WO2024053276A1

WO2024053276A1 - Steel cast slab, continuous casting method, and method for producing steel cast slab

Info

Publication number: WO2024053276A1
Application number: PCT/JP2023/027675
Authority: WO
Inventors: 陽一伊藤; 一貴西中; 則親荒牧; 祐也佐藤
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-09-09
Filing date: 2023-07-28
Publication date: 2024-03-14
Also published as: JPWO2024053276A1; TW202413666A; JP7563626B2

Abstract

Provided is a steel cast slab that contains from 2.0 mass% to less than 7.5 mass% of Ni and has few surface cracks. The steel cast slab containing Ni is composed of, in mass%, C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 0.10% to 1.00%, P: 0.001% to 0.010%, S: 0.0001% to 0.0050%, Ni: 2.0% to less than 7.5%, Al: 0.010% to 0.080%, N: 0.0010% to 0.0050%, and O: 0.0005% to 0.0040%, with the remainder comprising Fe and unavoidable impurities. The density of solidified nuclei in the surface of the steel cast slab is 0.35/mm2 or more.

Description

Steel slab, continuous casting method, and manufacturing method of steel slab

The present invention relates to a steel slab containing Ni (nickel), a continuous casting method, and a method for producing a steel slab.

It is known that adding Ni to steel improves low-temperature toughness. Steel containing around 9% by mass of Ni (hereinafter also referred to as Ni-containing steel) is also referred to as 9% Ni steel. 9% Ni steel can withstand use at temperatures below -160°C, and is therefore widely used as a welded structural steel for low-temperature applications such as LNG tanks, for example.

By the way, it is known that Ni-containing steel is prone to scratches on its surface. For example, a slab after being cast has many cracks (hereinafter also referred to as surface cracks) on and near the surface.

It has been known that surface cracks in steel slabs containing Ni occur along the grain boundaries of the coarse solidified structure. Specifically, surface cracking is caused by tensile stresses such as straightening stress, bulging stress, and thermal stress in the temperature range of 600 to 900°C, where the ductility of Ni-containing steel decreases, that is, in the secondary cooling zone section of continuous casting. It is thought that this is caused by the addition of carbon to steel slabs.

More specifically, when tensile stress is applied to a steel slab, S (sulfur), P (phosphorus), etc. contained in the steel slab become concentrated in a specific region. An increase in the concentration of S (sulfur) and P (phosphorus) causes grain boundary embrittlement of the steel slab. Therefore, in this region, surface cracks occur because the embrittled grain boundaries are destroyed by tensile stress.

In order to prevent such surface cracks, the temperature of the steel slab is controlled during secondary cooling when casting the steel slab. For example, Patent Document 1 discloses controlling the cooling rate and the surface temperature of the slab in a secondary cooling zone when continuously casting molten steel containing 5 to 10% by mass of Ni. ing.

In addition, Patent Document 2 describes that when continuously casting Ni-containing steel containing 8 to 10% by mass of Ni, the reduction of area at the time of casting is estimated, and the secondary Controlling the cooling intensity is disclosed.

Japanese Patent Application Publication No. 8-10919 Japanese Patent Application Publication No. 8-33964

As described above, many continuous casting methods have been proposed to suppress surface cracks, but it is difficult to completely suppress the occurrence of surface cracks. When surface cracks occur in a steel slab, the surface cracks are removed by so-called maintenance treatment, such as by grinding the surface using a grinder or the like. For this reason, when surface cracks increase, the range of cleaning treatment and treatment time become longer, resulting in lower productivity and higher manufacturing costs.

Incidentally, in recent years, as the price of Ni alloys has risen, efforts have been made to reduce the Ni content in steel. For example, as an alternative to 9% Ni steel, 7% Ni steel is being expanded as an alternative steel type. Furthermore, 5% Ni steel is used for ethylene liquefied fuel containers. As described above, the demand for Ni-containing steels having a Ni content of less than 7.5% by mass is increasing significantly.

Even for steel types with a relatively low Ni content (less than 7.5% by mass), the occurrence of surface cracks in steel slabs along the grain boundaries of the coarse solidified structure described above is a manufacturing issue. It becomes.

The present invention has been made in view of the above problems, and provides a steel slab containing 2.0% by mass or more and less than 7.5% by mass of Ni and less surface cracking, a continuous casting method, and a continuous casting method for producing a steel slab. The purpose is to provide a manufacturing method.

In order to solve the above problems, the present invention has the following features.

