JPH021209B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing low-temperature steel with excellent weld toughness. Low-temperature toughness is an important material property for steel used in low-temperature structures such as LPG tanks and tankers. High toughness materials are required. However, conventional welded zones and weld heat-affected zones of steel for low-temperature structures have had the problem of coarsening of crystal grains due to the effects of high temperatures during welding, resulting in a significant decrease in toughness. This problem is caused by grain coarsening and the formation of island-like martensite, and in order to prevent this problem, efforts are being made to reduce the N content in the steel and make the crystal grains finer. However, there is a limit to the reduction of N in steel due to problems in steel manufacturing technology, and grain refinement at the weld bond cannot be expected due to the thermal effects of high temperatures. The purpose of the present invention is to solve the above-mentioned problems by providing a method for producing low-temperature steel that has excellent toughness in both the base metal and the welded part. This is a manufacturing method in which steel containing steel is subjected to controlled rolling under certain conditions and then tempered. For details, C: 0.02-0.16%, Si: 0.05-0.7%, Mn:
0.5-1.6%, P: 0.015% or less, S: 0.0005-0.004
%, Al: 0.01~0.1%, B: 0.0005~0.002%,
Contains N: 0.004% or less, Ti: 0.003 to 0.02%,
Ceq: A steel billet with 0.4% or less and the remainder being substantially iron is heated to a temperature of 1100℃ or higher as necessary, rolled, and cooled to a temperature of 1250℃ or higher.
Heat to 1350℃ for 60 minutes or more, let it stand to cool or roll it to a temperature below the _A3 transformation point, and then
Heating to 900-1100℃, rolling reduction rate below 800℃ is 30
After rolling at 300°C or more,
This is a method for producing low-temperature steel with excellent weld toughness, in which the steel is cooled at ~50°C/second, then heated to 400-650°C, and then tempered. The manufacturing method of the present invention will be explained in detail below. In the present invention, steel billets are manufactured by ingot-making and blooming methods or continuous casting methods from molten steel produced using a conventional melting furnace such as a converter or an electric furnace. As mentioned above, C is added as an effective component to improve the strength of steel, and if it is less than 0.02%, the strength required for welded structural steel cannot be obtained.
If the content exceeds 0.16%, island-like martensite will precipitate in the weld, significantly deteriorating the toughness of the steel. Although Si is effective as a deoxidizing element for molten steel, in rolled steel such as the one of the present invention, it is necessary to limit it to 0.7% or less as a weld zone toughness component. Mn needs to be added in an amount of 0.5% or more as it improves the strength of steel, but excessive content impairs the toughness of the weld. Therefore, Mn was regulated to 0.5 to 1.6% in consideration of strength and toughness. P is a harmful component that promotes the precipitation of island martensite and deteriorates the toughness of the weld, and is regulated to 0.015% or less. B is an effective component that improves the toughness of the weld zone. There are three reasons for this, as explained below. Compound B dissolves in the steel when subjected to high-temperature welding heat, but precipitates during cooling and becomes the nucleus of cementite, promoting pearlite transformation from retained austenite and inhibiting the precipitation of island-like martensite. prevent. This improves toughness. That is, a welded part of steel to which B is added exhibits a ferrite-pearlite structure, and a welded part to which B is not added exhibits a ferrite island-shaped martensite.
It is thought that this difference in structure affects the toughness of the weld. A part of B becomes free boron (FreeB) and segregates at grain boundaries, lowers the temperature at which grain boundary ferrite is formed, promotes intragranular transformation, shortens the effective fracture surface unit for fracture, and improves toughness. do.
B converts into BN during cooling after welding, reduces free nitrogen (FreeN) in the steel, and prevents toughness from decreasing due to the presence of free nitrogen. In order to obtain the effects of B as described above, it is necessary that B be contained in the steel in an amount of 0.0005% or more. Moreover, excessive content increases B precipitates and deteriorates toughness. Therefore, the present invention takes into consideration the effect of B and the toughness of steel, and the content of B is 0.0005 to 0.002%.
regulated. Al was regulated to 0.1% or less because it has the same effect as Si. In order to bring about the effective effect of B, S needs to be contained within a certain range. S in steel exists as MnS, and a part of it is welded during the welding heat cycle, but during cooling it becomes fine MnS and precipitates.
It acts to fix the BN around it, improving the toughness of the weld. If the amount of S is too large, it will be difficult to melt due to welding heat, and if it is too small, the effect will not be exhibited. Therefore, it is preferable to contain S at 0.0005 to 0.004% and to disperse MnS as finely as possible. The lower the N content, the better the toughness. The reasons for this can be attributed to the following factors. By lowering the N content, the dislocation density decreases during welding cooling, lowering the strength and improving the toughness. This improves the toughness of the ferrite material itself. N
