JP2898455B2 - Manufacturing method of high strength steel with excellent weldability - 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 manufacturing a high tensile strength steel for architectural use having excellent weldability. The present invention relates to a method for producing a 780 MPa class high strength steel (hereinafter, referred to as HT-80) having excellent toughness.

[0002]

2. Description of the Related Art Building structures are being multi-storey and have a height of 580.
Although MPa-class high-strength steels have already been put into practical use, it is considered inevitable that ultrahigh-strength materials with higher strength will be used in the near future. However, the current HT-80 has a high yield ratio and requires preheating during small heat input welding such as tacking.
There are problems such as inferior HAZ toughness during large heat input welding such as electroslag welding. In addition, the present inventors,
As disclosed in Japanese Patent Application Laid-Open No. 62-142723, a means for producing high-strength steel was studied, and a component system in which the quenching critical diameter Di was 35 to 65 due to a component serving as an index of quenchability was investigated. By using work hardening generated during cold bending and age hardening generated by subsequent aging heat treatment, it is possible to secure a strength of 780 MPa or more, and thereby, the toughness of the weld bond portion can be reduced by the conventional HT- 80
It was much improved compared to. However, the above-mentioned invention is further improved from the fact that HT-80 is used with cold working, the yield ratio is high, and the HAZ toughness is not considered even for large heat input such as electroslag welding. is necessary.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a new high-strength steel having a low yield ratio and excellent weldability and HAZ toughness during electroslag welding in HT-80. Is what you do.

[0004]

Means for Solving the Problems The present inventors first examined the weldability of high strength steel by carbon dioxide welding using a 580 MPa class welding material in an oblique y-type welding crack test.
It has been found that if C + (Mn + Cr + Mo + V) / 20 is set to 0.20% or less, no crack occurs at room temperature. As a result, a high-strength steel utilizing the precipitation hardening of Cu and Nb can obtain strength at a low P value and has excellent weldability.
-80 can be obtained. Next, the HAZ toughness during electroslag welding was examined. In the conventional HT-80, the HAZ structure has large proeutectoid ferrite at the former γ grain boundary,
It was also found that the grains were coarse bainite with island martensite and reduced toughness. On the other hand, B is added to suppress grain boundary ferrite. A small amount of Ti and N is added to reduce old γ grains. In order to further suppress the island-like martensite structure, C
And that it is improved by regulating the amount of Si. Further, in order to achieve both strength and a low yield ratio, the heating temperature of the steel slab is set to a high temperature within a range in which TiN does not dissolve, and after rolling, water-cooling is performed.
It has been found that compatibility can be achieved by performing aging heat treatment by heating to 00 to 550 ° C.

As a result, a new method of manufacturing a high-strength steel which can satisfy the above three conditions at the same time has been established. That is, in weight (%), C: 0.06 to 0.10%, Si: 0.1%
Hereinafter, Mn: 0.8 to 1.5%, Ni: 0.5 to 1.2
%, Cu: 1.0 to 2.0%, Cr: 0.2 to 0.6
% , Mo: 0.2-0.5%, Al: 0.01-0.0
5%, Nb: 0.02% or less, V: 0.1% or less, T
i: 0.03% or less, B: 0.002% or less, N: 0.
004-0.007%, and C, Mn, C
The weldability parameter P by r, Mo, V is 0.20%
10
Heated to 00 ° C to 1150 ° C, rolled and cooled with water.
This is a method for producing a high-strength steel excellent in weldability, characterized by heating to a temperature of 80 to 830 ° C, water-cooling, and further performing aging heat treatment at a heating temperature of 500 to 550 ° C. However, P = C + (Mn + Cr + Mo + V) / 20

[0006]

The present invention will be described below in detail. First, as the steel material composition, C: 0.06 to 0.10 so that the weldability and the HAZ toughness during electroslag welding are excellent.
%, Si: 0.1% or less, Mn: 0.8 to 1.5%, N
i: 0.5 to 1.2%, Cu: 1.0 to 2.0%, C
r: 0.2 to 0.6% , Mo: 0.2 to 0.5%, A
l: 0.01 to 0.05%, Nb: 0.02% or less,
V: 0.1% or less, Ti: 0.03% or less, B: 0.0
Not more than 02%, N: 0.004 to 0.007%,
In addition, the present invention is intended for a steel which has a weldability parameter P of C, Mn, Cr, Mo, and V of 0.20% or less, and the balance is Fe. However, P = C + (Mn + Cr + Mo + V) / 20

In the present invention, the chemical components are limited as described above for the following reasons. First, if C is less than 0.06%, strength cannot be obtained. Further, if it exceeds 0.10%, the weld HAZ toughness decreases, so the content was set to 0.06 to 0.10%. Si
Is required as a deoxidizing element during steelmaking, but if it exceeds 0.1%, the HAZ toughness is reduced.
Mn is an element effective in ensuring hardenability, and also has the property of shifting the age hardening time of Cu to a short time. Therefore, it is effective for steel utilizing age hardening, and the addition of 0.8% or more is effective. It is a target. However, the addition of more than 1.5% is not sufficient for weldability and HA.
Since the Z toughness deteriorates, the content is set to 0.8 to 1.5%.