[1]
In mass%, C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.1% or more and 1.0% or less, P: 0.001% or more 0.010% or less, S: 0.0001% or more and 0.0050% or less, Ni: 2.0% or more and less than 7.5%, Al: 0.010% or more and 0.080% or less, N: 0.0010 % or more and 0.0050% or less, O: 0.0005% or more and 0.0040% or less, and the remainder is Fe and inevitable impurities, the steel slab containing Ni. A steel slab having a density of solidification nuclei on the surface of 0.35 pieces/mm ² or more.
[2]
Further, in mass%, Cu: 0.03% or more and 1.50% or less, Cr: 0.03% or more and 1.00% or less, Mo: 0.02% or more and 1.00% or less, Nb: 0.003 % or more and 0.100% or less, V: 0.003% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, B: 0.0002% or more and 0.0025% or less, Ca: 0 The steel slab according to [1], containing one or more selected from .0005% to 0.0050%, Mg: 0.0005% to 0.0030%.
[3]
A continuous casting method for casting the steel slab according to [1] or [2],
A continuous casting method including a step of adding mold powder having a viscosity of 0.5 Pa·s (5 poise) or more at 1300°C into a mold.
[4]
A continuous casting method for casting the steel slab according to [1] or [2],
A continuous casting method that includes the step of vibrating a mold at a frequency of 80 cycles or more per minute.
[5]
A continuous casting method for casting the steel slab according to [1] or [2],
A step of adding mold powder having a viscosity of 0.5 Pa·s (5 poise) or more at 1300° C. into the mold,
A continuous casting method comprising the step of vibrating a mold at a frequency of 50 cycles or more per minute.
[6]
A continuous casting method for casting the steel slab according to [1] or [2],
Adding mold powder with a viscosity of 0.15 Pa·s (1.5 poise) or more at 1300°C into the mold;
A continuous casting method comprising the step of vibrating a mold at a frequency of 80 cycles or more per minute.
[7]
A care step of performing care treatment on a slab manufactured by the continuous casting method described in any one of [3] to [6];
A method for producing a steel slab containing Ni, including a heating step of heating the slab at a heating temperature of 1100° C. or lower in a heating furnace after the care step.

According to the present invention, since the density of solidification nuclei on the surface of the steel slab is 0.35 pieces/mm ² or more, the size of the solidification cells can be made smaller than the conventional size. Thereby, segregation of S (sulfur) and P (phosphorus) at the interface of the solidification cell can be reduced more than before. As a result, embrittlement at the interface of the solidified cells can be suppressed. Moreover, the stress acting on the interface of the coagulation cell can also be dispersed. Thereby, it is possible to suppress the occurrence of cracks at the interface of the solidification cells, and it is possible to reduce the occurrence of cracks on the surface of the steel slab. As a result, it is possible to reduce the processing time for the treatment for removing cracks on the surface of the steel slab, and it is possible to improve productivity and reduce manufacturing costs.

The Ni-containing steel slab of the present invention (hereinafter also simply referred to as steel slab) contains 2.0% by mass or more and less than 7.5% by mass of Ni. The steel slab can be used, for example, as a low-temperature steel used in a temperature range lower than room temperature.

The Ni-containing steel slab of the present invention has, in mass %, C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1 .00% or less, P: 0.001% or more and 0.010% or less, S: 0.0001% or more and 0.0050% or less, Ni: 2.0% or more and less than 7.5%, Al: 0.010% N: 0.0010% to 0.0050%, O: 0.0005% to 0.0040%, and the remainder consists of Fe and inevitable impurities.

By containing C (carbon) as a composition, the steel slab can ensure the strength of the base material. In particular, by setting the C content in the steel slab to 0.03% by mass (hereinafter simply referred to as "%") or more, the strength of the base material can be improved. When the content of C contained in the steel slab becomes excessive, cementite and island martensite, which become the starting point of brittle fracture, increase, and there is a possibility that suitable toughness cannot be obtained. By controlling the content of C contained in the steel slab to 0.10% or less, appropriate toughness of the steel slab can be obtained.

By containing Si (silicon) as a composition, the steel slab can enhance the deoxidizing effect of removing oxygen contained in the steel slab. Moreover, the strength of the base material can be ensured by containing Si as a composition of the steel slab. When the amount of Si added increases, island-like martensite is generated in the structure of the weld heat-affected zone (HAZ), and good toughness of the HAZ tends to not be obtained.

That is, by controlling the Si content of the steel slab to 0.50% or less, suitable toughness of the HAZ can be ensured. By setting the Si content of the steel slab to 0.01% or more, an excellent deoxidizing effect can be obtained and the strength of the base material can be improved.

By containing Mn as a composition, the steel slab can ensure the strength of the base material. When the amount of Mn added increases, there is a tendency that good HAZ toughness cannot be obtained.

In particular, by setting the Mn content of the steel slab to 0.10% or more, the strength of the base material can be improved. Further, by controlling the Mn content of the steel slab to 1.00% or less, suitable toughness of the HAZ can be ensured.

When steel slabs contain P (phosphorus) as a composition, they tend to cause grain boundary embrittlement. For this reason, the steel slab preferably contains P (phosphorus) in the lowest possible amount. When the P (phosphorus) content of the steel slab is 0.010% or less, it is possible to suppress the promotion of surface cracking due to grain boundary embrittlement. Thereby, the toughness of the base material and the HAZ can be improved.

In addition, by setting the P (phosphorus) content of the steel slab to 0.001% or more, it is possible to suppress an increase in the load of dephosphorization refining in the steelmaking process, and to suppress an increase in manufacturing costs. Can be done.

