JPH03202422A - Production of thick high tensile steel plate excellent in toughness in weld heat-affected zone - Google Patents

Abstract

PURPOSE:To produce a thick high tensile steel plate excellent in toughness at low temp. in a weld heat-affected zone by properly regulating respective contents of Ti, Zr, N, and O in a steel and dispersing multiple oxides of Ti and Zr finely and uniformly. CONSTITUTION:Ti is added by 0.010-0.040% to a molten steel containing 0.005-0.020% dissolved oxygen and further Zr is added by 0.005-0.030% within 60min, and this molten steel is solidified within 60min, and a slab of the resulting steel which has a composition containing, by weight, 0.03-0.15% C, <0.6% Si, 0.8-2.0% Mn, <0.025% P, <0.005% S, <0.004% Al, 0.003-0.060% Nb, 0.0020-0.0060% N, 0.005-0.020% Ti, 0.003-0.015% Zr, and 0.001-0.006% O or further containing one or more kinds among specific amounts of B, Ni, Cu, Cr, V, Mo, and Ca and also has a structure dispersedly containing multiple oxides of Ti and Zr, such as ZrO2 and Ti2O3, of 0.05-10mum grain size by 30-300 pieces/mm<2> is reheated up to <=1250 deg.C and rolled, by which the thick steel plate can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a high-tensile steel plate having excellent low-temperature toughness in a weld heat-affected zone (hereinafter referred to as HAZ).

Steel produced using this method can be used in pressure vessels, shipbuilding, bridges, architecture, etc.
Can be used for welded steel structures such as line pipes.

(Prior art) The HAZ toughness of low alloy steel is determined by (1) grain size, (2)
) High carbon island martensite (M'L upper bainite (
It is controlled by various metallurgical factors such as the dispersion state of hardened phases such as (Bu), (3) presence or absence of grain boundary embrittlement, and (4) micro-segregation of elements. In particular, it is known that the grain size of HAZ has a large effect on low-temperature toughness, and a number of techniques have been developed and put into practical use to refine the HAZM weave.

For example, finely disperse TiN, 50kg f/m
A method for improving the HAZ toughness during welding of J1 class high tensile strength steel is disclosed ("Tetsu to Hagane" No. 65, June 1973).
Vol. 8, p. 1232). However, most of these precipitates are dissolved near the fusion line (hereinafter referred to as FL) during welding, resulting in coarse graining of the HAZ structure and deterioration of HAZ toughness.

In response to this problem, some of the inventors of the present invention finely dispersed Ti oxide in steel and generated intragranular acicular ferrite (hereinafter referred to as IFP) in the HAZ during welding, thereby making the HAZ structure finer. It was shown in JP-A-63-210235 and JP-A-1-15121 that the HAZ toughness can be significantly improved.

However, as a result of intensive investigation into the relationship between the welded HAZ structure and toughness, we found that even in steel with finely dispersed Ti oxides, the HAZ toughness of steel sheets used in extremely low-temperature environments such as northern sea areas and LPG tanks It became clear that in order to make dramatic improvements, it was necessary to introduce new technological ideas.

(Problems to be Solved by the Invention) The present invention proposes a method for manufacturing a high tensile strength steel plate with excellent weld HAZ toughness. The high-strength steel of the present invention has a fine structure throughout the HAZ including the vicinity of the molten wire trough, and has excellent low-temperature toughness.

(Means for Solving the Problems) The gist of the present invention is that the amount of dissolved oxygen is 0.003 to 0.020.
% of molten steel by adding 0.010 to 0.040% of Ti,
Furthermore, after adding 0.005 to 0.030% Zr within 60 minutes, coagulation was completed within 80 minutes, and c:
o, oa to a, 15%, Si: 0.8% or less, M n
: 0, 8-2, 0%, P; 0.025% or less,
S:0. (105% or less, Al: 0.004% or less, Nb: 0.003~o, oeo%, N: 0.00
20-0.0060%, Ti: 0.005-0.020
%, Zr: 0.003-0.015%, 0:0.
A composite oxide mainly composed of Ti and Zr with a particle size of 0.05 to 10% and a particle size of 0.05 to 10% is
A steel plate is manufactured by reheating a substantially Al-free slab containing 0 pieces/mm2 and the remainder consisting of iron and unavoidable impurities at a temperature of 1250° C. or lower, and having a dissolved oxygen amount of 0. 003~0.020% Ti in molten steel
．． 010 to 0.040% and further Z
Coagulation is completed within 60 minutes after adding 0.005-0.030% of r, C: 0.03-0.15% by weight%
, s 1: 0.6% or less, Mn: 0.8-2.0%, P
: 0.025% or less, S: 0.005% or less, Al
: 0.004% or less, Nb: 0.003 to 0.080
%, N: 0.0020-(1, (1060%, T 0.005-0.020%, Zr: 0.003-0.
015%, 0:o, ooi~o, ooe% further B
: 0.0003-0.0010%, NI: 0.0
5-4.00%, Cu: 0.05-1.50%, C"
:0.05~1.00%, V:0.005~0.08
0%, Mo: 0.05-0.40%, Ca: o, 000
Contains 30 to 300 pieces/1Ilj of a composite oxide containing 5 to 0.005% of one or two of Ti and Zr with a particle size of 0.05 to 10-1, the remainder being iron and The purpose is to produce a steel plate by reheating a cast slab containing essentially no Al, which is made up of inevitable impurities, at a temperature of 1250° C. or lower.

