JPS6145683B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength, high-toughness steel for low temperature use, and more specifically, for example, an ethylene storage tank with an operating temperature of -104°C, or an ethylene storage tank with an operating temperature of -60 to -104°C.
The present invention relates to a method for producing high-strength, high-toughness steel for low-temperature use, which is used in low-temperature regions such as ethylene fractionation towers at 0.degree. C. and has excellent low-temperature toughness at welded joints. Conventionally, ethylene-related steel materials have been ASTM
−A203 standard 3.5% Ni steel has been used. However, the tensile strength of this steel type is as low as 45 to 63 Kgf/ ^mm2 , and the toughness value of the heat affected zone during welding is also low. On the other hand, 9% Ni of ASTM-A553 standard containing 9% Ni
Although steel has high strength and excellent toughness in the weld heat affected zone, it is economically expensive. From the above viewpoints, the present inventors have developed an inexpensive low-temperature steel that has a tensile strength of 70 Kgf/mm ² or more and has excellent base metal and weld heat-affected zone toughness at low temperatures of -60 to -104°C. I started. As a result, it was found that by reducing the C content to 0.03% or less and further adding Nb, the low-temperature toughness of the weld heat affected zone was significantly improved. In addition, by reducing the amount of C to 0.03% or less, Mn and Ni
If 1.5% or more is added, precipitation strengthening due to Nb will be noticeable during quenching and tempering treatment, and therefore the amount of C will be reduced.
It was found that it is possible to produce steel with a tensile strength of 70 Kgf/mm ² or more even though the content is reduced to 0.03% or less. The present invention was established based on the above-mentioned findings, and specifically proposes a method for manufacturing steel that has excellent low-temperature toughness of the base metal and weld heat-affected zone and high tensile strength. The method of the present invention will be explained below in order, starting from the composition of the steel. First, C is 0.005 to 0.03%, and this component range is one of the characteristics of the present invention. Weld toughness is significantly improved when the C content is reduced to 0.03% or less. However, if it is less than 0.005%, the crystal grains will become coarse and toughness will be impaired. In addition, the amount of Ni and the amount of Mn are within the range of the present invention.
If the C content is 0.03% or less, Nb precipitation strengthening will be significant during normal quenching-tempering treatment, and if you want to obtain similar Nb precipitation strengthening with a C content of 0.03% or more,
It is necessary to set the quenching temperature to 1100℃ or higher, and the crystal grains become coarse, making it impossible to use as a practical steel. The reason for this is
It is thought that when the amounts of Mn and Ni are within the range of the present invention, Nb is dissolved as a solid solution during heating at 800° C. due to the solubility product relationship, and is precipitated during tempering to strengthen the steel. Next, Si is an essential element for deoxidizing during steel refining, and is also an inexpensive steel-strengthening element. But 0.50
%, the cleanliness of the steel deteriorates, leading to a decrease in weldability and toughness. Therefore, the range of Si addition is set at 0.50
% or less. Next, Mn was limited to 1.5% or more and 2.5% or less.
This component range is one of the characteristics of the present invention.
When the Mn content is 1.5% or more, precipitation strengthening of Nb becomes noticeable when the C content is reduced to 0.03% or less and normal quenching-tempering treatment is performed. However, if it exceeds 2.5%, weldability and toughness will be impaired, so the addition range was set to 1.5% or more and 2.5% or less. Next, due to the synergistic effect of Ni with Mn, 1.5 to 4.5
When the amount of C is 0.03% or less within the addition range of 0.03%, the precipitation strengthening of Nb becomes noticeable when normal quenching-tempering treatment is performed. At the same time, Ni improves the strength and low-temperature toughness of steel. However, addition of 4.5% or more is economically disadvantageous. Next, Mo improves the strength of steel and prevents tempering embrittlement. For this purpose, it is necessary to add 0.05% or more, but if it exceeds 0.5%, low temperature toughness deteriorates. Therefore, the addition range of Mo is
0.05% or more and 0.5% or less. Next, Nb is added in order to obtain Nb precipitation strengthening through normal quenching-tempering treatment when the C content is 0.03% or less, the Mn content is 1.5% or more, and the Ni content is 1.5% or more. This precipitation strengthening can sufficiently compensate for the decrease in strength due to the reduction in the amount of C. To obtain this effect, 0.01% or more Nb is required,
If it exceeds 0.05%, the toughness of the weld will be impaired. However, if it is less than 0.05%, the weld toughness is rather
Improved by adding Nb. Therefore, the amount of Nb is 0.01%
0.05% or less. Next, since adding more than 0.01% of N causes deterioration of the weld toughness, the amount added was set to 0.01% or less. Next, Al is an essential element for deoxidizing molten steel and refining crystal grains, and for this purpose, Al is required in an amount of 0.005% or more. However, if it exceeds 0.10%, toughness and ductility will be impaired, so the amount added should be 0.005% or more and 0.10%.
The following was made. In addition to containing each component as described above,
In addition to this, one or more of V, Cr, and Cu can be contained. That is, Cu and Cr improve strength by increasing hardenability. For that purpose, Cu is
It is necessary to add 0.1% or more of Cr, and 0.1% or more of Cr.
However, when Cu is added in an amount of 1.5% or more and Cr is added in an amount of 2.0% or more, the toughness rapidly deteriorates. Therefore, Cu is 0.1
% or more and 1.5% or less, and Cr 0.1% or more and 2.0 or less. Further, V increases the strength by forming precipitates. For this purpose, it is necessary to add 0.02% or more, and if it exceeds 0.15%, the toughness will deteriorate rapidly. Therefore, the addition range of V is 0.02% or more and 0.15% or less. Therefore, after hot rolling, the steel material having the above composition is heated to a temperature range of 800°C to 930°C for quenching, and further heated to a temperature range of 550°C to Ac ₁ to be tempered. In other words, when quenching after hot rolling, if the quenching temperature is 800°C or lower, Nb will not be sufficiently dissolved at this temperature, and therefore Nb will be reduced during tempering.
It is not possible to aim for precipitation strengthening due to precipitates. On the other hand, if the quenching temperature is 930° C. or higher, problems such as coarsening of crystal grains occur, which is not preferable. Further, the tempering temperature during further tempering after quenching is set to 550°C to ₁ point Ac, but if the temperature is 550°C or less, the toughness of the base material deteriorates. Moreover, even if the Ac is ₁ point or more, the base material toughness deteriorates, and from this point, the toughness of the base material is improved by tempering at 550° C. to the Ac ₁ point. In addition, in the above steel material composition, the remainder is essentially
It is made of Fe, and it is allowed that other impurities are unavoidably mixed in. Next, examples will be described. First, 16 types of steel materials with chemical compositions A to P shown in Table 1 were hot-rolled using the normal method, and then rolled at 820°C
When quenched for 60 minutes and tempered at 610°C for 60 minutes, the mechanical properties shown in Table 1 were obtained. In addition, in Table 1, B, D, F, H, and J all belong to the scope of the present invention, and these steels are C
The amount is 0.03% or less, the amount of Mn is 1.5 to 2.5%, the amount of Ni is 1.5
~4.5% and added Nb. During the quenching and tempering process, the reinforcement by Nb was significant, and despite the low C content of 0.03% or less, the strength was reduced to 70 kg.
It can be seen that f/mm ² can be secured. In addition, the experimental method used to examine the weld toughness using the above steel was to apply a thermal history equivalent to 30 KJ/cm to the bond using a high-frequency induction heating reproduction thermal cycle device, and to evaluate vTrs (°C) using a Charpy impact test. The results were as shown in FIG. From FIG. 1, it can be seen that when Nb is added with a C content of 0.03% or less, the weld toughness is significantly improved. In addition, in Figure 1, the dotted line
The solid line indicates Nb-free steel, the solid line indicates Nb-added steel, and the symbol A indicates the area of the present invention.

