JP2001355044A - High strength steel sheet excellent in formability and hole expandability and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a steel plate applicable to a member requiring hole expandability, such as a member of an automobile or brackets, and a method of manufacturing the same. SOLUTION: In mass%, C: 0.05% or more, 0.2%
%, Si: 1% or more, 2.5% or less, Cu: 0.5
% Or more, 2.5% or less, Mn: 0.5% or more, 3.0%
The following are contained, and further, Al: 0.01% or more,
2.5% or less, and / or Ni: less than 2.0%,
Mo: less than 0.2%, Cr: less than 1.0%, V: 0.3
%, B: less than 0.01%, Ti: less than 0.06%,
Nb: less than 0.06%, Ca: less than 0.05%, and Mg: at least one of less than 0.05%, and the hardness of the ferrite phase is 150 or more and 240 or less in Hv. A high-strength steel sheet having excellent formability and hole expandability, wherein the volume fraction of retained austenite in the steel structure is 2 to 20%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet applicable to a member requiring hole expandability, such as a member of an automobile or a bracket, and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, as represented by members of automobiles, there has been an increasing demand for weight reduction of members, and high-strength steel sheets have been increasingly used in order to meet the demand. Requires high formability and hole expandability. Conventionally, as a high-strength steel sheet that requires hole expandability,
A structure-reinforced steel sheet with a composite structure of ferrite (F) + martensite (M) or ferrite (F) + bainite (B) is often used. However, F + M
Composite structure steel (DP [= Dual Phase] steel plate)
However, although high strength and relatively good moldability can be obtained, voids tend to be generated from the interface between the two phases, which have significantly different strengths, and there is a problem that the hole expandability is poor.

[0003] Further, in the case of F + B DP steel,
As shown in Japanese Patent No. 101649, although the hole-expanding property is excellent, 700
It is difficult to obtain a strength of not less than MPa, and there is a problem that the moldability is inferior in the direction of further weight reduction in automobiles. Similarly, Japanese Patent Application Laid-Open No. 6-172924 discloses a structure-strengthened steel made of Beinetec ferrite. This steel mainly reinforces by the precipitation of TiC in ferrite, but it is difficult to secure high formability even with this reinforcement method because the method is a precipitation strengthening method.

[0004] Also, Japanese Patent No. 2844136 and Japanese Patent Application Laid-Open No. Hei 5-335991 produce a high-strength steel sheet excellent in hole expandability by adding Cu to precipitate fine Cu in ferrite. The method is shown.
However, even with this method, it is difficult to ensure high formability because the method is a precipitation strengthening method. On the other hand, in order to obtain the formability, the TR was changed to ferrite (F) + bainite (B) + retained austenite (γ).
IP (Transformation Induced Plasticity) steel plate
It is disclosed in Japanese Patent Publication No. 7-74378. This steel sheet has the highest workability among conventional steel sheets due to transformation induced plasticity (TRIP effect).
This steel sheet also has a problem that the hole expandability is inferior due to the remarkable difference in the strength between the martensite in which the retained austenite is the transformation-inducing composition and the ferrite.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the problems of the prior art, and has as its object to provide a high-strength steel sheet having both excellent hole expandability and formability, and a method of manufacturing the same. It is to provide.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to solve the above-mentioned problems found in conventional materials, and as a result, by increasing the strength of ferrite, the difference in strength between the different phases has resulted. It has been found that it is possible to prevent the deterioration of the hole expandability, and to achieve both excellent formability and hole expandability by having a required amount of retained austenite in the steel structure. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In mass%, C: 0.05% or more,
0.2% or less, Si: 0.5% or more, 2.5% or less, C
u: 0.5% or more and 2.5% or less, Mn: 0.5% or more and 3.0% or less, and the hardness of the ferrite phase is 150 or more and 240 or less in Hv, A high-strength steel sheet having excellent formability and hole expandability, characterized in that the volume fraction of retained austenite in the steel structure is 2 to 20%.

(2) In mass%, C: 0.05% or more,
0.2% or less, Si: 0.5% or more, 2.5% or less, A
l: 0.01% or more and 2.5% or less, Cu: 0.5% or more and 2.5% or less, Mn: 0.5% or more and 3.0% or less, and a ferrite phase Hardness is 15 in Hv
A high-strength steel sheet excellent in formability and hole expandability, characterized in that the volume ratio of retained austenite in the steel structure is from 0 to 240 and not more than 240.

(3) The high-strength steel sheet excellent in formability and hole expandability according to the above (1) or (2) further comprises, by mass%, Ni: less than 2.0% and Mo: 0.2. %, C
r: less than 1.0%, V: less than 0.3%, B: 0.01%
, Ti: less than 0.06%, Nb: less than 0.06%,
A high-strength steel sheet excellent in formability and hole expandability, characterized by containing at least one of Ca: less than 0.05% and Mg: less than 0.05%.

(4) In the high-strength steel sheet excellent in formability and hole expandability according to (1), (2) or (3), the amount of precipitated Cu having a size of 0.1 μm or less is 0.2. A high-strength steel sheet excellent in formability and hole expandability, characterized in that it is not less than mass%. (5) A steel satisfying the steel composition described in the above (1), (2) or (3) is subjected to hot rolling at a temperature of 800 ° C. or higher, and then wound at 350 to 600 ° C. A method for producing a high-strength hot-rolled steel sheet having excellent formability and hole expandability.

