JPH11293383A - Thick steel plate minimal in hydrogen induced defect, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent hydrogen induced defects liable to occur in an extra thick steel plate after rolling or after accelerated cooling and hardening treatment with respect to the extra thick steel plate. SOLUTION: This steel plate is a thick steel plate in which the proportion of the elements (excluding 0) constituting oxides satisfies (Ti+Mg)>=60 atomic % and the oxides having 0.05 to 5.0 μm grain size are contained by 10 to 500 pieces per mm<2> . This thick steel plate has a composition consisting of, by weight, 0.04-0.2% C, 0.02-0.5% Si, 0.6-2% Mn, <=0.02% P, <=0.02$ S, 0.05-0.025% Ti, 0.0002-0.005% Mg, <=0.01% Al, 0.001-0.006% N, 0.005-0.008% O, and the balance Fe with inevitable impurities. Further, Cu, Ni, Cr, Mo, Nb, V, B Ca, and REM can be incorporated into the composition, if necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for preventing hydrogen-induced defects, which are likely to occur in an extremely thick steel sheet or a high-tensile steel sheet after rolling or accelerated cooling or quenching, in the steel sheet.

[0002]

2. Description of the Related Art There have been many studies on techniques for preventing hydrogen-induced defects in thick steel plates. For example, "Iron and Steel", 62nd year (1976), No. 13, 1708-1719
Pp. 64 (1978), No. 9, 1343-1352
The page reports the results of a detailed study of the behavior of hydrogen in steel and hydrogen deficiencies. Hydrogen defects are caused by pseudo-brittle fracture caused by the action of the internal stress of the steel sheet due to the local increase in the concentration of diffusible hydrogen due to the accumulation of hydrogen existing in the steel in the micro-segregated portion. Alternatively, cracks may be generated by accumulating on non-compressed Zaku or coarse inclusions such as MnS and generating a high pressure as a gas.

The hydrogen deficiency prevention methods in the prior art described in the above-mentioned documents and the like are summarized in the following four points. (1) reduce the hydrogen concentration in the molten steel in the steelmaking stage; (2) reduce segregation generated during casting as much as possible; or reduce segregation by soaking diffusion treatment.
(3) A slab or a rolled thick steel plate is subjected to a hydrogen diffusion heat treatment in a ferrite region. (4) A high-pressure ratio or a high-shape-ratio rolling is performed in order to eliminate a non-compression-bonded zac that becomes a hydrogen accumulation site.

[0004]

As described above, the prior art has established means for preventing hydrogen defects in thick steel plates. Among them, in (1), the hydrogen concentration in molten steel is reduced. Stable control at a low value causes an increase in cost in the steel making stage, and there is an industrial limit. In (2), segregation can be reduced by adopting a special casting method in continuous casting, but productivity is liable to be impaired, such as lowering the casting speed. Furthermore, the soaking diffusion treatment for reducing segregation requires, for example, treatment at 1250 ° C. or more for 10 hours or more, resulting in productivity loss and cost increase. In (3), it is necessary to diffuse hydrogen in the ferrite region. For example, heat treatment is performed at a low temperature of 600 ° C. or less, so hydrogen diffusion is slow, and heat treatment is necessarily performed for a long time. Cheap. In the case of (4), particularly, a very thick steel plate which is difficult to roll under high pressure or a high tensile strength steel which is difficult to roll at a high shape ratio (for example, having a tensile strength of 78
(0 MPa or more), it is difficult to perform such rolling, and in order to prevent hydrogen defects, a dehydrogenation heat treatment of a slab or a rolled steel sheet is essential.

[0005] The present invention is an epoch-making technology that can prevent hydrogen defects in a thick steel sheet without reducing productivity or increasing costs.

The gist of the present invention is as follows.

(1) Elements constituting oxide (however, O
Is a steel that satisfies (Ti + Mg) ≧ 60% and contains 10 to 500 oxides per square mm having a particle size of 0.05 to 5.0 μm. Steel plate with few hydrogen defects.

