JPH02194115A - Production of high-strength steel for low temperature service containing titanium oxide and excellent in toughness at weld zone - Google Patents

Abstract

PURPOSE:To provide a steel appropriately used for a structure to be used in a extremely-low-temp. environment by preliminarily deoxidizing molten iron to adjust its composition into molten steel, finally deoxidizing the obtained molten steel with Ti, tapping the molten steel in a specified time, solidifying the steel, and rolling the obtained ingot having specified contents of Ti oxide having specified grain diameter and multicomponent deposit grains. CONSTITUTION:Molten iron is preliminarily deoxidized and smelted to reduce the dissolved oxygen to 0.0030-0.0100wt.% to adjust its composition, and molten steel contg., by weight, 0.02-0.18% C, 0.03-0.25% Si, 0.4-2.0% Mn, 0.0007-0.0060% S, 0.005-0.030% Ti, 0.0010-0.0043% N, <0.015% P, and <0.003% Al is obtained. The molten steel is finally deoxidized, then tapped, and solidified in 30min. The obtained steel contg. 40-170grains/mm<2> of the Ti oxide and the multicomponent deposit of Ti oxide, TiN, and MnS having 0.1-3.0mum grain size in total is rolled. By this method, the safety of a structure is secured, and the weldability is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a strong, high-strength steel with excellent weldability, and particularly to a structure with excellent low-temperature toughness in a weld heat affected zone (hereinafter referred to as HAZ). This relates to the manufacturing method of industrial steel.

(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. A5, No. 8, p. 1232 (published June 1978), TiN is finely dispersed to reduce the HA during adult heat welding of 50 kg-f/d high-strength steel.
Measures have been taken to improve Z toughness. However, most of these precipitates are dissolved during adult heat welding,
There is a drawback that the HAZ structure becomes coarser and solute N increases, resulting in deterioration of HAZ toughness.

On the other hand, some of the inventors of the present invention have proposed that TI can be used instead of Al deoxidation of molten iron.
Unexamined Japanese Patent Application Publication No. 2003-110007 has disclosed that HAZ toughness can be significantly improved by finely dispersing T1 oxide in steel through deoxidation and developing intragranular ferrite transformation group tm (hereinafter referred to as IFP) in the HAZ area during welding. Showa 00-245768
No., JP-A-Sho (io-79745, JP-A-61-11)
No. 7245 and Japanese Unexamined Patent Publication No. 82-1842.

Furthermore, the present inventors revealed in Japanese Patent Application No. 131313/1983 that in steel containing TI oxides, the HAZ toughness improves as the number of TJl2 oxides in the steel increases. However, when a large steel ingot is melted, the number of T1 oxides decreases in the center of the steel ingot, and there are cases where the number necessary to ensure high heat input HAZ toughness cannot be obtained.

(Problem to be Solved by the Invention) The decrease in the number of TI oxides in the center of a large steel ingot is mainly due to the fact that T1 oxides precipitate as secondary deoxidation products during solidification. It was found that the central part of the steel ingot was caused by agglomeration and coarsening.

Ensure the required number of TI oxides even in the center of the steel ingot,
In addition to secondary deoxidation products to improve HAZ toughness,
It was concluded that a deoxidizing method that utilizes the primary deoxidizing TI oxide that precipitates during the molten steel stage is effective.

However, with T1 deoxidation, which takes place in the normal steelmaking process, the residence time until tapping is long (more than 30 minutes), resulting in the problem that secondary deoxidation TI oxides float to the surface and decrease. .

The present invention solves this problem and provides a method for manufacturing high-strength steel for low temperature use with excellent weld zone toughness.

(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 0.0030-0.0100% by weight, and by adjusting the composition by adding alloys, C: 0.02-0.18%, S
j: 0.03-0.25%, Mn: 0.4-2.0%
, S: 0.0007-0.0060%, T1: 0.00
5~o, oao%, N: o, ooto~0.0040%
P < 0.015%, Al < 0.00
3%, Cr < 1.0%, Nl < 360%, M
o <0.5%, y<o, t%, Nb <0.05%
, B < 0.002%, Cu < 1.5%, and the remainder is Fe and unavoidable impurities. The steel is tapped and solidified within minutes, and the particle size is mainly 0.1~
3. T1 oxide and T1 oxide and TiN in Otm,
This is a method for producing a low-temperature high-strength steel with excellent weld toughness, which is characterized by producing a steel ingot containing 40 to 170 MnS composite precipitate particles in total.

