JPH02220735A - Production of high tensile strength steel for welding and low temperature including titanium oxide - Google Patents

Abstract

PURPOSE:To produce a high tension strength steel for low temp. having excellent low toughness at a welding part by adding low m.p. Ti-Cu granule into a continuous casting mold to finish deoxidized molten steel having the specific composition and rolling a cast steel slab containing Ti oxide and the combined precipitating granule at the specific condition in the center part of the cast steel slab. CONSTITUTION:The molten steel is pre-deoxidized to make 0.0030-0.0100wt.% oxygen content in this and alloys are added to produce the molten steel containing 0.02-0.18% C, 0.03-0.25% Si, 0.4-2.0% Mn, 0.0007-0.0060% S, 0.0010-0.0040% N, <=0.015% P, <=0.003% Al and the balance of Fe with inevitable impurities. Further, as the finish deoxidation, the alloy wire or granule of the low m.p. Ti-Cu, etc., is added into the continuous casting mold so as to contain 0.005-0.030wt.% Ti, and the cast steel slab containing Ti oxide having 0.1-0.3mu grain diameter and 40-170 pieces/cm<3> total combined precipitating granules of Ti oxide and TiN, MnS, is rolled.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to strong, high-strength steel with excellent weldability, and particularly to a method for manufacturing structural steel with excellent low-temperature toughness of the weld heat-affected zone (hereinafter referred to as HAZ). It is. (Prior art) The HAZ toughness of a welded joint of low alloy steel is determined by: (1) effective grain size (austenite grain size, microstructure); (2)
It is governed by metallurgical factors such as the grain size and volume fraction of the hardened phase (carbides, high carbon martensite, inclusions), (3) the hardness and toughness of the matrix (solid solution C, N in ferrite). There is. Among these methods, a simple method to improve HAZ toughness is to refine the HAZ structure and refine the effective crystal grains.
Various methods have been proposed that utilize various precipitates that are stable at high temperatures. For example, in Tetsu to Hagane, Vol. 65, No. 8, page 1232, published in June 1971, TiN was finely dispersed and 50k
Measures have been taken to improve the HAZ toughness of g-f/@J high-strength steel during high heat input welding. However, most of these precipitates are dissolved in high heat input welding, resulting in coarse graining of the HAZ structure and increase in solute N, which has the disadvantage of degrading the HAZ toughness. On the other hand, some of the inventors of the present invention have proposed that T
Unexamined Japanese Patent Application Publication No. 2003-110022 has shown that HAZ toughness can be significantly improved by finely dispersing Ti oxides in steel through deoxidation and developing an intragranular ferrite transformation structure (hereinafter referred to as IFP) in the HAZ during welding. Showa 80-245768
No., JP-A-Sho [io-79745, JP-A-81-11
No. 7245 and JP-A-62-1842. Furthermore, the present inventors revealed in Application No. 83-13813 that in Ti oxide-containing steel, HAZ toughness improves as the number of Ti oxides in the steel increases. However, when continuous casting is used, Ti is formed in the center of the slab.
There were cases where the number of oxides decreased and the number necessary to ensure high heat input HAZ toughness could not be obtained. (Problem to be solved by the invention) The decrease in the number of Ti oxides in the central part of the steel ingot during continuous casting is mainly due to the fact that Ti oxides precipitate as secondary deoxidation products during solidification. It was found that this was caused by coarsening of agglomerates. In order to secure the necessary number of Ti oxides even in the central part of the slab and improve HAZ toughness, in addition to secondary deoxidation products, temporary deoxidation Ti oxides precipitated during the molten steel stage are utilized.
It was concluded that a Ti deoxidizing method using a continuous casting mold is effective, and the present invention was completed. (Means for solving the problem) The present invention has been made based on the above findings,
The gist is that molten iron is pre-deoxidized to reduce dissolved oxygen to o, ooao to 0.0100% by weight, and by adjusting the components by adding alloys, C: 0.02 to 0.18%, S.
j: 0.03-0.25%, Mn: 0.4-2.0%,
S: 0.0007-0.0080%, N
: Contains 0.0010-0.0040%, P≦
0.015%, Al 50.003%, Cr≦
1.0%, Nl≦3.0%, Mo≦0.5%, ■≦0.
