JPH1180832A - Production of high tensile strength steel with low yield ratio, excellent in weldability and toughness at low temperature - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel material combining excellent weldability and tough ness at low temp. with characteristics of high strength and low yield ratio by specifying steel components and heat treatment conditions, respectively. SOLUTION: A steel having a composition which consists of, by weight, 0.05-0.15% C, <=0.4% Si, 1.0-2.0% Mn, <=0.02% P, $0.01% S, 0.05-1.0% Ni, 0.005-0.02% Nb, 0.005-0.025% Ti, <=0.06% Al, 0.001-0.005% N, and the balance iron and contains, if necessary, 0.05-0.5% each of Cu, Cr, and Mo and 0.01-0.05% V and in which the value of Pcm =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is regulated to <=0.25, is used. This steel is heated to 1,000 to 1,250 deg.C, and cumulative draft in the austenite unrecrystallization temp. region is regulated to >=30% and hot rolling is finished at >=800 deg.C. Successively, the steel is subjected to accelerated cooling from >=(Ar3 -50 deg.C) at (5 to 50) deg.C/sec cooling rate. Further, the steel is reheated to 750 to 870 deg.C, hardened, successively heated to a temp. not higher than the Ac1 point, and then tempered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to the production of a low yield ratio high tensile strength steel excellent in weldability and low temperature toughness as a multipurpose tank steel mainly loaded with other kinds of liquefied gas such as liquid ammonia (LAG) and LPG. It is about the method.

[0002]

2. Description of the Related Art Steel materials used for liquefied gas storage tanks vary depending on the type of liquefied gas. However, the liquefaction temperature of gas is generally low at normal pressure (-48 ° C. for LPG). Of course, excellent low-temperature toughness is also required in the welded joint. In contrast, JP-A-63-290
No. 246 discloses a method of adding 6.5 to 12.0% Ni, and Japanese Patent Application Laid-Open No. 58-153730 discloses a method of quenching and tempering a steel having a specific composition to obtain a toughness of tempered martensite and bainite. A method utilizing the property is disclosed.

[0003] Liquid ammonia is known to cause stress corrosion cracking (SCC) of steel materials.
DE 17.13 (International Co
default the Construction an
d Equipment of Ships Carry
ing Requested Gases inBul
In k), operating conditions during storage, such as oxygen partial pressure and temperature, are regulated, and the Ni content of steel is limited to 5% or less, and the actual yield strength is suppressed to 440 N / mm ² or less. Stipulates. For this reason, Japanese Unexamined Patent Publication No.
No. 3 discloses a steel sheet in which only the surface layer is softened,
Japanese Patent Application Laid-Open No. 139493 discloses a tank manufacturing method using mild steel clad steel and a soft welding final layer.

However, the tank in which the above-mentioned LPG and liquid ammonia are mixed and loaded needs to satisfy the usage required for both. On the other hand, large-capacity tanks and tanks of this type, which are often mounted on ships, are required to have high tensile strength, and are subject to superior low-temperature toughness from LPG and the upper limit of yield strength from liquid ammonia. At the same time, achieving a low yield ratio at the same time has been a major issue.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and is an excellent welding method as a method for producing a steel material for a multi-purpose tank in which other kinds of liquefied gas such as liquid ammonia (LAG) and LPG are mixed. It is an object of the present invention to provide a method for producing a steel material having both high strength and low yield ratio as well as high strength and low temperature toughness.

[0006]

DISCLOSURE OF THE INVENTION The present invention is intended to provide excellent weldability and low-temperature toughness as well as high strength and a low yield ratio.
It is characterized by sufficiently controlling the structure by limiting the steel components and the heat treatment conditions.

According to the present invention, liquid ammonia and LP
A steel material having excellent weldability can be supplied in large quantities and inexpensively for use as a mixed tank with G or the like, and particularly high strength can be provided, so that the tank can be easily mounted on a ship.

The gist of the present invention is as follows.

