JPH10317097A - High tensile strength steel excellent in weld crack resistance, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mass-produceable high tensile strength steel improved in weld crack resistance without restrictions on P cm. SOLUTION: This high tensile strength steel excellent in weld crack resistance has a chemical composition containing, by weight, 0.03-0.2% C, <0.1% Si, 0.5-3% Mn, 0.005-0.03% Ti, 0.002-0.007% O (oxygen), 0-2% Cu, 0-2% Ni, 0-2% Cr, 0-1% Mo, 0-0.2% V, 0-0.2% Nb, 0-0.003% B, and Al in an amount satisfying 0.5<=Al/O (oxygen) <=1.5. The high tensile strength steel can be produced by casting a steel having the above chemical composition by continuous casting and then performing hot rolling.

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 having excellent weld cracking resistance used for bridges, buildings, penstocks, pressure vessels, shipbuilding and the like.

[0002]

2. Description of the Related Art In order to increase the size of welded structures and improve construction efficiency, steel materials used for welded structures are gradually shifting to high-strength steels having higher strength. A problem in using high-strength steel is weld cracking. Weld cracks are caused by hydrogen dissolved in the weld metal,
HAZ) and cracks occur several hours to several days after welding. It is known that welding cracks are affected by three factors, namely, the hardening ability determined by the chemical composition of the steel material, the amount of hydrogen mixed into the weld metal during welding, and the stress. Conversely, welding cracks can be prevented by appropriately reducing the effects of the above three factors. For example, preheating before welding for the purpose of reducing the amount of hydrogen and stress is effective for preventing welding cracks, but inevitably impairs welding work efficiency. In order to solve such a problem, various studies have been made on high-strength steel having low weld cracking properties.

For example, there is disclosed a technique for lowering C and increasing the strength without increasing the weld cracking property by the precipitation hardening action of VN (JP-A-56-123350). However, the VN used in the present technology is thermally unstable, and in a HAZ heated to a high temperature, the VN dissolves in the base material, so that a softened region is generated in the HAZ, and sufficient strength cannot be secured. . In addition, a large amount of V and N has a problem that the toughness of HAZ is reduced.

In addition to limiting Pcm, Ti /
TiN is produced by limiting the range of N, and HA
A steel has been disclosed which improves weld cracking resistance by suppressing the hardness of Z to a low value (Japanese Patent Publication No. 5-55584). However, although suppressing the upper limit of Pcm is effective in improving weld cracking resistance, there is a problem that it is difficult to achieve high strength of steel. In addition, although the hardness of the HAZ is reduced due to the structure refining action of the fine TiN, TiN is also reduced to VN near the melting line of the HAZ heated to a high temperature.
In this position, the hardness does not decrease and the resistance to welding cracking is not improved.

[0005] A technique for improving the resistance to weld cracking by using a Ti oxide has also been disclosed (JP-A-8-175724). According to the present technology, hydrogen is trapped at the interface between Ti ₂ O ₃ and the base material, and hydrogen is substantially reduced, so that welding crack resistance is improved. T used in this technology
Since the i-oxide (Ti ₂ O ₃ ) is more thermally stable than the nitride, its effect does not disappear even near the melting line of the HAZ exposed to a high temperature. However, although it is possible to finely and uniformly disperse Ti ₂ O ₃ in steel in a laboratory, it is not easy to actually produce Ti ₂ O ₃ at a manufacturing site.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-tensile steel which can be mass-produced and has improved weld cracking resistance without limiting Pcm. Specifically, the object is to secure the following performance.

[0007] 1. Weld cracking resistance: The experimental value of the crack stop preheating temperature is lower than the temperature predicted from Pcm by the Japan Welding Society (WES) standard WES 3002.

[0008]

[0009]

Means for Solving the Problems The inventors of the present invention carry out a test focusing on the effect of preventing a composite oxide from being welded cracking stably and uniformly and finely dispersed in steel, and confirm the following items. Was completed.

(A) When one or more of Al-Mn-based, Mn-Ti-based or Al-Mn-Ti-based composite oxides are formed in steel, weld cracking resistance is significantly improved. You.

(B) In order to finely disperse the composite oxide of (a) in steel, it is necessary to include Si, O (oxygen), Al, Ti and the like in an appropriate range. In particular, it is essential to reduce Si.

