JPH05320763A - Production of structural steel reduced in yield strength - Google Patents

Abstract

PURPOSE:To produce a structural steel reduced in yield strength. CONSTITUTION:A steel slab or cast slab having a composition containing <=0.005% C, <=0.04% Si, <=0.20% Mn, 0.1-3.0% Al, <=0.004% N, and further one kind among 0.03-0.07% Ti, 0.01-0.1% Nb, and 0.01-0.2% V is heated up to 1050-1250 deg.C, hot-rolled at >=750 deg.C finishing temp., and normalized at 910-960 deg.C. By this method, the structural steel reduced in yield strength can be economically provided.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a structural steel having a low yield strength.

[0002]

2. Description of the Related Art In recent years, in the fields of shipbuilding, industrial machinery, etc., the welding work has been reduced to improve competitiveness, the steel material properties have been pursued to the limit, as represented by bending workability, the weldability has been improved and the steel material cost has been reduced. Various demands such as improvement of the are increasing. In order to improve the bending workability of thick steel plates, it is necessary to develop thick steel plates with low yield strength. Further, in the fields of construction and bridges, it is desired to lower the yield strength in order to improve the safety of structures, especially in order to improve seismic resistance, seismic isolation and seismic control.

Conventionally, in order to reduce the yield strength, a method of reducing the additive element as much as possible was considered, but this method has a limit to the reduction of the yield strength and cannot satisfy various requirements. It was a problem. Up to now, there is JP-A-3-31467 as a known document concerning structural steel,
Further lowering of yield strength is required.

[0004]

SUMMARY OF THE INVENTION An object of the present invention was made in view of the above points and is to provide a structural steel having a low yield strength.

[0005]

The present invention, in% by weight, comprises C:
0.005% or less, Si: 0.04%, Mn: 0.20
% Or less, Al: 0.1 to 3.0%, N: 0.004% or less, further Ti: 0.03 to 0.07%, N
b: 0.01 to 0.1%, V: 0.01 to 0.2%, and a balance of a steel slab having a steel composition substantially consisting of iron or a slab of 1050 to 1250. It is a method for producing a structural steel having a low yield strength, which comprises heating to ℃, hot rolling to a finishing temperature of 750 ° C. or higher, and normalizing at 910 to 960 ° C.

[0006]

In order to reduce the yield strength of steel, it is considered that the amount of reinforcing elements added to steel is reduced. Steel strengthening elements include elements related to solid solution strengthening and precipitation strengthening.
Examples of solid solution elements include C, Si, Mn, Cu, and Ni.
Examples of the precipitation strengthening element include Cr, Mo and V. From the above, in order to reduce the yield strength of steel, it has been considered to reduce these various additive elements.

As a result of various investigations to develop a steel for further lowering the yield strength, the inventors have found that it is necessary to coarsen the crystal grains to the limit.

To make the crystal grains as coarse as possible, A
Addition of 1 significantly increases the grain growth ability of crystal grains. As a result, abnormal grain growth of crystal grains is achieved during the temperature rising process during normalization after hot rolling. 0.002% C-0.03 in FIG.
% Si-0.09% Mn Steel with Al Addition Amount and Ferrite Grain No. Shows the relationship. It can be seen that crystal grains are significantly coarsened by the addition of Al having an Al content of 0.10% or more.

[0009] Although the crystal grain is coarsened to the utmost limit, the yield strength can be considerably reduced, but further studies were made with the aim of further lowering the yield strength. As a result, even in the extremely small amounts of C and N, which have been reduced to the limit, they fix dislocations, which increases the yield strength. These minute amounts of C,
By preventing dislocations from sticking to N, the yield strength can be lowered. It has been found that addition of a proper amount of Ti is effective for preventing dislocations from being fixed by C and N. If the amount of Ti added is too small, the dislocation sticking due to C and N is not sufficiently prevented and the yield strength is high. Also, T
If the amount of i added is too large, Ti precipitates become coarse, and conversely precipitation hardening increases yield strength. In FIG. 2, 0.003
% C-0.02% Si-0.10% Mn-0.3% Al
The effect of Ti addition amount on the yield strength and elongation in steel is shown. When the amount of Ti added is less than 0.03%, dislocations are not firmly fixed by C and N, and the yield strength is high. When the Ti addition amount exceeds 0.07%, Ti precipitates become coarse and the yield strength is high.

As for the effect of preventing dislocation sticking, as a result of various studies other than Ti, it was found that Nb and V also have similar effects. As shown in FIGS. 3 and 4, at 0.01% respectively, dislocation fixation by C and N is not sufficient, and when Nb exceeds 0.1% and V exceeds 0.2%,
Precipitates of Nb or V become coarse and conversely the yield strength becomes high.