(1) In weight%, C: 0.05 to 0.15% Si: 0.4% or less Mn: 1.0 to 2.0% P: 0.02% or less S: 0.01% Ni: 0.05 to 1.0% Nb: 0.005 to 0.02% Ti: 0.005 to 0.025% Al: 0.06% or less N: 0.001 to 0.005% and Pcm = C + Si / 30 + Mn / 20 + Cu / 20
+ Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5
B is 0.25% or less Steel whose balance is iron and unavoidable impurities is 1000 to 1000%.
It was heated to 1250 ° C., the cumulative reduction ratio of austenite non-recrystallization temperature region is 30% or more, after the completion of the hot rolling at 800 ° C. or higher, followed by 5 ° C. from Ar ₃ -50 ° C. or more temperature / sec. ５０50 ° C./sec. Low-yield excellent in weldability and low-temperature toughness characterized by performing accelerated cooling at a cooling rate of, quenching after reheating to 750 to 870 ° C, and subsequently tempering by heating to a temperature of ₁ point or less. Manufacturing method for high tensile strength steel.

(2) The steel described above further comprises: Cu: 0.05-0.5% Cr: 0.05-0.5% Mo: 0.05-0.5% V: 0.01- Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B satisfies 0.25% or less, The method for producing a low-yield-ratio high-strength steel excellent in weldability and low-temperature toughness according to the above (1).

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the steel composition and the heat treatment conditions as a manufacturing method according to the present invention will be described.

C has the most remarkable effect on hardenability, and the lower limit of 0.05% is the minimum amount that enables the control of the structure during quenching as described later. However, if the C content is too large, the hardenability increases more than necessary, and the steel material has an adverse effect on the inherent strength, toughness balance, weldability, etc., so the upper limit was made 0.15%.

[0013] Si is an element contained in the deoxidized upper steel,
If a large amount is added, the weldability and the HAZ toughness deteriorate, so the upper limit is limited to 0.4%. Deoxidation of steel is sufficiently possible only with Ti and Al, and the lower the better, from the viewpoint of HAZ toughness, hardenability and the like, the more preferable, and it is not always necessary to add.

Mn is an indispensable element for securing strength and toughness, and its lower limit is 1.0%. However, Mn
If the amount is too large, the hardenability increases and not only deteriorates the weldability and HAZ toughness, but also promotes the center segregation of the continuous cast slab, so the upper limit was made 2.0%.

P is an impurity in the steel of the present invention.
Since a reduction in the amount tends to reduce grain boundary fracture in the HAZ, a smaller amount is preferable. If the content is high, the base material,
The upper limit is set to 0.02% in order to deteriorate the low-temperature toughness of the weld.

S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low-temperature toughness of the base material. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit was made 0.01%.

If Ni is not added excessively, weldability, H
Improves the strength and toughness of the base material without adversely affecting AZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. On the other hand, excessive addition is not only expensive but also unfavorable for welding surnames. In addition, it has been pointed out that adding a large amount of Ni may induce stress corrosion cracking (SCC) in liquid ammonia. According to the experiments by the inventors, the addition of up to 1% does not significantly deteriorate the weldability and SCC in liquid ammonia and has a greater effect of improving the strength toughness, so the upper limit was made 1.0%.

Nb is an essential element for raising the unrecrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling, and requires at least 0.005% of addition. It also contributes to the refinement of heated austenite during quenching. However, excessive addition causes toughness degradation of the welded portion, so the upper limit was made 0.02%.

Ti is essential for improving the base material and HAZ toughness. Because, when the amount of Al is small (for example, 0.003% or less), Ti combines with O to form a precipitate containing Ti ₂ O ₃ as a main component, becomes a nucleus for the formation of intragranular transformed ferrite, and improves the HAZ toughness. Let it. Further, Ti is combined with N and finely precipitated as TiN in the slab, and γ during heating is obtained.
This is because it suppresses coarsening of the grains and is effective in reducing the grain size of the rolled structure, and the fine TiN present in the steel sheet refines the HAZ structure during welding. To get these effects,
Ti must be at least 0.005%. However, if too much, TiC is formed and the low-temperature toughness and weldability are deteriorated.
The upper limit is 0.025%.

Al is an element generally contained in the deoxidized upper steel, but the deoxidation is sufficient with only Si or Ti, and the lower limit is not limited in the steel of the present invention. But A
When the amount of l increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

N, which is contained in steel as an unavoidable impurity, combines with Nb to form a carbonitride to increase the strength, and forms TiN to form a steel as described above. Enhance the nature. Therefore, the amount of N is at least 0.001.
%is necessary. However, an increase in the amount of N is extremely harmful to HAZ toughness and weldability, and the upper limit of the steel of the present invention is 0.005%.

Next, C which can be optionally contained
The reason for adding u, Cr, Mo, and V will be described.

The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be naturally restricted.

