JPH0610043A - Production of 600n/mm2 class steel plate for construction use having low yield ratio and excellent in toughness in large heat input weld heat-affected zone - Google Patents

Abstract

PURPOSE:To inexpensively produce a 600N/mm<2> class steel plate for construction use having low yield ratio and excellent in toughness in large heat input weld heat-affected zone by subjecting a steel, having a specific composition containing Cu, Ni, Nb, Ti, etc., to reheating up to specific temp., to specific rolling, and then to respectively specified hardening and tempering treatments. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.05-0.11% C, <=0.5% Si, 0.6-1.6% Mn, <=0.03% P, <=0.01% S, 0.80-1.60% Cu, 0.30-1.0% Ni, 0.005-0.02% Nb, 0.005-0.025% Ti, <=0.005% Al, 0.001-0.004% N, 0.001-0.006% O, and the balance Fe with inevitable impurities and further containing, if necessary, prescribed amounts of V, Cr, Mo, and Ca is reheated up to 1000-1250 deg.C and then rolled at >=50% cumulative draft at <=1000 deg.C. The resulting steel plate is hardened from >=750 deg.C to ordinary temp. and further reheated up to 700-850 deg.C and hardened. Then this steel plate is tempered at a temp. in the range not higher than the Ac1 transformation point.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a thickness of 50 mm used for various structures in the fields of construction, civil engineering and marine structures.
The present invention relates to a method for producing a low yield ratio 600 N / mm ² grade steel sheet excellent in toughness of the heat affected zone (HAZ) particularly in large heat input welding (welding heat input = 500 to 1500 kJ / cm) such as electroslag welding.

[0002]

2. Description of the Related Art Generally, HAZ toughness of low alloy steel is (1)
Crystal grain size, (2) high carbon island martensite, dispersed state of hardened phase such as upper bainite (Bu), (3)
It is governed by various metallurgical factors such as grain boundary embrittlement and (4) elemental microsegregation. In particular, it is known that the size of the crystal grains of HAZ has a great influence on the low temperature toughness, and many techniques have been developed and put to practical use for refining the HAZ structure.

The technique of finely dispersing a nitride, such as TiN, which is relatively stable even at high temperatures in steel, and thereby suppressing the coarsening of the austenite (γ) grains of the HAZ, is particularly famous. However, in the region where HAZ is 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 greatly deteriorated in the vicinity of the melting line, and it is impossible to stably obtain high toughness in the entire HAZ. That is, in a Charpy test in which a notch is formed near the fusion line, the frequency is low, but a low value appears, which is not preferable from the viewpoint of safety of the welded structure.

On the other hand, Ti oxide (mainly Ti ₂ O
_The steel in which ₃ ) is finely dispersed (Japanese Patent Laid-Open No. 61-79745) can reduce the HAZ structure by forming intragranular acicular ferrite (hereinafter referred to as IGF) even in the vicinity of the melting line. Excellent low temperature toughness compared to

However, the conventional 600 N / mm ² grade steel plate has high hardenability, and particularly large heat input welding (welding heat input = 500 to 1).
In the case of 500 kJ / cm), it is difficult to form IGF, and even with this method, sufficient HAZ toughness cannot be obtained.

[0007]

DISCLOSURE OF THE INVENTION The present invention has a low yield ratio of 600 N / mm, which is extremely excellent in HAZ toughness in high heat input welding.
^It provides a technology for manufacturing ^second- grade steel sheets at low cost.
The steel produced by the method of the present invention has a fine HAZ structure even in the vicinity of the fusion line during high heat input welding, and exhibits excellent low temperature toughness throughout the HAZ.

[0008]

[Means for Solving the Problems] Specific means of the present invention are shown in the following (1) and (2).

