JPH06293936A - Production of welding steel for low temperature use, dispersedly containing ti-al compound oxide and excellent in toughness in weld zone - Google Patents

Abstract

PURPOSE:To produce a welding steel for low temp. use, excellent in toughness in a weld zone by regulating the amount of dissolved oxygen in a molten steel having a composition consisting of C, Si, Mn, N, and Fe, deoxidizing the molten steel by Mn and Si, and then adding specific amounts of Ti and Al. CONSTITUTION:In a molten steel which has a composition consisting of, by weight, 0.03-0.09% C, 0.05-0.5% Si, 0.4-2.0% Mn, 0.002-0.006% N, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among <=1.0% Cu, <=1.0% Ni, <=0.05% Nb, and <=0.1% V, the amount of dissolved oxygen is regulated to 20-200ppm. This molten steel is preliminarily deoxidized by the addition of prescribed amounts of Mn and Si, by which Mn and Si oxides are formed. Then, Ti is added to the molten steel, in which the Mn and Si oxides are present, by 0.005-0.030% to deoxidize the molten steel, by which fine Ti oxide is formed. Subsequently, Al is added to the molten steel by 0.005-0.020%, by which Ti-Al compound oxide is formed and finely and uniformly dispersed. Then this molten steel is cast and rolled.

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 used for ships, marine structures, storage tanks and the like and a welded portion.

[0002]

2. Description of the Related Art Recently, steel containing a Ti-based oxide has been used in various fields such as thick plate and shaped steel. For example, in the field of thick plates, as illustrated in Japanese Patent Application Laid-Open No. 61-79745, steel containing a Ti-based oxide is very effective in improving the toughness of a large heat input welded portion, and requires low temperature toughness. Application to high strength steel is promising. This principle is based on the fact that fine ferrite is generated by using Ti-based oxides and precipitates such as TiN and MnS which are precipitated by using them as nuclei, and as a result, the generation of coarse ferrite that is harmful to toughness is suppressed and deterioration of toughness is prevented. It can be done. As a method for dispersing such a Ti-based oxide in steel, the mainstream method is to add Ti to molten steel that does not substantially contain a strong deoxidizing element such as Al.

However, it is difficult to control the number and degree of dispersion of Ti-based oxides in the steel by simply adding Ti to the molten steel, and the number and dispersion of precipitates such as TiN and MnS. It is also difficult to control the degree. As a result, in the steel in which the Ti-based oxide is dispersed only by Ti deoxidation, problems such as variation in toughness of the thick plate in the plate thickness direction are recognized. In addition, JP-A-4-944
As exemplified in Japanese Patent No. 8, a method of adding Al into a tundish or a mold after adding Ti has been devised. However, this method is a method for effectively generating AlN, and Ti-based oxides and TiN,
It is not a method for dispersing precipitates such as MnS in steel.

[0004]

Problems to be Solved by the Invention JP-A-67-1974
There is room for further improvement of the characteristics by establishing a more uniform and fine dispersion method of the Ti-based oxide than the conventional method disclosed in Japanese Patent No. 5 or the like. In addition to the conventional Ti deoxidizing method, the present inventors performed Mn and Si deoxidizing as preliminary deoxidizing, and
By performing deoxidation and further adding Al, as a substitute for the Ti-based oxide, a Ti-Al composite oxide and TiN,
An attempt was made to uniformly and finely disperse precipitates such as MnS.

[0005]

The present invention has been made in order to solve the above-mentioned problems, and the gist of the present invention is C: 0.03 to 0.09 Si: 0 by weight%. .05 to 0.5 Mn: 0.4 to 2.0 N: 0.002 to 0.006, the balance of Fe and unavoidable impurities was adjusted to a dissolved oxygen content of 20 to 200 ppm. , S
i is added to deoxidize to produce Mn and Si-based oxides, and in the state where Mn and Si-based oxides exist in molten steel, 0.005 to 0.030% of Ti is added to deoxidize, After that, molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled, and welding having excellent toughness of a welded portion in which a Ti-Al composite oxide is uniformly finely dispersed. Method for manufacturing low temperature steel.

% 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, and Cu: ≤1.0%, Ni: ≤1.0
%, Nb: ≤0.05%, V: 0.1% of 1 or 2
A molten steel containing at least one species and the balance consisting of Fe and unavoidable impurities is adjusted to a dissolved oxygen amount of 20 to 200 ppm, and M
In the state where Mn and Si based oxides are present in molten steel, T
Ti-Al, characterized in that 0.005 to 0.030% of i is added to deoxidize and then molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled.
This is a method for producing low temperature welding steel having excellent toughness in a welded portion in which a composite oxide is uniformly and finely dispersed.

[0007]

The present invention will be described in detail below. First,
The present inventors have developed 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 were attempted in various orders using various deoxidation elements. As a result, the initial dissolved oxygen amount is 20 to 200 ppm
After adjusting to, Mn and Si, which are weaker deoxidizing elements than Ti
After deoxidizing the steel to generate Mn and Si-based oxides, 0.005 to 0.030% of Ti is added to deoxidize it, and then A
The most numerous method is to add 1 to 0.005 to 0.020% of Ti-Al complex oxides and precipitates such as TiN and MnS are uniformly finely dispersed, and when the obtained steel material is subjected to large heat input welding, welding is performed. The result is a welded low temperature steel with extremely excellent toughness.