When steel slabs contain S (sulfur) as a composition, they tend to cause grain boundary embrittlement. For this reason, the steel slab preferably contains as low a content of S (sulfur) as possible. When the S (sulfur) content of the steel slab is 0.0050% or less, it is possible to suppress the occurrence of grain boundary embrittlement and the promotion of surface cracking. Thereby, the toughness of the base material and the HAZ can be improved. In particular, S (sulfur) reduces the toughness of steel slabs as inclusions such as MnS. For this reason, it is desirable that the S (sulfur) content of the steel slab be low.

In addition, by setting the S (sulfur) content of the steel slab to 0.0001% or more, it is possible to suppress an increase in the load of dephosphorization refining in the steelmaking process, and to suppress an increase in manufacturing costs. Can be done.

As mentioned above, the steel slab contains Ni (nickel) in a composition of 2.0% or more and less than 7.5%. By containing the above-mentioned components, the steel slab can realize physical properties equivalent to 9% Ni steel with a lower Ni content than 9% Ni steel. For this purpose, the Ni content of the steel slab is preferably 2.0% or more and less than 7.5%, and preferably 6.5% or more and less than 7.5%. Note that if the Ni content is less than 2.0%, there is a tendency that the toughness at low temperatures due to Ni cannot be obtained.

By containing Al (aluminum) as a composition, the steel slab can enhance the deoxidizing effect of removing oxygen contained in the steel slab. Moreover, the strength of the base material can be ensured by containing Al (aluminum) as a composition of the steel slab. When the amount of Al (aluminum) added increases, the base material and HAZ toughness tend to decrease due to coarse AlN.

That is, by controlling the Al (aluminum) content of the steel slab to 0.080% or less, suitable HAZ toughness can be ensured. In particular, by setting the Si content of the steel slab to 0.010% or more, an excellent deoxidizing effect can be obtained.

When a steel slab contains N (nitrogen) as a composition, coarse metal nitrides such as AlN are generated, which tends to reduce the toughness of the base material and HAZ. For this reason, the steel slab preferably contains as low a content of N (nitrogen) as possible.

By setting the N (nitrogen) content of the steel slab to 0.0050% or less, suitable toughness of the base material and HAZ can be ensured. In addition, by setting the N (nitrogen) content of the steel slab to 0.0010% or more, it is possible to suppress the increase in the burden of denitrification treatment and nitrogen absorption prevention treatment in the steel manufacturing process, which leads to an increase in manufacturing costs. This can be suppressed.

When a steel slab contains O (oxygen) as a composition, inclusions tend to form and the base metal and HAZ toughness tend to decrease. For this reason, the steel slab preferably contains as low a content of O (oxygen) as possible. When the O (oxygen) content of the steel slab is 0.0040% or less, suitable toughness of the base material and HAZ can be ensured.

In addition, by setting the O (oxygen) content of the steel slab to 0.0005% or more, it is possible to suppress an increase in the load of inclusion removal treatment in the steel manufacturing process, and to suppress an increase in manufacturing costs. be able to.

Furthermore, in addition to the above alloying elements, the steel slab is selected from Cu, Cr, Mo, Nb, V, Ti, B, Ca, and Mg in order to improve the strength and toughness of the base metal and joint. It is preferable to contain one or more kinds.

The steel slab preferably contains Cu (copper) as a composition. By containing Cu (copper) as a composition, the steel slab can ensure the strength of the base material. When the amount of Cu (copper) added increases, there is a tendency that good HAZ toughness cannot be obtained.

That is, by setting the Cu (copper) content of the steel slab to 0.03% or more, good base material strength can be obtained. Further, by controlling the content of Cu (copper) in the steel slab to 1.50% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains Cr (chromium) as a composition. By containing Cr (chromium) as a composition, the steel slab can ensure the strength of the base material. When the amount of Cr (chromium) added increases, there is a tendency that good HAZ toughness cannot be obtained.

That is, by setting the Cr (chromium) content of the steel slab to 0.03% or more, good base material strength can be obtained. Further, by controlling the content of Cr (chromium) in the steel slab to 1.00% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains Mo (molybdenum) as a composition. By containing Mo (molybdenum) as a composition, the steel slab can ensure the strength of the base material. When the amount of Mo (molybdenum) added increases, there is a tendency that good HAZ toughness cannot be obtained.

That is, by setting the content of Mo (molybdenum) in the steel slab to 0.02% or more, good strength of the base material can be obtained. Further, by controlling the content of Mo (molybdenum) in the steel slab to 1.00% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains Nb (niobium) as a composition. By containing Nb (niobium) as a composition, the steel slab can ensure the strength of the base material and can achieve finer crystal grains. When the amount of Nb (niobium) added increases, there is a tendency that good HAZ toughness cannot be obtained.

In other words, by setting the content of Nb (niobium) in the steel slab to 0.003% or more, it is possible to obtain good strength of the base metal and to make the crystals in the slab finer. . Further, by controlling the content of Nb (niobium) in the steel slab to 0.100% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains V (vanadium) as a composition. By containing V (vanadium) as a composition, the steel slab can ensure the strength of the base material and can achieve finer crystal grains. When the amount of V (vanadium) added increases, there is a tendency that good HAZ toughness cannot be obtained.

