JPH05247531A - Manufacture of steel excellent in low temperature toughness in weld-heat affected zone - Google Patents

Abstract

PURPOSE:To manufacture steel whose weld-heat affected zone is excellent in low temp. toughness by forming steel having a specified comps. in which the content of Ti and O is prescribed into a slab by a continuous casting method, reheating it at a specified temp. and executing thick plate rolling. CONSTITUTION:Steel contg., by weight, 0.06 to 0.15% C, <=0.4% Si, 0.8 to 2.0%, Mn, <=0.020% P, <=0.005% S, <=0.004% Al, 0.004 to 0.013% Ti, 0.0035 to 0.0079% N and 0.0010 to 0.0030% O and in which D1*=0.316<1/2> (1+0.7Si) (0.35+4.1Mn) (1+2.2Cr) (1+3Mo) (1+0.36Ni) (1+0.37Cu) satisfies 0.6 to 1.0 as well as -0.020%<=[Ti]-2[O]-3.4[N]<=-0.010, and the balance iron with inevitable impurities, and, substantially, incorporated with is formed into a slab by continuous casting. This slab is reheated at <=1200 deg.C and is thereafter subjected to thick plate rolling.

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 steel having excellent low temperature toughness in the heat affected zone (HAZ) of small heat input welding to medium heat input welding.

[0002]

2. Description of the Related Art HAZ toughness of a low alloy steel depends on (1) grain size, (2) dispersed state of hardened phases such as high carbon martensite (M ^* ) and upper bainite (Bu), and (3) precipitation. It is governed by various metallurgical factors such as the hardening state, (4) presence or absence of grain boundary embrittlement, and (5) elemental microsegregation. It is known that these factors have a great influence on the low temperature toughness, and many techniques have been developed and put to practical use in order to improve the HAZ toughness.

A steel (Japanese Patent Application No. 62-42769) in which Ti oxide and TiN are finely dispersed is known as a particularly excellent technique. In this steel, ferrite is generated from the Ti oxide in the γ grains in the cooling process after welding to refine the microstructure and improve the toughness.

However, in a very narrow region near the fusion line exposed to a temperature of 1400 ° C. or higher, the oxide does not dissolve due to welding heat, but TiN dissolves and is produced as TiC by the subsequent welding heat, so that the toughness is reduced. Deterioration occurs.
As described above, when a local embrittlement region is present, the adverse effect thereof is extremely small in the Charpy test.
The test shows a big difference. Therefore, although the CTOD characteristics of the multi-pass weld of this steel depend on the composition system of the steel,
The limit was to satisfy the CTOD value of about 0.25 mm at about -30 ° C.

[0005]

In the prior art, HA is used.
The toughness deteriorates in a very narrow region near the melting line of Z, and the CTOD property of the multipass weld depends on the composition system of the steel, but it was limited to satisfying about 0.25 mm. At present, in the small to medium heat input welding, there is no steel manufacturing technology capable of satisfying good CTOD characteristics at -40 ° C or lower, and development of new steel based on new knowledge has been strongly desired.

The present invention is applicable to HAZ in small to medium heat input welding.
The present invention provides a technique for inexpensively manufacturing steel having extremely excellent toughness (particularly CTOD characteristics).

[0007]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
C: 0.06 to 0.15%, Si: 0.4% or less,
Mn: 0.8-2.0%, P: 0.020% or less, S:
0.005% or less, Al: 0.004% or less, Ti:
0.004-0.013%, N: 0.0035-0.0
070%, O: 0.0010 to 0.0030% is contained,

[Equation 2] Is in the range of 0.6 to 1.0, and −0.020% ≦ [T
i] −2 [O] −3.4 [N] ≦ −0.010, with the balance being iron and inevitable impurities.
In a method for producing steel having excellent low-temperature toughness in the weld heat-affected zone, which is characterized in that steel without addition of 1 is made into a slab by a continuous casting method, and this is reheated at a temperature of 1200 ° C. or lower, and then thick plate rolling is there.

Further, in the present invention, C: 0.06% by weight.
0.15%, Si: 0.4% or less, Mn: 0.8-2.
0%, P: 0.020% or less, S: 0.005% or less,
Al: 0.004% or less, Ti: 0.004 to 0.01
3%, N: 0.0035 to 0.0065%, O: 0.0
010 to 0.0030%, and further Nb: 0.005
0.020%, V: 0.005-0.030%, Ni:
0.05-1.0%, Cu: 0.05-0.5%, C
a: 0.0005 to 0.005%, REM: 0.005
~ 0.05% containing one or more,

[Equation 3] Is in the range of 0.6 to 1.0, and −0.020% ≦ [T
i] −2 [O] −3.4 [N] ≦ −0.010, with the balance being iron and unavoidable impurities, and substantially A
In a method for producing steel having excellent low-temperature toughness in the weld heat-affected zone, which is characterized in that steel without addition of 1 is made into a slab by a continuous casting method, and this is reheated at a temperature of 1200 ° C. or lower, and then thick plate rolling is there.

