JPH0610309B2 - Post-heat treatment method for welding heat affected zone of ERW steel pipe - Google Patents

Description

The present invention relates to a post heat treatment method for softening a weld heat affected zone of an electric resistance welded steel pipe.

<Prior art and its problems> In recent years, the improvement of the fuel efficiency of automobiles has been remarkably improved, and various measures have been taken to reduce the weight of the vehicle body. Among these, the study of hollowing parts that were conventionally manufactured from steel bars is remarkable, and the replacement of steel bars with electric resistance welded steel pipe is being actively promoted.

This tendency is especially remarkable for small diameter ERW pipes,
Many of these methods, such as bending and expanding, are used alone or in combination to form parts with complicated shapes.

On the other hand, the weld bead portion of the electric resistance welded steel pipe is heated to a high temperature during welding and is cooled after pressure welding. Therefore, the higher the C content of the pipe material, the higher the alloying elements (Si, Mn, Cr, Mo, B) that enhance the hardenability.
Element, etc.) is contained in one kind or two kinds or more, and the larger the added amount, the more remarkable the hardening of the weld bead part,
The hardening of the weld heat affected zone after cooling becomes more remarkable than the hardness of the base metal part of the blank tube.

Therefore, without softening the heat affected zone,
When processing a steel pipe as it is welded, cracks occur in the welded part during processing and it cannot be processed, and the difference in deformability between the weld heat affected zone and the base metal causes the weld heat affected zone to become thicker after processing. As a result, uneven thickness occurs, which causes a problem that the precision of the processed parts cannot be obtained.

Therefore, it is the current situation that the welded heat-affected zone of the electric resistance welded steel pipe used for processing is annealed after welding. But,
The conventional annealing method, that is, the post-heat treatment method is described in Japanese Patent Publication No. 59-42731, 59-48848, and ERW steel pipe weld annealing equipment (Steelmaking Research No. 295 (1978), p12673-12678). As can be seen, since the weld heat affected zone is heated to a temperature of A ₃ point or higher after welding, the temperature near the transformation point is set so that the weld zone does not harden during cooling after reheating, that is, it transforms into a ferrite + pearlite structure. It is necessary to gradually cool the area. Therefore, the apparatus requires a long annealing zone after the reheating zone, and the pipe forming machine line is also very long and large-scale equipment. Further, when the slow cooling zone is short, the pipe forming speed must be slowed down so as not to rapidly cool the temperature range near the transformation point, which significantly lowers the production efficiency.

Furthermore, in the pipe making from a material having a high C content or a material containing an alloying element (Mn, Cr, Mo, B, etc.) that improves hardenability, the time required to transform from austenite to ferrite + pearlite structure becomes long. , These problems become more prominent.

Therefore, the current post-heat treatment method has major drawbacks in terms of both equipment and production efficiency.

<Means for solving the problem> In view of the present situation, the object of the present invention is to examine various cooling conditions and reheating conditions after welding, and as a result, the martensite formation start temperature ( It will be abbreviated as Ms point below.) By rapidly quenching to the temperature below, and then tempering, the welding heat-affected zone can be obtained in a short time without the need for a long annealing zone and without slowing the pipe forming speed. We have found a post-heat treatment method that efficiently softens the hardness.

<Structure of the Invention> The present invention is a method for manufacturing an electric resistance welded steel pipe by low-frequency and high-frequency welding.
It is rapidly cooled to a temperature lower than the point by 50 ° C or more, and cooled so that 20% or more of the austenite structure generated by high-temperature heating during welding does not remain, forming a martensite structure.

After that, reheat from 500 ℃ to (Ac ₁ + 50 ℃),
This is a post-heat treatment method characterized by softening the weld heat affected zone by tempering martensite produced by quenching.

In this method, since 20% or more of the retained austenite does not remain after reheating, the weld heat affected zone does not soften even after rapid cooling, and it is not necessary to keep the annealing zone long. It is also unnecessary to slow down the pipe forming speed to complete the transformation.

The reasons for limitation of the present invention will be described below.

If the cooling temperature of the electric resistance welded portion after welding is higher than (Ms-50 ° C), the structure of the heat-affected zone of the welding tends to be a structure containing untransformed retained austenite. Since this retained austenite transforms to martensite in the cooling after reheating,
If the amount of retained austenite is 20% or more, the weld will be hardened. In addition, after reheating, in order to transform and soften 20% or more of retained austenite into a ferrite + pearlite structure, it is necessary to slow the pipe forming speed or provide a long annealing zone.

