JPH0649596A - Steel plate reduced in yield strength in surface layer and excellent in bendability and its production - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a surface low yield strength steel sheet excellent in bending workability. [Structure] Ac Steel strip or steel plate with a temperature of ₃ points or more,
When the plate thickness during water cooling during rolling is t ₀ and the product plate thickness is t, 1 × t ₀ / t (mm) at least in the plate thickness direction from the surface layer
Above, Ar in the region of 0.3 × t ₀ at a cooling rate of 2 ° C / sec or more
And rapidly cooled to below ₁ point, then, the surface portion starts or resumes rolling from Ar ₃ temperature below, (Ac ₃ -5
0) Rolling is completed within the range of 0 ° C to Ac ₃ ° C.
_The surface layer portion is cooled at a cooling rate of 1 ° C./sec or more to at least Ar ₁ point without reheating to ₃ points or more, and further (A
c ₁ -100) ° C. from (Ac ₁₎ is retained ° C. ranging in three minutes, the yield strength of the front and back layer portions of the steel plate over a range of thickness of 1mm or more to 30% or less than the yield strength of the sheet thickness inside 5 kg /
A method for producing a surface low-yield strength steel sheet having a low mm ² or more and excellent bending workability with continuity between the front and back layers and the inside.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface layer low yield strength steel sheet excellent in bending workability and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a structure was constructed by welding. However, due to a decrease in the number of welding engineers and environmental measures, the demand for molding instead of welding has increased. In press molding, besides the fact that the blank is molded without breaking,
Dimensional accuracy of product shape is required. This requirement is stronger as the product shape is larger. The ease of obtaining this dimensional accuracy is the shape. Generally, a steel sheet having a good shape is required to have a low yield point, that is, a so-called low yield point steel sheet. Further, there is a demand for a steel sheet that can have a low capability of pressing and the like during bending and that can satisfy a predetermined strength as a whole sheet thickness.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a steel sheet and a method for producing the same, which reduces the required pressing capacity during press forming by lowering the yield strength of the front and back layers of the steel sheet.

[0004]

According to the present invention, the structure of the front and back layer portions of the steel sheet has a grain size of 3 times or more the ferrite grain size inside the sheet thickness over a range of 1 mm or more and 30% or less of the sheet thickness. In addition, it is a surface low yield strength steel sheet having excellent bending workability, characterized by having continuity at the interface between the front and back layers and the inside.

Further, according to the present invention, the yield strength of the front and back layer portions of the steel sheet over the range of 1 mm or more and 30% or less of the sheet thickness is 5 kg / mm ² or more lower than the yield strength inside the sheet thickness, and the interface between the front and back layers is internal. It is a surface low yield strength steel sheet excellent in bending workability characterized by having continuity.

Further, according to the present invention, when a steel slab or a steel plate having a temperature of Ac ₃ or higher is used, the plate thickness during water cooling during rolling is t ₀ , and the product plate thickness is t, at least 1 × from the surface layer in the plate thickness direction. t
A region of ₀ / t (mm) or more and 0.3 × t ₀ is rapidly cooled at a cooling rate of 2 ° C./sec or more to ₁ point or less of Ar, and then the surface layer portion starts rolling at a temperature of Ar ₃ or less. Or restart,
(Ac ₃ -50) ° C. Exit rolled in the range of Ac ₃ ° C. from then cooled to at least Ar ₁ point without recuperation than Ac ₃ point the surface layer portion at 1 ° C. / sec or more cooling rate and further _{(Ac 1 -100) (Ac 1} ) is retained in a range of ° C. 3 minutes or more from ° C., the plate thickness of 1mm or more, the range over yield strength of the front and back layer portions of the steel sheet thickness inside the following 30% It is a manufacturing method of a surface low yield strength steel sheet which is lower than the yield strength by 5 kg / mm ² or more and has excellent bendability with continuity at the interface between the front and back layers and the inside.

The present invention will be described below. In the present invention, the target structural steel is a steel that includes the types and amounts of elements that are added to the ordinary structural steel to obtain the required material.

The respective constituent elements, the reasons for adding them, and the amounts thereof are shown below. C is one in which the addition as an effective ingredient for improving the strength of the steel, the excessive content of more than 0.20%, Ac ₁ -100~Ac grain growth of ferrite at the holding at ₁ ℃ range It cannot be expected, and island martensite is precipitated in the welded portion, which significantly deteriorates the toughness of the steel.

