JPH06264175A - Steel sheet excellent in dent resistance, surface distortion resistance, and workability and its production - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a steel sheet having excellent dent resistance, surface strain resistance, and workability, which is mainly used for automobile parts to be pressed, and a manufacturing method thereof. [Structure] Two kinds of molten steels A and B containing at least one of Ti and Nb and further regulated the amounts of C, Si, Mn, P, S, Al and N are mixed in the order of A, B and A. A slab or steel ingot having a layered structure and continuously solidifying from the surface side and having a transition layer of 1 to 10% is used as a material, hot rolled at Ar ₃ -50 ° C or higher, then cold rolled, and re-rolled. By annealing at a crystallization temperature or higher, a clad steel plate having a strength of 30 kgf / mm ² or higher and excellent in dent resistance, surface strain resistance and workability is obtained.
A predetermined amount of B may be added to the inner layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly applied to press-processed automobile parts, etc., and has a tensile strength of 30 kgf / mm ² or more, and is excellent in dent resistance, surface strain resistance and workability. And its manufacturing method.

[0002]

2. Description of the Related Art In recent years, steel sheets have been required to have high strength from the viewpoint of weight reduction and safety improvement of automobiles. In particular, since automobile outer panel panels are required to have dent resistance, there is a strong demand for higher strength steel sheets for outer panel. However, the increase in strength is accompanied by a decrease in formability, and particularly in the case of increasing the strength of the outer plate, there arises a problem that surface strain occurs due to an increase in yield strength. In order to overcome such problems, so-called BH steel sheets have been developed, which have a relatively low yield strength during processing, but increase the yield strength after coating baking. For example, Ti, Nb, etc. may be added to ultra-low carbon steel to limit the amount of solute C (Japanese Patent Laid-Open No. 59-38337 and Japanese Patent Laid-Open No. 59-3182).
No. 7), a method of controlling the formation of precipitates by adjusting the amounts of N and S (JP-A 61-26757 and JP-A 62-78).
No. 22) is known, but these steel sheets are excellent in surface strain resistance, but insufficient in dent resistance. Although high-strength BH steel sheets have been developed to secure dent resistance, it is difficult to apply them to members having severe formability, or the application is limited due to occurrence of surface strain.

The present invention solves the problems of ensuring dent resistance and surface strain resistance / workability with a so-called cast clad steel plate having a surface layer (both sides) having high strength and an inner layer having softness.

Japanese Unexamined Patent Publication No. HEI 3-Claims has attempted to secure both dent resistance and surface distortion resistance / workability with a clad steel plate.
There is a technique described in Japanese Patent No. 133630, but a casting method has not been studied. During rolling of clad steel, cracks and peeling may occur at the boundaries of dissimilar steels.
In addition, cracking or peeling may occur at the boundary between dissimilar steels during processing by the user after manufacturing. Appropriate casting method is necessary for stable production of steel sheet and stabilization of workability.

[0005]

DISCLOSURE OF THE INVENTION The present invention is a cast clad steel sheet comprising a high-strength steel for the surface layer and a soft steel for the inner layer and having a transition layer, which is difficult to achieve with ordinary steel sheets. The purpose of the present invention is to provide a steel sheet having both strainability and workability, and a manufacturing method thereof.

[0006]

In order to solve the above problems, the present invention uses, as a raw material, a clad slab or a clad steel ingot which has a specific surface layer and an inner layer component and has a predetermined transition layer by casting. The present invention aims to solve the problem by cold rolling and subjecting it to specific annealing, and the gist thereof is as follows.

(1) Steels of two kinds of component systems A and B are A, B,
In a steel sheet having a multilayer structure in the order of A, the components of the steel A located in the surface layer are C: 0.05 to 0.2% Si: 3.0% or less Mn: 0.5 to 3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.02 to 0.2% Nb: 0.01 to 0.1 %, Containing at least one of Fe and unavoidable impurities, and the composition of steel B located in the inner layer being C: 0.01% or less Si: 0.5% or less Mn: 0.5 % Or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.006 to 0.2% Nb: 0.003 to 0.1 % Of at least 1%, and the balance is Fe and unavoidable impurities, and the steel plate thickness is 1 to 1 at the boundary between the A layer and the B layer.
A cold-rolled steel sheet excellent in dent resistance, surface strain resistance, and workability, which is characterized by having a transition layer having a thickness of 0%.

