JP3426036B2 - Martensitic stainless steel excellent in strength and toughness and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention has excellent strength and toughness,
The present invention relates to a martensitic stainless steel used as a structural material or a machine part for building materials, vehicles, ships, power generation facilities, etc., and a manufacturing method thereof.

[0002]

2. Description of the Related Art Martensitic stainless steel typified by SUS420 has a large C content as compared with ferritic and austenitic stainless steels and is inferior in manufacturability and workability. Therefore, it is generally manufactured by the following process. The steel is heated to the austenite single phase region at the highest temperature possible and hot rolled. High-temperature heating is effective in preventing the decrease in hot ductility due to the presence of C in the steel as coarse carbide. Further, when austenite containing a large amount of solute C undergoes martensite transformation by rapid cooling, quench cracking easily occurs. Therefore, quenching cracks are prevented by gradually cooling the steel material after hot rolling. On the other hand, in the cold rolling or the working process as a molded product, the steel material is softened by annealing prior to rolling or working to secure the rollability or workability. The strength required for the final product is ensured by quenching and tempering after rolling or processing. This type of heat treatment is performed according to JIS G4303, JIS G430.
4, JIS G4305 and the like are specified to achieve both manufacturability and product characteristics.

[0003]

In the martensitic stainless steel, δ ferrite is formed depending on the balance between the ferrite forming element and the austenite forming element, and the toughness of the steel material is lowered. Also, the Ms point drops,
Austenite, which is a cause of strength reduction, may remain after cooling to room temperature. In particular, in a system containing a large amount of C for the purpose of improving strength, the austenite phase tends to remain during quenching as the Ms point decreases, and the amount of solid solution of C is large even when transformed to martensite. Due to this, quench cracking easily occurs during quenching. Quench cracks are C in steel
Can be prevented by slow cooling so as to precipitate as chromium carbide. Further, an annealing treatment or the like for increasing the Ms point and ensuring manufacturability is adopted.

The carbides precipitated by heat treatment such as annealing are coarse and non-uniformly dispersed, and make the properties of the final product unstable. When the precipitated carbide is re-solidified by solutionizing / quenching after rolling / working, the characteristics of the steel material are stabilized. Moreover, the material characteristics may be adjusted by tempering. Although hardness and material strength are improved by quenching and tempering, austenite grains grow violently during high temperature solution treatment, and the toughness of the final product deteriorates. As a result, satisfactory material properties have not been obtained as a structure or the like. Since the annealed martensitic stainless steel is softened, its rolling property and workability are secured. However, as it is, the hardness and strength are insufficient, and the material properties suitable for the application cannot be obtained. Therefore, various heat treatments have been conventionally proposed in order to improve the strength, manufacturability, toughness, etc., but it is difficult to satisfy the contradictory characteristics.

For example, as methods for strengthening steel and improving toughness, grain refinement, work hardening by hot rolling, solid solution strengthening by precipitation of subelements, precipitation strengthening, etc. are known. For steel grades such as those that undergo a structural change with temperature and do not contain elements that promote age hardening, these means are not adopted, and only C
Only solid solution strengthening by is used. As a result, the above-mentioned harmful effects caused by C cannot be avoided. The present invention was devised to solve such a problem, by uniformly dispersing fine carbides prior to solution treatment, by quenching at a temperature at which precipitated carbides are not completely solid solution,
It is intended to suppress the growth of austenite grains and improve the strength and toughness of the martensite phase generated after quenching.

