JPH0128827B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a high-toughness martensitic stainless steel for cryogenic use that has a 0.2% proof stress of 100 Kgf/mm ² grade and a large cryogenic impact value. For example, compressor parts, chemical plant parts, high-strength bolts, etc. often require high strength, high toughness, and corrosion resistance at the same time. High-strength stainless steels for such applications include martensitic stainless steels and precipitation-hardening stainless steels. The former is typically 13Cr and is inexpensive, but is relatively easy to rust and Poor weldability, low-temperature toughness, and cryogenic toughness. The latter is typically 17−4PH (JIS/SUS630).
17Cr−4Ni−4Cu−Nb system and FV520B (Firth
Vicker standard) is a typical 14Cr−5Ni−2Mo−
There is a 1Cu-Nb series steel, which has better weldability and corrosion resistance than 13Cr series steels, but has a problem of low low temperature toughness and cryogenic toughness, and also has a low yield strength ratio (0.2% yield strength/tensile strength) among tensile properties. is large, making it undesirable for use. Another problem is that FV520B contains Mo, which increases the cost accordingly. In view of the above circumstances, the present invention has developed a 100Kgf/mm ^2nd grade
The purpose is to provide an inexpensive martensitic stainless steel with 0.2% yield strength and higher cryogenic toughness than 17-4PH or FV520B, with weight percentages of C0.05~0.1%, Si1% or less, Mn1%
Below, Ni4~6%, Cr15~17.5%, V0.1% or less,
Martensitic stainless steel with Al0.005~0.04%, the balance substantially Fe and impurities, has a metal structure mainly composed of fine acicular martensite, and has a high cryogenic impact value that has been quenched and tempered. Pertains to. In this specification, chemical compositions are expressed in weight % as usual. Next, the chemical composition of the stainless steel according to the present invention will be described. It has been known that adding Ni to 13Cr martensitic stainless steel improves weldability and toughness. However, the corrosion resistance is poor, so in order to improve this, in the present invention,
Increase the Cr content to 15 to 17.5%. In order to form unstable austenite that is easily transformed into martensite by quenching from the austenitizing temperature range, 4 to 6% of Ni is further contained. In this way, by quenching and tempering, it is possible to obtain a metal structure in which the base is martensite, a small amount of fine ferrite is dispersed, and at the same time, a small amount of residual austenite is also mixed. As C content increases, weldability deteriorates, so the upper limit is set at 0.1%. In order to obtain high toughness, it is desirable that the content be approximately 0.03%, which is the lowest value obtained by atmospheric dissolution. Since this 0.03% is the boundary between atmospheric melting and vacuum melting, the lower limit is
Set it to 0.05%, which is close to 0.03%. Si is added for deoxidation as usual, but it is a ferrite-forming element and also has the effect of strengthening the base. However, if the amount increases, the toughness will be impaired, so the upper limit should not exceed 1%, and preferably
0.2-0.5%. Mn is also added for deoxidation and desulfurization, but it is an austenite-forming element, and if its amount increases, it accelerates anode dissolution and impairs corrosion resistance, so the upper limit should not exceed 1%, and preferably It should be 0.4-0.6%. V exhibits remarkable temper softening resistance and is an important element for maintaining strength after tempering. Furthermore, it also has the effect of making crystal grains finer, which is rather effective for improving toughness. However, even if the amount is increased, the effect will not change, so in the present invention, the upper limit is set at 0.1%, and 0.02 to 0.06%.
It is desirable to add some amount. As is well known, Al has a strong degassing effect and also has the effect of refining crystal grains, so the content should be 0.005 to 0.04%, preferably 0.02 to 0.04%.
%. In this way, even if the content of oxygen and nitrogen increases due to melting in the atmosphere, the desired effect can be sufficiently obtained. Cu is a precipitation-hardening alloying element, and Mo is an alloying element that exhibits resistance to temper softening and affects the hardness after tempering, but it has no effect on increasing toughness, so the smaller the amount, the better. Nb+Ta is a carbide-forming element and has a small effect on improving toughness, so the smaller the amount, the better. In the present invention, the amounts of Cu, Mo, and Nb+Ta are below the allowable amount that allows industrial control of contamination from raw materials during melting, and Cu is approximately 0.3% or below.
Desirably 0.1% or less, Mo approximately 1% or less, Nb+Ta approximately 0.1% or less, preferably 0.05
% or less. As for P and S, they are elements that are mixed incidentally during dissolution as usual, and the less the better, so in the present invention, the contamination in atmospheric dissolution is set to about 0.025% or less, which can be industrially controlled. is desirable. The melting may be carried out by ordinary atmospheric melting, and does not necessarily require vacuum melting or vacuum degassing. Heat treatment is simpler than FV520B, requiring only quenching and tempering, with quenching at 930-970°C, oil cooling, and tempering at 430-470°C, with air cooling being suitable. There is no need for preheating or postheating during welding, and the weldability is good, making it easy to perform repair welding. Next, Table 1 shows the chemical compositions of Examples along with the comparison materials, and Table 2 shows the heat treatment and tensile properties.