(2) Two types of component systems A and B steels are A, B,
In a steel sheet having a multilayer structure in the order of A, the components of the steel A located in the surface layer are C: 0.05 to 0.2% Si: 3.0% or less Mn: 0.5 to 3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.02 to 0.2% Nb: 0.01 to 0.1 %, Containing at least one of Fe and unavoidable impurities, and the composition of steel B located in the inner layer being C: 0.01% or less Si: 0.5% or less Mn: 0.5 % Or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less B: 0.0001 to 0.002% and Ti: 0.006 to 0.2 % Nb: 0.003 to 0.1% of at least one of them, and the balance Fe and unavoidable impurities, and the boundary between the A layer and the B layer. Parts to the steel plate thickness 1 to 1
A cold-rolled steel sheet excellent in dent resistance, surface strain resistance, and workability, which is characterized by having a transition layer having a thickness of 0%.

(3) An electroplated steel sheet excellent in dent resistance, surface strain resistance and workability obtained by electroplating one or both surfaces of the steel sheet described in (1) above.

(4) An electroplated steel sheet excellent in dent resistance, surface strain resistance, and workability obtained by electroplating one or both surfaces of the steel sheet described in (2) above.

(5) Steels of two kinds of composition systems A and B are A, B,
In the production of a steel sheet having a multilayer structure in the order of A, the components of the steel A located in the surface layer are C: 0.05 to 0.2% Si: 3.0% or less by mass ratio Mn: 0. 5 to 3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.02 to 0.2% Nb: 0.01 to 0 The content of steel B contained in the inner layer is 1% or more, and the balance is Fe and unavoidable impurities. The components of steel B located in the inner layer are C: 0.01% or less Si: 0.5% or less Mn: 0. 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.006 to 0.2% Nb: 0.003 to 0 1% of 1% or more, and the balance Fe and unavoidable impurities. Among these molten steels, the steel of A is first from the front side. At the point where solidification is started and this ends solidification, the steel of B subsequently begins to solidify, and the two layers A and B formed as a result are macroscopically separated from the point of component analysis, and A 1 to 1 of the slab thickness at the boundary between the B layer and the B layer
When using a slab or steel ingot having a 0% thick transition layer formed as a raw material, it is 11 when reheating during hot rolling.
Roll after heating to 80 ° C or higher, and if not reheated, start rolling at 1050 ° C or higher, finish hot rolling at Ar ₃ -50 ° C or higher, and after pickling, cold rolling, anneal at recrystallization temperature or higher. Is a method for producing a steel sheet having excellent dent resistance, surface strain resistance, and workability.

(6) Steels of two kinds of composition systems A and B are A, B,
In the production of a steel sheet having a multilayer structure in the order of A, the components of the steel A located in the surface layer are C: 0.05 to 0.2% Si: 3.0% or less by mass ratio Mn: 0. 5-3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less and Ti: 0.002-0.2% Nb: 0.003-0 The content of steel B contained in the inner layer is 1% or more, and the balance is Fe and unavoidable impurities. The components of steel B located in the inner layer are C: 0.01% or less Si: 0.5% or less Mn: 0. 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less B: 0.0001 to 0.002% and Ti: 0.006 to 0 0.2% Nb: 0.003 to 0.1%, containing at least one kind, and the balance Fe and unavoidable impurities. In molten steel, first, the steel of A starts to solidify from the front side, and at the point where it finishes solidification, the steel of B subsequently starts to solidify, and both layers A and B formed as a result are analyzed from the viewpoint of component analysis. It is macroscopically separated and has a slab thickness of 1 to 1 at the boundary between the A layer and the B layer.
When using a slab or steel ingot having a 0% thick transition layer formed as a raw material, it is 11 when reheating during hot rolling.
Roll after heating to 80 ° C or higher, and if not reheated, start rolling at 1050 ° C or higher, finish hot rolling at Ar ₃ -50 ° C or higher, and after pickling, cold rolling, anneal at recrystallization temperature or higher. Is a method for producing a steel sheet having excellent dent resistance, surface strain resistance and workability.

(7) In the method described in (5) above, a steel sheet having excellent dent resistance, surface strain resistance and workability, characterized in that one or both surfaces of the obtained steel sheet are electroplated. Is a manufacturing method.

(8) In the method described in the above item (6), a steel sheet having excellent dent resistance, surface strain resistance and workability, characterized in that one or both surfaces of the obtained steel sheet are electroplated. Is a manufacturing method.

[0015]

[Function] Since the strength of the surface has a strong influence on the dent resistance, it is necessary to highly strengthen the surface layer portion. On the other hand, if the strength is increased to the inner layer, surface strain tends to occur during pressing,
Further, since the workability is deteriorated, the inner layer needs to be soft. This makes it possible to achieve both dent resistance and surface distortion resistance / workability.

Next, the reasons for limiting the respective constituents of the present invention will be described in detail.

First, the reasons for limiting the chemical composition of the steel A located on the surface will be described in detail.

C: C is one of the strengthening elements, and is 0.2%.
%, The spot weldability deteriorates, so the upper limit of C is made 0.2%. Furthermore, in the present invention, TiC, Nb (C,
N) or the like is used as a strengthening means by precipitation strengthening and grain refinement of the structure. From this meaning, C must be at least 0.05%.

Si: Si is a strengthening element, so Si may be added to secure the strength of the surface layer portion, but an excessive addition causes deterioration of workability and weldability, so the upper limit is made 3%. A desirable upper limit is 2%. The lower limit is not particularly specified and may be lowered to 0.005%.

Mn: Mn is a strengthening element like Si, and the lower limit is 0.5% from the viewpoint of ensuring strength. The upper limit is 3% from the viewpoint of workability and weldability.

P: Since P is a strengthening element, it may be added to secure the strength of the surface layer portion, but it impairs secondary workability and weldability, so the upper limit is made 0.1%. The lower limit is not specified and may be lowered to about 0.001%.

S: S deteriorates workability and weldability and promotes hot cracking, so the lower the better, the upper limit is 0.02%.

Al: Al is used as a deoxidizing agent. Al
If the content is too large, alumina-based inclusions increase and the workability of the steel deteriorates, so the upper limit was made 0.1%. In the present steel, even if Si or Ti is used as a deoxidizing agent, it does not violate the gist of the present invention, so the lower limit of Al does not need to be specified. A desirable range is 0.002 to 0.1%.

N: N is 0.01% or less from the viewpoint of workability.

In the present invention, a precipitation strengthening element is added to secure the surface strength. Because of this 0.02% to 0.2%
One or more of Ti to 0.01 to 0.1% Nb are added. If it is less than the lower limit, precipitation strengthening is insufficient.
Addition exceeding the upper limit deteriorates workability and is not economical.

Although addition of Ca, REM and Cr is not an essential condition of the present invention, Ca and REM are spheroidized inclusions,
That is, since it contributes to the reduction of the adverse effects of inclusions on workability and Cr contributes to the prevention of C contamination during annealing, the selective addition is not against the gist of the present invention. However, the addition of these elements causes an increase in manufacturing cost, so the upper limit of the total addition amount of these elements is 0.5%.

Next, the reasons for limiting the chemical composition of the B steel located in the inner layer will be described in detail. In order to secure workability and surface strain resistance, the inner layer must basically be soft.

C, N: C and N are strengthening elements, and
It is an interstitial solid solution element and inhibits the formation of texture that contributes to the improvement of the r value. Therefore, it is necessary to reduce it as much as possible. Therefore, the upper limit is 0.01%.

Si: Si is also a strengthening element and deteriorates workability, so the upper limit is made 0.5%. The lower limit need not be specified in particular, and may be as low as about 0.005%.

Since Mn: C and Si deteriorate workability similarly to Si, the upper limit is made 0.5%. In addition, since extreme reduction of Mn promotes hot cracking and is not economical, Mn is preferably set to 0.05% or more.

P: As an strengthening element, it promotes central segregation and has the effect of lowering weldability, so the upper limit is set to 0.
1% It is preferably 0.06% or less. The lower limit is not particularly specified and may be lowered to about 0.001%.

S: Increases A-type inclusions and deteriorates workability. Also, since it promotes hot cracking, the upper limit is 0.03.
%. It is preferably 0.02% or less.

