JP3039369B2 - Method for producing Ni-containing steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous casting method and a continuous casting-hot direct rolling method for Ni-containing steel suitable as a material for low temperature.

[0002]

2. Description of the Related Art Conventionally, it has been known that the addition of Ni to steel improves the low-temperature toughness, and steel containing about 2 to 10% of Ni has been used as a low-temperature material. Above all, Ni 9
% Steel is used in tank materials such as liquefied natural gas because it can withstand temperatures below -160 ° C.

On the other hand, continuous casting in the steel manufacturing process
It has significant effects on improving yield, saving labor and increasing productivity. Currently, in the production of steel slabs, except for special materials and dimensional restrictions, etc.
100% is continuously cast. However, Ni 5.5 ~ 10
% Steel is more susceptible to surface lateral cracks, subcutaneous cracks and corner cracks than ordinary carbon steel and low alloy steel, making it difficult to produce by continuous casting.

[0004] These cracks occur when the surface temperature of the slab reaches 600 to 850 ° C at the time of secondary cooling in continuous casting, where the hot ductility is reduced. Since the steel containing 5.5 to 10% Ni solidifies with the γ phase as the primary crystal, segregation of S and P at the grain boundaries becomes remarkable, and as a result, compared with ordinary carbon steel and low alloy steel, ~ 850 ° C
It is believed that the ductility of the steel in the steel decreases and the susceptibility to cracking during continuous casting increases.

[0005] In order to continuously cast such Ni-containing steel, several proposals have been made to improve the cooling method.

The method disclosed in Japanese Patent Application Laid-Open No. 57-32862 is
Take a cooling pattern of weak cooling such that the surface temperature at the aforementioned correction point can avoid the temperature range where ductility decreases on the high temperature side,
In addition, the surface temperature of the slab is made uniform. In addition to the above publication, the use of an oval type nozzle or a steam-water nozzle (mist nozzle) as the secondary cooling water nozzle makes the slab surface temperature uniform, thereby reducing the thermal stress generated on the slab surface. As a result, it is described that it is possible to prevent a slab surface flaw. However, even if these measures are taken, the ends in the slab width direction (slab slab corners) are easily cooled, and the effect of recuperation from the inside of the slab is small.
It is difficult to stably maintain the surface temperature at the correction point at a temperature equal to or higher than the temperature at which ductility decreases.

In Japanese Patent Publication No. 5-4169, 1150-95
Based on the finding that controlling the cooling rate to 20 ° C / min or less in the temperature range of 0 ° C improves ductility, the cooling rate during continuous casting is reduced to 20 ° C / min or less in the temperature range of 1150 to 950 ° C. A method has been proposed in which surface cracking during continuous casting is prevented by control.

Although these methods can effectively prevent surface cracks, it is actually difficult to stably control the cooling of continuous cast slabs in actual operation.

Further, it is known that P and S are liable to be broken when segregated at the grain boundary, so that it is better to reduce their contents. In addition, it is described that the effect of preventing cracks is stabilized by increasing the purity to S: 0.003% or less, P: 0.010% or less, and N: 0.004% or less.

The present inventors have disclosed in Japanese Patent Application Laid-Open No. 7-90504 that, in addition to purifying the contents of P and S in steel to a level much lower than conventionally required, N and A Ni-containing steel for low-temperature use in which high-temperature ductility is further improved by limiting the Al content to a certain range, and a secondary cooling method for a continuous cast slab thereof have been disclosed.

As described above, various methods such as improvement of the cooling method and change of the composition have been tried for low-temperature steel, but the generation of surface cracks cannot be reduced or completely prevented. Have difficulty.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a continuous casting of a Ni-containing steel excellent in low-temperature toughness and a production method by continuous casting and hot direct rolling.

[0013]

The gist of the present invention is as follows.
(1) and (2) in the method for producing a Ni-containing steel.

(1) C: 0.1% or less, Si: 0.5% by weight
%, Mn: 1.0% or less, Ni: 5.5-10%, P: 0.002
% Or less, S: 0.002% or less, Al: 0.02% or less, and N: 0.001 to 0.004%. A method for producing a steel slab using a curved or vertical bending type continuous casting machine, comprising: The superheat of the molten steel supplied to the
In the next cooling zone, the casting speed Vc (m / min)
The water density W [liter / (min · c)
m ² )), between the rolls in the casting direction, cooling so that the relationship of the following formula (2) is established, and the thickness of the columnar γ grain layer on the surface layer of the slab is 25 mm or less A method for producing a Ni-containing steel slab. Hereinafter, this is referred to as a first method of the present invention.

