JP2003193186A - High strength steel sheet and high strength electroplated steel sheet having excellent ductility, stretch-flanging property and impact absorbing property and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet and a high strength electroplated steel sheet which have a tensile strength of 400 to 540 MPa, excellent ductility and stretch-flanging properties, and excellent impact absorbing properties, and to provide a production method therefor. <P>SOLUTION: The steel sheets have a componential composition containing, by mass, 0.02 to 0.08% C, ≤0.50% Si, 0.5 to 1.5% Mn, ≤0.04% P, ≤0.005% S, ≤0.1% Al, and ≤0.025% N, and further containing Cr and Mo so as to satisfy Cr+1.2Mo≥0.2%, and have a structure in which the fraction of a ferritic structure as the main phase is ≥85% in an area ratio, and the ratio of a martensitic structure is 2 to 10% in an area ratio, and the total fraction of the second phase containing the above martensite is ≤15% in an area ratio. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel sheet (also referred to as a high-strength steel sheet) and a high-strength electroplated steel sheet having high ductility, stretch flangeability, and shock absorption characteristics, which are used as steel sheets for automobiles and the like. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Various thin steel sheets are used for automobile bodies, but cold-rolled steel sheets having excellent formability are often used. Further, recently, the amount of high-tensile steel plates used has been increasing in order to reduce the weight of automobile bodies. However, when the conventional high-strength steel sheet is adopted instead of the cold-rolled steel sheet having relatively low strength, there are the following problems. 1) Ductility and stretch flangeability are poor. 2) In-plane anisotropy of mechanical properties is large. 3) In particular, a steel sheet having a tensile strength TS in the strength range of 400 MPa to 540 MPa has a large variation in material and is difficult to manufacture stably. 4) There is little margin for improving shock absorption characteristics, and there is little merit in using high-tensile steel plate.

In order to solve the above-mentioned problems, conventionally,
Measures such as reduction of C amount and reduction of S amount are taken.
However, these measures alone could not solve all the above problems. As another measure, a technology has been proposed to improve the stretch flangeability by adding a small amount of elements such as Nb and Ti, but this technology is because these elements (Nb, Ti, etc.) Since the deformation resistance is increased, there is a problem that hot rolling becomes difficult. Moreover, the addition of these elements makes it difficult to produce a relatively low strength and high formability steel sheet having a tensile strength (TS) of 540 MPa or less due to the precipitation strengthening action, and further, annealing after cold rolling. However, there is a problem in that the recrystallization is delayed and the material changes greatly depending on the annealing conditions. Still other conventional technologies include miniaturization of matrix structure and single phase structure.
A method for improving stretch-flangeability by (mainly bainizing) is also proposed. However, this proposal is for relatively thick materials (thickness> 3 mm) such as those used for undercarriage parts of automobiles.
It is intended for steel sheets of high strength (about 540 MPa or more) and its ductility is not sufficient.

[0004]

By the way, among automobile members, for example, front and rear side members,
Depending on the application, such as various members such as cross members and various reinforcing members connected to them,
Although not subjected to deep drawing, there are some which require relatively high strength and high ductility and stretch flangeability. These members have a plate thickness of 0.8 mm or more and 2.3 mm or less and a tensile strength of 40 mm.
High-strength cold-rolled steel sheets of 0 MPa or more and 540 MPa or less are mainly used. Further, conventionally, only static strength was required for these members, but even in the group of parts which have been required to have excellent shock absorption characteristics due to the recent demand for ensuring safety against automobile collisions. is there. However, as described above, in the conventional technology, a steel plate satisfying all these required characteristics is
There was no industrially inexpensive and stable manufacturing technology.

The object of the present invention is to have a tensile strength of 400 MPa or more.
A high-strength steel sheet having excellent ductility and stretch-flangeability, and excellent impact absorption characteristics while having a medium strength of 0 MPa or less, a high-strength electroplated steel sheet obtained by plating the steel sheet, and their production. To propose a method.