(for production) The present invention will be explained in detail below.

According to the inventors' research, HAZ toughness is determined by (1) the chemical composition of the steel, (2) the structure (grain size and hardened phase distribution!
r! ,), and it is considered that optimization of steel components and the resulting refinement of grain size are essential for achieving high toughness. Some of the inventors have published Japanese Patent Application Laid-Open No. 63210235, Japanese Patent Application Publication No. 1999
-15321, the HAZ structure is refined by generating IFP in the HAZ (especially near FL) during welding, and the HAZ toughness is significantly reduced. It can be improved.

On the other hand, as shown in Japanese Unexamined Patent Publication No. 1-159356, the inventors have discovered that the composite oxide of Ti and Z can be finely and uniformly dispersed over the entire thickness of the slab, and that IFP is generated with the oxide as a nucleus. Therefore, it was found that the structure was refined throughout the HAZ at all thickness positions including the center of the plate thickness, and extremely excellent low-temperature toughness was obtained.

However, as a result of intensive study to develop a steel plate with excellent HAZ toughness even at extremely low temperatures, Ti and Z
By specifying the method of adding Ti and Zr to steel,
The present inventors have discovered that it is possible to finely and uniformly disperse a larger amount of the composite oxide (an oxide mainly composed of zrO2 and Ti2O3), and that the HAZ toughness can be dramatically improved, leading to the present invention.

In other words, in steel in which a large amount of composite oxides of Ti and 2' are finely dispersed, even in the region heated to 1400°C or higher near the FL, the Ti and Zr present in the γ grains are removed during the γ-α transformation.
Using the composite oxide as a core, IFP is generated and the HAZ structure is significantly refined.

Furthermore, in the region away from the FL (region up to about 5 mm from the FL), the HAZ toughness can be improved by containing fine TiN. This is because fine TiN suppresses coarsening of γ grains and refines the structure.

In order to finely disperse a large amount of composite oxide of Ti and Zr having such an effect in steel, first, the amount of dissolved oxygen in the molten steel to which Ti and Zr are added is reduced to 0.003 to 0.020%.
It is necessary to This is because if the amount of dissolved oxygen is less than o, ooa%, the amount of oxygen after deoxidation by Ti or Zr will be small, and the number of final fine oxides will be small. However, if the amount of dissolved oxygen exceeds 0.020%, deoxidation will not be performed sufficiently even if Ti or Zr is added, and the cleanliness will drop and the toughness of the base material will deteriorate.

The amount of Ti and Zr added is determined by the amount of Ti and Zr that is ultimately required because approximately 50% of the amount added is released as slag by deoxidation. Zr
It is necessary to add twice the amount.

The order in which Ti and Z are added is to add Ti and then Z.
It is necessary to add r. This is because Zr has a stronger affinity with oxygen than Ti, so if Zr is added before Ti, the oxide produced in the steel will be a Z-based oxide.

IFP is not generated at all from the oxide of Zr, and no effect on refining the HAZ structure can be expected.
When adding Zr, it is necessary to add Zr within 60 minutes after adding Ti. If 60 minutes or more elapse after adding Ti, agglomeration, coalescence, and floating of Ti oxide will proceed.
This is because the fine oxides cannot be uniformly dispersed in the steel.

By adding Zr after adding Ti, the fine Ti oxides present in the molten steel are converted to Zr, which has a strong affinity for oxygen.
Since the oxides are collided and reduced by r, they are made finer and furthermore, agglomeration, coalescence, and flotation are suppressed, so that finally a large amount of fine composite oxides can be uniformly dispersed in the steel.

