JP2944842B2 - Method for producing low-temperature welding steel having excellent toughness in welds in which Ti-Al composite oxide is dispersed - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low temperature welding steel having excellent toughness of a base material and a welded portion used for ships, marine structures, storage tanks and the like.

[0002]

2. Description of the Related Art Recently, steels containing Ti-based oxides have been used in various fields such as thick plates and shaped steels. For example, in the field of thick plates, as exemplified in JP-A-61-79745, a steel containing a Ti-based oxide is very effective in improving the toughness of a large heat input weld and is required to have low-temperature toughness. Promising application to high strength steel. This principle is based on the principle that fine ferrite is generated using Ti-based oxides and precipitates such as TiN and MnS that precipitate as nuclei, thereby suppressing the formation of coarse ferrite harmful to toughness and preventing deterioration of toughness. It is possible. As a method of dispersing such a Ti-based oxide in steel, a method of adding Ti to molten steel substantially not containing a strong deoxidizing element such as Al is mainly used.

However, it is difficult to control the number and the degree of dispersion of Ti-based oxides in steel simply by adding Ti to molten steel, and further, the number and dispersion of precipitates such as TiN and MnS. It is also difficult to control the degree. As a result, in a steel in which a Ti-based oxide is dispersed only by Ti deoxidation, problems such as a change in toughness in a thickness direction of a thick plate are observed. Also, Japanese Unexamined Patent Publication No.
As exemplified in Japanese Patent Publication No. 8, a method of adding Al to a tundish or a mold after adding Ti has also been devised. However, this method is a method for effectively producing AlN, and is a method for producing Ti-based oxides and TiN,
This is not a method for dispersing precipitates such as MnS in steel.

[0004]

SUMMARY OF THE INVENTION Japanese Patent Application Laid-open No. Sho 61-7974
By establishing a method of uniformly dispersing the Ti-based oxide in a finer and finer manner than the conventional method disclosed in Japanese Patent Application Laid-Open No. 5-105, there is room for further improving the characteristics. The present inventors performed Mn and Si deoxidation as preliminary deoxidation in addition to the conventional Ti deoxidation method,
By performing deoxidation and further adding Al, Ti-Al composite oxide and TiN as a substitute for Ti-based oxide,
A method for uniformly and finely dispersing precipitates such as MnS was attempted.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is that C: 0.03 to 0.09 Si: 0% by weight. 0.05 to 0.5 Mn: 0.4 to 2.0 N: 0.002 to 0.006, the balance being Fe and unavoidable impurities, and adjusting the dissolved oxygen amount to 20 to 200 ppm. , S
i is added to deoxidize to generate Mn and Si-based oxides, and in a state where Mn and Si-based oxides are present in the molten steel, 0.005 to 0.030% of Ti is added to deoxidize; Thereafter, a molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled, and the welding is excellent in the toughness of a weld portion in which a Ti-Al composite oxide is uniformly and finely dispersed. Method for producing low-temperature steel.

C: 0.03 to 0.09 Si: 0.05 to 0.5 Mn: 0.4 to 2.0 N: 0.002 to 0.006% by weight, and Cu: ≦ 1.0%, Ni: ≦ 1.0
%, Nb: ≤0.05%, V: 0.1%
The molten steel containing at least the seeds and the balance consisting of Fe and unavoidable impurities is adjusted to a dissolved oxygen content of 20 to 200 ppm,
n and Si are added and deoxidized to form Mn and Si-based oxides. In a state where Mn and Si-based oxides are present in molten steel, T
i is added by 0.005 to 0.030% to deoxidize, and thereafter, the molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled to obtain Ti-Al.
This is a method for producing a low-temperature welding steel in which a composite oxide is uniformly finely dispersed and has excellent toughness in a welded portion.

[0007]

Hereinafter, the present invention will be described in detail. First,
The present inventors have proposed a Ti—Al composite oxide and TiN, M
In order to effectively and uniformly disperse a large number of precipitates such as nS,
Deoxidation experiments in various orders were attempted using various deoxidizing elements. As a result, the initial dissolved oxygen amount is 20 to 200 ppm
After adjusting to Ti, Mn and Si, which are weaker deoxidizing elements than Ti,
To generate Mn and Si-based oxides, then add 0.005 to 0.030% of Ti to deoxidize, and then
The most common method is to add 0.005 to 0.020% of l. The Ti-Al composite oxide and the precipitates such as TiN and MnS are uniformly and finely dispersed. The result was that the welded low-temperature steel had extremely excellent toughness in the part.

This principle is considered as follows. First, when Mn and Si are deoxidized, Mn and Si-based oxides are generated in the molten steel. Further, the amount of dissolved oxygen is reduced by deoxidation with Mn and Si. When Ti is added in such a state, dissolved oxygen and Ti combine to form a Ti-based oxide. At this time, the dissolved oxygen has already been reduced by deoxidation of Mn and Si, but it is generally said that the smaller the amount of dissolved oxygen, the finer the oxide to be generated. Therefore,
The Ti-based oxide generated at this time is very fine, having a diameter of 1 μm or less, and if the same amount of Ti-based oxide is generated, the number increases. Further, since it is fine, it does not float from the molten steel and always stays in the molten steel.