[0006]

In order to achieve the above object, the inventor manufactured steel sheets having various component compositions and conducted a material evaluation experiment. As a result, in order to obtain a steel sheet having high ductility and excellent stretch-flangeability and impact absorption characteristics, it is particularly effective to control the composition of components within an appropriate range, and particularly to control the microstructure of the steel sheet. I found that there is. That is, in order to satisfy the above-mentioned required characteristics, the main phase of the steel sheet structure is a soft ferrite phase, and the remaining structure is a second phase containing a martensite phase of 2% or more and 10% or less and the martensite phase. It was found that it is necessary to reduce the value to 15% or less.

The present invention has been developed on the basis of the above findings, wherein C: 0.02 to 0.08 mass%, Si: 0.50 mass% or less, Mn: 0.5 to 1.5 mass%, P: 0.04 mass% or less, S: : 0.
005mass% or less, Al: 0.1mass% or less, N: 0.025mass%
Contains the following, and further contains Cr and Mo: singly or in combination so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2 mass%, and the balance is Fe and inevitable impurities, and is the main phase The area ratio of the ferrite structure is 85% or more, the area ratio is 2% or more and 10% or less, and the total ratio of the second phase containing the martensite is 15% or less. %, Ductility,
It is a high-strength steel sheet with excellent stretch flangeability and impact absorption characteristics.

In the present invention, in order to improve the mechanical properties, in addition to the above components, B: 0.0005 to 0.00
40mass%, Ca: 0.0010-0.01mass%, REM: 0.0010-
0.01mass%, Ti: 0.005-0.10mass%, Nb: 0.002-0.10
mass%, V: 0.005-0.10mass%, Cu: 0.01-1.0mass
%, Ni: 0.01 to 1.0 mass%, and preferably one or more of them.

The present invention also proposes a high strength electroplated steel sheet obtained by forming an electroplated film on the surface of the above steel sheet.

The present invention also provides C: 0.02 to 0.08 mass%, S
i: 0.50mass% or less, Mn: 0.5 to 1.5mass%, P: 0.04mass
s% or less, S: 0.005 mass% or less, Al: 0.1 mass% or less,
N: 0.025 mass% or less, and Cr, Mo: added alone or in combination so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2 mass%, or further, B: 0.0005 to
0.0040 mass%, Ca: 0.0010 to 0.01 mass%, REM: 0.00
10-0.01mass%, Ti: 0.005-0.10mass%, Nb: 0.002-
0.10mass%, V: 0.005-0.10mass%, Cu: 0.01-1.0mass
s%, Ni: 0.01 to 1.0 mass% of any one type or two or more types of steel slabs, the balance of which is Fe and unavoidable impurities, is hot-rolled, then pickled and cold-rolled. Rolled, then heated above 750 ℃ to below 900 ℃, 20 ℃ / s
Characterized by continuous annealing with cooling at the above rate,
We propose a method for manufacturing high-strength steel sheets with excellent ductility, stretch-flangeability and impact absorption characteristics.

Further, according to the present invention, C: 0.02 to 0.08 mass
%, Si: 0.50 mass% or less, Mn: 0.5 to 1.5 mass%, P: 0.
It contains 04 mass% or less, S: 0.005 mass% or less, Al: 0.1 mass% or less, N: 0.025 mass% or less, and further contains Cr, Mo:
Single or in combination, so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2mass%, or further, B: 0.0005 to
0.0040 mass%, Ca: 0.0010 to 0.01 mass%, REM: 0.00
10-0.01mass%, Ti: 0.005-0.10mass%, Nb: 0.002-
0.10mass%, V: 0.005-0.10mass%, Cu: 0.01-1.0mass
s%, Ni: 0.01 to 1.0 mass% of any one type or two or more types of steel slabs, the balance of which is Fe and unavoidable impurities, is hot-rolled, then pickled and cold-rolled. Rolled, then heated above 750 ℃ to below 900 ℃, 20 ℃ / s
We propose a method for producing a high-strength electroplated steel sheet having excellent ductility, stretch flangeability, and shock absorption characteristics, which is characterized by performing continuous annealing that cools at the above rate and then performing electroplating.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the component composition in the present invention will be described below. C: 0.02 to 0.08 mass% C is an important element for ensuring strength. However, when the amount of C exceeds 0.08 mass%, the stretch-flange formability of the steel sheet is significantly deteriorated due to an increase in the fraction of carbides in the steel. More importantly, the amount of C is 0.08
If it exceeds mass%, the spot weldability, arc weldability, etc. are significantly reduced. Therefore, from the viewpoint of formability and weldability, the C content is 0.08 mass% or less. In order to improve the moldability, it is 0.06 mass% or less, more preferably 0.05 mass% or less. On the other hand, when the C content is less than 0.02 mass%, the fine structure is remarkably coarsened and the stretch flangeability is deteriorated. Furthermore, it becomes difficult to stably obtain the martensite phase, which is one of the important requirements of the present invention.
ss% Preferably, 0.03 to 0.05 mass% is good.

