JPH0718387A - Precipitation hardening type stainless steel excellent in wear resistance and production of precipitation hardening type stainless steel - Google Patents

Abstract

PURPOSE:To obtain a steel having prescribed hardness and wear resistance in a water environment by applying a two stage heat treating method in which temp. and time conditions are specified to stainless steel having a specified compsn. in which the contents of Nb and Cu are limited after hot working and solution treatment. CONSTITUTION:An ingot having a compsn. contg., by weight <=0.05% C, <=1.00% Si, <=1.0% Mn, <=0.04% P, <=0.01% S, 3.5 to 5.5% Cu, 3.0 to 5.5% Ni, 14.0 to 17.5% Cr and 0.15 to 0.35% Nb so as to satisfy Nb<=CX7.8, and the balance Fe is melted. This ingot is subjected to hot rolling to form into a hot rolled steel, which is subjected to solution treatment, is subjected to primary aging treatment of executing heating at 430 to 500 deg.C for 30min to 2hr and is subjected to secondary aging treatment of executing heating at 380 to 450 deg.C for 1 to 10hr. In this way, sufficient preciptation hardening occurs, by which the objective stainless steel can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardenable stainless steel and a precipitation hardenable stainless steel excellent in wear resistance which require high wear resistance in water environments such as for ships, dam earth and sand discharge channels, and floodgates. The present invention relates to a method for manufacturing a stainless steel material.

[0002]

2. Description of the Related Art In recent years, high-strength stainless steel materials have been used for hydrofoils and screws of high-speed ships, and earth and sand discharge channels of dams. The features of these applications are that they are required to have corrosion resistance because they are used naked in an aquatic environment, and that they need to have high wear resistance to prevent the impact caused by water bubbles and wear due to soil and sand. .

Conventionally, it has been known that the wear resistance has a certain degree of correlation with the hardness of the material. Therefore, a high hardness material is used for the wear resistance purpose, and the purpose is to endure the use especially in a water environment. For this purpose, the hardness and corrosion resistance of martensitic stainless steel are said to be suitable.

However, for the above-mentioned applications that have been recently demanded, even the martensitic stainless steel lacks in wear resistance, so that the 17Cr-4Ni-Cu system (17-4PH) is used.
Steel), 15Cr-5Ni-Cu series (15-5PH steel), and the like, are considered to be applied to precipitation hardening stainless steels in which martensitic ground is further hardened with precipitates. Of these, the 17-4PH steel is a general-purpose steel type that has the highest hardness level among stainless steels with the same Cr and Ni levels, and JIS
It is standardized as G 4303 SUS 630. Also, 15-5
PH steel is a steel type that has been improved to improve the hot workability and toughness of 17-4PH steel.

However, even the 17-4PH steel having a high hardness of Vickers hardness of 400 or more cannot be said to have sufficient wear resistance, and a short life due to abrasion is a problem. Taking a dam application as an example, as a corrosion-resistant metallic material that can withstand a 20-year earth and sand discharge channel life, 17-4PH steel, which is currently being considered for application, can almost satisfy both the corrosion resistance and ductility / toughness, but the wear resistance The higher is better, and the one with the same or higher economic efficiency is optimal, but there has been no such alternative material in the past. For other uses, it is common that the higher the abrasion resistance, the better as long as the corrosion resistance and the ductility / toughness are not impaired.

[0006] As a conventional knowledge, it is known that even if a small amount of Al, Ti or the like is added to Ni-containing stainless steel, the steel can be hardened relatively easily by precipitation of an intermetallic compound phase. It can be said that improvement in wear resistance can be expected to some extent. However, curing by such means results in a significant decrease in toughness, and the desired overall performance cannot be obtained.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a martensite precipitation hardening material for 17-4PH steel or 15-5PH steel. By revising the composition of type stainless steel and applying a two-step heat treatment method to the new composition system after hot working and solution heat treatment, the corrosion resistance and elongation / toughness of conventional precipitation hardening type stainless steel are maintained. It is an object of the present invention to provide a method for producing a stainless steel and a stainless steel material, each of which has performance far superior in hardness and wear resistance as compared with a conventional precipitation hardening stainless steel.

