JP2018118277A - CONTINUOUS CASTING METHOD FOR Ni-CONTAINING STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for Ni-added steel preventing the surface cracks of a slab.SOLUTION: Provided is a continuous casting method for Ni-containing steel characterized in that, upon the continuous casting of Ni-containing steel, the concentration of Ni in the steel is controlled to 0.2 to 3%, and the concentration of S in the steel is controlled to 0.001% or lower.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a continuous casting method for Ni-containing steel, and more particularly to a continuous casting method for preventing the occurrence of surface cracks in Ni-added steel manufactured using a vertical bending type or curved type continuous casting machine.

In order to improve the toughness of steel, Ni is generally added to the steel. However, when Ni-added steel is cast with a vertical bending mold or a curved continuous casting machine, cracks (hereinafter, also simply referred to as surface cracks) may occur on the surface of the slab, which causes an increase process such as a maintenance process. It becomes. Such surface cracks are hot ductile when the surface temperature of the slab becomes near the temperature (γ → α transformation temperature) (700 ° C. to 900 ° C.) at the time of secondary cooling in continuous casting. Is generated by receiving stress due to slab correction within such a temperature range.
Therefore, in order to improve the productivity of the Ni-added steel, it is a problem to prevent such slab surface cracks.

  As means for solving such a problem, Patent Document 1 discloses that the time required for drawing the slab from the meniscus portion of the molten steel in the mold to the lower end of the mold is within 1 minute, and immediately after the drawing, the secondary cooling is performed. The method of suppressing slab surface cracking in continuous casting of steel, characterized by cooling the slab surface temperature to A3 transformation temperature or less within 1 minute, and further cooling the slab surface temperature to A3 transformation temperature or less In continuous casting of steel, the temperature of the slab at the bending point and the straightening point is set to 850 ° C. or more, and the straightening of the slab is finished within 20 minutes after passing through the meniscus of the molten steel in the mold. A method for suppressing slab surface cracking is disclosed.

Patent Document 2 contains Ni: 5.5 to 10% by mass, C containing 0.1% or less, Si containing 0.5% or less, and Mn containing 1.0% or less. In the molten steel, P is 0.0010% or less, S is 0.0010% or less, Al is 0.002 to 0.030%, N is 0.0040% or less, and Al is further added. % Concentration product [Al] × [N] of N and N is less than 6 × 10 −5, and in the secondary cooling of the slab, the slab size ratio R expressed by the following formula (1) and the following (2) A continuous casting method for Ni-containing steel is disclosed in which casting is performed under the condition that the relationship with the ratio WR of the total amount of cooling water expressed by the formula satisfies the following formula (3).
R = W / T (1)
WR = WW / WN (2)
R <WR (3)
Where W: slab width (mm), T: slab thickness (mm), WW: total amount of cooling water on the long side of the slab (l / min), WN: short side of the slab Total amount of cooling water (l / min)

Japanese Patent Laid-Open No. 9-47854 JP-A-10-166126

However, the above method has the following problems.
In the method for suppressing slab surface cracking in continuous casting of steel described in Patent Document 1, secondary cooling is performed immediately after drawing from the mold, and the slab surface temperature is cooled to A3 transformation temperature or less within 1 minute. However, there is a problem that a larger amount of cooling water than usual must be used, cooling in the width direction of the slab and in the casting direction becomes uneven, and surface cracking may be promoted.
In the continuous casting method described in Patent Document 2, not only S but also the P concentration must be 0.0010% or less, and there is a problem that the refining load is very high.

  Therefore, the present invention has been made in view of such circumstances, and in a Ni-added steel manufactured by using a vertical bending type or a curved type continuous casting machine, it is possible to suppress the occurrence of surface cracks. An object is to provide a casting method.

