KR100381523B1 - Manufacturing method of high nickel equivalent austenitic stainless alloy with excellent surface quality - Google Patents

Abstract

The present invention relates to a high Ni equivalent austenitic stainless steel and a method for producing the same, in which solidification starts with a primary austenite phase, δ-ferrite is formed during the solidification process, and δ-ferrite is transformed into an austenite phase after solidification is completed. In the case of continuous casting of a stainless alloy, it is an object of the present invention to provide a method for producing a high Ni equivalent austenitic stainless alloy having excellent cast quality in which the δ-ferrite content is controlled to 1 to 3% within the cast steel.

Description

Manufacturing method of high nickel equivalent austenitic stainless alloy with excellent surface quality

The present invention relates to a method for producing a high nickel (Ni) equivalent austenitic stainless alloy, and in particular, the surface quality of the continuous cast slab to prevent the surface cracks, and to prevent the line defects generated during hot rolling excellent A method for producing a high nickel equivalent austenitic stainless alloy.

In general, high nickel (Ni) equivalent austenitic stainless alloys solidify with primary austenite, causing segregation during the solidification process, and solidification cracks are liable to occur during the process due to the large amount of solidification shrinkage. Hot workability falls at the time of cracking, and it is easy to produce a crack.

In particular, cracks occurring at the slab edges and hot-rolled sheet edges, which are called edge cracks, cause problems that make manufacturing impossible, such as greatly reducing the error rate. In order to prevent such cracks, cracks generated during the hot rolling process have been extensively studied in the past, and there are fewer cases in which manufacture is impossible due to the optimization of components and rolling conditions.

The crack that determines whether it can be manufactured is a linear defect detected after pickling in a hot rolling process. Since the linear defects are fatal defects in stainless steel where the casting cast is a place where the surface quality is important, an additional process of eliminating the linear defects through correction processes such as property tax and grinding is required. In terms of quality control, products may not be available, which is a factor of price increase in the manufacturing process. In order to prevent such line defects, various studies have been made from the continuous casting process to the hot rolling and annealing processes.

In view of the structural improvement of the surface layer of cast steel, Japanese Unexamined Patent Publication No. 2-9651 discloses that a surface blast is applied to the slab which regulates the Si content of the austenitic stainless steel before the furnace is heated. A technique for preventing cracks by making microfabricated layers by refining the solidified structure of the slab surface layer during heating is made.

Also, in view of scale at the time of heating, Japanese Patent Publication No. 4-48865 proposes a technique for preventing shipboard defects by regulating the oxygen concentration at heating slab that regulates sol.Al to 0.5 to 5%. Doing. However, the method described above requires an additional process and also increases the process load, so it is difficult to completely eliminate the line defects by the above method.

The present invention has been made to solve the above-mentioned problems of the prior art, and aims to provide a method for producing a high nickel equivalent austenitic stainless alloy having excellent surface quality by controlling the δ-ferrite content of the cast steel casting. .

Hereinafter, the present invention will be described in detail.

In the present invention, when the high Ni equivalent austenitic stainless steel is manufactured by continuous casting, the delta ferrite (δ-ferrite) content of the cast steel is investigated by investigating the correlation between the occurrence location of the coil linear defect and the crack of the cast wire during hot rolling of the continuous casting cast steel. To control the range.

In the metallurgical examination of the linear defects occurring on the coil surface during hot rolling, there are two types of linear defects during hot rolling, which occur at the δ / gamma (γ) interface and at the γ grain boundary. In case of δ / γ interface, a significant amount of δ-ferrite remains in the hot rolling process. In case of cracking due to deterioration of hot workability at the δ / γ interface because the annual triangular rate of δ-ferrite and γ phase is different during hot rolling. In order to prevent cracking, it is required to control the δ-ferrite from the slab state to less.

On the other hand, δ-ferrite is directly related to hot workability, but because δ-ferrite acts as nucleation site of recrystallization when heated, it also affects growth and coarsening of γ grains, causing irregular growth of γ grains. Cracks may occur at grain boundaries. Therefore, it is necessary to prevent abnormal generation of γ grains upon heating after finely distributing δ-ferrite.

In the present invention, when solidification is started in the primary austenite phase, δ-ferrite is formed during the solidification process, and δ-ferrite is transformed into an austenite phase after solidification is completed, the following mathematical formula The δ-ferrite content represented by Equation 1 is characterized by controlling to 1-3% in the interior of the cast steel.

[Equation 1]

Equation 1 is a generalized formula, and its source is (Ironmaking Steelmaking, 1986, vol. 13, no. 5, P248). In order to control the δ-ferrite content by weight%, Cr is 20 to 30%, Ni is 15 to 25% in the following Equation 2 and Equation 3, that is, Hammer and Sevensson relation, source It is an alloy system in the range which belongs to [Cr / Ni] eq. = 1.2-1.4 from Soliditication Technology in the Foundary and Cast House, 1983, P401).