[0008] Ni is effective in improving the toughness of the base material and the weld bond, but if its content is less than 0.5%, its effect is small, but if it exceeds 1.2%, its effect is saturated. Therefore, the upper limit was set to 1.2%. Cu is a remarkable age hardening element and is effective for steel utilizing age hardening.
The addition of ~ 2.0% is most effective. If it is less than 1.0%, the age hardening is small, and if it is more than 2.0%, cracks occur during rolling. Cr is
It is effective in increasing tempering softening resistance and increasing strength.
%, Sufficient strength cannot be obtained. More than 0.6%
Addition decreases the weldability and HAZ toughness. Therefore,
The amount was set to 0.2 to 0.6%. Also, Mo and Cr
Similarly, it is effective in increasing the temper softening resistance and increasing the strength, and if it is less than 0.2%, sufficient strength cannot be obtained. Also,
Addition of more than 0.5% reduces weldability and HAZ toughness. Therefore, the amount was set to 0.2 to 0.5%.

Further, Al is not only effective for deoxidation, but also an effective alloying element that fixes N to become AlN and also plays a role of grain refinement, so the lower limit is made 0.01%. On the other hand, if the content exceeds 0.05%, the effect is saturated, so the upper limit is set to 0.05%. Nb is a remarkable element of age hardening like Cu, but if it exceeds 0.02%, the HAZ toughness is reduced, so Nb is made 0.02% or less. V is a remarkable element of age hardening like Nb,
If it exceeds 0.1%, the weldability and the HAZ toughness decrease, so the content is made 0.1% or less. Ti is an effective element for fixing N, and TiN contributes to refinement of γ grains in the welded HAZ to improve the toughness of the HAZ. The amount is 0.03%
Is sufficient, the upper limit is made 0.03%. B improves the hardenability of the steel material in a very small amount and suppresses the formation of coarse ferrite at the former γ grain boundary of the HAZ, thereby improving the HAZ toughness. 0.002% is sufficient, so the upper limit is 0.002%
And Further, N combines with Ti described above to form TiN.
Which is essential for improving the HAZ toughness. Therefore 0.0
It is required to be 04% or more, but if it is too much, it binds to B and reduces the effect of B, so the upper limit was made 0.007%.

[0010] The above are the basic components of the steel targeted by the present invention. In order to further improve the weldability in the present invention, the weldability parameter P based on C, Mn, Cr, Mo, V is 0.20% or less. Satisfying is one of the main points. As described above, as a result of examining carbon dioxide gas welding using a 580 MPa class welding material in an oblique y-type welding crack test, P = C + (Mn + Cr + Mo + V) / 20 was set to 0.1.
If the content is set to 20% or less, no crack occurs at room temperature. Therefore, the upper limit of the P value is set to 0.20%.

Next, the production conditions according to the present invention will be described. The slab of the above-mentioned composition is heated to 1000 ° C. to 1150 ° C., rolled, cooled with water, further heated to a temperature of 780 to 830 ° C., and then cooled with water. Aging heat treatment is performed at a heating temperature of 500 to 550 ° C. First, the heating temperature during hot rolling is set to 1150 ° C. or less.
This is because TiN generated during casting becomes coarse. The lower limit is set to 1000 ° C. for the purpose of solutionizing precipitated elements such as Cu and Nb. If the temperature is lower than 1000 ° C., the solution becomes insufficient, and the increase in strength due to the subsequent aging treatment cannot be sufficiently expected. .

