JPH04297522A - Production of ultrahigh tensile strength steel for construction use - Google Patents

Abstract

PURPOSE:To produce an ultrahigh tensile strength steel for construction use in which a yield stress of >=95kgf/mm<2> is secured and which has <=95% yield ratio and >=15% uniform elongation. CONSTITUTION:A slab of a steel having a composition consisting of, by weight, 0.07-0.20% C, <=0.25% Si, 0.25-1.00% Mn, 0.20-0.40% Cu, 0.5-1.5% Ni, 0.40-0.70% Cr, 0.30-0.60% Mo, 0.010-0.030% Nb, 0.010-0.050% V, 0.015-0.060% sol.Al, <=0.0070% N, and the balance Fe with inevitable impurities is heated up to 1000-1150 deg.C, hot-rolled at 700-800 deg.C finishing temp., and then subjected, after hot rolling, to hardening at 840-900 deg.C and to tempering treatment at a temp. not higher than the Ac1 temp.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high-strength steel, particularly ultra-high-strength steel for construction in consideration of earthquake-resistant structures.

0002

[Prior Art] Conventionally, for example, the yield stress is 95 kgf/m.
Several ultra-high tensile strength steels with a tensile strength of m2 or more have already been proposed.

For example, Japanese Patent Application Laid-Open No. 56-9358 discloses a yield stress of 110 k for a Ni-Cr-Mo-V system characterized by C+Mo/8+V>0.26 and Cr>0.8 Mo.
A high-strength, high-toughness steel with a gf/mm2 or higher is disclosed.

JP-A-57-188655 discloses a Ni-Cr-Mo-V system with a yield stress of 110 kg, which can provide high strength and high toughness over a wide cooling rate range during quenching treatment.
Ultra-high tensile strength steels with f/mm2 or higher are disclosed.

[0005] Furthermore, many types of manufacturing methods have already been proposed, including methods for manufacturing highly tough quenched and tempered Ni-containing steels treated with extremely low P and extremely low sulfur.

[0006] Any of these manufacturing methods is effective in improving toughness. However, no study has been conducted considering structural members for buildings, and this poses a problem in use.

[0007] When applied to building structures, it is necessary to consider seismic design. This is especially important in earthquake-prone countries like Japan.

Generally, steel materials manufactured by quenching and tempering have a high yield ratio and have poor plastic deformability in members with stress gradients, which poses problems in terms of seismic design.

In order to increase the plastic deformability of a member with a stress gradient, it is important to increase the supplementary energy A of the stress-strain curve, as shown in FIG. In order to increase the supplementary energy A, it is important to reduce the yield ratio and increase the uniform elongation.

[0010] In recent years, as building structures have become larger, the tensile strength of steel materials used has been increasing. The advantages of using high-strength steel include reducing the amount of steel used.

[0011] High tensile strength steel has a high yield ratio, so
Plastic deformability is inferior to mild steel. For this reason, it is said to be unsuitable for earthquake-resistant design.

[0012] Furthermore, since welding is common in building structures, it is required that there be no problems in welding quality.

[0013]

SUMMARY OF THE INVENTION It is a general object of the present invention to provide a method of manufacturing ultra-high strength steel for architectural structures.

A specific object of the present invention is to provide an ultra-high tensile strength for construction that has a yield stress of 95 kgf/mm2 or more in a quenched and tempered state, a yield ratio of 95% or less, and a uniform elongation of 15% or more. The object of the present invention is to provide a method for producing steel, and furthermore, a method for producing ultra-high tensile strength steel for construction, which has the above-mentioned properties and has excellent welded part toughness.

[0015]

[Means for Solving the Problems] In order to achieve the above object, the present inventors have developed a method that has ultra-high strength and has a YR content of 95% or less.
As a result of our study with the goal of securing uniform elongation of 15% or more, we discovered that the specified performance could be satisfied by using Ni-containing steel and applying a specific quenching and tempering treatment, and thus completed the present invention. .

[0016] Here, in the present invention, in weight%, C: 0.0
7 ~0.20%, Si≦0.25%, Mn:0.25
~1.00%, Cu:0.20 ~0.40%, Ni
:0.5~1.5%, Cr:0.40~0.70%
, Mo: 0.30 to 0.60%, Nb: 0.010 to
0.030%, V: 0.010-0.050%,
sol. Al: 0.015 to 0.060%, N≦0
．． 0070%, Ti: 0.005-0.0 as necessary
A steel billet having a composition of 25% Fe, the balance being Fe and unavoidable impurities is heated to 1000-1150°C, hot rolled at a finishing temperature of 700-800°C, and after hot rolling, it is rolled at a temperature of 840-900°C. This is a method for producing ultra-high tensile strength steel for construction, which is characterized in that, after quenching, a tempering treatment is performed at a temperature of Ac1 temperature or lower.