is an austenite stabilizing element. Therefore, the hardenability of the remaining austenite transformed from ferrite is increased and island-like martensite grows. In other words, by reducing the amount of N, the island-like martensite is reduced, thereby improving the toughness. Due to this effect, in the present invention, N contained in steel is reduced.
It was limited to 0.004% or less. Add a small amount of Ti. In order to improve the toughness of welds, Ti nitride has traditionally been used to refine austenite grains and serve as a nucleation site for ferrite, and for this purpose a large amount of Ti has been required to be added. However, as a result of detailed study by the present inventors, the part heated to near the melting point of the bond part,
It has been found that these nitrides not only do not melt and have no effect on grain refinement, but rather deteriorate due to the precipitation effect of these carbides. Therefore, a small amount of Ti is added. As a result, a part of the bond re-precipitates and becomes a core site for intragranular transformation, and also serves as a necessary grain refinement in the heat-affected zone.
The amount of Ti added was regulated to 0.003 to 0.02%. Furthermore, the Ceq of steel composed of the above ingredients and composition was set to 0.40% or less. Ceq is a value calculated by the following formula, and if it exceeds 0.40%, the susceptibility to weld cracking increases and the toughness deteriorates significantly. Ceq (%)≡C ⁽ % ⁾ +Mn/6 ⁽ % ⁾ +Si/24 ⁽ % ⁾ +Ni/40 ⁽ % ⁾ +Cr/5 ⁽ % ⁾ +Mo/4 ⁽ % ⁾ +W/14 ⁽ % ⁾ Ingredients as above The steel billet configured to the composition is heated to a temperature of 1250-1350℃ for 60 minutes or more, and then allowed to cool or hot-rolled to a temperature below the _A3 transformation point.
This heating is performed together with the steel (base material) manufactured according to the present invention.
Significantly improves the toughness of welds. The cause of this improvement is currently not clear, but according to the invention of the present inventors, in addition to the effects of B mentioned above,
During cooling, the re-dissolved Mn and Ti become fine MnS,
It is thought that this is because it forms in TiN, becomes an intragranular transformation nucleus, produces intragranular ferrite, and exhibits a fine steel structure. FIG. 1 shows the effect of heating temperature on the toughness of the steel after welding. i.e. heating temperature (heating time 120
) increases the intragranular transformation area ratio and improves low-temperature toughness. However, this effect cannot be obtained at low temperatures below 1250°C or at high temperatures above 1350°C, and when heated for a short time of less than 60 minutes,
Because the formation of intragranular transformation nuclei such as MnS and TiN is insufficient, it does not appear conspicuously as shown in the figure. In addition, when the temperature is lowered from the heating temperature, in order to further promote the generation of intragranular transformation nuclei, the generation of intragranular transformation nuclei is allowed to occur, or while rolling is performed to diffuse intragranular segregation of intragranular transformation nuclei. A ₃ that does not affect the dispersion shape
By cooling to a temperature below the transformation point, the steel for which the invention is directed is produced. This temperature drop is preferably faster than air cooling because of fine dispersion. The steel billet treated in this way is then heated again to
Heat to 900-1150℃ and hot-roll. The high heating temperature during rolling is due to the fact that, in the present invention, among the components in the steel, free N is reduced in particular in consideration of the toughness of the weld zone, so that the austenite grains during the base metal heat treatment tend to become coarse. , Moreover, the effect of the pretreatment process also disappears. Therefore, high heating temperatures exceeding 1150°C should be avoided. But 900
Heating at a low temperature below .degree. C. causes a problem in that the deformation resistance of the steel increases and rolling workability deteriorates. Further, in the present invention, in this hot rolling, the temperature
It is necessary to perform controlled rolling at a temperature of 800°C or less and a reduction rate of 30% or more. In this controlled rolling, rolling is carried out in a non-recrystallized region where free B exists, so that austenite becomes fine grained, ferrite transformation is delayed, ferrite after transformation becomes fine grained, and toughness is improved.
The temperature and rolling reduction in this case are boundary values at which toughness is improved. In this way, the steel billet that has been rolled within the range where its toughness is improved is heated to a low temperature of 300℃ or less.
Cool at a rate of 10-50°C/sec. This cooling rate is ferrite + bainite or ferrite +
It exhibits a fine mixed structure of pearlite. However, as it is, the toughness is low, and this steel is heated at temperatures of 400 to 650℃.
Heat to improve tempering toughness. The temperature in this case is within a range where improvement in toughness can be obtained. Figure 2 shows the temperature of each steel piece with the composition shown in Table 1.
Heated to 1300℃ for 120 minutes, left to cool to 200℃, then heated to 1100℃ again and rolled to 800℃.
As-rolled toughness when water-cooled at 30℃/sec up to a temperature of 300℃ with a rolling reduction of 60%, and a temperature of 500℃.
The toughness of each steel when tempered by heating for 1 minute is plotted in relation to the falling strength of each steel.
In other words, steel that has been tempered after rolling has high strength and significantly improved low-temperature toughness.

