JPH0578740A - Method for producing steel with excellent low temperature toughness in the heat affected zone - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a high-strength steel sheet having excellent low-temperature toughness in a heat-affected zone (hereinafter referred to as HAZ) from small heat input welding to large heat input welding. Further, this steel can be used for welded steel structures such as pressure vessels, shipbuilding, bridges, constructions, and line pipes. [Composition] C: 0.01 to 0.15% by weight, S
i: 0.5% or less, Mn: 0.5 to 2.0%, P: 0.
025% or less, S: 0.005% or less, Al: 0.00
7% or less, Ce: 0.0001 to 0.030%, N:
A steel having a composition containing 0.0065% or less, the balance being iron and unavoidable impurities and containing substantially no Al is 1
A method for producing steel having excellent low-temperature toughness in a weld heat-affected zone, characterized by performing hot working after reheating in a temperature range of 000 to 1250 ° C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet having excellent low temperature toughness in a heat-affected zone (hereinafter HAZ) from small heat input welding to large heat input welding. In particular, this steel is a pressure vessel,
It can be used for welded steel structures such as shipbuilding, bridges, construction and line pipes.

[0002]

2. Description of the Related Art HAZ toughness of low alloys depends on (1) grain size, (2) dispersed state of hardened phase such as high carbon island martensite (M *) upper bainite (Bu), (3) grain It is governed by various metallurgical factors such as the presence or absence of field embrittlement and (4) elemental microsegregation. It is known that the size of the crystal grains of HAZ has a great influence on the low temperature toughness.
Many techniques have been developed and put into practical use for making the Z structure finer. The technique of finely dispersing a relatively stable nitride such as TiN in the steel even at a high temperature and thereby suppressing the coarsening of the austenite (γ) grains of HAZ is particularly famous. However, in the region of HAZ heated to 1400 ° C. or higher, TiN coarsens or dissolves, and the ability to suppress coarsening of γ grains disappears. For this reason, the toughness is largely deteriorated in the vicinity of the melting line, and stable high toughness cannot be obtained in the entire HAZ. That is, although the frequency is low in a Charpy test or a CTOD test in which a notch is provided near the melting line, a low characteristic value appears, which is not preferable from the viewpoint of safety of the welded steel structure.

[0003]

On the other hand, the steel in which Ti oxide is finely distributed (Japanese Patent Laid-Open No. 61-79745) can reduce the HAZ toughness even in the vicinity of the melting line.
Excellent low temperature toughness is obtained as compared with N steel. However, in the steel using this Ti oxide, the toughness of the high heat input welding HAZ is not sufficient because the Charpy transition temperature is about -15 to -35 ° C. Further, according to the subsequent research conducted by the present inventors, in the steel in which the Ti oxide is finely dispersed, in a region where the γ grains near the melting line are coarsened (coarse grain region: a region heated to 1400 ° C. or higher). Although the effect of reducing the HAZ structure is large, a region in which γ grains in which TiN is partly coarsened is slightly large (sub-coarse grain region: region heated to 1200 to 1350 ° C)
It has been found that the effect of refining the HAZ structure by Ti oxide is smaller than that in the coarse-grained region and the HAZ toughness is reduced. As described above, at present, a technique for stably refining the structure in all HAZ regions in small heat input and large heat input welding is required, and development of new steel based on new knowledge is strongly desired. The present invention provides a technique for producing a steel having extremely excellent HAZ toughness in low heat input welding and high heat input welding. In the steel produced by the method of the present invention, the HAZ structure is refined even in the vicinity of the fusion line during welding, and H
It exhibits excellent low temperature toughness throughout the AZ.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to produce a steel having excellent low temperature toughness in the entire HAZ region with Ce oxide. We have found that Ce and REM
As a result of detailed examination of the relationship between the low temperature toughness of the heat-affected zone and the structure formation by the oxide, the following was found. Ce oxide (Mn, which exists in the γ grains during the γ-α transformation)
Si is included as a nucleus to generate acicular ferrite (AF). On the other hand, REM (L other than Ce
a, Y, Hf, etc.), almost no AF formation is observed. As a result, excellent low temperature toughness is obtained over the entire HAZ. Ce oxide has a higher fine distribution effect than Ti oxide and other REMs. That is, Ce oxide is a good AF
It becomes a nucleus, and the HAZ structure is refined to obtain excellent low temperature toughness. Ce oxide has a small ability to suppress the coarsening of γ grains,
With Ce oxide existing in γ at the time of γ-α transformation as a nucleus,
Radially fine AF is generated, and the HAZ structure is remarkably refined. The Ce oxide is stable in a region (coarse grain region) heated to 1400 ° C. or higher in the vicinity of the melting line, and even in this region, it is effective in refining the structure. As a result, the HAZ structure is refined over the entire area, and extremely excellent low temperature toughness is obtained.