According to the research conducted by the present inventors, the HAZ toughness of a small to medium heat input weld where CTOD characteristics are a problem is found to be
1) Chemical composition of steel, 2) Microstructure (size of crystal grains and distribution of hardening phase), 3) Precipitation hardening (precipitation hardening element that is once dissolved by welding heat re-precipitates and hardens by post-welding heat) 4) It was clarified that the microstructure largely depends on the amount of oxides that have the same effect as the hardening phase.

For this reason, the present inventors have made various studies in order to disperse oxides in steel more finely than in the past, to dissolve TiN and to reduce TiC generated thereafter as much as possible.
It was found that an appropriate balance of Ti, O and N contents is important.

At the same time, in order to make use of the above knowledge, it was also clarified that a proper base component is also essential, and its proper range was identified. Ti oxide (mainly Ti ₂ O ₃ ) forms fine acicular ferrite to improve toughness, but when the test conditions are severe (-
In the case where the CTOD characteristic at a low temperature of 40 ° C. or lower is a problem), the Ti oxide has a bad effect of promoting the occurrence of brittle cracks. Therefore, it is necessary to suppress the size and number of Ti oxides so that brittle cracks do not occur.

As a result of various studies, the present inventors have solved the above-mentioned problems by setting Ti and O within proper ranges.
That is, the Ti amount is 0.004% to 0.013% in the proper range, and the O amount is 0.0010 to 0.0030% in the proper range. When the amount of Ti or O is larger than this range, the number of oxides increases, the size also increases, and brittle cracks easily occur. On the other hand, if the amount is less than this range, an effective oxide is not produced, so that the microstructure is not refined and the toughness is not improved.

Ti oxide is preferentially formed during the solidification of molten steel, but Ti that does not combine with oxygen combines with N to form TiN. TiN improves the toughness by refining the microstructure at a temperature of 1350 ° C. or lower during welding.
Melts at temperatures above ° C.

The melted Ti is cooled by TiN and TiC during cooling.
To form. In this case, when the amount of Ti is large and the amount of N is small, TiC is formed and the toughness is significantly deteriorated. Therefore, since the toughness is not deteriorated, the Ti content, Ti content, O content, N content, and N content are regulated.
It is a necessary condition to regulate the balance of the amount within an appropriate range, and the N amount is 0.0035 to 0.0065%, Ti,
The balance of O and N is -0.020% ≤ [Ti] -2
[O] -3.4 [N] ≤-0.010% is the conforming range.

However, even if the amounts of Ti, O, and N are regulated to finely disperse Ti oxide and TiN in the steel, excellent HAZ toughness cannot be obtained if the basic components are not proper. This point will be described below.

The lower limit of 0.06% of C is necessary to secure the strength of the base material and the welded portion. However, if the amount of C is too large, the low temperature toughness, weldability, and HAZ toughness of the base material deteriorate, so the upper limit was made 0.015%.

Si is an element contained in steel for deoxidation, but if added in a large amount, the weldability and HAZ toughness deteriorate, so the upper limit was limited to 0.4%. From the viewpoint of improving the HAZ toughness, 0.15% or less is desirable.

Mn is an essential element for securing strength and toughness, and its lower limit is 0.8%. Mn has the effect of preventing coarse proeutectoid ferrite generated at γ grain boundaries and improving the HAZ toughness, but if Mn is too much, the weldability and HAZ toughness are deteriorated, so the upper limit was made 2.0%.

The reason why the impurities P and S in the steel of the present invention are 0.020% and 0.005% or less, respectively, is to further improve the low temperature toughness of the base metal and the welded portion. Reduction of P reduces grain boundary fracture in HAZ, and S
Reduction tends to suppress the formation of grain boundary phyllite.

Although Al is an element generally contained in deoxidized upper steel, it is an unfavorable element in the steel of the present invention, and its upper limit is set to 0.004%. This is because when Al is contained in the steel, it bonds with O earlier than Ti and Ti oxide is not formed.

Next, Nb, V, Ni, Cu, Ca, RE
The reason for adding M will be described. Nb has a function of suppressing ferrite generated in the γ grain boundary and promoting generation of fine phyllite having Ti oxide as a nucleus. However, if it is too large, the formation of fine ferrite is adversely affected.
The regulation range was 005 to 0.020%.

V is an element having almost the same effect as Nb, but if 0.005% or less, the effect is small, and the upper limit is 0.0.
Up to 30% is acceptable. Ni has little adverse effect on weldability and HAZ toughness and improves the strength and toughness of the base material, but if it exceeds 1.0%, it is not preferable for weldability, so 1.0% was made the upper limit. Cu has almost the same effect as Ni and is also effective in corrosion resistance, hydrogen induced cracking resistance, etc., but if it exceeds 0.5%, Cu cracks occur during hot rolling, which makes manufacturing difficult. Therefore, the upper limit is set to 0.5%.