(6) A cold-rolled steel sheet satisfying the steel composition described in the above (1), (2) or (3) is prepared at 650-850 ° C.
After annealing for 10 seconds or more and 10 minutes or less in the two-phase coexisting temperature range, the sample is cooled to 350 to 600 ° C. at a cooling rate of 4 to 200 ° C./s, and then held for 10 seconds or more and 10 minutes or less in this temperature range. And then cooling at a cooling rate of 5 ° C./s or more to 250 ° C. or less. A method for producing a high-strength cold-rolled steel sheet having excellent formability and hole expandability.

(7) A cold-rolled steel sheet satisfying the steel composition described in the above (1), (2) or (3) is prepared at 650 to 850 ° C.
After annealing for 10 seconds or more and 10 minutes or less in the two-phase coexisting temperature range, the sample is cooled to 350 to 600 ° C. at a cooling rate of 4 to 200 ° C./s, and then held for 10 seconds or more and 10 minutes or less in this temperature range. And then hot-dip galvanized, then 5 ° C /
By cooling to a temperature of 250 ° C. or less at a cooling rate of at least s, a Zn plating layer comprising Al: 1% or less and unavoidable impurities is formed on the surface of the steel sheet. Manufacturing method of high strength galvanized steel sheet.

(8) A cold rolled steel sheet satisfying the steel composition described in the above (1), (2) or (3) is prepared at 650 to 850 ° C.
After annealing for 10 seconds or more and 10 minutes or less in the two-phase coexisting temperature range, the sample is cooled to 350 to 500 ° C. at a cooling rate of 4 to 200 ° C./s, and then hot-dip galvanized, and
After holding for 5 seconds or more and 2 minutes or less in a temperature range of 00 ° C., and then cooling to 250 ° C. or less at a cooling rate of 5 ° C./s or more, Fe: 8 to 15%, Al: : 1
%. A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent formability and hole expandability, characterized by forming a Zn alloy plating layer comprising at most% and an unavoidable impurity.

(9) The cold-rolled steel sheet is heated at 650 to 850 ° C.
After annealing for 10 seconds or more and 10 minutes or less in the two-phase coexisting temperature range of above, it is cooled to 350 to 500 ° C. at a cooling rate of 4 to 200 ° C./s, and then maintained at this temperature range for 10 minutes or less. The method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent formability and hole expandability according to the above (8).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a steel sheet comprising C, S
By limiting i, Mn, Cu content, hardness in ferrite, austenite volume ratio, hot rolling conditions, etc.,
An object of the present invention is to provide a high-strength steel sheet having both excellent hole expandability and formability, and the reasons for the limitation will be described below.

C is an austenite stabilizing element,
In the two-phase coexisting temperature range and the bainite transformation temperature range, it moves from ferrite and concentrates in austenite. As a result, 2-20% of chemically stabilized austenite remains even after cooling to room temperature, and improves formability by transformation induced plasticity. If C is less than 0.05%, it is difficult to secure 2% or more of retained austenite, and the object cannot be achieved. If C exceeds 0.2%, the weldability deteriorates and must be avoided.

Si does not form a solid solution in cementite and suppresses its precipitation, thereby delaying the transformation from austenite at 350 to 600 ° C. During this time, the concentration of C in the austenite is promoted, so that the chemical stability of the austenite is increased, transformation-induced plasticity is caused, and retained austenite which contributes to improving the formability can be secured. If the Si content is less than 0.5%, the effect cannot be obtained. On the other hand, if the Si concentration is increased, the weldability deteriorates, so the upper limit needs to be 2.5%.

Mn is an austenite-forming element,
Further, since the austenite is prevented from decomposing into pearlite during cooling to 350 to 600 ° C. after annealing in the two-phase coexisting temperature range, it contributes to the austenite remaining in the metal structure after cooling to room temperature. If the addition is less than 0.5%, it is necessary to increase the cooling rate so that industrial control cannot be performed in order to suppress the decomposition into pearlite, which is not appropriate as an addition amount. On the other hand, if it exceeds 3.0%, the band structure becomes remarkable and the characteristics are deteriorated, and the spot welded portion is easily broken in the nugget, which is not preferable. Therefore, Mn is set to 0.5% or more and 3.0% or less.

Cu is the most important element in the present invention.
In the present invention, by increasing the hardness of the ferrite by precipitating Cu in the ferrite, deterioration of hole expandability due to a difference in strength between different phases is prevented. Further, Cu is an austenite stabilizing element and works effectively to contain residual austenite in the steel sheet. The required amount of Cu depends on the relationship with the hardness of the ferrite phase.

In the steel sheet of the present invention, excellent formability is obtained by utilizing the fact that retained austenite is transformed into martensite when deformed. The martensite structure is very hard (Hv: about 800) due to the concentration of C, and the difference in hardness from the main phase (ferrite) is very large. However, in the steel sheet of the present invention, the hardness of the ferrite phase is increased. By doing so, the hardness difference from martensite is reduced. For this purpose, the hardness of the ferrite phase needs to be 150 or more in Hv.
% Must be added.