1%, Nb 50.05%, B≦0.002
%, Cu ≦1.5%, and the remainder is Fe and unavoidable impurities.Furthermore, as final deoxidation, low melting point Ti-CuSTi- is used in a continuous casting mold for final deoxidation. Add Nl, Ti-Fe alloy wire or particles, Ti: 0.005 to 0.005% by weight
Ti oxide and Ti with a particle size of 0.1 to 3.0 μs are mainly contained in the central part of the slab.
The total of composite precipitate particles of oxide, TiN, and MnS is 40
This is a method for producing high-strength steel for low temperature use with excellent low-temperature toughness of welds, which is characterized by producing by rolling a steel ingot containing -170 pieces/lll1. (Function) The present invention will be explained in detail below. First, the reason for limiting the basic component range of the steel of the present invention will be described. First, C is added as an effective component to improve the strength of steel. If it is less than 0.02%, the strength necessary for structural steel cannot be obtained, and if it is added in excess of 0.1896, it may cause weld cracks. The upper limit was set at 18% because it significantly lowers the hardness, HAZ toughness, etc. Next, Sl is necessary to ensure the strength of the base metal and to preliminarily deoxidize molten steel, but if it exceeds 0.25%, a hardened structure of high carbon martensite (hereinafter referred to as M*) will be formed in the heat-treated structure. and significantly reduce toughness. Further, if it is less than 0.03%, preliminary deoxidation of molten steel necessary for dispersing Ti oxide cannot be performed, so the S1 content was limited to this range. Although it is necessary to add Mn in an amount of 0.4% or more to ensure the strength and toughness of the base metal, the upper limit was set to 2.0% within an allowable range such as the toughness and crackability of the welded part. Regarding S, in order to precipitate MnS of the complex, 0.
0.0007% or more is necessary, but excessive addition of more than 0.0060% will form coarse sulfide inclusions, leading to a decrease in ductility and an increase in anisotropy of the base material, so 0.0007 to 0. .0
080%. Ti is an essential element for the formation of Ti oxide and Ti nitride, and if it is less than 0.005%, it will not be possible to form the necessary Ti oxide and Ti nitride.
0.0 because the amount of nitrides cannot be obtained and the amount of IFP generated is reduced.
It is necessary to add 0.05% or more, but addition of more than 0.03% causes precipitation of excessive Ti carbides, increases hardness due to precipitation hardening, and reduces toughness.
It was set to 3% or less. When the content of N exceeds 0.0040%, the matrix becomes brittle even in the absence of M*, reducing the toughness. Further, if N is less than 0.0010%, hardly any nitrides are generated in the steel, the amount of IFP structure generated is reduced, and the toughness is lowered. P should be reduced as much as possible to prevent deterioration of weld cracking properties, toughness, etc. due to solidification segregation, and the upper limit should be set at 0. Old 5
%. Ap is a strong deoxidizing element, and if it is added in an amount of 0.003% or more, Ti oxide formed by Ti deoxidation will not be formed, IFP will not be formed, and the toughness will decrease. % or less. The above are the basic components of the steel of the present invention.
and Cr for the purpose of improving the toughness of the base material. It can contain one or more of Ni, Mo, V, Nb, B, and Cu. First, Ni is an extremely effective element that improves the toughness of the base metal, but adding more than 3.0% increases hardenability and suppresses the formation of IFP structure. The upper limit was set to 3 to reduce toughness due to the formation of
．． It was set to 0%. Cr and Mo are effective in strengthening the base material by improving hardenability and precipitation hardening. Furthermore, adding an appropriate process such as TMCP is effective in improving the low-temperature toughness of the base material. However, excessive addition exceeding the upper limit of each component is harmful from the viewpoint of toughness and hardenability, so Cr
, Mo, the upper limits were set to 1.0% and 0.5%, respectively. V and Nb are effective in improving toughness by strengthening the base material and suppressing the formation of grain boundary ferrite, but excessive addition exceeding the upper limit of each component is harmful from the viewpoint of toughness and hardenability. , V, and Nb, the upper limit is set to 0.
