JP3011536B2 - Method of manufacturing high elongation structural steel sheet with low yield strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel sheet having a low yield strength and a high elongation.

[0002]

2. Description of the Related Art In recent years, in various fields such as shipbuilding and industrial machinery, welding work has been reduced in order to improve competitiveness. There are growing demands for various improvements. In order to improve the bending workability of thick steel plates, it is necessary to develop thick steel plates having low yield strength and high elongation.
In the field of construction and bridges, it is desired to lower the yield strength and increase the elongation in order to improve the safety of the structure, in particular, to improve seismic resistance, seismic isolation and seismic control.

Conventionally, in order to lower the yield strength and increase the elongation, a method has been considered in which the added elements are reduced as much as possible. However, in this method, the reduction in the yield strength and the increase in the elongation are limited. Yes, various requirements could not be satisfied, which was a problem. Until now, as a publicly known document on a structural steel sheet, Japanese Unexamined Patent Publication No.
However, there is a demand for a further decrease in yield strength and an increase in elongation.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a structural steel sheet having a low yield strength and a high elongation.

[0005]

SUMMARY OF THE INVENTION The present invention provides, in weight percent, C:
0.005% or less, Si: 0.04% or less, Mn: 0.
20% or less, Al: 0.060% or less, N: 0.004
% Nb: 0.01 to 0.1%,
V: A steel slab or cast slab containing any one of 0.01 to 0.2% and having a balance substantially consisting of iron having a composition of 105
Structural steel sheet with low yield strength and high elongation characterized by being heated to 0 to 1250 ° C and hot-rolled to a finishing temperature of 750 ° C or higher and then normalized at 910 to 960 ° C. It is a manufacturing method of.

[0006]

In order to lower the yield strength and increase the elongation of the steel, it is conceivable to reduce the amount of the reinforcing element added to the steel. As elements for strengthening steel, there are elements related to solid solution strengthening and precipitation strengthening. As solid solution elements, C, Si, M
As precipitation strengthening elements such as n, Cu, Ni, etc., Cr, M
o, V, etc. From the above, in order to lower the yield strength and increase the elongation of steel, it has been considered to reduce these various additional elements.

The inventors of the present invention have conducted various studies to develop a steel having a lower yield strength and a higher elongation. As a result, even trace amounts of C and N which have been reduced to the utmost limit, these dislocations are disturbed. It was found that this resulted in higher yield strength and lower elongation. By preventing the dislocation from being fixed by the trace amounts of C and N, the yield strength is reduced,
Elongation can be increased.

Next, it has been found that the addition of an appropriate amount of Nb or V is effective for preventing the dislocation from being fixed by C and N. If the amount of Nb added is too small,
The dislocation is not sufficiently fixed by C and N, and the yield strength is high.
Low elongation. On the other hand, if the added amount of Nb is too large, the precipitate of Nb becomes coarse, and conversely, precipitation hardening results in high yield strength and low elongation. In FIG. 1, 0.002% C-0.03% S
i-0.12% Mn-0.028% Al-0.0029
The effect of the amount of Nb added on the yield strength and elongation in a% N steel is shown. If the amount of Nb is less than 0.01%, dislocation is not sufficiently fixed by C and N, yield strength is high, and elongation is low. If the added amount of Nb exceeds 0.1%, the precipitate of Nb becomes coarse, the yield strength is high, and the elongation is low.

On the other hand, if the added amount of V is too small, the dislocation is not sufficiently fixed by C and N, and the yield strength is high and the elongation is low. On the other hand, if the added amount of V is too large, the precipitates of V become coarse, and conversely, precipitation hardening results in high yield strength and low elongation. FIG. 2 shows that 0.004% C-0.03% Si-0.0
The effect of the amount of V addition on the yield strength and elongation in 7% Mn-0.033% Al-0.0034% N steel is shown.

If the V content is less than 0.01%, the dislocation is not sufficiently fixed by C and N, the yield strength is high, and the elongation is low. If the amount of V added exceeds 0.2%, V precipitates become coarse, yield strength is high, and elongation is low.

The reasons for limiting the components are described below. C is an element that increases the yield strength and lowers the elongation by strengthening the solid solution and fixing the dislocations.
The upper limit is 05%.

Si is an element that increases the yield strength and lowers the elongation by solid solution strengthening, and must be suppressed as much as possible.
0.04% is made the upper limit.

Mn is an element that increases the yield strength and lowers the elongation by solid solution strengthening, and it is necessary to suppress it as much as possible.
The upper limit is 0.20%.

Al is an element necessary for deoxidation.
If added in excess of 60%, the toughness decreases, so the upper limit is made 0.060%.