Therefore, the gist of the present invention is as follows: 1) wt% C: 0.01 to 0.15% Si: 0.5% or less Mn: 0.5 to 2.0% P: 0.0. 025% or less S: 0.005% or less Al: 0.007% or less Ce: 0.0001 to 0.030% N: 0.0065% or less, and the balance substantially consisting of iron and inevitable impurities. Steel with a composition not containing Al is 1000 to 1250 ° C.
A method for producing steel with excellent low-temperature toughness in the heat-affected zone of welding, characterized by performing hot working after reheating in the temperature range of. 2) In% by weight C: 0.01 to 0.15% Si: 0.5% or less Mn: 0.5 to 2.0% P: 0.025% or less S: 0.005% or less Al: 0. 007% or less Ce: 0.0001 to 0.030% N: 0.0065% or less Nb: 0.005 to 0.04% Ti: 0.005 to 0.03% V: 0.005 to 0.1 % Ni: 0.05 to 2.0% Cu: 0.05 to 1.0% Cr: 0.05 to 1.0% Mo: 0.05 to 0.40% B: 0.0003 to 0.002 % Ca: 0.0005 to 0.005% REM: 0.0005 to 0.005% of one or two or more types, the balance of which is iron and inevitable impurities and is substantially free of Al Welding heat characterized by performing hot working after reheating steel in a temperature range of 1000 to 1250 ° C. A method for producing steel with excellent low temperature toughness in the affected zone.

[0006]

The reason for limiting the steel composition as described above in the present invention is as follows. C: C is 0.01 in order to guarantee the strength of the base material and the welded part.
% Or less, the effect is weakened, so the lower limit is made 0.01%.
Furthermore, if the amount of C is too large, not only the low temperature toughness of the heat affected zone (HAZ) is adversely affected but also the base metal toughness and weldability are deteriorated, so 0.15% is the upper limit. Si: Si is an element contained in the deoxidized upper steel. Since the weldability and HAZ toughness deteriorate when Si is increased, the upper limit was made 0.5%. In the steel of the present invention, Al deoxidation is sufficient,
Further, Ti deoxidation may be used. From the viewpoint of HAZ toughness, the content of Si is preferably about 0.15%. Mn: Mn is an unavoidable element for securing strength and toughness, and its lower limit is 0.5%. However, if the Mn content is too large, not only the hardenability increases and the weldability and HAZ toughness deteriorate, but also the base metal strength that meets the target specifications cannot be obtained. Therefore, the upper limit of the amount of Mn is set to 2.0%.

[0007] P: If too much P, toughness and strength in the plate thickness direction are deteriorated, so the upper limit was made 0.025% or less. S: S, like P, too much deteriorates toughness and strength in the plate thickness direction, so the upper limit was made 0.005% or less. Al: Al is generally an element contained in steel for deoxidation, but it is an unfavorable element in the steel of the present invention, and its upper limit was made 0.004%. This is because when Al is contained in steel, it is combined with O to form Ce oxide. Deoxidation is also possible with Si, Mn, and Ce, and the less Al in the steel of the present invention, the better.

Ce: Ce is an essential element in the steel of the present invention because it forms Ce oxide, and it is impossible to obtain excellent HAZ toughness without adding Ce. At least 0.002% to produce Ce oxide
Ce is required. However, if it exceeds 0.03%, the cleanliness of the steel decreases due to the formation of non-metallic inclusions, leading to deterioration in toughness, so the upper limit is made 0.03%. N: N is generally contained in steel as an unavoidable impurity. When the amount of N is large, the HAZ toughness is deteriorated and the surface defects of the continuously cast slab are promoted. Therefore, the upper limit was made 0.006%. The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved, but in order to further improve the characteristics for the purpose, the elements described below, namely Nb, V, Ni, Cu, Cr,
By selectively adding one or more of Mo and Ti, more preferable results can be obtained with respect to improvement of strength and toughness.