Si: 0.50 mass% or less Si has the desirable effect of improving stretch flangeability while solid-solution strengthening the steel sheet, so it is appropriately added if necessary. However, addition of a large amount has a risk of deteriorating the surface properties. Therefore, without such problems, 0.
Limit to 50 mass% or less. Preferably 0.05 to 0.30 mass
% Is good.

Mn: 0.5 to 1.5 mass% Mn is an element effective in preventing hot embrittlement due to S, and it is necessary to add Mn according to the amount of S contained. Also Mn
In the present invention, is an essential additional element for stably obtaining the martensite phase. This effect is 0.5mass
% Becomes remarkable. Further, by increasing the amount of Mn added, there is an advantage that the sensitivity of mechanical properties to hot rolling conditions is significantly improved. However, if Mn is added excessively,
The ductility tends to decrease, the annealing temperature dependency of the martensite fraction becomes remarkable, the material variation due to the manufacturing conditions increases, and the stretch flangeability tends to deteriorate. Therefore, the upper limit is set to 1.5 mass%. It is preferably 0.7 to 1.4 mass%.

P: 0.04 mass% or less P is effective as a solid solution strengthening element for steel, and can be appropriately added depending on the required strength. However, if added excessively, the steel is embrittled and the welded portion is embrittled. further,
P is an element that easily segregates and reduces the stretch flangeability of the steel sheet. From the above, the upper limit was set to 0.04 mass%. When these deteriorations in characteristics are particularly important,
It is preferably 0.02 mass% or less.

S: 0.005 mass% or less S is present in the steel as inclusions, reduces ductility and stretch flangeability of the steel sheet, and further deteriorates corrosion resistance.
In a microstructured steel containing martensite as in the present invention, this tendency is particularly remarkable, so it is preferable to reduce S as much as possible. For these reasons, the upper limit is set to 0.005 mass%. In applications where particularly good stretch flangeability is required, 0.003 mass% or less is desirable.

Al: 0.1 mass% or less Al is added as a deoxidizing element, is effective for improving the cleanliness of steel, and is also an element which is desirable for refining the structure of steel. However, even if added in excess of 0.1 mass%, the refinement effect of the structure will be saturated and the cost will increase by the amount of alloy added, so the upper limit is 0.1 m.
Ass%. From the viewpoint of material stability, 0.02 to 0.0
6 mass% is more desirable. Further, excessive reduction of Al addition amount may lead to coarsening of crystal grains, but in the present invention, other alloying elements such as Mn are set to appropriate ranges, and hot rolling conditions are limited to optimum ranges. Can be prevented.

N: 0.025 mass% or less N has the effect of making the structure mainly composed of ferrite uniform and fine by adding an appropriate amount thereof, whereby proper strength and excellent stretch flangeability can be obtained. . Also,
N also has the effect of lowering the transformation point of steel, and addition of N is effective when it is desired to avoid rolling with the transformation point greatly interrupted with a thin product. 0.0015mass to get these effects
% Or more, more preferably 0.0020 ma
ss% or more. However, if added in excess of 0.025 mass%, the occurrence rate of internal defects in the steel sheet increases and causes slab cracking during continuous casting, so the upper limit was made 0.025 mass%. Even if N is added,
Within the range of the present invention, weldability and the like are not adversely affected. More preferably, it is 0.003 to 0.015 mass%.