Si is necessary as a deoxidizing element for molten steel,
Although it is useful as a strength increasing element, if it is added in excess of 1.0%, the workability of steel is deteriorated and the toughness of the welded portion is deteriorated. Further, if less than 0.01%, the deoxidizing effect is insufficient, so the addition amount is regulated to 0.01 to 1.0%.

Mn is also necessary as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is lowered and the workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, since Mn lowers the transformation temperature, the excessive addition of Mn lowers the two-phase region rolling temperature too much.
Since it causes an increase in deformation resistance, the upper limit is 2.0%.

Al and N are added in order to make the steel effective by matching the crystal orientation of the steel and recrystallization by solid solution and precipitation in the rolling process in addition to the refinement of the steel by Al nitride. However, when the addition amount is small, the effect is not exerted, and when it is excessive, the toughness of the steel is deteriorated, so Al: 0.0
01 to 0.20%, N: 0.020% or less.
The above is the basic composition of the steel targeted by the present invention, for the purpose of increasing the base metal strength or improving the joint toughness,
Depending on the required properties, when adding alloy elements,
If the transformation temperature is lowered too much, the deformation resistance in the two-phase region increases and rolling becomes difficult.
r, Mo, Cu, W, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
One or more kinds of B may be added, but the total amount is restricted to 4.5% or less.

In the present invention, the structure of the front and back layer portions of the steel sheet has a grain size three times or more the ferrite grain size inside the sheet thickness over a range of 1 mm or more and 30% or less of the sheet thickness. It is also assumed that the yield strength of the front and back layers of the steel sheet is 5 kg / mm ² or more lower than the yield strength inside the sheet thickness, and that the interface between the front and back layers and the interior is continuous.

That is, the yield strength of the surface of the steel sheet of the present invention is important, but considering the corrosion allowance, it is necessary to be 1 mm or more even in a normal environment. If the yield strength is set to a plate thickness of 30% or more in the central portion of the plate, the strength of the steel plate cannot be maintained.

Therefore, the range of 1 mm or more and 30% or less of the plate thickness is defined as the front and back layer portions. Generally, in the clad steel, it has been conventionally performed to make the inner part and the surface layer part have different strengths.
However, it may peel off at the interface during bending or brittle fracture, and it is difficult to maintain and guarantee the characteristics of the interface. In the present invention, since there is no heterogeneous interface as described later, there is no problem as described above.

In order to form the structure of the present invention on the surface layer of the steel sheet, the steel sheet surface is water-cooled during rolling and the temperature is reduced to ₁ point or less to cause ferrite transformation, and the temperature is almost reduced even by rapid cooling. Not using the sensible heat of the central portion of the plate thickness, the ferrite structure of the surface layer portion is further heated while being heated to form a modified structure having a high residual dislocation density in the surface layer portion.

From (Ac ₁ -100) ° C. to (Ac ₁ )
The yield strength of the front and back layer portions of the steel sheet over the range of 1 mm or more and 30% or less of the plate thickness is 5 kg / mm ² or more lower than the yield strength inside the plate thickness by allowing the steel sheet to stay for 3 minutes or more in the range of ℃ It is possible to obtain a surface layer low-yield strength steel sheet which has continuity at the internal interface and is excellent in bending workability.

That is, from (Ac ₁ -100) ° C. to (A
c ₁ ) Grain growth of the processed ferrite in the surface layer by retention for 3 minutes or more in the range of ₁ ° C to secure coarse and highly pure ferrite. Although the yield strength is determined by the grain size of the ferrite, in the present invention, the surface ferrite grains are three times as large as the inside. Therefore, in the present invention, there is no heterogeneous interface between the front and back layers of the steel sheet and the inside thereof, and they are structurally continuous. In addition, the tempering treatment at high temperature realized the reduction of the concentration of solute carbon in ferrite necessary to secure the workability, so that the elongation was improved.

[0018] Figure 1 is a ferrite grain size and yield strength, the relationship between the elongation, FIG. 2 is a ferrite grain diameter and the Ac ₁ -50～Ac ₁
It is the relationship with the residence time at temperature.

[0019]

[Examples] Table 1 shows the components of the sample steel, and Table 2 shows the manufacturing conditions and the obtained materials.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

In Table 2, test numbers 1 to 12 show examples of the present invention, and test numbers 13 to 24 show comparative examples.

