JPH0124205B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) A method for producing steel for high heat input welding, in which a steel slab is subjected to a specific heat treatment and then rolled again, so that the low-temperature toughness of the joint produced by high heat input welding is excellent. This proposal aims to expand the uses of this type of steel. (Prior art) In recent years, when manufacturing welded structures, in order to reduce welding man-hours and welding costs, single-sided single-layer submerged arc welding (SAW), electrogas welding (EGW), or electroslag welding ( There is a growing momentum to adopt automatic welding that uses large heat input, such as ESW). 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. 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 produce steel materials with excellent welding properties under large heat input. Incidentally, as a steel suitable for the aforementioned high heat input welding, TiN is often used for the purpose of refining austenite grains during welding thermal cycles. According to the inventors' research, in order to improve the low-temperature toughness of joints welded with high heat input, it is necessary to
It is useful to have a large amount of TiN with a diameter of 0.05 μm or less. In this regard, in the past, as in Japanese Patent Publication No. 57-11944 and Japanese Patent Publication No. 58-14848, Ti-added steel was continuously cast and the cooling rate from the molten steel temperature at the time of pouring to 1100°C was controlled (0.1 There is an example of improving the toughness of a high heat input weld (by dispersing TiN of less than μm). However, in any of these -
There is nothing that can satisfy the toughness at 40℃ or -60℃. The inventors previously proposed a method that is completely different from the conventional method for continuous casting of Ti-added steel, but this method cannot be applied to blooming slabs, and it is difficult to continuously cast Ti-added steel. In some cases, it was not possible to carry out the process depending on the continuous casting machine. (Problem to be solved by the invention) To dramatically improve the toughness of high heat input welded parts of steel plates manufactured from steel slabs or cast slabs by finely dispersing TiN to 0.05 μm. is the purpose of this invention. Incidentally, as seen in Japanese Patent Publication No. 51-16890, there have been attempts to specify the heating conditions during the process from steel ingot to steel plate, but according to the inventors' research, it was found that TiN could be dispersed to a size of 0.05 μm or less. It is not enough to simply specify the heating conditions; in particular, the cooling conditions after heating are important, and the investigation of this point led to the completion of the present invention. (Means for solving the problem) This invention has C: 0.01~0.12wt%, Ti: 0.005~
When cooling a steel slab containing 0.015 wt% and N: 0.001 to 0.006 wt% after heating it to 1400 to 1200°C, the average cooling rate at the center in the thickness direction of the slab is 1100°C.
℃ up to 10℃/min or more, then 1100℃ or less 800
℃ to 5℃/min or less, and then the slab is reheated to a temperature of 1200℃ to Ac ₃ and rolled, thereby producing a steel for high heat input welding characterized by excellent low-temperature toughness. It is. (Operation) To achieve the purpose of this invention, a steel slab is first heated to 1400-1200°C. This heating is to melt the TiN produced during the production of the bloomed steel slab or cast slab. Conventionally, at temperatures above 1200℃,
Although it has been believed that TiN does not dissolve, it has been newly discovered that when the amounts of C and N are reduced according to the present invention, a large amount of TiN can be dissolved by heating to 1200° C. or higher. However, TiN did not dissolve at temperatures below 1200°C, so the heating temperature was limited to 1200°C or higher. The upper limit should be below the temperature at which the steel slab begins to melt, but it is set at 1400°C from the economical point of view of the heating furnace. Thus, the steel slab is heated to 1400-1200℃ and the
Rapid cooling up to 1100℃ at 10℃/min or more, then 1100℃
From ℃ to 800℃, slowly cool at a rate of 5℃/min or less. In other words, the purpose is to finely disperse and precipitate dissolved TiN at a low temperature of 1100° C. or lower, and this is based on the following experimental facts. Using steel pieces with the composition of the test steel shown in Table 1,
After heating to 1250℃, cooling rate up to 1100℃,
Table 2 shows the results of measuring the particle size r (μm) and volume fraction f of TiN while varying the cooling rate from 1100°C to 800°C.

