JP6589096B2 - Continuous casting method of Ni-containing steel - Google Patents

Description

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

  In order to improve the toughness of steel, Ni is generally added to the steel. However, when Ni-containing 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, causing an increase process such as a care 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-containing steel, it is a problem to suppress such slab surface cracks.

As means for solving such a problem, Patent Document 1 discloses that the time required for drawing a slab from the meniscus portion of molten steel in a mold to the lower end of the mold is set within 1 minute, and immediately after the drawing, the secondary cooling is performed. performed, method for inhibiting the cast slab surface cracks during continuous casting of steel, characterized in that the 1 minute within the billet surface temperature is cooled to below a ₃ transformation temperature, further, the cast slab surface temperature a ₃ transformation temperature or less The steel is characterized in that it is reheated, the slab surface temperature at the bending point and the straightening point is 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 during continuous casting 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. And the% concentration product [Al] × [N] of N and N is less than 6 × 10 ⁻⁵ , and in the secondary cooling of the slab, the dimensional ratio R of the slab expressed by the following formula (1) and the following (2) continuous casting method of the Ni-containing steel, wherein a relationship between the ratio W _R of the sum of the cooling water of the formula is cast under conditions satisfying the following formula (3) is disclosed.
R = W / T (1)
W _R = W _W / W _N (2)
R <W _R (3)
Where, W: slab width (mm), T: slab thickness (mm), _WW : total cooling water amount on the long side of the slab (liters / min), W _N : slab short side Total amount of cooling water on the surface (liter / min)

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

  However, the above method has the following problems.

Method for inhibiting the cast slab surface cracks during continuous casting of Patent Document 1 of the steel, after withdrawal from the mold, immediately subjected to secondary cooling, cooling the billet surface temperature within 1 minute or less A ₃ transformation temperature However, it is necessary to use a larger amount of cooling water than usual, and cooling in the slab width direction and the casting direction becomes non-uniform, which may promote surface cracks.

  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 it is possible to suppress the occurrence of surface cracks in Ni-containing steel manufactured using a vertical bending mold or a curved continuous casting machine. An object is to provide a continuous casting method.

  Here, as described above, the surface cracking of the Ni-containing steel produced by using the vertical bending die or the curved continuous casting machine occurs when the slab surface temperature is corrected at 700 ° C. to 900 ° C. Sometimes, it is known to crack along an austenite grain boundary (hereinafter sometimes referred to as a γ grain boundary). 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 Si concentration in the steel, embrittlement of the γ grain boundary can be prevented and surface cracking can be suppressed.

  The present invention has been made by further study based on the obtained knowledge, and the gist thereof is as follows.

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 Si concentration is 0.00. A continuous casting method of Ni-containing steel, characterized in that it is 25 to 0.5% and the slab is straightened at a surface temperature of 800 ° C. or higher without reheating.

  According to the present invention, in a high toughness Ni-containing steel manufactured using a vertical bending type or a curved type 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 graph which shows the influence of the tensile temperature and the steel component which have on the 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-containing 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 discovered that the slab surface crack of Ni-containing steel can be suppressed by making Si concentration in steel into 0.25-0.5%. This will be described in detail below.

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

  As can be seen from the graph shown in FIG. 1, in the steel containing 0.2% Si, the drawing value was 40% or less in the range of 700 ° C. to 900 ° C., which is the γ → α transformation temperature range. In addition, when the drawing value evaluated with a hot tensile tester is 40% or less, it is said that surface cracking occurs in the continuous cast slab. On the other hand, when the Si concentration is 0.25%, the drawing value at 800 to 900 ° C. is more than 40%, and the ductility is recovered to the extent that the slab is hardly cracked. Furthermore, when the Si concentration is 0.5%, the aperture value at 800 to 900 ° C. exceeds 40%.

  Although the mechanism of preventing embrittlement of the γ grain boundary due to Si has not been clarified sufficiently, the present inventors have found that the γ grain boundary is notable when Ni> 0.2% and Si <0.25%. Found to be brittle. This is considered that the grain boundary embrittlement due to the interaction of Ni and S segregated at the γ grain boundary is prevented by Si suppressing the grain boundary segregation of S.

  Based on the above investigation results, the continuous casting method of Ni-containing steel of the present invention will be described in detail.

  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%, Si concentration is set to 0.25 to 0.5%, and the surface temperature of the slab is 800 ° C. or more, and the slab is corrected.

  Hereinafter, the reason which limited the chemical composition of steel of this embodiment is demonstrated. Hereinafter, the notation “%” means “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. 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. It becomes difficult to reduce the depth. Therefore, the upper limit is 3%. If the Ni concentration is less than 0.2%, surface cracks will not occur in normal continuous casting without adding Si.

  Here, Ni is a noble element than Fe and has the property of being difficult to be oxidized. On the other hand, oxidizable elements such as Al 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 not easily oxidized, and the oxidation of the γ grain boundary where the elements that are easily oxidized tend to segregate increases. Since only the γ grain boundary is greatly oxidized in this way, the steel plate surface appears as if a notch (notch) has been formed, and when a tensile stress is applied to the slab surface, the notch is the starting point and is prone to cracking. Become.

  As described above, in the present invention, it is important to set the Ni concentration to 0.2 to 3%.

[Si: 0.25 to 0.5%]
Si is an element that prevents grain boundary segregation of S, which embrittles austenite grain boundaries, and effectively acts to suppress surface cracking of Ni-containing steel. By containing 0.25% or more, grain boundary segregation of S can be prevented. Even if added over 0.5%, the effect is the same, but the peelability of the scale on the steel sheet surface deteriorates, so the upper limit is made 0.5%.

  The composition demonstrated above can be implemented by adjusting by a conventional method in the molten steel stage until it starts casting. For example, Si can be contained in an amount of 0.25 to 0.5% by adding, for example, metal Si or ferrosilicon in the converter process and the secondary refining process.

  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. Therefore, in order to suppress 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, when the Si concentration is 0.25 to 0.5%, which is the range of the present invention, a drawing value exceeding 40% can be secured when the tensile temperature is 800 ° C. or higher. 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 is about 0.8 to 1.5 mpm and the specific water amount of the secondary cooling with respect to the casting speed and the specific water amount of the secondary cooling (the amount of secondary cooling water per slab unit weight). Is preferably 0.7 to 1.5 liter / 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 7 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 conditions 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. 1-3 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. 4-No. 7 is a comparative example. No. In No. 4, since the Si concentration was low, cracks requiring maintenance occurred. No. In No. 5, since the Ni concentration was too high, deep cracks of 1 mm or more were generated and had to be scrapped. No. In No. 6, 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. In No. 7, cracks did not occur, but the Ni concentration was too low, so the steel was insufficient in strength and toughness.

Claims (1)

A method of continuously casting Ni-containing steel using a vertical bending type continuous casting machine or a curved type continuous casting machine,
By mass%, the Ni concentration in the molten steel is 0.2-3%, the Si concentration is 0.25-0.5%, and the surface temperature of the slab is 800 ° C or higher without reheating , and the slab is corrected. A continuous casting method of Ni-containing steel, characterized in that it is performed.

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