Al: Al is used as a deoxidizing agent. Al
If the content is too large, alumina-based inclusions increase and the workability of the steel deteriorates, so the upper limit was made 0.1%. In the present steel, even if Si or Ti is used as a deoxidizing agent, it does not violate the gist of the present invention, so the lower limit of Al does not need to be specified. A desirable range is 0.001 to 0.1%.

Further, in the present steel, Ti 0.006 to 0.2%,
It is necessary to contain one or more of Nb 0.003 to 0.1%.

Ti and Nb fix N and C to form solid solution C and N
Prevent the adverse effects of. If the amount is less than the lower limit, these effects are not exhibited, and if the amount exceeds the upper limit, adverse effects as impurities become large and the workability deteriorates. Desirably, the formula 1 is used.

[0036]

[Formula 1] C (%) / 12 + N (%) / 14 ≦ Ti (%)
/ 48 + Nb (%) / 93 + Al (%) / 27

B: B is 0.0 to improve secondary workability.
You may add 001-0.002%. Below the lower limit, there is no effect, and above the upper limit, the recrystallization temperature rises,
Workability deteriorates.

This steel is hot-rolled or cold-rolled from a slab or ingot having such surface A steel and inner layer B steel as a raw material, but peeling or cracking occurs at the boundary between the A and B steels during rolling. Instead, it is necessary to perform uniform plastic deformation. For that purpose, it is necessary to have a transition layer when casting a slab or a steel ingot. The transition layer is a layer formed at the boundary between the A layer and the B layer, and is a region where the steel components of the A layer and the B layer are mixed. The transition layer formed during casting is retained even after rolling and annealing. FIG. 1 exemplifies the change in composition near the transition layer of a 0.75 mm thick product. In the transition layer, the components continuously change as shown in FIG. 1, and the structure also changes in accordance with this component change.

This transition layer is obtained by the casting method in which the steel A first starts to solidify from the front side, and the steel B subsequently starts to solidify at the portion where the solidification ends. However, it is necessary that the A and B layers sandwiching the transition layer are solidified continuously without being largely mixed and are macroscopically separated from the viewpoint of component analysis.

As a method of obtaining a steel sheet having such a structure of A steel-B steel-A steel, a method of joining by rolling or a method of melting once on a steel piece or steel sheet of B steel once solidified is employed. Although methods of solidifying A steel and the like are known, in the materials manufactured by these methods, the inner layer and the outer layer are structurally discontinuous. Further, unlike the casting method, since the boundary surface between the A layer and the B layer is always exposed to an oxidizing atmosphere in these methods, there are macro- or micro-defects at this boundary. If strong processing is applied by, for example, stress concentration occurs at a part of the boundary, rolling becomes unstable, and peeling or cracking easily occurs.

The thickness of the transition layer is 1 of the casting thickness or the steel plate thickness.
10% is preferable. The transition layer is formed on both sides of the inner layer (layer B), and the thickness of the transition layer here is the thickness per one side.

As mentioned above, the transition layer is necessary for stabilizing the rolling work, but is also necessary for ensuring the stability of the workability of the product. Generally, thin steel sheets are processed by shear shearing or punching with punches and dies, but if the transition layer is too thin, separation-like cracks that divide the plate thickness into shear planes of the steel sheet, or microcracks that become the starting points thereof, are generated. Occur. This cracking or microcrack occurs because the deformation during shearing differs between the surface layer portion and the inner layer portion. These defects cause cracks and necking in the subsequent processing even if they are not a problem by themselves.

FIG. 2 is a graph showing the defect rate (separation-like cracking or necking occurrence rate within 40% of hole expansion) and the transition layer thickness when punching and punching an initial hole of 20φ with a punch and a die. It shows the relationship. Here, the surface layer of the material has a tensile strength of 50 to 60 kgf / mm ² and the inner layer has a tensile strength of 27 to 35 kgf / mm ² , and the surface layer has a thickness of 10 to 15% per side. If the thickness of the transition layer is less than 1% of the plate thickness of the steel plate, the defective rate increases rapidly. On the other hand, if the transition layer is 1% or more, the transition layer serves as a deformation interference layer and the occurrence of separation at the time of punching is eliminated, so that defective processing can be prevented. Therefore, the thickness of the transition layer is 1% or more of the steel plate thickness.

On the other hand, when the thickness of the transition layer is increased, the thickness ratio of the inner layer portion is substantially reduced, so that it becomes difficult to secure workability. Therefore, the thickness ratio of the transition layer is 10% or less.

As for the cladding rate, it is preferable to control the thickness ratio of inner layer / surface layer to 2 to 10. Here, the surface layer thickness means a portion in which the front and back sides are combined, and the transition layer portion is excluded. If the thickness ratio of the inner layer / surface layer is less than 2, the ratio of the inner layer is small, the workability as a whole is insufficient, and surface strain easily occurs due to the influence of the surface layer portion. On the other hand, when it exceeds 10, the surface layer is too thin and it becomes difficult to secure the dent resistance.

The slab or ingot produced in this way is
If necessary, it is slab-rolled and then hot-rolled. At the time of hot rolling, it may be hot-rolled after being inserted into a heating furnace and reheated, or may be directly hot-rolled without being inserted into the heating furnace. In the present invention, since it is necessary to secure the strength of the surface layer portion by precipitation strengthening of Ti and Nb, when reheating, 118
Rolled after heating to 0 ℃ or higher, and 1 if not reheated
It is necessary to start rolling at 050 ° C or higher. The finish rolling finish temperature in hot rolling needs to be Ar ₃ −50 ° C. or higher. When the temperature is lower than this temperature, the workability of the product is deteriorated, and coarse precipitation of TiC and Nb (C, N) during hot working of the surface layer portion is increased, which is disadvantageous for strengthening. Cooling and winding after finishing rolling may be performed by ordinary methods. The coiling temperature is 550 to 7 in order to sufficiently precipitate TiC or Nb (C, N).
00 ° C is preferred.

After hot rolling, the steel is pickled and cold rolled.
Cold rolling reduction is 60 to 90% to secure workability of inner layer
Is desirable.

Next, annealing is performed. Here, workability is secured through recrystallization and grain growth. Therefore, the annealing temperature is set to the recrystallization temperature or higher. The annealing method may be box annealing or continuous annealing. Note that if the annealing temperature is too high, the precipitation strengthening effect of the surface layer portion decreases, so the annealing temperature is 8 for box annealing.
It is preferably 50 ° C or lower, and 900 ° C or lower in the case of continuous annealing. The temper rolling after annealing is performed under normal conditions, for example, a pressure regulation rate of 0.5.
~ 2% is good.

In order to impart corrosion resistance to the steel sheet, zinc or the like may be electroplated after annealing. This plating may be applied to only one side or both sides.

[0050]

EXAMPLES The surface layer and the inner layer were adjusted to the chemical composition shown in Table 1 (A to G) and the method disclosed in JP-A-63-108947, that is, two kinds of molten steel were injected with two nozzles. Then
Moreover, it was melted by continuous casting by a method of applying a magnetic flux in a direction crossing the thickness of the slab. In both slabs, the surface layer and the inner layer are macroscopically separated from the point of component analysis, and a transition layer having a thickness of 1 to 10% of the casting thickness is formed at the boundary of each layer. It was confirmed that the thickness did not change even after annealing. AF are the components of the present invention. G has a low C in the surface layer, and Ti and Nb are not added to the inner layer. In addition, H and I are comparative single-layer slabs manufactured by a normal continuous casting method.

[0051]

[Table 1]

After hot rolling, these slabs were subjected to descaling treatment in an pickling line, then cold rolled at a rolling reduction of 75 to 85% and annealed. The annealing was continuous annealing or box annealing.
In the case of continuous annealing, the temperature is raised at 5 to 20 ° C./s, the annealing temperature is maintained for 40 to 120 s, and then rapidly cooled, and the overaging is performed at 150 to
It went for 300s. In box annealing, high temperature holding was performed for 8 to 12 hours. Some parts were galvanized through the electroplating line. Conditions such as hot rolling, annealing, and coating weight are shown in Tables 2 and 3. Skin pass elongation is 1 to 1.5
%Met. Tables 2 and 3 also show the tensile properties, dent resistance, and surface strain resistance of the products.