Further, since Ti is a more strongly deoxidizing element than Mn and Si, the previously generated Mn and Si-based oxides are reduced by Ti and changed to Ti-based oxides. This also increases the number of Ti-based oxides. Further, when an appropriate amount of Al is added thereafter, since Al is a more strongly deoxidizing element than Ti, a part of the Ti-based oxide that has been generated earlier becomes Al.
To change to a Ti—Al composite oxide. The Ti-based oxides originally present are very fine and many exist for the above-mentioned reason, and thus the Ti-Al composite-based oxides generated here are also fine and many exist.

On the other hand, since a part of the Ti-based oxide is reduced by Al, the dissolved Ti increases, and the dissolved Ti precipitates as TiN as the temperature after solidification decreases. This amount increases when Al is added, as compared with when Al is not added, by the amount of dissolved Ti.

The reasons for limiting the range of the basic components of the present invention will be described. C is an effective component for improving the strength of steel, with the lower limit being 0.03%. Excessive addition exceeding 0.09% significantly reduces the weldability of steel materials and the HAZ toughness at low temperatures below -60 ° C. Therefore, the upper limit was set to 0.09%. Si is a component necessary for securing the strength of the base material, preliminarily deoxidizing, etc., but the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ. If the content is less than 0.05%, a necessary Si-based oxide cannot be generated during the preliminary deoxidation, so the lower limit is set to 0.05%.

Mn must be added in an amount of 0.4% or more to ensure the strength and toughness of the base material and to form a Mn-based oxide during preliminary deoxidation. The upper limit is set to 2.0% within an allowable range such as the above. N is an extremely important element for the precipitation of TiN, and if it is less than 0.002%, the amount of TiN deposited is insufficient, and a sufficient amount of ferrite structure cannot be obtained. The upper limit of 0.006 is set because an increase in solid solution N causes a decrease in HAZ toughness.

[0013] Cu is effective in improving the strength of the steel material, but if it exceeds 1.0%, the HAZ toughness is reduced. Therefore, the upper limit is made 1.0%. Ni is effective for improving the strength and toughness of the steel material, but if it exceeds 1.0%, the HAZ toughness is reduced. Therefore, the upper limit is set to 1.0%.

Nb is an element effective for improving the strength and toughness of the base material by improving the hardenability. However, in the HAZ portion, excessive addition significantly decreases toughness, so that 0.05% is required. Capped. N for V
0.1% was made the upper limit from the same effect as b.

[0015]

EXAMPLES Tables 1 and 2 show the components of the steel of the present invention and the comparative steel, the method of deoxidation, and the toughness of HAZ in large heat input welding (Charpy test results). The prototype steel was melted in a converter and the dissolved oxygen in the molten steel was measured during vacuum degassing.
i, Ti, and Al were added to perform deoxidation, cast into a 280 mm thick slab by continuous casting, and rolled into a 30 mm thick steel plate. The obtained steel sheet was subjected to large heat input welding and evaluated by the Charpy test result at a half thickness position. A, E, and
H, J, K, L, and M all exhibited excellent characteristics of 50 J or more at -60 ° C. On the other hand, B, C, D,
F, G and I all exhibited low toughness values of less than 50 J at -60 ° C.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

According to the present invention, there is provided a steel sheet to which a large heat input welding method is applied, which is used at a low temperature and which satisfies strict toughness requirements for breaking of ships, marine structures, storage tanks and the like. The effect on certain industrial fields is extremely large,
Furthermore, the contribution to society is very large in terms of structural safety.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the timing of adding a deoxidizing element and the transition of the number of generated oxides during deoxidation.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/16 C22C 38/16 (72) Inventor Seiji Nishimura 1 Nishinosu, Oita, Oita City, Oita Prefecture Nippon Steel Corporation Oita Steel In-house (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 33/04 C21C 7/06

Claims (2)

(57) [Claims] C .: 0.03 to 0.09 Si: 0.05 to 0.5 Mn: 0.4 to 2.0 N: 0.002 to 0.006% by weight, with the balance being the balance Adjusts molten steel composed of Fe and unavoidable impurities to a dissolved oxygen content of 20 to 200 ppm, Mn, S
i is added to deoxidize to generate Mn and Si-based oxides, and in a state where Mn and Si-based oxides are present in the molten steel, 0.005 to 0.030% of Ti is added to deoxidize; After that, the molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled, and the welding is excellent in the toughness of a weld portion in which a Ti-Al composite oxide is uniformly and finely dispersed. Method for producing low-temperature steel. 2. The composition contains, by weight%, C: 0.03 to 0.09 Si: 0.05 to 0.5 Mn: 0.4 to 2.0 N: 0.002 to 0.006, Cu: ≦ 1.0%, Ni: ≦ 1.0
%, Nb: ≤0.05%, V: 0.1%
The molten steel containing at least the seeds and the balance consisting of Fe and unavoidable impurities is adjusted to a dissolved oxygen content of 20 to 200 ppm,
n and Si are added and deoxidized to form Mn and Si-based oxides. In a state where Mn and Si-based oxides are present in molten steel, T
i is added by 0.005 to 0.030% to deoxidize, and thereafter, the molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled to obtain Ti-Al.
A method for producing a low-temperature steel for welding having excellent toughness in a weld portion in which a composite oxide is uniformly finely dispersed.