JPS61106722A - Production of high tensile steel for large heat input welding - Google Patents

Abstract

PURPOSE:To produce a high-tensile steel having excellent low-temp. toughness for large heat input welding by subjecting a molten steel to cooling and reheating under specific conditions then to hot rolling in the stage of casting continuously the molten steel contg. a specific ratio each of C, Ti and N. CONSTITUTION:The molten steel which contains 0.01-0.15wt% C, 0.005-0.015wt% Ti and <=0.006wt% N and in which the ratio Ti/N of Ti and N is >=1.0 and <=3.5 is prepd. Such molten steel is cooled at >=10 deg.C/min average cooling rate in the final solidifying part from the molten steel temp. in the stage of pouring up to 800 deg.C to produce a billet or steel ingot. The billet or ingot is reheated to >=900 deg.C and <=1,000 deg.C and is then subjected to hot rolling. The steel having the excellent low-temp. toughness at the joint formed by large heat input welding is thus obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) The technical content described in this specification regarding the manufacturing method of high-strength steel for high heat input welding used in harsh environments such as polar regions and deep seas is particularly applicable to low-temperature dust welding. The objective is to obtain superior steel materials by controlling the cooling rate during the casting or ingot making process.

The soaring prices of oil and natural gas triggered by the oil crisis are spurring the development of energy resources in harsh environments such as the polar regions and the deep sea. Various structures and ships used for this energy development, including welded parts, are required to undergo severe low-temperature dust injection.

(Prior Art) A conventional technique for improving pouring, especially pouring of welded parts, is disclosed in Japanese Patent Publication No. 51-44088.This technique is based on continuous casting or similar solidification, Under cooling conditions, T1-added steel is heated to 11% from the injection temperature of molten steel.
The average cooling rate to 00℃ is 5℃ at the center of the steel ingot or slab.
The TiN particles are cooled at a temperature of 1100° C. or more and finely dispersed at a temperature of 1100° C. or more. However, added Ti
is o, ozwt% or more, H content is 0.008 wt%
This applies under the above conditions, and does not apply to cases where the amount of TitNN is less than this. Also, the amount of Tl is 0.02wt
If the amount of N is more than o, oos wt%, the amount of solid solute N in the high heat input weld will increase, and sufficient injection cannot be obtained at a low temperature such as -60°C, for example.

(Problems to be Solved by the Invention) To provide a method for manufacturing steel with excellent low-temperature dust injection, especially at welded joints, by controlling the cooling rate during continuous casting or ingot-making process and by cryogenic heating. This is the object of this invention.

(Means for solving the problem) The inventors have determined that 0.015 wt% (hereinafter simply referred to as 俤)
As a result of investigating the relationship between the solidification and cooling conditions of molten steel and the amount of precipitated TiN particles using the following low N steel with a low i Ti of 0.006% or less, it was found that the amount of precipitated TiN particles depends on the cooling rate. It was found that there is a dependence on For example, molten steel containing o, oos Ti is heated to 1100°C from the temperature at the time of injection.
The average cooling rate up to ](,:・5.
. 7°. ,,-L a. . If 1□□□□ゎゎ, 0.0
0.3% of Ti was dissolved in the steel.

Similarly, when cooling at a rate of 10"C/Din, 0.0
In this way, when cooling at a rate of s'Q/min or more, T1 of 05chi was dissolved in the steel. It has been newly discovered that this is not the case because the TiN particles are all precipitated during the cooling process.

In addition, the average cooling rate from the molten steel temperature at the time of injection to 1100°C was set at 5°C/min in the final solidification zone, and the solid solution Ti of the slab or steel ingot rapidly cooled from 1100°C was measured.
,006% of the added T1 of 0.000% was in the same bath. Similarly, when cooling at a rate of 10°C/min, B, 0
．． 007 was dissolved in solid solution. Therefore, in the case of a steel with a trace amount of Ti1 and a trace amount of violet N, the temperature at which TiN particles precipitate is 1 i
o It is not above O-0, but from 800°C to 1100°C.

Furthermore, the inventors have discovered that the 800'C of slab or steel ingot! '1
Ti, which was not precipitated during cooling after solidification by increasing the cooling rate as much as possible, was
It has been found that extremely fine precipitates can be obtained by re-heating to a temperature below :C. Using this steel plate, welded joints were exploded and the dust injection was investigated, and it was found that it was excellent at low temperatures.