[0017] In the above invention, Ti: 0.005
By containing up to 0.025%, it is possible to produce ultra-high tensile strength steel for construction with excellent weld joint toughness.

[0018]

[Operation] The reason why the range of components is limited as described above in the present invention is as follows. In this specification, "%" means "% by weight" unless otherwise specified.

[0019] C requires a minimum content of 0.07% from the viewpoint of required strength, and if it exceeds 0.20%, the effect is not so pronounced, so the upper limit is set at 0.20%. Therefore, in the present invention, the C content is 0.07% or more and 0.20%
Limited to the following.

[0020]Si is added for the purpose of deoxidizing, but it is also effective as a stable reinforcing element, but if it exceeds 0.25%, it is not so noticeable, so the upper limit is set at 0.25%.
shall be. Therefore, in the present invention, the Si content is limited to 0.25% or less.

[0021] Mn is an essential element for increasing strength, and at least 0
．． 25% is necessary. However, if it exceeds 1.00%, the effect is small, so the upper limit was set at 1.00%. Therefore, in the present invention, the Mn content is 0.25% or more.
．． 00% or less.

[0022] Cu causes precipitation hardening during aging treatment. In the present invention, the effect is significant between 0.20 and 0.40%. Therefore, in the present invention, 0.20% or more
．． Limited to 40% or less.

[0023] Ni forms a matrix structure with good toughness, and exhibits a precipitation hardening effect by partially combining with Al. The effect is insufficient below 0.5%, and 1.5%
If it exceeds 1.5, the strength during aging will decrease, so the upper limit is set at 1.5.
%. Therefore, in the present invention, the Ni content is limited to 0.5% or more and 1.5% or less.

[0024] Cr has the same effects as Ni, but
Furthermore, Cu forms stable martensite and lower bainite structures independent of the cooling rate. This effect is 0.4
If it is less than 0%, it is insufficient, and if it exceeds 0.70%, it is saturated. Therefore, in the present invention, the Cr content is 0.4
Limited to 0% or more and 0.70% or less.

[0025] When Mo is 0.30% or more, aging hardening is observed, but when it exceeds 0.60%, the effect is saturated. Therefore, in the present invention, the Mo content is 0.30%
It is limited to 0.60% or less.

[0026] Nb forms carbonitrides during the tempering treatment, and is effective in securing strength through precipitation hardening. 0.010
The effect is recognized above 0.030%, and is not noticeable above 0.030%. Therefore, in the present invention, the Nb content is limited to 0.010% or more and 0.030% or less.

Like Nb, V is effective in securing strength by forming carbonitrides during tempering treatment and precipitation hardening. The effect is observed between 0.010 and 0.050%. Therefore, in the present invention, the content is limited to 0.010% or more and 0.050% or less.

[0028] sol. Al combines with Ni and exhibits a precipitation hardening effect. The effect was recognized at 0.015% or more, and 0.
．． If it exceeds 0.060%, the effect is saturated. Therefore,
sol. Al is limited to 0.015% or more and 0.060% or less.

[0029]N is necessary to refine the austenite grains, but if it exceeds 0.0070%, no effect will be observed, so the upper limit was set at 0.0070%. Therefore,
N is limited to 0.0070% or less.

The above are the basic components of the steel in the present invention, but in the present invention, an appropriate amount of Ti may be added in order to improve the toughness of the welded joint.

[0031]Ti is effective in improving toughness because it prevents coarsening of the weld heat affected zone. Therefore, 0.005%
It is necessary to add more than 0.02%, but if it exceeds 0.025%, the toughness of the base material will deteriorate, so the upper limit should be set at 0.02%.
5%. Therefore, it is desirable to limit the Ti content to 0.005% or more and 0.025% or less.

Furthermore, the reasons for limiting the manufacturing conditions will be explained.

First, N melted to the above component composition
After the i-containing steel is melted into a steel billet by continuous casting or agglomeration and blooming, heating and cooling are repeated directly or as necessary for the purpose of diffusing the segregated components.

[0034] After performing such pretreatment, it is heated to a temperature of 1000°C or more and 1150°C or less, and hot rolled. This heating is a process that fixes N with Al, causes fine precipitation of AlN, and improves hardenability prior to hot rolling.

Furthermore, in order to utilize the strengthening caused by the precipitation of fine carbonitrides such as Mo and V during the tempering process, it is also a process in which carbonitrides such as Mo and V present in the steel slab are sufficiently converted into a solid solution.