(Function) 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 0.602%, the strength necessary for structural steel cannot be obtained, and if it is added in excess of 0.18%, it may cause weld cracking. , HAZ toughness etc. are significantly reduced, so the upper limit was set at 0.18%.

Next, Si 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 martensite) will be formed in the heat-treated structure. and significantly reduce toughness.

Furthermore, if it is less than 0.03%, preliminary deoxidation of molten steel necessary for dispersing TI oxides cannot be performed, so the Si content is 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 required, but excessive addition of more than 0.0060% will form coarse sulfide-based inclusions, leading to a decrease in the ductility and an increase in the anisotropy of the base material. 00
It was set at 60%.

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 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.
However, addition of more than 0.03% causes precipitation of excessive T1 carbides, increases hardness due to precipitation hardening, and reduces toughness.
A% or less.

When the N content exceeds 0.0040%, the matrix becomes brittle even in the absence of M atoms, and the toughness decreases.

Furthermore, if N is less than 0.0010%, hardly any nitrides are formed in the steel, the amount of IFP and ill weave formed is reduced, and the toughness is lowered.

P should be reduced as much as possible in order to prevent deterioration of weld cracking properties, toughness, etc. due to solidification segregation, and the upper limit should be set at 0.01.
It was limited to 5%.

Al 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 be reduced. % 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, N1 is an extremely effective element that increases the toughness of the base metal, but adding more than 3.0% increases hardenability, suppressing the formation of IFP structure, and causing the formation of M fibers. This causes a decrease in toughness, so the upper limit is set to 3.
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 strengthening the base material and improving toughness by suppressing the formation of grain boundary ferrite, etc., 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 00.
1% and 0.05%.

B is expected to increase the strength of the base metal by improving hardenability and improve the toughness of high-temperature heat-treated steel materials by suppressing the growth of grain boundary ferrite.
23 (CB)6 precipitation causes a decrease in toughness and hardening during rapid cooling treatment, so the upper limit was set at 0.002%.

Although Cu strengthens the base material, it hardens HAz less,
Although it is an effective element, the upper limit was set at 1.5% in consideration of temper brittleness due to stress relief annealing, weld cracking property, etc.

Next, HAZl:IFP is generated and the structure is refined to form HAZl:IFP.
The IFP nuclear precipitates that serve as a base for improving toughness will be explained below.

IFP is mainly made of T12o3. T1305 titanium oxide and composites of these oxides with TiN and MnS, T
It is produced from a complex of i N+Mn S. The particle size is 0
．． Below 1, the IFP generation effect is extremely weak;
．． If it exceeds 〇-, it has the ability to generate IFP, but it is 0.
The body is likely to become a site of fracture, resulting in a decrease in HAZ toughness.

Regarding the number of particles in the center of a large steel ingot, T
If the number of particles of the composite of I oxide and TI oxide and TiN+MnS is small, the IFP will be sufficient in the high heat input HAZ part.
cannot be generated, so the total is 40
It is necessary that there be at least 1/- or more.

As the number of particles increases, the number of IFPs also increases,
Excessive presence of more than 170/111 particles in total tends to reduce the ductility of the base metal and welded part.
The upper limit of the number of particles must be 170 particles/mJ.

Adding TI as final deoxidation to increase the number of TI oxides in the central part of the steel ingot, which is the basis of the present invention in the above,
A steel manufacturing method for tapping and solidifying steel within 30 minutes will be explained.

The reason for setting the restriction that the steel should be tapped and solidified within 30 minutes after adding T1 as final deoxidation is that if the molten steel is kept for any longer, the primary deoxidized T1 oxide that precipitates in the molten steel stage will float to the surface and decrease. However, since the necessary TI oxide could not be secured, the time was limited to 30 minutes. For this purpose, it is necessary to take measures such as adding T1 to the tundish.