This invention is derived from the above knowledge.

j That is, this invention is suitable for G 0.01 to 0. +1.5%・
, Ti 0.005~o, Qll 5°%, and NQ
, (1+06% or less), the ratio Tj of T1 and N,
Molten steel with /N of 1.0 or more and 3.5 or less is cooled at an average cooling rate of 10" or more in the final solidification zone from the molten steel temperature at the time of injection to SOO℃ to obtain slabs or steel ingots. This is a method for producing high-strength steel for large heat input bath welding, which comprises reheating the cast slab or ingot to a temperature of 900° C. or more and 1000° C. or less, and then hot-sharpening it.

It should be noted that conventional cooling methods are insufficient to achieve an average cooling rate of 10°C/min or more from the molten steel core temperature during injection to 5oo-c in the final solidification zone.For example, in the case of continuous casting, cooling The final solidification part is 800mm by making the band longer than the conventional one.
In the case of a steel ingot, as soon as the surface solidifies, it can be pulled out from the ingot case, and the steel ingot with an unsolidified center can be water-cooled as it is, or after blooming and rolling, it can be water-cooled until it reaches sou' OC or less. The method of water cooling depends on whether you spray water with a nozzle to cool it or put it in water.

(Function) First, the limitation of the composition of steel in the method of this invention will be explained.

0: If it is less than 0.014, it is not possible to obtain the necessary strength as a welded structural steel, and the upper limit is set at 0.15% from the viewpoint of crack susceptibility during heat welding and injection of welded parts. It was limited to a range of .01-0.15%.

T1: Finely dispersed TiN in the steel not only improves dust injection near the weld bond, but also prevents coarsening of austenite grains during slab heating and improves dust injection into the base metal. In order to achieve this, a minimum of 0.005% is required, and if it exceeds 0.015%, it will deteriorate the base material.
Next is 15%.

N: Contained in normal steelmaking processes, but if it exceeds 0.006%, it becomes solid solution and impairs injection of base metal and high heat input welding parts, so it is limited to 0.006% or less.

Ti/N: When hot welding is performed, the temperature near the bond of the joint becomes around 1400"C and the austenite grain size grows. In the case of low C steel, the austenite grain size determines the grain size near the bond. The main factor is that TiN has the effect of suppressing the growth of austenite grains, and its suppressing ability depends on the ratio r/f of the grain size r of TiN and its volume fraction f, and furthermore, this r/f is equal to T1. It depends on the ratio Ti/N.

In Figure 1, 0.06%O-0,26%Si-1,50
1Kn-0,004%P-0,0007%S, T1 0.005-0.015%, N 0.0020-0.0
Changes within a range of 0.058%.

Molten steel with a Ti/N ratio of 0.86 to 7,50,
The average cooling rate from the molten steel temperature at the time of pouring to 800°C was set at 14.5°C/min in the final solidification section to form a slab, which was then heated to 960°C K7J [I] and then hot rolled to produce a steel plate. However, the heat input on the finishing side of this steel plate is 1
19 KJ 7cm double-sided single layer submerged door”
,gyaichi,,□8oeyeya. 5,1. . , 4□□,.

Note that the numerical values in the figure are the values of r/f. Bond injection at -60°C is excellent when T-+-'N is 1.0 or more and 3.5 or less, so Ti/N was limited to 1.0 or more and 3.5 or less.

In addition, the average cooling rate from the molten steel temperature at the time of injection to 800'C is set to io℃/min or more in the final solidification section, and then this slab or steel ingot is reheated to 900''C or more and 1000℃ or less, and then hot rolled. The reason for doing this is as follows.

Figure 2 shows the castings obtained by varying the average cooling rate (hereinafter referred to as average cooling rate) from the molten steel temperature at the time of pouring to 800''C of steel having the components of fragrances 1 to 4 in Table 1. After heating the piece and the steel ingot at varying heating temperatures in the range of 880°C to 1250'O, hot rolling was performed to form a 32mm thick steel plate, and this steel plate was single-layer submerged arc welding on both sides (heat input 1x9xJ/m). It is clear from Figure 2, which shows the absorbed energy (hereinafter simply referred to as absorbed energy) at 160°C when a Charpy test was performed using a 2 tnm J notch full-size test piece cut from the bond at the center of the plate thickness in a joint with The average cooling rate is lO℃/
Absorbed energy is excellent when mj-n or more and heating temperature is 1000° C. or less, but absorbed energy is poor under other conditions. This means that the average cooling rate is lθ℃
/min or more, there is a lot of 1 per solid volume, and this T
Fine Ti is produced at a slab force temperature of less than 1000-0.
This is because it precipitates as N.

