JPH0453940B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is characterized by quenching, tempering or normalizing.
Regarding martensitic chromium stainless steel used after tempering heat treatment and its manufacturing method, more specifically, martensitic chromium stainless steel that exhibits excellent softening resistance and high strength even when tempered at high temperatures. The present invention relates to stainless steel and a method for producing the same at low cost. Martensitic chromium stainless steel is widely used in various applications requiring strength and corrosion resistance, and its proportion in the overall production of stainless steel is quite large. Further, this martensitic chromium stainless steel is often used as a so-called structural steel, but in that case, it is generally used after being quenched and tempered or normalized and tempered. However, although martensitic stainless steel has high strength and corrosion resistance in the as-hardened state or, depending on the size and shape of the material, in the normalized state, it has low notch toughness and is prone to problems such as stress corrosion cracking. Easy to occur. Therefore, in order to impart excellent performance in terms of notch toughness and other impact performance as a structural steel in addition to corrosion resistance, a tempering treatment is required. By the way, tempering treatment of martensitic stainless steel is not suitable for cutlery, tableware, etc.
So-called low-temperature tempering is often performed at 150 to 180℃, and 400 to 450℃ for springs, etc., but for structural members such as those mentioned above, emphasis is placed on toughness, so at least High temperature tempering at 600°C or higher, preferably 650°C or higher is required. However, when tempering is performed at such a high temperature, the strength of conventional martensitic stainless steel is significantly reduced, and this is called temper softening. Therefore, in order to provide the desired strength and toughness, it is necessary to impart temper softening resistance. Conventionally, in order to provide this resistance to temper softening, elements such as Mo and V were specifically added (e.g., Mo≧0.2%, V≧0.15%), and in order to utilize the solid solution strengthening effect of N. Consciously high N
It was considered essential to use steel. However, unnecessarily increasing the content of alloying elements is a problem from the viewpoint of resource saving and cost reduction, although it is effective for improving temper softening resistance. Further, since Mo and V are so-called ferrite-forming elements, adding a large amount of them may cause the formation of δ-ferrite, which may impair hot workability. On the other hand, intentional addition of a large amount of N may also cause cracks in the steel material during blooming, rolling, or heat treatment, resulting in a reduction in yield, and it is difficult to say that there are no problems. Thus, the object of the present invention is to create a martensitic material that has excellent strength, toughness, and corrosion resistance even when tempered at a high temperature of 650°C or higher, and has a good yield, with the addition of alloying elements being minimized. The purpose of the present invention is to provide chrome stainless steel and a method for manufacturing it at low cost. The present inventors have continued their intensive research to achieve this objective, and have found that the current state of the art can be improved by adjusting the chemical composition of the steel itself, and preferably by further restricting the quenching or normalizing conditions. They found that the problem was solved and completed the present invention. That is, in the present invention, C: 0.10 to 0.10 by weight%
0.40%, Si: 1.0% or less, Mn: 0.1% to 1.0%,
Ni: 1.0% or less, Cr: over 11.0% and 16.0% or less, V:
0.01-0.10%, Ti: 0.050% or less, Al: 0.10% or less, and if necessary, Mo: 0.02-0.02% or less in the above composition.
0.15%, Nb: 0.005 to 0.050%, and Cu: 0.5% or less, with the balance consisting of Fe and unavoidable impurities, and the following two formulas: Ti (%) - 3.5 × N (%) + 0. 0070, and 25×C (%) - Cr (%) - Ti (%) + 9.50. A martensitic chromium stainless steel with excellent resistance to temper softening at high temperatures. be. Furthermore, the present invention preferably quenches or normalizes the steel having the above composition at a heating temperature of 925°C or higher and 1100°C or lower, and then heats the steel at a heating temperature of 650°C or higher and Ac ₁
This is an inexpensive method for producing martensitic chromium stainless steel that has excellent resistance to temper softening at high temperatures, and is characterized by tempering at a temperature below 100%. In summary, the present invention can be summarized as follows:
Adding a small amount of V to high Cr steel with Ti added;
Furthermore, the main feature is that the heating temperature during quenching or normalizing is limited if necessary. That is, according to the findings of the present inventors, when martensitic chromium stainless steel contains a trace amount of V, strength is ensured and hot workability is improved by keeping the free N content low and suppressing ferrite formation. In order to improve, when the following two formulas are satisfied; Ti (%) - 3.5 x N (%) + 0.0070, and 25 x C (%) - Cr (%) - Ti (%) + 9.50. During tempering after normalization, depending on the size and shape, a large resistance to temper softening can be obtained, especially against high-temperature tempering at 650° C. or higher. Therefore, according to the present invention, a steel with particularly excellent strength can be obtained by utilizing this excellent temper softening resistance, and remarkable effects can be seen as shown in the Examples below. Next, the reason why the composition ratio of each additive element is limited in the present invention is as follows. C: C is an effective element for increasing the strength of steel and suppressing the formation of δ-ferrite, but if it exceeds 0.40%, carbon dioxide corrosion susceptibility increases significantly and it also causes deterioration of low temperature toughness. bring. Further, if the content exceeds 0.40%, cracks are likely to occur during the blooming and rolling heat treatment processes, resulting in a decrease in yield, which is not preferable. On the other hand, 0.10%
