JPS62238351A - Martensitic stainless steel for cold forging - Google Patents

Abstract

PURPOSE:To improve cold forgeability, corrosion resistance, and hardenability, by limiting Cr equivalent in the composition of a martensitic stainless steel for cold forging to a specific range. CONSTITUTION:The martensitic stainless steel as steam-turbine and blade materials and as material for screws, bolts, etc., has a composition in which <0.07%, by weight, C, <0.40% Si, 0.10-1.0% Mn, 10.0-13.0% Cr, and <0.02% N are contained, or further, in order to prevent the formation of impurities deteriorating cold forgeability, such as MnS, Al2O3, etc., S content and O content are reduced, respectively, to <=0.003% and <=0.0040%, or further, in order to improve corrosion resistance, 1 or >=2 kinds among <0.50% Ni, <0.50% Cu, <1.5% Mo, <0.2% Ti, <0.15% V, and <0.10% Nb are incorporated. Moreover, the Cr equivalent of this stainless steel represented by Cr+2Si-30C-25 N-2Ni-0.3Cu+1.5(Mo+Ti)+1.75Nb+5V is limited to 9.0-10.5.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention has excellent cold forgeability in an annealed state and is used for steam turbines for ships, blade materials, screws, bolts, etc. This invention relates to high-strength martensitic stainless steel with outstanding corrosion resistance and quench hardenability.

(Prior art) SOS is generally used as martensitic stainless steel.
403 (0, I C-12Cr) Strength (It is widely used in steam turbines for ships, blade materials, screws, bolts, etc. due to its edibility and strength.

(Problems to be Solved) However, since the SUS 403 has a high tensile strength of about 55 kg/mm in an annealed state, its cold forgeability is not necessarily sufficient.

There was also a need for improvement in corrosion resistance, as rust occurs under severe corrosive conditions.

(Means for Solving the Problems) In view of the above-mentioned drawbacks of SO3403, the present invention provides:
As a result of intensive research on the influence of alloying elements such as CX5l-, Mn, and CrsN on the tensile strength in the annealed state of 2%Cr stainless steel, we found that Cl was 01.
By reducing the tensile strength to 0.7% or less, the tensile strength in the annealed state is 47 kg/mrJ or less, improving cold forgeability, and the tensile strength in the quenched and tempered state is 60%.
kg/mm or more, and has a low C content, but the current SU
It was possible to obtain strength equivalent to that of S403, and also succeeded in significantly improving corrosion resistance.

That is, as shown in Figure 1, the relationship between c4 and tensile strength in an annealed state was investigated for 12% Cr steel, and by suppressing the C content to 0.07% or less, the tensile strength 47kg/mtr? Excellent cold S without the following
(Figure 2 shows the relationship between the amount of C and the corrosion rate in seawater at 50℃, and the amount of C is 0.07.
% or less, the corrosion rate can be reduced to 25g/rr?・1
) "By setting the Cr equivalent to 10.5 or less and improving the corrosion resistance, and furthermore, Fig. 3 investigates the relationship between the Cr equivalent and the amount of γ-M phase transformation, and by setting the Cr equivalent to 10.5 or less, the γ-M phase transformation is Fig. 4 shows that the amount of γ-M phase transformation can be 100%.
The relationship between the amount of C and the tensile strength in the quenched and tempered state was investigated for 0.00% steel, and the tensile strength was 6 when ci was 0.01%.
0kg/mm or more to achieve high strength.Furthermore, Figure 5 investigates the relationship between Cr equivalent and 1) aperture value at 00°C. It has been found that the value is 85% or more and hot workability is improved.

Based on these findings, the present invention aims to determine the optimum content of C, Sis Crs N, in order to obtain a steel that has excellent cold forgeability in an annealed state and has significantly excellent corrosion resistance and quench hardenability. This is what we discovered.

The present invention has C0.07% or less and Si0.4% by weight.
0% or less, Mn 0.10-1.0%, Cr 10.0
~13.0%, N 0.02% or less, balance Fe
and impurity elements, and has a Cr equivalent of 9.0 to
10.5, and the second invention steel has S of the first invention steel of 0.003% or less and O of 0.0040% or less, and
This is a product that further improves the cold forgeability of the invented steel, and is the third
The invention steel is the first invention steel plus Ni 0.50% or less,
CuO 150% or less, Mo 1.5% or less, Tj 0.
The corrosion resistance of the first invention steel is further improved by containing one or more of Nb 20% or less, Nb 0.15% or less, and Nb 0.10% or less.

Note that the Cr equivalent was calculated using the following formula. C
r hitfi=cr+2 Si-Mn-30C-25N
-2Ni-0,3Cu+ 1.5 (Mo+Tt)
÷1.75Nb+5V The reason for limiting the composition of the steel of the present invention will be explained below.

C is an element that increases tensile strength in an annealed state and reduces cold forgeability, and its content must be reduced as much as possible, and the upper limit was set at 0.07%.

Although Si is an effective deoxidizing element, it is also a strong ferrite-forming element and also an element that reduces cold forgeability, so its upper limit was set at 0°40%.

Mn is an element effective in deoxidizing and is also a strong austenite-forming element, and the lower limit was set at 0.10%. However, if it is contained in a large amount, the tensile strength in an annealed state increases, so the upper limit is set as i,oo%.

Cr is a basic element that imparts corrosion resistance to stainless steel, and to obtain this effect, it must be contained in an amount of 10.0% or more, and the lower limit was set at 10.0%. However, Cr is also a strong ferrite-forming element and impairs hardenability, so the upper limit was set at 13.0%.

N is an element that increases the tensile strength in an annealed state, and it is necessary to regulate its content, so the upper limit was set at 0.020%.