[0006]

In order to achieve the object, the martensitic stainless steel of the present invention has a C content of 0.05 to 1.5% by weight, a Si content of 2% by weight or less,
Mn content is 2% by weight or less and Cr content is 10 to 20
It has a composition of wt% and has a grain size of 2 due to the carbide precipitation treatment.
It is characterized by a fine martensite structure having a prior austenite grain size of 30 μm or less, which is obtained from a structure before quenching in which fine carbides of μm or less are uniformly dispersed at a ratio of 1 to 30% by volume. The steel material targeted by the present invention may contain 7% by weight of Ni in order to improve the toughness. This steel material is heated to a temperature of Ac ₁ point or less to uniformly precipitate fine carbides, and then heated to a temperature of Ac ₃ points or more and below the temperature at which the carbides are completely dissolved, and then to a temperature of Ac ₁ point or less. It is manufactured by quenching. As the carbide precipitation treatment, after hot rolling at a finishing temperature of 500 to 900 ° C., the temperature may be maintained for 250 seconds or more. Quenching is A
_{c 3 ≦ [T] ≦ 6100} / [4.32-0.0186 × (Cr%
+ Ni%)-log C%]-273 is preferably performed at a temperature T.

[0007]

[Function] As a result of various investigations and studies focusing on the refinement of crystal grains among the various strengthening methods, the present inventors have found that the dispersed state of precipitated carbide before quenching greatly affects the material properties after quenching / tempering. I found out what I was doing. The dispersed state of the precipitated carbide is adjusted by a combination of an appropriate component range and treatment conditions, and excellent material properties not found in conventional steel materials can be obtained. In a general steel material, the crystal grain size increases as the annealing temperature increases. However, in the martensitic stainless steel targeted by the present invention, by controlling the solid solution / precipitation state of the carbide prior to the quenching treatment, the metal structure at the time of heating for quenching (austenite grain size),
As a result, the metal structure after quenching (former austenite grain size)
The inventors have found that is controllable. C in steel is precipitated as chromium carbide after heat treatment such as annealing. When this carbide is finely and uniformly distributed, coarsening of crystal grains is suppressed during quenching. On the other hand, carbides that are distributed nonuniformly or coarsely do not exhibit the effect of suppressing coarsening of crystal grains. In the present invention, the grain size of the former austenite is reduced by controlling the precipitation morphology of the carbide, and the toughness is improved.

The form of carbide precipitation can be adjusted by the temperature of the carbide precipitation treatment prior to quenching. The C solid solution limit is very small in the ferrite phase and large in the austenite phase. Therefore, when the steel material is heated to a temperature of 500 ° C. to Ac ₁ point, C in the steel becomes fine carbide and is uniformly precipitated. If the heating temperature at this time does not reach 500 ° C., the precipitation rate of the carbide is slow, and it takes a long time to obtain the required precipitation state, which is not practical. On the contrary, at a heating temperature exceeding the Ac ₁ point, C forms a solid solution in the austenite phase and the matrix becomes two phases of ferrite and austenite. The steel material in which the carbide is precipitated exhibits substantially the same mechanical properties as those after the annealing treatment, does not generate martensite even by rolling or working, and is soft, and thus is excellent in rollability and workability. By utilizing this characteristic, the steel material after the carbide precipitation treatment can be subjected to appropriate cold rolling and forming as necessary.

The steel material in which the carbide is precipitated is heated to a temperature range above the Ac ₃ point and where the carbide is not completely dissolved. This temperature range is an austenite single-phase range, and the carbide is slightly dissolved in the austenite phase. However, the carbide remains as a fine precipitate without completely forming a solid solution, and suppresses the growth of austenite grains. By controlling conditions such as the temperature rising rate, the holding time, and the temperature lowering rate in the above-mentioned temperature range, the required fine carbide can be uniformly dispersed. In order to make the finally obtained martensite into fine crystal grains effective for improving strength and toughness, it is necessary to adjust the precipitated carbide to have a grain size of 2 μm or less and a precipitation amount of 1 to 30% by volume. The grain size and the amount of precipitation of the carbide are controlled by the conditions such as the temperature rising rate, the holding time, and the temperature lowering rate in the above temperature range. A carbide having a grain size of more than 2 μm or a precipitation amount not reaching 1% by volume is not effective for refining the martensite structure. On the contrary, a precipitation amount exceeding 30% by volume is effective for refining crystal grains, but adversely affects the toughness of steel. When the precipitation treatment according to the present invention is applied, for example, 6% by volume of M ₂₃ C in 0.3% C steel is used.
_It has been confirmed that 20% by volume of M ₂₃ C ₆ is precipitated in ₆ , 1.0% C steel.