(C) In order to maintain the strength and toughness of the base material and the HAZ in a well-balanced manner while maintaining excellent weld crack resistance, the other alloy elements of the base material must be in an appropriate range.

The present invention has been completed by combining the above-mentioned items and conducting tests such as smelting and casting of high-tensile steel at a manufacturing site. The gist of the present invention lies in steel having the following chemical composition and a method of manufacturing the same. .

(1) C: 0.03-0.2% by weight
%, Si: less than 0.1%, Mn: 0.5 to 3%, Ti:
0.005 to 0.03%, O (oxygen): 0.002 to
0.007%, Cu: 0 to 2%, Ni: 0 to 2%, C
r: 0 to 2%, Mo: 0 to 1%, V: 0 to 0.2%, N
b: 0 to 0.2%, B: 0 to 0.003%, and a high-strength steel containing Al satisfying the following formula and having excellent weld cracking resistance.

0.5 ≦ Al / O (oxygen) ≦ 1.5 (2) After the steel having the chemical composition described in the above (1) is cast by a continuous casting method, A method for producing high-strength steel that has excellent resistance to weld cracking that is rolled.

Here, the "high-tensile steel" includes a thick steel plate, a hot-rolled steel plate (hot coil), a steel bar, a bar steel, and the like. "Hot rolling" includes both hot rolling for simply adjusting the shape and hot rolling for obtaining a fine structure by working. After hot rolling, (a) accelerated cooling without cooling, (b) tempering after accelerated cooling or direct quenching, or (c) after cooling, Heat treatment such as quenching and tempering may be performed.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The most important feature of the present invention is that Al-M
n-based, Mn-Ti-based and Al-Mn-Ti-based
One or two or more of the complex oxides are finely dispersed in steel. These three types of oxides can be uniformly and finely dispersed in steel by limiting the chemical composition of steel, particularly Si, O, Al, and Mn, to the following ranges.

1. Chemical composition Si: less than 0.1% Si is usually added to steel as a deoxidizing agent, and is also an effective element for increasing the strength. However, in the present invention, Si is not desirable from the viewpoint of improving weld cracking resistance. 0.1% of Si
When it is contained as described above, a Si-Mn-based oxide is generated, which inhibits formation of a composite oxide Al-Mn-based, Mn-Ti-based or Al-Mn-Ti-based oxide effective for weld cracking resistance.
As a result, sufficient welding crack resistance cannot be secured, so that Si
Is less than 0.1%. Desirably, it is 0.07% or less. On the other hand, deoxidation and strength increase are possible with elements other than Si, so Si need not always be added,
Therefore, the lower limit is not particularly limited.

Mn: 0.5 to 3% Mn is an essential element in the present invention for securing a sufficient amount of the composite oxide and ensuring strength. If it is less than 0.5%, the amount and strength of the composite oxide cannot be sufficiently secured. On the other hand, if it exceeds 3%, the hardenability is excessively increased and the weld crack resistance is deteriorated. In addition, since the center segregation is generated in the continuous casting slab manufactured by the continuous casting method commonly used for manufacturing a high-tensile steel sheet, the upper limit is set to 3%.

Ti: 0.005 to 0.03% Ti is an essential element in forming a composite oxide in the present invention. Residual T other than that of forming composite oxide
i also has the effect of forming TiN and improving the toughness of HAZ. If Ti is less than 0.005%, these effects cannot be sufficiently obtained. On the other hand, if it exceeds 0.03%, HAZ
0.005 to 0.03
%.

O (oxygen): 0.002 to 0.007% Oxygen needs to be contained in an amount of 0.002% or more to form a composite oxide. If it is less than 0.002%, even if a desired composite oxide is obtained, a sufficient amount of the composite oxide to improve weld cracking resistance cannot be obtained. On the other hand, 0.00
Since oxygen in excess of 7% impairs the cleanliness of the steel and causes a deterioration in ductility and toughness, the oxygen content should be 0.002 to 0.002%.
7%.

0.5 ≦ Al / O (oxygen) ≦ 1.5... Al has a higher affinity for oxygen than Ti and Mn. Therefore, when Al / O exceeds 1.5, almost all of the oxygen in the steel reacts with Al to form Al ₂ O ₃ . For this reason, a composite oxide cannot be obtained, and the effect of improving weld cracking resistance is lost. On the other hand, when Al / O is less than 0.5, deoxidation is not sufficiently performed due to lack of Al, and the toughness of the base material is deteriorated.