S is an element that generates Mr+S and significantly reduces cold forgeability, and the upper limit of its content is set to 0.003%.

0 is an element that produces oxides of n l )9 and significantly reduces cold forgeability, and the upper limit of its content is set to 0.00.
It was set at 40%.

Ni and Cu are elements that improve corrosion resistance, but on the other hand,
It is also an element that increases the tensile strength in an annealed state, and the upper limit was set to 0.50% for Ni and 0.50% for Cu.

MO% i'i, v, Nb contributes to improving the tensile strength in the tempered state! Although it is a j,z element, on the other hand, it is a strong ferrite-forming element and impairs hardenability, so the upper limit is set to N.

is 1.5%, Ti is 0.20%, V4. t0.15%,
Nb&;to, 10%.

C “The reason why the 5 position is less than 10.5 is r-M phase transformation!10
This is because if it is less than 9.0, hot workability deteriorates, and if it exceeds 1005, the desired tempering hardness cannot be obtained.

(Example) Next, the characteristics of the steel of the present invention will be clarified by comparing it with conventional steel and comparative steel through examples.

Table 1 shows the chemical composition of these test steels.

In Table 1, A-S steel is the invention steel, A-F steel is the first invention steel, G-i steel is the second invention steel, J-S steel is the third invention steel, and T-W steel is the third invention steel. In the comparison steel, X is the conventional steel 5l15403
It is.

Table 2 shows the test steel in Table 1 at 900°C x 1) 1"
and furnace cooling to 600°C at a cooling rate of 25°C/Hr.
Those that were annealed under the condition of air cooling, Table 3, were held at 970°C for 30 minutes, soaked in oil, and then heated to 600°C.
〜750℃×2■ This shows the hardness and mechanical properties of those that have been tempered.

0.2%, J for yield strength, tensile strength, elongation, and reduction of area.
An IS4 test piece was prepared and measured.

As is clear from Table 2, Table 3, and Table 2, the conventional steel X steel is
Hardness in a better state is Ilv 1 (iQ, 0□2% proof stress is 32.5 kg/w, tensile strength is 55.6 kg/mm
Furthermore, the cold forgeability is poor at 73.1%, and the comparative steel X steel has a reduction of area of 78.1%.
% and poor cold forgeability, and furthermore, tJ, V
, W steel has a hardness of Hv145-156 and a tensile strength of 4
The weight is as high as 5.6 to 51.9 kg/dragon, and the cold forgeability is poor like the conventional steel I steel.

In contrast, A-5 steel, which is the steel of the present invention, has a hardness of 1) ■ 1) 1 to 139.0.2% proof stress in an annealed state of 2
1.5-25.7 b/n+s tensile strength 39.3-46
．． It's as low as 5kg/Yoiso, and the aperture value is 79.8-85.
It has a high cold forgeability of 1%.

Furthermore, as is clear from Table 3, the corrosion rate of conventional steel X steel in the quenched and tempered state is 65.3 g/m.
h and r, and the corrosion resistance was not satisfactory. In addition, the comparison steels T, U, and ■ steels also have a fast corrosion rate of 29.5 to 42.0 g/mhr, and their corrosion resistance is low, and the corrosion rate of W steel is 6
It is as fast as conventional steel at 0.5 g/m hr, but its corrosion resistance is inferior.

In contrast, the A''-3 steel, which is the steel of the present invention, has a high hardness of LLV 205 to 232, and a 0 and 2% yield strength of 4.
5.1~56.1kg/mm, tensile strength 66
, 1” 74.9 kg lrn m, excellent strength, and corrosion rate of 12
．． It has a low corrosion resistance of 0 to 22.0 g/mhr and has excellent corrosion resistance.

(Effects of the present invention) As mentioned above, the present invention has a tensile strength of 4 in the annealed state.
It has a low strength of less than 7 kg/mm, and has excellent cold forgeability. It also has a high tensile strength (iQ kg/mm or more) in the quenched and tempered state, and has excellent strength. Furthermore, it has excellent corrosion resistance, and the steel of the present invention is a martensitic stainless steel suitable for use in steam turbines for ships, blade materials, screws, bolts, etc.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between Cl and tensile strength in the annealed state, and Figure 2 is a diagram showing the relationship between cH and corrosion rate in the quenched and tempered states. Figure 3 is a diagram showing the relationship between the Cr equivalent and the amount of phase transformation.
This is a diagram showing the relationship between tensile strength in a tempered state, and FIG. 5 is a diagram showing the relationship between Cr equivalent and 1) reduction of area value at 00°C.

Claims (3)

[Claims] (1) C 0.07% or less, Si 0.40% by weight
Below, Mn0.10-1.0%, Cr10.0-13.
A martensitic stainless steel for cold forging, characterized in that it contains 0.0% and 0.02% or less of N, the balance is Fe and impurity elements, and has a Cr equivalent of 9.0 to 10.5. (2) C0.07% or less, Si 0.40% by weight
Below, Mn0.10-1.0%, Cr10.0-13.
0%, N0.02% or less, and S0.003
% or less, O0.0040% or less, the balance consists of Fe and impurity elements, and the Cr equivalent is 9.0 to 10.
A martensitic stainless steel for cold forging characterized by a temperature of 5. (3) C 0.07% or less, Si 0.40% by weight
Below, Mn0.10-1.0%, Cr10.0-13.
0%, N0.02% or less, and further Ni0.50
% or less, Cu 0.50% or less, Mo 1.5% or less, Ti
Contains one or more of 0.2% or less, V 0.15% or less, and Nb 0.10% or less, with the balance consisting of Fe and impurity elements, and has a Cr equivalent of 9.0 to 10.5.
Martensitic stainless steel for cold forging, characterized by: