JPH01262048A - Production of high corrosion resistant stainless steel having excellent hot workability and reducing segregation - Google Patents

Abstract

PURPOSE:To produce a high corrosion resistant stainless steel having excellent hot workability and reducing segregation by casting he specific composition of a high alloy containing much quantities of Cr, Ni, Mo, N, etc., into a cast slab having suitable structure and executing hot-rolling after executing the specific soaking. CONSTITUTION:The alloy composed of 18-55wt.% Cr, 13-70% Ni, <=5% Si, <=10% Mn, <=0.10% C, <=0.5% N, 3-20% Mo, and <=0.006% S, <=0.005% B, <=0.010% O, <=0.030 P as elements having small equilibrium distribution factor and particularly easy-to-segregation and one or more kinds among <=0.06% Y, <=0.02 Ce, <=0.02% Mg, <=0.01% Ca as existing in the slanting line range in the figure, and if necessary, further one or more kinds among <=4% Cu, <=1% Al, <=4% Nb, <=2% Ti, <=0.2% Zr and the balance Fe with inevitable impurities, is cast. Then, by one side face solidifying method, ESR method or both face solidifying method, over-heating degree is changed to make <=30% of uniaxial crystal part in cross section of the cast slab. Successively, after executing soaking in the slanting line range in the figure, the hot rolling is executed, to obtain the high corrosion resistant stainless steel.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to high-alloy stainless steel with excellent hot workability and corrosion resistance, and particularly includes a method for reducing segregation and corrosion resistance in seawater environments and chemical plants. The present invention relates to a method for manufacturing high-alloy stainless steel that has excellent corrosion resistance against the environment.

(Prior Art) High-alloy stainless steel is used in cases where particularly severe corrosion resistance, heat resistance, and oxidation resistance are required, and its importance tends to increase in the future. These alloys are often C
It contains large amounts of r, Ni, Mo, Si, etc., and N is an element that is actively desired to be used as an element to improve the strength and corrosion resistance of stainless steel. Highly alloyed stainless steel has poor workability in high-temperature ranges, and conventionally the ingot method had to be used. The inventors of the present invention and others have
As presented in Publication No. 47), continuous casting of these high-alloy steels has been promoted, but it has been found that these continuously cast slabs (hereinafter referred to as CC slabs) pose particular problems when the alloys are made particularly high. There was found.

(Problem to be solved by the invention) As the demand for high-alloy stainless steel increases, Cr +
There is an increasing need for a method for stably and economically producing stainless steel containing a large amount of alloying elements such as L+Mo+N+Cu. However, there are the following problems when manufacturing these high alloy stainless steels in large quantities using a continuous casting method.

(1) As the alloy becomes higher, hot workability in a high temperature range becomes poor.

(2) Since it contains a large amount of alloying elements, segregation of the alloying elements is likely to occur at the same time, and depending on the components, segregation cannot be easily reduced even if a subsequent soaking treatment is applied.

(3) Deformation resistance increases with high alloying, and deformation resistance is large and processing is difficult at conventional processing temperatures.

In order to solve these problems, the present inventors have developed a high-alloy stainless steel containing 6% Mo, and in order to solve the problem (1), for example, the method described in JP-A-61-163247 ( 2) Regarding the problem, the patent application No. 62-314
The method according to No. 834 was clarified. When these developments are further developed and the content of Ni + Mo, Cr, etc. increases, the problem (3) becomes more serious, and a method to solve this problem was studied and the method of the present invention was arrived at.

(Means for solving problems) The gist of the present invention is as follows.

