JPH09176730A - Production of thick steel plate excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a thick steel plate having high toughness even if direct rolling is executed. SOLUTION: (1) A slab having a compsn. contg., by weight, 0.01 to 0.25% C, 0.3 to 3% Mn, 0.001 to 0.007% O, 0.003 to 0.03% Ti, <=0.02% Al, other optional elements and Fe and having the following (a), (b) and (c) oxides (A grains) by >=10 pieces/mm<2> is subjected to direct rolling at >=900 deg.C and is cooled for 2 to 90 deg.C/sec to <=500 deg.C. (2) A thick plate producing method in which the slab with the compsn. in the above (1) having the following (a), (d) and (e) oxides (B grains) by >=10 pieces/mm<2> is applied with the method in the above (1) is provided. (3) A slab with the compsn. in (1) having the A grains or/and B grains by >=10 pieces/mm<2> is applied with the method in (1):(a) in the case each metallic element occupied in the total metallic elements in the oxides is expressed by at%, Ti+Mn+Al>70at%, (b) in the case Ti+Mn+Al=100at% (the same shall be apply hereinafter), Al+Mn>=40at%, (c) 1<= Al/Mn}<=5, (d) Ti+Mn>=80at% and (e) 50at%>=Mn>=7at%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thick steel plate having high toughness, which is used for steel structures such as bridges, shipbuilding, pressure vessels, low temperature storage vessels, line pipes, and marine structures, after reheating after casting. The present invention relates to a manufacturing method of hot rolling without rolling.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for improvement in productivity and cost reduction in the production of steel materials. In order to meet such demands, even for the production of thick steel plates, rolling is performed immediately after casting without reheating the continuous casting slab,
So-called direct rolling is under consideration.

However, when direct rolling is applied to the production of thick steel plates, since an extremely coarse structure as cast is rolled, there is a problem that the structure becomes coarse after rolling and the low temperature toughness deteriorates. Especially in high-strength steel, deterioration of low temperature toughness is a serious problem.

In order to avoid this, a method has been proposed in which controlled rolling in a low temperature range and controlled cooling are combined (JP-A-57-131320, etc.). This method is excellent in refining the structure and improving the toughness, and can be expected to improve the strength by increasing the cooling rate of controlled cooling.

However, rolling in the two-phase region below the Ar ₃ point requires a great deal of time to decrease the temperature during rolling, and there is a problem that productivity is significantly impaired. further,
The plate thickness of the target thick steel plate is also limited to a thin range in which the reduction ratio (cast slab thickness / thick steel plate thickness) can be made large so that the structure can be effectively refined by controlled rolling. Therefore, it was not possible to manufacture a steel plate having a large plate thickness (extra-thick steel plate) using the thin slab by direct rolling. In addition, there is a problem that the controlled rolling causes anisotropy of mechanical properties.

[0006]

SUMMARY OF THE INVENTION According to the present invention, even if the reduction ratio is small and the controlled rolling is not performed, the thick steel sheet excellent in toughness is also included in the high-tensile steel sheet and the high productivity is maintained. It is an object of the present invention to provide a method for producing by direct rolling.

[0007]

The present inventors have focused on the following facts in order to solve the above problems.

In order to improve the toughness of the weld metal, there is a method of dispersing oxide particles in the weld metal. According to this method, acicular ferrite (hereinafter abbreviated as AF) is generated from oxide particles dispersed regardless of grain boundaries, and coarse austenite grains are divided, so that the austenite grain size itself is extremely small. Despite being coarse, toughness is remarkably good.

If the oxide is also dispersed in the cast slab and cooling is carried out at an appropriate cooling rate, a high toughness steel plate can be produced by the austenite grain fragmentation action of AF without relying on the refinement of the austenite grains by rolling. Can be expected. However, unlike the weld metal, the amount of oxygen in the thick steel plate is a very small amount of less than 100 ppm, so it is possible to obtain a dispersed state of oxides that can exert the above-mentioned effects.
It was difficult.

The present inventor has, in order to overcome this difficulty,
The test was repeated by producing a huge number of slabs having different compositions of O (oxygen), Al, Ti and Mn in steel. As a result, it was confirmed that the oxide exhibiting the above-mentioned effects had a specific oxide composition, and that excellent toughness was obtained only when the oxide having the composition was dispersed in steel. .

The present invention provides a proper composition of the above oxides,
It was completed by examining the rolling conditions and cooling conditions when performing direct rolling.

The present invention has the following alloy composition:
The gist is a method for producing a thick steel plate characterized by the composition of oxide particles, rolling and cooling conditions.

