JPS6023905B2 - Method for manufacturing high carbon chromium nickel steel with excellent hot workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to the production of high carbon chromium nickel steel, which has fine precipitated carbides by slowing down the cooling rate during casting, thereby significantly improving hot workability. It is about the method.

H, which has excellent high-temperature strength, especially creep rupture strength, is used for chemical industry re-former tubes, cracking tubes, etc.
High chromium-high nickel heat-resistant steel, of which K40 is a typical example, is used.

These tubes are currently manufactured by centrifugal casting, melting, etc., but each has the following drawbacks. In other words, with centrifugal casting, there is an unhealthy layer on the inner surface of the product.
There are restrictions on the dimensions that can be manufactured (diameter, thickness, length). There are many welding points when maximizing the size, and productivity is low when using the melting method. These drawbacks can be improved by making tubes by the process of steel ingot → blooming rolling → hot extrusion → heat treatment, but high carbon chromium nickel heat-resistant steel has extremely poor hot workability, This process was disadvantageous because the steel ingot or tube cracked during the extrusion process, resulting in a very low yield.

The purpose of the present invention is to have C: 0.26 to 1.0% by weight, Si:
5% or less, Mm: 15% or less, Cr: 15-40%, M
i: When making a tube of high carbon chromium nickel heat-resistant steel containing 10 to 50% as the main component and the remainder consisting of Fe and unavoidable impurities by ingot → blooming → hot extrusion → heat treatment process, during ingot casting. (7 from the liquidus line)
By setting the average cooling rate to 00 qo or higher, the shape of the precipitated carbide is made finer, thereby reducing hot work cracking after blooming in the tube manufacturing process. High-carbon chromium heat-resistant steel pipes have traditionally been used as cast in chemical industry equipment. The reason for this is that a large amount of carbon is included in order to increase the high temperature creep strength, and as a result, many huge eutectic carbides precipitate at the grain boundaries and within the grains in the cast state. This is because cracking occurs from carbides (particularly grain boundary carbides), making hot working extremely difficult. Cracking during hot working of this type of heat-resistant steel occurs below 1150 qo. Further, at high temperatures of 1250 qo or more, grain boundary carbides melt and become unworkable. Therefore, the temperature range with good workability is extremely narrow. In normal blooming rolling, cracks generated from eutectic carbides occur at the corners and faces of the ingot when rolling below 1150 qo. It would be possible to perform multi-heat rolling in a temperature range that provides good workability, but this is not possible when considering actual operation. However, according to the present invention, the average cooling rate (from liquidus to 700°C) during ingot casting is reduced to 30q
If the giant crystalline carbides in the ingot are made finer by increasing the rolling time to 0/min or more, hot workability will be significantly improved and there will be no cracking during blooming.

Therefore, it is possible to perform crack-free blooming rolling in one heat rolling. Cracks during hot working of this type of heat-resistant steel occur from grain boundary carbides, and cracks propagate through austenite grain boundaries, interfaces between grain boundary carbides and austenite matrix, grain boundary carbides, and the like.

The reasons why workability is improved by making carbides finer are as follows: 1) Since carbides are small, cracks are less likely to occur. 2) Even if grain boundary carbides are destroyed and cracks occur, the finer the grain boundary carbides, the more likely the fracture will propagate.

Next, the reasons for limiting the chemical composition in the present invention will be explained. C is essential for increasing the strength of mirror steel.

However, if it is less than 0.26%, strength cannot be obtained and 1.0%
%, the workability during hot and cold working deteriorates, so the upper limit was set at 1.0%. The most preferred range is 0
．． It is 35-0.6%. Si is added as a deoxidizing agent and to improve oxidation resistance at high temperatures, but if its amount exceeds 5%, processability and acid solubility are inhibited, so the upper limit of Si is set at 5%.

The preferred range is 0.5-3%. Mn is added for deoxidation and prevention of hot oxidation, but it has a strong effect of stabilizing austenite and can also be used as a substitute element for expensive Ni. However, if too much Mn is added, the oxidation resistance will be deteriorated, so the amount is set to 15% or less. The preferred range is 1.0-10%. Although 15% or more of Cr is required to improve oxidation resistance, too much Cr tends to cause sigma arm formation, so it is set to 40% or less.

Among these, 20 to 28% is most preferable. Ni is contained in an amount of 10% or more to stabilize austenite.

Further, Ni has the effect of preventing sigma armization, carburization, and nitriding, but if too much Ni is added, the effect reaches almost saturation, and the material cost increases, so it was set at 10 to 50%. Among these, the most preferable range is 18 to 35%. Example 1 Test specimens with the compositions shown in Table 1 were prepared by changing the average cooling temperature during casting (from the liquidus line to 70 mm), and were subjected to a Greeple test (high temperature and high distance tensile test) and a small block test. Hot workability was investigated by a continuous 8-bus continuous rolling test using a piece (120 x 120 x 190).

Table 1 Figures 1, 2, and 3 show how the cooling rate during solidification affects hot workability using the Grieple test.

It can be seen that steels A, B, and C, which were cooled at a faster rate by the method of the present invention, had significantly improved ductility (reduction ratio) than steels ○, E, F, G, 1, and 1 by the conventional method. A, B
It has been empirically found that blooming is possible if the reduction ratio is comparable to that of C steel. Therefore, steels A, B, and C, which have a far cooling rate, can be subjected to soul rolling. On the other hand, steels with slow cooling rates of ◯, E, F, G, 1 and 1 have a narrow temperature range in which blooming can be carried out, and it is difficult to carry out blooming on an industrial scale in practice. Figure 4 shows the ASCAST structure of steels B, F, and G. It can be seen that steel B, which has a fast cooling rate, has significantly smaller carbides than steels F and G, which have a slower cooling rate. The reference figure is a photograph showing cracks in a small block material cut from A and E steels after continuous 8-pass rolling (12 yards → 2 yards).

As expected from the Greeple test results, no cracks were observed in Steel A, but significant cracks were observed in Steel E. As explained above, according to the method of the present invention, high carbon chromium nickel heat-resistant steel, which was conventionally considered difficult to be rolled, can be rolled.
Since hot processing such as hot extrusion can be carried out, it is possible to easily manufacture reformer tubes, cracking tubes, etc. for the chemical industry, and the effect thereof is extremely large.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, and FIG. 3 are diagrams showing the results of the Greeple test (high temperature and high distance tensile test), and FIG. 4 is a photographic diagram of the ASCAST structure of B steel, F steel, and G steel. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1 C: 0.26 to 1.0% by weight, Si: 5% or less,
Mn: 15% or less, Cr: 15-40%, Ni: 10-
When casting high carbon chromium nickel steel containing 50% as the main component and the remainder consisting of Fe and unavoidable impurities,
A method for producing high carbon chromium nickel steel with excellent hot workability, characterized by making the average cooling rate from the liquidus line to 700°C at 30°C/min or more.