[0008]

[Means for solving the problems] 17-4PH steel and 15-5PH steel are
It is a steel type that obtains high strength by strengthening the martensitic ground by precipitation of Cu-rich phase, but there is no history of component design with particular attention to wear resistance, and precipitation strengthening components such as Nb and Cu. However, the conventional heat treatment method was empirically determined by considering the strength / toughness balance of steel.

First, we started by understanding the details of the precipitation behavior of the same steel type, and created a detailed time / temperature / precipitate diagram (TTP diagram) within the range of 300 to 900 ° C. investigated. As a result, Nb precipitates as carbonitride, but its strengthening effect is not only small, but excessive addition adversely affects toughness, and Cu is an element that gives very fine spherical precipitation as a Cu-rich phase. However, the addition amount has a range that significantly improves the wear resistance, and as described later, in order for the precipitation hardening to sufficiently occur, it is necessary to perform a two-step aging treatment with carefully controlled temperature conditions and time conditions. I found out.

The present invention was made on the basis of the above findings, and the stainless steel of the first invention is C: 0.0.
5 wt.% Or less, Si: 1.00 wt.% Or less, Mn: 1.0 wt.% Or less, P: 0.04 wt.% Or less, S: 0.01 wt.% Or less, Cu:
3.5 to 5.5 wt.%, Ni: 3.0 to 5.5 wt.%
, Cr: 14.0 to 17.5 wt.%, Nb: 0.15 to 0.3
5 wt.%, But Nb ≦ C × 7.8, and the balance: Fe and unavoidable impurities. The method for producing a stainless steel material according to the second aspect of the present invention is as follows: C: 0.05 wt.% Or less, Si: 1.0 wt.% Or less, M
n: 1.0 wt.% or less, P: 0.04 wt.% or less, S: 0.01 w
t.% or less, Cu: 3.5 to 5.5 wt.%, Ni: 3.0 to 5.5 wt.%, Cr: 14.0 to 17.5 wt.%, Nb:
0.15 to 0.35 wt.%, With the proviso that Nb ≤ C × 7.8 and the balance: Fe and unavoidable impurities, a steel ingot having a chemical composition is melted, and then the steel ingot is hot-rolled. A hot-rolled steel material is obtained, and then the hot-rolled steel material is subjected to a solution heat treatment, and then subjected to a first aging treatment of heating at a temperature in the range of 430 to 500 ° C. for 30 minutes to 2 hours, and then, It is characterized in that it is subjected to a second aging treatment in which heating is carried out at a temperature in the range of 380 to 450 ° C. for 1 to 10 hours.

[0011]

Next, the reason why the chemical composition of the stainless steel according to the first aspect of the present invention is limited to the above range will be described below.

C (Carbon): C is one of the elements that improves the wear resistance by increasing the strength of the parent phase of the invention steel. If the C content exceeds 0.05 wt.%, It is harmful to corrosion resistance and reduces the ductility and toughness after aging heat treatment. Therefore,
The C content should be 0.05 wt.% Or less.

Si (Si): Si is an element effective for deoxidation, but if the Si content exceeds 1.0 wt.%, Precipitation of an embrittlement phase occurs and hot workability is impaired. Therefore, the Si content is
It should be 1.0 wt.% Or less.

Mn (manganese): Mn suppresses precipitation of a ferrite phase together with Ni and stabilizes a martensite phase,
It also acts as a desulfurizing agent to fix harmful residual S in steel. However, if the Mn content exceeds 1.0 wt.%, It is harmful to hot workability. Therefore, the Mn content is 1.
It should be 0 wt.% Or less.

P (phosphorus): P segregates at the grain boundaries, deteriorates hot workability, and causes deterioration of ductility and toughness after aging. Therefore, its upper limit should be 0.04 wt.%. is there.

