JPS59163063A - Production of continuously cast billet of 9% ni steel - Google Patents

Abstract

PURPOSE:To obtain a defectless titled billet having no cracking on and under the surface by drawing continuously a molten 9% Ni steel regulated in the content of P and S by an LD-AOD method, solidifying thorougyly the unsolidified billet in the core part thereof by the mist spray cooling water and subjecting the billet to light draft rolling at a prescribed temp. or above. CONSTITUTION:The 9% Ni steel regulated to <=0.005wt% P and <=0.0010wt% S is melted by the above-described method and is cast into a continuous casting mold, from which the cast steel is drawn continuously. The resulting billet having the unsolidified core part is thoroughly solidified by the mist spray of cooling water. The satisfactory distribution in the flow rate of said mist spray is obtd. by selecting preferably the conditions of 2mm.X10mm. nozzle size, 10-13l/sec water flow rate of each one spray nozzle and 1.5-3.5kg/cm<2> air pressure. The thoroughly solidified billet is subjected to the light draft rolling by a hot charge at the temp. not lower than 500 deg.C and the defectless continuously cast billet of the 9% Ni steel having no cracking on and under the surface is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing continuously cast slabs of 9% Nl steel with good properties, free from surface cracks and subcutaneous cracks, by a continuous casting method.

In recent years, 9%Nl has been used for piping for liquefied gas storage and transportation equipment such as LP() and LNG, low-temperature heat exchangers, and other low-temperature equipment because of its extremely good low-temperature toughness.
BACKGROUND OF THE INVENTION Low-temperature steel, called steel, has come into widespread use, and its demand is on the rise.

By the way, 9% Ni steel is itself a very expensive material, so even though efforts are being made to reduce costs as much as possible at the material manufacturing stage and meet the demands of customers. there were. An attempt to manufacture 9% Nl steel by casting all steel materials is also part of this effort.

However, conventionally, when trying to continuously cast 9%N1 steel, because the primary crystal is γ, surface cracks due to grain boundary segregation of P and S, grain boundary precipitation of AεN, or martensite formation during slag cooling occurred. There was a problem in that subcutaneous cracking occurred due to transformation stress due to transformation and ferrite-bainite transformation, making it impossible to obtain slabs with sufficiently satisfactory properties.

From the above-mentioned viewpoints, the present inventors have realized a continuous casting method for 9%Nl steel that allows the production of healthy slabs without surface cracks or subcutaneous cracks, and have improved the manufacturing efficiency of 9%N1 steel. In order to reduce costs associated with
We conducted a detailed study on how each process in the continuous casting of 9%Ni steel affects the quality of the resulting slab, and found that: Partly because the method for refining molten steel had not been established, L D -'
The DH method was adopted, but in this LD-DH method,
Even if the double slag method is implemented in an LD converter, 1
Since steel must be tapped at a high temperature of 750°C or higher, the P content in the steel can only be reduced to about 0010% by weight at most, and the S content must be reduced by strengthening hot metal desulfurization. Even if it were, the limit was to keep it to about 0.003 weight factor. Therefore, the 9 obtained by this method
When %Nl steel is continuously cast, it is impossible to prevent P and S from segregating at the γ grain boundaries, which causes cracks on the slab surface. A flat spray as shown in the schematic diagram in Figure 1 was used as a cooling water spray to solidify the unsolidified core slab that was pulled out, but the water flow density was too high in such a flat spray. This results in thermal shock being applied, which can also cause surface flaws in the slab. In addition, Figure 1 (
1(a) is a schematic side view showing a state in which the slab 2 is being cooled by the flat spray 1, and FIG. 5 is the support roll, and 5 is the area to be cooled.

■ In the conventional continuous casting method, after the slab is completely solidified, it is left to cool, sand cooled, or gradually cooled by force and pressure. Subcutaneous cracking of the cast slab cannot be prevented unless transformation stress is applied and this is prevented. They came to the conclusion that there is a risk of subcutaneous cracking occurring during precipitation.

