, the remaining part 1 in ferrite and bainite Kind or two kinds, The maximum length of the crystal grains of the microstructure is 10 microns or less, A composite steel sheet having excellent hole expandability, characterized in that the number of inclusions having a size of 20 microns or more in the cross section of the steel sheet is 0.3 or less per square millimeter. The composite tissue steel sheet having excellent pore expandability according to claim 1 or 2, wherein the micro-Vickers hardness of bainite is less than 240. The composite structured steel sheet having excellent pore expandability according to claim 1 or 2, further comprising 0.3% or less of Nb, V, and Ti in a total amount of at least one of Nb, V, and Ti as the mass% of the chemical component. The composite-structured steel sheet having excellent pore expandability according to claim 1 or 2, further comprising B as 0.01% or less as the mass% of the chemical component. The composite tissue steel sheet having excellent pore expandability according to claim 1 or 2, further comprising one or two of Ca of 0.01% or less and REM of 0.05% or less as the mass% of the chemical component. . As mass% of chemical component, C: 0.03 to 0.15%, P: 0.010% or less, S: 0.003% or less, 0.5 to 4% of one kind or two kinds of Si and Al are included in the total amount, 0.5 to 4% of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of The remainder is a method for producing a steel sheet composed of Fe and unavoidable impurities, When refining molten steel having the above components, the molten steel is refluxed at least 1.5 times after the deflux flux is added during the deflowing of the molten steel, When the steel sheet is produced by hot rolling the slab obtained after casting the molten steel, finish rolling is performed by controlling the finish rolling inlet side temperature to be at least 950 ° C. and the finish rolling outlet side temperature in the range of 780 ° C. to 920 ° C., The steel sheet thus obtained is wound at a temperature of 500 ° C. or less, wherein the method for producing a composite structural steel sheet having excellent hole expandability. The method for producing a composite structural steel sheet having excellent hole expandability according to claim 7, characterized in that the steel sheet further contains, as mass% of the chemical component, one or more of Nb, V, and Ti in a total amount of 0.3% or less. . The method for producing a composite structural steel sheet having excellent hole expandability according to claim 7 or 8, wherein the steel sheet further contains B as 0.01% or less as the mass% of the chemical component. The steel sheet further comprises one or two kinds of 0.01% or less of Ca and 0.05% or less of REM as the mass% of the chemical component, wherein the steel sheet further has excellent hole expandability. Method of manufacturing composite structural steel sheets.

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