[0036] If the heating temperature is less than 1000°C, this effect is insufficient. On the other hand, heating above 1150°C re-decomposes the fine precipitates of AlN, reducing the hardenability during hardening. For this reason, the heating conditions are set to 1000°C or more and 1150°C or less, preferably 1000°C or more and 1100°C or less.

[0037] The finishing temperature of hot rolling is set to 700°C or higher and 80°C.
The reason for limiting the temperature to 0° C. or below is to make the austenite grains finer at all positions in the thickness direction and to ensure hardenability at the center of the thickness. If the temperature exceeds 800°C, the austenite grains will not be sufficiently refined. Furthermore, if the temperature is lower than 700°C, the formation of deformation bands within the austenite grains increases, resulting in a decrease in hardenability.

[0038] After hot rolling, cold rolling is performed if necessary, but the cold rolling conditions themselves are not particularly limited in the present invention.

The reason why the quenching temperature is limited to 840° C. or higher and 900° C. or lower is that 840° C. or higher is necessary to sufficiently austenitize, but if it exceeds 900° C., the austenite grains become coarse.

After quenching, tempering is performed by heating to a temperature below Ac1.

Next, the mechanical properties and weld joint properties of steel produced by the method according to the present invention will be shown as examples.

[0042]

[Example 1] Steel with the chemical composition shown in Table 1 was prepared from 990 to 12
It was heated to 50°C and hot rolled at a finishing temperature of 688 to 825°C, and after cold rolling, it was reheated and quenched at each temperature, followed by tempering at 550°C.

Table 2 shows the mechanical properties of the obtained test steel. In addition, A to D in Table 1 indicate the steel compositions of the examples of the present invention, and E to K indicate the steel compositions of the comparative examples.

[0044] All steels according to the present invention have a weight of 95 kgf.
YS of /mm2 or more is ensured. Furthermore, it can be seen that both uniform elongation and YR have better performance than the steel of the comparative example.

FIG. 2 shows Run No. This is a graph showing the influence of tempering temperature on YS for cases 1 and 7.
It can be seen that this is significantly improved. This tendency is particularly noticeable in high-temperature tempering.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Example 2] Steel with the chemical composition shown in Table 3 was
After heating to 50°C, hot rolling was performed at a finishing temperature of 780 to 825°C, and after cold rolling, the material was reheated and quenched at each temperature, followed by tempering at 550°C.

Table 4 shows the mechanical properties of the obtained test steel.

[0050] All of the steels according to the present invention have a strength of 95 kgf/m
m2 or more YS is secured. Also, uniform elongation, Y
It can be seen that both R and R have better performance than the comparison steel.

Furthermore, it can be seen that by adding Ti, the toughness of the welded joint in the steel of the present invention is better than that of the comparative steel.

[0052]

[Table 3]

[0053]

[Table 4]

[0054]

[Effects of the Invention] According to the present invention, an ultra-high tensile strength steel having the following characteristics can be obtained, which is of great significance.

■At room temperature, YS is 95 kgf/m
m2 or more, uniform elongation is 15% or more, and YR is 9
Ultra-high tensile strength steel with less than 5% strength.

(2) The toughness of the weld joint is good.

[0057] In earthquake-resistant construction, this steel has better deformability than ordinary high-tensile steel.

[Brief explanation of the drawing]

FIG. 1 is a graph representing a stress-strain curve illustrating complementary energy.

FIG. 2 is a graph showing the correlation between tempering temperature and YS.

Claims (2)

[Claims] [Claim 1] C: 0.07 to 0.20 in weight%
%, Si≦0.25%, Mn: 0.25 to 1.00%
, Cu: 0.20-0.40%, Ni: 0.5-1.
5%, Cr: 0.40 to 0.70%, Mo: 0.3
0 to 0.60%, Nb: 0.010 to 0.030%
, V: 0.010-0.050%, sol. Al:
0.015 to 0.060%, N≦0.0070%,
A steel piece having a composition consisting of the balance Fe and unavoidable impurities is heated to 1000 to 1150°C, and the finishing temperature is 700°C.
Hot rolling was performed at ~800°C, and after hot rolling, the temperature was 840°C.
A method for producing ultra-high tensile strength steel for construction, which comprises quenching at a temperature in the range of ~900°C and then tempering at a temperature below Ac1 temperature. 2. The manufacturing method according to claim 1, wherein the steel piece further contains Ti: 0.005 to 0.025 in weight percent.
A method for producing ultra-high tensile strength steel for construction, which has excellent toughness at welded joints, characterized by containing %.