JPS6026645A - Martensitic stainless steel with high toughness - Google Patents

Abstract

PURPOSE:To obtain an inexpensive martensitic stainless steel with high toughness at low temp. by increasing the Cr content of a high strength 13Cr stainless steel, adding a proper amount of Ni, and forming a specified structure by heat treatment. CONSTITUTION:A steel consisting of, by weight, 0.03-0.1% C, <=1% Si, <=1% Mn, 4-6% Ni, 15-17.5% Cr, <=0.1% V, 0.005-0.04% Al and the balance Fe is refined. The steel is hardened at about 930-970 deg.C and tempered at about 430- 470 deg.C to form a structure contg. a small amount of fine-grained ferrite and a very small amount of residual austenite dispersed in a fine acicular martensite matrix. The desired steel with much superior toughness at low temp. is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a martensitic stainless steel having a 0.2% yield strength of 100 kgf/fin'' class and a large low-temperature 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 uses include martensitic stainless steels and precipitation-hardening stainless steels. The former is typically 13Cr-based and is inexpensive, but is relatively easy to rust and Poor weldability and low temperature toughness.

The latter is a 17Cr-4Ni-4Cu-Nb system, of which 17-4PH (JIS-3US63G) is a typical example, and FV520.
B (Firth Vicker standard) is a typical one.
There is a 4Cr-5Ni-2Mo-lCu-Nb series, which has better weldability and corrosion resistance than the 13C1r series steel, but has a problem of low low-temperature toughness. % proof stress/tensile strength) is large, making it undesirable for use.

Furthermore, since FV520B contains MO, there is a problem in that the cost is correspondingly high.

In view of the above circumstances, the present invention has been developed to
It has a 0.2% yield strength of 17-4PH or FV5
The aim is to provide an inexpensive martensitic stainless steel with higher low-temperature toughness than 20B.
Below, Ni 4-6%, Cr 15-17.5%, v 001% or less A1 0.0
05 to 0.04% The remainder consists essentially of Fe and impurities, and relates to a martensitic stainless steel with a high low-temperature impact value and a metal structure mainly composed of fine acicular martensite. 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.

Conventionally, Ni has been applied to 13Cr martensitic stainless steel.
It is known that adding Cr improves weldability and toughness. However, since the corrosion resistance is poor, in order to improve this, in the present invention, the content of Cr is increased to 15
~17.5%.

In order to form unstable austenite which 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 retained austenite is also mixed.

As C content increases, weldability deteriorates, so the upper limit is set to 0.1%. In order to obtain high toughness, it is desirable that the toughness be approximately 0.03% of the minimum value obtained by atmospheric melting.

St 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 to 0.
．． 5%.

Mn is similarly 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 The content should be 0.4 to 0.6%.

(2) 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 to 0.
It is preferable to add 1% and about 0.02 to 0.06%.

As is well known, eye A has a strong degassing effect and also has the effect of refining crystal grains, so its content should be reduced to 0.
005-0.04%, preferably 0.02-0.04%
shall be. 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 hardenable alloying element, and MO is an alloying element that exhibits resistance to temper softening and affects the hardness after tempering, but has no effect on improving 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, Cu s
The amounts of Mo and Nb+Ta are below the allowable amount that allows industrial control of contamination from raw materials during melting, Cu is about 0.3% or less, preferably 0.1% or less, Mo is about 1% or less, Nb+Ta is approximately 0.1% or less, preferably 0.05% or less.

As for P and S, they are elements that are mixed incidentally during melting as usual, and the smaller the better, so in the present invention, the mixing is controlled to be approximately 0.025% or less in atmospheric melting, which is industrially controllable. It is desirable to do so.

The melting may be carried out by ordinary atmospheric melting, and does not need to be vacuum melting or vacuum membrane gas.

The heat treatment is simpler than FV520B, and requires only quenching and tempering, and the quenching is 930 to 970°C, oil cooling, and the tempering temperature is 430 to 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.

Note: Invention steel, V%=0.03211h2:
17-4PII, Na3:FV520BCu%: f
f12 = 3.28, No, -3 = 1.68 M0%: 隘3 = 1.57, Nb-1-Ta: I'h2 = 0.26, N13
= Q, 3C1 Table 2 Note, Quenching is oil cooling, tempering is air cooling, however, if fish 3 is quenched, 1050℃Xlh air cooling, 850℃xih air cooling, yield strength and tensile strength kgf / **2, *GL = 5D No. As can be seen from Table 2, when the 0.2% yield strength of Nagi according to the present invention is approximately 100 kgf/**2, the tensile strength, elongation, and area of area are all larger than the comparison #Oka, and the yield strength is It has the smallest ratio and is advantageous in use.

Next, two notch mechanical impact test pieces were prepared from each of the above samples and subjected to a low temperature impact test. 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 phase according to the present invention is significantly superior to that of the conventional products 17-4PH or FV520B, and the 0.2% proof stress is approximately 100 kgf/dragon2.
In this case, even at -120°C, it shows high toughness with a shock value of 2m force <5 kgf/2m force.

Figure 2 shows the microscopic structure of the N11l test piece that was heat treated as shown in Table 2 above and had a hardness of 1Iv385.
360 is shown in Figure 3. Contrast material size 2/1
The structure of 7-4PH is a lath-like martensite structure, whereas the orange according to the present invention has a structure in which a small amount of fine ferrite particles are mixed in a base of fine needle-like tempered martensite by quenching and tempering. This is thought to improve low-temperature toughness.

As explained above, the steel according to the present invention increases the Cr content of the conventional 13Cr-based high-strength stainless steel mesh, and also increases the N
Since the steel contains 4 to 6% i and has a small amount of fine ferrite dispersed in the acicular martensite structure by quenching and tempering, it exhibits an outstanding low-temperature 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, and has good weldability, so it does not require preheating or postheating during welding. It has better corrosion resistance than other stainless steels, so it does not require rust removal when used in actual equipment, and its simple chemical composition makes it easy to manage when scrapped. It is a high-strength stainless steel that is both economical and highly effective in practical use.

[Brief explanation of the drawing]

Figure 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. Figure 2 is a photograph (100x magnification) showing the microscopic structure of the stainless steel according to the present invention. Figure 3 shows the microscopic structure of the contrast material 17-4PH (10
0x). Applicant's agent Patent attorney Kamoshi 1) Second son Yakugen Onjuncho ('cn Figure 1 (xlooo) Figure 2 Figure 3 07)

Claims (1)

[Claims] C0.03-0.1%, St 1% or less, Mn 1% or less, Ni 4-6%, Cr 15-17.5%, v 0.1% or less, A1 0
．． 005 to 0.04% A martensitic stainless steel with a high low-temperature impact value and having a metal structure mainly consisting of fine acicular martensite, with the remainder essentially consisting of Fe and impurities.