On the other hand, as the amount of Cu increases, the ferrite layer becomes harder and the strength of the steel sheet increases, but if the strength of the ferrite phase becomes too high, the formability deteriorates. The present inventors have conducted intensive investigations and have found that good moldability can be obtained if the Hv is 240 or less. And the upper limit of the amount of Cu required for that was set to 2.5%. In FIG. 1, C:
0.1%, Si: 1.2%, Mn: 1.5%, Cu:
A hot rolled steel sheet (thickness 2) rolled at 880 ° C. after hot rolling a steel sheet containing 0.8% and finishing hot rolling at 880 ° C.
mm, strength, hardness H of ferrite at about 600MPa)
The relation between v and the hole expansion rate and the total elongation El is shown. Here, the hole expansion ratio was a ratio d / d ₀ between the hole diameter (d) at the time when the crack penetrated the plate thickness and the initial hole diameter (d ₀ ).

The steel sheet of the present invention has the above-mentioned basic components, but may contain Al having the same function as Si. Al
Is used as a deoxidizing agent, does not form a solid solution in cementite like Si, suppresses the precipitation of cementite at 350 to 600 ° C., and delays the progress of transformation. However, since the ferrite-forming ability is stronger than that of Si, the onset of transformation is earlier, and even for a very short holding time, C is enriched in austenite as compared with annealing at a temperature where two phases coexist, and the chemical stability of austenite increases. . Therefore,
Only a small amount of martensite, which deteriorates the formability, is present in the metal structure after cooling to room temperature. For this reason, when Al and Si coexist, changes in strength and elongation due to holding conditions at 350 to 600 ° C. become small, and it becomes easy to obtain high strength and good press moldability.

Therefore, when Al is added, 0.01%
% Or more is desirable. On the other hand, when the Al concentration is increased, the spot welded portion is easily broken in the nugget, and at the same time, cracks caused by Al inclusions occur frequently and the manufacturability deteriorates. There is a need to. In the steel sheet of the present invention, by utilizing Cu precipitates, excellent formability and hole expandability can be achieved at the same time, but a uniform ferrite reinforced layer can be obtained by finely depositing Cu, thereby further improving the properties. Can be. As a result of a detailed investigation, the present inventors have confirmed that the effect of precipitates of 0.1 μm or less is large. If the amount of fine precipitation is small, the effect of strengthening ferrite is not exhibited, so that
The u precipitate is required to be 0.2% by mass or more.

Further, Ni is an austenite stabilizing element, Mo, Cr, V, B, Ti and Nb are elements for increasing the strength, and Ca and Mg are good because they bind to S in steel and reduce inclusions. Ni: less than 2.0%, Mo: 0.2
%, Cr: less than 1.0%, V: less than 0.3%, B:
Less than 0.01%, Ti: less than 0.06%, Nb: 0.0
Less than 6%, Ca: less than 0.05%, and Mg: 0.
Adding at least one or more of the components less than 05% as needed does not impair the spirit of the present invention.

The effect of these elements saturates at each of the above upper limits, so further additions will increase the cost. Further, since Ni prevents a surface defect called “Cu scab” by the complex addition with Cu, it is desirable to contain Ni by １ ／ Cu. The steel sheet of the present invention has the above basic components, but contains, in addition to these elements and Fe, other elements that are inevitably mixed into general steel.

The content of S is desirably low in order to enhance the hole expandability of the steel sheet. If it exceeds 0.01%, MnS inclusions are formed and a crack generation point is formed at the time of hole expansion. It is desirable to set it to 01%. P is Si, A
Like l, it is an element that does not dissolve in cementite, but is preferably less than 0.03% from the viewpoint of preventing weldability and intergranular fracture embrittlement.

The formability of the steel sheet of the present invention as a final product depends on the volume fraction of retained austenite contained in the product. The retained austenite contained in the metal structure is stable when not deformed, but transforms into martensite when deformed and exhibits transformation-induced plasticity, so that good formability with high strength can be obtained. If the volume fraction of retained austenite is less than 2%, no clear effect is observed.

On the other hand, the volume fraction of retained austenite is 20
%, When extremely severe molding is performed, a large amount of martensite is present in a pressed state, which may cause problems in secondary workability and impact properties. Therefore, in the steel sheet of the present invention, the volume ratio of retained austenite is set to 20% or less. The metal structure further includes ferrite, bainite, martensite, and carbide.

Next, the reasons for limiting the methods for producing the hot-rolled steel sheet, cold-rolled steel sheet, hot-dip galvanized steel sheet and galvannealed steel sheet of the steel of the present invention will be described. The hot-rolled steel sheet of the steel of the present invention, when hot rolling after casting a steel material satisfying the requirements of the above component composition, finishes hot rolling at a temperature of 800 ° C or more, and thereafter, at 350 ° C to 600 ° C. Obtained by winding.

If the rolling end temperature is lowered, the processed structure remains in the steel, so that the retained austenite cannot be secured and the ductility is deteriorated. Therefore, the rolling end temperature must be 800 ° C. or higher. Although the upper limit of the rolling end temperature is not particularly defined, it is desirably 1000 ° C. or lower which can be produced industrially stably. Cu
The strength of the ferrite phase and the form of Cu precipitates of the steel sheet to which is added greatly changes depending on the winding temperature, and when wound in an appropriate temperature range, it is possible to achieve both excellent hole expandability and formability. .

At a winding temperature of less than 350 ° C., the amount of fine particles of the Cu particles is not sufficient, and the bainite transformation does not proceed, so that retained austenite cannot be secured. Therefore 3
A winding temperature of 50 ° C. or higher is required. On the other hand, 600
When wound at a temperature exceeding ℃, Cu particles are coarsely precipitated,
In addition, carbides are precipitated and retained austenite cannot be secured, resulting in poor hole expandability and ductility. Therefore, 600
A winding temperature of less than ℃ is required. Under the above manufacturing conditions, a hot-rolled steel sheet excellent in hole expandability and formability can be manufactured.

