JP2014005490A - Continuous cast slab for high toughness steel having excellent surface crack resisting sensitivity - Google Patents

Abstract

Provided is a slab that suppresses the occurrence of slab surface cracks that occur during continuous casting and reduces the load of slab maintenance work when an alloying element such as Ni is added to improve toughness at low temperatures. .
The composition of the slab is as follows: C: 0.02 to 0.20%, Si: 0.01 to 0.6%, Mn: 0.3 to 2.5%, Ni: 0.2 to 5.0%, P: 0.030% or less, S: 0.020% or less, Al: 0.005-0.03%, O: 0.001-0.005%, N: 0.0001-0.006%, Ti: 0.0075-0.2%, V: 0.005-0.1%, Nb: 0.005-0.05%, Mo: 0.05- One or two or more of 1.0%, and 4 / 47.9 × Ti + 3 / 50.9 × V + 2 / 52.0 × Cr + 3 / 92.9 × Nb + 2 / 95.9 × Mo> 2 / 58.7 / 20 × Ni By doing so, a continuous cast slab for high toughness steel having excellent surface cracking resistance is obtained.
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Description

  The present invention relates to a technique for producing a slab, which is a rolling material when producing a high-strength steel material used for a structural member that requires low-temperature toughness, under high productivity in a continuous casting process.

Since high-tensile steel materials represented by thick steel plates are used as structural members, low-temperature toughness is required from the viewpoint of ensuring the safety of structures. As one of the methods for improving the low temperature toughness of high strength steel materials, a metallurgical method for increasing the Ni content has been proposed in the past.
However, as described in Non-Patent Document 1, since Ni is a γ stabilizing element, but A ₃ transformation temperature decreases the addition of Ni, accordingly, to the cold side of the high temperature brittle temperature range It is known that there is a case where surface cracking may occur during continuous casting due to a change in cooling characteristics due to enlargement and scale fixing to the slab surface.

Further, in Patent Document 1, in continuous casting, the time required for drawing from the meniscus portion of the molten steel in the mold to the lower portion of the mold is set to 1 minute or less, and immediately after drawing, secondary cooling is performed and the surface temperature is set to A within 1 minute. A method of cooling to ₃ transformation temperatures or lower is disclosed.
However, depending on the steel material component, even if the continuous casting process shown in Patent Document 1 is used, cracks may occur on the surface of the slab, reducing its occurrence and reducing the load on the slab maintenance work. It was necessary.

Japanese Patent Laid-Open No. 9-47854

Kato et al., Iron and Steel, 84 (1998) 20

  The present invention is to produce a slab as a rolling material under high productivity in a continuous casting process when producing a high-tensile steel material used for a structural member that requires low-temperature toughness. It is an object of the present invention to provide a slab that reduces the susceptibility of the slab surface by reducing the susceptibility of the slab surface, suppresses the occurrence of slab surface cracks in continuous casting, and reduces the load of the slab maintenance work.

  In order to solve the above-mentioned problems, the present inventors have clarified an essential cause of surface cracks during continuous casting by adding Ni by using computer simulation and the like, and have found means for avoiding surface cracks. The present invention has been reached. That is, this invention makes the summary the continuous cast slab for steel toughness steel materials as follows.

[1] By mass%, C: 0.02 to 0.20%, Si: 0.01 to 0.6%, Mn: 0.3 to 2.5%, Ni: 0.2 to 5.0% , P: 0.030% or less, S: 0.020% or less, Al: 0.005-0.03%, O: 0.001-0.005%, N: 0.0001-0.006% Further, Ti: 0.0075 to 0.02%, V: 0.005 to 0.1%, Cr: 0.05 to 1.0%, Nb: 0.005 to 0.05%, Mo : 0.05-1.0% of 1 type or 2 or more types, balance Fe and unavoidable impurities, and the content of Ti, V, Cr, Nb, Mo, Ni is the following formula (1 ), A continuous cast slab for steel toughness steel with excellent surface cracking resistance.
4 / 47.9 × Ti + 3 / 50.9 × V + 2 / 52.0 × Cr + 3 / 92.9 × Nb +
2 / 95.9 × Mo> 2 / 58.7 / 20 × Ni (1)
However, in Formula (1), an element symbol shall mean the mass (%) of each element.
[2] The steel toughness having excellent surface cracking resistance according to [1] above, wherein the contents of Ti, V, Cr, Nb, Mo, and N satisfy the following formula (2): Continuous casting slab for steel.
1.0 ≦ (Ti / 47.9 + V / 50.9 + Cr / 52.0 + Nb / 92.9 + Mo /
95.9) / (N / 14.0) ≦ 2.0 (2)
However, in Formula (2), an element symbol shall mean the mass (%) of each element.

  By using the present invention, it is possible to manufacture a slab that becomes a rolling material when manufacturing a high-strength steel material used for a structural member that requires low-temperature toughness with high productivity in a continuous casting process. In addition, by optimizing the steel components, it is possible to reduce the cracking susceptibility of the slab surface, suppress the occurrence of slab surface cracking in continuous casting, and reduce the load of the slab maintenance work.

