JP2721773B2 - Method for producing surface layer low yield strength steel sheet with excellent bending workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, structures have been constructed by welding. However, due to a decrease in welding engineers and environmental measures, the demand for forming by pressing instead of welding has increased. In press molding, besides that the base plate is formed without breaking,
Dimensional accuracy of product shape is required. This requirement is stronger as the product shape is larger. The ease with which this dimensional accuracy can be obtained is the shape. Generally, a steel sheet having 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 which requires a small capacity such as a press at the time of bending and can satisfy a predetermined strength in the entire thickness.

[0003]

SUMMARY OF THE INVENTION The present invention provides a method for producing a surface layer low yield strength steel sheet excellent in bending workability , in which the required pressing capacity during press forming is reduced by lowering the yield strength of the front and back layers of the steel sheet. Is what you do.

[0004]

According to the present invention, a steel slab or a steel sheet having a temperature of not less than ₃ points of Ac is obtained from the surface layer at least from the surface layer when the thickness during water-cooling during rolling is t ₀ and the thickness of the product sheet is t. thickness direction to _{1 × t 0 / t (mm} ) or more, and rapidly cooled to below ₁ point Ar an area of 0.3 × t ₀ at 2 ° C. / sec or more cooling rate, then the surface layer portion Ar ₃ Rolling is started or restarted from the following temperature, rolling is completed in the range of (Ac ₃ -50) ° C. to Ac ₃ ° C., and thereafter, at least up to Ar ₁ point without reheating to more than Ac ₃ points. Is cooled at a cooling rate of 1 ° C./sec or more, and further cooled from (Ac ₁ -100) ° C. to (A
c ₁ ) Stay at least 3 minutes in the range of ° C, and the yield strength of the front and back layers from the surface layer and the back layer of the steel sheet over the range of 1 mm or more and 30% or less of the sheet thickness, respectively, is more than the yield strength inside the sheet thickness. This is a method for producing a surface-layer low-yield-strength steel sheet excellent in bending workability, characterized in that it is 5 kg / mm ² or less and has continuity between the front and back layers and the inside.

Hereinafter, the present invention will be described. In the present invention, the target structural steel is a steel containing the type and amount of an element added to a normal structural steel to obtain a required material. The constituent elements, the reasons for their addition, and their amounts are shown below.

C is added as an effective component for improving the strength of the steel. However, if the content is excessively more than 0.20%, ferrite is not retained in the range of Ac ₁ -100 to Ac ₁ ° C. Since no grain growth can be expected, and island martensite precipitates in the welded portion, significantly deteriorating the toughness of the steel, the content is restricted to 0.20% or less.

[0007] Si is necessary as a deoxidizing element of molten steel,
Further, it is useful as a strength increasing element, but if added in excess of 1.0%, the workability of the steel is reduced, and the toughness of the weld is deteriorated. Further, if the content is less than 0.01%, the deoxidizing effect is insufficient, so the addition amount is restricted to 0.01 to 1.0%.

[0008] Mn is also required as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is reduced 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 lowers the two-phase rolling temperature too much,
Since deformation resistance increases, the upper limit is 2.0%.

[0009] Al and N are added to refine the steel by means of Al nitride, and also to provide a solid solution and precipitation during the rolling process, thereby making the crystal orientation of the steel effective and effective for recrystallization. However, when the addition amount is small, the effect is not obtained, and when the addition amount is excessive, the toughness of the steel is deteriorated.
01 to 0.20%, N: 0.020% or less.

The above are the basic components of the steel targeted by the present invention. For the purpose of increasing the base material strength or improving the joint toughness, when adding an alloy element according to the required properties, If the transformation temperature is too low, the deformation resistance in the two-phase region increases, and rolling becomes difficult.
Ni, Cr, Mo, Cu, W, Co, V, Nb, Ti,
Zr, Ta, Hf, rare earth element, Y, Ca, Mg, T
One, two or more of e, Se and B may be added, but a total of 4.
Regulate within 5%.

According to the present invention, a ferrite structure having a grain size of at least three times the ferrite grain size inside the sheet thickness is provided in a range of 1 mm or more and 30% or less of the sheet thickness from the surface layer and the back layer of the steel sheet. Shall be. Further, it is assumed that the yield strength of the front and back layers of the steel sheet is lower than the yield strength inside the sheet thickness by 5 kg / mm ² or more, and that the interface between the front and back layers and the inside has continuity.

That is, the yield strength of the steel sheet surface of the present invention is important, but in consideration of the corrosion allowance, it is required to be 1 mm or more even in a normal environment. Further, when the plate thickness at the central portion of the plate is 30% or more with low yield strength, 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.

[0013] Generally, in the clad steel, the inside and the surface layer are made different in strength from each other. 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 such problem as described above.

In order to form the structure according to the present invention on the surface layer of the steel sheet, the surface of the steel sheet is water-cooled during rolling, and the ferrite transformation is performed once by setting the temperature to Ar ₁ point or less. Using the sensible heat of the central part of the sheet thickness, the rolling is further performed while raising the temperature of the ferrite structure in the surface layer to form a modified structure having a high residual dislocation density in the surface layer.