[Equation 2]

[Equation 3]

If the δ-ferrite content distributed in the continuous casting slab is less than 1%, the δ-ferrite does not absorb excessive generation of segregation of the primary austenite phase during coagulation, and segregation remains at the grain boundaries and the surface of the cast steel during playing Γ-grains grow irregularly and cause linear defects during hot rolling because δ-ferrite remaining in the slab during the hot rolling is completely extinguished and the γ-crystal recrystallization is delayed.

In addition, when the δ-ferrite content in the continuous cast slab is distributed over 3%, it does not affect the quality of the slab, but the δ-ferrite in the slab transforms into a weak sigma phase and causes line defects during hot rolling.

On the other hand, when explaining the reason for limiting the component range in the present invention.

The stainless material of the present invention is intended to be used in chemical plants and incinerators used at high temperatures. [Cr / Ni] eq. Should be in the range of 1.2 to 1.4, where Ni is an essential element for obtaining an austenite phase at high temperature and room temperature. However, since the use environment is required to suppress high temperature oxidation and excellent high temperature strength, the Ni content should be in the range of 15 to 25%.

Cr is an essential element for improving corrosion resistance. At least 20% Cr is required to have corrosion resistance in the use environment. However, since Cr is a ferrite forming element, if it is too high, a large amount of δ-ferrite phase will be produced at high temperature. Therefore, austenite forming elements (C, N, Ni, Mn, Cu, etc.) must be added to suppress the δ-ferrite phase, but the excessive addition of such elements causes problems such as grain boundary corrosion, so the upper limit of Cr is 30%. Is set to.

In addition, the reason for limiting the ratio of [Cr / Ni] eq. To 1.2 to 1.4 is that when [Cr / Ni] eq. Is 1.2 or less, no ferrite phase is formed in the casting structure in the continuous casting process. If the purpose of controlling δ-ferrite to 1 to 3% cannot be achieved, and the [Cr / Ni] eq. is 1.4 or more, the ferrite phase is formed as a primary crystal upon solidification, and thus the primary austenite phase (γ phase) to be achieved in the present invention is achieved. Because it does not form.

Through the following examples will be described in detail the effects and features of the present invention.

(Example)

After experimentally preparing a high Ni equivalent austenitic stainless alloy, the quality characteristics of the final product were shown as shown in Table 1 below. The comparative material A alloy system exhibited poor corrosion resistance with less Cr concentration than the reference value, while the comparative material B alloy system showed poor corrosion resistance with less Ni concentration than the reference value. On the other hand, the inventive alloys C and D showed that the Cr and Ni concentrations correspond to the reference values, indicating that the corrosion resistance and the high temperature strength were good.

In addition, Table 2 summarizes the problems occurring after continuous casting and hot rolling for the alloy system falling within the scope in the present invention. Continuous casting and hot rolling were prepared under normal conditions, and solidification cracks and hot rolled coils of the continuous casting cast after operation were examined closely for linear defects. Comparative A alloy had a δ-ferrite content of 3.6% in the cast steel, which did not cause solidification cracks in the cast steel, but caused linear defects on the hot rolled coil, which lowered the error rate of the hot rolled coil. Since the process of removing defects by grinding is required, the process load is taken during production.

In addition, the comparative material B has a problem of causing defects in the continuous cast steel and the hot rolled coil when the δ-ferrite content in the cast steel is 0.8%. On the other hand, the C and D alloys according to the present invention had a solidity crack in the continuous cast slab with δ-ferrite content of 1.7% and 2.9, and a good cast with no linear crack on the surface of the hot rolled coil. Could get

Through the above embodiment, the present invention confirmed that the quality of the continuous cast steel and hot rolled coil as well as stable operation can be obtained by controlling the δ-ferrite content in the cast steel casting.

TABLE 1

Ingredient (% by weight) Corrosion resistance High temperature strength Cr Ni Comparative material A 18.7 22.1 Bad Good B 25.3 14.5 Good Bad The present invention C 21.6 24.5 Good Good D 28.7 16.2 Good Good

TABLE 2

Example Ingredient (% by weight) δ-ferrite content (%) Cast quality Hot rolled coil quality Cr Ni Comparative material A 28.2 15.7 3.6 Good Bad B 20.7 24.5 0.8 Bad Bad The present invention C 23.7 19.8 1.7 Good Good D 27.6 16.7 2.9 Good Good

As described above, according to the manufacturing method of the high nickel equivalent austenitic stainless alloy having excellent surface quality, the high nickel equivalent (Ni) equivalent austenitic stainless alloy without process load for crack prevention during continuous casting and hot rolling There is an effect that can be prepared.

Claims (1)

Austenitic stainless alloys containing 20% by weight or less of Cr, 15% to 25% by weight, residual Fe and other unavoidable impurities, and having a ratio of [Cr / Ni] eq. In the range of 1.2 to 1.4. A method for producing a high nickel (Ni) equivalent austenitic stainless alloy having excellent surface quality, characterized by controlling the delta-ferrite content in the continuous cast slab to 1 to 3%.