Therefore, in order to keep the amount of martensite in the heat affected zone after welding to less than 20%, the cooling temperature is (Ms-50 ℃).
The temperature was limited to the following.

Reheating after cooling is to temper the martensite formed in the weld heat affected zone during cooling, and the postheating temperature is 500 ° C.
If it is less than 1, the martensite cannot be sufficiently tempered under the post heat treatment condition for a short time, and if heated above the temperature of Ac ₁ + 50 ° C, it transforms into an austenite structure again, and this austenite structure transforms into martensite during cooling. Therefore, the weld heat affected zone does not soften. Therefore, in order to soften the heat affected zone, the post-heating temperature is 500 ℃ ~ (Ac ₁ +50
(° C) temperature range.

Example 1 In order to examine cooling conditions after the post heat treatment according to the conventional method and the method of the present invention, using materials having the chemical components shown in Table 1 (a) and the transformation points shown in Table 1 (b), The transformation cycle measuring device (Formaster-F) was used to apply a heat cycle simulating the production of ERW steel pipes (welding and post heat treatment). The heat cycles corresponding to the conventional method and the method of the present invention are shown in FIGS. 2 and 1. In the conventional method, as shown in FIG. 2, the reheater (post-annealer) heats the weld to a temperature of A ₃ or higher.
The transformation structure and the obtained hardness also differ depending on the subsequent cooling rate. Table 2 shows the hardness when cooled by changing the cooling rate in the temperature range from 900 ° C to 500 ° C.

The hardness obtained when the test material A equivalent to S45C is cooled at a cooling rate of 20 ° C./sec is H _R C35, and the hardness when cooled at 40 ° C./sec is H _R C52. On the other hand, test material B equivalent to SCM435
The hardness obtained when the steel is cooled at a cooling rate of 4 ° C / sec is H
_R C33, and the hardness when cooled at 40 ° C / sec is H _R C54. In other words, to obtain a hardness of H _R C33~35 test materials A
About 20 seconds after heating with post-annealer, 100 for sample B
It takes time to transform the seconds. Now, when producing at a pipe making speed of 30 m / min, the length of the annealing zone is 10
m, the length of 50 m is required for test material B. To obtain even lower hardness, the length of the annealing zone becomes longer.

On the other hand, in the method of the present invention shown in FIG.
The material was rapidly cooled to a temperature of 250 ° C lower than the point by 250 ° C, transformed into a martensite structure, reheated to 720 ° C for 5 seconds, and cooled to room temperature in 3 seconds. The resulting hardness test specimen A in H _R C32, was H _R C36 in the test material B.

Now, if the pipe making speed is 30 m / min, the cooling zone needs to be 1.5 m, and the length of the cooling zone after post-annealing in the present invention method may be significantly shorter than that in the conventional method as compared with the conventional method. .

Example 2 In producing an electric resistance welded steel pipe having a size of 22.2φ × 2.6 t (mm) by using a material having the chemical composition shown in Table 3 (b) and the transformation point shown in Table 3 (b), After forming the steel strip into a tubular shape, the edge of the tubular body was heated by high frequency heating and pressure welding was performed with a squeeze roll. After that, the welded part is cooled with water in about 5 seconds,
The sample was cooled to 250 ° C., which is about 180 ° C. lower than the Ms point, which is the range of the method of the present invention, and heated to various temperatures by a post annealer for 3 seconds. Further, after reheating, it was cooled to about 150 ° C within 15 seconds by water cooling. The pipe forming speed is 30 m / min.

Figure 3 shows the hardness distribution in the weld heat affected zone due to reheating at 600 ° C.

Table 4 shows the maximum hardness of the heat affected zone, the post heat temperature, and the bending test result of the electric resistance welded steel pipe shown in FIG. From the table, when the post heat temperature is less than 500 ° C, the hardness of the welding heat affected zone does not tend to soften, but in the temperature range of 500 ° C or higher (Ac ₁ + 50 ° C), the higher the heating temperature, the harder the welding heat affected zone becomes. Sa significantly softened.

Further, when the temperature exceeds (Ac ₁ + 50 ° C.) and is heated, the heated portion is transformed into austenite again, so that martensite is generated by subsequent cooling and the weld heat affected zone becomes harder.

Further, in the post-heat temperature range of the method of the present invention, cracks do not occur in the weld heat affected zone in a bending test with a bending angle of 180 °, a product having a complicated shape can be processed, and good processing characteristics are exhibited.