For the tensile test, a No. 5 test piece according to JIS Z2201 was used. The average r value is 0 with respect to the rolling direction.
In-plane average of r values in each direction of °, 45 °, 90 ° (= (0
R value in ° direction + r value in 90 ° direction + r value in 45 ° direction x 2) /
It is represented by 4).

The dent resistance was measured by subjecting a steel plate to a cylindrical flat bottom overhang forming process with a diameter of 100 mm, applying a prestrain of 2.5% to the flat bottom face, and then applying a steel indenter with a radius of 25 mm to a load of 20 kgf. Plastic deformation (dent) by pressing with
Was given, and the dent amount was measured by a three-point method with a span of 40 mm.

The surface strain resistance was evaluated by visually forming a 600 × 600 mm steel plate into a kamaboko shape (with embossing) with a creasing force of 60 tons and visually evaluating the periphery of the embossing.

In Tables 2 and 3, No. 1, 2,
Steel sheets 4, 5, 6, 7, 8, and 9 are manufactured according to the method of the present invention, and are excellent in workability (elongation / r value), dent resistance, and surface strain resistance as compared with the comparative material.

FIG. 3 shows the dent resistance of the steels of the present invention shown in Tables 2 and 3 with the tensile strength taken along the horizontal axis and the normal single layer plate (No. 1).
1, 12) and other comparative materials. The thicker the plate, the higher the strength.
The steel of the present invention has a smaller amount of dents than a single-layer plate having the same strength even if the plate thickness is thin. This means that the present invention can reduce the plate thickness from the viewpoint of dent resistance.

Other steel plates will be described. No. Three
Is inferior in r value and dent resistance to the steel of the present invention because the heating temperature and finishing temperature are too low. No. Sample No. 10 has a low surface layer strength, a clad ratio that is too high (the surface layer portion is too thin), and is therefore inferior in dent resistance, and a low r value because Ti or Nb is not added to the inner layer.

[0059]

[Table 2]

[0060]

[Table 3]

In Tables 2 and 3 above, they are as follows. Note 1) Cladding ratio (ratio) = inner layer thickness / front and back layer thickness. Note 2) Annealing method: “Continuous” indicates continuous annealing, and “Box” indicates box annealing. Note 3) Annealing temperature: The value in parentheses for continuous annealing indicates the overaging temperature.

[0062]

EFFECTS OF THE INVENTION According to the present invention, it becomes possible to achieve both high strength, dent resistance, surface strain resistance and workability, which are increasingly required for automobile parts and the like. This leads to a reduction in fuel consumption (preventing waste of natural fuel) by reducing the thickness of the steel plate, and an improvement in safety due to an increase in collision strength, which has great social significance.

[Brief description of drawings]

FIG. 1 is a graph showing an example of component changes in the plate thickness direction of a surface layer / transition layer / inner layer portion.

FIG. 2 is a graph showing a relationship between a ratio of a transition layer and a processing defect rate.

FIG. 3 is a graph showing dent resistance (indentation amount) and tensile strength.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C21D 9/46 Z C22C 38/14

Claims (8)