(1) C: 0.01 to 0.2 in% by weight
5%, Si: 0.6% or less, Mn: 0.3 to 3%, O
(Oxygen): 0.001 to 0.007%, Ti: 0.00
3 to 0.03%, Al: 0.02% or less, B: 0.00
3% or less, P: 0.03% or less, S: 0.01% or less,
N: 0.01% or less, Cu: 2% or less, Ni: 3% or less, Cr: 1.5% or less, Mo: 1.5% or less, Nb:
0.25% or less, V: 0.5% or less, Zr: 0.02%
Below, Ca: 0.004% or less, Mg: 0.004% or less, Hf: 0.02% or less, Y: 0.02% or less and RE (rare earth element): 0.02% or less, and the balance is F
900 ° C. for a slab which is composed of e and unavoidable impurities, and which has an average of 10 particles / mm ² or more of oxide particles dispersed in the slab after solidification that satisfies the following conditions.
Thus, the method for producing a thick steel sheet having excellent toughness, characterized in that the direct rolling is completed and the temperature is cooled to 500 ° C. or less at a cooling rate of 2 to 90 ° C./sec (referred to as [invention 1]).

The proportion of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti
(At%) + Mn (at%) + Al (at%) + [other metal elements] (at%) = 100at%, Ti (at%) + Mn (at%) + Al (at%)> 70at% Regarding Ti, Mn, and Al among the metal elements forming the oxide, Ti (at%) + Mn (at%) + Al (at
%) = 100 at%, Al (at%) + Mn (at%) ≧ 40 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100 at%, 1 ≦ {Al (at%) / Mn (at%)} ≦ 5 (2)% by weight, C: 0.01 to 0.25%, Si:
0.6% or less, Mn: 0.3 to 3%, O (oxygen): 0.
001 to 0.007%, Ti: 0.003 to 0.03
%, Al: 0.02% or less, B: 0.003% or less,
P: 0.03% or less, S: 0.01% or less, N: 0.0
1% or less, Cu: 2% or less, Ni: 3% or less, Cr:
1.5% or less, Mo: 1.5% or less, Nb: 0.25%
Hereinafter, V: 0.5% or less, Zr: 0.02% or less, C
a: 0.004% or less, Mg: 0.004% or less, H
f: 0.02% or less, Y: 0.02% or less and RE
(Rare earth element): 0.02% or less, the balance consisting of Fe and unavoidable impurities, and an average of 10 oxide particles satisfying the following conditions in the slab after solidification.
Direct casting rolling is completed at 900 ° C or higher for the slabs dispersed at a rate of at least ² pieces / mm ² and 50 at a cooling rate of 2 to 90 ° C / sec.
A method for producing a thick steel sheet having excellent toughness, which comprises cooling to 0 ° C. or less (referred to as [invention 2]).

The proportion of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti
(At%) + Mn (at%) + Al (at%) + [other metal elements] (at%) = 100at%, Ti (at%) + Mn (at%) + Al (at%)> 70at% Regarding Ti, Mn, and Al among the metal elements forming the oxide, Ti (at%) + Mn (at%) + Al (at
%) = 100 at%, Ti (at%) + Mn (at%) ≧ 80 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100 at%, 50 at% ≧ Mn (at%) ≧ 7 at% (3) wt%, C: 0.01 to 0.25%, Si:
0.6% or less, Mn: 0.3 to 3%, O (oxygen): 0.
001 to 0.007%, Ti: 0.003 to 0.03
%, Al: 0.02% or less, B: 0.003% or less,
P: 0.03% or less, S: 0.01% or less, N: 0.0
1% or less, Cu: 2% or less, Ni: 3% or less, Cr:
1.5% or less, Mo: 1.5% or less, Nb: 0.25%
Hereinafter, V: 0.5% or less, Zr: 0.02% or less, C
a: 0.004% or less, Mg: 0.004% or less, H
f: 0.02% or less, Y: 0.02% or less and RE
(Rare earth element): Oxide particles containing 0.02% or less, the balance being Fe and inevitable impurities, and satisfying the following conditions in a cast piece after solidification, and:
An average of 10 oxide particles satisfying the conditions and
Direct casting rolling is completed at 900 ° C or higher for the slabs dispersed at a rate of at least ² pieces / mm ² and 50 at a cooling rate of 2 to 90 ° C / sec.
A method for producing a thick steel sheet having excellent toughness, which comprises cooling to 0 ° C. or lower (referred to as [invention 3]).

The proportion of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti
(At%) + Mn (at%) + Al (at%) + [other metal elements] (at%) = 100at%, Ti (at%) + Mn (at%) + Al (at%)> 70at% Regarding Ti, Mn, and Al among the metal elements forming the oxide, Ti (at%) + Mn (at%) + Al (at
%) = 100 at%, Al (at%) + Mn (at%) ≧ 40 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100 at%, 1 ≦ {Al (at%) / Mn (at%)} ≦ 5 Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn ( at%) + Al (at
%) = 100 at%, Ti (at%) + Mn (at%) ≧ 80 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100at%, 50at% ≧ Mn (at%) ≧ 7at%

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION First, conditions to be satisfied by an oxide will be described.

1. Oxide FIG. 1 is a drawing showing the composition range of oxides limited by the method of the present invention by ternary representation. Region A is surrounded by straight lines 1, 2 and 3, and is a range that simultaneously satisfies the composition ranges described in [Invention 1] and [Invention 3], that is, the limiting items regarding the oxide composition. Also,
Region B is surrounded by straight lines 4, 5 and 6 and is a range that simultaneously satisfies the composition ranges described in [Invention 2] and [Invention 3], that is, the limiting items of the oxide. The limiting items will be described later, but when a metal element that is neither Al, Ti nor Mn is contained in the oxide, Al, T
It is a condition that i and Mn must satisfy.

It should be noted that (atomic shown in FIG.
ratio) represents the atomic ratio, and multiplying it by 100
It becomes at%.

Region A is Galaxite (Al ₂ Mn
Al, Mn containing O ₄ ) as a main component and also containing Ti
And an oxide containing Ti as a main constituent element. In the present description, oxygen is omitted when referring to the composition of the oxide.
The region B includes a region having a composition in which the proportion of Ti is higher than that of the region A.

A physical property common to the regions A and B is that it has extremely high electric conductivity as compared with the insulators Al ₂ O ₃ and Ti ₂ O ₃ . In the regions E, D and C other than the regions A and B, the electric conductivity of the oxide is low. Besides the composition, depending on the electrical conductivity,
The oxide according to the present invention can be specified, and the electric conductivity of the oxide according to the present invention reaches 1.0 to 10 (Ωcm) ⁻¹ or more at room temperature, while that in the F region is It is 10 ^-2 (Ωcm) ^-1 or less. The present inventor has empirically confirmed the fact that an oxide having a high electric conductivity acts as a nucleus for generating ferrite or AF, and the composition range of the oxide in the present invention is consistent with this fact. Although not as effective as AF, ferrite, especially ferrite formed from oxides in the grains, has a microstructural destructive action and is effective in improving toughness.

The oxide in the composition range of the region B may be dispersed alone, or may be combined with the oxide in the composition range of the region A to form composite particles. In both cases, the oxide particles act as good ferrite nuclei and / or AF nuclei.

In the following description, the oxide particles having the composition of the region A are referred to as A particles. Oxide particles having a composition such as the region B and the region C are also referred to as B particles, C particles and the like. Further, the above-mentioned composite particles are referred to as composite (A + B) particles. In [Invention 3], composite (A + B) particles are also counted as oxide particles. The counting is performed by a method described later.

For B particles or composite (A + B) particles,
In some cases, E particles or C particles having a composition range shown below are attached, and in this case as well, these composite particles (combined (B + E) particles or composite (A + B) according to the above notation are used.
+ E) particles etc.) function as ferrite or / and AF generating nuclei. However, E particles or C particles alone do not function as transformation nuclei of ferrite and / or AF, and the desired result cannot be obtained.

Further, in the case of F particles and G particles, in order to form these particles in the steel, it is necessary to suppress the amount of Al and excessively increase the amount of Mn, so that the total amount of oxygen in the steel becomes high,
Since the toughness deteriorates, it cannot be used for thick steel plates where safety is the highest priority. Although the reason for the D particles is not clear, it was difficult to form them in the steel with good reproducibility, so they were excluded from the scope of the present invention.

That is, the straight lines 1, 2, 3, 4, 5 and 6 which define the boundaries of the composition range of the A particles and the B particles have been determined as a result of enormous experiments. Those exceeding the range defined by these do not act as AF or / and ferrite generating nuclei or have weak effects except for the above-mentioned special reason.

Next, refining conditions for obtaining A particles and / or B particles will be described.

The deoxidizing power per unit amount of Al, Ti and Mn follows this order. After a sufficient amount of Al is contained, Ti and Mn do not form an oxide. Therefore, A
In order to form particles or / and B particles in the steel, the composition of molten steel immediately before casting is about 1% Mn content and about 0.01% Ti content, and then a small amount of Al is added immediately before casting. Supply, cast and solidify. At this time, after a trace amount of Al is contained in the molten steel, it is necessary to secure about 10 ppm of dissolved oxygen in the molten steel.
By performing such a treatment, a sufficient amount of A particles and / or B particles is formed, and thereafter, the ratio of the formation amount of A particles and B particles changes according to the ratio of the Ti amount and the Al amount. Al can also be used for rough oxygen control in preliminary deoxidation. However, the Al deoxidation in the preliminary deoxidation must be carried out after that by adding a trace amount of Al to the molten steel immediately before casting, and about 10 ppm of oxygen still remains in the molten steel.

Conventionally, in the case of Al killed steel, since the technical idea was to remove oxygen by fixing and floating oxygen as an oxide with Al, the conditions for forming A particles and / or B particles are not satisfied. It was Even with Ti deoxidized steel, A
Since l was not controlled within the above range, the conditions for forming A particles and / or B particles were not satisfied.

Adding deoxidizing elements such as Al, Ca, Mg, Y, Zr, and Hf in the preliminary deoxidizing step up to the addition of Al just before casting as described above is performed when Al is added just before casting. It is acceptable if the dissolved amount of element (3) in the molten steel is 5 ppm or less and is substantially within the range that can be handled as an impurity. 10p after adding a trace amount of Al just before casting
This is because dissolved oxygen of about pM can be secured.

B particles which do not contain Al tend to be stably dispersed in the steel.

Therefore, 0.5% by weight of Al (at% with respect to the sum of Al, Ti and Mn in the oxide. That is, at%, Al (at%) + Ti (at%) + Mn (at
%) = 100 at%, 0.5 at%) or more is preferably contained. However, even if it is less than 0.5 at%, it is effective for refining austenite grains, regardless of the ferrite or / and AF transformation nuclei, so the lower limit of Al is not set in the range of B grains.