1% and 0.05%. B is expected to improve the toughness of high-temperature heat-treated steel materials by increasing the strength of the base material by improving hardenability and suppressing the growth of grain boundary ferrite, but addition of more than 0.002%
The upper limit was set at 0.002% because the precipitation of (CB)e causes a decrease in toughness and hardening during rapid cooling treatment. Although Cu strengthens the base material, it hardens the HAZ less.
Although it is an effective element, the upper limit was set at 1.5% in consideration of temper embrittlement caused by stress relief annealing, weld cracking resistance, etc. Next, the IFP nuclear precipitates that form the basis of IFP generation in the HAZ, refinement of the structure, and improvement of HAZ toughness will be described below. IFP mainly consists of titanium oxides such as Ti2O3 and TiO5 with a particle size of 0.1 to 3.0 μm and a few percent of Mn as a solid solution, and composites of these oxides with TtN and MnS, and Ti.
It is produced from a complex of N+MnS. The particle size is 0.1-
Below 3.0a, the IFP generation effect is extremely weak.
If it exceeds m, although it has the ability to generate IFP, it itself tends to become a point where fracture occurs, resulting in a decrease in HAZ toughness. Regarding the number of particles in the central part of the continuous casting slab, if the number of particles of Ti oxide and the composite of Ti oxide and TiN + MnS is small, sufficient IFP cannot be generated in the high heat input HAZ part. , it is necessary that a total of 40/- or more of them be present. As the number of particles increases, the number of IFPs also increases,
The presence of an excessive number of particles exceeding 1708/mm2 in total tends to cause a decrease in the ductility of the base metal and welded part.
The upper limit of the number of particles is 170 pieces/lII! Must. In order to increase the number of Ti oxides in the central part of the slab, which is the basis of the present invention, in addition to the secondary deoxidation product,
The primary deoxidizing Ti oxide that precipitates during the molten steel stage must be utilized. Therefore, after the addition of Ti for final deoxidation, it is necessary to tap and solidify the steel as quickly as possible. The most effective method for this purpose is to add Ti to the mold during continuous casting, and this method will be explained below. To deoxidize Ti in a continuous casting mold, the added Ti
must be uniformly diffused into molten steel in the shortest possible time. Ti alloys with low melting points are effective for this purpose, and as a result of considering processability and economic efficiency, Ti-Cu, Ti
-Nl, Ti-Fe alloys were found to be superior. The composition of the alloy is [L% Ti: 4Q to 60%, the balance is Cu 1Ti: 60 to 80%, the balance is Ni, Ti: 8
5 to 75%, and the remainder is Fe, which is an alloy with a lower melting point than pure Ti. Addition is a method of processing these alloys into wires and granules and continuously adding them in a mold. Also, before Ti deoxidation

[0] When the concentration exceeds 0.0100%, even if other conditions are met, the Ti oxide becomes coarse grained and becomes the starting point of brittle fracture, and the toughness is not improved. The effects of the present invention will be further illustrated by Examples below. (Example) Table 1 shows the chemical composition of trial steel, and steels 1 to 6 are T
It is based on the method of the present invention in which mold addition is made of i-alloy.
fR7 and fR8 are comparative steels made by the conventional method of deoxidizing with sponge Ti in a vacuum degassing device for steelmaking. Table 2 shows the composition of the added Ti/alloy, the added shape, the slab thickness, the number of Ti oxides in the center of the slab, and the welding reproduction HAZ toughness. Note that the number of Ti oxides is Ti. Characteristic X-rays of the O element were determined by image analysis processing (CMA device) using a computer. These prototype steels were made by rolling 300mm thick slabs to 50u.
mm steel plate, plate thickness 1/2t to 12 x 12 x
A test piece of 60 um+m was taken, and the HAZ toughness was evaluated by a welding reproduction thermal cycle test. In the welding reproduction thermal cycle test, the central part of the specimen was rapidly heated to 1400°C by high-frequency induction heating, and
Cooling was performed at 00° C. for a cooling time of 101 seconds. This condition corresponds to a welding heat input of 130 kJ/cm, and a heating temperature of 1400° C. corresponds to a heating region near the fusion line of the actual HAZ. Furthermore, the toughness was determined by processing this test piece to a 211 mV Notch Charpy and impact fracture transition temperature (hereinafter referred to as v).