In other words, by setting the content of V (vanadium) in the steel slab to 0.003% or more, it is possible to obtain good base material strength and to make the crystals in the slab finer. . Further, by controlling the content of V (vanadium) in the steel slab to 0.100% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains Ti (titanium) as a composition. By containing Ti (titanium) as a composition, the steel slab can ensure the strength of the base material and can achieve finer grains of crystals in the slab. When the amount of Ti (titanium) added increases, HAZ toughness tends to decrease due to coarse TiN.

That is, by setting the content of Ti (titanium) in the steel slab to 0.005% or more, it is possible to obtain good strength of the base material and to achieve finer grain size of the crystals. Further, by controlling the content of Ti (titanium) in the steel slab to 0.020% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains B (boron) as a composition. By containing B (boron) as a composition, the steel slab can improve hardenability even in a very small amount. As a result, when performing controlled cooling and quenching heat treatment, a significant increase in strength can be achieved. When the amount of B (boron) added increases, HAZ toughness tends to decrease due to the precipitation of coarse boron nitrides and carbides.

That is, good strength can be obtained by setting the B (boron) content of the steel slab to 0.0002% or more. Further, by controlling the B (boron) content of the steel slab to 0.0025% or less, suitable HAZ toughness can be ensured.

The steel slab preferably contains Ca (calcium) as a composition. By containing Ca (calcium), the steel slab can control the form of inclusions and improve toughness. Ca (calcium) combines with S to become CaS. CaS suppresses ductility-degrading cracking at grain boundaries and reduces surface cracking. When the amount of Ca (calcium) added increases, HAZ toughness decreases due to the formation of coarse Ca-containing inclusions. There is a tendency to

That is, by setting the Ca (calcium) content of the steel slab to 0.0005% or more, good strength can be obtained. Further, by controlling the content of Ca (calcium) in the steel slab to 0.0050% or less, suitable toughness of the HAZ can be ensured.

The steel slab preferably contains Mg (magnesium) as a composition. By containing Mg (magnesium), the steel slab can control the form of inclusions and improve toughness. Mg (magnesium) combines with S to become MgS. MgS can suppress ductility reduction cracking at grain boundaries.

Furthermore, MgS has a large effect of making the austenite grain size fine, and can reduce surface cracking during continuous casting or rolling. When the amount of Mg (magnesium) added increases, the HAZ toughness tends to decrease due to the formation of coarse Mg-containing inclusions.

That is, by setting the Mg (magnesium) content of the steel slab to 0.0005% or more, good strength can be obtained. Further, by controlling the content of Mg (magnesium) in the steel slab to 0.0030% or less, suitable toughness of the HAZ can be ensured.

The density of solidification nuclei on the surface of the steel slab is 0.35 pieces/mm ² or more. The density of coagulation nuclei on the surface is preferably 0.35 pieces/ ^mm2 or more and less than 5.00 pieces/ ^mm2 , and preferably 0.50 pieces/mm2 ^or more and less than 5.00 pieces/ ^mm2. More preferred.

If the density of solidified nuclei is 5.00 pieces/mm ² or more, it is not preferable because it is necessary to use mold powder that is cooled more intensely and to set the mold vibration frequency to a very high value. That is, if the cooling of the mold becomes too strong, there is a tendency for vertical cracks to occur more prominently due to non-uniform cooling of the steel slab within the mold. In addition, operational problems such as breakouts due to insufficient inflow of mold powder are likely to occur. From the above, it cannot be said that it is effective to extremely increase the density of coagulation nuclei (5.00 pieces/mm ² or more).

The density of solidification nuclei on the surface of a steel slab can be measured by the following method. For example, on the surface of a steel slab, a lump (solidification cell or dendrite cell) in which dendrite branches are oriented in approximately the same direction can be considered to have grown from one solidification nucleus. That is, by calculating the number of the lumps per predetermined area, the density of the solidified core can be calculated.

As a result of detailed investigation of actual Ni-containing steel slabs, it is desirable that the number of solidification nuclei be as large as possible in order to suppress surface cracking. Specifically, if the density of solidification nuclei on the surface of the steel slab is 0.35 pieces/mm ² or more, surface cracking can be effectively suppressed.

The continuous casting method for casting steel slabs explained above will be explained.
One way to increase the density of solidification nuclei on the surface of a continuously cast steel slab is to strengthen the cooling during the initial solidification of continuous casting, that is, to strengthen the cooling within the mold. .

Generally, continuous casting of steel slabs containing Ni includes a step of adding mold powder from above the surface of molten steel in a mold. The mold powder functions as an antioxidant, a heat insulator, and a lubricant between the mold and the solidified shell.

Since this mold powder flows into the gap between the solidified shell and the mold, there is no direct contact between the molten steel and the mold, and the molten steel is indirectly cooled by the mold via the inflow layer of mold powder.

Therefore, by adjusting the viscosity of the mold powder, the thickness of the inflow layer can be made thin, and the heat removal ability of the mold can be increased, thereby increasing the density of the solidification cores.

For example, the mold powder is composed of CaO, _SiO2, _Na2O , CaF2 _, _Al2O3 _, etc. The thermal conductivity of the mold powder is significantly lower than that of the metals molten steel and copper that constitutes the continuous casting mold.