Furthermore, if the [0] concentration before TI deoxidation exceeds o, oioo%, even if other conditions are met, the T1 oxide becomes coarse grained and becomes the starting point of brittle fracture, and the toughness is not improved.

(Example) Table 1 shows the chemical composition and slab thickness of the prototype steel, the holding time in molten steel after adding TI in steel manufacturing, the number of TI oxides, and the welding reproduction HAZ toughness. The comparative steel was cast into a 250 to 300 μl thick slab within 30 minutes of holding time in molten steel, while the comparative steel was held in molten steel for 60 minutes.

Note that the number of TI oxides was determined by image analysis processing (CMA device) of characteristic X-rays of TI and 0 elements using a computer.

These trial steels were rolled into 50 mffJ plates, test pieces of 12 x 12 x 60 mm were taken from the plate thickness of 1/2 t, and the HA2 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 the temperature was increased from 800°C to 50°C.
Cooling was performed at 0° C. for a cooling time of 161 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 measured by processing this test piece into two-layered V-notch Charpy, and measuring the impact fracture transition temperature (hereinafter referred to as vTrs).
was determined and evaluated.

As shown in Table 1, the steel according to the present invention contains 40 TI oxides at the center of the thickness of the slab, while the steel according to the comparative method has TI oxides reduced to 20 to 30 oxides, which meets the purpose. 40 particles/d or more cannot be dispersed.

Therefore, the welding reproduction HAZ toughness (v
Trs) is improved compared to steel made by the comparative method, and vTrs is shifted to a lower temperature side of 20 to 40°C.

FIG. 1 and Table 2 show the influence of holding time in molten steel after addition of T1 for Steels 1 and 2.

FIG. 1 shows a characteristic xliI elemental image obtained by computer-processed image analysis (CMA device).

As is clear from these, the number of particles becomes 40/- or more when the holding time in molten steel is less than 30 minutes, and 250/- or more.
Sufficient high heat input HAZ toughness is exhibited even in the center of a thick slab with a thickness of ~300+n.

That is, when all the requirements of the manufacturing method of the present invention are satisfied, even in the 1/2 thickness part of a thick steel plate as shown in Steel 7 shown in Table 1, an excellent temperature of vTrs of -75°C can be obtained. Heat input HA
It becomes possible to manufacture low-temperature steel materials with Z toughness.

(Effects of the Invention) The present invention makes it possible to manufacture a low-temperature steel material with excellent high heat input HAZ toughness even at 1/2 part of the thickness of a thick steel plate.
It can be applied to steel materials such as marine structures, line pipes, and cryogenic vessels used in cryogenic environments such as the North Sea. As a result, industrial effects such as ensuring the safety of structures and economic effects due to improved welding performance are extremely significant.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the number of particles versus the retention time in molten steel after T1 addition until tapping.

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: 0.0007-0.0060%, Ti: 0.005-0.030%, N: 0.0010-0.0040%, P<0.015%, Al<0.003 %, and the remainder consists of Fe and unavoidable impurities.Furthermore, Ti is added for final deoxidation, and the steel is tapped and solidified within 30 minutes, mainly with a particle size of 0.1 to 3. .0μ
Weld joint toughness characterized by manufacturing by 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 in m. An excellent method for producing high-strength steel for low temperatures. 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: 0.0007-0.0060%, Ti: 0.005-0.030%, N: 0.0010-0.0040%, P<0.015%, Al<0.003 %, and Cr<1.0%, Ni<3.0%, Mo
<0.5%, V<0.1%, Nb<0.05%, B<0
．． 002%, Cu<1.5%, and the remainder consists of Fe and unavoidable impurities.Furthermore, Ti is added as final deoxidation, and the steel is tapped within 30 minutes. , steel containing a total of 40 to 170 particles/mm^2 of Ti oxide and composite precipitate particles of Ti oxide, TiN, and MnS, which are solidified and mainly have a particle size of 0.1 to 3.0 μm. A method for producing high-strength steel for low temperature use with excellent weld toughness, which is produced by rolling a lump.