If it is less than this, the proportion of δ-ferrite that appears during quenching or normalizing increases, making it difficult to homogenize the material and impairing hot workability, so the lower limit of the C content is set to 0.10%. Si: In addition to being effective as a deoxidizer, Si is also effective in increasing the strength of steel. However, if a large amount of Si is contained, δ-ferrite is likely to occur.
Even if heated above ₃ points of Ac, it becomes difficult to homogenize the steel, and addition of a large amount of Si also deteriorates toughness.
It is necessary to keep it below 1.0%. Mn: Mn expands the austenite region and increases strength and
It is also effective in improving toughness, but if it is less than 0.1%, the effect will not be sufficiently increased.On the other hand, if it is added in an amount exceeding 1.0%, the effect will be saturated and there will be no particular effect, so 1.0% is the upper limit. did. Ni: Ni is an element that is effective in suppressing δ-ferrite and improving the structure, as well as improving strength and toughness, but it is expensive and does not necessarily need to be added in large amounts for economic reasons. The upper limit was set at 1.0%. Cr: It is necessary to contain more than 11.0% Cr in order to satisfy the inherent rust resistance of chromium stainless steel and to reduce the carbon dioxide corrosion rate. On the other hand, if it exceeds 16.0%, a large amount of δ-ferrite is generated when heated to the quenching or normalizing temperature, resulting in a non-uniform structure, which is not preferable because hot workability deteriorates, and the strength also decreases. Therefore, in the present invention, the Cr content is
More than 11.0% and less than 16.0%. V: V is particularly effective in improving the strength of martensitic chromium stainless steel with low free N content. However, if it is less than 0.01%, the effect is small, and if it exceeds 0.10%, the tendency to form δ-ferrite increases and the toughness deteriorates, so the upper limit is set to 0.10%. Ti: Ti is added to reduce free N, in other words to fix N in steel as TiN, but if it exceeds 0.050%, toughness deteriorates and there is a tendency to form δ-ferrite. Therefore, the upper limit of Ti addition is limited to 0.050%. Al: Al is effective as a deoxidizing agent, but if it exceeds 0.10%, its effect will be saturated, and instead it will cause defects due to increased inclusions and deteriorate toughness.
The content should be 0.10% or less, but it is desirable to keep the content to a level necessary and sufficient for deoxidation to suppress δ-ferrite during quenching or normalizing. Mo, Nb, Cu: Mo, Nb, and Cu are all effective in improving strength, and solid solution strengthening is remarkable. Mo
Since it is expensive, it is not necessarily necessary to add it in a large amount, and addition of a large amount promotes the formation of δ-ferrite and saturates or reduces the effect on strength. Therefore, the upper limit when adding Mo was set at 0.15%. On the other hand, if it is less than 0.02%, the effect on improving strength is small. The upper limit of Nb is set at 0.05% since addition of a large amount of Nb promotes the formation of δ-ferrite and saturates or reduces the effect on strength. on the other hand,
If it is less than 0.005%, the effect on improving strength is small. Furthermore, Mo and Nb also have the effect of finely precipitating carbides during tempering. In addition to improving strength, Cu is also effective in suppressing ferrite, but if it exceeds 0.5%, toughness deteriorates, so the upper limit is set at 0.5%. Note that Cu itself precipitates during tempering and strengthens by precipitation. Regarding these additive elements Mo, Nb, and Cu, Mo
and Nb or Cu.
Add. Furthermore, regarding Ti and N, the formula: Ti
(%) - 3.5 × N (%) + 0.007 becomes a negative value, the effect of a small amount of V on tempering softening resistance disappears,
Also, for C, Cr, and Ti, the formula: 25 x C (%) - Cr
When (%) - Ti (%) + 9.5 becomes a negative value, a large amount of δ-ferrite is generated, resulting in a non-uniform structure, and hot workability is impaired, so Ti (%) - 3.5 × N ( %) +0.0070 and 25 x C (%) - Cr (%) - Ti (%) + 9.50 After melting the steel composed of the above ingredients, quenching after processing into steel plates, steel pipes, etc. Tempering or normalizing - subject to tempering treatment. The heating temperature for quenching or normalizing at this time is preferably 925°C or higher and 1100°C or lower, more preferably 950 to 1050°C. This is based on the fact that the tendency to form δ-ferrite decreases in this temperature range; on the other hand, heating above 1100°C also causes coarsening of crystal grains, leading to a decrease in strength and toughness. Furthermore, after heating and maintaining in the above temperature range, quenching in an appropriate cooling medium or normalizing will result in δ
- Since the formation of ferrite is suppressed and the composition has been adjusted to have excellent tempering softening resistance particularly in high temperature ranges, high strength can be obtained even during subsequent tempering at high temperatures of 650°C or higher and Ac ₁ point or lower. In addition, the structure mainly composed of homogeneous fine tempered martensite obtained in this way has excellent low-temperature toughness, and therefore has high strength, high toughness, and excellent carbon dioxide corrosion resistance in addition to rust resistance. It also has Thus, according to the present invention, martensitic chromium stainless steel that exhibits excellent softening resistance and high strength even when tempered at a high temperature of 650° C. or higher can be produced at low cost and with a high yield. Next, the effects of the present invention will be explained in more detail with reference to Examples. Example 1 Each steel type having the chemical composition shown in Table 1 below was oil quenched at 975°C after hot rolling, and then heated at 600°C to
It was tempered at 760°C and its tensile strength was examined. The test results are summarized in Table 2. According to the results shown in Table 2, it is clear that the steel according to the present invention exhibits excellent softening resistance during high temperature tempering, and therefore high strength can be obtained. These results show that even with an inexpensive component system with reduced alloying elements, it is possible to produce martensitic chromium stainless steel with high toughness, high strength, and high corrosion resistance, especially through high-temperature tempering at 650°C or higher. This shows that it is possible, and therefore the present invention has very high industrial value.