(2) 0.04 ≦ C ≦ 0% by weight.
2, 0.02 ≦ Si ≦ 0.5, 0.6 ≦ Mn ≦ 2, P ≦
0.02, S ≦ 0.02, 0.005 ≦ Ti ≦ 0.02
5, 0.0002 ≦ Mg ≦ 0.005, Al ≦ 0.0
1, 0.001 ≦ N ≦ 0.006, 0.0005 ≦ O ≦
The steel sheet according to the above (1), wherein the steel sheet contains 0.008 and the balance is Fe and inevitable impurities.

(3) In addition to the steel, a group of elements for increasing the strength of the base material are further added in weight% to 0.05 ≦ Cu ≦ 1.5, 0.05 ≦ N
i ≦ 2, 0.02 ≦ Cr ≦ 1, 0.02 ≦ Mo ≦ 1,
0.005 ≦ Nb ≦ 0.05, 0.005 ≦ V ≦ 0.
(1) The thick steel sheet having few hydrogen defects as described in (2) above, wherein one or more kinds of 0.0004 ≦ B ≦ 0.004 are contained.

(4) In steel, 0.00% by weight is added.
05 ≦ Ca ≦ 0.003, 0.0005 ≦ REM ≦ 0.
003, wherein the steel plate has a small number of hydrogen defects as described in the above (2) or (3).

(5) A slab having the composition described in any of the above (2) to (4) is produced by continuous casting,
After cooling to the Ar ₁ transformation point or lower, the cooling temperature is changed from the Ac ₃ transformation point to 13
After heating to 50 ° C. or lower, and performing hot rolling at a reduction ratio of 1.5 or higher at 700 ° C. or higher, the steel plate is cooled to room temperature by cooling in the air, and is characterized in that it has a low hydrogen-defect property. Production method.

(6) A slab having the composition described in any of the above (2) to (4) is produced by continuous casting,
After cooling to the Ar ₁ transformation point or lower, the cooling temperature is changed from the Ac ₃ transformation point to 13
After heating to 50 ° C. or lower, and performing hot rolling at a temperature of 700 ° C. or higher and a reduction ratio of 1.5 or higher, it is cooled to the transformation end temperature or lower and further reheated to the Ac ₃ transformation point or higher and 1000 ° C. or lower. A method for producing a thick steel plate having few hydrogen-induced defects, characterized by performing post-cooling.

(7) A slab having the composition described in any of the above (2) to (4) is produced by continuous casting,
After cooling to the Ar ₁ transformation point or lower, the cooling temperature is changed from the Ac ₃ transformation point to 13
After heating to 50 ° C. or lower and performing hot rolling at a temperature of 700 ° C. or higher and a reduction ratio of 1.5 or higher, cooling to the transformation end temperature or lower, and further reheating to the Ac ₁ transformation point or higher and 1000 ° C. or lower. A method for producing a thick steel sheet having a small number of hydrogen defects, wherein the steel sheet is subjected to a post-quenching treatment, cooled to a transformation end temperature or lower, and then tempered to 500 ° C. or higher and an Ac ₁ transformation point or lower.

(8) A slab having the composition described in any of the above (2) to (4) is produced by continuous casting,
After cooling to the Ar ₁ transformation point or lower, the cooling temperature is changed from the Ac ₃ transformation point to 13
After heating to 50 ° C. or less and performing hot rolling at a reduction ratio of 1.5 or more at 700 ° C. or more, the average cooling rate at 800 to 500 ° C. by direct quenching or accelerated cooling is 2 to 2.
A method for producing a thick steel sheet having few hydrogen defects, wherein the steel sheet is cooled to room temperature at a rate of 100 ° C./sec and tempered to a temperature of 500 ° C. or more and an Ac ₁ transformation point or less.