(1) By weight, Cr: 18~55%, Ni: 13~
70%, Si55%, Mn510%, C≦0.10%,
S, which contains N≦0.5% and Mo: 3 to 20%, has a small equilibrium distribution coefficient during solidification and is particularly prone to segregation; 13. P,
For 0, S≦0.006%, B≦0.005%, 0
≦0.010%, P≦0.030%, and the amount of Ni+30xN in the alloy and ΔS (however, ΔS=S−0.8×C
a-0.5×Y-Q, 3×Mg-Q, 3×
Ce) so that the relationship with Y is within the shaded area in Figure 1.
≦0.06%, Ce6002%, Mg≦0.02%, C
a≦0.01% of one or more types, the remainder F
When an alloy consisting of e and unavoidable impurities is processed by the single-sided solidification method or the ESR method, or by the double-sided solidification method, the superheating degree of the molten alloy (the difference between the temperature of the molten alloy and the solidification temperature of the alloy) is controlled. The ratio of the equiaxed crystal portion in the cross section of the obtained slab is reduced to 30% or less, and after soaking is carried out so that the holding time and temperature relationship are within the shaded area shown in Figure 2, hot heating is carried out. A method for producing highly corrosion-resistant stainless steel that has excellent hot workability and reduced segregation, which involves rolling.

(2) By weight, Cr: 18-55%, Ni: 13-70
%, Si55%, Mn510%, C≦0.10%, N≦
Regarding S, B, P, and 0, which contain Mo: 3 to 20% and have a small equilibrium distribution coefficient during solidification and are particularly easy to segregate, S≦0.006%, B≦0.005%, 0≦0.01
0%, P≦0.030%, and Ni+3 in the alloy
0×N amount and ΔS (however, △5=S-0, +3 ×Ca-0
, 5×Y −0.3×Mg−0°3×Ce) is within the shaded area in FIG.
Contains one or more of e≦0.02%, ceramic≦0.02%, Ca≦0.01%, further Cu54%, A151
%, 1% Nb, 52% Ti, and one or more of Zr≦0.2%, with the balance consisting of Fe and unavoidable impurities, by one-sided solidification method, ESR method, or double-sided solidification method. By controlling the degree of superheating of the molten alloy (the difference between the temperature of the molten alloy and the solidification temperature of the alloy), the ratio of the equiaxed crystal portion in the cross section of the resulting slab is increased to 30%.
% or less, followed by soaking with holding time and temperature within the shaded area in Figure 2, followed by hot rolling. Method of manufacturing corrosion-resistant stainless steel.

The present invention will be explained in detail below. The present inventors have developed various Cr
-Investigated and considered the solidification, hot workability, and corrosion resistance of Ni-based alloys.

High-alloy steel, especially CC slabs alloyed with large amounts of Cr, Ni, Mo, N, etc., has poor hot workability and cracks such as edge cracks and face cracks occur during hot working, making subsequent processes difficult. There may be cases where the product is lost, or there may be a case where the cost increases due to a decrease in yield due to each maintenance to remove the restriction. The present inventors conducted a detailed investigation into the hot workability of CC slabs, particularly the occurrence of cracks, and found that cracks tend to occur at grain boundaries and interfaces with the second phase. In addition, when we investigated the segregation behavior and hot deformability of various alloys during solidification, we found that S, B, P, and O
It has been found that the hot workability of CC877 pieces of high alloy steel is largely controlled by the elements . In order to reduce the effects of these elements, it is important to control the content of S, BXP, and 0.
It is necessary to satisfy P≦0.010% and P≦0.030%.

In addition to elements that segregate at grain boundaries, N1 and N also degrade hot workability, but in order to prevent hot workability from deteriorating, addition of Y is effective, and Ce +' Ca 1M
Addition of g etc. is also effective. Especially when the amount of Ni increases, ΔS =S-0.8×Ca-0.5×Y-0,3×
Y≦0 so that the relationship between ΔS determined by Mg-0, 3×Ce and Ni+30×N falls within the shaded area shown in FIG.
06%, Ce≦0.02%, Mg≦0.02%, Ca≦
It has been found that hot workability can be improved by selectively adding one or more kinds within a range of 0.01%.

Furthermore, the desirable soaking conditions (described below) for these alloys (
Soak at 1100°C or higher as shown in Figure 2), and
It has become clear that ductility at high temperatures can be greatly improved by diffusing and eliminating phases and microsegregation. For this reason, 1
It has become possible to start hot working from a temperature range higher than the temperature range of 000°C, and rolling can now be performed without cracking in a temperature range where deformation resistance is low.