Cu exhibits almost the same effects and phenomena as Ni. The upper limit of 0.5% is not only deteriorated in weldability, but excessive addition is regulated because Cu-cracks are generated during hot rolling and production becomes difficult. You. The lower limit should be 0.05%, which should be the minimum for a substantial effect to be obtained. This is the next Cr, M
The same applies to o.

Cr and Mo both improve the strength and toughness of the base material. However, if the added amount is too large, the toughness and weldability of the base material and the welded portion are deteriorated, and the control of the structure described later becomes difficult.

V has almost the same effect as Nb, but its effect is smaller than that of Nb. Further, V also affects the hardenability, and is added from the viewpoint of controlling the structure as in the case of the above-mentioned elements. The effect similar to Nb is 0.0
If it is less than 1%, the effect is small, and the upper limit is allowable up to 0.05%.

Even if the individual components of the steel are limited, excellent properties cannot be obtained unless the entire component system is appropriate. Therefore, P
The value of cm is limited to 0.25% or less. Pcm is an index indicating weldability, and the lower the value, the better the weldability. In the steel of the present invention, if Pcm is 0.25% or less, excellent weldability can be ensured.

The reasons for limiting the steel composition as described above and further limiting the manufacturing conditions as in the present invention will be described.

The heating temperature prior to rolling is 1000 to 125
The reason for limiting the temperature to 0 ° C. is to keep the austenite grains small during heating and to make the rolling structure finer. 125
0 ° C. is the upper limit temperature at which austenite during heating does not become extremely coarse, and if the heating temperature exceeds this, austenite grains become coarsely mixed and the structure after transformation also becomes coarse, thereby significantly deteriorating the toughness of steel. . On the other hand, if the heating temperature is too low, it is difficult to secure a rolling end temperature (Ar ₃ point or more) described later. In addition, when precipitation hardening elements such as Nb and V are added, they do not form a solid solution and the balance between strength and toughness is deteriorated. For this reason, the lower limit was limited to 1000 ° C.

The steel slab heated under the above conditions was subjected to a cumulative rolling reduction in the austenite non-recrystallization temperature range of 30% or more, and after hot rolling at 800 ° C. or more, the A
r ₃ -50 ° C. or higher to 5 ° C./sec. ~ 50 ° C /
sec. Accelerated cooling at a cooling rate of By performing rolling in the austenite non-recrystallization temperature range, austenite grains are remarkably refined.
% Or more is required. After the completion of the hot rolling at 800 ° C. or higher, the temperature is continuously increased from Ar ₃ −50 ° C. or higher to 5 ° C./sec. ５０50 ° C./sec. The reason for performing the heating and cooling at the cooling rate of the above will be described below. By generating pro-eutectoid ferrite in the steel sheet by accelerated cooling from the Ar ₃ point or less, it is possible to achieve a low yield ratio of the steel sheet, but below Ar ₃ -50 ° C, coarse pro-eutectoid ferrite precipitates. Deteriorates low-temperature toughness. Therefore, the lower limit is Ar ₃ −
It was limited to 50 ° C. The cooling rate was 10 to 8
This corresponds to a cooling rate at which a steel sheet of 0 mm is mainly composed of bainite.

After the completion of the hot rolling at 800 ° C. or higher, subsequently, the temperature is reduced from Ar ₃ -50 ° C. to 5 ° C./sec.
c. ５０50 ° C./sec. The thermal processing control method of performing accelerated cooling by the cooling rate of the above has a metallurgical effect equal to or higher than reheating and quenching, and is extremely effective from the viewpoint of energy saving.

After the accelerated cooling, it is necessary to reheat to 750 to 870 ° C. and then to perform quenching, and then to perform tempering by heating to a temperature not higher than the Ac ₁ point.

The reason for reheating and quenching to 750 to 870 ° C. is to reduce the yield ratio. Generally, the Nb-added steel has a high yield ratio as it is rolled, and the yield ratio further increases when Nb precipitates as carbonitride and precipitates and hardens.

Thus, reheating and quenching are performed in the γ + α two-phase region at 750 to 870 ° C. By partially transforming the γ, the microstructure is substantially refined and the toughness is improved, and the untransformed region is softened, and the γ transformed region is hardened to form a two-phase microstructure (a soft phase and a hard phase). ), And the yield ratio can be reduced. If the reheating temperature is lower than 750 ° C, γ
The above-mentioned effects cannot be obtained because the region transformed into the sphere is small. On the other hand, if the temperature exceeds 870 ° C., the transformation region to γ becomes too large, and a desired two-phase structure cannot be obtained, so that a low yield ratio and excellent toughness cannot be achieved.