Next, the reason for water cooling after rolling is that if not water cooled after rolling, a mixed structure of ferrite and upper bainite having a large grain size is formed, and strength is reduced. Then 780
Heating to 830 ° C. and cooling with water is such that heating at this temperature partially austenites the micro-segregated portion, and C is concentrated in that portion by diffusion, and becomes a harder martensite structure by subsequent water cooling. . On the other hand, the portion that does not become austenite is softened because it has a high-temperature tempered structure. As a result, the yield ratio can be significantly reduced. If the temperature is 780 ° C or lower, austenitization is insufficient, and if it exceeds 830 ° C, the austenitizing region is excessive and the initial purpose cannot be achieved. Furthermore, the aging heat treatment combines the strength increase due to the precipitation hardening of Cu, Nb, and V, and the tempering treatment of the hard martensite structure generated by the previous heat treatment. If the heating temperature is lower than 500 ° C., sufficient precipitation cannot be obtained. If the temperature exceeds 550 ° C., overaging occurs and the strength is reduced.

Hereinafter, the effects of the present invention will be more specifically described with reference to examples.

EXAMPLES Steel having the chemical composition of A, B, C, D and E shown in Table 1 was melted in a 100 ton converter and continuously cast to a thickness of 300.
mm slabs were manufactured, and each of these slabs was made into a steel plate having a thickness of 100 mm by changing the manufacturing conditions to 1 to 3 respectively. From these steel sheets, JIS No. 4 tensile test pieces were sampled from 1/4 t to examine the tensile properties, and an oblique y-type crack test of carbon dioxide welding using a 580 MPa class welding material was performed. Furthermore, JIS regarding the HAZ toughness of electroslag welding
Absorbed energy at 0 ° C. was determined using a No. 4 full-size Charpy test piece.

A-1, B-1 and C-1 have a tensile strength (TS) of 780 MPa or more and a yield ratio of 85% or less according to the method of the present invention.
Satisfies crack stop preheating temperature of 20 ° C or less in y-type crack test,
Further, the HAZ toughness of electroslag welding was 20 J or more. In A-2, since the heating temperature was high and the reheating temperature was low, the yield ratio exceeded 85%, and the toughness of the HAZ portion was reduced. In the case of A-3, the heating temperature was low, and air-cooling was performed after rolling, so that the TS was low at 650 MPa. In B-2, since the reheating temperature was high, a high TS value of 900 MPa was obtained, but the yield ratio was a very high value of 96%. B-3
TS is 6 because the reheating temperature and the aging heat treatment temperature are low.
The yield was as low as 80 MPa and the yield ratio was as high as 92%.

In C-2, reheating was omitted, and the aging heat treatment temperature was too high, so that the yield ratio was abnormally high at 97%. C
In the case of -3, air-cooling was performed after rolling, and subsequent heat treatment was omitted, so that TS reached only 580 MPa. D steel is C, Si, A
1 is high, Cu is low, and Nb and Ti are not added. Also, the P value is high and the weldability is poor. Further, since C and Si are high and Ti is not added, the HAZ toughness is low. And D
The yield ratio is high when this material is air-cooled after rolling and reheated at 890 ° C. as shown in -1, and the same result is obtained even if the reheat treatment is omitted after rolling and water-cooled as shown in D-2. there were. Further, as shown in D-3, when water-cooled and reheated at 770 ° C., the yield ratio was reduced, but TS was 750 M
Pa was reached only. E steel has low C and N and Si,
Cr and Nb are high, and V, Ti and B are not added.
Therefore, the weldability is very good, but E-1 and E-
Both E-2 and E-3 have low TS and poor HAZ toughness.

[0016]

[Table 1A]

[0017]

[Table 1B]

[0018]

According to the present invention, it is apparent from the above embodiments that the present invention provides a high-strength steel in which the strength, the low yield ratio, the weldability, and the toughness of the HAZ portion of the electroslag welding are significantly improved as compared with the conventional material. The effect is extremely remarkable in industry.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-79716 (JP, A) JP-A-64-52023 (JP, A) JP-A-3-162518 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C21D 8/00-8/02 C21D 6/00

Claims (1)

(57) [Claims] 1. In weight (%) C: 0.06 to 0.10% Si: 0.1% or less Mn: 0.8 to 1.5% Ni: 0.5 to 1.2% Cu: 1 0.0 to 2.0% Cr: 0.2 to 0.6% Mo: 0.2 to 0.5% Al: 0.01 to 0.05% Nb: 0.02% or less V: 0.1% Ti: 0.03% or less B: 0.002% or less N: 0.004 to 0.007%, and the weldability parameter P by C, Mn, Cr, Mo, V is 0.20% or less. The steel slab composed of other unavoidable impurities is heated to 1000 ° C. to 1150 ° C., rolled and water-cooled, further heated to a temperature of 780 to 830 ° C., then water-cooled, and further heated to 500 to 550 ° C.
A method for producing high-strength steel with excellent weldability, characterized by subjecting to aging heat treatment. However, P = C + (Mn + Cr + Mo + V) / 20