(1) C: 0.05 to 0.11 by weight ratio
%, Si: 0.5% or less, Mn: 0.6 to 1.6%,
P: 0.03% or less, S: 0.01% or less, Cu: 0.
80 to 1.60%, Ni: 0.30 to 1.0%, Nb:
0.005 to 0.02% or less, Ti: 0.005 to 0.
025%, Al: 0.005% or less, N: 0.001 to
0.004%, O: 0.001 to 0.006%, with the balance essentially iron and unavoidable impurities
After reheating the steel not containing 1 in the temperature range of 1000 to 1250 ° C., it is rolled so that the cumulative rolling reduction of 1000 ° C. or less is 50% or more, and then quenched from the temperature of 750 ° C. or more to room temperature immediately. Then, it is reheated to a temperature range of 700 to 850 ° C., quenched, and tempered in a temperature range of Ac ₁ transformation point or lower, and a thickness of 50 mm or more (preferably 50 mm or more).
Large heat input welding heat-affected zone of 600 mm / mm ² class steel sheet with low yield ratio for construction with excellent toughness.

(2) C: 0.05 to 0.11 by weight ratio
%, Si: 0.5% or less, Mn: 0.6 to 1.6%,
P: 0.03% or less, S: 0.01% or less, Cu: 0.
80 to 1.60%, Ni: 0.30 to 1.0%, Nb:
0.005 to 0.02% or less, Ti: 0.005 to 0.
025%, Al: 0.005% or less, N: 0.001 to
0.004%, O: 0.001-0.006%, V: 0.005-0.10%, Cr: 0.05-0.5
%, Mo: 0.05 to 0.5%, Ca: 0.001 to
The steel containing 0.006% and the balance of iron and unavoidable impurities and containing substantially no Al is 1000 to 12
After reheating in the temperature range of 50 ° C, rolling is performed so that the cumulative rolling reduction of 1000 ° C or less is 50% or more, and then 750 ° C.
Immediately quench from the above temperature to room temperature, 700 ~ 850
High heat input welding heat affected zone toughness with a thickness of 50 mm or more (preferably 50 to 100 mm) characterized by reheating and quenching in a temperature range of ℃, and tempering in a temperature range below the Ac ₁ transformation point. Excellent manufacturing method for low yield ratio 600N / mm ² grade steel sheet for construction.

[0011]

The present invention will be described below. In order to maintain the performance stipulated in the rolled steel for welded structure (JIS G3106) and to improve the toughness of the heat-affected zone with high heat input welding,
It is important to refine the structure together with the steel composition. According to the study by the present inventors, in order to obtain excellent high heat input welding HAZ toughness, it is essential to add low C and B and to use Ti oxide. Therefore, there is a refinement of crystal grains and precipitation hardening by Cu. Has invented a method for ensuring the strength and toughness of 600 N / mm ² .

The features of the present invention are low C, no B addition and an appropriate amount of C.
When u is added and Ti, N and O are added, Ti ₂ O ₃ and Ti
A steel slab in which N is finely dispersed is 1000 to 1250 ° C.
After reheating in the temperature range of, the cumulative rolling reduction below 1000 ° C is 5
After rolling to 0% or more, it is immediately quenched from a temperature of 750 ° C or higher, reheated to a temperature range of 700 to 850 ° C, quenched, and tempered in a temperature range of the Ac ₁ transformation point or lower. It is to obtain a strength of 600 N / mm ² , a low yield ratio, and an excellent large heat input welding HAZ toughness at the same time by performing a treatment for precipitation hardening of Cu and a fine ferrite-bainite structure.

The minimum amounts of Cu and Nb necessary to obtain a strength and toughness of 600 N / mm ² are 0.80% and 0.
It is 005%. However, if these elements are excessively added, the HAZ toughness deteriorates, so the upper limits of Cu and Nb are 1.60% and 0.02%, respectively.

Next, in order to fully exert the effects of Cu and Nb as described above and to obtain excellent HAZ toughness, it is necessary to make the manufacturing conditions of the steel sheet appropriate. First, set the reheating temperature to 1
It is limited to the range of 000 to 1250 ° C. Reheating temperature is N
In order to form a solid solution of precipitates such as b and V and to secure the rolling end temperature, the temperature must be 1000 ° C. or higher. However, if the reheating temperature exceeds 1250 ° C., the austenite grains are remarkably coarsened, and it is impossible to completely refine them even by rolling, so that excellent low temperature toughness cannot be obtained. Therefore, the reheating temperature needs to be 1250 ° C or lower.