Heat removal from molten steel to the mold depends on the thickness of the inflow layer of mold powder. The thinner the mold powder inflow layer, the higher the cooling efficiency of the mold. As the thickness increases, cooling efficiency decreases.

The thickness of the inflow layer of mold powder can be estimated from the consumption amount of mold powder. The thickness of the molding powder inflow layer is usually about 0.1 to 0.3 mm.

In order to form a thin mold powder inflow layer, it is preferable to use a mold powder with high viscosity. Therefore, the steel slab is preferably continuously cast by adding mold powder having a viscosity of 0.5 Pa·s (5 poise) or more at 1300° C. into the mold.

The viscosity of the mold powder at 1300°C is preferably 0.5 Pa·s (5 poise) or more and 5.0 Pa·s (50 poise) or less, and 1.0 Pa·s (10 poise) or more and 5.0 Pa·s (50 poise) or less is more preferable.

By setting the viscosity of the molding powder at 1300° C. to 0.5 Pa·s (5 poise) or more, it is possible to make it difficult for the molding powder to flow into the gap between the solidified shell and the mold. Therefore, the thickness of the inflow layer of mold powder can be reduced, and the heat removal from the molten steel to the mold can be increased. Thereby, the density of solidification nuclei can be increased, that is, 0.35 pieces/mm ² or more, and the occurrence of surface cracks can be suppressed.

The density of solidified nuclei can also be controlled by vibrating the mold at a predetermined frequency (oscillation cycle). For example, when the mold vibrates, some of the dendrite dendrites in the middle of solidification dissociate and adhere to the surface of the mold powder inflow layer. When dendrite dendrites adhere to the surface of the mold powder inflow layer, solidification nuclei are generated from that location. Thereby, the density of coagulation nuclei can be increased.

Continuous casting preferably includes, for example, a step of vibrating the mold at a frequency of 80 cycles or more per minute. The frequency at which the mold is vibrated is preferably 80 to 400 cycles, more preferably 100 to 400 cycles.

If the frequency of vibration of the mold is less than 80 cycles, there is a possibility that sufficient solidification core density cannot be ensured. Furthermore, if the frequency of vibration of the mold exceeds 400 cycles, the mold tends to resonate and casting becomes unstable.

The viscosity of the mold powder can be changed depending on the frequency (oscillation cycle) of the mold. For example, when the frequency (oscillation cycle) of the mold is 50 cycles per minute, it is preferable to use a molding powder with a viscosity of 0.5 Pa·s (5 poise) or more at 1300°C. Even in this case, the density of solidification nuclei on the surface of the Ni-containing steel slab can be set to 0.35 pieces/mm ² or more.

Further, for example, when the vibration frequency (oscillation cycle) of the mold is 80 cycles per minute, it is preferable to use a molding powder having a viscosity of 0.15 Pa·s (1.5 poise) or more at 1300°C. Even in this case, the density of solidification nuclei on the surface of the Ni-containing steel slab can be set to 0.35 pieces/mm ² or more.

In particular, it is preferable to use a molding powder with a mold frequency (oscillation cycle) of 80 cycles or more per minute and a viscosity of 0.5 Pa·s (5 poise) or more at 1300°C. Under such conditions, surface cracking of the steel slab can be significantly reduced.

By the way, on the surface of a steel slab containing Ni, there are a surface containing fine dimples and an extremely flat surface. It is thought that the surface containing dimples was formed when grain boundaries were destroyed in the ductility decreasing temperature range of 600 to 900°C.

Based on its shape, the flat surface is thought to have been formed by a type of solidification cracking. More specifically, when molten steel solidifies, C, S, P, etc. are concentrated in the final solidified part, and the melting point is lowered. The flat surface is formed by shrinking the surrounding area that has already been solidified while a low melting point liquid phase is present in the final solidified part.

More specifically, surface cracking occurs when solute elements such as C, S, and P are concentrated at the boundary between two solidified cells that make up the solidified shell when the solidified shell grows within the mold. As a result, a liquid phase with a low melting point is generated, resulting in solidification cracking. Starting from this solidification crack, it is thought that the crack progresses further due to thermal stress in the secondary cooling zone, stress due to bending straightening, etc.

Therefore, it is not possible to sufficiently suppress surface cracks simply by relaxing the thermal stress and bending straightening stress in the secondary cooling zone, which has been conventionally practiced. In order to suppress surface cracking, it is important and necessary to suppress solidification cracking during initial solidification in the mold.

Solidification cracking is less likely to occur as the concentration of solute elements in the final solidification zone decreases, and as the thermal stress acting on the final solidification zone decreases. For example, when the size of the solidification cell becomes smaller, the cooling rate naturally increases and concentration of solute elements is suppressed. Moreover, when the size of the solidification cells is small, thermal stress is dispersed, and the thermal stress acting on the interface of each solidification cell becomes small. Reducing the size of the solidification cells is effective in preventing solidification cracking.

It is generally known that there is a correlation between the number of coagulation cells and the number of coagulation nuclei, and the size of the coagulation cells becomes smaller as the density of coagulation nuclei increases. Therefore, the size of the solidification cell can be reduced by increasing the density of solidification nuclei in the portion where the molten steel contacts the mold.