[Table] In Table 1, ΔTS _Nb indicates the increase in strength due to the addition of Nb. As explained in detail above, the method of the present invention reduces the amount of C to 0.03% or less and adds Nb to aluminum killed steel for low temperature use, and then heats the steel to 800°C to 930°C after hot rolling.
According to the present invention, a steel material with significantly improved low-temperature toughness of the weld zone is produced by quenching in a temperature range of 550℃ to Ac ₁ and then heating and annealing in a temperature range of 550℃ to Ac 1. High-strength, high-toughness steel materials can be manufactured even with Ni levels. Therefore, according to the method of the present invention, steel materials used for applications such as pipe materials that require excellent weld zone low-temperature toughness and a high-strength base material can be manufactured at low cost.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount of C and the toughness of the weld zone.

Claims (1)

[Claims] 1 C: 0.005 to 0.03%, Si: 0.05 to 0.50%,
Mn: 1.5~2.5%, Ni: 1.5~4.5%, Mo: 0.05~
After hot rolling a steel material containing 0.5%, Nb: 0.01~0.05%, N: 0.01% or less, and Al: 0.005~0.10%, the balance being iron and unavoidable impurities, 800%
It has a tensile strength of 70 Kgf/mm ² or more, and is characterized by being quenched by heating to a temperature range of ℃ to 930 ℃, and then tempered by heating to a temperature range of 550 ℃ to _{Ac 1} . An excellent method for producing low-temperature, high-strength, high-toughness steel. 2 C: 0.005-0.03%, Si: 0.05-0.50%,
Mn: 1.5~2.5%, Ni: 1.5~4.5%, Mo: 0.05~
0.5%, Nb: 0.01-0.05%, N: 0.01% or less and Al: 0.005-0.10%, as well as V,
V: 0.02 to 0.15 of one or more of Cr and Cu
%, Cr: 0.1~2.0%, Cu: 0.1~1.5%, with the balance consisting of iron and unavoidable impurities. After hot rolling, the steel material is heated to a temperature range of 800°C to 930°C and quenched. , and further heated to a temperature range of 550℃ to Ac ₁ to achieve a tensile strength of 70Kgf/
A method for producing a high-strength, high-toughness steel for low-temperature use having a diameter of 2 mm ² or more and excellent low-temperature toughness at welded parts.