Subsequently, in rolling, it is essential to set the cumulative rolling reduction at 1000 ° C. or less to 50% or more. This is to refine the γ grains and obtain excellent low temperature toughness. The cooling conditions for the steel sheet following rolling are quenching from a temperature of 750 ° C. or higher to room temperature, reheating and quenching in a temperature range of 700 to 850 ° C.,
After that, it is necessary to perform tempering treatment in a temperature range below the Ac ₁ transformation point.

The reason for this is that the ferrite-austenite two-phase coexisting region is reheated to cause the enrichment of C from ferrite to austenite, and the austenite enriched in C and the ferrite phase depleted in C are brought into the quenching state. This is to obtain a two-phase mixed structure of a phase having extremely fine carbides and a phase having coarse carbides by carrying out. The reduction of the yield ratio is achieved by this two-phase mixed structure.

However, the quenching temperature after rolling is 7
If the temperature is less than 50 ° C, the transformation proceeds and the desired strength cannot be obtained.
Quenching from a reheating temperature of more than 0 ° C. has few ferrite phases, and the yield ratio reduction effect cannot be expected. Quenching from less than 700 ° C. reduces the strength and fails to achieve the object. Tempering is essential to improve the toughness of steel and to prevent softening due to welding, stress relieving, etc. However, the temperature is Ac
_If it exceeds ₁ point, the strength will remarkably decrease, so Ac must be ₁ point or less.

Next, the reasons for limiting the components in the present invention will be explained. The lower limit of 0.05% of C is the minimum amount for ensuring the strength of the base material and the welded portion and exerting these effects when Nb and V are added. However, if the C content is too large, the HAZ toughness of large heat input welding will be significantly deteriorated.
It was set to 11%.

Since Si is an element contained in the deoxidized upper steel, the weldability and HAZ toughness deteriorate when the amount of Si increases, so the upper limit was made 0.5%. Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.8%. However, if the Mn content is too large, the hardenability increases and the weldability and HA
Since the Z toughness deteriorates, the upper limit of Mn is set to 1.6%.

Ni improves strength and toughness without adversely affecting HAZ toughness, and is also effective in preventing Cu-cracks. However, if 1.0% or more, the cost becomes extremely high and the economy is lost, so the upper limit was made 1.0%.

Al is an element generally contained in deoxidized upper steel, but is an element not preferred in the present invention and is 0.005%.
Limited to: This is because if Al is contained in the steel, it will combine with oxygen and Ti oxide will not be generated.
Since deoxidation is also performed by Si and Ti, the less Al is, the better the steel of the present invention is, and 0.003% or less is desirable.

Ti is added in the form of 0.
005% or more is required, and if 0.025% is exceeded, T
Since the HAZ toughness is deteriorated due to the generation of iC, 0.0
It is limited to 05 to 0.025%. N is an element necessary to secure TiN, and 0.001 to secure the minimum amount.
% Or more is required, and when the amount of N increases, H due to solid solution N
Since the AZ toughness is deteriorated, the range is 0.001 to
It was set to 0.004%.

O is an element necessary for producing Ti ₂ O ₃ , and the minimum necessary amount is 0.001%, and 0.0
If it exceeds 06%, the cleanliness and toughness of steel will be deteriorated.
It is limited to 0.001 to 0.006%. The steel of the present invention contains P and S as unavoidable impurities. P, S
Since the effect on the high temperature strength is small, its amount is not particularly limited, but generally, the properties of steel such as toughness and strength in the plate thickness direction are improved as the amounts of P and S are smaller. Desirable P and S contents are 0.02% and 0.00, respectively
It is 3% or less.

The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved. However, in order to further improve the characteristics for the purpose, the elements described below, namely V, Cr, Mo,
Regarding the improvement of strength and toughness by selectively adding Ca,
Further favorable results are obtained.

Next, the above-mentioned additional element and its addition amount will be described. V is an element that has almost the same effect as Nb,
The effect on high temperature yield strength is smaller than Nb, but 0.0
No effect if less than 05%, HAZ if more than 0.10%
It has an unfavorable effect on toughness.

Cr is an element that enhances the strength of the base material and the welded portion. If it exceeds 0.5%, the high heat input welding HAZ toughness deteriorates, and if it is 0.05% or less, the effect is small. Therefore, the Cr content is 0.05 to 0.5%. Next, Mo is an element that improves both the strength and toughness of the base material.
% Or less, the effect is small, and if it exceeds 0.5%, the high heat input welding HAZ toughness is deteriorated, which is not preferable, so that
Limited to 0.5%.