The elements constituting the oxide in the steel material are A
In addition to 1, Ti and Mn, Ca, Mg, Y, Zr,
There are Hf etc. When these elements other than Al, Ti and Mn (hereinafter referred to as “elements other than Al”) exist in the steel as oxides, there are the following three types.

(A) It exists as a solid solution in A particles or B particles.

(B) An oxide other than A particles or B particles is formed to form composite particles with A particles or B particles.

(C) An oxide other than A particles or B particles is formed to exist separately from A particles or B particles.

Regarding the state of (a), Al, Ti, Mn
And elements other than Al and the like are added, and the elements other than Al and the like are 30 at% (at at%, Al (at%) + Ti (at%) +
Mn (at%) + [elements other than Al = other metal elements]
(At%) = less than 30 at% when 100 at%) is allowed. It has been confirmed that if the elements other than Al and the like are less than 30 at%, the effects are exhibited as long as the conditions of the method of the present invention are satisfied for the A particles and the B particles. Al
Elements other than, etc. are permitted up to less than 30 at% in A particles or B particles, that is, Al (at%) + Ti (at
%) + Mn (at%) exceeding 70 at% corresponds to the contents of the limited items of oxides described in [Invention 1] to [Invention 3].

FIG. 2 shows that, in addition to Al, Ti and Mn,
FIG. 3 is a drawing showing a composition range of A particles and B particles according to the present invention in a quaternary representation including elements such as Ca and Mg other than Al. The plane 7 in the figure defines the boundary of the above-mentioned limited items.

Even in the states of (b) and (c), A
The composition of particles and B particles is within the above range, and the dispersion density is 10 particles / m for A particles and / or B particles.
m ² or more.

The dispersion density of the oxide can be adjusted by the cooling rate after casting. In particular, cooling immediately after casting affects the solidification rate, and by increasing the solidification rate,
Dispersion density tends to increase. In the case of a large steel ingot, it becomes difficult to increase the cooling rate and the dispersion density decreases.

In order to secure sufficient toughness, a dispersion density of 10 pieces / mm ² or more is required. It is easy to obtain a dispersion density of 10 pieces / mm ² by performing the process immediately before casting and performing continuous casting under the above-mentioned conditions.

A particles or B particles or composite (A +
B) The number of particles is 10 particles / mm ² is determined by the following measurement. Set an arbitrary 1 mm x 1 mm square on the polished sample surface (scribing with a scribing needle),
By grasping the approximate oxide distribution in the field of view under a 100-fold optical microscope, EDX (Energy Di
The composition of the metal element is determined by means of energy dispersion X-ray analysis). When the composition of the precipitate is measured by observing the extraction replica etc. with an electron microscope equipped with an analysis function, the analysis is also EDX.
Therefore, this measurement method is basically applied. Instead of this method, another measurement method may be applied.

Such a measurement can be performed at an arbitrary 1 mm × 1 mm
Do 5 times for a square and 3 times or more for A particles or B
If there were 10 or more particles or composite (A + B) particles, the average was 10 or more. Further, the composite (A + B) particles are
Count as one particle and one B particle.

2. Chemical Composition of Steel Next, the chemical composition of the entire steel including oxide particles will be described. In the following explanation of the chemical composition, all the percentages of the content of the chemical element are represented by weight%. Also,
Even in the case of the metal element forming the oxide, the% shown here represents the weight% in the entire steel.

C: C is an element necessary for ensuring strength,
If it does not contain 0.01% or more, it is impossible to produce steel having practical strength. However, since C is a deoxidizing element, if it exceeds 0.25%, the formation of oxide particles containing Mn is hindered. Therefore, the range of the amount of C is 0.0
1 to 0.25%.

Si: Si may not be added. However, since it is effective for pre-deoxidizing molten steel, it is added when pre-deoxidizing with Si. However, if it exceeds 0.6%, the formation of island martensite, which is harmful to the toughness, is promoted in the weld heat affected zone, so the content is made 0.6% or less.

Mn: Mn is necessary for securing strength, and also necessary for preliminary deoxidation and formation of an oxide used in the method of the present invention. If it is less than 0.3%, these effects cannot be obtained, but if it exceeds 3%, the toughness of the weld heat affected zone deteriorates significantly, so it is made 0.3 to 3%.

O (oxygen): Oxygen is contained in an amount of 0.001% or more in order to form oxide particles. On the other hand, if the oxygen content exceeds 0.007%, the cleanliness deteriorates and practical toughness cannot be obtained even if oxygen is sufficiently fixed by Al, Ti, etc., so 0.001 to 0. 007%.

Ti: Ti is necessary as a constituent element of oxide particles and for preventing cracks in continuously cast slabs. If it is less than 0.003%, these effects cannot be obtained, but if it exceeds 0.03%, Ti ₂ O ₃ that does not work for AF or / and ferrite nucleation is increased and the toughness is deteriorated. It is set to 0.03%.