If the heating temperature during the quenching treatment does not reach the Ac ₃ point, the steel material is in the two-phase region of ferrite + austenite, and since the volume ratio of austenite is small, the amount of C in the austenite phase is rather increased. . Therefore, the austenite phase is stabilized even at low temperatures, and the amount of retained austenite after quenching increases. On the contrary, in high-temperature heating where the carbide is completely dissolved, the austenite grains become coarse with the disappearance of the carbide. The temperature at which the carbide completely dissolves is 6
100 / [4.32-0.0186 × (Cr% + Ni
%)-Log C%]-273. This formula is
As described in the examples described later, it is obtained as a result of investigations and studies by the present inventors. Ac
By heating in a temperature range of ₃ points or more and in which carbides are not completely dissolved, a structure in which fine carbides that are effective as austenite grain precipitation sites are uniformly dispersed can be obtained. Since the austenite phase precipitates from the interface of the finely divided fine carbide, a large number of austenite grains are generated. Also,
Since the carbide exhibits a pinning effect of suppressing grain growth, the generated austenite grains maintain a fine grain size of 30 μm or less, preferably 15 to 25 μm, and do not grow into coarse crystal grains. In this respect, the conventional steel material has a large crystal grain size of 45 μm or more.

When a steel material in which a large number of fine austenite grains are formed in this way is quenched, martensitic transformation occurs. At this time, the generated martensite phase is derived from fine prior austenite grains having a grain size of 30 μm or less, so that the steel material has a metal structure in which crystal grains are refined. The properties of the obtained steel material such as impact toughness and low temperature toughness are remarkably improved. Although the reason why the toughness and the like are improved by refining the crystal grains is not clear, it is presumed that the grain boundary fracture is suppressed from the result of observing the fracture surface after the impact test. Moreover, since the precipitated carbides are quenched from the dispersed state, the amount of solid solution C of the former austenite grains is low, and retained austenite and quench cracks that cause strength reduction are suppressed.

Fine chromium carbide can also be uniformly deposited by controlling the conditions of hot rolling prior to the quenching treatment. That is, the hot rolling finishing temperature is 500 to
When the temperature is set to 900 ° C. and is held in this temperature range for 250 seconds or more, the chromium carbide is finely and uniformly precipitated due to the strain introduced into the austenite. This hot rolled material is Ac
_{If the material} is heated to ₃ points or more and below the temperature at which the carbide is completely melted and quenched, the former austenite grain size is similarly 30μ.
m or less, good properties are obtained, and the amount of solid solution C at the time of martensitic transformation is reduced, so that quench cracking is prevented. Even when the above carbide precipitation treatment and quenching treatment are performed, it is expected that the toughness will decrease as the C content increases. Therefore, as a result of various studies from the aspect of the composition in order to further improve the toughness, it was found that the addition of Ni is very effective in a system having a high C content. That is, when the addition of Ni is combined with the carbide precipitation treatment and the quenching treatment, a steel material having significantly excellent strength and low temperature toughness as compared with the conventional case can be obtained.

The components of the steel material used in the present invention, heat treatment conditions, etc. will be described below. C: 0.05 to 1.5 wt% The crystal grains are refined by the formation of chromium carbide, and the tensile strength of martensite obtained at room temperature is increased by forming a solid solution in austenite. Such an effect becomes remarkable when the C content is 0.05% by weight or more. However, a large amount of C content exceeding 1.5% by weight is
Not only does this deteriorate the manufacturability, but it also lowers the Ms point and increases the amount of retained austenite. Si: 2% by weight or less It is an alloying element effective in improving corrosion resistance and oxidation resistance.
Excessive Si content in excess of wt% reduces manufacturability. Mn: 2 wt% or less Effective for improving manufacturability and weldability. However, 2% by weight
A large amount of Mn content exceeding 0.1 deteriorates corrosion resistance and oxidation resistance.