【table】

【table】

【table】

[Table] Example 2 Each steel type having the chemical composition shown in Table 3 below was heated and rolled, then quenched in water or oil depending on the steel composition from 1000°C, and then tempered in a temperature range of 710 to 740°C. A tensile test and an impact test were conducted on each material. The test results are summarized in Table 4. According to the results shown in Table 4, the steel according to the present invention can obtain high strength without adding particularly large amounts of elements for imparting resistance to temper softening, and
It is clear that the material has an extremely good balance of toughness.

【table】

【table】

[Table] Example 3 of Zshyarupy test piece For each steel type having the chemical composition shown in Table 5, after hot rolling, water or oil quenching was performed at a temperature of 900 to 1000°C depending on the steel composition, and then tempering treatment was performed at 700°C. A sample material was obtained, with a thickness of 3 mm and a width of 40 mm.
A test piece with a length of 50 mm was cut out, polished with No. 320 emery, and subjected to a corrosion test under the following conditions. That is, the polished test piece was degreased and dried, then placed in an autoclave and degassed under vacuum.
Degassed artificial seawater was added to the autoclave. After that, pressurize to 30 atmospheres with carbon dioxide gas and lower the liquid temperature.
The temperature was maintained at 120°C for 500 hours. During this time, approximately
Stirring was carried out to give a flow rate of 1.5 m/sec. After the test, the weight difference before and after the test was determined after removing the deposits. Furthermore, as for the corrosion characteristic values, steel number 22, which is a comparative steel,
It is expressed as the corrosion rate when the amount of corrosion of the test piece is set as 100. The test results are summarized in Table 6. According to this, it is clear that the steel type according to the present invention also has excellent carbon dioxide corrosion resistance.