Al: As described above, B containing no Al
Since the particles are also effective as oxide particles in the method of the present invention, the lower limit of Al is not specified. However, since almost all oxide particles contain Al, Al is added in principle. However, if it exceeds 0.02%, alumina (Al ₂ O ₃ ) becomes the majority as an oxide, so it must be 0.02% or less.

B: A small amount of B improves the hardenability by suppressing the ferrite transformation from the austenite grain boundaries. Therefore, it is effective in promoting the nucleation of ferrite and / or AF from the inside of the grains. In the experiment for completing the method of the present invention, this effect was clearly recognized even when the B content was 0.0001%. Therefore, in order to obtain more stable toughness and strength, it is added.
However, if over 0.003%, the toughness deteriorates, so even if added, it should be 0.003% or less.

P: P is preferably as low as possible.
If it exceeds 0.03% as an unavoidable impurity, it will cause cracking in the heat-affected zone of the weld, so it must be made 0.03% or less.

S: S is preferably as low as possible.
If the content of unavoidable impurities exceeds 0.01%, welding hot cracks and those in the form of MnS become the starting points of cracking, so the content must be 0.01% or less. further,
In order to reduce the segregation of the slab, it is desirable to set it to less than 0.005%.

N: If a large amount of N is contained, the toughness deteriorates. Normally, Ti is added to steel to fix it as TiN to render it harmless, but if N exceeds 0.01%, the toughness of the weld heat affected zone deteriorates regardless of the Ti amount, so 0.01%.
The following is assumed. On the other hand, it is very difficult to reduce N to less than 0.0005% in actual production, and 0.0005% or more is allowed from the economical viewpoint.

Cu: Cu can increase the strength without degrading the toughness. It is added to further increase the strength, but if it exceeds 2%, minute cracks are generated on the surface during hot rolling. Therefore, even if it is added, the content is made 2% or less.

Ni: Ni can improve the toughness of the (base iron) of the steel itself, so it is added to further improve the toughness. However, if it exceeds 3%, the toughness does not improve despite the increase in alloy cost, so even if it is added, it is made 3% or less.

Cr: Cr increases the strength, and if it is an appropriate amount, the hardenability is improved, the structure is refined, and the toughness is also improved. Therefore, Cr is added when the strength is further increased.
However, if it exceeds 1.5%, a hardened structure is formed in the weld heat affected zone and the toughness deteriorates, so even if added, it should be 1.5%.

Mo: Mo enhances the hardenability and is effective in increasing the strength, so it is added when high strength steel is targeted. However, if it exceeds 1.5%, the toughness of the heat-affected zone of welding will be greatly deteriorated, so even if it is added, it should be kept to 1.5% or less.

Nb: Nb has a great effect on refinement of structure, improvement of hardenability, and increase of strength by precipitation hardening. It is added to increase the strength, but if it exceeds 0.25%, the toughness deteriorates.
Should be:

V: V is effective for improving hardenability and increasing strength by precipitation hardening. It is added when high-strength steel is targeted, but if it exceeds 0.5%, the toughness deteriorates significantly, so even if it is added it should be kept to 0.5% or less.

Zr: Zr can increase the dispersion density of A particles and / or B particles by adding it to molten steel prior to addition of Al or the like. Further, the effect of fixing excess S as sulfide can be obtained. Therefore, it is added to obtain these effects, but if it exceeds 0.02%, A particles and / or B particles cannot be obtained. Therefore, even if it is added, it is 0.02% or less.

Ca: Ca can increase the dispersion density of A particles and / or B particles by adding it to molten steel before adding Al or the like. Further, an extremely strong effect of fixing excess S as sulfide can be obtained. 0.004% is added if these effects are further obtained.
If it exceeds 0.1%, an oxide having the composition of the method of the present invention cannot be obtained.

Mg: Mg can increase the dispersion density of A particles and / or B particles by adding it to molten steel before adding Al or the like. When these effects are obtained, it is added, but if it exceeds 0.004%, A particles and / or B particles essential for the method of the present invention cannot be obtained, so even if added, it is made 0.004% or less.

Hf: Hf can increase the dispersion density of A particles and / or B particles by adding it to molten steel prior to addition of Al or the like. Further, an effect of fixing S as sulfide can be obtained. It is added to further obtain these effects, but if it exceeds 0.02%, the toughness deteriorates, so even if added, it is made 0.02% or less.

Y: Y can increase the dispersion density of A particles and / or B particles by adding it to molten steel prior to addition of Al or the like. In addition, the effect of fixing S as a sulfide is also obtained. When these effects are obtained, it is added, but if it exceeds 0.02%, the cleanliness deteriorates and ductility deteriorates.

RE (rare earth element): RE (rare earth element)
Is added to the molten steel prior to the addition of Al or the like, whereby the dispersion density of A particles and / or B particles can be increased. Further, an extremely strong action of fixing S as a sulfide can be obtained. The rare earth may be added, for example, in a mixed state of rare earths (referred to as misch metal or the like), or may be added as separated rare earth elements Ce and Nd. When these effects are obtained, it is added, but if it exceeds 0.02%, toughness deteriorates, so even if it is added, it is made 0.02% or less.