W = VL / S (1) 0.02 Vc> W (2) where W: water density of secondary cooling [liter / ( mincm
² )] VL: The amount of water sprayed on the slab surface between the rolls in the secondary cooling zone (liter / min) S: Slab surface area between the rolls (cm ² ) Vc: Casting speed Vc (m / min) (2) A method for producing a steel slab by producing a Ni-containing steel slab by the method for producing a Ni-containing steel slab according to the above (1), and then directly sending the steel to a rolling step while hot to perform rough rolling. Then, the slab during continuous casting is secondarily cooled to a temperature of 600 ° C. or higher, and then the slab is charged into a heating furnace while maintaining the temperature at a temperature of 600 ° C. or higher, and then subjected to rough rolling. A method for producing a Ni-containing billet, which is a feature. Hereinafter, this is referred to as a second method of the present invention.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The first method of the present invention employs a continuous casting machine of a curved or vertical bending type (hereinafter referred to as a continuous casting machine) to superheat a molten steel supplied into a mold (hereinafter referred to as ΔT). And the secondary cooling conditions were adjusted appropriately, and the thickness of the columnar γ grain layer was 25 mm in the surface layer of the Ni-containing steel slab having the above composition.
The following is assumed.

First, the details of the study for limiting the ΔT and the secondary cooling conditions and the results thereof will be described. Hereinafter,% in the notation of a chemical composition means a weight ratio.

As described above, Ni represented by 9% Ni steel
The steel containing 5.5 to 10% causes more slab surface cracks than ordinary carbon steel and low alloy steel, and is difficult to manufacture by continuous casting. The cracks occur along the γ grain boundary due to the strain of the slab during bending or straightening, and fine AlN precipitated at the grain boundary affects the crack. In addition, Ni
%, The cracking sensitivity is extremely increased. Fe-Ni
In the phase diagram of the system, since Ni is γ-solidified at 5.5% or more, it is clear that cracking sensitivity is increased due to solidification with the γ phase as a primary crystal.

In order to evaluate such cracking susceptibility,
Investigating the hot ductility of steel is a common practice and it has been found that there is a good correlation. It is also known that high-temperature ductility can be improved by reducing the size of γ grains, as shown in, for example, Iron and Steel, 67 (1981), P. 1180. However, it is not clear how to control the growth of γ grains in the continuous casting process for steel types that solidify with γ primary crystals.

The present inventor has set forth Table 1 in order to study the effect of casting conditions on the growth of γ grains when solidifying with γ primary crystals.
9% Ni steel having the composition shown in the following table was smelted, and a slab having a cross section of 40 cm wide × 18 cm thick was cast by a vertical test continuous caster having a machine length of 4 m, and then subjected to secondary cooling.

[0021]

[Table 1]

In this test, the Δ
T and the secondary cooling pattern (temperature history) were used as parameters. The obtained slab was cut and etched to examine the γ grain growth behavior of the surface layer of the slab. The result is shown in FIG.

FIG. 1 shows that ΔT is 60 ° C. and 15 ° C., the casting speed is 1.0 m / min, and the specific water volume of the secondary cooling is 0.63 liter / (kg ·
FIG. 4 is a schematic view of a macro structure photograph of a surface layer portion of a slab under the condition of (steel). FIG. 1 (a) shows that ΔT is 60 ° C.
(b) is a case where ΔT is 15 ° C. The specific water amount of the secondary cooling is 0.019 liter / (min · cm ² ) in terms of the water density W of the secondary cooling described later.

As shown in FIG. 1A, when ΔT is as high as 60 ° C., a columnar γ-grain layer (columnar γ-grain layer) is formed from the slab surface to a region of 30 mm or more. On the other hand, ΔT
When the temperature is as low as 15 ° C., as shown in FIG. 1 (b), the thickness of the columnar γ-grain layer is clearly thin, and a chill-like fine γ-grain layer (fine γ-grain layer) Had been generated.