However, even in this case, it is necessary to complete coagulation within 60 minutes after adding Zr. This is because if 60 minutes or more elapse after Zr is added, agglomeration, coalescence, and flotation occur, and the dispersion effect of fine oxides due to the collision and reduction action of Zr cannot be obtained.

In the continuous casting steel manufacturing method, after adding Zr, 6
In order to complete solidification within 0 minutes, it is desirable to add Zr to the tundish or mold in continuous casting.

In order to obtain the effect of refining the HAZ structure by the composite oxide of Ti and Zr, the oxide of an appropriate size must be uniformly dispersed in the steel. The particle size of the composite oxide of Ti and Zr is 0.05 to 10 m5, and the number is 30 to 30.
0 pieces/mm2 is required.

When the particle size of the composite oxide of Ti and Zr is less than 0.5, or when the number of particles is less than 30 particles/mm2, the ability to generate IFP becomes weak. Furthermore, if the particle size of the composite oxide of Ti and Zr exceeds 10 or the number of particles exceeds 300 particles/mm2, the composite oxide of Ti and Zr itself may become a point of initiation of brittle cracks, or the cleanliness of the steel may deteriorate. decreases, and not only the HAZ but also the low-temperature toughness of the base metal deteriorates.

The number and size of Ti and 2'' composite oxides were measured using an optical microscope after polishing the cross section of the slab.

In normal steelmaking methods, composite oxides of Ti and Zr are added to steel,
In order to secure TiN, especially Ti.

It is essential to optimize the amounts of Zr, N, and O. For this reason, Ti,
The amounts of Zr, N, and O are each Tf: 0.005 to 0.02.
0%, Zr: 0.003-0.015%, N: 0.0
020-0.0035%, 0:0.001-o, oo
It is necessary to limit it to e%.

The lower limit of the amounts of Ti, Zr, N, and 0 is the minimum amount necessary to generate TiN, a composite oxide of Ti and Z.
, Zr is set to prevent deterioration of HAZ toughness due to formation of TiC and ZrC, and the upper limit of N amount is set to prevent deterioration of HAZ toughness due to solid solution N. Further, the upper limit of the amount of O is set to prevent deterioration of the cleanliness and toughness of the steel due to the formation of nonmetallic inclusions.

However, even if steel contains composite oxides of Ti and Zr, TiN
If the basic components are not suitable even if HAZ is produced
Toughness cannot be obtained. The reasons for limiting the other basic components will be explained below.

The lower limit of 0.03% for the amount of C is the minimum amount to ensure the strength of the base metal and welded part and to exhibit these effects when adding Nb, V, etc. However, if the amount of C is too large, it not only adversely affects the HAZ toughness but also deteriorates the base metal toughness and weldability, so the upper limit was set at 0.15%.

St is an element contained in deoxidized steel, and as the amount of Sl increases, weldability and HAZ toughness deteriorate, so the upper limit is set at 0.6%.
And so. In the steel of the present invention, Ag deoxidation is sufficient, and T
i Deoxidation may also be used. 1 for St (from the point of view of AZ toughness, it is desirable that the content be about 0.15%).

Mn is an essential element for ensuring strength and toughness.
Its lower limit is 0.8%. However, if the amount of Mn is too large, not only will hardenability increase and weldability and HAZ toughness deteriorate, but also it will not be possible to obtain base metal strength that meets the target standards. Therefore, the upper limit of the Mn content was set to 2.0%.

In the steel of the present invention, the reason why the unavoidable impurities P and S are set to 0.025% or less and 0.010% or less, respectively, is to further improve the low-temperature toughness of the base metal and HAZ. A reduction in the amount of P tends to reduce the tendency for grain boundary fracture at the joint, and a reduction in the amount of B tends to suppress the formation of grain boundary ferrite. The most preferable amounts of p and s are each 0.
．． 01%, 0.0050% or less.

Al is an element generally contained in deoxidized steel, but in the present invention, it is an undesirable element and is limited to 0.004% or less. This is because when Ag is contained in steel, it combines with oxygen and no Ti oxide is produced. Deoxidation is T
It is possible to use only i and Sl, and in the present invention, Af
The smaller the amount of I, the better, and desirably 0.003% or less.

Nb has the function of suppressing ferrite generated at the γ grain boundaries and promoting the generation of fine IFFs with Ti oxides as nuclei. In order to obtain this effect, a minimum amount of Nb of o, ooa% is required. However, if the amount of Nb is too large, it will hinder the formation of fine IFPs, so the upper limit should be set at o, oeo%.
And so.

