JP2001064749A - Hic-resisting non-heat treated high tensile strength steel product excellent in toughness in weld heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively improve toughness in weld heat-affected zone by a simplified process without deteriorating HIC resistance and welding efficiency by providing a specific composition in which Si content is regulated to values in a proper range and also ACR as an index to fixation of S is controlled to values in a specific range. SOLUTION: The HIC-resisting non-heat treated high tensile strength steel product has a composition which consists of, by weight, 0.03-0.08% C, 0.05-0.15% Si, 0.30-1.60% Mn, <=0.020% P, <=0.015% S, 0.01-0.10% Al, 0.005-0.06% Nb, 10-40 ppm Ca, <=0.007% N, further one or more kinds among 0.05-1.30% Cu, 0.05-10.0% Ni, 0.05-1.50% Cr, 0.03-0.50% Mo, 0.010-0.15% V, and 0.005-0.050% Ti, and the balance iron and in which the value of ACR defined by equation ACR =0.8×Ca-(130Ca+0.18)×O)/S} is regulated to 1.0 to 3.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-HIC non-refined high-strength steel material having a tensile strength of 500 MPa or more used for applications such as line pipes, pressure vessels and tanks used in low-temperature environments below freezing. In particular, it relates to a HIC-resistant non-tempered high-strength steel with excellent toughness in the welded HAZ.

[0002]

2. Description of the Related Art Normally, when welding high strength steel, the heat affected zone (HAZ), particularly the portion in contact with the weld metal called BOND, undergoes γ transformation and grain growth due to its heat history. The organization becomes significantly coarse. For this reason, the low-temperature toughness of the welded HAZ is significantly reduced as compared with the base metal, causing brittle fracture.

[0003] As measures to prevent this problem, conventionally, TiN or RE
There is known a method in which M-based inclusions are finely distributed in steel, the growth of γ grains is suppressed by the Pinning effect, and as a result, the structure of the BOND portion is refined. In addition, efforts have been made to ensure the low-temperature toughness of the BOND portion by modifying the welding method, for example, by reducing the welding heat input.

[0004] In addition, Japanese Patent Application Laid-Open No. 8-165518 discloses that a steel having a component composition in consideration of sour resistance is subjected to a strong working at a central portion of a sheet thickness by combining temperature difference rolling in the thickness direction and large reduction in each pass. A method for producing a sour-resistant steel sheet having excellent HIC resistance and low-temperature toughness, which comprises applying accelerated cooling after rolling, has been proposed.

[0005]

However, in the method using TiN, since the temperature at which TiN dissolves is lower than the maximum temperature in the BOND portion, a sufficient effect cannot be obtained. If used, it becomes the starting point of HIC, and there is a problem that the HIC resistance is reduced. Further, the method relying on the welding method has a problem that the welding efficiency is reduced and the method is not economical. Further, the method disclosed in Japanese Patent Application Laid-Open No. 8-165518 discloses that, after heating, after rolling to a thickness of 2 t or more, the water density is 0.5 to 2.0 m ³ / m ² · min at a temperature of 950 ° C. or more, and the water cooling time is Cooling of 10 to 50 s is performed, and after that, it is necessary to perform rolling in which the rolling reduction per pass is 10% or more for 60% or more of the number of passes within 300 s. I was

The present invention has been developed in view of the above circumstances, and can effectively improve the toughness of a welded HAZ portion without deteriorating HIC resistance and welding efficiency and by a simple process. The purpose of this study is to propose a HIC-resistant non-tempered high-strength steel material with excellent toughness in the welded HAZ.

[0007]

Means for Solving the Problems Now, the present inventors have conducted various experiments to achieve the above object, and as a result, the deterioration of the toughness in the welded HAZ portion is caused by the formation of island-like martensant in the BOND portion. It turned out to be due. Therefore, next, we conducted intensive research to suppress the formation of such island-like martensant, and the intended purpose was advantageously achieved by adjusting the composition of steel materials, especially by limiting the Si content to an appropriate range. The knowledge of what is achieved is obtained. In the course of the above experiment, it was also found that controlling the ACR, which is an index for fixing S, to a predetermined range is extremely effective in securing HIC resistance. The present invention is based on the above findings.