(9) A slab having the composition described in any of the above (2) to (4) is produced by continuous casting,
After cooling to the Ar ₁ transformation point or lower, the cooling temperature is changed from the Ac ₃ transformation point to 13
After heating to 50 ° C. or less, and performing hot rolling at 700 ° C. or more and a reduction ratio of 1.5 or more, 800 to 800 ° C. by accelerated cooling
The average cooling rate at 500 ° C. is 2 to 100 ° C./sec, and the cooling is stopped at 700 ° C. or less and 500 ° C. or more,
A method for producing a thick steel plate having few hydrogen defects, which is cooled to room temperature.

(10) A slab having the composition described in any one of the above (2) to (4) is produced by continuous casting, cooled to an Ar ₁ transformation point or lower, and then an Ac ₃ transformation point or more and 1350 ° C. or less. At 700 ° C. or more and a reduction ratio of 1.
After performing hot rolling of 5 or more, it is cooled to room temperature at an average cooling rate of 800 to 500 ° C. at 2 to 100 ° C./sec by direct quenching or accelerated cooling, and is cooled to an Ac ₁ transformation point or more and an Ac ₃ transformation point or less. A method for producing a thick steel sheet having few hydrogen defects, wherein the steel sheet is cooled to room temperature by quenching after reheating, and further tempered to a temperature of 500 ° C. or more and an Ac ₁ transformation point or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors re-examined the factors relating to the generation of hydrogen defects, and found that if hydrogen trap sites were finely dispersed and hydrogen was trapped therein, the diffusivity in ground iron could be reduced. By reducing the hydrogen concentration, pseudo-brittle fracture can be less likely to occur, and even if the accumulated hydrogen becomes gaseous and generates high pressure, it is so fine that it does not grow into hydrogen defects. It was thought that hydrogen defects could be prevented.

The problem here is how to finely disperse the particles capable of trapping hydrogen. The present inventors have studied various oxides and found that a composite oxide mainly composed of Ti and Mg is most suitable for this purpose. Ti and M
The oxide containing g as a main component has an ability to trap hydrogen as a gas at the interface between the oxide and the base iron in addition to effectively trapping hydrogen in the oxide. Although the details of the mechanism are unknown, hydrogen is trapped in lattice defects in the oxide, Ti or Mg forming the oxide generates hydride, or the oxide has a catalytic action. It is estimated that diffusible hydrogen is converted to gaseous hydrogen. These hydrogen trapping actions are based on carbonitrides such as V
It is stronger than the hydrogen trapping action of C and VN. Typical crystal structures of oxides mainly composed of Ti.Mg are Ti ₂ O ₃ , TiO, TiO ₂ , MgO, MgTiO ₅ ,
MgTiO ₃ , Mg ₂ TiO ₄ , MgTi ₂ O ₄ , MgTi ₂
O _{5 and the} like are conceivable, but the crystal structure is not particularly limited in the present invention.

Such hydrogen trapping ability differs greatly depending on the type of oxide, and an oxide containing Ti and Mg as main components has a remarkable effect. Assuming that, except for O, among the elements constituting the oxide, the larger the amount of Ti and Mg, the higher the hydrogen trapping ability. With respect to the element excluding O, the total of Ti and Mg in atomic% is 60% or more. If so, the effects of the present invention can be exhibited. The elements constituting the residue are not particularly limited, but may contain Mn, Al, Ca, REM, and the like.

Further, an oxide containing Ti and Mg as main components (hereinafter, referred to as a Ti.Mg-based oxide) is finely crystallized in the molten steel in a deoxidizing step of the molten steel, and this crystallizes during solidification until the solidification. Although a part is coagulated, coalesced, and floated, many remain fine in the molten steel, and finely remain in the slab of continuous casting. Even in the case of Ti / Mg-based oxides, the time required for solidification is long in mold casting, so that fine dispersion becomes difficult.