The columnar γ grain layer is a layer composed of γ grains grown in the thickness direction of the slab, and can be clearly distinguished from a portion where the γ grains are more granular on the inner side of the slab. The γ grain boundaries can be revealed by etching with, for example, nital. The thickness of the columnar γ-grain layer was not affected by the arrangement of the secondary cooling nozzles and the like, and had grown with a substantially uniform thickness in the width direction. If ΔT is reduced, overcooling is likely to occur,
Since the generation frequency of solidification nuclei increases, a chill-like fine γ-grain layer is formed on the surface layer of the slab, and the growth of columnar γ-grain is suppressed inside.

Further, in this continuous casting test, γ grain boundary cracks were generated along the columnar γ grains although no mechanical stress was applied to the slab from the outside. This γ grain boundary crack occurs only in the columnar γ grain layer, and does not occur at all in other parts such as the inside of the cast slab or the surface chill-like fine γ grain layer generated when ΔT is low. Did not. When this crack appears on the surface of the slab, it becomes a problem as a surface crack.

Therefore, if the thickness of the columnar γ grain layer in the surface layer of the slab is reduced, it is possible to suppress grain boundary cracking. Also, if chill-like fine γ grains are present in the surface layer of the slab, no cracks appear on the surface, which is effective in suppressing cracks.

Then, ΔT was variously changed under the same secondary cooling conditions, and the thicknesses of the columnar γ grain layer and the fine γ grain layer on the surface layer were examined. The result is shown in FIG.

FIG. 2 is a diagram showing the effect of ΔT of molten steel on the thickness of the columnar γ grain layer and the fine γ grain layer on the surface layer of the slab. As shown in the figure, it is clear that the reduction of ΔT suppresses the growth of columnar γ grains, and that when the temperature is lowered to 30 ° C. or lower, chill-like fine γ grains are generated on the surface layer of the slab.

Next, the effect of secondary cooling conditions on the growth of γ grains in the slab was investigated. That is, the thickness of the columnar γ-grain layer was investigated when ΔT was substantially constant at 25 to 30 ° C. and the water density W was variously changed at various casting speeds. However, 2
The next cooling was performed uniformly over the entire length of the slab.

The water amount density W for secondary cooling described above [liter / liter]
(min · cm ² )] is defined by the following equation (1).

W = VL / S (1) where VL is the amount of water (liter / min) injected onto the slab surface between the rolls in the secondary cooling zone. The slab surface area between the rolls (cm ² ) The water density W varies between the nozzle front and the roll,
For the sake of simplicity, the ratio between the amount of water VL injected onto the slab surface between the rolls in the secondary cooling zone and the slab surface area S between the rolls was defined as in the above equation (1). The reason for using the water density W is as follows.

Usually, when expressing the strength of the secondary cooling, a specific water amount is used, which means an amount of cooling water used per 1 kg of the weight of the slab. This specific water amount is appropriate as an index indicating the cooling state of the entire slab such as the average temperature of the cross section of the slab, but compares the cooling state near the surface layer of the slab with different slab thickness and casting speed. It is not a possible indicator. Therefore, in the present invention, the water density W representing the amount of water per unit area
Was used. FIG. 3 shows the results of investigation on the thickness of the columnar γ grain layer.

FIG. 3 is a graph showing the influence of the water density W and the casting speed Vc on the thickness of the columnar γ grain layer on the surface layer of the slab. As shown in the drawing, the thickness of the columnar γ-grain layer increased as the water density W increased at any casting speed, and the columnar γ-grain layer grew at a lower casting speed Vc at the same water density W. On the other hand, changing the casting speed Vc or the water density W has no effect on the chill-like fine γ grains on the surface layer of the slab. When the water density W is increased, the heat flow velocity from the slab surface increases, so that the directional solidification of the γ grains becomes remarkable, and the thickness of the columnar γ grain layer on the surface layer of the slab increases. FIG. 4 is obtained by rearranging the results in relation to the casting speed Vc and the water density W.

FIG. 4 is a diagram showing the thickness of the columnar γ-grain layer on the surface layer of the slab, organized according to the relationship between the casting speed Vc (m / min) and the water density W. According to FIG. 4, there is a good linear relationship between the casting speed Vc and the water density W, and if control is performed so that the relationship of the following formula (2) is established, when ΔT is 25 to 30 ° C. It can be seen that the thickness of the columnar γ grain layer can be suppressed to 25 mm or less.

0.02Vc> W (2) Further, as shown in FIG. 2, the thickness of the columnar γ-grain layer on the surface layer of the slab becomes larger as ΔT increases. From above (2)
If the relationship of the formula is satisfied, when ΔT is 30 ° C. or less, the thickness of the columnar γ grain layer can be suppressed to 25 mm or less.