3. Rolling Conditions and Cooling Conditions Finally, rolling conditions will be described.

The present invention does not expect much for making the structure fine by rolling. Therefore, the reduction ratio is expected only to crush the porosity in the slab. Therefore, a reduction ratio of about 1.5 to 3 or more is sufficient. Therefore, 10
It is possible to produce thick steel plates of normal thickness from thin slabs of 0 mm or less.

Further, since the main purpose of rolling is not to make the structure fine, rolling at a low temperature is not performed, and the rolling temperature is 900 ° C. or higher. This is because if rolling is performed at less than 900 ° C. and the austenite grains are refined halfway, the ratio of grain boundary areas increases and acicular ferrite is less likely to be generated, resulting in deterioration in toughness. Rather, finishing the rolling in the high temperature range improves the toughness and significantly improves the productivity as compared with the controlled rolling. Therefore, it is preferably set to 925 ° C. or higher.

By rolling at 900 ° C. or higher, anisotropy of mechanical properties does not occur and porosity can be easily pressure-bonded. Therefore, when using a thin slab,
Alternatively, when the thickness of the cast slab cannot be increased to a certain thickness or more, for example, 100 mm or more, it is an extremely effective method for manufacturing an extra-thick steel plate having a thickness of more than 30 mm. That is, since it is not necessary to prepare a specially thick slab for manufacturing the extra-thick steel plate, it is possible to simplify the equipment specifications and reduce the slab inventory, which brings a great advantage in reducing the manufacturing cost.

After rolling, it must be cooled from Ar ₃ point or more to 500 ° C. or less at a rate of 2 ° C./sec or more.
As long as the rolling is completed at 900 ° C. or higher and the usual cooling method after rolling is applied, the water cooling start temperature of Ar ₃ or higher can be satisfied with some margin.

If the cooling rate is lower than 2 ° C./sec, coarse ferrite is generated and the toughness deteriorates. As the cooling rate increases, AF + becomes
The martensite mixed structure and martensite structure change. Sufficient toughness is obtained except for the martensitic structure. With respect to a thick steel plate having a normal thickness in the composition range of the present invention, a martensite structure is not formed, so that the cooling rate is 9
The rate is 0 ° C./second or less. The cooling rate of 2 to 90 ° C./second is a cooling rate that can be easily achieved by using a normal control cooling device for a thick steel plate having a normal plate thickness.

The cooling end temperature is set to 500 ° C. or lower because
If the cooling end temperature is higher than this, coarse ferrite is likely to be generated, and it becomes difficult to secure toughness.

[0074]

EXAMPLES Tables 1 and 2 are tables showing the chemical compositions of the present invention and comparative examples. Steels 1 to 1 used in the examples of the present invention
The chemical compositions of 8 and 17 to 22 are the same as those of the steels 9 to 16 and 28 to 33 used in Comparative Examples, except for the elements constituting the oxide.
They are arranged so that they substantially correspond to each other. The steels 23 to 27 have almost the same composition as the steel 34 used in the comparative example. Steels 23 to 27 have an oxide composition within the range of the present invention, but elements other than Al and the like contained in the oxide are changed. In Tables 1 and 2, the only composition outside the scope of the present invention is the S content of steel 34. As described above, the difference between the present invention example and the comparative example is that Al is added immediately before casting so that the dissolved oxygen is 10 p in the molten steel.
It depends on whether or not about pm is left.

[0075]

[Table 1]

[0076]

[Table 2]

In the steel used in the examples of the present invention, a plurality of types of oxides in the steel coexist, and often 2 to 3 types of oxides are adhered to each other as composite particles of about 0.5 to 10 μm across. It was dispersed. The composition of each oxide particle constituting the composite particle was analyzed by EDX analysis. For particles with a size of 0.5 μm or less, an extraction replica was sampled and observed with a transmission electron microscope. Most of such fine particles are carbonitrides, and oxides are extremely rare. Met. The oxide particles are mostly 200
It can be immediately distinguished from it in the field of view of an optical microscope at a magnification of 2 × or 200 × or more.

When S is detected, all Mn
From the measured value of Mn obtained from EDX assuming that S was formed, this amount was corrected to obtain the amount of Mn in the oxide.

Tables 3 and 4 are tables showing the analysis results of oxides in the slabs of these steels, the rolling conditions of direct rolling and the cooling conditions after rolling, and the mechanical properties of thick steel plates. Is. In the table, A particles and B particles (A
The composition such as (1 + Ti + Mn) represents the average value of the respective measured particles measured for the A particles or the B particles. In Tables 3 and 4, the dispersion densities of A particles and B particles of all comparative examples are outside the scope of the present invention.

The mechanical properties were evaluated by a tensile test and an impact test using JIS No. 4 test pieces.

Table 5 shows, as a comparative example, the results obtained by applying the conditions for direct rolling and cooling after rolling other than the method of the present invention to the slab of steel 3 used in the above-mentioned example of the present invention. is there. In Table 5, all the rolling conditions and cooling conditions of the steel sheets A to E are outside the scope of the present invention.

Table 6 is a table comparing the anisotropy of steel 3 with respect to the method of the present invention and the rolling conditions outside the scope of the present invention.