Nb: Effective for ensuring toughness, 0.005
% Or less, there is no effect, and if it exceeds 0.05%, the toughness of the welded portion deteriorates. Therefore, the Nb content is limited to 0.005 to 0.05%. Ti: TiN is effective for refining the structure and producing very fine and uniform γ grains at the time of reheating and tempering. If the content is 0.005% or less, the effect is weakened, so the lower limit is 0.005. %. On the other hand, if it exceeds 0.03%, the toughness is significantly impaired, so the upper limit is made 0.03%. V: Improves the HAZ toughness in the range of 0.005 to 0.1%. However, 0.005% or less has no effect, and more than 0.1% has an unfavorable effect on the HAZ toughness.

Ni: Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but is less effective at 0.05% or less, and is preferable for weldability at more than 2.0%. Therefore, the upper limit was set to 2.0%. Cu: Cu has almost the same effect as Ni, and also has an effect of increasing high strength due to Cu precipitates and improving corrosion resistance and weather resistance. However, if the amount of Cu exceeds 1.0%, Cu cracking occurs during hot rolling, which makes manufacturing difficult, and further,
If it is less than 05%, there is no effect, so the amount of Cu is 0.05-0.
Limited to 1%. Cr: Cr enhances the strength of the base material and the welded portion, and is 0.05
% Or more improves the corrosion resistance and weather resistance, but if it exceeds 1.0%, the weldability and HAZ toughness are deteriorated, and if it is 0.05% or less, the effect is small. Therefore, the Cr content is 0.05 to 0.1.
Limited to%. Mo: Mo is an element that improves both strength and toughness,
If it exceeds 1.0%, the toughness and weldability of the base material and the welded portion are deteriorated, which is not preferable, and if it is 0.05% or less, the effect is small. Therefore, the amount of Mo is limited to 0.05 to 1.0%.

B: B is an element that increases hardenability and strength. The solid solution B segregated at the γ grain boundary of HAZ suppresses the formation of ferrite and helps the formation of fine AF.
In addition, BN combined with N has a function as a ferrite generation nucleus and miniaturizes the HAZ structure. To obtain such an effect of B, a minimum amount of 0.0003% B is necessary. However, if the amount of B is too large, coarse precipitates are deposited on the γ grain boundaries and the low temperature toughness is deteriorated. Therefore, the upper limit of the amount of B is limited to 0.002%. Ca: Ca has the effect of controlling the morphology of MnS, increasing Charpy absorbed energy, and improving low temperature toughness.
However, if it is less than 0.0005%, it has no practical effect, and if it exceeds 0.005%, large amounts of CaO and CaS are formed to form large inclusions, which not only impairs the toughness of the steel but also the cleanliness. Since the weldability is also adversely affected, the range of the Ca addition amount is set to 0.0005 to 0.005%. REM: REM has the same effect, and its appropriate range is 0.0005 to 0.005%.

Next, the reasons for limiting the manufacturing conditions are as follows. This steel is industrially required to be manufactured by a continuous casting method. This is because in the continuous casting method, the solidification cooling rate of molten steel is fast and a large amount of fine Ce oxide is obtained in the slab. It is difficult to finely disperse Ce oxide in a slab by the ingot-segmentation method using a large steel ingot. In the case of continuous casting, the cooling rate depends on the slab thickness, but H
From the viewpoint of AZ toughness, the thickness is preferably 350 mm or less. Further, the reheating temperature of the slab needs to be 1250 ° C or lower. This is because if heated at a temperature higher than this, the γ grains become coarse and it becomes impossible to secure the toughness of the base material. Also,
Reheating the slab for a long time at 1000 ° C or lower is not realistic. In the present invention, it is not always necessary to reheat the slab, and there is no problem even if hot charge rolling or direct rolling is performed. Next, so-called thermo-mechanical treatment is essential for the rolling method after the slab is reheated. This is because even if excellent HAZ toughness is obtained, if the toughness of the base material is poor, it is insufficient as a steel material. In order to improve the low temperature toughness of the base material, it is necessary to refine the crystal grains of steel by thermomechanical treatment. As the hot rolling method, 1) controlled rolling, 2) controlled rolling-accelerated cooling, 3) quenching immediately after rolling-tempering, etc. can be raised, but controlled rolling and accelerated cooling are the most preferable for improving the base material properties. Is a combination of. Even if the steel is reheated to a temperature not higher than the Ac ₁ transformation point for the purpose of dehydrogenation after manufacturing, it does not impair the present invention.