That is, the present invention relates to a method for producing C: 0.03 to 0.08 wt.
%, Si: 0.05 to 0.15 wt%, Mn: 0.30 to 1.60 wt%, P: 0.
020 wt% or less, S: 0.0015 wt% or less, Al: 0.01 to 0.10 wt
%, Nb: 0.005 to 0.06 wt%, Ca: 10 to 40 ppm and N:
0.007 wt% or less and specified by the following formula (1)
R satisfies the range of 1.0 to 3.0, ACR = 0.8 × {Ca− (130 Ca + 0.18) × O) / S} (1) Further, Cu: 0.05 to 1.30 wt%, Ni: 0.05 to 10.0 wt%, Cr:
0.05-1.50 wt%, Mo: 0.03-0.50 wt%, V: 0.010-0.
15% by weight and at least one selected from the group consisting of 0.005 to 0.050% by weight of Ti, and the balance being iron and unavoidable impurities. Tempered high-tensile steel.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the composition of the steel material is limited to the above range in the present invention will be described. C: 0.03 to 0.08 wt% C is an element necessary for securing hardenability and strength, and therefore, it is necessary to add at least 0.03 wt%. However, if the added amount exceeds 0.08 wt%, HIC resistance
Since the characteristics are deteriorated, the upper limit of the amount of addition is 0.08 wt%.

Si: 0.05-0.15 wt% Si is a particularly important element in the present invention. That is, Si is related to the formation of island-like martensite in the BOND part, and by reducing the amount of addition, it is possible to suppress the formation of island-like martensite, which lowers the toughness. The toughness can be improved.
Figure 1 shows that 0.06wt% C-1.5wt% Mn-0.010wt% P-0.0009wt% S
In the component system of the present invention based on -0.020wt% Al-0.015wt% Nb-200ppmCa-0.004wt% N, when the Si content is variously changed, the Si content and the heat history of the BOND part are FIG. 9 shows the results of an investigation on the relationship between the simulated (reproduced) thermal cycle and the transition temperature of the Charpy test. As shown in the figure,
By suppressing the Si content to 0.15 wt% or less (preferably 0.11 wt% or less), the toughness of the BOND portion can be significantly improved.

Further, in order to improve the HIC resistance, it is necessary to control the ACR value to be described later within a predetermined range. For this purpose, it is important to reduce the oxygen content in the steel. FIG. 2 shows the relationship between Si content and oxygen content in steel.
The result of the examination using the same steel as in the case of is shown. Here, from the viewpoint of such deoxidation, the Si content is important, and the content is 0.
If it is less than 05 wt%, sufficient deoxidation cannot be expected. For the above reasons, the Si content is preferably 0.05 to 0.15 wt%, more preferably 0.05 to 0.
It was limited to the range of 11 wt%.

Mn: 0.30-1.60 wt% Mn is effective for improving the strength without impairing the toughness. For this purpose, at least 0.30 wt% must be added. However, an excessive addition deteriorates the HIC resistance, so the upper limit is set to 1.60 wt%.

P: not more than 0.020 wt% P segregates in the continuous segregation center segregation part and hardens the structure, thereby deteriorating the HIC resistance. The allowable value changes depending on the balance with C and Mn, which degrade the HIC resistance by the same mechanism. However, in the range of C and Mn described above, the allowable upper limit is 0.020 wt%.
It is.

S: 0.0015 wt% or less S mainly combines with Mn to form a MnS compound.
Is the starting point of HIC, so it is necessary to minimize its contamination. This S is harmless to HIC by addition of Ca
Although it is possible to make a CaS compound, the content is 0.00
If the content exceeds 15 wt%, there is a possibility that MnS may be generated due to insufficient Ca, so the S content was limited to 0.0015 wt% or less.

Al: 0.01 to 0.10 wt% Al must be contained in an amount of 0.01 wt% or more to deoxidize the steel and refine the structure. Is generated in a large amount and the toughness of the steel is greatly deteriorated. Therefore, in the present invention, the upper limit is set to 0.10 wt%.