[0083]

[Table 3]

[0084]

[Table 4]

[0085]

[Table 5]

[0086]

[Table 6]

FIGS. 3, 4 and 5 show Example 2 of the present invention.
The drawing which ternarily displayed the composition of each oxide in the cast pieces of 1, 8 and 4 is represented. On the other hand, FIGS. 6, 7 and 8 show drawings in which the compositions of the respective oxides in the cast pieces of Comparative Examples 10, 14 and 11 are ternary displayed. The oxide is arbitrarily selected. Each point (■ and ○) in these figures
Corresponds to one oxide. A point {circle around (2)} indicates that the composition of the oxide is within the range of the present invention, and a point ○ indicates that the composition of the oxide is outside the range of the present invention.

It can be seen from FIGS. 3, 4 and 5 that the particles (A particles and B particles) having the oxide composition according to the present invention are present in a very high ratio. That is, the refining conditions described above (deoxidation conditions with Mn, Ti and Al)
It is found that most of the oxides produced are A particles and / or B particles. Further, as shown in Tables 3 and 4, the densities of A particles and B particles of the examples of the present invention are within the scope of the present invention. this is,
This is because the above-mentioned dissolved oxygen concentration was satisfied and the above-mentioned cooling rate after solidification was satisfied immediately after the addition of a slight amount of Al immediately before casting.

From the mechanical properties summarized in Tables 3 and 4, in the examples of the present invention, excellent toughness was obtained in comparison with the same strength, whereas in the comparative examples, the oxide was dispersed. However, it is clear that the toughness is poor. The reason why the toughness is poor in the comparative example is that the oxide dispersed in the slabs of these steels is a Ti oxide or a TiAl oxide, and these oxides do not function as transformation nuclei of ferrite and AF. Is.

In Comparative Example 10 shown in FIG. 6, since the dissolved oxygen concentration after adding all the deoxidizing elements was small, Al
And an oxide mainly composed of Ti is formed, and the toughness is not good. In Comparative Example 14 shown in FIG. 7, since the amount of Al was small, the oxide had an extremely high electric resistance, Ti ₂
Most of them have a composition close to O ₃ , and the toughness is also poor. In Comparative Example 11 shown in FIG. 8, the amount of Ti was outside the range of the present invention, and the amount of Al added during deoxidation was small, so Mn
An oxide mainly composed of is formed, and the toughness is deteriorated.

According to Table 5 showing the results of examining the rolling and cooling conditions for the steel 3 used in the invention examples, all the steels except the steel sheet C are inferior in toughness to the steel sheet according to the method of the present invention. Only the steel sheet C exhibits the toughness equivalent to that of the method of the present invention, but this is because the steel sheet C is once cooled to room temperature and then subjected to normal reheating rolling, not direct feed rolling. Therefore, even if the toughness is equivalent to that of the method of the present invention, it is a economically disadvantageous manufacturing method.

According to the method of the present invention, the results in Table 6 are
Although a slight anisotropy remains in the direction perpendicular to the plate surface (Z direction), very uniform characteristics independent of the test piece sampling position are obtained.

[0093]

According to the method of the present invention, even if direct rolling with a small reduction ratio and finishing at 900 ° C. or higher is performed, it is possible to obtain high toughness over the entire plate thickness due to the effect of the structure segmentation by AF and / or ferrite. it can. The method of the present invention can perform not only the economic effect of direct-feed rolling itself but also high-productivity rolling, and can apply a general-purpose slab to the production of extremely thick steel sheets, simplifying equipment specifications and reducing slab inventory. Weight loss can be achieved. The effects related to the basics of iron and steel production are extremely large as they contribute to the development of the industry.

[Brief description of the drawings]

FIG. 1 is a drawing showing the composition range of oxides limited by the method of the present invention by ternary representation.

FIG. 2 shows C in addition to Al, Ti and Mn.
FIG. 3 is a drawing showing the composition range of A particles and B particles according to the present invention in a quaternary representation including elements other than a, such as a, Mg, Zr, Hf, Y, and Rem.

FIG. 3 is a drawing ternary indicating the composition of each oxide in the cast slab of Inventive Example 21.

FIG. 4 is a drawing ternary indicating the composition of each oxide in the cast slab of Example 8 of the present invention.

FIG. 5 is a drawing ternary indicating the composition of each oxide in the cast slab of Example 4 of the present invention.

FIG. 6 is a drawing in which the composition of each oxide in the cast slab of Comparative Example 10 is displayed in ternary form.

FIG. 7 is a drawing in which the composition of each oxide in the cast slab of Comparative Example 14 is displayed in ternary form.

FIG. 8 is a drawing in which the composition of each oxide in the cast slab of Comparative Example 11 is displayed in ternary form.