Therefore, the present inventors have developed a continuous 9% Ni steel that can reliably and efficiently obtain slabs with good properties without employing any of the above-mentioned individual means that impede the production of sound slabs. As a result of searching for a method for manufacturing cast slabs and conducting further research, we found that (a) the LD-A OD method, which combines the LD converter steel manufacturing method with the AOD refining process, was adopted as the method for producing 91Ni steel; Low-temperature tapping of LD converter became possible (LD
- Possible to tap steel at a temperature approximately 100°C lower than in the case of the DH method) Extremely low phosphorus of 9% Ni steel (actually 0.00
The S content in the steel can be further reduced by strong stirring in the next AOD process, which can process low-temperature steel (according to actual results, ○ooo7N).
(b) When a cooling water mist spray is used to cool the slab pulled out of the continuous casting mold, the water flow density is 1/2 to 1/3 that of a conventional flat spray. In addition, as shown in Figure 2, the area to be cooled becomes a dog, so even if the specific water volume is lowered by about 20%, it is possible to secure the same cooling capacity as a flat spray. . FIG. 2 is a schematic diagram showing the condition in which the slab 2 is cooled by the mist spray 6. FIG. 2(a) is a side view thereof, and FIG. 2(b) is a front view thereof. Shown is a cooling water spray stream. Components common to those in FIG. 1 are designated by the same reference numerals.

(C) By combining the means shown in paragraphs (a) and (b) above, it is possible to completely eliminate surface flaws in continuous cast slabs of 9% Ni steel; (d) and that After solidification is completed, if the slab is hot-charred at a temperature not lower than 500°C and subjected to light reduction rolling in the γ region such as blooming or hot rolling, the solidified structure is crushed and the grains become finer. ,
It is also possible to prevent subcutaneous cracking of slabs due to ferrite precipitation; (e) When all of the measures shown in items (a) to (d) above are combined, the additive effect is to prevent surface cracking and subcutaneous cracking. We have obtained the knowledge shown in (a) to (e) above that 9% Ni steel continuous casting slabs, which are free from carbon dioxide, can be manufactured efficiently, with high yield, and at low cost and labor costs. .

This invention was made based on the above findings, By L'D-AO,D method, P: 0.005 graviton.

S: 0.0010 weight factor or less.

After melting 9% Nl steel and casting it into a continuous casting mold,
The core of the unsolidified slab that is continuously pulled out of the mold is completely solidified with a mist spray of cooling water, and then subjected to light reduction rolling using a hot charge at a temperature not lower than 500°C. It is also characterized by continuous continuous casting of 9% Nl continuous steel, which is sound and free of cracks both under the skin and under the skin.

In addition, in the method of this invention, P and S of 9% Ni steel
The content is less than 0005 weight ratio and o, o 6 respectively.
The reason why we limited the content to 10% or less is that if the content exceeds the above value, they will precipitate at the γ grain boundaries of the solidified slab, causing surface cracks in the slab, and making it impossible to maintain a sound continuously cast slab. Because you won't be able to get anything.

For example, Figure 3 is a diagram examining the relationship between the S content and the degree of S segregation to the γ grain boundaries for 9% Nl steel.
It is clear from FIG. 3 that if the S content is as low as O,001 weight ratio, segregation becomes so small that it hardly becomes a problem.

In addition, Figure 4 shows the results of investigating the influence of P and S contents on the occurrence of surface cracking in continuous casting of 9% Nl steel, and this figure also shows that a healthy slab can be obtained. For P:
It is clear that it is required that the weight ratio is 0O05 or less, S: 0, and Coo 10% or less. and,
To achieve this value, it is essential to combine the LD converter steelmaking method and the AOD refining method.

Any mist spray that is used for general cooling can be used, but in order to obtain a good water volume distribution, nozzle hole size: 27117+1X 10M.

Water amount per spray nozzle: 10-151/min
.