That is, since the Ni-containing steel slab of the present invention has a high density of solidification nuclei, the size of the solidification cells can be reduced. As a result, a Ni-containing steel slab with less surface cracking can be provided.

According to the continuous casting method of the present invention, initial solidification on the surface of a steel slab can be controlled. As a result, a large number of solidification nuclei are generated, and the concentration of impurity elements such as P (phosphorus) and S (sulfur) and C (carbon) on the interface of the solidification cell is reduced. Therefore, solidification cracking at the interface of the solidification cells can be suppressed. Therefore, so-called surface cracks occurring on the surface of the steel slab can be suppressed.

As described above, the Ni-containing steel slab according to the present invention has a density of solidification nuclei on the slab surface of 0.35 pieces/mm ² or more. Therefore, the size of the coagulation cell can be reduced. This reduces the concentration of S and P at the interface of the solidification cells, so that embrittlement at the interface of the solidification cells can be suppressed. Moreover, the stress acting on the interface of the solidification cell is also dispersed, and solidification cracking at the interface of the solidification cell can be suppressed. As a result, the occurrence of cracks on the surface of the steel slab can be reduced.

Further, in the present invention, a method for manufacturing a steel slab using a slab produced by the above-mentioned continuous casting will be described. The method for manufacturing steel slabs containing Ni includes a care process in which a slab manufactured by a continuous casting method is treated, and after the care process, the slab is heated at a heating temperature of 1100°C or less in a heating furnace. and a heating step.

In the method for manufacturing steel slabs, steel slabs are manufactured using slabs produced by continuous casting of molten steel. Conventionally, the generated slab is heated at 1000 to 1200° C. (first heat treatment). The slab subjected to the first heating is subjected to preliminary rolling (light blooming) to a thickness of about 60 to 90%. The pre-rolled slab is ground until there are no flaws, and a care process is performed to remove the flaws. The treated slab is heated at 1000 to 1200°C (second heat treatment). Rolling (main rolling) is performed on the slab that has been subjected to the second heat treatment.

It has been confirmed that steel slabs manufactured by conventional manufacturing methods have cracks accompanied by scale at austenite grain boundaries. The composition of this scale includes Fe ₂ SiO ₄ (firelite). The presence of Fe ₂ SiO ₄ (firelite) is cited as one of the causes of cracks accompanied by scale.

Fe ₂ SiO ₄ scale is produced in steel containing 0.05% or more of silicon (Si) as Fe ₂ SiO ₄ is produced. Note that the eutectic temperature of Fe ₂ SiO ₄ with wustite (FeO) is 1170°C. Fe ₂ SiO ₄ is a liquid phase oxide above the eutectic temperature.

When Fe ₂ SiO ₄ scale is generated, grain boundaries become brittle. Since Fe ₂ SiO ₄ scale is in a liquid phase at high temperatures, it easily diffuses into grain boundaries and deep into the matrix.

After Fe ₂ SiO ₄ scale is generated, intergranular cracking occurs due to thermal stress or strain during rolling. Therefore, even if the occurrence of surface flaws is suppressed in continuous casting, flaws are formed on the surface of the product steel slab due to the presence of Fe ₂ SiO ₄ (firelite).

Therefore, when manufacturing a steel slab, it is preferable that the manufacturing process be performed at a temperature equal to or lower than the eutectic temperature of Fe ₂ SiO ₄ . In other words, it is preferable that the heating temperature of the heating furnace be equal to or lower than the eutectic temperature of Fe ₂ SiO ₄ .

Note that the formation of Fe ₂ SiO ₄ scale at grain boundaries is also related to the segregation of elements such as P and S in the regions. For this reason, even if the manufacturing process is performed at a temperature below the eutectic temperature of about 1100° C., for example, liquid phase Fe ₂ SiO ₄ scale may be generated in a part of the region.

Therefore, in order to suppress the occurrence of cracks accompanied by Fe ₂ SiO ₄ scale, it is preferable that the heating step is performed at a temperature of 1100° C. or lower, and it is more preferable that the heating step is performed at a temperature of 1050° C. or lower. . Further, it is preferable that the rolling step is performed after the slab is heated in the heating step under such conditions.

By the way, when a slab is produced by controlling the density of solidified cores by the continuous casting method of the present invention, it has superior toughness than conventional slabs, so it can be used to produce steel slabs without pre-rolling such as light blooming. can be manufactured.

Therefore, when a slab is produced by controlling the density of solidification nuclei by the continuous casting method of the present invention, a steel slab can be produced by performing the following (1) to (3).

(1) A care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws (care process).
(2) Heating the treated slab at 1100° C. or lower (heating step).
(3) The slab subjected to the heating process is rolled (main rolling).

In this way, by performing the heat treatment at a temperature of the slab of 1100° C. or lower, it is possible to suppress the flaws on the surface of the steel slab (product) to a level that does not pose a problem as a product. In other words, by producing steel slabs in this way, the formation of Fe ₂ SiO ₄ scale can be suppressed, and steel slabs (products) with suppressed occurrence of defects can be produced in one rolling process. It becomes possible to manufacture.