If the thickness of the columnar γ-grain layer on the surface layer of the slab is suppressed to 25 mm or less, fine AlN precipitated at the grain boundaries may exist to some extent, or the slab surface temperature may be reduced to a region where the hot ductility decreases. Even if it is, it is possible to reduce these effects and reduce or prevent cracks generated along the γ grain boundary due to distortion of the slab during bending or straightening.

To suppress the thickness of the columnar γ-grain layer on the surface layer of the slab to 25 mm or less, a more desirable condition of ΔT is 20 ° C. or less, and a desirable lower limit in consideration of avoiding nozzle clogging is about 5 ° C. .

As described above, when ΔT is reduced, the growth of columnar γ grains is suppressed, and when the temperature is lowered to 30 ° C. or lower, chill-like fine γ grains are generated on the surface layer of the slab. Further, when the water density of the secondary cooling is reduced, the growth of columnar γ grains is suppressed. This is γ
This is a unique behavior when solidifying with primary crystals. Even if the method of the present invention is applied to a steel type that solidifies with δ primary crystals, such as ordinary carbon steel, there is no effect on changes in γ grain growth.

In this casting test, secondary cooling was performed uniformly over the entire length of the slab. However, according to the water density distribution in the actual production by ordinary continuous casting, it was divided into several zones from immediately below the mold to the end of the continuous casting machine. The water amount is controlled so that the water amount density is gradually reduced for each zone immediately below the mold. However, from the viewpoint of the growth behavior of the solidified shell, the growth of columnar γ grains has been completed from the mold to the portion immediately below the mold, and if the relationship of 0.02Vc> W holds for the portion having the highest water density,
The growth of columnar γ grains can be suppressed.

Further, when a portion having a high water amount density is generated due to poor water amount control or the like, cracks along the columnar γ-grain layer are significantly increased due to thermal stress. Therefore, it is necessary that the relationship of the above formula (2) is established even in the portion having the highest water density, and in the method of the present invention, the relationship of 0.02Vc> W of the above formula (2) is established for all the rolls. And limited.

Further, a rolling test was performed on the slab obtained in the above test. As a result, it was found that if fine γ grains in the form of chill exist in the surface layer of the slab and the thickness of the columnar γ grain layer is 25 mm or less, no flaw is generated on the surface of the hot-rolled material.

It is desirable that the thickness of the columnar γ-grain layer on the surface layer of the slab be as thin as possible, but it is practically difficult to reduce the thickness to about 15 mm or less as shown in FIG. 2.

Next, the reason why the composition of the steel slab to be subjected to the first method of the present invention is limited as described above will be described.

C: 0.1% or less C is inevitably contained in the steel production process and is an element necessary for ensuring the strength of the steel. If it exceeds 0.1%, the strength becomes too high, and the low-temperature toughness is adversely affected. Desirable lower limit is
It is about 0.02%.

Si: 0.5% or less Si is added in a refining process for deoxidation. A desirable lower limit is about 0.1%.

On the other hand, if the content exceeds 0.5%, the low-temperature toughness is adversely affected.

Mn: 1.0% or less Mn is also added in the refining process for deoxidation and the like. A desirable lower limit is about 0.4%. In addition, there are effects of improving hardenability and securing strength, but if the content exceeds 1.0%, these effects are saturated.

Ni: 5.5 to 10% Ni is effective in improving low-temperature toughness as described above. 5.5%
If it is less than 1, since the primary crystal at the time of solidification is the δ phase, the form of solidification is different, and the effect of controlling the γ particle size by ΔT cannot be obtained. On the other hand, even if it exceeds 10%, the effect of improving low-temperature toughness is not recognized.

P: 0.002% or less P is known as a typical element that segregates during solidification of a slab. In steels that solidify with the γ phase as primary crystals, such as steel containing 5.5 to 10% Ni, the final solidification position coincides with the γ grain boundary.
That is, if P exceeds 0.002%, segregation during solidification at the γ grain boundary significantly embrittles the γ grain boundary, so the upper limit was made 0.002%. On the other hand, it is desirable to reduce the P content as much as possible. When the content is 0.001% or less, cracking along the columnar γ grains is reduced, and high-temperature ductility is also improved. However, P is also an unavoidable impurity, and a desirable lower limit in terms of cost is 0.00.
It is about 03%. Therefore, the preferable range of the P content is 0.
About 0003% to 0.002%, and more preferably about 0.0003% to 0.001%.