Nb: 0.005 to 0.06 wt% Nb is an element that prevents the austenite grains from becoming coarse during slab heating, and is effective in reducing the grain size and strengthening during rolling.
In order to exhibit these effects, 0.005 wt% or more is required. However, the addition of more than 0.06 wt% deteriorates the toughness of the welded HAZ, so the Nb content is 0.005%.
Limited to the range of ~ 0.06 wt%.

Ca: 10 to 40 ppm Ca combines with S to form CaS harmless to HIC,
Thereby, generation of MnS can be suppressed. And
To ensure HIC resistance, 10 ppm or more is required. However, excessive addition causes an increase in the amount of oxide-based compounds, degrades HIC resistance and extremely reduces the life of refractories, so the upper limit was set to 40 ppm.

N: 0.007 wt% or less N combines with Al to form AlN, and has an effect of preventing the crystal grains from becoming coarse during slab heating. However, if contained in a large amount, the toughness of the welded HAZ is deteriorated. Therefore, in the present invention, the N content is set to 0.007 wt% or less.

ACR [0.8 × ΔCa− (130 Ca + 0.18) ×
O) / S｝] = 1.0 to 3.0 The ACR represented by the above formula is an index indicating the susceptibility of the HIC to cracking, and indicates the excess or deficiency of Ca effective for fixing S as CaS. If this index is small, MnS will be generated
HIC crack susceptibility increases. To prevent this, set the ACR
Must be at least 1.0. On the other hand, if this index is large, the amount of oxide inclusions increases, and as a result, the susceptibility to HIC cracking increases. To prevent this, the ACR must be less than 3.0. Therefore, the ACR value was controlled in the range of 1.0 to 3.0.

In the present invention, in addition to the above elements, at least one selected from the group consisting of Cu, Ni, Cr, Mo, V and Ti is further contained. Cu: 0.05-1.30 wt% Cu is an element useful for solid solution strengthening and precipitation strengthening.
To obtain the former effect, it is necessary to add 0.05 wt% or more, while to obtain the latter effect, it is necessary to add 0.5 wt% or more. However, if it exceeds 1.3 wt%,
Neither enhancement can be expected to have any further effect, but rather is costly.

Ni: 0.05-10.0 wt% Ni is useful not only for improving toughness but also for effectively producing low temperature steel by shifting the ductile / brittle transition temperature to a lower temperature side. In order to obtain these effects, it is necessary to add 0.10 wt% or more. However,
Even if more than 10.0 wt% is added, more useful low-temperature steel cannot be obtained.

Cr: 0.05-1.50 wt% Cr is an element useful for securing the strength of steel, and for this purpose, 0.05 wt% must be added. However, excessive addition degrades weldability, so the upper limit is 1.
50 wt%.

Mo: 0.03 to 0.50 wt% Mo is an element useful for improving the strength and toughness of steel with a small amount of addition, and for that purpose, it is necessary to add 0.03 wt% or more. . However, excessive addition causes deterioration of weldability, so the upper limit was made 0.50 wt%.

V: 0.010 to 0.15 wt% V is an element useful for increasing the strength by precipitation strengthening.
In order to exhibit this effect, it is necessary to add 0.010 wt% or more. However, the addition exceeding 0.15 wt% causes deterioration of the weldability and the toughness of the welded HAZ.

Ti: 0.005 to 0.050 wt% Ti is useful not only for preventing coarsening of the welded portion, but also for improving the strength by precipitation strengthening. To exert these effects, at least 0.005 wt% must be added, but if it exceeds 0.050 wt%, the toughness is deteriorated.

Next, preferred production conditions of the present invention will be described. In the present invention, when rolling the steel, it is preferable to finish the rolling at _three or more Ar points. This is because, when rolling is completed at a temperature lower than the Ar ₃ point, the precipitated ferrite is processed to increase the transition density, thereby increasing the hardness of the ferrite and increasing the crack susceptibility of the HIC. .