[Explanation of symbols]

1 ... Al (at%) + Mn (at%) = 40 at% straight line, 2 ... Al (at%) / Mn (at%) = 5 straight line, 3 ... Al (at%) / Mn (at%) ) = 1 straight line, 4 ... Ti (at%) + Mn (at%) = 80at% straight line, 5 ... Mn (at%) = 7at% straight line, 6 ... Mn (at%) = 50at% A straight line representing 7 ... Ti (at%) + Mn (at%) + Al (at%) = 70
plane representing at%

Claims (3)

[Claims] 1. C: 0.01 to 0.25% by weight,
Si: 0.6% or less, Mn: 0.3 to 3%, O (oxygen): 0.001 to 0.007%, Ti: 0.003 to
0.03%, Al: 0.02% or less, B: 0.003%
Hereinafter, P: 0.03% or less, S: 0.01% or less, N:
0.01% or less, Cu: 2% or less, Ni: 3% or less, C
r: 1.5% or less, Mo: 1.5% or less, Nb: 0.2
5% or less, V: 0.5% or less, Zr: 0.02% or less,
Ca: 0.004% or less, Mg: 0.004% or less, H
f: 0.02% or less, Y: 0.02% or less and RE
(Rare earth element): 0.02% or less, the balance consisting of Fe and unavoidable impurities, and an average of 10 particles / mm ² or more of oxide particles satisfying the following conditions in the solidified slab: For the dispersed slab, the direct feed rolling was completed at 900 ° C. or higher, and
A method for producing a thick steel sheet having excellent toughness, which comprises cooling to 500 ° C or less at a cooling rate of 90 ° C / sec. The ratio of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti (at%) + Mn
(At%) + Al (at%) + [other metal elements] (at%) =
Ti (at%) + Mn (at%) + Al (at%)> 70at% When Ti is 100%, Ti (at%) + Mn (at% ) + Al (at
%) = 100 at%, Al (at%) + Mn (at%) ≧ 40 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100 at%, 1 ≦ {Al (at%) / Mn (at%)} ≦ 5 2. In% by weight, C: 0.01 to 0.25%,
Si: 0.6% or less, Mn: 0.3 to 3%, O (oxygen): 0.001 to 0.007%, Ti: 0.003 to
0.03%, Al: 0.02% or less, B: 0.003%
Hereinafter, P: 0.03% or less, S: 0.01% or less, N:
0.01% or less, Cu: 2% or less, Ni: 3% or less, C
r: 1.5% or less, Mo: 1.5% or less, Nb: 0.2
5% or less, V: 0.5% or less, Zr: 0.02% or less,
Ca: 0.004% or less, Mg: 0.004% or less, H
f: 0.02% or less, Y: 0.02% or less and RE
(Rare earth element): 0.02% or less, the balance consisting of Fe and unavoidable impurities, and an average of 10 particles / mm ² or more of oxide particles satisfying the following conditions in the solidified slab: For the dispersed slab, the direct feed rolling was completed at 900 ° C. or higher, and
A method for producing a thick steel sheet having excellent toughness, which comprises cooling to 500 ° C or less at a cooling rate of 90 ° C / sec. The ratio of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti (at%) + Mn
(At%) + Al (at%) + [other metal elements] (at%) =
Ti (at%) + Mn (at%) + Al (at%)> 70at% When Ti is 100%, Ti (at%) + Mn (at% ) + Al (at
%) = 100 at%, Ti (at%) + Mn (at%) ≧ 80 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100at%, 50at% ≧ Mn (at%) ≧ 7at% 3. C: 0.01 to 0.25% by weight,
Si: 0.6% or less, Mn: 0.3 to 3%, O (oxygen): 0.001 to 0.007%, Ti: 0.003 to
0.03%, Al: 0.02% or less, B: 0.003%
Hereinafter, P: 0.03% or less, S: 0.01% or less, N:
0.01% or less, Cu: 2% or less, Ni: 3% or less, C
r: 1.5% or less, Mo: 1.5% or less, Nb: 0.2
5% or less, V: 0.5% or less, Zr: 0.02% or less,
Ca: 0.004% or less, Mg: 0.004% or less, H
f: 0.02% or less, Y: 0.02% or less and RE
(Rare earth element): 0.02% or less, the balance consisting of Fe and inevitable impurities, and oxide particles satisfying the following conditions in the solidified slab and satisfying the conditions and For the slab in which the oxide particles to be dispersed are 10 particles / mm ² or more on average, direct rolling is completed at 900 ° C. or more,
A method for producing a thick steel sheet having excellent toughness, which comprises cooling to 500 ° C or less at a cooling rate of 90 ° C / sec. The ratio of each metal element in the metal elements constituting the oxide is expressed in atomic% (at%), and Ti (at%) + Mn
(At%) + Al (at%) + [other metal elements] (at%) =
Ti (at%) + Mn (at%) + Al (at%)> 70at% When Ti is 100%, Ti (at%) + Mn (at% ) + Al (at
%) = 100 at%, Al (at%) + Mn (at%) ≧ 40 at% For Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100 at%, 1 ≦ {Al (at%) / Mn (at%)} ≦ 5 Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn ( at%) + Al (at
%) = 100 at%, Ti (at%) + Mn (at%) ≧ 80 at% Regarding Ti, Mn and Al among the metal elements constituting the oxide, Ti (at%) + Mn (at%) + Al (At
%) = 100at%, 50at% ≧ Mn (at%) ≧ 7at%