【table】

【table】

[Table] As mentioned above, in the slab before manufacturing the steel plate
Dispersing a large amount of TiN with a diameter of 0.05 μm or less is useful for improving the low-temperature toughness of high heat input welds. The nature is excellent. Here, the state of TiN in the form of a steel ingot and bloom is indicated by ε, and a small amount of very large TiN with an average grain size r of 1.5 μm is dispersed. On the other hand, from the comparison of conditions γ to ε, 1250 to 1100
It can be seen that the faster the cooling rate at 1100°C to 800°C is, the smaller γ becomes and the larger f becomes. After repeated experiments with many steels, the result was 1400~
It was found that a large amount of TiN of 0.05 μm or less can be dispersed if the heating rate is 10°C/min or more from 1200°C to 1100°C, and 5°C/min or less from 1100°C to 800°C. Further, the slab subjected to the above treatment is reheated for rolling, but this temperature range is limited to 1200~ _Ac3 . The reason is that the lower limit is the lowest temperature at which the steel can be austenitized. On the other hand, the upper limit is TiN, which finely precipitates below 0.05μm when heated above 1200℃.
This is because the particles undergo re-dissolution and reprecipitation, becoming coarse and ineffective. C is required to be at least 0.01wt% in order to obtain the strength required for this type of welded structural steel;
If it exceeds 0.12wt%, not only will it have a negative effect on the toughness of the high heat input welded part, but especially during the heating of this invention.
TiN becomes difficult to dissolve and its purpose cannot be achieved. N is contained in the normal steelmaking process, but 0.006wt
If it exceeds %, the toughness of the high heat input weld will be impaired;
If TiN is less than 0.001wt%, sufficient TiN will not be released, so the lower limit was set at 0.001wt%. Ti not only improves the bond toughness of the weld joint through the finely dispersed TiN in the steel, but also prevents austenite grains from coarsening during slab heating and improves the toughness of the base metal. The minimum amount of Ti that exhibits these effects is 0.005wt%, and
On the contrary, Ti exceeding 0.015wt% deteriorates the toughness of the base material. Components other than the above are not particularly limited, but suitable specific examples include Si: 1wt% or less, Mn: 2wt%
The following examples include those containing Al: 0.1 wt% or less, with the balance being Fe. In addition, to this basic component system, one or two types of Nb of 0.1wt% or less, V of 0.1wt% or less and/or Ni, Cr, Mo, Cu, etc. are added for the purpose of increasing strength and toughness. 2wt each
% or less or 0.005wt% or less B or
It may contain 0.01et% of Ca and/or one or more of REM. (Example) In order to further clarify the effects of the present invention, an example will be described below. After melting steel with various components shown in Table 1, ingots are formed.
A steel slab was created by a blooming process or a continuous casting process, followed by heat treatment with varying heating and cooling conditions, and then a steel plate was manufactured by a normal process. So-called high heat input welding was performed on these steel plates, and the toughness of the bonded portion was measured in a Shalby impact test. The conditions and toughness values are summarized in Table 3. Note that the symbol 1A in Table 3, for example, indicates that steel 1 in Table 1 was used. The steel sheets heat-treated before rolling by the method of the present invention have excellent toughness at the high heat input weld bond, whereas the comparative examples do not have sufficient toughness. Steels 1A and 4A are examples in which steel plates were manufactured from slabs made by the ingot-blowing process, and the toughness of the high-heat-input weld bond is low. Steel 3A is an example in which a steel plate was manufactured from a slab made in the continuous casting process, and although the toughness of the high heat input welding bond is superior to that in the ingot-blowing process, the value is low. Next, for Steel 5A and 5B, C is or Steel 6A,
6B is 5A and 5B because Ti is not in the component range of this invention.
And as is clear from the comparison between 6A and 6B, the toughness is not improved regardless of whether or not the method of the present invention is applied. Next, Steel 1C has a cooling rate of 1100 to 800 degrees, Steel 1D has a cooling rate from heating temperature to 100 degrees, Steel 1E has a heating temperature, and Steel 1F has a reheating temperature for rolling. Since it is out of the range, its toughness is far inferior to that of steel 1A. In contrast, 1B, 2A, 3B, and
4B has extremely high toughness in the high heat input welded area, which clearly demonstrates the effectiveness of this invention. (Effects of the Invention) According to the method of the present invention, a steel plate with excellent toughness at a large heat input weld can be reliably obtained. In particular, it is possible to significantly improve the toughness of high heat input welded parts of steel plates, which involve the ingot making process, where it has been difficult to obtain excellent toughness in the past.

【table】

【table】

Claims (1)

[Claims] 1 C: 0.01 to 0.12wt%, Ti: 0.005 to 0.015wt
%, N: When cooling a steel slab containing 0.001 to 0.006wt% after heating it to 1400 to 1200℃, the average cooling rate at the center in the thickness direction of the slab is 1100℃.
℃ up to 10℃/min or more, then 1100℃ or less 800
℃ to 5℃/min or less, and then the slab is reheated to a temperature of 1200℃ to Ac ₃ and rolled, thereby producing a steel for high heat input welding characterized by excellent low-temperature toughness. .