【table】

[Table] As can be seen from Table 2, the steel according to the present invention is 0.2%
When the proof stress is approximately 100Kgf/ ^mm2 , the tensile strength is
Both elongation and reduction of area are larger than the comparison steel, and the proof stress ratio is the smallest, making it advantageous in terms of use. Next, 2 mm V-notched pylon impact test pieces were prepared from each of the above samples and cryogenic impact tests were conducted. The results are shown in FIG. 1 in relation to absorbed energy and test temperature. As can be seen from Figure 1, the low-temperature impact value of the steel according to the present invention is significantly superior to that of conventional products 17-4PH or FV520B, and the 0.2% proof stress is approximately 100Kgf/
mm ² , it shows high toughness with a 2 mm V notch shear py impact value of 5 Kgf/mm ² even at -120°C. Figure 2 shows the microscopic structure of the No. 1 test piece that was heat-treated and had a hardness of Hv385 as shown in Table 2 above.
Figure 3 shows that of Hv360 No. 2. Contrast material No.
The structure of 17-4PH in No. 2 is a lath-like martensite structure, whereas the steel according to the present invention has a fine needle-like tempered martensite base mixed with a small amount of fine ferrite due to quenching and tempering. It is thought that this structure improves the cryogenic toughness. As explained above, the steel according to the present invention is different from the conventional steel.
In addition to increasing the Cr content of 13Cr-based high-strength stainless steel, it also contains 4 to 6% Ni, and is quenched and tempered to create a structure in which a small amount of fine ferrite is dispersed in the acicular martensite structure, so it can withstand extremely low temperatures. It shows an outstanding toughness value. In addition, it has excellent workability in forging, cutting, welding, heat treatment, etc., can be melted in the atmosphere, and does not contain Mo, so it is inexpensive.Also, it has good weldability, so it does not require preheating or postheating during welding, and 13Cr It has excellent cryogenic toughness and yield strength ratio, such as better corrosion resistance than other stainless steels, so no rust removal is required when used in actual equipment, and its simple chemical composition makes it easy to manage when scrapped. It is an economical high-strength stainless steel that has extremely high practical effects.

[Brief explanation of drawings]

Fig. 1 is a graph showing the low-temperature impact properties of the stainless steel according to the present invention in comparison with the properties of a comparison material, Fig. 2 is a photograph (100x magnification) showing the microscopic structure of the stainless steel according to the present invention, and Fig. 3 The figure shows the contrast material 17−4PH.
This is a photograph showing the microscopic structure (100x magnification).

Claims (1)

[Claims] 1% by weight, C0.05-0.1%, Si1% or less, Mn1
% or less, Ni 4-6%, Cr 15-17.5%, V 0.1% or less, Al 0.005-0.04%, the remainder consisting essentially of Fe and impurities, with a metal structure mainly consisting of fine acicular martensite. A high-toughness martensitic stainless steel for cryogenic use that has been quenched and tempered and has a high cryogenic impact value.