In the present invention, the particle diameter of the oxide having the above composition is limited to 0.05 to 5.0 μm. From the viewpoint of the hydrogen trap, it is better that oxides are finely and numerously present. Since it is industrially difficult to disperse the Ti / Mg main oxide smaller than 0.05 μm, the lower limit is set to 0.1 μm.
It was set to 05 μm. If it exceeds 5.0 μm, cracks are likely to occur in the base iron due to the pressure of gaseous hydrogen trapped in the oxide, and on the contrary, the possibility of increasing the number of hydrogen defects increases. Further, brittle fracture is likely to occur starting from the coarse oxide, and the toughness is reduced. Therefore, the upper limit is set to 5.
It was set to 0 μm.

The above-mentioned Ti / Mg-based oxide is 1 square mm
It is necessary that there be 10 or more per unit. If the number is less than 10, the total amount of hydrogen traps decreases. If it exceeds 500, the possibility of lowering the ductility of the steel sheet increases. Here, the number of Ti.Mg-based oxides is EPMA or CM
Measure using A. By making the electron beam diameter 0.5 μm or less per 1 mm square of the polished surface of the steel sheet, Ti, Mg, and further elements (Ca, Al, Mn,
REM), and a two-dimensional map of O is created, the composition of each oxide is determined, and the number of oxides whose atomic percentage (Ti + Mg) is 60% or more may be counted.

Since the above-mentioned oxide mainly composed of Ti.Mg also has a function as a ferrite forming nucleus, it has a function of finely crystallizing delta ferrite during solidification, and as a result, an effect of reducing micro segregation. Also have. As a result, the probability of occurrence of hydrogen defects is reduced. Further, it is easily used as a nucleus of intragranular ferrite transformation in the heat-affected zone of the large heat input welding, and improves the HAZ toughness.

In order to form the above-mentioned Ti.Mg-based oxide in steel, it is desirable that Ti, Mg, Al and O be in the following ranges by weight%.

Ti is an element necessary for generating a Ti.Mg-based oxide. If it is less than 0.005%, the number of oxides decreases. If the content exceeds 0.025%, a coarse oxide is formed, and also a large amount of TiC is formed to lower the toughness.
Therefore, the range of the Ti amount is set to 0.005 to 0.025%.

Mg is an element necessary for forming a Ti.Mg-based oxide. If it is less than 0.0002%, the number of oxides decreases. If it exceeds 0.005%, a coarse oxide is formed to lower the toughness. Therefore, the range of the amount of Mg is set to 0.1.
0002 to 0.005%.

Since Al has a stronger deoxidizing power than Ti and Mg,
The lower the better, in order to generate a Ti.Mg-based oxide. If the content exceeds 0.01%, the amount of the oxide mainly containing Ti and Mg decreases, and the amount of alumina increases. Therefore, the upper limit is set to 0.01%.

N forms TiN and has a pinning effect of γ grains, but TiN is preferably 0.001% or more. If N exceeds 0.006%, coarse AlN may be formed, which is not preferable in surface properties, toughness, and processing (for example, bending) of a thick steel plate.

O is an element necessary for producing a Ti.Mg-based oxide. If it is less than 0.0005%, the number of oxides decreases. If it exceeds 0.008%, a coarse oxide is formed, and the toughness is reduced. Therefore, the range of the amount of O is 0.00
05 to 0.008%.

As described above, if an oxide having a specific composition is dispersed in steel, it is effective in preventing hydrogen defects.
No matter how steel containing such an oxide is contained, it is difficult to prevent hydrogen defects when the hydrogen concentration in the steel is high. From such a viewpoint, the hydrogen concentration in steel is desirably 3 ppm or less.

The object of the present invention is a thick steel plate, and the range of component elements is determined as follows in order to secure the strength and the base metal / HAZ toughness as a steel plate.

C is an element capable of increasing the strength of the base material.
If it is less than 0.04%, the base material strength cannot be secured, so 0.04% was made the lower limit. Conversely, if a large amount of C is contained,
To increase cementite, which is the starting point of brittle fracture,
Decreases the toughness of base metal and HAZ. If the content exceeds 0.2%, the toughness is significantly reduced. In order to further improve the base material and HAZ toughness,
It is desirable to set it to 0.04 to 0.15%.