Here, as described above, the surface crack of the Ni-added steel manufactured using the vertical bending mold or the curved continuous casting machine occurs when the slab surface temperature is corrected when the slab surface temperature is 700 ° C. to 900 ° C., At this time, it is known that cracking occurs along austenite grain boundaries (hereinafter sometimes referred to as γ grain boundaries). Therefore, the inventors have conceived that by preventing embrittlement of the γ grain boundary, the crack depth can be reduced, and shallow cracks that do not require maintenance can be suppressed.
Therefore, the present inventors diligently studied a steel composition that embrittles the γ grain boundary in order to suppress surface cracking. As a result, it was found that by controlling the S concentration in steel, embrittlement of the γ grain boundary can be prevented and surface cracking can be suppressed. In addition to the S concentration in steel, it has been found that Ca can also effectively act on surface cracks.
The present invention has been made by further study based on the obtained knowledge, and the gist thereof is as follows.

[1] A method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, wherein the Ni concentration in the molten steel is 0.2 to 3% and the S concentration in mass%. Is a continuous casting method of Ni-containing steel, characterized in that the slab is straightened when the surface temperature of the slab is 800 ° C. or higher.
[2] A method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, wherein the Ni concentration in the molten steel is 0.2 to 3% and the S concentration in mass%. More than 0.001 and 0.002% or less, Ca concentration is 0.001% or more and 0.004% or less, the surface temperature of the slab is 800 ° C. or more, and the slab is corrected. Steel continuous casting method.

  According to the present invention, in a high toughness Ni-added steel manufactured using a vertical bending mold or a curved continuous casting machine, surface cracks can be controlled by controlling the steel composition without increasing the manufacturing cost and the number of manufacturing steps. Generation can be suppressed.

It is a figure which shows the influence of the tensile temperature and steel component which acts on an aperture value in this embodiment.

Hereinafter, the Ni-containing steel continuous casting method according to the present invention will be described.
First, the results of studying the steel composition that affects the embrittlement of the γ grain boundary in order to suppress surface cracks will be described.

As described above, in the Ni-added steel produced by using a vertical bending type or a curved type continuous casting machine, the present inventors have studied earnestly about the steel composition in order to suppress surface cracks generated on the surface of the slab. Attention was focused on preventing embrittlement of the γ grain boundaries, which causes surface cracking. As a result, it has been found that cracking of the slab surface of Ni-added steel can be prevented by setting the S concentration in the steel to 0.001% or less. Moreover, even if the S concentration in the steel is more than 0.001% and 0.002% or less, Ca is controlled to 0.001% or more and 0.004% or less to prevent slab surface cracking of Ni-added steel. I found out that I can do it.
This will be described in detail below.

  As a result of examining elements that embrittle γ grain boundaries, it was found that S has the greatest influence on Ni-containing steel. FIG. 1 shows the results of investigating the effects of S concentration and tensile temperature on the drawing value of Ni-added steel using a hot tensile tester. A specimen having a diameter of 10 mm and a length of 100 mm was taken from the Ni-added steel with the steel components adjusted, heated to 1350 ° C. at 20 ° C./second, and held for 10 minutes. The temperature was lowered to a predetermined temperature at 5 ° C./second, held for 2 minutes, and then pulled and broken at a strain rate of 0.005 / second to obtain a drawing value.

As can be seen from the graph shown in FIG. 1, in the steel containing 0.002% S, the drawing temperature was 40% or less in the range of 700 ° C. to 900 ° C., which is the γ → α transformation temperature range. It is said that surface cracking occurs in a continuous cast slab when the drawing value evaluated by a hot tensile tester is 40% or less. On the other hand, when the S concentration is 0.001%, the drawing value at 700 to 900 ° C. exceeds 40%, and the ductility is recovered to the extent that the slab does not crack. Furthermore, when the S concentration is 0.002% and the Ca concentration is 0.003%, the aperture value at 700 to 900 ° C. exceeds 40%.
Although the mechanism of embrittlement of the γ grain boundary due to S has not been clarified sufficiently, the present inventors have found that the γ grain boundary becomes extremely brittle when Ni> 0.2% and S> 0.001%. I found out that This is thought to be due to the increased embrittlement due to the interaction between Ni and S segregated at the γ grain boundaries.