Ca and REM control the morphology of sulfide (MnS), and have an effect on improving low temperature toughness and hydrogen-induced cracking resistance. However, when the amount of Ca is 0.0005% and REM is 0.005% or less, the effect is small, so the respective lower limits are set. Further, Ca and REM impair the toughness and cleanliness if the addition amounts are too large, so the respective upper limits are set to 0.005.
% And 0.05%.

Even if the steel composition is limited as described above, good toughness of the welded joint cannot be obtained unless the manufacturing method is appropriate. Therefore, it is necessary to limit the manufacturing conditions. In order to obtain the characteristics of the invention steel, it is essential industrially to manufacture it by a continuous casting method. The reason for this is that in the continuous casting method, the molten steel has a high cooling rate and a large amount of fine Ti oxide is obtained in the slab. In a large steel ingot, fine Ti oxide cannot be obtained because the cooling rate during solidification is slow.

In the case of the continuous casting method, the cooling rate becomes slower due to the increase in the slab thickness, so that the fine Ti oxide is reduced. Therefore, the applied slab thickness is preferably 350 mm or less. The reheating temperature of the slab needs to be 1200 ° C or lower. When reheated at a temperature higher than this, TiN becomes coarse, so that the microstructure refinement of the HAZ at 1350 ° C. or lower for welding becomes insufficient.

Next, the rolling method after the slab is reheated is preferably manufactured by a thermo-mechanical treatment method. The reason for this is that even if excellent HAZ toughness is obtained, if the toughness of the base material is poor, it will be insufficient as a steel material. Examples of the method of thermomechanical treatment include 1) controlled rolling, 2) controlled rolling-accelerated cooling, 3) direct quenching after rolling-tempering, and the most preferable method is a combination of controlled rolling and accelerated cooling. After manufacturing this steel, Ac ₁
Reheating to a temperature below the transformation point does not impair the characteristics of the steel of the present invention.

[0027]

[Examples] Steel plates of various steel components are manufactured in a converter-continuous casting-thick plate process, and actual welded joints are prepared to obtain Charpy or CTO.
D test was performed. Welding is performed using the latent arc welding (SAW) method which is generally used as test welding.
It carried out by K groove so that L) might become vertical. In addition, the Charpy test was performed with a Bond notch (weld metal and HAZ 50%) from 1/4 t of the plate thickness, and the CTOD test was t
(Plate thickness) × 2t The cross-sectional fatigue notch was performed.

Table 1 shows examples.

[0029]

[Table 1]

[0030]

[Table 2]

The steel sheets produced according to the present invention (invention steels) all have good HAZ toughness, whereas the comparative steels not according to the present invention have poor HAZ toughness and are not suitable as steel sheets used in harsh environments.

Among the comparative steels, Steel 11 had a large amount of Al, so the microstructure did not become finer.
4 of the 5 OD values were low. Further, in Steel 12, since the oxygen content was large, all five CTOD values were low values. Steel 13 had a poor Ti, N, O balance and f (Ti) did not fall within the regulation range, so that a low CTOD value occurred in 4 out of 5 steels. For steel 14, D ₁
^{Since the *} is low, the microstructure does not become fine, and Charpy, C
A low TOD value was generated. Furthermore, for steel 15, D
_{Since 1} ^* was too high, both Charpy and CTOD values were low. The steel of the present invention is most preferably applied to thick plates, but is also applicable to hot coils, shaped steel and the like.

[0033]

The steel produced according to the present invention has a fine HAZ structure even in the vicinity of the melting line during welding, and exhibits excellent low temperature toughness throughout the HAZ. As a result, the extremely low temperature range (-
(40 ° C or less), low temperature tanks, line pipes, and other steel products that can be used in harsh environments.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Isoda 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Works

Claims (2)

[Claims] 1. By weight%, C: 0.06-0.15%, Si: 0.4% or less, Mn: 0.8-2.0%, P: 0.020%
Below, S: 0.005% or less, Al: 0.004%
Hereinafter, Ti: 0.004 to 0.013%, N: 0.0035
.About.0.0070%, O.sub.2: 0.0010 to 0.0030%, and Is in the range of 0.6 to 1.0, and −0.020% ≦ [T
i) -2 [O] -3.4 [N] ≦ −0.010 is satisfied, and a balance of iron and unavoidable impurities, which is substantially free of Al, is formed into a slab by a continuous casting method. A method for producing steel having excellent low-temperature toughness in a weld heat-affected zone, which comprises reheating at a temperature of 1200 ° C. or lower and then performing plate rolling. 2. In% by weight, Nb: 0.005 to 0.020%, V: 0.00
5 to 0.030%, Ni: 0.05 to 1.0%, Cu: 0.05
~ 0.5%, Ca: 0.0005 to 0.005%, REM: 0.00
The method for producing a steel having excellent low temperature toughness in a weld heat affected zone according to claim 1, wherein the steel contains 5 to 0.05% of one kind or two or more kinds.