JPH0353367B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technical content described in this specification regarding the reduction of variations in toughness in rolled base materials and welded parts of high-strength steel for welded structures to which so-called high heat input welding of 50000 J/cm or more is applied. This paper proposes the development results of composition adjustment corresponding to the recent trends in the production of high tensile strength steel for high heat input welding and compatibility with heating conditions for hot rolling. In recent years, when manufacturing welded structures, in order to reduce welding man-hours and reduce welding costs, methods such as single-sided single-layer submerged arc welding (SAW), electrogas welding (EGW), or electroslag welding (ESW), which require high heat input, have been adopted. There is a growing momentum to adopt automated welding. However, when high heat input welding is performed on steels of 40Kgf/mm ² or higher, which have been conventionally used for welded structures, the structure of the weld heat affected zone (HAZ), especially at the weld bond, is mainly composed of coarse upper bainite. As the structure deteriorates, the toughness becomes extremely poor, making it difficult to carry out high heat input welding. (Prior Art) Since then, various steels suitable for high heat input welding have been developed, and some of them are now being put into practical use. On the other hand, recently, technologies related to so-called processing heat treatment such as controlled rolling, controlled cooling after rolling, and direct quenching have been developed, and high heat input welded steel can also be manufactured using these methods. This is because by adopting such processing heat treatment technology, it is possible to reduce the amount of C, Mn, and other alloying elements that are considered to be bad for the toughness of high heat input welds. This is why it is expected to be a manufacturing method for steel materials with excellent welding properties under large heat input. In addition to further improving the toughness of the base material, these new manufacturing methods aim to reduce fuel costs for the heating furnace, and rolling is often carried out at a low temperature of about 1150°C. An example of this can be seen in, for example, Japanese Patent Publication No. 55-30047. (Problems to be solved by the invention) Although the base metal and weld zone toughness of high-strength steel welded with high heat input by combining low-temperature heating and processing heat treatment technology is certainly excellent, the toughness values are sometimes abnormally low. A phenomenon indicating this was experienced. In other words, it is a variation in toughness between the base metal and the high heat input welded part, and such unstable toughness is a serious problem from the perspective of structural stability, and there is a need for improvement from the side of users of this type of steel. This is a new issue that must be resolved as soon as possible, in light of not only the demands but also the mission of material manufacturers to stably manufacture and reliably supply steel products of stable quality. When low carbon equivalent steel is heated at low temperature, coarse grains as large as 200 μm are generated in the fine austenite grain size of 20 to 50 μm, and such abnormally coarse grains cannot be refined by rolling. is inherited in the structure after metamorphosis, resulting in variations in base material toughness. Furthermore, it is inherited by the structure of the high heat input welding affected zone, resulting in variations in its toughness.The combined addition of Ti and REM is particularly effective as a component to suppress the generation of abnormal grains, and the base metal It was found through the following experiments that variations in toughness in high heat input welds can be effectively reduced. Generally, when steel is heated above the A _c3 temperature, the austenite grain size becomes coarser as the heating temperature increases, but in a certain temperature range, extremely coarse austenite grows among the fine austenite grains. Figure 1 shows the results of the experiment that led to the idea of this invention.
The relationship between heating temperature and austenite grain size of −1.36%Mn−0.014%Al steel at each temperature from point A _c3 to 1200℃.
