JPS6362566B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for improving the toughness of a weld metal surface. High heat input welding methods such as submerged arc welding are often used to assemble large structures by welding steel plates, steel pipes, etc.; ) also deteriorates toughness. Recently, in order to improve the toughness of HAZ, a method has been developed in which the narrow groove method is used to reduce the heat input to restore toughness. has a dendrite structure, so the improvement effect is small. On the other hand, in order to ensure the toughness of the weld metal, alloy elements such as Si, Mn, and Mo added to the flux and welding wire must be appropriately adjusted, or the toughness can be improved by making the structure finer and more uniform with additives such as Ti and B. However, the use of such alloying elements increases costs, and the dendrite structure makes it difficult to obtain toughness equivalent to that of the base metal.Furthermore, when heat treated after welding, the toughness deteriorates. It had many drawbacks. On the other hand, it is well known in the rolling of steel plates that when a material with a dendrite structure is subjected to hot rolling, the dendrites are destroyed and the toughness is improved by turning into fine ferrite pearlite. However, the present invention aims to improve the dendrite structure of the weld metal to a fine ferrite pearlite structure or a fine bainite martensite structure, by reducing the excess weld metal and increasing the surface toughness to prevent brittle fracture. At the same time, it also has the effect of reducing the height of the weld bead, and the surface temperature of the weld metal extra layer after cooling or reheating is between just above the Ar ₃ transformation point and the Ar ₁ transformation point. It is characterized by a reduction of 10% or more. Generally, in order to destroy dendrites and make the structure finer by rolling, the heating temperature and rolling temperature are lowered to make the austenite grains finer. An effective method is to increase the ratio and refine the austenite grains. When reducing the weld metal extra layer as in the present invention, reheating may not be necessary, and it is also difficult to increase the total reduction rate, but
% or more between just above the Ar ₃ transformation point and the Ar ₁ transformation point, the effect of dendrites is almost eliminated and the desired toughness is improved. Furthermore, if a large reduction is applied during rolling, the dendrites will be destroyed, so when the rolling reduction is relatively small, such as 10% or slightly more than 10%, it is desirable to perform rolling in one pass. Next, when heat treatment is performed after cooling to room temperature, when reduction is applied, the austenite is finely divided, so higher toughness can be obtained compared to the conventional heat treatment without reduction. Next, examples of the present invention will be shown. Using SM50A steel plate with a thickness of 50 mm, welding was performed at a bevel angle of 30° at intervals of 5 mm under the welding conditions shown in Table 1.
Performed MAG welding. Table 2 shows the chemical composition (wt%) of the test welding wire. At this time, the Ar ₃ -point temperature and Ar _1- point temperature of the weld metal were 874°C and 698°C, respectively. Three-point bending COD test pieces (10t×
20w×100) was sampled and subjected to fatigue cracking, then COD
I conducted a test. Table 3 illustrates the implementation conditions and results.

【table】

【table】

[Table] In the above table, No. 1 to 7 are comparative methods, and No. 1
Items 4 to 4 were left as welded or were only heat treated, and their COD values were low. No. 5 is a case where the reduction is performed at a high temperature, No. 6 is a case where the reduction is performed at a low temperature, and No. 7 is a case where the reduction ratio is as low as 5%. Nos. 8 to 14 are methods of the present invention, and the COD values at -30°C are sufficiently high, 0.15 to 0.32 mm, and are sufficiently durable for use at low temperatures of -30°C or lower. Although the above embodiments were based on MAG welding, the improvement effect of the present invention is the same even if the welding method is different, such as submerged arc welding or TIG welding.

Claims (1)

[Claims] 1 Cool or reheat the weld metal excess area, reduce the excess area by 10% or more between the surface temperature just above the Ar ₃ transformation point and the Ar ₁ transformation point temperature, and then cool it as it is or then quench, temper, or quench it. A method for improving the toughness of weld metal characterized by smoothing.