Cr, Mo: Cr + 1 when added alone or in combination.
2Mo ≧ 0.2 mass% Cr, Mo is one of the important additional elements in the present invention, and has a desirable effect of expanding an annealing condition in which an appropriate amount of martensite remains after annealing. That is,
Addition of Cr and Mo makes it possible to secure stable mechanical properties over a wider range of conditions and reduce variations in mechanical properties. These elements alone or (Cr + 1.2Mo)
Then, by adding 0.20 mass% or more, the above desired effects can be obtained. The upper limit is determined from the upper limit of strength, but it is generally (Cr + 1.2Mo) and may be 1 mass% or less. Preferably, it is 0.25 to 0.8 mass%.

In the present invention, in addition to the above-mentioned essential elements, the following components can be selectively added in order to improve various mechanical properties. B: 0.0005 to 0.0040 mass% B has the effect of stabilizing the production of martensite,
This effect is exhibited by adding 0.0005 mass% or more. On the other hand, when it is added in an amount of more than 0.0040 mass%, the effect is saturated and in addition, surface cracking of the slab is caused, which is not desirable.

Ca: 0.0010 to 0.01 mass% Ca is an additional element effective for fixing S and improving stretch flangeability. It was also confirmed that the addition of Ca has an effect that the distribution of the second phase such as martensite in the cold rolled steel sheet becomes more isotropic. This desirable effect is exhibited by adding 0.0010 mass% or more, but 0.01 ma
Even if it is added in excess of ss%, the effect is saturated, and problems such as deterioration of corrosion resistance occur.

REM: 0.0010 to 0.01 mass% REM, like Ca, has the effect of stably fixing S and improving mechanical properties. Similarly, 0.0010 to 0.01 mass% is the optimum addition amount range. To get this effect,
The content is preferably 0.0010 mass% or more, but even if added in excess of 0.01 mass%, the effect is saturated and leads to an increase in manufacturing cost. Therefore, the REM content should be 0.0010 to 0.01 mass%. preferable.

Ti: 0.005 to 0.10 mass% Ti contributes to the improvement of ductility and stretch flangeability by the effect of making the structure uniform and fine. Ti also has a desirable effect of stably fixing S. To obtain these effects, it is preferable to contain 0.005 mass% or more,
Even if added in excess of 0.10mass%, its effect saturates, leading to an increase in manufacturing costs, so Ti is 0.005 to 0.10mass.
The content is preferably s%.

Nb: 0.002 to 0.10 mass% Nb has the effect of making the distribution of martensite particularly fine and uniform, and is effective in increasing the strength of the steel sheet without impairing the weldability. To get these effects, 0.
It is preferable to contain 002mass% or more, but 0.10mass%
If added in excess, the effect will saturate, leading to an increase in manufacturing cost. Therefore, the Nb content is preferably 0.002 to 0.10 mass%.

V: 0.005 to 0.10 mass% V has a desirable effect of compensating for the effects of Cr and Mo. By adding V, an appropriate amount of martensite can be stably ensured, and thus the mechanical properties finally obtained will be excellent and stable. To get these effects, 0.
It is preferable to contain 005 mass% or more, but 0.10 mass
Even if added in excess of%, the effect is saturated, leading to an increase in manufacturing cost. Therefore, it is preferable that the content of V be 0.005 to 0.10 mass%.

Cu: 0.01 to 1.0 mass% Cu can be added to increase the strength of the steel sheet. In addition, there is a merit that ductility and stretch flangeability are less deteriorated while a large increase in strength is obtained. Such a desired effect is achieved by adding 0.01 mass% or more, but even if added over 1.0 mass%, the effect tends to saturate, so the content is 0.01 to 1.0 mass%.
%.

Ni: 0.01 to 1.0 mass% Ni has the effect of enabling the martensite fraction, which is important in the present invention, to be controlled stably without largely depending on the manufacturing conditions. Such a desired effect is achieved by adding 0.01 mass% or more, but even if added over 1.0 mass%, the effect tends to be saturated, so the content is 0.01 to 1.0 m
It is preferably as%. In addition, even if these selective addition elements described above are added in a complex manner, their desired effects are not offset.