This principle is considered as follows. When Mn and Si are first deoxidized, Mn and Si-based oxides are generated in the molten steel. In addition, the amount of dissolved oxygen decreases by deoxidizing with Mn and Si. When Ti is added in such a state, dissolved oxygen and Ti are bonded to each other to form a Ti-based oxide. At this time, the dissolved oxygen is already reduced by Mn and Si deoxidation, but it is generally said that the smaller the dissolved oxygen amount, the finer the oxide produced. Therefore,
The Ti-based oxide generated at this time is extremely fine with a diameter of 1 μm or less, and when the same amount of Ti-based oxide is generated, the number thereof 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 stronger deoxidizing element than Mn and Si, the Mn and Si type oxides generated earlier are reduced by Ti and changed to Ti type oxides. This also increases the number of Ti-based oxides. Furthermore, when an appropriate amount of Al is added thereafter, since Al is a stronger deoxidizing element than Ti, part of the Ti-based oxide that was previously formed is Al.
Is converted to a Ti-Al composite oxide. The Ti-based oxides that were originally present are extremely fine and abundant for the reasons described above, so the Ti-Al composite oxides produced here are also fine and abundant.

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

The reasons for limiting the range of basic components of the present invention will be described. C is a lower limit of 0.03% as an effective component for improving the strength of steel, and excessive addition exceeding 0.09% remarkably increases the weldability of steel materials and the HAZ toughness at low temperatures of -60 ° C or lower. Therefore, the upper limit was made 0.09%. Si is a component necessary for securing the strength of the base material, pre-deoxidizing, etc., but the upper limit was made 0.5% in order to prevent deterioration of toughness due to hardening of the HAZ. Further, if it is less than 0.05%, the necessary Si-based oxide cannot be generated during the preliminary deoxidation, so the lower limit was made 0.05%.

Mn must be added in an amount of 0.4% or more in order to secure the strength and toughness of the base metal and to generate Mn-based oxide during preliminary deoxidation, but the toughness and cracking property of the welded portion are required. The upper limit was set to 2.0% within the allowable range such as. N is an extremely important element for precipitation of TiN, and if it is less than 0.002%, the amount of precipitation of TiN is insufficient and a sufficient amount of ferrite structure is not formed. Further, since an increase in solute N causes a decrease in HAZ toughness, the upper limit was made 0.006.

Cu is effective for improving the strength of the steel material, but if it exceeds 1.0%, the HAZ toughness is lowered, so 1.0% was made the upper limit. Ni is effective for improving the strength and toughness of the steel material, but if it exceeds 1.0%, the HAZ toughness decreases, so 1.0% was made the upper limit.

Nb is an element effective for improving the strength and toughness of the base material by improving the hardenability, but in the HAZ portion, excessive addition significantly lowers the toughness, so 0.05% is added. Capped. N for V
Since it has the same effect as b, 0.1% was made the upper limit.

[0015]

EXAMPLES Tables 1 and 2 show the components of the present invention steel and comparative steel, the deoxidizing method, and the HAZ toughness (Charpy test result) in high heat input welding. Prototype steel was melted in a converter and the dissolved oxygen in the molten steel was measured during vacuum degassing.
Deoxidation was performed by adding i, Ti, and Al, continuous casting was performed to cast a 280 mm thick slab, 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 the 1/2 thickness position. Examples A, E of the present invention,
All of H, J, K, L, and M showed excellent characteristics of 50 J or more at -60 ° C. On the other hand, Comparative Examples B, C, D,
F, G and I all showed low toughness values of less than 50 J at -60 ° C.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

INDUSTRIAL APPLICABILITY The present invention is to provide a steel plate to which a large heat input welding method is applied, which is used at a low temperature, and which satisfies a strict toughness requirement against destruction of ships, marine structures, storage tanks, etc. The effect of seeds on the industrial field is extremely large,
In addition, the contribution to society is very large in terms of the safety of structures.

[Brief description of drawings]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Nishimura 1 Nishinosu, Oita City, Oita Prefecture Nippon Steel Co., Ltd. Oita Steel Works Ltd.

Claims (2)

[Claims] 1. 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 the balance The molten steel consisting of Fe and inevitable impurities is adjusted to a dissolved oxygen amount of 20 to 200 ppm, and Mn and S are added.
i is added to deoxidize to produce Mn and Si-based oxides, and in the state where Mn and Si-based oxides exist in molten steel, 0.005 to 0.030% of Ti is added to deoxidize, After that, molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled, and welding having excellent toughness of a welded portion in which a Ti-Al composite oxide is uniformly finely dispersed. Method for manufacturing low temperature steel. 2. 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% of 1 or 2
A molten steel containing at least one species and the balance consisting of Fe and unavoidable impurities is adjusted to a dissolved oxygen amount of 20 to 200 ppm, and M
In the state where Mn and Si based oxides are present in molten steel, T
Ti-Al, characterized in that 0.005 to 0.030% of i is added to deoxidize and then molten steel obtained by further adding 0.005 to 0.020% of Al is cast and rolled.
A method for producing a steel for low-temperature welding, which has excellent toughness in a welded portion in which a composite oxide is uniformly and finely dispersed.