An example of measurement after quenching after holding for 30 minutes is shown. When heated below 920°C, it becomes regular austenite with an average grain size of 20 μm. On the other hand, even at temperatures above 1000°C, the average grain size becomes larger, but it is austenite with a regular grain size of about 65 to 130 μm. However, within the range of approximately 920°C or higher and lower than 1000°C, austenite with a grain size of 20 to 65 μm
Coarse particles as large as 200 μm are mixed in, and the area ratio of these coarse particles reaches a maximum of 30%. However, the upper limit temperature at which mixed grains do not occur depends on the steel composition, and must be lower than 1000°C within the composition range of this invention. This phenomenon can be seen to a greater or lesser degree in steel of any composition, but in particular, the fewer alloying elements and impurity elements contained in iron, the larger the number of coarse grains, or the area ratio. This has been newly discovered, and this means that steel with a lower C equivalent is more likely to produce mixed grains containing abnormal grains when heated at a low temperature. If a steel plate is produced by rolling starting from such austenitic state, not only will the base metal have an abnormally low toughness value, but also the toughness of the joint will become worse when high heat input welding is performed. The phenomenon of abnormally low values of toughness was also observed. Such variations in toughness tend to occur when low C-equivalent steel is manufactured by low-temperature heating.
It was found that a particularly coarse structure was present at the bottom of the notch in the test. Here, the base metal has a coarse bainite structure, and the particularly coarse structure after welding is due to the coarse bainite structure that existed in the steel sheet before welding, and that bainite structure is formed at a low temperature before rolling. Coarse austenite grains of up to 200 μm that were generated during heating and grew abnormally were transformed without being sufficiently refined even during subsequent rolling. Therefore, the inventors conducted research on a component system that can suppress the above-mentioned unstable toughness that tends to occur in the base metal of low carbon equivalent steel obtained by low-temperature heating method and in high heat input welded parts, and conducted numerous experiments. After repeated consideration,
The combined addition of REM and Ti helps to prevent abnormal grain growth of austenite during low-temperature heating, regularizes the austenite during low-temperature heating, and reduces variations in toughness in the base metal and high heat input welds. The present invention was completed based on the discovery that it can effectively contribute to the In other words, the purpose of this invention is to appropriately realize such a contribution, and
Due to the combined addition of REM, TiN and REM oxysulfide act together effectively, especially (A _c3 point −30
It is based on the basic recognition that abnormal growth of austenite is inhibited during low-temperature heating at temperatures below 1000°C. (Structure of the invention) This invention comprises: C: 0.03 to 0.15wt%, Si: 0.05 to 0.6wt%,
Contains Mn: 0.5 to 2.0 wt% and Al: 0.01 to 0.08 wt% as basic components, or further contains Nb:
0.005-0.10wt%, V: 0.005-0.15wt%, Ni: 0.1
~2.0wt%, Cu: 0.1~1.0wt%, Cr: 0.1~1.0wt
% and Mo: 0.05 to 0.5 wt%,
When producing high-strength steel for high heat input welding by hot rolling, the composition of which is suppressed to N: 0.001 to 0.007wt% in the steel, Ti: 0.005 to 0.005 at the material melting stage.
The composition is adjusted to include a composite of 0.025wt% and REM: 0.002~0.01wt%, and during hot rolling, the slab is rolled at a temperature ranging from (A _c3 point - 30℃) to a temperature below 1000℃. This is a method for stabilizing the toughness of high-strength steel for welded structures, which is characterized by suppressing the heating temperature to avoid the generation of abnormal grains accompanying the heating. First, the reason for limiting the range of the steel composition in this invention will be explained. C: 0.03 to 0.15% If the C content is less than 0.03%, the required strength cannot be obtained, and the weld heat affected zone may become softened.