It is desirable to select conditions of air pressure crab 15 to 3.5 kg/ca T.

Figure 5 shows 9% Ni with width: Y260 and thickness: 1888.
This is a diagram comparing the water flow density between flat spray and mist spray when continuously casting steel slabs.As is clear from this diagram, water flow density can be significantly lowered by using mist spray. I understand that.

The temperature at which light reduction rolling is performed needs to be 500°C or higher, but if the temperature is lower than 500°C, coarse ferrite is formed and there is a risk of subcutaneous cracking of the slab.

Furthermore, it is desirable that the rolling reduction rate of light reduction rolling is within the range of 20 to 40%. This is because if the rolling reduction is less than 20%, the effect of grain refinement by rolling is insufficient, while if the rolling reduction exceeds 40%, the temperature drop due to the increase in passes is large.

Next, the present invention will be explained using Examples and in comparison with Comparative Examples.

Example First, 9% Ni steel was produced by combining the LD converter steel making method, the DH degassing method, and the AOD refining method as shown in Table 1, and molten steel having the composition shown in Table 1, respectively. A-G was obtained.

Next, these are cast into a continuous casting mold and continuously pulled out from the mold. Width: 1260 myn, Thickness: 18
The 8rx slab was sprayed with water to promote complete solidification. Table 2 shows the type of water spray type used at this time.

After the slab was completely solidified, it was cooled in the same manner as shown in Table 2, and test numbers 1-3 and 9! For the second one
It was subjected to light reduction rolling under the conditions shown in the table. In addition, when performing light reduction rolling, a hot charging method was adopted in which the slab was charged into a soaking furnace before the temperature dropped below 500°C.

The surface defects and internal defects of the continuously cast 91Ni steel slab obtained in this way were examined, and the product properties when rolled into thick plates were examined. Also listed in the table.

The results shown in Table 2 also show that the 9% Ni steel continuous cast slab manufactured by the method of the present invention has excellent quality with no defects, whereas the manufacturing conditions shown in Table 1 The slabs obtained by the comparative method, which is outside the scope of the present invention as indicated by * in Table 2, have undesirable defects such as surface cracks or subcutaneous cracks, and cannot be considered to be of good quality. It was something I couldn't say.

As described above, according to the present invention, it is possible to reliably produce defect-free, high-quality continuously cast slabs with high efficiency from 9% Ni steel, which has conventionally been considered unsuitable for continuous casting. Kadeta 1. ” Reducing the cost of N1 copper products will bring about industrially useful effects such as further expanding the field of application.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the conventional water cooling method for slabs in continuous casting, with Figure 1 (a) being a side view and Figure 1 (b) being a side view.
2 is a front view thereof, FIG. 2 is a schematic diagram showing the slab water cooling method in the present invention, and FIG. 2(a) is a side view thereof.
Figure 2(b) is a front view of the same, Figure 3 is a diagram examining the relationship between the S content and the degree of S segregation to the γ grain boundaries for 91Ni steel, and Figure 4 is a continuous 90%Ni steel. FIG. 5 is a plot diagram examining the influence of P and S contents on the occurrence of surface cracks in cast slabs, and FIG. 5 is a diagram comparing the water flow density between flat spray and mist spray. In the drawings, 1. Flat spray, 2. Slab, 3. Cooling water jet, 4. Support roll, 5.
Area to be cooled, 6... Mist spray, 7... Cooling water spray flow. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person
Dimensions 3 Diagrammatics 4 Diagram 2 4 6 8 1OrP]X IQ
-(further 1% Figure 5)

Claims (1)

[Claims] According to the LD-AOD method, P:0005 weight ratio or less. S: O, 0O10 weight factor or less. After melting 9% Ni steel and casting it into a continuous casting mold, the slab with an unsolidified core that is continuously pulled out of the mold is completely solidified with a cooling water mist spray. A method for producing a continuously cast 9% Nl steel slab, which comprises subsequently performing light reduction rolling by hot charging at a temperature not lower than 500°C.