JP2528561B2 - Low yield ratio 70kgf / mm2 class 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 producing a low yield ratio 70 kgf / mm ² class high strength steel (hereinafter referred to as HT70) having excellent weldability.

[0002]

^2. Description of the Related Art Most conventional high-strength steels exceeding 60 kgf / mm ² grade have been manufactured by quenching and tempering B-added steel. However, the weldability of the B-added high-strength steel was significantly inferior to that of HT60. For this reason, at the time of welding, preheating of about 200 ° C. (maintaining the temperature of the steel sheet at a constant temperature at the time of welding) is required to prevent welding cracks, resulting in a remarkable reduction in the working efficiency.

On the other hand, in the 60 kgf / mm ² ultra high strength steel, an increase in the yield ratio (ratio of the yield strength to the tensile strength) was unavoidable as the strength increased. Regarding the weldability among the above-mentioned problems, B as disclosed in JP-A-02-129317 is used.
An additive-free HT80 has been invented.

However, although this manufacturing method can improve the weldability, it could not be applied to a building having a high yield ratio and requiring earthquake resistance. That is, the characteristic of the manufacturing method of the present invention steel is that after rolling, it is reheated to the austenite region, quenched, and tempered, so that the carbide becomes fine and the yield strength inevitably increases and the yield ratio rises. Was there. Recently, buildings have been becoming more and more huge, and there has been a strong demand for research and development of low-yield ratio, high-strength steel plates with good weldability.

[0005]

SUMMARY OF THE INVENTION The present invention provides a technique for producing a low yield ratio HT70 having excellent weldability.
The HT70 produced according to the method of the present invention has a low yield ratio of the base metal, is excellent in strength and toughness, and, under normal welding conditions,
Hardening of the welding heat affected zone (HAZ) is small, and preheating during welding can be reduced.

[0006]

Means for Solving the Problems The gist of the present invention is (1) C: 0.04 to 0.08% by weight, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.02% or less,
S: 0.008% or less, Cu: 0.9 to 1.8%, N
i: 0.3 to 2.0%, Mo: 0.3 to 0.7%, N
b: 0.005-0.030%, V: 0.02-0.0
8%, Ti: 0.005 to 0.020%, Al: 0.0
6% or less, N: 0.0015% to 0.0060%, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 +
Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(%) Satisfies 0.28% or less, and the balance is 1000% steel containing essentially B and iron and inevitable impurities.
Reheat to a temperature range of ℃ ~ 1250 ℃, and rolled so that the cumulative rolling reduction of 1050 ℃ or less is 20% or more, then air-cooled to room temperature, or 800 after rolling.
A steel sheet that has been quenched from a temperature above ℃ to room temperature is reheated to 700 ℃ to 850 ℃ in a heat treatment furnace, then immediately quenched from this temperature to room temperature, then reheated to a temperature below 700 ℃ and tempered. A low yield ratio 70 kgf / mm ² class high-strength steel with excellent weldability, characterized by being treated. (2) C: 0.04 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.02% or less,
S: 0.008% or less, Cu: 0.9 to 1.8%, N
i: 0.3 to 2.0%, Mo: 0.3 to 0.7%, N
b: 0.005-0.030%, V: 0.02-0.0
8%, Ti: 0.005 to 0.020%, Al: 0.0
6% or less, N: 0.0015% to 0.0060%, Cr: 0.05 to 0.4%, Ca: 0.0005 to 0.
Contains 0050% of one or two, Pcm = C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
20 + Mo / 15 + V / 10 + 5B (%) is 0.28%
The steel satisfying the following conditions and having the balance consisting essentially of iron and unavoidable impurities and containing substantially no B is reheated to a temperature range of 1000 ° C to 1250 ° C, and a cumulative reduction amount of 1050 ° C or less is 2
Roll the steel sheet to 0% or more, and then air cool it to room temperature or quench it to a room temperature from 800 ℃ or more to room temperature.
Reheat to 0 ° C, then quench from this temperature to room temperature,
Then, a method for producing a high yield steel with a low yield ratio of 70 kgf / mm ² class, which has excellent weldability, is characterized by reheating to a temperature of 700 ° C or lower and tempering.

The present invention will be described in detail below. According to the research conducted by the inventors, the conventional HT60 super grade steel components were basically B-added systems or those with a relatively high C content because of the need to secure the strength and toughness of the base metal. However, the addition of B and the high amount of C markedly deteriorate the weldability, and therefore HT6
Compared with 0 grade steel, the welding work efficiency was greatly reduced, and its improvement was required. In addition, JP-A-02-12931
The invention steel of Japanese Patent No. 7 has strength of HT80 and good weldability. However, since the invention steel is hardened from the austenite region to secure its strength, it is difficult to apply it to a building structure having a high yield strength and requiring earthquake resistance.

The present inventors have diligently studied the possibility of developing an HT60 super-grade steel having both good weldability and a low yield ratio. By setting the strength of the low weld ratio steel with good weldability to 70 kgf / mm ^2. I found what I could achieve. In the present invention,
1) To ensure good weldability, the amount of C is reduced and B is not added, and the Pcm value of the steel composition is suppressed to 0.28% or less. 2) Precipitation hardening of Cu, Nb, and V is used to secure the strength of the base metal. 3) Achieving a low yield ratio was solved by reheating and quenching into the austenite-ferrite two-phase coexisting region. The above three points are the gist of the present invention.