Ca controls the morphology of sulfide (MnS),
It has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, CaO, C
A large amount of aS is generated and becomes large inclusions, which not only impairs the toughness of steel but also the cleanliness and adversely affects the weldability and lamella tear resistance. Therefore, the range of Ca addition amount is 0.001 to 0.0
It is set to 06%.

[0028]

EXAMPLE A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and the strength, toughness, and high heat input welding HAZ toughness of the base material were investigated.

[0029]

[Table 1]

[0030]

[Table 2]

Tables 1 to 8 show the chemical compositions of the present invention steels, and 9 to 21 show the chemical compositions of the comparative steels. Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the present invention steel and the comparative steel. Inventive Steels 1 to 8 in Table 2
Satisfies the standard strength and yield ratio of HT60 of 80% or less, obtains excellent low temperature toughness of the base material, and obtains excellent high heat input welding HAZ toughness. On the other hand, in the comparative steel 9, the standard strength of the base material is not satisfied because the C content is small. In Comparative Steel 10, the HAZ toughness is low due to the large amount of C. Comparative Steel 11 does not satisfy the standard strength of the base material because the amount of Cu is small. Comparative Steel 12 has a high HAZ toughness due to the large amount of Nb. Comparative steel 13 has a small amount of Ti, and comparative steel 14 has a large amount of Ti.
Z toughness is low. In Comparative Steel 15, since the amount of Al is large, Ti
No oxide is generated and HAZ toughness is low. In Comparative Steel 16, the reheating temperature was low and Nb was not sufficiently solid-dissolved, so the standard strength of the base material was not satisfied. Comparative steel 17
However, the reheating temperature is too high, which causes coarsening of the structure, which deteriorates the low temperature toughness of the base material and lowers the HAZ toughness.

In Comparative Steel 18, the cumulative rolling reduction at 1000 ° C. or lower is low, the grain refinement of the structure is not sufficiently performed by rolling, and the low temperature toughness of the base material and the HAZ toughness are lowered.
In Comparative Steel 19, the water cooling start temperature is low and the amount of ferrite is too large, and the standard strength of the base material is not satisfied. In Comparative Steel 20, the quenching temperature is low and the strength of the base material is low, and the standard value is not satisfied. Since the comparative steel 21 has a high quenching temperature and does not have a sufficient two-phase structure, the yield ratio of the base metal exceeds 80%.

[0033]

EFFECTS OF THE INVENTION Steel materials manufactured by the chemical composition and manufacturing method of the present invention, such as thick steel plate, shaped steel, and steel bar, are excellent in high heat input welding.
It is a steel with AZ toughness and can greatly enhance the safety of construction, civil engineering, and marine structures.

Claims (2)

[Claims] 1. By weight ratio, C: 0.05 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03%.
Hereafter, S: 0.01% or less, Cu: 0.80-
1.60%, Ni: 0.30 to 1.0%, Nb: 0.005
~ 0.02% or less, Ti: 0.005 to 0.025%, Al: 0.005
% Or less, N: 0.001 to 0.004%, O: 0.001
After reheating a steel containing 0.006% to 0.006% and the balance of iron and unavoidable impurities and containing substantially no Al in the temperature range of 1000 to 1250 ° C, the cumulative rolling reduction at 1000 ° C or less is 50% or more. After being rolled so that the temperature becomes 750 ° C. or higher, it is immediately quenched to a room temperature from 750 ° C. or higher, reheated to a temperature range of 700 to 850 ° C., quenched, and tempered in a temperature range of Ac ₁ transformation point or lower A method for producing a steel plate with a low yield ratio of 600 N / mm ^{2 which has} excellent toughness and a high heat input welding heat affected zone. 2. V: 0.005 to 0.10% by weight ratio, Cr: 0.05 to
0.5%, Mo: 0.05 to 0.5%, Ca: 0.001
To 0.006% of one or two or more of them are contained, and the method for producing a low yield ratio 600N / mm ² grade steel sheet for construction having a high heat input weld heat affected zone toughness according to claim 1, .