The reasons for limiting the components will be described below. C is an element that increases the yield strength by solid solution strengthening and dislocation fixing, and it is necessary to suppress it as much as possible, so 0.005% is made the upper limit.

Si is an element that enhances the yield strength by solid solution strengthening, and it is necessary to suppress it as much as possible.
The upper limit is 4%.

Mn is an element that strengthens the solid solution to increase the yield strength, and it is necessary to suppress it as much as possible, so the upper limit is 0.20%.

Ti is an important element for preventing the fixation of dislocations due to C and N. As shown in FIG.
Limited to 0.07%.

Nb is an important element for preventing the fixation of dislocations due to C and N, and its addition amount is limited to 0.01 to 0.1%.

V is an important element for preventing the fixation of dislocations due to C and N, and its addition amount is limited to 0.01 to 0.2%.

Al is an element which lowers the yield strength through graining, and it is necessary to add 0.1% or more for graining. The toughness of the steel sheet is significantly reduced, so the upper limit is made 3.0%.

N is an element for increasing the yield strength and lowering the elongation by solid solution strengthening and dislocation fixing, and it is necessary to suppress it as much as possible, so 0.004% is made the upper limit.

In melting this steel, an electric furnace,
Any of the converters may be used. In the case of a steel sheet, if the heating temperature of hot rolling is less than 1050 ° C or the rolling finishing temperature is less than 750 ° C, the yield strength is high and the elongation is low due to the refinement of crystal grains. Also, 12
Heating above 50 ° C increases fuel costs, so
The heating temperature is 1050-1250 ° C, and the rolling finishing temperature is 7
The temperature is 50 ° C or higher. After hot rolling, normalization is performed in order to promote abnormal grain growth in the temperature rising process and prevent an increase in yield strength due to the residual strain introduced by hot rolling. Regarding normalization, the lower limit is 910 because it has an austenitic one-phase structure.
Above ℃, and to prevent excessive rise in fuel cost,
The temperature is 960 ° C or lower.

[0020]

EXAMPLES Among the chemical components shown in Table 1, 1 to 5 are steels of the present invention, and 6 to 15 are comparative steels. The steel was melted by a converter, made into slabs by a conventional method, and then manufactured under the heating, rolling and heat treatment conditions shown in Table 1 to the plate thickness shown in Table 1. Table 1 shows the tensile test results of these steels.

[0021]

[Table 1]

In the steels 1 to 5 of the present invention, in order to reduce various elements such as C and N, and at the same time coarsen the crystal grains to the limit, an appropriate amount of Al is added and an extremely small amount of C and N is added. In order to prevent dislocation sticking, Ti, Nb or V is added in an appropriate range, and heating, rolling and heat treatment conditions are also appropriate, so that the yield strength is low.

Next, Steel 6 has a high C, Steel 7 has a high Si, Steel 8 has a high Mn, Steel 9 has no Ti, Nb and V added, Steel 10 has a high Ti, Steel 11 has a high Ti content. Low Al, steel 1
No. 2 has a high N, steel 13 has a low heating temperature, steel 14 has a low rolling finish temperature, and steel 15 has no heat treatment, and therefore has a high yield strength.

[0024]

EFFECTS OF THE INVENTION According to the present invention, in order to reduce various elements to the utmost and further to make the crystal grains coarse to the utmost, an appropriate amount of Al is added, and dislocations are fixed by a very small amount of C and N. In order to prevent this, by adding an appropriate amount of Ti, Nb or V and performing appropriate heating, rolling and heat treatment, it is possible to economically provide structural steel with low yield strength and high elongation. There is a great effect on the industry.

[Brief description of drawings]

[Fig. 1] Ferrite grain size No. 3 is a chart showing the influence of the amount of Al added on the temperature.

FIG. 2 is a chart showing the influence of the Ti addition amount on the yield strength.

FIG. 3 is a chart showing the effect of the amount of Nb added on the yield strength.

FIG. 4 is a chart showing the influence of the amount of V added on the yield strength.

Claims (1)

[Claims] 1. By weight%, C: 0.005% or less Si: 0.04% or less Mn: 0.20% or less Al: 0.10 to 3.0% N: 0.004% or less, Further, a steel composition containing Ti: 0.03 to 0.07%, Nb: 0.01 to 0.1%, V: 0.01 to 0.2%, and the balance substantially consisting of iron. The steel slab or the slab of No. 1 is heated to 1050 to 1250 ° C., hot rolling is performed so that the finishing temperature is 750 ° C. or higher, and then the yield strength is characterized by normalizing at 910 to 960 ° C. Low structural steel manufacturing method.