After the above-mentioned rolling is completed, in the vicinity of the segregated portion, C, which cannot be completely dissolved, is discharged from the precipitated ferrite, and the untransformed segregated portion (the segregated portion is such that C, Mn and the like are densely segregated). C is moved to a lower Ar ₃ transformation point), the C concentration in the segregated portion increases, and the hardness of the segregated portion increases. If the cooling rate after the end of rolling is low, the above phenomenon becomes remarkable and the HIC sensitivity increases. Therefore, it is preferable to accelerate cooling at a rate of 5 ° C./s or more after rolling. However, even if the cooling rate is too high, a low-temperature transformed structure with high HIC sensitivity such as mantensite is formed.
/ s is desirable.

In order to obtain a high-strength material having a tensile strength of 500 MPa or more in the above-mentioned accelerated cooling, the cooling stop temperature must be
The temperature is preferably set to 650 ° C. or lower. Nevertheless, if the cooling stop temperature is too low, a low-temperature transformed structure having high HIC sensitivity such as mansite is generated, so the lower limit is preferably set to about 350 ° C.

[0029]

EXAMPLES Steels having the composition shown in Table 1 were hot-rolled under the conditions shown in Table 2. Test pieces were taken from each of the steel sheets thus obtained and subjected to a tensile test and an HIC resistance test. In addition, a welded joint was manufactured using each steel plate, and a welded joint impact test was performed. The obtained results are also shown in Table 2. In the tensile test, A
PI rectangular test pieces were sampled in the direction perpendicular to the rolling direction and performed. In addition, the weld joint impact test was conducted using the JI
An S 4 test piece was sampled, and a 2 mm V notch was applied to the BOND portion. Welding is about 40 kJ / cm with SAW welding method at K groove
It carried out on condition of. Furthermore, the HIC resistance test is based on the NACE standard
The method was performed according to the method specified in [0284]. Three test pieces were collected from the steel plate at a width of 1/4, and the test solution (solution A) was used.
After immersion in 96Hr, measure CLR (crack length ratio) and measure HIC resistance
The properties were evaluated.

[0030]

[Table 1]

[0031]

[Table 2]

As shown in Table 2, all of the conforming examples according to the present invention have excellent HIC resistance, and have excellent low-temperature toughness with a fracture transition temperature (vTrs) of -31 ° C or less in a welded joint impact test. On the other hand, none of the comparative examples was able to achieve both the HIC resistance and the impact toughness of the welded joint.

As described above, in the examples, the tensile strength is mainly
Although a 600 MPa class steel material has been described, it goes without saying that the present invention is not limited to this, but can be applied to a wide range of steel materials having a tensile strength of 500 to 800 MPa class.

[0034]

As described above, according to the present invention, it is possible to effectively improve the toughness of the BOND portion while securing the HIC resistance, and as a result, the weld HAZ has excellent low-temperature toughness and HIC resistance. A non-heat treated high-strength steel sheet can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of Si in steel and vTrs.

FIG. 2 is a graph showing the relationship between the amount of Si and the amount of O in steel.

Claims (1)

[Claims] 1. C: 0.03 to 0.08 wt%, Si: 0.05 to 0.15 wt%, Mn: 0.30 to 1.60 wt%, P: 0.020 wt% or less, S: 0.0015 wt% or less, Al: 0.01 to 0.10 wt% , Nb: 0.005 to 0.06 wt%, Ca: 10 to 40 ppm, and N: 0.007 wt% or less, and the ACR defined by the following formula (1) satisfies the range of 1.0 to 3.0, and ACR = 0.8 × {Ca− (130 Ca + 0.18) × O) / S} --- (1) Cu: 0.05-1.30 wt%, Ni: 0.05-10.0 wt%, Cr: 0.05-1.50 wt%, Mo: 0.03 0.50 wt%, V: 0.010-0.15 wt% and Ti: at least one selected from the group consisting of 0.005-0.050 wt%, with the balance being a composition of iron and unavoidable impurities. Excellent HAZ toughness
HIC Non-heat treated high strength steel.