If it exceeds 0.15%, weldability will be impaired, so 0.03~
It was set at 0.15%. Si: 0.05-0.60% Si promotes deoxidation of steel and increases strength, so it must be added at least 0.05%, but 0.6%
If the amount exceeds 0.05 to 0.60%, the toughness and weldability will be significantly impaired. Mn: 0.5~2.0% Mn less than 0.5% reduces the strength and toughness of the steel sheet and causes HAZ softening;
If the amount exceeds 2.0%, the toughness of the HAZ deteriorates, so it was set at 0.5 to 2.0%. Al: 0.01 to 0.08% Al requires a minimum content of 0.01% for deoxidizing steel, but if solid solution Al exceeds 0.08%, it will cause HAZ
Not only this, but also the resiliency of the weld metal is significantly deteriorated, so Al was set at 0.01 to 0.08%. The above are the basic components necessary to advantageously apply the method of this invention, but in addition to these, it is already necessary to suppress the N content, which is unavoidably accompanied in ordinary steelmaking and refining, to a range of 0.001 to 0.007%. The synergy of REM with oxysulfide is essential for the production of TiN. In this invention, TiN generated in the steel due to the inclusion of Ti and REM, together with oxysulfide of REM, prevents abnormal grain growth of austenite. However, if the content of Ti and N is too large, TiN will become coarse and will not only lose its effect of suppressing abnormal austenite grain growth, but will also impair the toughness of high heat input welded joints, so the upper limit for Ti is 0.025%, and the upper limit for N is 0.025%. ,0.007%
And so. On the other hand, even if the Ti and N contents are too small, there is no effect of suppressing abnormal austenite grain growth.
The lower limits of and N were set to 0.005% and 0.001%, respectively.
REM, as an oxysulfide, exhibits the effect of suppressing abnormal austenite grain growth in the coexistence of TiN, but excess REM exceeding 0.01% impairs the cleanliness of the steel and causes internal defects, so the upper limit is
It is set at 0.01%, while less than 0.002% has no effect. In addition to the above, this invention further includes:
Nb: 0.005-0.10wt%, V: 0.005-0.15wt%,
Ni: 0.1-2.0wt%, Cu: 0.1-1.0wt%, Cr: 0.1
~1.0wt% and Mo: 0.05~0.5wt%.The main purpose of including these elements is to improve the strength and toughness of the board without losing the characteristics according to the present invention. This makes it possible to increase the thickness, and the amount added is limited by the following reasons. Nb is included to refine the grains of the rolled structure and harden it by precipitation, and is an important element that improves both strength and toughness, but if it exceeds 0.10%, it will not only affect weldability but also the toughness of the weld metal. The upper limit is set to cause deterioration.
It was set at 0.10%. V has almost the same effect as Nb, but the upper limit is
Acceptable up to 0.15%. Ni is added because it improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of the HAZ, but it is expensive, so the upper limit was set at 3.0%. Cu not only has almost the same effect as Ni, but also
It also improves corrosion resistance, but if it exceeds 1.0%, hot embrittlement tends to occur and the surface quality of the steel sheet deteriorates.
The upper limit is 1.0%. Mo improves strength and toughness by making the austenite grains finer and more regular during rolling and producing fine bainite and martensite, but it is expensive, so the upper limit was set at 0.50%. Cr generates fine bainite and martensite to improve strength and toughness, but addition of more than 1.0% impairs weldability, so the upper limit was set at 1.0%. Note that the minimum required amount for the addition effect of these elements to be significant is 0.005% for Nb and V;
Ni, Cu and Cr are 0.1%, and Mo is 0.05wt%. The steel whose composition is limited as described above has almost no variation in toughness in the base material and the high heat input welded part even if it is heated and rolled at a low temperature, and can fully achieve the effects aimed at by the present invention. The upper limit of heating temperature for this hot rolling is 1000℃
If the temperature exceeds 1000°C, the average austenite grain size becomes too large, which is not preferable. Further, as mentioned above, the heating temperature at which mixed grains occur is 1000°C or higher. On the other hand, from the point of view of austenitizing steel
The temperature is A _c3 , but in practical terms, the desired effect can be achieved up to A _c3 -30°C. Note that the conditions for rolling and cooling after heating to a temperature below (A _c3 -30℃) to 1000℃ are not particularly stipulated, but for the purpose of this invention, controlled rolling method, accelerated cooling method or It is best to use the direct hardening method. Next, embodiments of the present invention will be described. Using slabs manufactured in the converter-continuous casting process with the components shown in Table 1, the plate thickness was determined by changing the heating-rolling-cooling conditions.