The details will be described below. In order to secure good weldability, it is extremely important to add no B and reduce the amount of C. In addition to this, the total of the constituent elements must be 0.28% or less in Pcm. Within this limited range, the hardening of the welding heat affected zone (HAZ) is small under normal welding conditions, and preheating during welding can be reduced.

Further, in order to reduce the amount of C without adding B, it is necessary to add Cu, Nb or V to the strength of the base material. These are known as precipitation hardening elements, but they exert their effect under appropriate manufacturing conditions. In the steel of the present invention, steel having a predetermined composition is reheated to 1000 ° C to 1250 ° C, and Cu, N
Since b and V are completely solutionized and are not precipitated as much as possible in the subsequent rolling, it is necessary to avoid reduction in a low temperature region (preferably rolling is completed at 780 ° C. or higher). However, in order to secure the toughness of the base metal, it is necessary to reduce the grain size by rolling to some extent,
A reduction amount of 1050 ° C. or lower is required to be 20% or more (a preferable range is 30 to 70%).

Further, in order to obtain a low yield ratio, quenching from the austenite-ferrite two-phase coexisting region is essential. Although this treatment has a lower strength than that obtained by quenching from directly above austenite which is generally performed, it has a low yield ratio. That is, C is enriched from ferrite to austenite in the coexistence state of two phases of reheating, and an austenite enriched with C and a ferrite phase reduced in C are formed. By quenching from this state, a two-phase mixed structure of a phase having extremely fine carbide and a phase having coarse carbide is formed. The reduction of the yield ratio is achieved by this two-phase mixed structure.

However, the purpose cannot be achieved unless the reheating temperature and the quenching temperature are appropriate. 850
Quenching from a temperature higher than ℃ cannot be expected to reduce the yield ratio because the proportion of ferrite phase is small. Also, 700 ° C
Quenching from less than this lowers the strength and cannot achieve the purpose. Furthermore, re-tempering is necessary to decompose martensite and recover toughness as stable carbides.
At 00 ° C., carbide was abnormally coarsened and strength could not be secured, so the temperature was set to 700 ° C. or less.

Although the main points of the present invention have been described, it is necessary to control the basic components within an appropriate range in order to obtain the low yield ratio HT70 having excellent weldability. This point will be described below. The lower limit of 0.04% of C is the minimum amount for ensuring the strength of the base material and the welded portion and exerting the effect of V. However, if the amount of C is too large, the weldability is deteriorated, so the upper limit was made 0.08%. If a large amount of Si is added, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.5%. Mn is an element essential for securing strength and toughness, and its lower limit is 0.8%. However, if the Mn content is too large, the hardenability increases and the weldability and HAZ toughness deteriorate, so the upper limit was made 1.5%.

In the steel of the present invention, the upper limits of P and S which are impurities are 0.02% and 0.008%, respectively.
This is to further improve the HAZ toughness. The reduction in the amount of P is to prevent grain boundary destruction during tempering, and the reduction in the amount of S is to prevent deterioration of toughness due to MnS. Cu is an important element for suppressing the deterioration of weldability and ensuring the strength of the base metal. However, when the amount added exceeds 1.8%, H
Since the AZ toughness is impaired, the upper limit was made 1.8%. Further, since the amount of C in the component is kept low, the lower limit of the amount of Cu was set to 0.9% in order to secure the strength.

Ni has little adverse effect on weldability, improves strength and toughness, and is also effective for preventing Cu cracks. However, if it exceeds 2.0%, it is not preferable for weldability, so the upper limit was made 2.0%. If it is less than 0.3%, the effect is small, so the lower limit was made 0.3%. Mo is an element that improves both strength and toughness of the base material, and 0.3% or more is essential. However, if too much, the weldability is deteriorated, so the upper limit was made 0.7%. Nb is an important element for ensuring the toughness of the base material, and 0.005% is the lower limit. Further, if the addition amount is too large, not only the toughness of the base material is deteriorated but also the HAZ toughness is deteriorated, so the upper limit is 0.0.
It was set to 30%.

V is important for securing the strength of the base material, and 0.02% is the lower limit. If it exceeds 0.08%, HAZ toughness is impaired, so 0.08% was made the upper limit.
When the amount of Al is small, Ti combines with O to form an oxide containing Ti ₂ O ₃ as a main component and improves HAZ toughness.
In addition, Ti combines with N to form TiN, suppresses coarsening of austenite grains during reheating, and exerts an effect on refining the structure after rolling. Minimum of 0.0 to obtain these effects
05% is required. However, if too much, TiC is formed and the toughness of the base material and HAZ toughness are impaired, so the upper limit is 0.02.
%.