Next, the reasons for limiting the structure of the steel sheet according to the present invention will be described. Area ratio of ferrite phase: 85% or more The present invention is intended for a steel sheet for automobiles which is required to have high stretch flangeability. In order to secure the stretch-flangeability required for this application, ferrite must be the main phase and the ferrite area ratio must be 85% or more in terms of the area ratio to the entire structure. When higher stretch flangeability is required, a ferrite fraction of 95% or more is desirable. The ferrite referred to here means a so-called polygonal ferrite structure that does not include strain due to processing. Further, bainitic ferrite and acicular ferrite which do not include precipitation of carbide also bring about an appropriate increase in strength, have no deterioration in stretch-flange formability, and have a desirable structure, so they are included in the category of ferrite in the present application. The measurement of the fraction of the structure can be evaluated as the area ratio by observing the cross-sectional structure of the steel sheet with an optical microscope.

Martensite Fraction: 2% or more and 10% or less Martensite is an extremely important structure for achieving the excellent balance between strength, ductility and stretch flangeability, which is the goal of the present invention. Further, when the area ratio of martensite is less than 2% in terms of the area ratio with respect to the entire structure, a sufficient ductility improving effect cannot be obtained, and the tensile strength is also low. However, if the fraction exceeds 10% in terms of area ratio, the strength is greatly increased, but the stretch flangeability is significantly reduced, which is not desirable. Although the detailed mechanism is unknown, even at the strength level as in the present invention,
In order to give excellent impact absorption resistance, it is essential to control the martensite fraction within the range of 2 to 10%.

Total fraction of the second phase: 15% or less If the second phase fraction containing the above-mentioned martensite exceeds 15% (the balance is the ferrite structure), ductility is lowered and stretch flangeability is lowered. There is a tendency. This is the second
This is because when the phase fraction increases, the phases are connected to each other and exist in a band shape. When the second phase fraction exceeds 15%, the impact resistance anisotropy becomes large, and when applied to a shock absorbing component, stable shock absorbing characteristics cannot be obtained. The structure contained in the second phase includes pearlite, bainite, retained austenite, cementite, etc. in addition to the martensite.
Different tissues can be selected depending on the target level of strength, elongation and the like.

Next, the mechanical properties of the steel sheet according to the present invention will be described. Tensile Strength TS: 400 to 540 MPa When the steel sheet of the present invention is applied to a cold rolled steel sheet having a tensile strength TS of 400 to 540 MPa, its excellent effect is remarkably exhibited. The reason for this is that for high strength steel plates of 540 MPa or more,
It is easy to obtain a finer structure, and it is relatively easy to improve the hole expandability, but in addition to the decrease in ductility, the shape fixability is significantly reduced, which is the object of the present invention. This is because it is not suitable for applications that require high performance. Also, TS
Is less than 400 MPa, it is possible to easily manufacture a steel sheet having excellent mechanical properties by a method which has been widely used only for solid solution strengthening.

Hole expandability: 80% or more Stretch-flangeability is represented by the hole expandability that corresponds well to flange cracks that occur in actual parts. The hole expandability test is
The method according to the Iron and Steel Federation Standard (JFST1001) was performed, the initial hole diameter was set to 10 mm, and the hole diameter D (mm) when the crack penetrates the plate thickness was measured, and the following formula ((D-10) / 10 ) × 100 (%) to calculate the hole expansion rate. The steel sheet of the present invention has a hole expansion ratio of 80.
% Or more. If this value is 80% or more, press molding can be performed on the part targeted by the present invention without causing flange cracks. 100% hole expansion rate
The above is more preferable. The steel sheet of the present invention is also characterized in that stable hole expanding workability (stretch flangeability) can be obtained even if the clearance during punching changes.
Is one.

Impact absorption characteristics In the steel sheet of the present invention, the impact absorption characteristics are also important.
This impact absorption property is evaluated by high-speed tensile property, that is, the ratio of dynamic strength (TS at tensile strain rate of 500 / s) and static strength (TS at normal tensile strain rate of 10 ^-3 / s). can do. If this ratio is 1.2 or more, it can be judged that the impact absorption characteristics are excellent.