In the present invention, Al is an essential element for forming a composite oxide and for obtaining excellent base metal toughness, and its content depends on the oxygen concentration in the steel as in the above formula. Limited. After refining molten steel in a converter or the like, the amount of Al to be added is determined according to the oxygen content obtained by online analysis, taking into account the yield, so that the formula is satisfied. Is desirably charged into molten steel. Recent advances in Al addition technology have improved the accuracy of the Al content to a range of 0.002% by weight. Completion of the present invention largely depends on such advances in Al addition technology.

C: 0.03-0.2% C is added to obtain the strength of the base material. C is 0.03
%, Sufficient strength cannot be obtained, while 0.2%
If it exceeds 0.2%, the hardness of the HAZ is significantly increased, and the resistance to weld cracking is significantly degraded.

In addition to the above alloying elements, the steel of the present invention
The following optional elements are contained in the following ranges mainly in order to secure the strength and toughness of the base material.

Cu: 0 to 2% Cu may not be contained. However, Cu increases the strength of the steel without impairing the low-temperature toughness, and is included when high strength and high toughness are required. If the content is less than 0.05%, the effect is small.
% Is desirable. On the other hand, if it exceeds 2%, the surface properties of the steel are significantly deteriorated, so the upper limit is made 2%.

Ni: 0 to 2% Ni may not be contained. However, Ni enhances both the strength and toughness of the steel, so it is included in the case of high tensile steel used in cold regions. If the content is less than 0.05%, the effect is small. Therefore, when Ni is contained, the content is preferably set to 0.05% or more. On the other hand, if it exceeds 2%, the flow of molten metal at the time of welding is hindered and the welding efficiency in large heat input welding deteriorates, so the upper limit is made 2%.

Cr: 0 to 2% Cr may not be contained. However, Cr is effective in improving the strength of steel and in improving the corrosion resistance to carbon dioxide gas, so Cr is included when these properties are secured. 0.05
%, The effect is small.
% Is desirable. On the other hand, if it exceeds 2%, the resistance to weld cracking is degraded due to excessive hardenability, so the upper limit is made 2%.

Mo: 0 to 1% Mo may not be contained. However, Mo improves the hardenability and improves the strength and toughness of the steel. Therefore, when the thickness of the steel is large, Mo is included up to the center to obtain a desired structure. 0.
If it is less than 02%, the effect is small, and if it is contained, it is desirable to make it 0.02% or more. On the other hand, if it exceeds 1%, the baking often occurs too much and the base material, HA
Since the toughness of both Z deteriorates, the upper limit is set to 1%.

V: 0 to 0.2% V may not be contained. However, V slightly increases the strength of steel, so it is included when high strength is required.
If the content is less than 0.005%, the effect is small. Therefore, when V is contained, the content is desirably 0.005% or more. On the other hand, if it exceeds 0.2%, precipitation hardening of V carbonitride occurs excessively and the strength is secured, but the toughness is deteriorated in a part of the base metal and HAZ, so the upper limit is made 0.2%.

Nb: 0 to 0.2% Nb may not be contained. However, since Nb has the effect of improving the strength and toughness of the base material and preventing the softening of the HAZ, Nb is included in the case of steel subjected to high heat input welding.
If the content is less than 0.005%, the effect is small. Therefore, when the content is included, the content is preferably 0.005% or more. On the other hand, if it exceeds 0.2%, the surface properties of the continuously cast slab are significantly deteriorated, so the upper limit is made 0.2%.

B: 0 to 0.003% B may not be contained. However, B improves the hardenability of steel in a very small amount, and is included when the thickness of steel is large.
If the content is less than 0.0001%, the effect is small. Therefore, if the content is included, the content is preferably 0.0001% or more.
On the other hand, if it exceeds 0.003%, the toughness of the base material is deteriorated, so the upper limit is made 0.003%.

In addition to the above-mentioned optional alloying elements, well-known alloying elements having a known effect, for example, Ca, La, Ce or the like may be added for controlling the form of inclusions.

It is desirable that P and S of the impurity elements be limited to the following ranges.