Trs) was determined and evaluated. As shown in Table 2, the steel according to the present invention contains 40 Ti oxides/- or more at the center of the thickness of the slab, and the steel according to the comparative method contains Ti oxides at least 40 Ti oxides/III! to the target of 40 pieces/ff.
Particles larger than ll1 cannot be dispersed. Therefore, the welding reproduction HAZ toughness (vT
rs) is improved by L and vTrs by 2 compared to the waste obtained by the comparative method.
Shift to the lower temperature side by 0 to 40°C. In this way, low melting point Ti
By adding the alloy in a mold, the number of particles becomes 40/- or more even in the center of a 300 mm thick slab, showing excellent adult thermal HAZ toughness. That is, when all the requirements of the manufacturing method of the present invention are satisfied, even a half part of a steel plate made by continuous casting as shown in Steel 6 shown in Table 1 can have an excellent large input of vTrs of -70°C. It becomes possible to manufacture low-temperature steel materials with thermal HAZ toughness. (Effects of the invention) The present invention makes it possible to manufacture low-temperature steel materials with excellent high heat input HAZ toughness even at 1/2 part thickness of thick steel plates by continuous casting, and can be used in cryogenic environments such as the North Sea. be done,
It can be applied to steel materials such as marine structures, line pipes, and low-temperature containers. As a result, industrial effects such as ensuring the safety of structures and economic effects due to improved welding performance are extremely significant. Agent Patent Attorney Tate Chanoki Continuing from page 1 [Shares] Int, C1,' Identification code Office reference number

Claims (1)

[Claims] 1. Preliminary deoxidation of molten iron to reduce dissolved oxygen to 0.00% by weight
C: 0.02-0.18%, Si: 0.03
~0.25%, Mn: 0.4-2.0%, S: 0.00
07-0.0060%, N: 0.0010-0.004
0%, P≦0.015%, Al≦0.003
%, with the remainder consisting of Fe and unavoidable impurities.Furthermore, as final deoxidation, low melting point Ti-Cu, Ti-Ni, Ti-Fe alloy wire or granules are cast in a continuous casting mold. Ti: 0.00 by weight%
Containing 5 to 0.030%, in the central part of the slab,
Rolling a steel ingot containing a total of 40 to 170 particles/mm^2 of Ti oxide and composite precipitate particles of Ti oxide, TiN, and MnS whose particle diameter is mainly 0.1 to 3.0 um. A manufacturing method for low-temperature high-strength steel with excellent low-temperature toughness in welded parts. 2. Preliminary deoxidation of molten iron reduces dissolved oxygen to 0.00% by weight
C: 0.02-0.18%, Si: 0.03
~0.25%, Mn: 0.4-2.0%, S: 0.00
07-0.0060%, N: 0.0010-0.004
0%, P≦0.015%, Al≦0.003
%, Cr<1.0%, Ni≦3.0%, Mo≦
0.5%, V≦0.1%, Nb≦0.05%, B≦0.
002%, Cu≦1.5%, one or more of them, and the remainder consists of Fe and inevitable impurities.
Furthermore, as final deoxidation, low melting point Ti-Cu, Ti-Ni, Ti-Fe alloy wire,
Or add granules and Ti: 0.005 to 0.00% by weight.
In the central part of the slab, a total of Ti oxide and composite precipitate particles of Ti oxide, TiN, and MnS with a particle size of 0.1 to 3.0 μm are contained in the central part of the slab.
A method for producing high-strength steel for low temperature use with excellent low-temperature toughness at welds, the method comprising rolling a steel ingot containing 70 pieces/mm^2.