Based on the above investigation results, the continuous casting method of Ni-containing steel of the present invention will be described in detail.
First, the reason for limiting the chemical composition of the steel of the present invention will be described (hereinafter, the notation “%” is “mass%”).

<First Embodiment>
The Ni-containing steel continuous casting method of the present embodiment is a method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, and the Ni concentration in molten steel in mass%. 0.2 to 3%, S concentration is 0.001% or less, and the surface temperature of the slab is 800 ° C. or more, and the slab is corrected.
Hereinafter, the reason why the chemical composition of the steel of this embodiment is limited will be described (hereinafter, the notation “%” is “mass%”).

[Ni: 0.2-3%]
Ni in the steel is an element added to improve the strength and toughness of the steel material. The addition amount necessary for improving the strength and toughness is 0.2% or more. If the Ni concentration is less than 0.2%, surface cracks will not occur in normal continuous casting without reducing the S concentration or adding Ca.
On the other hand, if added in excess of 3%, the austenite grain boundary oxidation becomes too large, and the starting point of grain boundary cracking occurs. Therefore, even if the S concentration described later is reduced and the embrittlement of the austenite grain boundary is prevented. Since it is difficult to reduce the crack depth, the upper limit is made 3%.
Here, Ni is a noble element than Fe and has the property of being difficult to be oxidized. On the other hand, easily oxidizable elements such as Si are segregated at the γ grain boundaries. In general, the grain boundaries are preferentially oxidized because the grains are not easily oxidized and the grain boundaries are easily oxidized. Therefore, when the amount of Ni added is large, it means that there is a large amount of Ni that is difficult to oxidize, and the oxidation of the γ grain boundary tends to segregate easily oxidizable elements. Since only the γ grain boundary is greatly oxidized in this way, the surface of the steel plate appears to have a notch (notch). When tensile stress is applied to the surface of the slab, the notch is the starting point and easily breaks. That's it.
As described above, in the present invention, it is important to set the Ni concentration to 0.2 to 3%.

[S: 0.001% or less]
S is an element that embrittles the austenite grain boundary, and its influence is remarkable in Ni-containing steel.
If the content exceeds 0.001%, cracks that require maintenance occur in the continuous cast slab, so the content is made 0.001% or less. The lower limit includes 0%.

Second Embodiment
Next, a second embodiment of the present invention will be described.
The Ni-containing steel continuous casting method of the present embodiment is a method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, and the Ni concentration in molten steel in mass%. 0.2 to 3%, S concentration is more than 0.001 and 0.002% or less, Ca concentration is 0.001% or more and 0.004% or less, and the surface temperature of the slab is 800 ° C. or more. It is characterized by performing correction.
Hereinafter, the reason which limited the chemical composition of steel of this embodiment is demonstrated.

[Ni: 0.2-3%]
Ni is an element added to improve the strength and toughness of the steel material as in the first embodiment described above, and the necessary addition amount is 0.2% or more. If the Ni concentration is less than 0.2%, surface cracks will not occur in normal continuous casting without reducing the S concentration or adding Ca.
On the other hand, if added in excess of 3%, the austenite grain boundary oxidation becomes too large and the starting point of grain boundary cracking occurs, so even if the S concentration is reduced and embrittlement of the austenite grain boundary is prevented, cracking occurs. Since it is difficult to reduce the depth, the upper limit is made 3%.

[S: more than 0.001 and 0.002% or less]
[Ca: 0.001% to 0.004%]
Ca easily binds to S in steel and easily forms CaS. For this reason, S existing in the γ grain boundary can be reduced. When the S concentration is more than 0.001% and not more than 0.002%, if the Ca concentration is 0.001% or more, the grain boundary will not be embrittled, so the continuous cast slab needs to be maintained. Does not occur. On the other hand, since Ca is an element that has a low boiling point and easily evaporates, it is expensive to contain a large amount in Ca. Furthermore, since the effect of detoxifying S as CaS is saturated, the upper limit is made 0.004%.