Further, from (Ac ₁ -100) ° C. to (Ac ₁ )
By staying for 3 minutes or more in the range of ° C, the yield strength of the front and back layers from the surface layer and the back layer of the steel sheet over the range of 1 mm or more and 30% or less of the sheet thickness is 5 kg more than the yield strength inside the sheet thickness. / Mm ² or more, and a surface layer low yield strength steel sheet excellent in bending workability with continuity at the interface between the front and back layers and the inside can be obtained.

That is, from (Ac ₁ -100) ° C. to (A
c ₁ ) Grain growth of the processed ferrite in the surface layer by staying for 3 minutes or more in the range of ° C. to secure coarse and high-purity ferrite. Although the yield strength is determined by the grain size of the ferrite, in the present invention, the surface ferrite grain is three times as large as the inside.
Accordingly, in the present invention, the boundary between each interface between the surface layer and the back layer and the inside is defined .
However, there is no heterogeneous interface between the front and back layers of the steel sheet and the inside, and it is systematically continuous. In addition, the tempering treatment at a high temperature achieved a reduction in the concentration of solute carbon in the ferrite necessary for ensuring workability, so that elongation was improved.

FIG. 1 is a chart showing the relationship between the ferrite grain size and the yield strength and elongation, and FIG. 2 is a chart showing the relationship between the ferrite grain size and Ac ₁ -5.
Is a table showing the relationship between the residence time in 0～Ac ₁ temperature.

[0018]

EXAMPLES The components of the test steel are shown in Table 1, and the manufacturing conditions and the obtained materials are shown in Table 2. Test Nos. 1 to 12 in Table 2 show the present invention examples, and Test Nos. 13 to 24 show the comparative examples.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

[0023]

[Table 5]

According to the present invention, the average yield strength of each part has a difference of 5 kgf / mm ² or more between the surface layer portion and the inside, so that the workability is excellent and the joint toughness can be secured according to the components. .
On the other hand, in Comparative Example 13, although the water cooling and the temperature raising rolling were performed during the predetermined rolling, the cooling speed after the rolling was slow, so that the residual dislocation density decreased during the slow cooling, and thereafter, the predetermined heat treatment was performed. The ferrite grain size of the surface layer did not increase so much even when the method was applied, and the elongation of the entire steel sheet and the required load at the time of bending at 90 ° were deteriorated as compared with the examples of the present invention.

Comparative Example 14 was applied up to a predetermined temperature-increasing rolling and subsequent cooling. However, since the heat treatment temperature was too high, the plate was austenitized to the inside and the ferrite was transformed by slow cooling. As the steel sheet became larger, the strength of the steel sheet as a whole was reduced, and the merit of reducing the required load during processing was small.

In Comparative Example 15, the residual dislocation density could not be increased in the surface layer portion because the rolling end temperature was high in the predetermined temperature-increased rolling, and the ferrite in the surface layer portion could not be coarsened even in the subsequent heat treatment. Could not secure a difference in strength. In Comparative Example 16, although the predetermined temperature-increasing rolling, intermediate cooling, and heat treatment were applied, since the thickness cooled by Ar ₁ was 30% or more, the strength of the entire steel sheet could not be maintained, and the merit of workability was small.

In Comparative Examples 17, 18, 21 to 24, since no heat treatment was applied, there was almost no difference in yield strength between the surface layer and the inside, and the advantage of workability was not obtained as a matter of course. 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 sheet surface, but it became 30% or more of the sheet thickness, and the strength of the steel sheet decreased.

[0028]

According to the present invention, processed ferrite is generated on the surface layer of a steel sheet, and the ferrite is grown by heat treatment to secure coarse and high-purity ferrite. Therefore, 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 interfacial delamination during bending workability or brittle fracture.

[Brief description of the drawings]

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

2 is a chart showing the relationship between the residence time in the ferrite grain size and Ac ₁ -50~Ac ₁ temperature.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-49595 (JP, A) JP-A-6-49597 (JP, A) JP-A-1-159316 (JP, A) JP-A-53-1979 80317 (JP, A) JP-A-58-19441 (JP, A) JP-A-64-47815 (JP, A) JP-B-60-57490 (JP, B2)

Claims (1)

(57) [Claims] 1. When a billet or a steel sheet having a temperature of ₃ or more Ac is subjected to water cooling during rolling, the thickness is t ₀ , and the product thickness is t, at least 1 × t _{0 in the} thickness direction from the surface layer. / T (m
m) or more, and rapidly cooled to below ₁ point Ar an area of 0.3 × t ₀ at 2 ° C. / sec or more cooling rate, then the surface layer portion Ar
Rolling is started or restarted from a temperature of ₃ or less, and (Ac ₃
Rolling is completed in the range of −50) ° C. to Ac ₃ ° C., and thereafter, the surface layer portion is cooled at least to the Ar ₁ point at a cooling rate of 1 ° C./sec or more without reheating to the Ac ₃ point or more. (Ac ₁ -100) ° C. to (Ac ₁ ) ° C. for 3 minutes or more, and the thickness of each of the steel sheet from the surface layer and back layer is 1
The yield strength of the front and back layers over a range of not less than 30 mm of the plate thickness is 5 kg / mm ² or more lower than the yield strength inside the plate thickness,
A method for producing a surface-layer low-yield-strength steel sheet excellent in bending workability, characterized in that there is continuity between the front and back layers and the internal interface.