Next, the reason for adding B, Ni, Cu, Cr, V, Mo, and Ca will be explained.

Furthermore, the main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the characteristics of the steel of the present invention.

Therefore, the amount added should be limited.

B segregates as solid solution B at the γ grain boundaries near the fusion line after welding, and suppresses grain boundary ferrite. In order to obtain this effect, a B content of at least 0.0003% is required. However, excessive addition of B causes coarse precipitates such as Fe (CB) e to precipitate at the γ grain boundaries and deteriorate low-temperature toughness, so the upper limit of the amount of B should be set to 0.
It is necessary to set it to 0015%.

Ni has no adverse effect on weldability or HAZ toughness,
Although it improves the strength and toughness of the base metal, additions of 0.05% or less have little effect, and additions of 4.0% or more are unfavorable for weldability, so the upper limit was set at 4.0%.

Cu has almost the same effects as NI, as well as corrosion resistance, hydrogen-induced cracking resistance, etc., but if it exceeds 1.5%, Cu-cracks will occur during hot rolling, making manufacturing difficult.

Therefore, the upper limit was set at 1.5%.

Cr is an element that increases the strength of the base metal and welded part,
If it exceeds 1.0%, weldability and HAZ toughness will deteriorate, and if it is less than 0.05%, the effect will be weak. Therefore, the amount of Cr is set to 0.05 to 1.0%.

V is an element that has almost the same effect as Nb, but 0.01
% or less, the effect is small, and an upper limit of 0.08% is permissible.

Mo is an element that increases the strength of the base metal and welded part,
If it exceeds 0.4%, like Cr, the toughness of the base material and joints will deteriorate.
This is undesirable as it causes deterioration of the welded part.

Further, if it is less than 0.05%, the effect is weak. Therefore M.

The amount should be 0.05 to 0.4%.

Ca controls the morphology of sulfides, increases Charpy absorbed energy, improves low-temperature toughness, and is also effective in improving hydrogen-induced cracking resistance. However, the amount of Ca is 0.0
If it is less than 0.005%, it has no practical effect;
If the amount exceeds 0.0%, a large amount of Cab and CaS will be generated and become large inclusions, which will impair not only the toughness but also the cleanliness of the steel, and will also have an adverse effect on weldability.
0005 to 0.005%.

Even if the components of the steel are limited as described above, if the manufacturing method is not appropriate, fine composite oxides of Ti and Zr or TiN cannot be finely dispersed in the steel before welding. For this reason, it is also necessary to limit the manufacturing conditions.

First, industrially, it is essential to manufacture steel by continuous casting. The reason for this is that the continuous casting method has a faster cooling rate during solidification than the ingot-blooming method using large steel ingots, and a large amount of fine composite oxides of Ti and Zr and TiN can be obtained in the slab. It is. In the ingot-blowing method using large steel ingots, T
It is difficult to finely disperse a composite oxide of i and Zr or TiN in a slab.

It is necessary to keep the reheating temperature of the slab to 1250°C or lower.

This is because if the steel is reheated at a temperature higher than this, TiN will become coarse and there will be no fine TiN in the steel before welding, making it impossible to refine the structure in the HAZ.

In addition, in the present invention, it is not necessary to reheat the slab, and there is no problem at all even if hot charge rolling or direct rolling is performed.

In the present invention, the rolling method after reheating the slab is not particularly limited, but so-called working heat treatment, quenching and tempering after rolling, and normalizing treatment are appropriate for ensuring strength and toughness. This is because even if excellent HAZ toughness is obtained, if the toughness of the base material is poor, the steel is insufficient.

In order to improve the low-temperature toughness of the base metal, it is necessary to refine the crystal grains of steel. The method of processing heat treatment is (
Examples include 1) controlled rolling, (2) controlled rolling-accelerated cooling, and (3) rolling direct quenching and tempering, but the most preferred is a combination of controlled rolling and accelerated cooling. Note that, after manufacturing this steel, the characteristics of the present invention are not impaired even if A c is reheated to a temperature below the transformation point for the purpose of dehydrogenation or the like.

(Example) Examples are shown in Table 1.

Steel plates of various steel compositions were manufactured using the well-known converter, continuous casting, and thick plate processes, and submerged arc welding (SAW) was performed, and HAZ toughness was investigated by the -60°C teno 2 mm V notch Charpy test. . 1/4 of the test piece was taken from the position, and the notch position was set at FL and HAZlmm.