[Claims] 1. In a steel sheet in which two kinds of steels of component systems A and B have a multi-layer structure in the order of A, B and A, the component of steel A located in the surface layer is C: 0 in mass ratio. 0.05 to 0.2% Si: 3.0% or less Mn: 0.5 to 3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less And, Ti: 0.02 to 0.2%, Nb: 0.01 to 0.1%, at least one kind is contained, the balance consists of Fe and unavoidable impurities, and the components of steel B located in the inner layer are mass proportions. C: 0.01% or less Si: 0.5% or less Mn: 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less And Ti: 0.006 to 0.2%, Nb: 0.003 to 0.1%, at least one kind, and the balance Fe and inevitable impurities. 1 to 1 of the steel plate thickness at the boundary between B and B layers
A cold-rolled steel sheet excellent in dent resistance, surface strain resistance, and workability, which has a transition layer having a thickness of 0%. 2. In a steel sheet in which two kinds of steels of component systems A and B form a multi-layer structure in the order of A, B and A, the component of steel A located in the surface layer is C: 0 in mass ratio. 0.05 to 0.2% Si: 3.0% or less Mn: 0.5 to 3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01% or less And, Ti: 0.02 to 0.2%, Nb: 0.01 to 0.1%, at least one kind is contained, the balance consists of Fe and unavoidable impurities, and the components of steel B located in the inner layer are mass proportions. C: 0.01% or less Si: 0.5% or less Mn: 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01% or less B: 0.0001 to 0.002% and Ti: 0.006 to 0.2% Nb: 0.003 to 0.1%. Consists fine unavoidable impurities, 1 and 1 of the steel sheet thickness at the boundary between the A layer and the B layer
A cold-rolled steel sheet excellent in dent resistance, surface strain resistance, and workability, which has a transition layer having a thickness of 0%. 3. An electroplated steel sheet excellent in dent resistance, surface strain resistance, and workability, which is obtained by electroplating one or both surfaces of the steel sheet according to claim 1. 4. An electroplated steel sheet excellent in dent resistance, surface strain resistance, and workability, which is obtained by electroplating one or both surfaces of the steel sheet according to claim 2. 5. In the production of a steel sheet in which steels of two kinds of component systems A and B form a multi-layer structure in the order of A, B and A, the components of steel A located on the surface layer are C by mass ratio. : 0.05-0.2% Si: 3.0% or less Mn: 0.5-3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01 % Or less and Ti: 0.02 to 0.2% Nb: 0.01 to 0.1%, at least one of which is composed of the balance Fe and inevitable impurities, and the composition of Steel B located in the inner layer is In mass ratio C: 0.01% or less Si: 0.5% or less Mn: 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01 % Or less and Ti: 0.006 to 0.2%, Nb: 0.003 to 0.1%, one or more of them are contained, and the balance is Fe and inevitable impurities. In these molten steels, the steel A first starts to solidify from the front side, and the steel B subsequently starts to solidify at the point where it solidifies, and the two layers A and B formed as a result are analyzed. Is macroscopically separated from the above point, and the slab thickness is 1 to 1 at the boundary between the A layer and the B layer.
When using a slab or steel ingot having a 0% thick transition layer formed as a raw material, it is 11 when reheating during hot rolling.
Roll after heating to 80 ° C or higher, and if not reheated, start rolling at 1050 ° C or higher, finish hot rolling at Ar ₃ -50 ° C or higher, and after pickling, cold rolling, anneal at recrystallization temperature or higher. A method for producing a steel sheet having excellent dent resistance, surface strain resistance, and workability. 6. In the production of a steel sheet in which steels of two kinds of component systems A and B form a multilayer structure in the order of A, B and A, the components of steel A located on the surface layer are C by mass ratio. : 0.05-0.2% Si: 3.0% or less Mn: 0.5-3% P: 0.1% or less S: 0.02% or less Al: 0.1% or less N: 0.01 % Or less and Ti: 0.02 to 0.2% Nb: 0.01 to 0.1%, at least one of which is composed of the balance Fe and inevitable impurities, and the composition of Steel B located in the inner layer is In mass ratio C: 0.01% or less Si: 0.5% or less Mn: 0.5% or less P: 0.1% or less S: 0.03% or less Al: 0.1% or less N: 0.01 % Or less B: 0.0001 to 0.002% and Ti: 0.006 to 0.2% Nb: 0.003 to 0.1% One or more kinds are contained, and the balance. In these molten steels, the steel A first starts to solidify from the front side, and the steel B subsequently starts to solidify at the site where the solidification ends. , B layers are macroscopically separated from the viewpoint of component analysis, and the slab thickness is 1 to 1 at the boundary between A layer and B layer.
When using a slab or steel ingot having a 0% thick transition layer formed as a raw material, it is 11 when reheating during hot rolling.
Roll after heating to 80 ° C or higher, and if not reheated, start rolling at 1050 ° C or higher, finish hot rolling at Ar ₃ -50 ° C or higher, and after pickling, cold rolling, anneal at recrystallization temperature or higher. A method for producing a steel sheet having excellent dent resistance, surface strain resistance, and workability. 7. The method for producing a steel sheet excellent in dent resistance, surface strain resistance and workability according to claim 5, wherein one or both surfaces of the obtained steel sheet are electroplated. . 8. The method according to claim 6, wherein the obtained steel sheet is electroplated on one side or both sides thereof, which is excellent in dent resistance, surface strain resistance and workability. .