Cr: 10 to 20% by weight It is an alloying element that produces chromium carbide and refines the crystal grains, and 10% by weight or more of Cr is necessary to maintain corrosion resistance. However, if a large amount of Cr exceeding 20% by weight is contained, manufacturability deteriorates. Also,
A large amount of Cr lowers the Ms point and increases the amount of retained austenite. Ni: 0 to 7% by weight It is an alloying element added as necessary to improve the toughness. It also has the effect of increasing the amount of solid solution C in the austenite phase. However, a large amount of N exceeding 7% by weight
When i is included, the Ms point is lowered and the amount of retained austenite is increased, which is a defect. The steel of the present invention further contains 0.5% by weight or less of Ti, Nb, V for grain refining, and 2.0% by weight for improving corrosion resistance.
Mo below, Cu below 3.0 wt%, Al below 0.5 wt% to improve oxidation resistance, V below 0.1 wt% to improve hot workability, 0.1 wt % Or less of a rare earth element (REM) may be included.

[0015]

【Example】

Example 1: The stainless steel whose components are shown in Table 1 was melted,
Under the conditions shown in Table 2, hot rolling, carbide precipitation treatment, and quenching and tempering were performed. For each stainless steel, old austenite grains, hardness after tempering, precipitation amount of carbides, etc. were investigated. As can be seen from Table 2 showing the investigation results, the steel materials according to the present invention all had the former austenite grains of 30 μm or less. It was confirmed that the crystal grain size was smaller than that of the conventional steel and the low temperature toughness was excellent even with the same hardness.

[0016]

[Table 1]

[0017]

[Table 2]

For the steel types A1 to A3 and A5, the temperature at which the precipitated carbides were completely dissolved and the austenite grain size at that time were investigated. As shown in FIG. 1, the austenite grains grew larger as the heating temperature increased, but the growth start temperature increased as the C content increased.
At any C content, a rapid growth of austenite grains was observed after C was completely dissolved. Therefore, it was confirmed that when heating in the temperature range of the complete melting temperature of Ac ₃ point to C, the austenite grains can be solutionized without growing significantly beyond 30 μm. When the temperature T _{c at} which C was completely melted was investigated for various steel types having different C contents, the relationship shown in FIG. 2 was established between the temperature T _c and the C content. It was found that the temperature T _c increased as the C content increased and the rate of increase decreased as the C content increased. It was also found that the temperature T _c increases as the Ni content increases, but the rate of increase is almost constant regardless of the Ni content, unlike C.

The temperature T _{c at which} C is completely dissolved is T _c = 61
00 / [4.32-0.0186 × (Cr% + Ni%)
-Log C%]-273. Therefore, when the quenching treatment at a temperature of between Ac ₃ through T _c the carbide precipitation treated steels, it is found that can suppress the growth of austenite grains in 30μm or less. Further, although the carbides precipitated in the steel are slightly dissolved in the austenite phase, the grain size is 2
It was uniformly dispersed as fine precipitates having a size of μm or less. A
After the steel 3 was subjected to a carbide precipitation treatment, it was quenched from various temperatures and the former austenite grains were measured. The measurement results are shown in FIG. 3 in comparison with the case where the carbide precipitation treatment is not performed. Figure 3
As is clear from the above, the former austenite grains are remarkably reduced by the carbide precipitation treatment.

The A3 steel was kept at 800 ° C. for 30 minutes to precipitate carbides and quenched from 900 ° C. And it tempered so that hardness might be set to HV235. For comparison, the same A3 steel not subjected to the carbide precipitation treatment was quenched from 1150 ° C. and then tempered so that the same hardness was obtained. Then, the influence of the presence or absence of the carbide precipitation treatment on the Charpy impact toughness was investigated. As can be seen in FIG. 4 showing the investigation result, the one subjected to the carbide precipitation treatment had a lower impact transition temperature than the one not subjected to the precipitation treatment, and the impact toughness was remarkably improved.