Si is an element effective for increasing the strength of the base material.
If less than 0.02%, this effect cannot be obtained, so the lower limit is set to 0.02%. Conversely, if the content exceeds 0.5%, HA
A large amount of island-like martensite is generated in the Z structure and further hardens the ferrite ground, thereby lowering the HAZ toughness. Therefore, the upper limit is set to 0.5%. In addition, HAZ
In order to improve toughness, it is desirable that the content be 0.3% or less.

Mn is an element effective for increasing the strength of the base material. If the content is less than 0.6%, this effect cannot be obtained, so the lower limit is set to 0.6%. Conversely, if the content exceeds 2%, the toughness is significantly reduced. Therefore, the upper limit is set to 2%.

P is an element that causes grain boundary embrittlement and is harmful to the toughness. If the content exceeds 0.02%, the toughness is significantly reduced, so the upper limit is 0.02%. However, in order to further improve the base material / HAZ toughness, the content is desirably 0.01% or less.

S generates elongated MnS, traps hydrogen, and easily causes hydrogen defects. Further, the characteristics in the thickness direction are reduced. If the content of S exceeds 0.02%, it becomes difficult to prevent hydrogen-induced defects, so the upper limit was made 0.02%. However, in order to prevent defects and further improve the base material / HAZ toughness, the content is desirably 0.01% or less.

Further, the limited range of the selected element effective for increasing the base metal strength was determined for the following reasons.

Cu is an element effective for increasing the base material strength.
In particular, a remarkable increase in strength can be obtained by precipitating a fine Cu phase by aging heat treatment. If less than 0.05%, no increase in strength can be obtained, so 0.05% was set as the lower limit.
Conversely, if the content exceeds 1.5%, the embrittlement of the base material and HAZ becomes remarkable, so the upper limit was set to 1.5%. However, in order to further improve the base material and the HAZ toughness, it is necessary to prevent hardening due to excessive Cu precipitation, and for this reason, it is desirable that the content be 1% or less.

Ni has the effect of increasing the strength of the base metal by increasing the hardenability, and further improves the toughness.
If the content is less than 0.05%, these effects cannot be obtained, so the lower limit is set to 0.05%. Conversely, if the content exceeds 2%, the hardenability becomes too high, so that a HAZ hardened structure is easily generated, and the HAZ toughness is reduced. Therefore, the upper limit is set to 2%. However, the curability of HAZ is suppressed and weldability and HAZ
In order to improve toughness, it is desirable that the content be 1.5% or less.

Cr is effective in increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1%, a hardened structure is generated in the HAZ, so that the HAZ toughness is reduced. Therefore,
The upper limit was set to 1%. However, in order to suppress the hardenability of HAZ and further improve the weldability and HAZ toughness, 0.5
% Is desirable.

Mo is effective in increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1%, a hardened structure is generated in the HAZ, so that the HAZ toughness is reduced. Therefore,
The upper limit was set to 1%. However, in order to suppress the hardenability of HAZ and further improve the weldability and HAZ toughness, 0.5
% Is desirable.

Nb is an element effective for increasing the strength and reducing the size of the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.
When the content exceeds 0.05%, precipitation of Nb carbonitride in the base material / HAZ becomes remarkable, and the toughness is remarkably reduced. Therefore,
The upper limit was set to 0.05%. However, in order to suppress excessive carbonitride precipitation and further improve toughness, 0.02
% Is desirable.

V is an element effective for increasing the strength and reducing the grain size of the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.1
%, The precipitation of carbonitrides in the base material and HAZ becomes remarkable, and the toughness is remarkably reduced. Therefore, the upper limit is set to 0.1%. However, in order to suppress excessive carbonitride precipitation and further improve toughness, the content is desirably 0.04% or less.

