JPH03104819A - Production of high chromium steel - Google Patents

Abstract

PURPOSE:To produce a high chromium steel having superior internal quality while obviating the necessity of complicated stages and operations by specifying molten steel superheating temp. in a tundish and carrying out rolling reduction working for a cast bloom strand, heating treatment, and rolling under respectively specified conditions at the time of producing a high chromium steel using a continuous bloom caster. CONSTITUTION:At the time of producing a high chromium steel by using a continuous bloom caster, molten steel superheating temp. in a tundish is regulated to 20-60 deg.C. Subsequently, a cast bloom strand prepared by continuous casting is subjected to rolling reduction 5-10 times unsolidification thickness in a region where the unsolidification thickness of the above cast bloom strand becomes 5-30% the thickness of this strand. Then, the above strand is heated up to 1150-1250 deg.C, held for 2-5hr, and rolled so that rolling reduction ratio is regulated to >=1.5. By this method, the high chromium steel can be obtained while obviating the necessity of complicated stages, without causing deterioration in quality.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention aims to produce high chromium steel without deteriorating its quality or going through complicated processes.

(Conventional technology) When a slab obtained by continuous casting of steel with a high chromium content is air-cooled after casting, most of it transforms into martensite, but some of it becomes retained austenite, which causes delayed fracture. First, it is charged into a soaking furnace and heated to about 600 ~ 8
After being maintained at a relatively low temperature of 00°C for about 20 to 30 hours, it is cooled to room temperature. After that, in order to prevent the center borosity existing inside the slab from being oxidized by later heat treatment, iron plates are welded to both end faces of the slab, and then heated to 1 liter at 50-1350℃ in a soaking furnace. The first rolling (diffusion rolling) to prevent the high concentration of P present in the center of the slab from turning into δ-ferrite after heating and soaking.
and heated to 1150-1250℃ again in a heating furnace.
After heating and soaking, diffusion annealing is performed for 10 to 30 hours to form a round billet, which is then sent to a process such as tube-making rolling.

High chromium steel requires complicated processes to ensure such good quality, and is also disadvantageous in terms of cost.

Here, as a method for improving the quality of slabs obtained by continuous casting, especially reducing center segregation, for example,
As seen in Japanese Patent No. 229450, there is known a technique for performing low-temperature casting in which the superheating temperature of the molten w4 in the tundish is lowered.

(Problems to be Solved by the Invention) The technology disclosed in the above publication performs continuous casting by lowering the degree of superheating of molten steel (molten steel temperature to liquidus temperature), and also performs continuous casting in the mold and in the final solidification region of the slab. '8 Steel is stirred. However, does such low-temperature casting work in the tundish nozzle? In addition to operational problems such as solidification of the molten steel, the increased viscosity of the molten steel prevented inclusions from floating, reducing the refining function of the tundish and making it difficult to produce highly clean steel. In this regard, JP-A-54-107831
The publication discloses a method in which high-temperature casting is performed and the resulting slab is rolled down using rolls to reduce inclusions and center deflection, but this method involves light rolling of the slab. Therefore, in order to obtain a high segregation reduction effect, it is essential to have a columnar structure in the center of the slab. There was a problem that the appropriate range was limited to the high temperature side and became very narrow. It is an object of the present invention to propose a new method for manufacturing high chromium steel with good internal quality without going through the complicated steps and operations described above.

(Means for Solving the Problems) This invention, when producing a high chromium stage using a bloom continuous caster, reduces the melting @ superheating temperature in the tundish to 20~20°C.
For a slab strand obtained by continuous casting at 60°C, the unsolidified thickness of the slab strand is 5% of the thickness of the strand.
A reduction of 1 to 10 times the unsolidified thickness is applied in the region where the thickness becomes ~30%, then heated to a temperature range of 1150 to 1250°C and held for 2 to 5 hours, and then rolled to a reduction ratio of 1.5 or more. This is a method of manufacturing high chromium steel characterized by applying

In this invention, high chromium steel is defined as containing 6% or more of chromium.

(Function) High-temperature casting in the production of high chromium steel is advantageous in reducing inclusions in the steel, but it also increases center segregation and center porosity, so conventional casting methods are not recommended. It was not possible to produce high-quality high-chromium steel simply by applying it. In this invention, high density casting is performed which is advantageous for reducing inclusions, but since the area near the completion of solidification of the slab is rolled down according to the above conditions, the center segregation of the slab is reduced, and the center There is no occurrence of porosity, so there is no need for the iron plates that were attached to both ends of the slab to prevent oxidation within the center borossie. Furthermore, since no segregation of P occurs due to the reduction of the unsolidified region, steps such as diffusion rolling and diffusion annealing are completely unnecessary.

In this invention, the degree of superheating of the molten steel in the dandysh is set to 20 to 6.
The reason for this is that as shown in Figure 1, which shows the relationship between the degree of superheating of molten steel and the inclusions in the slab, the inclusions in the slab are
At least ? This is because the degree of superheating of 8fjI needs to be 20°C. As described above, it is better to have a high degree of superheating of molten steel in order to reduce inclusions, but if the degree of superheating of molten steel is too high, the load on the converter increases due to the rise in tapping temperature. ? '? ! The upper limit of the wAA thermal degree is about 60° C. in view of the effect of reducing inclusions. Therefore, the degree of superheating of the molten steel in the tundish was set to 20 to 60°C.

