JPH0140903B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a steel plate for spiral fins in heat exchangers that has excellent workability and high temperature oxidation resistance, and a method for manufacturing the same. For example, high-temperature combustion exhaust gas or high-temperature steam is exchanged with heat through pipes of a heat exchanger. A spiral fin is provided on the outer periphery of the pipe through which the high-temperature combustion exhaust gas or high-temperature steam passes, in order to efficiently exchange heat. Incidentally, as shown in FIG. 1, the spiral fin is manufactured by spirally winding a steel bar 2 having a narrow width, for example, 10 to 30 mm, around a pipe 1, and then welding and joining the pipe 1. When manufacturing the spiral fin, the side _2-1 that is not joined to the pipe 1 is the side _2-2 that is joined.
Since it is subject to greater elongation deformation than that of steel, it is necessary to have workability that can withstand this. On the other hand, since the spiral fin of the heat exchanger is subject to high temperatures, it is necessary to have oxidation resistance at high temperatures. Additives that generally improve high-temperature oxidation resistance, such as
When Cr, Mo, etc. are contained in steel, the workability deteriorates significantly. For these reasons, conventionally it has been difficult to manufacture steel sheets with excellent high-temperature oxidation resistance that can be used in spiral fins of heat exchangers. In view of the above-mentioned circumstances, the present invention was developed based on a detailed study of steel components and manufacturing methods.
Contains 0.5 to 5% Cr to provide high temperature oxidation resistance,
On the other hand, C is 0.015% or less, Mn is 0.10-0.35%, S is
We have developed a steel plate for spiral fins in heat exchangers with N content of 0.01% or less and 0.0060% or less for improved workability, as well as a method for manufacturing the same. The present invention will be explained in detail below. First, let's talk about the steel components. If the C content increases, the workability deteriorates, so it is preferable that the C content is low. In particular, in the present invention, in order to achieve high elongation properties,
0.015% or less. As Si content increases, workability deteriorates and breakage or cracks occur when a steel strip is spirally wound, so the upper limit is set to 0.5% or less. Mn is an effective component for preventing embrittlement during hot working, but as its content increases, it becomes hard and elongation deteriorates, so it is set at 0.10 to 0.35%. If the content of P increases, it will make the steel brittle and the elongation will deteriorate significantly, so it should be kept at 0.03% or less. S forms inclusions and deteriorates elongation, especially local elongation, so its content is regulated to 0.01% or less. Cr is an effective component for providing high-temperature oxidation resistance, but on the other hand, if its content increases, workability may deteriorate and cracks may occur when the steel strip is wound in a spiral, so it should be 0.5 to 5%. shall be. A is a necessary component for deoxidizing during steel manufacturing, but if its amount increases, it becomes a cause of hot brittleness, so it is set at 0.01 to 0.08%. N is a component that deteriorates elongation, and if its content increases, processing cracks will occur when the steel strip is spirally wound, so it is regulated to 0.0060% or less. Steel comprising the above components is melted in a converter or electric furnace, made into a slab by continuous casting or ingot-blushing rolling, and then hot rolled. In this hot rolling, it is preferable to set the winding temperature after rolling to 650° C. or less in order to prevent coarsening of carbides and ensure workability. Then, after being descaled, it is cold rolled.
As shown in Figure 2, the rolling reduction in cold rolling has a large effect on the elongation properties of steel containing high-temperature oxidation-resistant components Si and Cr, and if the rolling reduction is not within an appropriate range, elongation may deteriorate. discovered. The steel in Figure 2 is C: 0.005%, Mn: 0.28%,
It is a cold-rolled steel plate with a thickness of 1.4 mm, based on Si: 0.25%, Cr: 1.0%%, with the balance being iron and other unavoidable components. From this knowledge, in order to obtain high elongation, the rolling reduction ratio is
40-70%. The reason for doing this is that even if the rolling reduction becomes low or high, the elongation value decreases and cracks occur during spiral fin manufacturing, so the lower limit was set at 40% and the upper limit was set at 75%. The preferred rolling reduction ratio to improve elongation properties is 50 to 70.
%. After cold rolling, the cold rolled sheet is slit into multiple narrow strips. The width of this slit steel strip is, for example, 10 to 30 mm. The steel strip slit into multiple strips is rolled up and then annealed in an annealing process. What should be noted in this annealing is that the steel of the present invention contains Cr, so depending on the annealing atmosphere, it absorbs N, which deteriorates workability, especially elongation. In order to avoid this, 30% or more of
It is necessary to include _H2 . When H ₂ is contained in an amount of 30% or more, the amount of N penetrating into the steel is small, and high elongation properties can be obtained. The annealing temperature in this annealing is 700℃ or higher, Ar ₃
Below the metamorphosis point. The reason for this is that when the annealing temperature is low, the crystal grains are small and the workability is low.
℃. Furthermore, if the annealing temperature exceeds the Ar ₃ transformation point, the crystal structure changes due to phase transformation, which deteriorates workability, so the upper limit is set to the Ar ₃ transformation point. Through the above steps, a steel bar plate for spiral fins of a heat exchanger is manufactured. Next, examples will be shown. Table 1 shows the chemical composition of the steel of the present invention. Steel having the chemical composition shown in Table 1 was manufactured under the hot rolling, cold rolling, slitting and annealing conditions shown in the table, and its mechanical properties, high temperature oxidation loss, and heat exchanger pipe properties were evaluated. Table 2 shows the results of investigating whether or not cracks occur when steel strips are spirally wound.

【table】

【table】

[Table] As is clear from this result, Samples 1, 2, 3, and 4 according to the present invention have large elongation, and when the steel strip is wound as a spiral fin, no cracks occur at all and the workability is excellent. This was confirmed. It was also found that high-temperature oxidation loss was small and high-temperature oxidation resistance was excellent.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of manufacturing a spiral fin for a heat exchanger, and FIG. 2 is a diagram showing the influence of cold rolling reduction on elongation. 1... Pipe, 2... Steel bar plate.

Claims (1)

[Claims] 1 C: 0.015% or less, Si: 0.5% or less, Mn: 0.10
~0.35%, P: 0.03% or less, S: 0.01% or less,
Cr: 0.5~5%, A: 0.01~0.08%, N: 0.0060
A steel plate for spiral fins in heat exchangers with excellent workability and high-temperature oxidation resistance, containing less than % iron and unavoidable impurities. 2 C: 0.015% or less, Si: 0.5% or less, Mn: 0.10
~0.35%, P: 0.03% or less, S: 0.01% or less,
Cr: 0.5~5%, A: 0.01~0.08%, N: 0.0060
% or less, the balance is iron and unavoidable impurities, is hot rolled, then cold rolled at a rolling reduction of 40 to 75%, slit into narrow widths, and the slit steel plate is subjected to Ar ₃ transformation at temperatures above 700°C. A method for producing a steel plate for use in spiral fins for heat exchangers, which has excellent workability and high-temperature oxidation resistance, and is characterized by annealing in an atmospheric gas containing 30% or more of H ₂ at a point temperature.