We manufactured steel plates from 25mm to 75mm.

[Table] Then, the strength and toughness values of the base material and the variation in toughness were measured for 10 pieces each of Charpey test and COD test pieces.
Table 2 shows the results for each book. Furthermore, the second
For example, the symbol 1A in the table indicates that steel numbered 1 in Table 1 was used. Note that 1000°C in the column of heating temperature in Table 2 is the rounded value of the actual temperature that is very close to 1000°C but does not reach 1000°C.

【table】

[Table] In addition, so-called large heat input welding was performed under the welding conditions shown in Table 3, and 10 shear pea test pieces were taken from this bond area and tested, and the results are shown as the average value and minimum value of the 10 pieces. .

[Table] The steel sheet whose toughness was stabilized by the method of this invention has excellent properties with no variation in toughness in the base metal and in the weld bond area with large heat input, whereas the comparative steel There is significant variation in toughness. Steel 1A does not have Ti and REM added, so
There is variation in the toughness of the base metal, and the toughness of joints made by high heat input welding is also low. Because Steel 2A has Ti added, its toughness in the base metal and large heat input welds is superior to Steel 1A, but since REM is not added in combination, there is still variation in the toughness in the base metal and joints. It remains unstable. The composition of Steel 3B is within the composition range of this invention, but
Since the heating temperature is outside this range, the toughness of the base material itself is poor. Also, since REM is not added to Steel 5A, there are variations in the toughness of the base metal and the welded part. In contrast to these, 3A, 3C, 4 according to the present invention
A, 4B, 6A, 7A, 8A, 9A, 10A, 1
1A, 12A, 13A, and 14A have high average values of the Charpy absorbed energy and COD value of the base metal, and do not show abnormally low values. Furthermore, both the average value and the minimum value of the high heat input welded joint are high. (Effects of the Invention) According to the method of the present invention, a high-strength steel material with excellent toughness and uniform toughness in the base metal and the high heat input weld zone can be reliably obtained.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between austenite grain size, coarse grain size, coarse grain area ratio, and heating temperature when steel is heated to an austenite region.

Claims (1)

[Claims] 1 Contains C: 0.03 to 0.15 wt% Si: 0.05 to 0.6 wt% Mn: 0.5 to 2.0 wt% and Al: 0.01 to 0.08 wt% as basic components,
When manufacturing high tensile strength steel for high heat input welding with a composition suppressed to N: 0.001 to 0.007 wt% in steel by hot rolling, Ti: 0.005 to 0.025 wt% is added at the melting stage of the material.
REM: 0.002 to 0.01wt% is mixed and the components are adjusted, and during hot rolling, the heating temperature of the slab is set in the range from (A _c3 point - 30℃) to a temperature that does not reach 1000℃. 1. A method for stabilizing the toughness of high-strength steel for welded structures, characterized by suppressing the occurrence of abnormal grains caused by heating. 2 Contains C: 0.03-0.15wt% Si: 0.05-0.6wt% Mn: 0.5-2.0wt% and Al: 0.01-0.08wt% as basic components, furthermore, Nb: 0.005-0.10wt%, V: 0.005 ~0.15wt%, Ni: 0.1~2.0wt%, Cu: 0.1~1.0wt%, Cr: 0.1~1.0wt%, and Mo: 0.05~0.5wt%. When manufacturing high tensile strength steel for high heat input welding with a composition suppressed to N: 0.001 to 0.007wt% by hot rolling, Ti: 0.005 to 0.0025wt% is suppressed at the material melting stage.
REM: 0.002 to 0.01wt% is mixed and the slab is heated to a temperature range from (A _c3 point - 30℃) to a temperature below 1000℃ during hot rolling. A method for stabilizing the toughness of high-strength steel for welded structures, the method comprising suppressing the generation of abnormal grains accompanying the heating.