The tempering treatment is indispensable for improving the toughness of the steel and preventing the steel from being softened by welding, stress removing treatment, or the like.
However, if the temperature exceeds the Ac ₁ point, the strength is remarkably reduced. Therefore, the temperature must be lower than the Ac ₁ point (a desirable tempering temperature is 400 to 650 ° C.).

[0036]

EXAMPLE A steel plate (thickness: 15 to 50 mm) of various steel components was manufactured in a converter-continuous casting-thick plate process, and its strength, yield ratio (YR), toughness, and weldability (oblique y-shaped weld cracking) Test) was investigated.

Table 1 shows the steel composition of the steel of the present invention together with the comparative steel, and Table 2 shows the properties of the steel sheet.

The steel sheets manufactured according to the method of the present invention (the steels of the present invention) all have good properties. In contrast,
Comparative steel not according to the present invention is inferior in either property.

The comparative steel 7 has a low C content and thus has good weldability but low strength. In addition, YR is high because quenching (two-phase region heating quenching) is not performed during the heat treatment. The comparative steel 8 has a lower rolling end temperature and thus has lower strength than the steel 1 of the present invention having the same Pcm, and also has a lower Ni content, and thus is inferior in low-temperature toughness. Comparative steel 9 is inferior in weldability due to its high Pcm. In addition, YR is high due to high quenching temperature,
Further, since the Nb addition amount is low, the grain refinement of the structure is not sufficient, and the low-temperature toughness is poor. The comparative steel 10 is inferior in low-temperature toughness because it has no Ti and has not been tempered. The comparative steel 11 is inferior in weldability because it has a high C content and a high Pcm.
Further, Comparative Steels 3-1 and 3-2 having the steel component of Invention Steel 3 have a high YR because the heat treatment conditions are not appropriate.
That is, the comparative steel 3-1 has no heat treatment, and the comparative steel 3-2 has no heat treatment.
Has not been quenched (two-phase heating quenching),
Both have high YR. Further, Comparative Steel 3-3 is inferior in low-temperature toughness because the cooling start temperature is lower than Ar ₃ -50 ° C. In Comparative Steel 3-4, the cooling rate was lower than the lower limit, and the structure before reheating was a ferrite + pearlite structure. Therefore, even after the heat treatment in the two-phase region, the low-temperature toughness was poor. Comparative steel 3-
In No. 5, the cooling rate exceeded the upper limit, and thus the YR was high even after the heat treatment by reheating.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

According to the present invention, it has become possible to produce a low-yield-ratio high-tensile steel excellent in weldability and low-temperature toughness. as a result,
A steel material having excellent weldability can be supplied in large quantities and at low cost for a mixed tank of liquid ammonia and LPG and the like, and particularly high strength can be provided, so that the tank can be easily mounted on a ship.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tsuyoshi Nakamura 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works

Claims (2)

[Claims] C: 0.05 to 0.15% Si: 0.4% or less Mn: 1.0 to 2.0% P: 0.02% or less S: 0.01% or less by weight% Ni: 0.05 to 1.0% Nb: 0.005 to 0.02% Ti: 0.005 to 0.025% Al: 0.06% or less N: 0.001 to 0.005% and Pcm = C + Si / 30 + Mn / 20 + Cu / 20
+ Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5
B is 0.25% or less Steel whose balance is iron and unavoidable impurities is 1000 to 1000%.
It was heated to 1250 ° C., the cumulative reduction ratio of austenite non-recrystallization temperature region is 30% or more, after the completion of the hot rolling at 800 ° C. or higher, followed by 5 ° C. from Ar ₃ -50 ° C. or more temperature / sec. ５０50 ° C./sec. Low-yield excellent in weldability and low-temperature toughness characterized by performing accelerated cooling at a cooling rate of, quenching after reheating to 750 to 870 ° C, and subsequently tempering by heating to a temperature of ₁ point or less. Manufacturing method for high tensile strength steel. 2. The steel according to claim 1, further comprising: Cu: 0.05 to 0.5% Cr: 0.05 to 0.5% Mo: 0.05 to 0.5% V: 0.01 to 0% Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
2. The method of claim 1, wherein i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B satisfies 0.25% or less.