However, if the heating temperature is below 900°C, the toughness of the joint will improve, but the base material will deteriorate, so the lower limit of the heating temperature is u9.
Limited to 00℃.

In addition, Fig. 8 shows the average cooling rates of the steels having the compositions of steel numbers 1-4 in Table 1, 16.3°C/min and 12.5°C/min, respectively.
Q/min, 7, 8°C/In1n, 1.
3'O/min, solid solution Ti1i when casting,
The cast slab or steel ingot is reheated at 960°C and then rolled into a steel plate, welded under the same conditions as in Fig. 2, and then Charpy processed. 1 Indicates the absorbed energy when conducting the test.

As shown in Figure 3, the average cooling rate is 10-Q/min.
In the above case, the bond injection is excellent, and the solid solution T1 in the final solidified part of the slab and steel ingot is 0.005 inch or more.

Furthermore, Table 2 shows the average cooling rate of the first stage from the molten steel temperature to 1100°C for perfume 1-4 in Table 1, and the average cooling rate at 1100°C.
Then, after reheating to 960°O, a steel plate with a thickness of 32 mm was obtained by welding and subsequent cooling under the same conditions as in the case of Fig. 2. From Table 2, which shows the absorbed energy when performing the Charpy test, the average cooling rate of the first stage is 10°C
/min or more, and the average cooling rate is 10-o/min.
Sample A% 0. E is 960 because the average cooling rate in the latter stage is slow and the amount of solid solution T1 in the final solidification part is small.
Grain size included in the core of the steel plate rolled after reheating to ℃ 0.04μ
It can be seen that the amount of TiN below m is small, and the injection around the bond at -60°C is poor. Therefore, the average cooling rate at all stages from the molten steel temperature at the time of injection to 800°C must be t-10°C/min or more.

As described above, in order to manufacture steel with excellent injection in the vicinity of the bond after bath welding, it is necessary to
0.05% or more of solid solution T1 must exist, and for this purpose, the average cooling rate must be 10"c/min, and in order to increase the amount of TiMi with a particle size of 0.04μm or less, the heating temperature must be further increased to 1000"c/min. ℃ or less),,・1 ′″ti“
3・'"080ゝ271"021 degrees l O"0/mi
n or more, and the slab or steel ingot is heated to 900°C or more and 10θ0°C.
(Example) Molten steel containing the components of fragrances 1 to B in Table 1 was manufactured under the conditions shown in Table 8. Table 3 shows the results of the 82-thickness steel plate.

Plate number 3.6 in the table is steel produced by this invention method, and plate number 1
．． No. 2 and No. 4.5 use the same slab or steel ingot as No. 3 and No. 6, but the heating temperature is outside the range of this invention and the absorbed energy is inferior.

In addition, for plate number 9.12, although the heating temperature is within the range of the present invention, the average cooling rate is outside the range of the present invention. The notes are inferior. Plate number 7.8 manufactured by conventional method
and No. 10.11 also have significantly inferior bond strength compared to board number 8.6.

Furthermore, plate numbers 13 to 16 were produced by the method of this invention containing melted w4 containing the components of fragrances 5 to 8 in Table 1.
-m or more, which is excellent.

In addition, V, Nl), B, O that increase the strength
The features of the present invention are not impaired by the addition of REM, which controls the morphology of u, '□゛°1 and sulfides to improve dust injection in the thickness direction.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide a steel that is excellent in low-temperature dust injection even in joints welded with high heat input.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Ti/N and absorbed energy. Figure 2 is a graph showing the relationship between slab heating temperature, average cooling rate of casting, and dust injection. Figure 3 is a graph showing the relationship between average cooling rate and solid solution. It is a graph showing the relationship between T1 amount and toughness.

Claims (1)

[Claims] 1. C 0.01 to 0.15 wt%, Ti 0.005 to 0.015 wt%, and N 0.
006wt% or less, and the ratio Ti/N of Ti and N is 1.0 or more and 3.5
The following molten steel is cooled at an average cooling rate of 10°C/min or more in the final solidification zone from the molten steel temperature at the time of injection to 800°C to form a slab or steel ingot, and the obtained slab or steel ingot is heated to 900°C. A method for producing high-strength steel for high heat input welding, which comprises reheating to 1000° C. or less and then hot rolling.