[0013]

EXAMPLES Next, the present invention will be further described by way of examples. Plate thicknesses (thickness: 32 mm) of various steel components were manufactured in a converter-continuous casting-thick plate process, and HAZ toughness was investigated using a welding heat cycle reproducing device. The simulated thermal cycle was performed using a test piece taken from a plate thickness of 1/4 t, and peak temperatures (maximum temperature reached) of 1400 ° C. and 1300 ° C., 800 to 50.
It was carried out at 0 ° C. for a cooling speed of 192 seconds. This condition is equivalent to a welding heat input of 200 kJ / cm, and a peak temperature of 14
00 ° C. and 1300 ° C. respectively show a coarse grain region (near the melting line) and a sub coarse grain region of the actual welded HAZ. A slab having the composition shown in Table 1 was manufactured, and the slab was manufactured under the conditions shown in Table 2 and subjected to a heat cycle test. The mechanical properties and HAZ toughness of the obtained steel are summarized in Table 2. The steels produced by the method of the present invention (invention steels) all have good base material properties and HAZ toughness, whereas the comparative steels not according to the present invention have poor base material properties or HAZ toughness and are subject to severe environment Not suitable as a steel plate used.

Comparative steel 16 is TiN steel and has a peak temperature of 130.
The toughness of the reproduced HAZ at 0 ° C is good, but the toughness of the reproduced HAZ at 1400 ° C deteriorates. In addition, steel 17 is TiO
Although it is steel, the toughness of the reproduced HAZ is inferior to that of the present invention.
Steel 18 has a large amount of Ce, so steel 19 has a small amount of HA.
Z toughness was poor. Further, in Steels 18 and 19, the formation of AF was significantly deteriorated. Steel 20 has poor toughness because it is a Ti- and Ce-free steel. The present invention is most preferably applied to a thick plate mill, but is also applicable to hot coils, shaped steel and the like. Further, the thick steel plate manufactured by this method can be used for welded steel structures used in harsh environments such as marine structures, pressure vessels, and line pipes.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

As described above, according to the present invention, it becomes possible to produce a large amount of steel having excellent toughness not only in the base metal but also in the entire weld zone. As a result, it was possible to greatly improve the safety of the welded steel structure used in a severe environment such as an extremely cold region or the sea.

Claims (2)

[Claims] 1. By weight%, C: 0.01 to 0.15% Si: 0.5% or less Mn: 0.5 to 2.0% P: 0.025% or less S: 0.005% or less Al : 0.007% or less Ce: 0.0001 to 0.030% N: 0.0065% or less, the balance of which is 1000 to 1250, which is a steel containing iron and inevitable impurities. ℃
A method for producing steel with excellent low-temperature toughness in the heat-affected zone of welding, characterized by performing hot working after reheating in the temperature range of. 2. By weight%, C: 0.01 to 0.15% Si: 0.5% or less Mn: 0.5 to 2.0% P: 0.025% or less S: 0.005% or less Al : 0.007% or less Ce: 0.0001 to 0.030% N: 0.0065% or less Nb: 0.005 to 0.04% Ti: 0.005 to 0.03% V: 0.005 to 0.005% 0.1% Ni: 0.05 to 2.0% Cu: 0.05 to 1.0% Cr: 0.05 to 1.0% Mo: 0.05 to 0.40% B: 0.0003 to 0.002% Ca: 0.0005 to 0.005% REM: 0.0005 to 0.005% One kind or two or more kinds are contained, and the balance is substantially Al containing iron and unavoidable impurities. A steel having a composition not containing is reheated in a temperature range of 1000 to 1250 ° C. and then hot worked. Excellent production method for steel low temperature toughness of the weld heat affected zone that.