P: 0.03% or less The impurity element P inevitably contained in steel is preferably as low as possible to secure toughness, and its upper limit is preferably set to 0.03%.

S: 0.03% or less The impurity element S inevitably contained in steel improves the steel quality,
It is desirable to set the upper limit to 0.03% in order to prevent lamella tear. 2. By setting the composite oxide Si, Al, Mn, O, and Ti within the above range, an Al—Mn system, a Mn—Ti system, or an Al—M
A steel in which the n-Ti-based composite oxide is finely dispersed can be obtained. Further, by adjusting the content of each alloy element within the above range, the amount of the composite oxide can be adjusted.

It is considered that the reason why the weld cracking property is improved when these oxides are formed is due to the refinement of the structure. The structure is refined by both the effect of suppressing grain growth when heated at a high temperature and increasing the density of transformed nucleus sites during cooling, and as a result, hardening ability is reduced and hardening is difficult. In ordinary Al-killed steel, almost all of the oxide is formed as Al ₂ O ₃ by a primary crystal reaction. On the other hand, in the case of steel having the above composition range, the above three types of composite oxides are formed by a monotectic reaction, so that these composite oxides are less likely to cause agglomeration and coarsening than Al ₂ O ₃ . Since these composite oxides are spherical in shape, they increase the interface area between the composite oxide and steel in addition to the effect of fine dispersion. For this reason, it is considered that the number of transformation nucleus generation sites increased, the hardening ability decreased, and the weld crack resistance improved.

In the production of the steel of the present invention, it is desirable that the cooling rate during solidification be high from the viewpoint of fine dispersion of the composite oxide. In the case of a continuous casting method in which the section thickness of a slab normally used for the production of this type of steel is 100 to 250 mm, the cooling rate falls within the range of a desired cooling rate. A finely dispersed composite oxide cannot be obtained, and sufficient welding crack resistance cannot be obtained. The composite oxide obtained in this way does not significantly change during rolling and heat treatment. The average density of the composite oxide is desirably in the range of 3 to 60 particles / mm ² within the visual field of the optical microscope as long as the above three types of composite oxides are used regardless of the type. If it is less than 3 pieces / mm ² , the resistance to weld cracking is insufficient, and if it exceeds 60 pieces / mm ² , ductility and toughness decrease, and it cannot be used for welded steel structures.

3. Manufacturing Method When the steel of the present invention is cast by a continuous casting method, the solidification rate is in an appropriate range, and the above three types of composite oxides are finely and uniformly dispersed, thereby greatly improving weld cracking resistance. The limit of the solidification rate at which the composite oxide is not uniformly and finely dispersed is estimated to be about 300 mm, the minimum crossing thickness of the slab section.
In the normal continuous casting method, the passing thickness of the slab section is 250.
mm or less, fine and uniform dispersion of the composite oxide is ensured. The composite oxide uniformly finely dispersed at the time of solidification does not substantially change by subsequent hot rolling or heat treatment, and effectively maintains weld cracking resistance.

The hot rolling is performed to adjust the shape of the steel or to refine the structure by rolling and recrystallization. In order to further enhance the balance between strength and toughness, accelerated cooling may be performed after hot rolling, or accelerated cooling or direct quenching may be followed by tempering, or, after hot rolling, left to cool and reheat. Heat treatment may be performed.

[0041]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

Tables 1 and 2 show the chemical composition of the test steel melted in a 200 kg vacuum refining furnace. 20 molten steel
Cast into 0kg square mold, minimum cross-section thickness 15
It was a 0 mm steel ingot.

[0043]

[Table 1]

[0044]

[Table 2]

The obtained steel ingot was hot rolled into a 30 mm steel sheet. The finishing temperature of the hot rolling was 850 ° C., after the finish rolling, accelerated cooling was performed at a cooling rate of 35 ° C./sec at the center of the thickness, cooling was stopped at 350 ° C., and the material was allowed to cool. Some test steels were further tempered at 625 ° C.

For the evaluation of weld cracking, a welding crack test was conducted in accordance with JIS Z3158, and a preheating temperature at which cracking did not occur in the HAZ was determined. A commercially available low hydrogen type 80 kgf / mm ² class coated welding rod was used for placing the test bead in the welding crack test, and the welding heat input was 17 kJ / cm. The diffusible hydrogen concentration of the deposited metal welded under the same conditions as this welding was determined according to JIS Z311.
As a result of measurement by the glycerin method described in 3,
It was 3.3 cc per 00 g.