B exerts a particularly remarkable increase in strength when subjected to controlled cooling and quenching heat treatment. 0.000
If the content is less than 4%, the effect of increasing the strength cannot be obtained, so the lower limit was made 0.0004%. Conversely, if the content exceeds 0.004%, coarse B nitrides and carbon borides are precipitated and serve as starting points for fracture, so that toughness is reduced. Therefore,
The upper limit was set to 0.004%. However, in order to suppress excessive carbonitride precipitation and further improve toughness, 0.0
Desirably, it is not more than 02%.

Ca and REM suppress the formation of elongation MnS by forming sulfides, and improve the properties in the thickness direction of the steel material, particularly the lamella tear resistance. If both Ca and REM are less than 0.0005%, this effect cannot be obtained, so the lower limit is set to 0.0005%. Conversely, 0.00
When the content exceeds 3%, oxides of Ca and REM increase,
The number of Ti.Mg-based oxides decreases. Therefore, Ca
And the upper limit of REM is set to 0.003%. Ca and RE
It is desirable to make the total of the M content lower than the Mg content. In order to suppress the formation of coarse oxides, Ca
And the content of REM are desirably 0.0015% or less.

The method of manufacturing a thick steel plate was limited for the following reasons.

In the present invention, the continuous cast slab can be heated and rolled without going through the steps of dehydrogenation heat treatment or soaking diffusion treatment. However, the slab may be hot-rolled to change the width and thickness before rolling into a slab due to dimensional restrictions of the steel sheet.

A fifth aspect of the present invention relates to a method for producing a steel sheet as rolled. Since the slab must be austenitized before rolling, it is necessary to heat the slab to a temperature higher than the Ac ₃ transformation point.
Heating at 1350 ° C. or higher is not preferable from the viewpoint of economy, and is not preferable because austenite grains become coarse.

The reduction ratio in hot rolling (steel plate thickness / slab slab thickness) is 1.5 or more. At a lower reduction ratio, zaku remains uncompressed, and even if hydrogen is trapped by the oxide of the present invention, defects tend to occur starting from zaku. Hot rolling is completed at 700 ° C. or higher. Rolling at a lower temperature results in insufficient compression of the zaku.

A sixth aspect of the present invention relates to a method for manufacturing a normalized steel sheet. Since it is necessary to completely austenite by reheating after rolling, the reheating temperature is set to the Ac ₃ transformation point or higher.
If the temperature exceeds 1000 ° C., the coarsening of austenite grains becomes remarkable, so the upper limit is made 1000 ° C.

A seventh aspect of the present invention relates to a method for manufacturing a quenched and tempered steel sheet, in which the transformation is temporarily terminated by cooling after rolling. Thereafter, quenching is performed after reheating to the Ac ₃ transformation point or higher. However, the reheating temperature is set to 1000 ° C. or less in order to suppress austenite grain coarsening. Tempering is less effective at less than 500 ° C., and reverse transformation occurs at more than Ac _1.
The temperature from 00 ° C. to the Ac ₁ transformation temperature is in the tempering range.

An eighth aspect of the present invention is a method for manufacturing a steel sheet in which direct quenching after rolling or tempering after accelerated cooling is performed. If the cooling rate is less than 2 ° C./sec, it is difficult to secure strength. Since it is industrially difficult to obtain a cooling rate exceeding 100 ° C./sec for a thick steel plate, the cooling rate was set to 2 to 100 ° C./sec.
When the tempering temperature is lower than 500 ° C., the recovery recrystallization is insufficient, and when the temperature exceeds the Ac ₁ transformation point, reverse transformation occurs. Therefore, the tempering temperature is set to 500 ° C. to the Ac ₁ transformation point.

A ninth aspect of the present invention is a method for manufacturing a steel sheet in which cooling is stopped halfway after accelerated cooling and self-tempering is performed.
The cooling rate range was limited for the same reason as above. If the cooling stop temperature is higher than 700 ° C., the cooling is stopped before the transformation is sufficiently advanced, and it is difficult to secure the strength. 5
If the temperature is lower than 00 ° C., the sensible heat of the steel sheet is insufficient and self-tempering cannot be performed. Therefore, the cooling stop temperature is set to 700 ° C. or less, 500 ° C.
It was above.