S: 0.002% or less S, like P, is an element that segregates during solidification of a slab. γ
In a steel type that solidifies with a phase as a primary crystal, if S exceeds 0.002%, the γ grain boundary becomes significantly embrittled, so the upper limit is 0.002%.
%. On the other hand, it is desirable to reduce the S content as much as possible. When the content is 0.001% or less, cracks along the columnar γ grains are reduced, and more excellent ductility is obtained. However, a desirable lower limit in terms of cost is about 0.0003%. Therefore, a desirable range of the S content is about 0.0003% to 0.002%, and a more desirable range is about 0.0003% to 0.001%.

Al: 0.02% or less Al is added in the refining process for deoxidation. The desirable lower limit is about 0.003%. On the other hand, if the Al content exceeds 0.02%, Al combines with N to generate excess AlN, weakening grain boundaries and increasing cracking susceptibility.

N: 0.001 to 0.004% N is combined with Al to form AlN as described above. If the N content exceeds 0.004%, an excessive amount of AlN is generated to weaken the grain boundaries and cause crack susceptibility. Enhance. Therefore, its content is preferably reduced as much as possible. However, the normal steel manufacturing process inevitably contains a certain amount of N,
% Is difficult.

Further, the above steel slab may further contain Ti, Mo or the like according to the purpose. Although Ti deteriorates the low-temperature toughness, it has the effect of reducing the γ grain size. Although Mo also deteriorates the low-temperature toughness, it improves the ductility from 600 to 700 ° C and also increases the strength. Composite addition of Ti and Mo can also be selected. Desirable content when containing Ti and / or Mo,
0.005 to 0.02% for Ti and 0.02 to 0.1% for Mo.

Next, the second method of the present invention and the reason for limiting the slab temperature will be described.

According to the second method of the present invention, (1) a Ni-containing steel slab is produced by the method of the first method of the present invention, and then directly sent to a rolling step while hot, subjected to heating and rough rolling. To manufacture billets. At this time, secondary cooling is performed so that the slab temperature is maintained at 600 ° C. or higher during continuous casting, and the slab is charged into a heating furnace while maintaining the slab temperature at 600 ° C. or higher, and is heated to a predetermined temperature.

In steel slabs containing 5.5 to 10% of Ni, cracks occur in the slabs during the cooling process after continuous casting, and these cracks appear on the surface during rolling. It may be. To elucidate the cause of this occurrence, the coefficient of linear expansion during cooling was investigated. The results obtained are shown in FIG.

FIG. 5 is a diagram showing the temperature change of the linear expansion coefficient E (%) when the temperature of a steel slab containing 5.5 to 10% of Ni is raised and lowered at 5 ° C./min.

In the steel slab containing 5.5 to 10% Ni, a large change in the coefficient of linear expansion was observed at about 550 ° C. as shown in the figure, and the slab passed through this temperature range during continuous casting. Transformation stress occurs. Therefore, if the structure is refined by rolling before the slab reaches about 600 ° C., cracks inside the slab can be prevented. For this purpose, it is necessary that the slab temperature before charging into the heating furnace be maintained at 600 ° C. or higher, and that the slab temperature be maintained at 600 ° C. or higher during continuous casting. The slab temperature refers to the surface temperature.

The specific method of maintaining the slab temperature during continuous casting at 600 ° C. or higher is as follows.

In the upper part of the continuous casting machine, since the water density is controlled by the above equation (2), the slab temperature does not fall below 600 ° C. Since the average temperature of the cross section of the slab decreases in the lower part, the slab temperature is 600 ℃
It may be less than. In such a case, the slab temperature is reduced to 600
It is possible to keep the temperature at or above ° C.

A specific method of maintaining the temperature of the slab before charging in the heating furnace at 600 ° C. or higher is as follows.

The slab temperature is usually at least 700 ° C. at the end of continuous casting. By rapidly charging the slab to the heating furnace, the slab temperature can be maintained at 600 ° C. or higher. Alternatively, if necessary, a method may be used in which a slab after continuous casting is covered with a cover, gradually cooled, and charged into a heating furnace while maintaining the temperature at 600 ° C. or higher.