Next, FIG. 2 shows a graph in which the relationship between the degree of superheating of molten steel and the central deflection of a slab was evaluated from the viewpoint of C content using C/Co. Second
From the figure, it is clear that the center segregation deteriorates as the molten steel superheat increases, and the center volocity also increases. Therefore, in this invention, it is necessary to apply reduction to the slab strand at this point. There is. The reason why we decided to apply a reduction of 1 to 10 times the unsolidified thickness of the slab strand in regions where the unsolidified thickness of the slab strand is 5 to 30% of the thickness of the strand is to reduce the reduction of the slab toss strand and prevent center segregation. As shown in Figure 3, by applying a reduction equivalent to the unsolidified thickness of the slab strand, it is possible to keep C/C0 within 1.1, while by applying a reduction twice the unsolidified thickness. This is because by adding C/Co<1.025, a sufficient center segregation relative effect can be obtained. Center volocity is also greatly reduced because the unsolidified portion is compressed, and furthermore, by applying a reduction that satisfies this range, it is possible to refine the crystal grains in the center of the slab. The area where the above-mentioned reduction is performed is the unsolidified thickness of the slab strand, which is 5 to 30% of the thickness of the strand.
% range because if the unsolidified thickness of the slab strand is less than 5% of the strand thickness, there are areas where center segregation has already occurred, and if the unsolidified thickness of the slab strand exceeds 30%, high pressure is applied. This is because if it is added, negative segregation may occur. Incidentally, in the case where the rolling reduction ratio is 0.5 or more in a region where the unsolidified thickness of the slab strand exceeds 35% of the thickness of the strand, C/Co<0.9. As is clear from Fig. 3 above, even if the rolling reduction ratio is increased in the rolling process of the slab strand during continuous casting, there is no change in the C/Co improvement effect. It can be said that a reduction ratio of 10 times is sufficient. Figure 4 shows the relationship between the center volocity area ratio and the pipe-making performance. By applying reduction to the unsolidified area of the slab during the casting process, internal oxidation caused by center volocity does not occur at all. This significantly improves rolling performance.

Next, the relationship between the furnace time and heating temperature of the slab is shown in Figure 5.
The reason for holding the temperature in the 250°C temperature range for 2 to 5 hours is that if the temperature is outside this range, δ-ferrite will occur on the surface of the slab and large surface flaws will occur. This is because the occurrence of scratches and the like can be minimized.

In this invention, this heat treatment is followed by rolling with a reduction ratio of 1.5 or more, but in relation to the pipe making results, the reduction ratio is set to 1.5 or more as shown in Figure 6. By doing so, the pipe manufacturing performance index can be made extremely small.

(Example) Molten steel having the composition shown in the table above was continuously cast under the conditions of applying a bloom continuous caster. When we investigated the internal quality of the obtained slabs, we found that slabs obtained by conventional continuous casting had a center volosity of 5 to 8%, whereas those manufactured according to the present invention had center volosity of 5 to 8%. It was confirmed that the center volosity was only a matter of degree and had no effect on quality.

(Effects of the Invention) Thus, according to the present invention, it is possible to omit the work of solution treatment in a soaking furnace, diffusion annealing, diffusion rolling, or attaching an iron plate to the end face of a cast slab, thereby advantageously reducing the fuel consumption rate. However, even if these steps are omitted, there will be no quality deterioration due to inclusions or center porosity, and the cost for manufacturing high chromium steel can be significantly reduced. Also, in the rolling of the seamless vibrator from Hillend, it is possible to prevent a decrease in the pipe production yield due to δ-ferrite, so that the manufacturing cost can be reduced as much as possible.

[Brief explanation of drawings]

Fig. 1 is a graph of the relationship between the molten steel heating temperature in the tundish and the slab inclusion index, and Fig. 2 is a graph of the relationship between the molten steel heating temperature in the tundish and the C/C. Figure 3 is a graph of the relationship between rolling reduction ratio and C/C0, Figure 4 is a graph of the relationship between center porosity area ratio and pipe making performance index, Figure 5 is a graph of the relationship between furnace time and heating furnace temperature. , FIG. 6 is a graph showing the relationship between the rolling reduction ratio and the pipe-making rate.

Claims (1)

[Claims] 1. When manufacturing high chromium steel using a bloom continuous caster, the superheating temperature of molten steel in the tundish should be set at 20 to 60℃.
The unsolidified thickness of the slab strand obtained by continuous casting is 5 to 30 times the thickness of the strand.
%, apply a reduction of 1 to 10 times the unsolidified thickness,
Next, heat to a temperature range of 1150 to 1250°C for 2 to 5 minutes.
A method for producing high chromium steel, which comprises rolling at a rolling reduction ratio of 1.5 or more after holding for a period of time.