A tenth aspect of the present invention is a method for manufacturing a steel sheet which is manufactured by reheating and quenching in a two-phase region after direct quenching, and further by tempering. The cooling rate was limited for the same reason as above. Since it is necessary to quench the austenite-ferrite two-phase region by the two-phase region heat treatment, the reheating temperature is set to Ac ₁ to Ac _3.
The range of the transformation point was set. The reason for limiting the tempering temperature range is the same as above.

As described above, in the case of steel in which the T.Mg-based oxide is not finely dispersed, it is necessary to perform dehydrogenation heat treatment in the ferrite region after rolling or quenching to prevent hydrogen defects. However, in the steel of the present invention, such heat treatment can be greatly reduced, but can be omitted.

The present invention is intended to prevent a hydrogen defect in a thick steel plate, but at the same time, is effective in improving the tensile properties in the thickness direction, particularly, the elongation and the reduction rate of the cross-section.

[0057]

Examples of the present invention will be described below. Steel was melted by a converter and cast pieces having a thickness of 240 mm were produced by continuous casting. Table 1 shows the chemical composition of the steel material.

[0058]

[Table 1] Table 2 shows the manufacturing method and thickness of the steel sheet, and the mechanical properties of the base material. As shown in the table, the production methods were as-rolled, normalizing, quenching and tempering, accelerated cooling after controlled rolling, direct quenching and tempering after rolling, and direct quenching in the two-phase region heat treatment and tempering after rolling.

[0059]

[Table 2] Table 3 shows the composition of the oxide (mean value of ten measured values) measured by CMA and the number of oxides satisfying claim 1. In the steels 1 to 8 of the components corresponding to claims 2 to 4, the composition of the oxide is (Ti + Mg) ≧ 60% in atomic%,
Claim 1 is satisfied. Also, the number is 1 per square mm.
Zero or more.

The presence or absence of hydrogen-based defects in the steel sheet was measured by an ultrasonic flaw detection test (according to JIS G 0801). In the invention steels 1 to 8, there is no defect, or even if there is a defect, it is a minor defect. On the other hand, in the comparative steel, a defect having a large size is generated because there is no oxide that occludes diffusible hydrogen. For these steels, the method shown in the prior art,
It is mainly necessary to prevent the occurrence of hydrogen defects by performing dehydrogenation treatment after rolling. The steel of the present invention does not require such a dehydrogenation treatment, and has a great advantage in omitting the steps.

[0061]

[Table 3]

[0062]

As described above, in the steel of the present invention, Ti in which hydrogen inevitably present in the slab is finely dispersed.
・ By storing with Mg-based oxides and reducing the diffusible hydrogen concentration of the base iron, it is possible to prevent the occurrence of hydrogen defects in the steel sheet without performing dehydrogenation heat treatment, etc. large.

Claims (10)