Since the cracks generated in the slab of the Ni-containing steel are caused by the distortion of the slab during bending or straightening, the method of the present invention is effective when using a curved or vertical bending type continuous caster. is there. These continuous casters may be those provided with ordinary various rolls and a secondary cooling zone in a certain range from directly below the mold to the casting direction. Further, in any of the methods of the present invention, operating such that the slab surface temperature at the position of bending or straightening of continuous casting avoids the brittle temperature range,
More preferred. Therefore, a desirable lower limit of the slab surface temperature during continuous casting is about 800 ° C.

Other desirable conditions in the method of the present invention are as follows.

Secondary Cooling Means and Coolant: A preferred means of secondary cooling is spraying. Desirable coolant is water or air-water mist, but from the viewpoint of uniform cooling in the width direction, this mist is more preferable. When an air-water mist is used, the influence of air on the cooling capacity is smaller than that of water, so that only the water density W need be specified.

Range of water density W: 0.01 to 0.06 liter /
(min · cm ² ) Range of casting speed Vc: 0.5 to 3.0 m / min Range of slab size: width: 1000 to 2500 mm, thickness: 100 to 350 m
m

[0068]

[Example] Melting of test steel is performed by hot metal pretreatment → converter refining →
Secondary refining for addition of Ni, removal of P and removal of S [LF (Lad
le furnace) treatment, and then use a three-point straightening curved continuous caster with a machine length of 23 m, width 2300 mm, thickness 240 mm
Slab. Table 1 shows the chemical composition of the cast steel slab.
And Table 2 below. Symbols B and C of the steel slabs shown in Table 2 are steel types in which the contents of P and S are increased or decreased.

[0069]

[Table 2]

In the secondary cooling, cooling conditions were set such that the slab surface temperature at the straightening point could be maintained at 850 ° C., and the cooling means was mist cooling so as to uniformly cool the slab surface. A radiation thermometer was used to measure the slab surface temperature during continuous casting. As a result, it was confirmed that the surface temperature near the corner of the slab at the correction point was about 850 ° C.

The cast slab was cooled slowly in order to prevent cracking due to thermal stress, and then the surface was ground to investigate the occurrence of cracking. The evaluation of the state of occurrence of cracks was carried out using a six-stage crack code index, where 0 was a case where no crack occurred and 5 was a case where deep cracks occurred on the entire surface and maintenance was impossible.

Further, the thickness of the columnar γ-grain layer of the surface layer of the cast slab was visually measured with respect to a sample which had been cut into a cast, cut in a transverse section, mirror-polished, and etched with nitric acid.

[Test 1: First Method Example and Comparative Example of the Invention] As shown in Table 3, the casting conditions of ΔT, casting speed Vc, and water density W of secondary cooling were changed. Table 3 also shows the thickness of the columnar γ grain layer and the state of occurrence of cracks.

ΔT is the temperature measured in the tundish,
The water amount density W of the secondary cooling showed the value of the largest part in the secondary cooling process of the slab.

[0075]

[Table 3]

As shown in Table 3, the thickness of the columnar γ grain layer had a good correlation with the state of occurrence of cracks, and corresponded well to the above-mentioned casting test results. In each of the examples of the present invention in which the casting conditions were within the range determined by the method of the present invention, the thickness of the columnar γ grain layer was clearly thinner than that of the comparative example, and cracks were reduced. ΔT is 30
A fine γ grain layer was formed on the surface layer of the slab cast at a temperature of not more than ℃.

In Example 2 of the present invention in which ΔT was further reduced to 17 ° C., no crack occurred. Therefore, ΔT is 20
It has been clarified that when the temperature is lower than or equal to ° C, a higher crack suppressing effect can be obtained. In Comparative Example 1 where ΔT exceeds 30 ° C.,
Apparently, the columnar γ-grain layer grew thickly, and no fine γ-grain layer on the surface was formed, which caused severe cracking.

In Comparative Examples 2 and 3 in which 0.02 Vc <W, the columnar γ grain layer was thickened and cracking was worsened due to an increase in water density or a decrease in casting speed.

Further, in Example 3 of the present invention for steel C in which both P and S contents were reduced and the content was less than 0.001%, cracks were generated in steel A for high P and S contents. It was clearly reduced compared to Inventive Example 1. The crack is
On the other hand, in Comparative Example 4 for steel B having a high P content of 0.0028%, the deterioration was worsened, and the extremely low P and extremely low S clearly showed the effect of suppressing cracking.