In this way, the content of S, B, P, and 0 is reduced in high alloy steel, and depending on the amount of Ni+30×N, Y2Ce,
The addition of Ca, Mg, etc. reduces the effects of elements that segregate at grain boundaries, and each soaking treatment of CC slabs eliminates the negative effects of micro-segregation and second phase on hot workability, thereby improving hot workability. By improving this, it has become possible to expand the temperature range suitable for hot working to the high temperature side, making it possible to work in a temperature range with low deformation resistance.

In addition, when high alloy steel contains a large amount of Mo or Ni, the elements may segregate during solidification, especially semi-macro segregation in the center of the thickness of CC slabs or semi-macro segregation in the center of the thickness of CC slabs. It was found that single-column segregation was significant. Mo and Mo are the main components of high alloy steel, and if their segregation is large, they deteriorate corrosion resistance.

The segregation of Mo in CC slabs deteriorates the corrosion resistance in seawater, and countermeasures for this problem were disclosed in Japanese Patent Application No. 62-201028.

The unidirectional solidification method is a casting method that eliminates or reduces the macro-segregation in the center of the thickness of CC slabs and the semi-macro-segregation in which spot-like segregation is dispersed, which causes a significant deterioration in corrosion resistance. has been studied, and this method is effective because it does not cause segregation in the center of high-alloy steel. Furthermore, the remelting method using the ESR method has also been effective in reducing segregation. As a result of research into a method for reducing segregation in the center using a normal continuous casting method, the present inventors have found that it is especially effective to reduce the center segregation by controlling the casting temperature from 1 to 100 to solidify into a structure consisting mainly of columnar crystals. It was confirmed that it is easy to reduce the segregation by soaking. The ratio of the casting temperature and the equiaxed crystal ratio of the CC slab is as shown in FIG. 4, and from this relationship, the casting temperature that makes the ratio of the equiaxed load 30% or less is determined. In this way, by performing casting to reduce the ratio of equiaxed crystals in the cross section of the slab to 30% or less, the degree of segregation in the center of the CC slab is light and the center segregation during subsequent soaking of 1100'c or more is reduced. It becomes possible to make it easier.

Next, the present invention will be explained in detail. Table 1 shows the composition of the high alloy and ratio M'FA according to the invention.

1 contains 25Cr-3ONi-3Mo-3Cu-0,IN as the basic component, reduces S, B, P, 0, and further Ca: 0
．． 003%, V: 0.006%, 2 is 20Cr-45N+-9Mo-I
S with Cu as the basic component.

Reduced B, P, O and further Mg: 0.007%, y
: High alloy steel containing 4% o, oo. These alloys were made of Sb into CC slabs with a thickness of 150 mm at a ΔT of 40 to 50°C during casting. In the cross section of the slab, columnar crystals were developed, and the equiaxed crystal ratio was less than 10%. After that, the slab was 125
Soaking was performed at 0°C with 511r and 151b, respectively, to diffuse and eliminate the segregation and microsegregation in the center and the second phase. In this state, the high-temperature ductility of the slab was improved, and segregation in the center, which affects corrosion resistance, was significantly reduced. These slabs are then heated at 1200°C in a conventional thick plate rolling furnace.
It was heated at 1300°C and subjected to hot rolling, but especially Ni
Regarding steel type 2 with a large content, since the deformation resistance was large, the heating temperature was increased, the rolling temperature was started at about 1240°C, and a high reduction was applied at the beginning of the rolling. As a result, both side slabs could be hot-rolled without cracking. After hot rolling, this rolled thick plate is water-cooled to prevent the formation of intermetallic compounds, and then
A thick plate was produced by annealing at 50°C. In this jV plate product, the segregation of Mo and Ni was slight and the pitting corrosion resistance was also good.