Example 3 In producing an electric resistance welded steel pipe having a size of 22.2φ × 2.6 t (mm) by using a material having the chemical composition shown in Table 3 (b) and the transformation point shown in Table 3 (b), After the steel strip was formed into a tubular shape, the edge portion of the tubular body was heated by high frequency heating and pressure welding was performed with a squeeze roll. After that, the welded portion was cooled to each temperature by water cooling in about 5 seconds and heated by a post annealer to a temperature of 670 ° C. for 3 seconds. Further, after reheating, it was cooled to about 150 ° C within 15 seconds by water cooling. The pipe forming speed is 30 m / min.

FIG. 4 is a diagram showing the relationship between the maximum hardness of the welding heat affected zone and the cooling temperature. From the figure, when cooled to a temperature lower than the Ms point by 50 ° C or more, the martensite produced during cooling is tempered and the hardness of the weld heat affected zone decreases. In contrast,
At a cooling temperature exceeding (Ms point −50 ° C.), the amount of retained austenite becomes 20% or more, martensite is generated after reheating, and high weld zone hardness is exhibited after post heat treatment.

Example 4 In manufacturing an electric resistance welded steel pipe using a material having the chemical composition shown in Table 5, the material strip steel is formed into a tubular shape, the edge of the tubular body is heated by high frequency heating, and pressure welding is performed with a squeeze roll. did. After that, it was cooled to each temperature by water cooling in about 5 seconds, and subsequently heated by a post annealer for 3 seconds and water cooled.

The pipe making speed is 30 m / min. The hardness measurement and bending test of the welding heat affected zone of the electric resistance welded steel pipe manufactured under such conditions were performed. The results are shown in Table 6.

For each of the ERW steel pipes of the test materials A (S35C), B (S55C) and C (SCM435), the cooling temperatures are higher than those of the method of the present invention A-1, B-1,
C-1 does not transform into martensite at the time of cooling,
Since it is continuously reheated, residual austenite transforms to martensite during cooling after post-heating and the weld heat affected zone becomes a hardened structure and is very hard. It cannot be used as a steel pipe.

Further, the cooling temperature is a condition of the method of the present invention, but the electric resistance welded steel pipes of A-2, B-2, C-2 whose post-heating temperature is lower than that of the method of the present invention are transformed into martensite during cooling. Since the post-heat temperature is low, the tempering of martensite is insufficient, the heat affected zone of welding is extremely hard, and cracks occur in the welded portion in the bending test.

On the other hand, in the electric resistance welded steel pipes of A-3, B-3, and C-3 of the method of the present invention, the martensite formed during cooling is tempered at a high temperature during post-heating, so that the heat-affected zone of welding is significantly softened. Even in the bending test, there is no crack in the welded part and it can be used as an electric resistance welded steel pipe for processed parts.

<Effect of the Invention> The present invention is to rapidly cool the electric resistance welded portion heated by welding to a temperature lower than the Ms point of the pipe material by 50 ° C. or more, and transform the austenite structure into the martensite structure by high temperature heating during welding. After that, by reheating at a temperature from 500 ° C to the maximum (Ac ₁ + 50 ° C) and tempering the martensite generated by quenching, the post heat treatment method softens the heat affected zone in a short heat cycle, There is no need to provide a long annealing zone after the conventional post-annealer, or to slow down the pipe making speed for manufacturing, thus improving the defects in terms of equipment and production efficiency. Further, the characteristics of the electric resistance welded steel pipe produced by the method of the present invention can be subjected to severe bending, and show good quality characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between temperature and time when a heat cycle imitating welding and post heat treatment of the present invention is applied to a steel material;
The figure shows the relationship between temperature and time when a heat cycle imitating welding and conventional post heat treatment is applied to a steel material.
FIG. 4 is a diagram showing the hardness distribution of the weld heat affected zone when reheated to 0 ° C., and FIG. 4 is a diagram showing the relationship between the cooling temperature and the hardness of the weld heat affected zone after the post heat treatment at 670 ° C.

Claims (1)

[Claims] 1. In a method for producing an electric resistance welded steel pipe by low frequency and high frequency welding, the electric resistance welded portion after welding is cooled once to a temperature lower than the Ms point of the pipe material by 50 ° C. or more, and then 500 ° C.
To (Ac ₁ + 50 ° C), a post-heat treatment method that softens the heat affected zone by reheating and cooling.