The cold-rolled steel sheet of the present invention is prepared by annealing a cold-rolled steel sheet satisfying the above-mentioned steel composition in a dual phase coexisting temperature range of 650 to 850 ° C. for 10 seconds or more and 10 minutes or less. s
At a cooling rate of 350 to 600 ° C., and then kept at this temperature range for 10 seconds or more and 10 minutes or less.
It is obtained by cooling to 250 ° C. or less at a cooling rate of s or more.

Further, the hot-dip galvanized steel sheet of the steel of the present invention comprises:
After annealing a cold-rolled steel sheet satisfying the above steel composition in a dual phase coexisting temperature range of 650 to 850 ° C for 10 seconds or more and 10 minutes or less,
It is cooled to 350 to 600 ° C. at a cooling rate of 4 to 200 ° C./s, and then kept at this temperature range for 10 seconds or more and 10 minutes or less, then subjected to hot-dip galvanizing, and then cooled at 5 ° C./s or more. By cooling to a temperature of 250 ° C. or less at a rate, a Zn plating layer made of Al: 1% or less and unavoidable impurities is formed on the surface of the steel sheet.

Further, the alloyed hot-dip galvanized steel sheet of the steel of the present invention is a cold-rolled steel sheet satisfying the above-mentioned steel composition in an amount of 650-8.
After annealing for 10 seconds or more and 10 minutes or less in a two-phase coexisting temperature range of 50 ° C., 350 to 500 ° C. at a cooling rate of 4 to 200 ° C./s.
℃, then hot-dip galvanized, 450
After holding for 5 seconds or more and 2 minutes or less in a temperature range of ６００600 ° C., and cooling to 250 ° C. or less at a cooling rate of 5 ° C./s or more, Fe: 8 to 15% A
l: It is obtained by forming a Zn alloy plating layer of 1% or less and inevitable impurities.

In the continuous annealing of a cold-rolled steel sheet after cold rolling, first, in order to obtain a two-phase structure of [ferrite + austenite], heating is performed in a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point. . At this time, if the heating temperature is lower than 650 ° C., it takes too much time for the cementite to re-dissolve in solid form, and the amount of austenite also becomes small, so the lower limit of the heating temperature is 650 ° C.

If the heating temperature is too high, the volume ratio of austenite becomes too large, and the C concentration in austenite decreases. Therefore, the upper limit of the heating temperature is set to 850 ° C. If the soaking time is too short, the possibility of the presence of undissolved carbides increases and the amount of austenite decreases. In addition, when the soaking time is increased, the possibility that the crystal grains become coarse increases, and the strength-ductility balance deteriorates. Therefore, in the manufacturing method of the present invention, the holding time is set to 10
The time is from 10 seconds to 10 minutes.

After soaking, it is cooled to 350 to 600 ° C. at a cooling rate of 4 to 200 ° C./s. This is because the austenite generated by heating to the two-phase coexistence temperature range is carried over to the bainite transformation region without transforming to pearlite, and by subsequent processing, to obtain predetermined characteristics as room temperature retained austenite and bainite at room temperature. . If the cooling rate at this time is less than 4 ° C./s, most of the austenite undergoes pearlite transformation during cooling, so that retained austenite cannot be secured. On the other hand, if the cooling rate exceeds 200 ° C./s, the cooling end point temperature is greatly shifted in the width direction and the longitudinal direction, and it becomes impossible to produce a uniform steel sheet.

Thereafter, when producing cold-rolled steel sheets and hot-dip galvanized steel sheets, at 350 to 600 ° C. for 10 seconds or more,
Hold for 0 minutes or less. At this time, bainite undergoes bainite transformation and contains almost no carbides, residual austenite in which C swept from the portion is concentrated and the Ms point is lowered to room temperature or less, and cleansing during heating to a two-phase coexisting temperature range. The emergence of a microstructure with mixed ferrite,
A high-strength steel sheet having both hole-expanding properties and formability is obtained by finely depositing Cu in ferrite at the same time as achieving both high strength and formability.

If the holding temperature is less than 350 ° C., the amount of fine particles of the Cu particles is not sufficient, and good hole expandability cannot be ensured. Therefore, a holding temperature of 350 ° C. or more is required. on the other hand,
If the temperature is maintained at a temperature exceeding 600 ° C., the Cu particles coarsely precipitate to deteriorate the hole expandability and ductility. Therefore, it is necessary to maintain the temperature at 600 ° C. or less. If the holding time is short, bainite transformation does not proceed sufficiently, so that austenite enriched with C cannot be obtained. As a result, when cooled to room temperature, most of the austenite is transformed into martensite, so that although the strength becomes high, the press formability deteriorates. In addition, when the holding time is increased, the retained austenite is decomposed to precipitate carbides. As a result, moldability deteriorates. From the above viewpoint, the holding time is defined as 10 seconds or more and 10 minutes or less.

When a hot-dip galvanized steel sheet is manufactured, hot-dip galvanizing is performed thereafter. Then, it is cooled to 250 ° C. or less at a cooling rate of 5 ° C./s or more. If the cooling rate after the holding is set to less than 5 ° C. or the cooling end point temperature is set to more than 250 ° C., the bainite transformation proceeds further, and the austenite enriched in C also precipitates carbides and decomposes into bainite in the preceding reaction. The amount of retained austenite, which improves workability due to induced plasticity, is reduced, and the original purpose cannot be achieved.