N is an element that increases the yield strength and lowers the elongation by solid solution strengthening and fixation of dislocations, and it is necessary to suppress as much as possible. Therefore, the upper limit is 0.004%.

In the present invention, in addition to the above components, Nb, V
It is important to add any one of them. Nb
Is an important element for preventing dislocation fixation due to C and N, and the addition amount is limited to 0.01 to 0.1% as shown in FIG. V is an important element for preventing the dislocation from being fixed by C and N, and the addition amount is limited to 0.01 to 0.2% as shown in FIG. In melting this steel, either an electric furnace or a converter may be used. In making a steel sheet, when the heating temperature of hot rolling is less than 1050 ° C. or the rolling finish temperature is less than 750 ° C., the yield strength is high and the elongation is low due to the refinement of crystal grains. Also, 1250
Heating exceeding ℃ causes an increase in fuel cost, so the heating temperature is 1050 to 1250 ° C, and the rolling finish temperature is 750.
C or higher. After the hot rolling, normalizing is performed to prevent an increase in yield strength and a decrease in elongation due to residual strain introduced by hot rolling. As for normalization, the lower limit is set to 910 ° C. or higher in order to form an austenite single phase structure, and to 960 ° C. or lower in order to prevent an excessive increase in fuel cost.

[0017]

EXAMPLES Example 1 Among the chemical components shown in Table 1, 1 to 3 are steels of the present invention and 4 to 1
2 is a comparative steel. The smelting of the steel was carried out by a converter and made into a slab by a conventional method, and then the steel was manufactured to the thickness shown in Table 1 under the conditions of heating, rolling and heat treatment shown in the table. Table 1 shows the tensile test results of these steels.

[0018]

[Table 1]

In the steels 1 to 3 of the present invention, Nb is added in an appropriate range, and heating and rolling are performed in order to reduce various elements such as C and N, and at the same time, prevent dislocation from being fixed by a trace amount of C and N. Also, since the heat treatment conditions are appropriate, the yield strength is low and the elongation is high.

Next, steel 4 has a high C, steel 5 has a high Si, steel 6 has a high Mn, steel 7 has no Nb added, steel 8 has a high Nb, steel 9 has a high N, Steel 10 has a low heating temperature, steel 11 has a low rolling finish temperature, and steel 12 has no heat treatment, and thus has high yield strength and low elongation, respectively.

Example 2 Among the chemical components shown in Table 2, 1 to 3 are steels of the present invention and 4 to 1
2 is a comparative steel. The steel was melted by a converter and formed into a slab by a conventional method. Then, the steel was manufactured to the sheet thickness shown in Table 2 under the heating, rolling and heat treatment conditions shown in the table. Table 2 shows the tensile test results of these steels.

[0022]

[Table 2]

In the steels 1 to 3 of the present invention, V is added in an appropriate range, and heating and rolling are performed in order to reduce various elements such as C and N and at the same time prevent dislocation from being fixed by a trace amount of C and N. ,
With the proper heat treatment conditions, the yield strength is low,
High elongation.

Next, steel 4 has a high C, steel 5 has a high Si,
Steel 6 has a high Mn, steel 7 has no added V, steel 8 has a high V, steel 9 has a high N, steel 10 has a low heating temperature, and steel 11 has a low heating temperature.
Has a low rolling finish temperature and steel 12 has no heat treatment.
Each has high yield strength and low elongation.

[0025]

According to the present invention, an appropriate amount of Nb or V is added to reduce the amount of various elements to the utmost and prevent the dislocations from being fixed by trace amounts of C and N. , Rolling, heat treatment, yield strength is low,
It provides economical steel sheet with high elongation,
It has a great industrial effect.

[Brief description of the drawings]

FIG. 1 is a chart showing the effect of the amount of Nb added on the yield strength and elongation.

FIG. 2 is a chart showing the effect of the amount of V addition on yield strength and elongation.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) C21D 6/00 C21D 8/00-8/02 C21D 9/46-9/48

Claims (1)

(57) [Claims] C: 0.005% or less Si: 0.04% or less Mn: 0.20% or less Al: 0.060% or less N: 0.004% or less in weight%, and further Nb : 0.01 to 0.1% V: 0.01 to 0.2% of any one of the following, and a steel slab or a cast slab having a steel composition substantially consisting of iron remaining at 1050 to 1250 ° C: A method for producing a structural steel sheet having a low yield strength and a high elongation, characterized by heating, performing hot rolling to a finishing temperature of 750 ° C or higher, and then normalizing at 910 to 960 ° C.