On the other hand, comparative steel types 3 and 4 did not satisfy the relationship shown in FIG. 1 in terms of Ni+30×N, had poor hot workability, and even after soaking treatment, edge cracks occurred during hot rolling. This thick plate was also water-cooled after rolling and subjected to solution heat treatment in the same manner as Steel Types 1 and 2 to produce a product, but the segregation in the center of the plate thickness did not decrease and the corrosion resistance was poor. The reason for this is that since the equiaxed load in the slab exceeds 30%, segregation did not decrease even after soaking for 15 hours.

(Effects of the Invention) According to the present invention, hot workability in the high temperature range, which is a problem with Fe-Ni-Cr-Mo-based high alloys, is improved, segregation during solidification is reduced, and excellent properties with no segregation are achieved. In addition, it is possible to process high alloys with high deformation resistance at a higher temperature than before, which has an extremely large effect on the production of high alloy steels.

[Brief explanation of the drawing]

Figure 1 shows Ni-1-30×N and ΔS (-3-0,8×
Figure 2 shows the good ductility region (shaded region) just below the melting point of Fe-Cr-Ni stainless steel in the relationship of Ca-0,5xY-0,3Mg-0,3Ce), and Figure 2 shows the soaking conditions of the present invention. Fig. 3 shows the effect of soaking on the hot workability of the steel slab of the invention, and Fig. 4 shows the relationship between the casting temperature and the equiaxed cross section of the invention steel. FIG. 3 is a diagram showing the relationship between crystal rates.

Claims (2)

[Claims] (1) By weight, Cr: 18-55%, Ni: 13-70
%, Si≦5%, Mn≦10%, C≦0.10%, N≦
0.5%, Mo: 3 to 20%, and S, B, P, and O, which have a small equilibrium distribution coefficient during solidification and are particularly easy to segregate, are S≦0.006%, B≦0.005%, O≦0.01
0%, P≦0.030%, and Ni+3 in the alloy
0×N amount and ΔS (However, ΔS=S-0.8×Ca-0.5
x Y - 0.3 x Mg - 0.3 x Ce) is within the shaded area in Figure 1, Y≦0.06%, Ce≦0
．． 02%, Mg≦0.02%, Ca≦0.01%, and the balance consists of Fe and inevitable impurities, by one-sided solidification method, ESR method, or double-sided solidification method. In this case, the degree of superheating of the molten alloy (the difference between the temperature of the molten alloy and the solidification temperature of the alloy) is controlled to keep the ratio of equiaxed crystals in the cross section of the obtained slab to 30% or less, and then The retention time and temperature relationship are the second
A method for producing highly corrosion-resistant stainless steel with excellent hot workability and reduced segregation, which comprises soaking within the shaded area in the figure and then hot rolling. (2) By weight, Cr: 18-55%, Ni: 13-70
%, Si≦5%, Mn≦10%, C≦0.10%, N≦
0.5%, Mo: 3 to 20%, and S, B, P, and O, which have a small equilibrium distribution coefficient during solidification and are particularly easy to segregate, are S≦0.006%, B≦0.005%, O≦0.01
0%, P≦0.030%, and Ni+3 in the alloy
0×N amount and ΔS (However, ΔS=S-0.8×Ca-0.5
x Y - 0.3 x Mg - 0.3 x Ce) is within the shaded area in Figure 1, Y≦0.06%, Ce≦0
．． 02%, Mg≦0.02%, Ca≦0.01%, and further contains Cu≦4%, Al≦1%,
An alloy containing one or more of Nb≦4%, Ti≦2%, and Zr≦0.2%, with the balance consisting of Fe and unavoidable impurities, is prepared by a single-sided solidification method, an ESR method, or a double-sided solidification method. In this case, the degree of superheating of the molten alloy (the difference between the temperature of the molten alloy and the solidification humidity of the alloy) is controlled to increase the ratio of equiaxed crystal parts in the resulting slab cross section to 30%.
A highly corrosion-resistant stainless steel with excellent hot workability and reduced segregation, which is characterized by being hot-rolled after soaking with the holding time and temperature within the shaded area in Figure 2. manufacturing method.