The toughness of HAZ was examined using a reproducible heat cycle material. 11 mm from each test steel in Tables 1 and 2
A reproducible heat cycle test piece of the corner was sampled, a reproducible welding heat cycle was applied using a high frequency heating device, and a Charpy impact test was performed. The given heat cycle is the maximum heating temperature 1400 ℃
(Holding time: 3 seconds), and the cooling time from 800 ° C. to 500 ° C. was 60 seconds. The Charpy impact test was performed at −40 ° C., and the absorbed energy at −40 ° C. was determined by the average value of three test pieces, and the HAZ toughness was evaluated. This condition is 30m thick
m corresponds to a heat cycle in the vicinity of the melting line when submerged arc welding is performed on a steel sheet having a welding heat input of 100 kJ / cm. Table 3 shows the mechanical properties of the base material, the crack initiation stop temperature, and the results of the Charpy test of the reproduced heat cycle material.

[0048]

[Table 3]

As shown in Table 3, in the comparative example, the base material toughness,
Either the crack initiation stop temperature or the reproduced thermal cycle toughness was inferior.

In steel number 3 (hereinafter referred to as “comparative example”) as a comparative example, since the Si exceeds the upper limit defined in the present invention, a desired composite oxide cannot be obtained, and thus the crack generation stopping temperature. No improvement was observed.

In Comparative Example 4, a sufficient amount of oxide was not obtained because oxygen was less than the lower limit of the present invention, and no improvement was observed in the crack generation stopping temperature.

In Comparative Example 6, Al / O exceeded the upper limit of the present invention, so that most of the oxides in the steel were Al ₂ O ₃ , and no improvement in the crack generation stop temperature was observed.

In Comparative Example 7, the oxide was not finely dispersed because Ti was less than the lower limit of the present invention, and no improvement was observed in the crack generation stopping temperature.

In Comparative Example 9, since the content of Ti exceeded the upper limit of the present invention, no improvement in weld cracking resistance was obtained.
The toughness also deteriorated.

In Comparative Example 10, since Al / O was less than the lower limit of the present invention, sufficient deoxidation was not performed, and the toughness of the base material was deteriorated, and the HAZ toughness was also deteriorated accordingly.

In Comparative Example 13, since the oxygen content exceeded the upper limit of the present invention, the oxide was coarsened and the toughness of the base material and HAZ was deteriorated.

In Comparative Example 14, since Al / O exceeded the upper limit of the present invention, most of the oxides in the steel were Al ₂ O ₃ , no improvement in the crack stop temperature was observed, and the HAZ toughness was deteriorated. .

In Comparative Example 16, the desired oxide was not obtained because Si exceeded the upper limit of the present invention, and no improvement was observed in the crack stop temperature.

On the other hand, in the steel of the present invention, the crack initiation temperature obtained by the experiment is shifted to the lower temperature side with respect to the crack initiation stop temperature obtained by the calculation, and it is clear that the steel has excellent weld cracking resistance. It is. Further, the steel of the present invention had excellent base metal toughness and HAZ toughness.

[0060]

According to the present invention, a high-strength steel having excellent resistance to weld cracking can be stably and inexpensively provided without limiting Pcm. As a result, it is possible to greatly improve the welding efficiency of a welded structure using high-tensile steel.

Claims (2)

[Claims] (1) C: 0.03 to 0.2% by weight, S
i: less than 0.1%, Mn: 0.5 to 3%, Ti: 0.0
05 to 0.03%, O (oxygen): 0.002 to 0.00
7%, Cu: 0 to 2%, Ni: 0 to 2%, Cr: 0 to 2
%, Mo: 0 to 1%, V: 0 to 0.2%, Nb: 0 to 0%
A high-strength steel excellent in weld crack resistance, characterized by being a steel containing 0.2%, B: 0 to 0.003% and Al satisfying the following formula. 0.5 ≦ Al / O (oxygen) ≦ 1.5 2. A method for producing a high-strength steel excellent in weld crack resistance, comprising: casting a steel having the chemical composition according to claim 1 by a continuous casting method, followed by hot rolling.