[Test 2: Second method example and comparative example of the present invention] A steel slab cast on steel C shown in Table 1 under the conditions (first method of the present invention) shown in Table 4 was subjected to heat. The steel sheet was sent to a rolling process and subjected to rough rolling to produce a 180 mm thick steel slab, and the relationship between the heating furnace charging temperature and the state of steel slab cracking after rough rolling was investigated.

[0081]

[Table 4]

The charging temperature of the heating furnace was changed by changing the charging timing of the slab of the same charge into the heating furnace. Evaluation of the state of occurrence of flaws in the rolled billet was performed based on the yield reduction rate due to care. Table 4 also shows the yield reduction situation.

As is clear from Table 4, the slab surface temperature was
Example 4 of the present invention charged into a heating furnace while hot at 700 ° C.
Did not show a decrease in yield due to maintenance. On the other hand, the yield was worse in Comparative Examples 5 and 6, which were charged into a heating furnace at a temperature lower than 600 ° C. As described above, the yield decreased even when the heating furnace was charged at room temperature or 400 ° C., and there was almost no difference between the two.

[0084]

According to the method of the present invention, a Ni-containing steel slab or a slab in which cracks are prevented or reduced can be manufactured by continuous casting or continuous casting-hot direct rolling.

[Brief description of the drawings]

[Fig. 1] The casting speed is 1.0m / min, the specific water volume for secondary cooling is 0.63.
It is a mimic view of a macro structure photograph of the surface layer portion of a slab under the condition of liter / (kg · steel). (a) shows that ΔT
60 ° C, (b) shows the case where ΔT is 15 ° C.

FIG. 2 is a diagram showing the effect of ΔT on the thickness of a columnar γ-grain layer and a fine γ-grain layer in a slab surface layer.

FIG. 3 is a view showing the influence of water density W and casting speed Vc on the thickness of a columnar γ grain layer on the surface layer of a slab.

FIG. 4 is a diagram showing the thickness of a columnar γ-grain layer on the surface layer of a slab, organized according to the relationship between casting speed Vc and water density W;

FIG. 5 is a diagram showing a temperature change of a coefficient of linear expansion E when a steel slab containing 5.5 to 10% of Ni is heated and lowered at a rate of 5 ° C./min.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-164446 (JP, A) JP-A-58-77756 (JP, A) JP-A-9-253814 (JP, A) 47854 (JP, A) JP-A-8-132207 (JP, A) JP-A-8-33964 (JP, A) JP-A-8-10919 (JP, A) JP-A-8-10920 (JP, A) JP-A-7-90504 (JP, A) JP-A-4-91854 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/124 B22D 11/00 B22D 11/12 B22D 11/22 C22C 38/08

Claims (2)

(57) [Claims] (1) C: 0.1% or less, Si: 0.5% by weight
Mn: 1.0% or less, Ni: 5.5 to 10%, P: 0.002%
Hereinafter, S: 0.002% or less, Al: 0.02% or less, and N:
A method for producing a steel slab containing 0.001 to 0.004% by using a curved or vertical bending type continuous casting machine, wherein a degree of superheat of molten steel supplied into a mold is set to 30 ° C or less, and a secondary cooling zone is provided. Between the casting speed Vc (m / min) and the water density W [liter / (min · cm ² )] defined by the following equation (1) for all the rolls in the casting direction:
A method for producing a Ni-containing steel slab, characterized in that cooling is performed so that the relationship of the formula is satisfied, and the thickness of the columnar γ grain layer on the surface layer of the slab is 25 mm or less. W = VL / S (1) 0.02Vc> W (2) where W: water density of secondary cooling [liter / (mincm)
² )] VL: The amount of water sprayed on the slab surface between the rolls in the secondary cooling zone (liter / min) S: Slab surface area between the rolls (cm ² ) Vc: Casting speed Vc (m / min) ) 2. A method for producing a steel slab by producing a Ni-containing steel slab according to the method for producing a Ni-containing steel slab according to claim 1, and then directly feeding the hot-rolled steel to a rolling step for rough rolling. Then, the slab during continuous casting is secondarily cooled to a temperature of 600 ° C. or higher, and then the slab is charged into a heating furnace while being maintained at a temperature of 600 ° C. or higher, heated, and then subjected to rough rolling. Characterized by
Manufacturing method for Ni-containing billets.