  The inventors have clarified the essential cause of the surface crack during continuous casting by adding Ni by using computer simulation or the like. As the computer simulation, first-principles calculation capable of calculating the electronic state from only the atomic number and the initial arrangement of atoms and calculating various physical property values was used.

In the computer simulation, a grain boundary of γ iron is prepared, S that is easily segregated is arranged at the grain boundary, and Ni that is an element that easily causes surface cracking during continuous casting is arranged near the grain boundary. The grain boundary embrittlement promotion mechanism was investigated.
As a result, when Ni is present in the vicinity of S where grain boundary segregation occurs, Ni has two more electrons than Fe, so that the electrons occupy unoccupied orbitals of S. However, it was found that when a plurality of S exist in the vicinity of the grain boundary, the antibonding orbitals between the S atoms are occupied, a repulsive force acts between the S atoms, and the grain boundary embrittlement is promoted. That is, it is thought that the phenomenon that the surface cracking during continuous casting is promoted by the addition of Ni is due to this mechanism.

  If this mechanism causes surface cracks during continuous casting, it is because Ni has two more electrons than Fe, and if other atoms can receive the electrons, this mechanism will not work, Surface cracking does not occur. Ti, V, Cr, Nb, and Mo are listed as elements having such functions, and a method for suppressing surface cracking by adding these elements has been found.

  Furthermore, since Ti, V, Cr, Nb, and Mo are elements that easily form carbides and nitrides, even if carbides and nitrides are generated, atoms that can accept Ni electrons are solid solution atoms in the steel. It was found to be important to add as it remains.

The present invention is based on the above findings, and the reason for limiting the chemical component of the slab will be described in detail below.
C is required to be 0.02% or more in order to ensure the strength of the base material of the thick steel plate or H-shaped steel that is a steel product. If C is too much, the toughness and weldability of the base material of the steel product are impaired, so 0.2% is the upper limit.
Si is contained in the steel for deoxidation, but if it is too much, the weldability and toughness of the steel product are deteriorated, so the upper limit is made 0.6%. Si is an element that improves strength, and for that purpose, it is necessary to contain 0.01% or more.

Mn is indispensable for securing the strength and toughness of the base material of the steel product, so 0.3% or more is necessary. However, if there is too much Mn, the hardenability increases and the weldability and toughness of the steel product deteriorate, so the upper limit of Mn is set to 2.5%.
Ni is an element added to improve the strength and toughness of the steel material. The addition amount necessary to improve the strength and toughness is 0.2% or more. If excessively added over 5.0%, Ni is an expensive element, so it is not economically preferable from the viewpoint of alloy cost, so 5.0% is the upper limit.

P and S are impurity elements in the present invention, and need to be reduced to 0.030% or less and 0.020% or less, respectively, in order to ensure the mechanical properties of the base material of the steel product.
Al is an element important for deoxidation, and in order to sufficiently reduce the oxygen concentration, it is necessary to contain at least 0.005%. On the other hand, even if added excessively exceeding 0.03%, not only the deoxidizing effect is small, but also a large amount of coarse oxides that cause the strength and toughness of the steel material to be reduced. 0.03%.

O is combined with a special element to form an oxide and contributes directly or indirectly to the fine dispersion of TiN, so 0.001% or more is necessary. However, if O exceeds 0.005%, the properties of the steel decrease and the mechanical properties of the base material of the steel product deteriorate, so 0.005% is the upper limit.
If N exceeds 0.006%, the toughness of the steel material is deteriorated, so the N content is made 0.006% or less. N is inevitably mixed, and the lower limit is 0.0001%.

  Ti, V, Cr, Nb, and Mo are effective elements for improving the strength or toughness of the steel material, and it is necessary to contain one or more of them. In addition, although these elements are required also in order to suppress the surface crack of a slab as mentioned above, the content requirements for that are mentioned later.

Ti is combined with N and contributes to improving the toughness of the steel material by generating fine TiN and TiC. The effect is manifested when 0.0075% or more is contained. On the other hand, if it exceeds 0.02%, coarse TiN and TiC are generated, and the toughness is easily deteriorated.
V contributes to improving the strength of the steel material. The effect appears when the content is 0.005% or more. However, if the content exceeds 0.1%, the weldability of the steel product deteriorates, so 0.1% is the upper limit.

Cr is effective in improving strength and corrosion resistance. The effect is manifested at 0.05% or more. However, if it exceeds 1.0%, the weldability of the steel product deteriorates, so 1.0% is the upper limit.
Nb is effective in improving the toughness by refining the structure of the base material of the steel product. The lower limit for expressing the effect is 0.005%. However, if it exceeds 0.05%, the slab surface cracks remarkably occur or the weldability of the steel product deteriorates, so 0.05% is the upper limit.
Mo is effective in improving the strength of the base material of the steel product. The lower limit for expressing the effect is 0.05%. However, if it exceeds 1%, the weldability of steel products deteriorates, so 1% is the upper limit.