According to the examples of the present invention, the average yield strength of each part has a difference of 50 kgf / mm ² or more between the surface layer part and the inside, and it is excellent in workability, and the joint toughness can be secured depending on the components.

On the other hand, Comparative Example 13 is water-cooled during the predetermined rolling,
Although temperature-controlled rolling was carried out, the cold speed after rolling was slow, so the residual dislocation density decreased during slow cooling, and even after applying the prescribed heat treatment, the ferrite grain size in the surface layer was too small. Without coarsening, the elongation of the entire steel sheet and the required load at 90 ° bending deteriorated as compared with the examples of the present invention.

Comparative Example 14 was applied to a predetermined temperature-rolling and subsequent cooling. However, since the temperature of the heat treatment was too high, the inside of the sheet was austenitized, and the ferrite was transformed by slow cooling thereafter, but the ferrite grain size was also inside. However, the strength of the steel sheet as a whole decreased, and the merit of making the necessary load during processing was small.

In Comparative Example 15, since the rolling end temperature was high in the predetermined temperature-rolling, the residual dislocation density could not be increased to the surface layer portion, and the ferrite in the surface layer portion could not be coarsened even in the subsequent heat treatment, so that the inside of the plate could not be coarsened. Could not secure a difference in strength.

In Comparative Example 16, predetermined temperature-rolling, intermediate cooling,
Heat treatment was applied, but the thickness cooled by Ar ₁ was 30%
Because of the above, the strength of the entire steel sheet could not be maintained and the merit of workability was small.

Further, in Comparative Examples 17, 18, 21 to 24, since no heat treatment was applied, there was almost no difference in the yield strength between the surface layer portion and the inside, and naturally no merit of workability was obtained.

Further, in Comparative Example 19, since the heat treatment temperature was set to Ac ₃ or more, coarsening of ferrite grains could be achieved over 9 mm from the plate surface, but it became 30% or more of the plate thickness, and the strength of the steel plate decreased.

[0033]

EFFECTS OF THE INVENTION According to the present invention, processed ferrite is generated in the surface layer of a steel sheet, and grain growth is carried out by heat treatment to secure coarse and high-purity ferrite, so that a heterogeneous interface is formed between the inner layer and the front and back layers. not exist. Therefore, a low yield strength steel material can be obtained without causing interfacial peeling during bending workability or brittle fracture.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between ferrite grain size, yield strength, and elongation.

FIG. 2 is a chart showing the relationship between the ferrite grain size and the residence time at Ac ₁ −50 to Ac ₁ temperature.

Claims (3)

[Claims] 1. The structure of the front and back layer portions of the steel sheet has a grain size of 3 times or more of the ferrite grain size inside the plate thickness over a range of 1 mm or more and 30% or less of the plate thickness, and A surface low yield strength steel sheet with excellent bending workability, characterized by having a continuous interface. 2. The yield strength of the front and back layers of the steel sheet over a range of 1 mm or more and 30% or less of the sheet thickness is 5 kg / mm ² or more lower than the yield strength inside the sheet thickness, and the continuity between the front and back layers and the inside is continuous. The surface layer low yield strength steel sheet having excellent bending workability according to claim 1, wherein _3. A steel slab or steel plate having a temperature of Ac ₃ points or more, where the plate thickness during water cooling during rolling is t ₀ and the product plate thickness is t, at least 1 × t _{0 in the} plate thickness direction from the surface layer. / T (m
m) or more and the area of 0.3 × t ₀ is rapidly cooled to a point of Ar ₁ or less at a cooling rate of 2 ° C./sec or more, and then the surface layer part is Ar.
Rolling is started or restarted from a temperature of ₃ or less, and (Ac ₃
-50) Finish the rolling in the range of ℃ to Ac ₃ ℃, and then cool the surface layer portion at least at the Ar ₁ point at a low cooling rate of 1 ℃ / sec or more without reheating to the Ac ₃ point or more, and further. (Ac ₁ -100) ℃ from (Ac ₁₎ in the range of ° C. 3 minutes or more residence is allowed thickness of 1mm or more, 5 yield strength of the front and back layer portions of the steel sheet over the range of 30% than the yield strength of the sheet thickness inside
A method for producing a surface low yield strength steel sheet having excellent bending workability, which is characterized by being lower than kg / mm ² and being continuous at the interface between the front and back layers and the inside.