[Table] Furthermore, as a pretreatment of the method of the present invention as described above, a steel billet produced by an ingot-making/blowing method or a continuous casting method is heated to a temperature of 1100°C or higher and then rolled. do. This hot rolling is the rolling of a material that is compatible with the subsequent rolling function, as well as the intragranular transformation nuclei that are segregated and generated during the manufacture of the steel billet, that is, MnS,
This is rolling for segregation and diffusion treatment of TiN, etc.
However, at low temperatures below 1100°C, such effects cannot be obtained at the same time. Next, examples of the present invention will be described. Test pieces of steel having the steel compositions shown in Table 2, melted in a converter, were rolled and tempered under the manufacturing conditions shown in Table 3. The mechanical properties of the steel at that time are shown in Table 4. Steel produced by the method of the present invention has superior low-temperature toughness of the steel (base metal) or weld heat-affected zone after production, compared to steel obtained by a comparative production method.

【table】

【table】

【table】

【table】 [Brief explanation of drawings]

Figure 1 is a graph of billet heating temperature, vTrs, and intragranular transformation area ratio, and Figure 2 is a graph of yield point and vTrs.

Claims (1)

[Claims] 1 C: 0.02-0.16%, Si: 0.05-0.7%, Mn:
0.5-1.6%, P: 0.015% or less, S: 0.0005-0.004
%, Al: 0.01~0.1%, B: 0.0005~0.002%,
Contains N: 0.004% or less, Ti: 0.003 to 0.02%,
A steel billet with Ceq 0.4% or less and the remainder substantially made of iron is heated to a temperature of 1250 to 1350°C for 60 minutes or more, and then allowed to cool or rolled to a temperature below _A3 transformation point,
Heating again to a temperature of 900 to 1150℃, rolling with a reduction rate of 30% or more at a temperature of 800℃ or less, cooling at a rate of 10 to 50℃/sec to a temperature of 300℃ or less, and then heating to a temperature of 400 to 1150℃.
A method for producing low-temperature steel with excellent weld toughness, which is characterized by heating and tempering to 650℃. 2 C: 0.02-0.16%, Si: 0.05-0.7%, Mn:
0.5-1.6%, P: 0.015%, S: 0.0005-0.004%,
Al: 0.01~0.1%, B: 0.0005~0.002%, N:
Contains 0.004% or less, Ti: 0.003-0.02%,
Ceq: A steel billet with 0.4% or less and the remainder being substantially iron is heated to a temperature of 1100℃ or higher, rolled, cooled, heated again to a temperature of 1250 to 1350℃ for 60 minutes or more, and then left to cool or rolled. cooled to a temperature below the _A3 transformation point,
Furthermore, it is heated to a temperature of 900 to 1150℃ and rolled with a rolling reduction of 30% or more at a temperature of 800℃ or less.
A method for producing a low-temperature steel with excellent weld toughness, characterized by cooling at a rate of 10 to 50°C/sec, and then heating and tempering to a temperature of 400 to 650°C.