3.5% Ni steel with a Ni content of 3.5% by mass, 5% Ni steel with a Ni content of 5.0% by mass, and 5% Ni steel with a Ni content of 7.0% by mass. Molten steel was created by melting steel. A converter and an RH vacuum degassing device were used to create the molten steel.

A test was conducted to cast this molten steel using a vertical bending type continuous slab casting machine. The vertical bending continuous slab casting machine had a thickness of 250 mm and a width of 2100 mm. The test was conducted in a total of 20 heats (Test Nos. 1 to 20). Table 1 shows the chemical components of Test Nos. 1 to 20. Table 2 shows the casting conditions in the continuous casting machine for test Nos. 1 to 20.

For Test Nos. 1, 4, 6, 9, 11, 14, and 16, the casting speed was 0.8 m/min. The amplitude of the mold oscillation was 8 mm. The frequency of the oscillation was 60 cycles per minute. The molding powder used had a viscosity of 0.20 Pa·s at 1300°C.

For Test Nos. 2, 3, 5, 7, 8, 10, 12, 13, 15, 17, and 18, the casting speed was 0.8 m/min. The amplitude of the mold oscillation was 8 mm. The frequency of the oscillation was 60 cycles per minute. The molding powder used had a viscosity of 0.06 to 2.00 Pa·s at 1300°C.

For test No. 19, the casting speed was 0.8 m/min. The amplitude of the mold oscillation was 8 mm. The frequency of the oscillation was 80 cycles per minute. The molding powder used had a viscosity of 0.20 Pa·s at 1300°C.

For test No. 20, the casting speed was 0.8 m/min. The amplitude of the mold oscillation was 8 mm. The frequency of the oscillation was 80 cycles per minute. The molding powder used had a viscosity of 0.5 Pa·s at 1300°C.

The cast steel slab was cut into a length of 300 mm. The cut samples were subjected to the following treatments and then evaluated for surface cracks. The surface of each sample was subjected to shot blasting to remove the oxide film on the surface. Thereafter, surface cracks were determined by penetrant testing. Regarding the determined surface cracks, the length and number of cracks were measured.

In order to investigate the depth of surface cracks, grinding was performed at a distance of 3 mm, 6 mm, and 9 mm from the surface. On each ground surface, surface cracks were determined by penetrant testing. Regarding the determined surface cracks, the length and number of cracks were measured.

The density of solidified nuclei on the surface of the slab was measured by the following method. A sample was taken from the surface of a steel slab, and the oxide film on the surface was removed by shot blasting. The surface of the steel slab from which the oxide film had been removed was mirror-ground and corroded with picric acid to reveal the solidified structure.

The coagulated tissue that appeared was photographed. In the photograph, a mass (coagulation cell or dendrite cell) in which dendrite branches were oriented in almost the same direction was considered to have grown from one coagulation nucleus. The density of coagulation nuclei was calculated by calculating the number of the lumps per predetermined area.

Specifically, the number of coagulated cells was counted as the mass shown in the photograph of the coagulated tissue, and the density was determined by dividing by the area occupied by the coagulated cells. Furthermore, the size of the coagulation cell tends to be small near the oscillation mark and to become large away from the oscillation mark. For this reason, the range in which coagulation cells were counted was from one oscillation mark adjacent to the other oscillation marks, and the average value was determined.

Table 3 shows test no. The results of the investigation of the density of solidified nuclei from 1 to 20 and the total crack length (crack length x number of cracks) are shown.

In Test Nos. 1, 6, 11, and 16, the density of coagulation nuclei was less than 0.35 pieces/mm ² . In Test Nos. 1, 5, 9, and 13, many surface cracks occurred. In addition, in Test Nos. 1, 6, 11, and 16, cracks occurred at a distance of 3 mm or 6 mm from the surface.

On the other hand, in Test Nos. 2 to 5, 7 to 10, 12 to 15, and 17 to 20, results in which the density of coagulation nuclei was higher than 0.35 pieces/mm ² were obtained. Referring to the results of Test Nos. 2 to 5, 7 to 10, 12 to 15, and 17 to 20, as the density of solidification nuclei increases, the occurrence of surface cracks decreases.

In particular, test No. 2 in which the density of coagulation nuclei exceeds 1.50 pieces/ ^mm2 . In samples No. 5, 10, 15, and 20, a significant reduction in surface cracking was confirmed. In addition, in the remarks column of Table 3, tests within the scope of the present invention are indicated as "invention examples", and other tests are indicated as "comparative examples".

(Manufacturing test of steel slabs)
Test No. Slabs 1 to 20 were subjected to a heating process and a rolling process in the following manner to produce steel slabs (hereinafter also referred to as products). The products were created using "Method A", which corresponds to the conventional manufacturing method, and "Method B", which corresponds to the manufacturing method of the present invention.