Al is an element generally contained in deoxidized upper steel, but Si or Ti is sufficient for deoxidation, and the lower limit is not limited. However, when the amount of Al increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal welded using this steel deteriorates, so the upper limit was made 0.06%. N is an unavoidable impurity contained in the steel, but it combines with Nb to form a carbonitride to improve toughness, and forms TiN to enhance the properties of the HT70 as described above. Therefore, it is necessary to add at least 0.0015%. However, an increase in the amount of N is harmful to the HAZ toughness, so the upper limit was made 0.0060%.

Next, the reason for adding Cr and Ca will be explained. The purpose of adding these elements to the basic components is to improve the strength and toughness without impairing the characteristics of the steel of the present invention. Cr increases the strength of the base material and the welded portion, but if it is too large, the weldability and HAZ toughness are significantly deteriorated. Therefore, the upper and lower limits are 0.05%,
It was 0.4%. Ca controls the sulfide morphology and improves the base metal toughness. However, if the amount of Ca is less than 0.0005%, there is no practical effect, and if it exceeds 0.005%, C
A large amount of aO and CaS are formed to form large inclusions, which lowers toughness. Therefore, the upper and lower limits of the addition amount are set to 0.
It was set to 005% and 0.005%. As described above, the steel containing Cu, Nb, and V without B addition is 700 ° C to 850 ° C.
By reheating to ℃, quenching, and then tempering, it became possible to manufacture a low yield ratio HT70 kgf / mm ² steel sheet with excellent weldability.

[0019]

[Examples] Various steel plates are manufactured by converter, continuous casting, thick plate and heat treatment process, and HAZ hardness is measured under the welding conditions of the base metal strength, toughness and small heat input (standard condition of manual welding). Etc. were carried out. Table 1 shows the chemical composition of the invention steel and the comparative steel, and Table 2 shows the measurement results of the steel plate manufacturing process, the strength and toughness of the base material, and the HAZ hardness under standard welding conditions.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

Steels 1 to 10 in Table 1 show the chemical compositions of the steels of the present invention, and steels 10 to 20 show the chemical compositions of comparative steels. Further, the results of measurement of base metal strength, toughness, and HAZ hardness under standard welding conditions are shown for steels 1 to 10 in Table 2 and invention steels for steels 11 to 20 as comparative steels. The invention steel controls the Pcm value to 0.28% or less, and therefore the maximum value of HAZ hardness was 313 or less under the condition of standard welding heat input of 17 kJ / cm. Furthermore, the strength and toughness of the base material were sufficient properties as HT70, and the yield ratio was a low value of 83% or less.

On the other hand, in the case of the comparative steel, steel 11, since the manufacturing method is not appropriate, the strength and toughness of the base material are sufficient, but the yield ratio exceeds 85% and the C addition amount and Pcm are high. The HAZ hardness was as high as Hv386, and the weldability was insufficient. Similarly, in Comparative Steels 12, 13, 14, 15, and 16, the manufacturing method is not appropriate and the C content and Pcm are high, so the yield ratio of the base material is high and the HAZ hardness is 354 or more, and the weldability is insufficient. there were. Further, in Comparative Steel 17, the yield strength was insufficient because the manufacturing method was appropriate but the amount of Cu was insufficient. In Comparative Steel 18, the yield strength was insufficient because the manufacturing method was appropriate but the Mo content was insufficient. The comparative steel 19 was also produced by an appropriate manufacturing method, but the yield strength was insufficient because the C content was insufficient. Further, in Comparative Steel 20, although the manufacturing method is appropriate, the yield strength was insufficient because V was not added.

[0026]

According to the present invention, it is possible to manufacture a low yield ratio HT70 having excellent base material strength, toughness and weldability.

Compared with the conventional HT60 super grade steel, it can be expected that the welding work efficiency will be greatly improved and the safety of the structure will be remarkably improved.

The thick steel plate manufactured by this method can be used for a welded structure such as a building structure which is required to have earthquake resistance (prevention of collapse of the building due to an earthquake) and good weldability.

Claims (2)

(57) [Claims] 1. By weight ratio, C: 0.04 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.02% or less, S: 0.0. 008% or less, Cu: 0.9 to 1.8%, Ni: 0.3 to 2.0%, Mo: 0.3 to 0.7%, Nb: 0.005 to 0.030%, V: 0.02-0.08%, Ti: 0.005-0.020%, Al: 0.06% or less, N: 0.0015% -0.0060%, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(%) Is 0.28% or less, and the balance of steel containing iron and unavoidable impurities and containing substantially no B is reheated to a temperature range of 1000 ° C to 1250 ° C. Rolling is performed so as to be 20% or more, and then air-cooled to room temperature, or after rolling, a steel sheet that has been quenched from a temperature of 800 ° C or more to room temperature is reheated to 700 ° C to 850 ° C in a heat treatment furnace, Immediately thereafter, quenching from this temperature to normal temperature, and then reheating to a temperature of 700 ° C. or less and tempering treatment, a method for producing a low yield ratio 70 kgf / mm ² class high strength steel with excellent weldability. 2. A low yield ratio with excellent weldability according to claim 1, which contains one or two of Cr: 0.05 to 0.4% and Ca: 0.0005 to 0.0050% by weight. 70
kgf / mm ² class high-strength steel manufacturing method.