  The composition described in each of the first embodiment and the second embodiment can be carried out by adjusting by a conventional method at the molten steel stage until the start of casting. For example, S can be reduced to 0.001% or less by using, for example, a flux containing lime in the hot metal preliminary treatment process, the converter process, and the secondary refining process. Ca can be contained by charging powdered Ca into the steel or by inserting a wire filled with Ca particles into the steel.

  In the continuous casting method according to the present invention, Ni-containing steel having the composition as described above is cast using a vertical bending type continuous casting machine or a curved type continuous casting machine.

[Slab surface temperature when straightening the slab: 800 ° C or higher]
As described above, when steel is cast using a vertical bending type continuous casting machine or a curved type continuous casting machine, there is a problem that surface cracks occur on the upper surface of the slab when the slab is straightened. For this reason, in order to prevent surface cracking, the secondary cooling and the casting speed are adjusted so as to avoid the temperature range in which the steel becomes brittle.
As shown in FIG. 1, in the method of the present invention, when the tensile temperature is 800 ° C. or higher, an aperture value of 40% or more can be secured. From this, the slab surface temperature at the time of correcting a slab shall be 800 degreeC or more. The upper limit of the surface temperature is not particularly limited. However, if the temperature is too high, an internal crack or an increase in the amount of slab oxidation occurs.

  In the present invention, the casting speed and the specific water amount of secondary cooling (the amount of secondary cooling water per slab unit weight) are 0.8 to 1.5 mpm at the casting speed, The specific water amount is preferably 0.7 to 1.5 l / kg.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

No. shown in Table 1. 1-No. Steels having 8 chemical components were cast using a vertical bending type continuous casting machine or a curved type continuous casting machine, respectively. At that time, the slab surface temperature in the straightening band was changed as shown in Table 1 by changing the cooling condition and casting speed of the secondary cooling.
In the steel after casting, the cross section perpendicular to the casting direction was observed, the depth of the deepest crack in the cross section was measured, and indexing was performed. If the crack depth is less than 0.2 mm, the slab surface crack index is 1 for the slab surface crack index, 2 if the depth is 0.2 mm or more and less than 1 mm and needs to be cleaned, and if the depth is 1 mm or more What was not required was set to 3.

No. 1-4 are examples of the present invention for both chemical components and production conditions. In both cases, the slab surface crack index was 1, and maintenance was unnecessary.
On the other hand, no. 5-No. 10 is a comparative example. No. Since No. 5 had a high S concentration, cracks requiring maintenance occurred. No. No. 6 had a Ni concentration that was too high, resulting in deep cracks of 1 mm or more, which had to be scrapped. No. In No. 7, the Ca concentration was too low, and therefore cracks requiring maintenance occurred. No. In No. 8, since the surface temperature of the slab in the straightening band was too low, deep cracks of 1 mm or more were generated and had to be scrapped. No. No. 9 had Ca of 0.004%, but the S concentration exceeded the upper limit, so cracking occurred. No. In No. 10, cracks did not occur, but the Ni concentration was too low, so the steel was insufficient in strength and toughness.

Claims (2)

  This is a method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, wherein the Ni concentration in the molten steel is 0.2 to 3% and the S concentration is 0. A continuous casting method of Ni-containing steel, characterized in that the slab is straightened when the surface temperature of the slab is 800 ° C. or higher.   This is a method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine, wherein the Ni concentration in the molten steel is 0.2 to 3% and the S concentration is 0. Continuous Ni-containing steel characterized in that the slab is straightened when the slab surface temperature is 800 ° C. or higher and the Ca concentration is 0.001% or more and 0.004% or less over 001. Casting method.

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