(Method A)
A care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws. The treated slab is heated at 1050°C or 1200°C (first heat treatment). The slab subjected to the first heating is pre-rolled (light blooming) to a thickness of 250 mm to 190 mm. The pre-rolled slab is cleaned by grinding approximately 3 to 6 mm from the surface to remove any flaws until there are no flaws. The treated slab is heated at 1050°C or 1200°C (second heat treatment). The slab subjected to the second heat treatment was subjected to main rolling, and its thickness was changed from 190 mm to 25 mm. Thereafter, flaws were inspected on surfaces at distances of 3 mm, 6 mm, and 9 mm from the surface (hereinafter also referred to as product inspection).

(Method B)
A care process is performed to remove the flaws by grinding approximately 3 to 6 mm from the surface until the generated slab is free of flaws (care process). The treated slab is heated at 1050°C or 1200°C (heating step). The slab subjected to the heating process was subjected to main rolling to a thickness of 25 mm from 250 mm. Thereafter, product inspection was performed on surfaces at distances of 3 mm, 6 mm, and 9 mm from the surface.

Product inspection was evaluated in the following three stages.
○: No product defects △: Small amount of product defects (can be used as a product with proper care)
×: Large amount of product flaws (Flaws remain even after care treatment, making it impossible to use as a product)

Table 4 shows the occurrence of surface flaws after rolling each product.

When the slab was manufactured using Method A at a heating temperature of 1200°C, some flaws remained in the product, but the flaws were within a range that did not hinder production.

5-7% Ni steel test No. For Nos. 1, 6, and 11, the amount of treatment was 9 mm. In contrast, the amount of care treatment for the other 5-7% Ni steel tests was 3-6 mm. Also, test No. 3.5% Ni steel. For No. 16, the amount of care treatment was 6 mm. In contrast, the amount of care treatment for these other tests for the 3.5% Ni steel was 3 mm. That is, in the example of the present invention, the amount of maintenance processing can be significantly reduced compared to the comparative example.

Test No. Slabs 2 to 5, 7 to 10, 12 to 15, and 17 to 20 heated at a heating temperature of 1050°C were evaluated as "○: No product defects" even if light blooming was not performed. It was done. Therefore, for these tests, process costs can be significantly reduced and product quality can be stably improved.

As a result of investigating product flaws (defects) using an EPMA (Electron Probe MicroAnalyzer), it was found that in a test example in which the slab was heated to 1200°C, a large amount of Fe ₂ SiO ₄ component scale was contained in the flaws. Ta. It was found that the influence of oxide scale during heating of the slab significantly contributed to the formation of defects.

Claims

In mass%, C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1.00% or less, P: 0.001% or more 0.010% or less, S: 0.0001% or more and 0.0050% or less, Ni: 2.0% or more and less than 7.5%, Al: 0.010% or more and 0.080% or less, N: 0.0010 % or more and 0.0050% or less, O: 0.0005% or more and 0.0040% or less, and the remainder is Fe and unavoidable impurities. A steel slab having a density of solidification nuclei on the surface of 0.35 pieces/mm 2 or more.
Further, in mass%, Cu: 0.03% or more and 1.50% or less, Cr: 0.03% or more and 1.00% or less, Mo: 0.02% or more and 1.00% or less, Nb: 0.003 % or more and 0.100% or less, V: 0.003% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, B: 0.0002% or more and 0.0025% or less, Ca: 0 The steel slab according to claim 1, containing one or more selected from .0005% to 0.0050%, Mg: 0.0005% to 0.0030%.
A continuous casting method for casting the steel slab according to claim 1 or 2,
A continuous casting method including a step of adding mold powder having a viscosity of 0.5 Pa·s (5 poise) or more at 1300°C into a mold.
A continuous casting method for casting the steel slab according to claim 1 or 2,
A continuous casting method that includes the step of vibrating a mold at a frequency of 80 cycles or more per minute.
A continuous casting method for casting the steel slab according to claim 1 or 2,
A step of adding mold powder having a viscosity of 0.5 Pa·s (5 poise) or more at 1300° C. into the mold,
A continuous casting method comprising the step of vibrating a mold at a frequency of 50 cycles or more per minute.
A continuous casting method for casting the steel slab according to claim 1 or 2,
Adding mold powder with a viscosity of 0.15 Pa·s (1.5 poise) or more at 1300°C into the mold;
A continuous casting method comprising the step of vibrating a mold at a frequency of 80 cycles or more per minute.
A care step of performing care treatment on the slab manufactured by the continuous casting method according to claim 3;
A method for producing a steel slab containing Ni, including a heating step of heating the slab at a heating temperature of 1100° C. or lower in a heating furnace after the care step.
A care step of performing care treatment on the slab manufactured by the continuous casting method according to claim 4;
A method for producing a steel slab containing Ni, including a heating step of heating the slab at a heating temperature of 1100° C. or lower in a heating furnace after the care step.
A care step of performing care treatment on the slab manufactured by the continuous casting method according to claim 5;
A method for producing a steel slab containing Ni, including a heating step of heating the slab at a heating temperature of 1100° C. or lower in a heating furnace after the care step.
A care step of performing care treatment on the slab manufactured by the continuous casting method according to claim 6;
A method for producing a steel slab containing Ni, including a heating step of heating the slab at a heating temperature of 1100° C. or lower in a heating furnace after the care step.