Simulate temperature history and tensile stress (corresponding to bending part or bending part) during continuous casting for the purpose of finding a component design method that can reduce slab surface cracking susceptibility under the component system of the present invention. A hot tensile test (Gleeble test) was performed.
Here, after holding the steel at 1400 ° C. for 600 s, the cooling process of continuous casting is simulated, a tensile test (strain rate: 5 × 10 ⁻³ / s, atmosphere: Ar) is performed at 1000 to 700 ° C., and the drawing value is When the ductility was less than 60%, the slab surface cracking sensitivity was judged to be high. On the other hand, it was judged that the slab surface cracking susceptibility was low when the ductility value was higher than 60%.
This criterion is a general relationship found during many tests in the past, and it is confirmed that it coincides with the actual occurrence tendency of slab surface cracks.

In the component range of the present invention, as a result of examining the above hot deformability in detail, in the component system of the present invention, the contents of Ti, V, Cr, Nb, and Mo are linked with the Ni content, and the following It was found that the slab surface cracking susceptibility can be stably kept low by adjusting so as to satisfy the formula (1).
(4 / 47.9) × Ti + (3 / 50.9) × V + (2 / 52.0) × Cr +
(3 / 92.9) × Nb + (2 / 95.9) × Mo> (2 / 58.7 / 20) × Ni
... (1)
However, in Formula (1), each element symbol shall mean the mass (in%) of each element.

Furthermore, Ti, V, Cr, Nb, Mo, and N have the effect of suppressing γ grain growth during slab heating by pinning carbide or nitride, and the content of these elements is expressed by the following formula ( By controlling to the ratio of 2), precipitates can be finely dispersed, and atoms sufficient to accept Ni electrons can be dissolved in the steel.
1.0 ≦ (Ti / 47.9 + V / 50.9 + Cr / 52.0 + Nb / 92.9 +
Mo / 95.9) / (N / 14.0) ≦ 2.0 (2)
However, in Formula (2), each element symbol shall mean the mass (in%) of each element.

  The present invention is configured as described above, and the effects obtained by implementing the present invention will be described below based on examples.

Table 1 shows the components and surface cracking conditions of the slab continuously cast after adjusting the components by the converter and secondary refining. The continuous casting machine used here was a vertical type, and the slab thickness was 240 mm. After completion of casting, the presence of cracks on the surface of the slab was visually measured to determine the necessity of maintenance work for crack removal.
When it was judged that maintenance was necessary, after further cleaning, the slab was heated to 1250 ° C. and a steel plate having a thickness of 80 mm was produced by thick plate rolling. The temperature at the end of rolling at this time was 860 ° C., and then cooled by air cooling, and no heat treatment after cooling was performed. All steel plates were produced under the same conditions.
A 2 mm V notch Charpy test was performed at −40 ° C. at the center of the thickness of each steel plate after cooling, and the toughness was evaluated with the average absorbed energy of three pieces.

Steels 1 to 14 are examples, and slab surface cracks did not occur.
Steels 15 to 22 were comparative examples, and slab surface cracks occurred.
Among the examples, steels 2, 4, 8, and 9 in which Ti, V, Cr, Nb, Mo, and N satisfy Formula (2) were excellent in toughness.

  The continuous cast slab according to the present invention is commercialized into a thick steel plate, steel pipe, H-shaped steel, etc. by hot working, and is generally used for structures such as marine structures, pressure vessels, ships, bridges, buildings, and line pipes. However, it is particularly useful as a structural steel material for offshore structures, ships, bridges, etc. that require low temperature toughness.

Claims (2)

% By mass
C: 0.02 to 0.20%,
Si: 0.01 to 0.6%,
Mn: 0.3 to 2.5%
Ni: 0.2 to 5.0%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.005 to 0.03%,
O: 0.001 to 0.005%,
N: 0.0001 to 0.006%
Further,
Ti: 0.0075 to 0.02%,
V: 0.005 to 0.1%
Cr: 0.05 to 1.0%,
Nb: 0.005 to 0.05%,
Mo: 0.05-1.0%
1 or 2 or more types, and the balance consists of Fe and inevitable impurities, and the content of Ti, V, Cr, Nb, Mo, N satisfies the following formula (1) Continuous cast slab for steel tough steel with excellent surface crack resistance.
4 / 47.9 × Ti + 3 / 50.9 × V + 2 / 52.0 × Cr + 3 / 92.9 × Nb +
2 / 95.9 × Mo> 2 / 58.7 / 20 × Ni (1)
However, in Formula (1), an element symbol shall mean the mass (%) of each element. Furthermore, content of Ti, V, Cr, Nb, Mo, and N satisfy | fills Formula (2), The continuous cast slab for steel toughness steel materials excellent in the surface cracking susceptibility of Claim 1 characterized by the above-mentioned. .
1.0 ≦ (Ti / 47.9 + V / 50.9 + Cr / 52.0 + Nb / 92.9 + Mo /
95.9) / (N / 14.0) ≦ 2.0 (2)
However, in Formula (2), an element symbol shall mean the mass (%) of each element.