Example 2 A1, A6, A8, A9 in Table 1 in which Ni was added to steel corresponding to SUS410 in various contents.
After the carbide precipitation treatment of maintaining the steel type at 600 to 800 ° C. for 60 minutes, the steel was hardened at 900 to 950 ° C. and tempered at 600 to 700 ° C. so that the hardness became HV235. For comparison, the same steel material was quenched and tempered without a carbide precipitation treatment. The Charpy impact toughness of the steel material after tempering was investigated. As shown in FIG. 5 by comparing the investigation results, the impact transition temperature becomes lower as the Ni content increases in any case. In particular, the carbide to which the carbide precipitation treatment was applied showed a marked improvement in impact toughness even with a small Ni content. From this, it is understood that the low temperature toughness is significantly improved when the carbide precipitation treatment and Ni addition are combined.

[0022]

As described above, the martensitic stainless steel of the present invention has a prior austenite grain size of 3 or less.
Since it has a fine structure of 0 μm or less, it has excellent impact toughness and sufficient strength. The refined crystal structure is obtained by performing a carbide precipitation treatment and then quenching from a temperature at which the precipitated carbide is not completely dissolved. At this time, since the amount of C contained in the old austenite is small, the Ms point is not lowered, and retained austenite that causes quench cracking and insufficient strength is suppressed. The carbide precipitation treatment exhibits a more remarkable effect in improving the toughness when combined with the addition of Ni. The steel material thus obtained is used as a mechanical part, a structural material or the like in a wide range of fields such as construction, vehicles, ships, and power generators by utilizing its excellent material.

[Brief description of drawings]

Fig. 1 Effect of heating temperature on austenite grain size and C dissolution

FIG. 2 Relationship between C content and complete melting temperature of C

[Fig. 3] Relationship between quenching temperature and prior austenite grain size

[Fig. 4] Effect of presence or absence of carbide precipitation treatment on impact toughness

FIG. 5: Effect of presence or absence of carbide precipitation treatment and Ni content on impact toughness

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (6)

(57) [Claims] 1. A C content of 0.05 to 1.5% by weight, Si
Content of 2 wt% or less, Mn content is 2 wt% or less and the Cr content has a composition 10 to 20 wt%, carbide
1 to 3 of fine carbides with a particle size of 2 μm or less
Obtained from the structure before hardening uniformly dispersed in a proportion of 0% by volume
Fine austenite grain size of 30 μm or less
Martensitic stainless steel excellent in strength and toughness characterized by having a tensitic structure. 2. The composition according to claim 1, further comprising 7% by weight or less of N.
A martensitic stainless steel containing i and having excellent strength and toughness. 3. A C content of 0.05 to 1.5% by weight, Si
A steel material having a composition in which the content is 2% by weight or less, the Mn content is 2% by weight or less, and the Cr content is 10 to 20% by weight is heated to a temperature of Ac ₁ point or less and a fine grain size of 2 μm or less is obtained.
After uniformly depositing the carbide with a dispersion amount of 1 to 30% by volume ,
A method for producing a martensitic stainless steel excellent in strength and toughness, which comprises heating to a temperature of Ac ₃ points or higher and below a temperature at which carbides are completely dissolved, and then rapidly cooling to a temperature of Ac ₁ point or lower. 4. The alloy composition according to claim 3, further comprising:
A method for producing a martensitic stainless steel excellent in strength and toughness, which uses a steel material containing Ni in an amount of not more than wt%. 5. A martensitic stainless steel excellent in strength and toughness, which is subjected to the carbide precipitation treatment according to claim 3 by hot rolling at a finishing temperature of 500 to 900 ° C. and then maintaining the temperature range for 250 seconds or more. Manufacturing method. 6. Ac ₃ ≤ [T] ≤6100 / [4.32-0.
The method for producing martensitic stainless steel according to claim 3, wherein quenching is performed at a temperature T satisfying 0186 x (Cr% + Ni%)-log C%]-273.