The effect of the present invention is remarkable when it is applied to a cold-rolled steel sheet having a plate thickness of 0.8 mm to 2.3 mm because of the use requiring the above characteristics. However, even if the present invention is applied to plate thicknesses other than the above, the effect is not impaired.

Next, the characteristic structure of the manufacturing method according to the present invention will be described. Hot rolling Hot rolling may be performed according to a conventional method and is not particularly limited. The preferable conditions will be described below. (1) Slab heating Steel slabs having the above component composition are generally preferably manufactured by a continuous casting method in order to reduce segregation of the components, but may be manufactured by a slab casting method or a thin slab casting method. . For heating the slab before hot rolling, in addition to the conventional method in which the slab is cooled to room temperature after it is manufactured and then reheated, it is charged into the heating furnace as it is without cooling or with a slight heat retention. Energy-saving processes such as direct rolling and direct rolling in which rolling is performed immediately after the rolling can be applied without any problems. As for the heating temperature of the slab, the initial state before hot rolling is N of solid solution state.
From the viewpoint that the above-mentioned effect is secured and the effect of N described above is easily obtained, it is preferably 1000 ° C. or higher. Further, in order to prevent loss due to increase in oxidized weight, it is desirable to set the temperature to 1280 ° C or lower. In order to effectively secure N in a solid solution state, direct-feed rolling is one of the useful techniques.

(2) Hot rolling finish temperature (FT) The finish rolling temperature is preferably 800 ° C. or higher. When hot rolling is performed at such a finishing temperature, it is possible to manufacture a hot-rolled base plate suitable for obtaining a cold-rolled steel sheet containing martensite in an appropriate range, which is an important requirement of the present invention. If the hot rolling finishing temperature is lower than 800 ° C, the anisotropy may occur in the final microstructure after cold rolling / annealing and the stretch flangeability may be deteriorated. Therefore, the hot rolling finishing temperature should be 800 ° C or higher. It is preferable. On the other hand, if the temperature exceeds 950 ° C, the risk of defects due to surface scale increases, so the upper limit is 95%.
About 0 ° C is preferable.

(3) Cooling after hot rolling The cooling after hot rolling is not particularly limited and may be within the range which is carried out by an ordinary method, but the average cooling rate is 40 ° C./s or more to refine the structure, It is preferable to make the particle diameters of the two phases small in order to ensure uniformity. The upper limit of the cooling rate is also not specified, but if it exceeds 300 ° C / s, it may be difficult to secure the uniformity of the material and the uniformity of the shape. Therefore, the upper limit is set to about 300 ° C / s. It is preferable.

(4) Winding temperature The winding temperature after hot rolling is preferably 500 ° C. or higher.
If this temperature is lower than 500 ° C., the shape of the steel sheet may be disturbed, which may cause rolling trouble during cold rolling. Further, since the ductility may be lowered, it is preferable to set the coiling temperature to 500 ° C. or higher. The upper limit is not particularly limited, but if it exceeds 750 ° C, the difference in material in the longitudinal direction of the coil tends to increase. If descaling is a problem, the temperature is preferably 700 ° C or lower.

Cold Rolling to Plating Step (1) Cold Rolling After the above hot rolling, pickling and cold rolling are performed according to a conventional method.
The reduction ratio of cold rolling is not particularly limited, but is preferably about 30 to 90% in order to improve stretch flangeability.

(2) Annealing process Annealing is performed by heating to a temperature of 750 to 900 ° C. after cold rolling at 20 ° C.
Continuous annealing is performed under the condition of cooling at a rate of / s or more. In the present invention, continuous annealing conditions are extremely important. At least a sufficient amount of martensite cannot be obtained unless it is heated to 750 ° C or higher. However, the heating temperature is 900
If the temperature exceeds ° C, the second phase becomes coarse, the stretch-flangeability deteriorates, the yield stress increases, and the workability deteriorates. In addition, as for the above cooling rate, in order to obtain a sufficient amount of martensite, it is important that the average cooling rate up to at least 400 ° C. is rapidly cooled at 20 ° C./s or more. The upper limit is not particularly limited, but it can be applied without problems up to about 100 ° C / s that can be achieved with normal gas jet cooling equipment, and even if high-speed water cooling (~ 1000 ° C / s) is performed,
Equivalent levels of product can be manufactured. The steel sheet after the continuous annealing can be subjected to skin pass rolling or leveler processing as required to obtain a product.

(3) Plating Step The steel sheet after continuous annealing may be further electroplated on the surface of the steel sheet to obtain an electroplated steel sheet. The effect of plating on the characteristics of the steel sheet of the present invention is small, and good characteristics can be secured at a strength level equivalent to that of a cold rolled steel sheet. The types of electroplating include Zn plating used for automobile steel sheets,
Zinc-based plating such as Zn-Ni plating is preferable, and the plating method may be a conventional method. The skin pass rolling or leveling process may be performed immediately after continuous annealing or after plating.

Next, other manufacturing conditions that are preferably adopted will be described. INDUSTRIAL APPLICABILITY The present invention is a cold-rolled thin steel plate or an electroplated steel plate for an automobile body. Naturally, in addition to the above characteristics, the shape and dimensional accuracy of the steel plate are improved, and the material is made uniform in the longitudinal and width directions of the steel plate. Is desired. In order to achieve this purpose, it is effective to apply a continuous rolling technique, which is currently partially put into practical use, in a hot rolling process for producing a mother plate. At the same time, in order to make the material uniform, it is also effective to use a sheet bar edge heater or a sheet bar heater which makes the rolling temperature uniform in the longitudinal direction and width direction of the coil. Further, in cooling after rolling, a technique of masking the cooling water in the width direction in order to prevent overcooling of the edge portion is also an important technique from the viewpoint of material uniformity. In addition, performing lubrication rolling to reduce the load during hot working is effective from the viewpoint of homogenizing the shape and homogenizing the material. In particular, applying lubrication rolling to the continuous rolling process described above. Is also desirable from the viewpoint of operation stability of hot rolling.

[0043]

EXAMPLES Next, examples of the present invention will be described. (Example 1) A steel containing the composition shown in Table 1 and the balance being substantially Fe was melted in a converter, and the steel slab was prepared as shown in Table 2.
After hot-rolling under the conditions shown in (1), pickling, cold-rolling to a sheet thickness of 0.8 to 1.6 mm, continuous annealing was performed to produce a cold-rolled steel sheet or a steel sheet electroplated therewith. In addition, the electrogalvanization in Table 2 is a double-sided plating with an adhesion amount per one surface of 60 g / m ² . In addition, electric Zn-Ni alloy plating is
Zn-Ni alloy plating with a content of 12 mass% was plated on both sides with an adhesion amount per one side of 20 g / m ² . With respect to these steel sheets, a structure investigation, a tensile test, a hole expansion test and an investigation of high-speed tensile properties were conducted in the following manner, and the results are shown in Table 3. -Structural investigation: The cross-sectional structure of the steel sheet is observed by an optical microscope, and the ferrite area ratio, martensite fraction and second
The phase fraction (including martensite) was measured. -Tensile test: JIS No. 5 test piece was used. -Hole expansion test: As described above, the test was performed in accordance with the standards of the Iron and Steel Federation. High-speed tensile property: The ratio of the TS at a tensile strain rate of 500 / s and the TS at a normal tensile strain rate of 10 ⁻³ / s was determined, and 1.2 or more was excellent and less than 1.2 was inferior.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

(Example 2) C: 0.055 mass%, Si: 0.005
mass%, Mn: 1.2mass%, P: 0.009mass%, S: 0.001m
ass%, Al: 0.055 mass%, N: 0.0025 mass%, Cr: 0.05
Mass%, Mo: 0.15 mass% (Cr + 1.2 Mo: 0.23 mass%) steel slab is used as a raw material, and as shown in Table 4, cold-rolled steel plate or electroplated steel plate is manufactured by widely changing the manufacturing conditions. Then, the mechanical properties were investigated in the same manner as in Example 1. The results are shown in Table 5. The electric Zn-Ni plating in Table 4 is a Zn-Ni alloy plating having a Ni content of 12 mass% and double-sided plating with an adhesion amount of 20 g / m ² per surface.

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

As described above, according to the present invention,
It becomes possible to stably manufacture a high-strength steel sheet having a tensile strength of 400 MPa or more and 540 MPa or less, excellent ductility and stretch flangeability, and excellent impact absorption characteristics. Further, according to the present invention, it is possible to manufacture a high-strength electroplated steel sheet by subjecting the steel sheet surface to a plating treatment. INDUSTRIAL APPLICABILITY The steel sheet of the present invention is suitable for use in collision-resistant parts such as side members and contributes to weight reduction of automobile bodies. Further, the steel sheet of the present invention can be processed in a hot dip galvanizing line to produce a (galvanized) hot dip galvanized steel sheet that is almost the same as the cold rolled steel sheet.

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Claims (5)

[Claims] 1. C: 0.02 to 0.08 mass%, Si: 0.50 mass% or less, Mn: 0.5 to 1.5 mass%, P: 0.04 mass% or less, S: 0.
005mass% or less, Al: 0.1mass% or less, N: 0.025mass%
Contains the following, and further contains Cr and Mo: singly or in combination so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2 mass%, and the balance is Fe and inevitable impurities, and is the main phase The area ratio of the ferrite structure is 85% or more, the area ratio is 2% or more and 10% or less, and the total ratio of the second phase containing the martensite is 15% or less. %, Ductility,
High strength steel sheet with excellent stretch flangeability and shock absorption characteristics. 2. In addition to the above components, B: 0.0005 to 0.0040 ma
ss%, Ca: 0.0010 to 0.01 mass%, REM: 0.0010 to 0.01
mass%, Ti: 0.005-0.10mass%, Nb: 0.002-0.10mass
%, V: 0.005 to 0.10 mass%, Cu: 0.01 to 1.0 mass%, N
i: The high-strength steel sheet according to claim 1, containing one or more of 0.01 to 1.0 mass%. 3. A high-strength electroplated steel sheet having an electroplated film on the surface of the steel sheet according to claim 1 or 2. 4. C: 0.02 to 0.08 mass%, Si: 0.50 mass% or less, Mn: 0.5 to 1.5 mass%, P: 0.04 mass% or less, S: 0.
005mass% or less, Al: 0.1mass% or less, N: 0.025mass%
Contains the following, and further contains Cr, Mo: alone or in combination so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2 mass%, or further contains B: 0.0005 to 0.0040 mass%, Ca:
0.0010-0.01mass%, REM: 0.0010-0.01mass%, T
i: 0.005 to 0.10 mass%, Nb: 0.002 to 0.10 mass%, V: 0.
005 to 0.10mass%, Cu: 0.01 to 1.0mass%, Ni: 0.01 to 1.
A steel slab containing one or more of 0 mass% and the balance consisting of Fe and unavoidable impurities is hot-rolled, then pickled and cold-rolled, and then 750 ° C or higher. A method for producing a high-strength steel sheet excellent in ductility, stretch-flangeability and impact absorption characteristics, characterized by comprising continuous annealing after heating to 900 ° C or lower and cooling at a rate of 20 ° C / s or higher. 5. C: 0.02 to 0.08 mass%, Si: 0.50 mass% or less, Mn: 0.5 to 1.5 mass%, P: 0.04 mass% or less, S: 0.
005mass% or less, Al: 0.1mass% or less, N: 0.025mass%
Contains the following, and further contains Cr and Mo: alone or in combination so as to satisfy the following formula, Cr + 1.2Mo ≧ 0.2 mass%, or further, B: 0.0005 to
0.0040 mass%, Ca: 0.0010 to 0.01 mass%, REM: 0.00
10-0.01mass%, Ti: 0.005-0.10mass%, Nb: 0.002-
0.10mass%, V: 0.005-0.10mass% Cu: 0.01-1.0mass
%, Ni: 0.01 to 1.0 mass% of any one or more of the steel slabs, the balance of which is Fe and unavoidable impurities, is hot-rolled, and then pickled and cold-rolled. Then, after heating at 750 ℃ to 900 ℃, 20 ℃ / s
A method for producing a high-strength electroplated steel sheet excellent in ductility, stretch-flange formability, and shock absorption characteristics, characterized by performing continuous annealing that cools at the above rate and then performing electroplating treatment.