[Claims] 1. An oxide having an atomic percentage of atomic percentage (excluding O), which satisfies (Ti + Mg) ≧ 60%, and an oxide having a particle diameter of 0.05 to 5.0 μm.
A thick steel plate with few hydrogen defects, wherein the steel plate contains 10 to 500 steels per square mm. 2. In% by weight, 0.04 ≦ C ≦ 0.2, 0.02 ≦ Si ≦ 0.5, 0.6 ≦ Mn ≦ 2, P ≦ 0.02, S ≦ 0.02, 0 0.005 ≦ Ti ≦ 0.025, 0.0002 ≦ Mg ≦ 0.005, Al ≦ 0.01, 0.001 ≦ N ≦ 0.006, 0.0005 ≦ O ≦ 0.008 and the balance Fe The steel plate according to claim 1, wherein the steel plate is made of steel comprising unavoidable impurities. 3. The steel further contains a base metal strength increasing element group in terms of% by weight: 0.05 ≦ Cu ≦ 1.5, 0.05 ≦ Ni ≦ 2, 0.02 ≦ Cr ≦ 1, 0.02 ≦ Mo ≦ 1, 0.005 ≦ Nb ≦ 0.05, 0.005 ≦ V ≦ 0.1, 0.0004 ≦ B ≦ 0.004. Item 4. A thick steel sheet having a small number of hydrogen defects according to Item 2. 4. The steel further comprises one or two of 0.0005 ≦ Ca ≦ 0.003 and 0.0005 ≦ REM ≦ 0.003 by weight%.
Or a thick steel sheet having few hydrogen-induced defects according to 3. 5. A slab having the composition according to claim 2 manufactured by continuous casting, cooled to an Ar ₁ transformation point or lower, and then heated to an Ac ₃ transformation point or more and 1350 ° C. or less, A method for producing a thick steel plate with few hydrogen defects, comprising: performing hot rolling at a temperature of 700 ° C. or more and a reduction ratio of 1.5 or more, and then cooling to room temperature by cooling in the air. 6. A slab having the composition according to claim 2 manufactured by continuous casting, cooled to an Ar ₁ transformation point or lower, and then heated to an Ac ₃ transformation point or more and 1350 ° C. or less. After performing hot rolling at a reduction ratio of 1.5 or more at 700 ° C. or more, cooling to a transformation end temperature or less, and further cooling after reheating to an Ac ₃ transformation point or more and 1000 ° C. or less. Method for producing thick steel sheets with few hydrogen defects. 7. A slab having the composition according to any one of claims 2 to 4 is produced by continuous casting, cooled to an Ar ₁ transformation point or lower, and heated to an Ac ₃ transformation point or more and 1350 ° C. or less. After hot rolling at a reduction ratio of 1.5 or more at 700 ° C. or more, the material is cooled to the transformation end temperature or less, and further quenched after reheating to an Ac ₁ transformation point or more and 1000 ° C. or less. After cooling below the end temperature, 50
A method for producing a thick steel sheet having a small number of hydrogen defects, wherein the steel sheet is tempered at a temperature of 0 ° C. or more and an Ac ₁ transformation point or less. 8. A slab having a composition according to any one of claims 2 to 4 manufactured by continuous casting, cooled to an Ar ₁ transformation point or lower, and heated to an Ac ₃ transformation point or higher and 1350 ° C. or lower, After hot rolling at a temperature of 700 ° C. or more and a reduction ratio of 1.5 or more, 8% by direct quenching or accelerated cooling.
The average cooling rate at 00 to 500 ° C is 2 to 100 ° C /
Cooled to room temperature in seconds, further, 500 ° C. or higher and Ac ₁
A method for producing a thick steel sheet having few hydrogen-induced defects, characterized by tempering to below the transformation point. 9. A slab having the composition according to claim 2 manufactured by continuous casting, cooled to an Ar ₁ transformation point or lower, and then heated to an Ac ₃ transformation point or more and 1350 ° C. or less, After hot rolling at 700 ° C. or more and a reduction ratio of 1.5 or more, the average cooling rate at 800 to 500 ° C. is 2 to 100 ° C./sec.
A method for producing a thick steel plate having a small number of hydrogen defects, wherein cooling is stopped at a temperature of 0 ° C. or less and 500 ° C. or more, and the mixture is allowed to cool to room temperature. 10. A slab having the composition according to claim 2 manufactured by continuous casting, cooled to an Ar ₁ transformation point or lower, and then heated to an Ac ₃ transformation point or more and 1350 ° C. or less, After performing hot rolling at a reduction ratio of 1.5 or more at 700 ° C. or more, the average cooling rate at 800 to 500 ° C. is 2 to 100 ° C. by direct quenching or accelerated cooling.
/ Second, cooled to room temperature by re-heating after cooling from Ac ₁ transformation point to Ac ₃ transformation point or less, and then quenching.
A method for producing a thick steel sheet having few hydrogen defects, wherein the steel sheet is tempered at a temperature of 500 ° C. or more and an Ac ₁ transformation point or less.