【table】

【table】

[Table] Example 4 Steels according to the present invention shown in Table 7 below were heated and rolled, then oil quenched at various temperatures, and the amount of δ-ferrite was measured. The results are shown in Table 8. As a result, in order to suppress the formation of δ-ferrite and homogenize the structure, the temperature must be 925-1100℃, preferably 950-1050
It is clear that quenching or normalizing from a temperature range of °C is effective.

【table】

【table】

Claims (1)

[Claims] 1% by weight: C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.050%
The following contains Al: 0.10% or less, the balance consists of Fe and unavoidable impurities, and the following two formulas: Ti (%) - 3.5 × N (%) + 0.007 ≧ 0, and 25 × C (%) - Cr (%) - Ti (%) + 9.5≧0 A martensitic chromium stainless steel with excellent resistance to temper softening at high temperatures, characterized by having a composition that satisfies the following. 2 In weight%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.050%
Below, Al: 0.10% or less, Cu: 0.5% or less,
The remainder consists of Fe and unavoidable impurities, and the following two
It is characterized by having a composition that satisfies the formula: Ti (%) - 3.5 × N (%) + 0.007≧0, and 25 × C (%) - Cr (%) - Ti (%) + 9.5≧0. A martensitic chromium stainless steel with excellent resistance to tempering and softening at high temperatures. 3 In weight%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.05%
Below, in addition to Al: 0.10% or less, Mo: 0.02 to 0.15
% and Nb: 0.005 to 0.050%, the balance consists of Fe and unavoidable impurities, and the following two formulas: Ti (%) - 3.5 × N (%) + 0.007 ≧ 0, and 25 × A martensitic chromium stainless steel with excellent resistance to temper softening at high temperatures, characterized by having a composition satisfying C (%) - Cr (%) - Ti (%) + 9.5≧0. 4 In weight%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.050%
The following contains Al: 0.10% or less, the balance consists of Fe and unavoidable impurities, and the following two formulas: Ti (%) - 3.5 × N (%) + 0.007 ≧ 0, and 25 × C (%) - Cr (%) - Ti (%) + 9.5≧0 Steel with a composition satisfying the following is quenched or normalized at a heating temperature of 925°C or higher and 1100°C or lower,
A method for producing martensitic chromium stainless steel having excellent resistance to temper softening at high temperatures, which is then tempered at a temperature of 650°C or higher and Ac ₁ point or lower. 5 In weight%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.050%
Below, Al: 0.10% or less, Cu: 0.5% or less,
The remainder consists of Fe and unavoidable impurities, and the following two
Formula: Steel with a composition satisfying the formula: Ti (%) - 3.5 x N (%) + 0.007≧0 and 25 x C (%) - Cr (%) - Ti (%) + 9.5≧0 at 925℃ Above, quenching or normalizing is performed from a heating temperature of 1100℃ or less,
A method for producing martensitic chromium stainless steel having excellent resistance to temper softening at high temperatures, which is then tempered at a temperature of 650°C or higher and Ac ₁ point or lower. 6 In weight%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 0.1 to 1.0%, Ni: 1.0% or less, Cr: 11.0
% over 16.0%, V: 0.01-0.10%, Ti: 0.050%
Below, in addition to Al: 0.10% or less, Mo: 0.02 to 0.15
% and Nb: 0.005 to 0.050%, the balance consists of Fe and unavoidable impurities, and the following two formulas: Ti (%) - 3.5 × N (%) + 0.007 ≧ 0, and 25 × Harden or normalize steel with a composition satisfying C (%) - Cr (%) - Ti (%) + 9.5 ≧ 0 at a heating temperature of 925 ° C or higher and 1100 ° C or lower,
A method for producing martensitic chromium stainless steel having excellent resistance to temper softening at high temperatures, which is then tempered at a temperature of 650°C or higher and Ac ₁ point or lower.