In the case of producing an alloyed hot-dip galvanized steel sheet, the steel sheet is cooled to 350 to 500 ° C., then hot-dip galvanized, and kept in a temperature range of 450 to 600 ° C. for 5 seconds or more.
Min. And then at a cooling rate of 5 ° C./s or more for 25 min.
Cool below 0 ° C. When the cooling end point temperature is lowered, most of austenite is transformed into martensite, so that although the strength is high, the press formability is deteriorated. In addition, when the cooling end point temperature is lowered, it is necessary to increase the temperature of the steel sheet at the time of Zn plating, and it becomes inefficient because it is necessary to apply extra heat energy.

On the other hand, when the cooling end point temperature is high, the alloying reaction between iron and zinc occurs vigorously when hot-dip galvanizing is performed, and the iron% of the galvanized layer becomes too high.
It becomes difficult to control the plating film thickness. Therefore, in the manufacturing method of the present invention, the cooling end point temperature is set to 350 to 500 ° C. The holding temperature after the hot-dip galvanizing and the subsequent cooling conditions are determined from the viewpoint of the alloying reaction between Fe and Zn and the organizational viewpoint. Since the steel sheet of the present invention contains Si and Al, it utilizes the fact that the transformation from austenite to bainite is separated in two steps, and bainite containing almost no carbide, and C that has been swept from that part is concentrated. A structure in which retained austenite in which the Mn point has dropped to room temperature or lower and ferrite that has been cleaned during heating in a two-phase coexisting temperature region is present is mixed, thereby achieving both high strength and formability.

At the same time, by precipitating fine Cu in the ferrite, it is possible to obtain excellent hole expandability. If the holding temperature exceeds 600 ° C., pearlite is generated, so that residual austenite is not included, and the alloying reaction proceeds too much, and the Fe concentration in the plating exceeds 15%, and at the same time, Cu precipitates are formed. It coarsens and also deteriorates hole expandability. On the other hand, when the heating temperature is 450
If the temperature is lower than ℃, the alloying reaction rate of plating becomes slow,
The Fe concentration during plating is reduced. On the other hand, if the holding time is less than 5 seconds, bainite is not sufficiently generated, C enrichment in untransformed austenite becomes insufficient, and martensite is generated during cooling to deteriorate the formability, and at the same time, plating The alloying reaction becomes insufficient. On the other hand, if the holding time exceeds 2 minutes, over-alloying of the plating occurs, and the plating tends to peel off during molding.

Further, when the cooling rate after the holding is set to less than 5 ° C. or the cooling end point temperature is set to more than 250 ° C., the bainite transformation further proceeds, and the austenite in which C is enriched in the preceding reaction also precipitates carbide. Broken down into bainite,
The original purpose cannot be achieved because the amount of retained austenite, which improves workability, is reduced by transformation induced plasticity.

The hot-dip Zn plating layer contains Al: 1% or less, with the balance being Zn and unavoidable impurities.
The reason why the Al content in the plating is 1% or less is that when the Al content exceeds 1%, the Al segregated during the plating constitutes a local battery, the corrosion resistance is deteriorated, and at the same time, the Fe-Al alloy layer is formed. This is because it is remarkably developed and the plating adhesion is deteriorated.

Further, as the Zn alloy plating layer, Fe:
8 to 15%, Al: 1% or less, with the balance being Zn and unavoidable impurities. The reason why the Fe content in the plating layer is 8% or more is that when the Fe content is less than 8%, the chemical conversion treatment (phosphate treatment) coating film adhesion becomes good. Also,
The reason why the Fe content is set to 15% or less is that if the Fe content exceeds 15%, the alloy becomes overalloy and the plating adhesion of the processed portion deteriorates.

The reason why the Al content in the plating is set to 1% or less is that, when the Al content exceeds 1%, Al segregated during the plating constitutes a local battery, the corrosion resistance is deteriorated, and at the same time, the Fe content is reduced. This is because the Al alloy layer remarkably develops and the plating adhesion deteriorates. The Zn plating layer and the Zn alloy plating layer in the present invention are as described above.
Mn, Pb, Fe, Sb, Ni, Cu, Sn, Co, C
It may contain elements inevitably mixed such as d and Cr. Moreover, Mg and Ca have the effect of reducing oxides on the surface layer of the steel sheet during Zn plating and improving the plating adhesion, and therefore may contain less than 8% and less than 1%, respectively.

In some cases, when manufacturing an alloyed hot-dip galvanized steel sheet, before hot-dip galvanizing,
The temperature may be maintained within a range of 350 to 500 ° C. for 10 minutes or less. By maintaining the temperature before Zn plating, bainite transformation can be advanced and the retained austenite in which C is concentrated can be stabilized, and a steel sheet having more stable strength and elongation can be manufactured more stably. At this time, if the holding time exceeds 10 minutes, the heating after the Zn plating causes deterioration of both strength and press formability due to precipitation of carbides and disappearance of untransformed austenite. Therefore, the holding time is desirably 10 minutes or less.

The hot-dip galvanizing temperature is desirably from the melting point of the plating bath to 500 ° C. or less. If the temperature exceeds 500 ° C., the vapor from the plating bath becomes large, and the operability deteriorates. The heating rate up to the holding temperature after plating does not need to be particularly specified.
C./s or more is desirable. Further, the thickness of the Zn alloy plating layer is not particularly limited, but from the viewpoint of corrosion resistance,
It is preferable that the thickness be 0.1 μm or more and 15 μm or less from the viewpoint of workability.

The material of the cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet is, in principle, manufactured through the refining, casting, hot-rolling, and cold-rolling steps, which are ordinary iron-making processes. A material manufactured by omitting a part or all does not impair the purpose of the present invention. It should be noted that the temperatures and cooling temperatures in the above-described steps do not need to be constant as long as they are within the specified ranges, and even if the temperatures fluctuate within the ranges, the characteristics of the final product do not deteriorate at all and may be improved. is there.

Further, in order to further improve the plating adhesion, the atmosphere during the annealing of the steel sheet is adjusted so that the steel sheet surface is oxidized at first and then reduced to clean the steel sheet surface before plating. Is also good. Further, in order to improve the plating adhesion, there is no problem if the oxide on the steel sheet surface is removed by pickling or grinding the steel sheet before annealing.
By performing these treatments, the plating adhesion is further improved.

[0052]

EXAMPLES Example 1 A steel having a component composition shown in Table 1 was melted in a converter and then continuously cast into a slab, and hot-rolled under the production conditions shown in Table 2, to obtain a hot-rolled steel sheet. Manufactured. About the manufactured hot-rolled steel sheet, the following "tensile test", "hole expansion test", "ferrite phase hardness test",
"Remaining austenite measurement test", "welding test", and "Cu precipitate observation" were performed. Here, the furnace temperature during heating was 200 minutes, and the soaking time was 60 minutes. The thickness of the manufactured hot-rolled steel sheet was 2 mm.

In the "tensile test", a JIS No. 5 tensile test piece was sampled and a gauge thickness of 50 mm and a tensile speed of 10 mm / min.
Then, a room temperature tensile test was performed. "Hole expansion test" is 2
The punched hole of 0 mm was pushed out from the surface without burrs with a 30 conical punch. The results, hole diameter at the time the crack penetrated the plate thickness (d) to the initial hole diameter: indicated by the ratio d / d ₀ with (d ₀ 20mm).

The "ferrite phase hardness" was measured by a Vickers hardness test using a diamond square indenter having a facing angle of 136 degrees and a test load of 500 g. The “remaining austenite measurement test” is a test in which α-Fe and γ are analyzed by X-ray diffraction using a Mo tube after chemically polishing a 1/4 inner layer from the surface layer.
-A method called a 5-peak method determined from the intensity of Fe.

The "welding test" was conducted with a welding current of 10 kA, a pressing force of 220 kg, a welding time of 12 cycles, and an electrode diameter of 6 m.
m, electrode shape: dome shape, spot welding was performed under welding conditions of 6φ-40R at the tip, and the number of continuous hitting points until the nugget diameter fell below 4 ° t (t: plate thickness) was evaluated. The evaluation criteria were: ○: Continuous hit points exceeding 1000 points, Δ: Continuous hit points: 500 to
1000 points, x: Continuous hit points were less than 500 points. Here, ○ was regarded as pass, and Δ · × was rejected.

The “observation of Cu precipitate” means that the extraction replica method obtained by the SPEED method is applied to a transmission electron microscope (TE).
M). Observation was performed at 20,000 times in 10 visual fields. Each field is 2.7 μm × 4 μm. The diameter and number of Cu precipitates were measured in each field, and C
The mass% of the amount of the precipitated u was calculated. Table 3 shows the performance evaluation test results.
Shown in Samples 1 to 10 of the present invention have a retained austenite of 2 to 20% and 550 MPa or more, a total elongation of 30% or more, and a hole expansion ratio of more than 1.3. A hot-rolled steel sheet that has both good formability and good hole expandability, and at the same time, has satisfactory weldability.

On the other hand, sample 11 has a low C concentration, sample 12 has a high C concentration, sample 13 has a low Si concentration, sample 14 has a high Si concentration, and sample 15 has a high Mn concentration. Because the concentration is low, sample 16 has a high Mn concentration, and sample 17 has a high Al concentration.
Sample No. 8 has a low Cu concentration, and Sample 19 has a high Cu concentration. Therefore, high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, and weldability are not all satisfied. Can not achieve its purpose.

Further, even in the steel sheet of the present invention, if there is a problem in one of the processing conditions, as in Samples 20 to 22, high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, All of the weldability is not satisfied, and the object of the present invention cannot be achieved.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

Example 2 A steel having the composition shown in Table 1 was melted in a converter and converted into a slab by continuous casting. Thereafter, the heating temperature was 1200 ° C., the furnace time was 200 minutes, and the finish rolling end temperature was 880. Hot rolling at a winding temperature of 600 ° C. to obtain a hot-rolled steel sheet of 4 mm. Thereafter, it was pickled and cold rolled to obtain a 1.4 mm material. Thereafter, annealing was performed under the manufacturing conditions shown in Table 4 to manufacture a cold-rolled steel sheet.

For the manufactured cold-rolled steel sheet, the “tension test”, “hole expansion test”, “ferrite phase hardness”, “remaining austenite measurement test”, “welding test” were performed in the same manner as in Example 1. And "Cu precipitate observation" was performed. Table 5 shows the results of the performance evaluation test. Samples 23 to 32 of the present invention have a residual austenite of 2 to 20% and a residual austenite of 5%.
Although it is 50 MPa or more, the total elongation is 30% or more, and the hole expansion ratio also exceeds 1.3, and at the same time, both high strength, good press moldability, and good hole expansion are achieved. It is a cold rolled steel sheet that also satisfies weldability.

On the other hand, sample 33 has a low C concentration, sample 34 has a high C concentration, sample 35 has a low Si concentration, sample 36 has a high Si concentration, and sample 37 has a high Mn concentration. Because the concentration is low, sample 38 has a high Mn concentration, and sample 39 has a high Al concentration.
The sample No. 0 has a low Cu concentration, and the sample 41 has a high Cu concentration. Therefore, all of the high strength, formability, hole expandability, residual austenite, ferrite phase hardness, and weldability are not satisfied. Can not achieve its purpose.

Even if the steel of the present invention has a problem in one of the processing conditions, high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, weldability are obtained as in Samples 42 to 52. Are not satisfied, and the object of the present invention cannot be achieved.

[0066]

[Table 4]

[0067]

[Table 5]

Example 3 A steel having the component composition shown in Table 1 was melted in a converter and converted into a slab by continuous casting. Thereafter, the heating temperature was 1200 ° C., the furnace time was 200 minutes, and the finish rolling end temperature was 880. Hot rolling at a winding temperature of 600 ° C. to obtain a hot-rolled steel sheet of 4 mm. Thereafter, it was pickled and cold rolled to obtain a 1.4 mm material. Thereafter, annealing and hot-dip galvanizing were performed under the manufacturing conditions shown in Table 6 to manufacture hot-dip galvanized steel sheets. Here, the bath temperature of the hot-dip galvanizing is 460 ° C.
And For the manufactured hot-dip galvanized steel sheet, in the same manner as in Example 1 and Example 2, “tensile test”, “hole expansion test”, “ferrite phase hardness”, “remaining austenite measurement test”, "Welding test" and "Cu precipitate observation" were performed. further,
Tests of “plating adhesion” and “measurement of concentration in plating layer” were performed.

The “plating adhesion” was evaluated according to the following criteria by conducting a tape test after performing a 60 ° V bending test on a plated steel sheet. Tape test blackness (%) Evaluation: ◎ ０〜 0 to 10 Evaluation:… 未 満 Less than 10 to 20 Evaluation:… 未 満 Less than 20 to 30 Evaluation: × 以上 30 or more (と and 合格 pass, Δ △ x fail) The "measurement of the concentration in the plating layer" was measured by dissolving the plating layer with 5% hydrochloric acid containing an amine inhibitor, followed by ICP emission spectrometry.

Table 7 shows the results of the performance evaluation test. Samples 53 to 62 of the present invention have a retained austenite of 2 to 20%.
The total elongation is more than 30% while it is 550MPa or more, and the hole expansion ratio also exceeds 1.3, and at the same time, both high strength, good press moldability and good hole expansion are achieved. It is a hot-dip galvanized steel sheet that also satisfies weldability and plating properties.

On the other hand, the sample 63 has a low C concentration, the sample 64 has a high C concentration, the sample 65 has a low Si concentration, the sample 66 has a high Si concentration, and the sample 67 has a high Mn concentration. Since the concentration is low, the sample 68 has a high Mn concentration, and the sample 69 has a high Al concentration.
0 is because the Cu concentration is low, and the sample 71 is because the Cu concentration is high, and all of the high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, weldability, and plating property are not satisfied. However, the object of the present invention cannot be achieved.

Further, even if the steel of the present invention has a problem in one of the processing conditions, high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, weldability are obtained as in samples 72 to 83. However, all of the plating properties are not satisfied, and the object of the present invention cannot be achieved.

[0073]

[Table 6]

[0074]

[Table 7]

Example 4 A steel having the composition shown in Table 1 was melted in a converter and converted into a slab by continuous casting. Thereafter, the heating temperature was 1200 ° C., the furnace time was 200 minutes, and the finish rolling end temperature was 880. Hot rolling at a winding temperature of 600 ° C. to obtain a hot-rolled steel sheet of 4 mm. Thereafter, pickling and cold rolling were performed.
A 4 mm material was obtained. Thereafter, annealing, hot-dip galvanizing, and alloying treatment were performed under the manufacturing conditions shown in Table 8 to produce an alloyed hot-dip galvanized steel sheet. Here, the bath temperature of the hot-dip galvanizing was 460 ° C. Regarding the manufactured galvannealed steel sheet, the same method as in Example 3 was used.
"Tension test", "Hole expansion test", "Ferrite phase hardness", "Residual austenite measurement test", "Welding test", "Cu precipitate observation", "Plating adhesion", and "Concentration in plating layer" Measurement ".

Table 9 shows the results of the performance evaluation test. Samples 84 to 93 of the present invention have a retained austenite of 2 to 20%.
The total elongation is more than 30% while it is 550MPa or more, and the hole expansion ratio also exceeds 1.3, and at the same time, both high strength, good press moldability and good hole expansion are achieved. It is a galvannealed steel sheet that also satisfies weldability.

On the other hand, sample 94 has a low C concentration, sample 95 has a high C concentration, sample 96 has a low Si concentration, sample 97 has a high Si concentration, and sample 98 has a high Mn concentration. Because the concentration is low, sample 99 has a high Mn concentration, sample 100 has a high Al concentration, sample 101 has a low Cu concentration, and sample 102 has a high Cu concentration. Properties, hole expandability, retained austenite content, ferrite phase hardness, and weldability are not all satisfied, and the object of the present invention cannot be achieved.