As is clear from Table 1, all the steels according to the present invention have good high heat input welding HAZ toughness. On the other hand, the comparison steel deteriorates in <HAZ.

Among the comparative steels, Steel 19 has a small amount of Ti and no IFP generation effect due to the composite oxide of Ti and Zr.
HAZ toughness deteriorates because there is no γ-grain suppressing effect. Steel 20 has a large amount of Ti, and the HAZ toughness deteriorates due to the precipitation of TiC. Steel 21 has a large amount of Ap and does not produce a composite oxide of Tl and Zr, so the amount of IFP produced is small and HAZ
Toughness deteriorates. Steel 22 has a small amount of Zr and does not produce a composite oxide of Ti and Zr, so the amount of IFP produced is small and the HAZ toughness deteriorates. Steel 23 contains too much Zr, so the HAZ toughness deteriorates due to the precipitation of ZrC. Steel 24 has a small amount of dissolved oxygen before addition of 1 and a small final amount of oxygen, so the number of oxides decreases and the HAZ toughness deteriorates. steel 25
Since the amount of dissolved oxygen before addition of Ti is too large, deoxidation is not performed sufficiently, the cleanliness of the steel is reduced, and the toughness of the base metal and HAZ is deteriorated. In Steel 26, since Zr was added and then Ti was added, Zr-based oxides were generated, and since IFP was not generated, the HAZ toughness deteriorated. Steel 27 takes a long time from adding Ti to adding Zr, so Ti and Zr
Since the number of composite oxides is small and the amount of IFP produced is small, HAZ toughness deteriorates. Steel 28 takes a long time to solidify after adding Zr, so the number of composite oxides of Ti and Zr is small, and the amount of IFP produced is small, so the HAZ toughness deteriorates. Steel 29 has a small number of composite oxides of Ti and Zr, and the amount of IFF produced is small, so HA
Z toughness deteriorates. Steel 30 has too many composite oxides of Ti and Zr, so the cleanliness of the steel deteriorates and the base metal and H
AZ toughness deteriorates.

(Effects of the Invention) According to the present invention, it has become possible to manufacture steel having extremely excellent low-temperature toughness not only in the base material but also in the entire welding HAZ in large quantities and at low cost. As a result, the safety of welded structures could be greatly improved.

Claims (1)

[Claims] 1. Ti is added in an amount of 0.010 to 0.040% to molten steel having a dissolved oxygen content of 0.003 to 0.020%, and Zr is added in an amount of 0.005 to 0.04% within 60 minutes. 60 after adding 030%
Coagulation was completed within minutes, C: 0.03-0.
15%, Si: 0.6% or less, Mn: 0.8-2.0%
, P: 0.025% or less, S: 0.005% or less, Al
: 0.004% or less, Nb: 0.003 to 0.060%
, N: 0.0020-0.0060%, Ti: 0.00
5-0.020%, Zr: 0.003-0.015%,
O: 0.001~0.006% and particle size 0.05~
30 μm of composite oxide mainly composed of Ti and Zr
~300 pieces/mm^2, the balance being iron and unavoidable impurities, and substantially containing no Al, 12
A method for producing a high tensile strength steel plate with excellent weld heat-affected zone toughness, which comprises producing the steel plate after reheating at a temperature of 50° C. or lower. 2. After adding 0.010 to 0.040% Ti to molten steel with a dissolved oxygen content of 0.003 to 0.020%, and further adding 0.005 to 0.030% Zr within 60 minutes,
Coagulation was completed within minutes, C: 0.03-0.
15%, Si: 0.6% or less, Mn: 0.8-2.0%
, P: 0.025% or less, S: 0.005% or less, Al
: 0.004% or less, Nb: 0.003 to 0.060%
, N: 0.0020-0.0060%, Ti: 0.00
5-0.020%, Zr: 0.003-0.015%,
O: 0.001-0.006% and further B: 0.000
3-0.0010%, Ni: 0.05-4.00%, C
u: 0.05-1.50%, Cr: 0.05-1.00
%, V: 0.005~0.080%, Mo: 0.05~
0.40%, Ca: 0.0005 to 0.005%, and particle size 0.05 to 10 μm
30 to 300 of a composite oxide whose main components are Ti and Zr.
Welding heat effect characterized by producing a steel plate by reheating a substantially Al-free slab containing aluminum/mm^2 and the remainder consisting of iron and unavoidable impurities at a temperature of 1250°C or less. A method for producing high-tensile steel plates with excellent partial toughness.