Further, even if the steel of the present invention has a problem in one of the processing conditions, high strength and formability, hole expandability, residual austenite amount, ferrite phase hardness, weldability are obtained as in samples 103 to 118. Are not satisfied, and the object of the present invention cannot be achieved.

[0079]

[Table 8]

[0080]

[Table 9]

[0081]

As described above, according to the present invention,
Since it is possible to achieve both good formability and hole expandability while having high strength, it can sufficiently respond to new demands from users of steel sheets, and is extremely useful industrially.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the hardness Hv of ferrite, the hole expansion ratio, and the total elongation El.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C22C 38/16 C22C 38/16 38/58 38/58 (72) Inventor Takehisa Senuma 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka -1 F-term in Nippon Steel Corporation Yawata Works (reference) 4K037 EA02 EA05 EA06 EA09 EA11 EA13 EA14 EA15 EA16 EA17 EA19 EA20 EA27 EA28 EA31 EA32 EB05 EB09 EB12 FC03 FE01 FE02 FJ05 F06K

Claims (9)

[Claims] 1. Mass%, C: 0.05% or more, 0.2% or less, Si: 0.5% or more, 2.5% or less, Cu: 0.5% or more, 2.5% or less , Mn: 0.5% or more and 3.0% or less, and
A high-strength steel sheet having excellent formability and hole expandability, wherein the ferrite phase has a hardness of 150 or more and 240 or less in Hv, and the volume ratio of retained austenite in the steel structure is 2 to 20%. 2. In mass%, C: 0.05% or more and 0.2% or less, Si: 0.5% or more and 2.5% or less, Al: 0.01% or more and 2.5% or less , Cu: 0.5% or more and 2.5% or less, Mn: 0.5% or more and 3.0% or less, and
A high-strength steel sheet having excellent formability and hole expandability, wherein the ferrite phase has a hardness of 150 or more and 240 or less in Hv, and the volume ratio of retained austenite in the steel structure is 2 to 20%. 3. The high-strength steel sheet excellent in formability and hole expandability according to claim 1 or 2, further comprising:
i: less than 2.0%, Mo: less than 0.2%, Cr: 1.0
%, V: less than 0.3%, B: less than 0.01%, T
i: less than 0.06%, Nb: less than 0.06%, Ca:
A high-strength steel sheet excellent in formability and hole expandability, characterized by containing at least one of less than 0.05% and Mg: less than 0.05%. 4. The high-strength steel sheet according to claim 1, which is excellent in formability and hole expandability, has a thickness of 0.1 μm.
A high-strength steel sheet excellent in formability and hole expandability, characterized in that the amount of precipitated Cu having the following size is 0.2% by mass or more. 5. A forming method characterized in that a steel satisfying the steel composition according to claim 1, 2 or 3 is subjected to hot rolling at a temperature of 800 ° C. or higher, and then wound at 350 to 600 ° C. For producing high-strength hot-rolled steel sheets with excellent heat resistance and hole expansion properties. 6. After annealing a cold rolled steel sheet satisfying the steel composition according to claim 1, in a dual phase coexisting temperature range of 650 to 850 ° C. for 10 seconds or more and 10 minutes or less, 4 to 200 minutes. ° C
/ S at a cooling rate of 350-600 ° C., and after holding at this temperature range for 10 seconds or more and 10 minutes or less,
A method for producing a high-strength cold-rolled steel sheet having excellent formability and hole expandability, characterized by cooling to 250 ° C or less at a cooling rate of 5 ° C / s or more. 7. A cold rolled steel sheet satisfying the steel composition according to claim 1, 2 or 3 is annealed in a dual phase coexisting temperature range of 650-850 ° C. for 10 seconds or more and 10 minutes or less, and then 4 to 200 minutes. ° C
/ S at a cooling rate of 350-600 ° C., and after holding at this temperature range for 10 seconds or more and 10 minutes or less,
By applying hot-dip galvanizing and then cooling to 250 ° C or less at a cooling rate of 5 ° C / s or more,
A method for producing a high-strength hot-dip galvanized steel sheet having excellent formability and hole expandability, characterized by forming a Zn plating layer comprising Al: 1% or less and inevitable impurities. 8. A cold rolled steel sheet satisfying the steel composition according to claim 1, 2 or 3 is annealed in a dual phase coexisting temperature range of 650-850 ° C. for 10 seconds or more and 10 minutes or less, and then 4 to 200 minutes. ° C
/ S at a cooling rate of 350-500 ° C., then hot-dip galvanized, and kept in a temperature range of 450-600 ° C. for 5 seconds or more and 2 minutes or less, and then 5 ° C./s or more. Formability and hole enlargement characterized by forming a Zn alloy plating layer comprising Fe: 8 to 15%, Al: 1% or less and unavoidable impurities on the surface of the steel sheet by cooling to a temperature of 250 ° C. or less at a cooling rate. Method for producing high-strength galvannealed steel sheet with excellent heat resistance. 9. After annealing the cold-rolled steel sheet in a two-phase coexisting temperature range of 650 to 850 ° C. for 10 seconds or more and 10 minutes or less,
The high-strength alloy according to claim 8, wherein the alloy is cooled to 350-500C at a cooling rate of -200C / s, and then kept at this temperature range for 10 minutes or less. Manufacturing method of galvannealed steel sheet.