S (sulfur): S, like P, is an element that segregates at the grain boundaries and deteriorates hot workability, and in particular, it is an element that induces cracking during rolling. The smaller the content, the better. Since the maximum allowable limit is 0.01 wt.%, Its upper limit should be 0.01 wt.%.

Cu (Cu): Cu is an element that plays a major role in precipitation strengthening and wear resistance improvement, and is precipitated as a Cu-rich phase during aging, and is effective in strengthening the matrix and improving wear resistance. . According to our research, there is a close relationship between the addition amount of Cu, the aging time, and the effect of improving wear resistance, and the effect of the two-step aging described below is remarkably exhibited when the Cu content is 3 .
Limited to 5 wt.% Or more. However, when the Cu content exceeds 5.5 wt.%, The above effect is saturated and the ductility and toughness of the steel are significantly impaired. Therefore, the Cu content is 3.5
To 5.5 wt.% Should be limited.

Ni (Nickel): Ni is an element that suppresses precipitation of a ferrite phase and enhances hardenability. This effect is insufficient when the Ni content is less than 3.0 wt.%. Conversely, Ni
If the content exceeds 5.5 wt.%, The retained austenite after quenching increases and the hardness and wear resistance decrease.
Therefore, the Ni content should be limited to the range of 3.0 to 5.5 wt.%.

Cr (Cr): Cr is a basic element that imparts corrosion resistance to stainless steel in an aqueous environment. Cr content is 14.0
If it is less than wt.%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content exceeds 17.5 wt.%, The phase balance is disturbed, δ ferrite increases, which impairs hot workability and deteriorates toughness. Therefore, the Cr content should be limited to the range of 14.0 to 17.5 wt.%.

Nb (niobium): Nb is an element that fixes C in steel, suppresses precipitation of Cr carbides at grain boundaries, and is effective in improving corrosion resistance. In addition to the above, precipitation strengthening as NbC has been additionally considered in conventional applications, but we have revealed that precipitation of NbC has little effect on improving wear resistance. If the Nb content is less than 0.15 wt.%, The effect of fixing C is not sufficient. On the other hand, if the Nb content exceeds 0.35 wt.%, It is harmful to the toughness. Therefore, the Nb content should be limited to the range of 0.15 to 0.35 wt.%. However, if the Nb content exceeds 7.8 times the C content, brittle intermetallic compounds and complex carbides are formed, and the toughness of steel is deteriorated. Therefore, the Nb content should be 7.8 times or less the C content (Nb ≦ C × 7.8).

Next, in the method for producing a stainless steel material according to the second aspect of the present invention, the reason why the heat treatment conditions are limited to the above-mentioned range will be described below.

After repeating a number of trial combinations of heat treatment conditions, we carried out the first aging treatment within a temperature range of 430 to 500 ° C. for 30 minutes to 2 hours, and subsequently,
It has been found that the wear resistance is best when the second aging treatment is performed for 1 to 10 hours in the temperature range of 380 to 450 ° C. That is, by performing the heat treatment with the combination as described above, nucleation of the precipitation phase is efficiently caused at a high temperature, fine and uniform dispersion is achieved, and subsequently, the aging treatment at a further lower temperature causes Oswald growth. It is possible to promote the suppressed and stable growth of the precipitation phase and complete the sufficient precipitation of the added Cu.

[0023]

EXAMPLES Next, the present invention will be described in more detail by way of examples. [Embodiment 1] An embodiment of the first invention will be described. Table 1 shows the chemical composition of the examined steels. In Table 1, Nos. 1 to 7 are steels of the present invention having chemical composition within the scope of the present invention, and Nos. 8 to 15 are comparative steels having chemical composition outside the scope of the present invention.

[0024]

[Table 1]

After melting these steels in a vacuum melting furnace, the steel ingots were soaked at 1250 ° C. and hot-rolled to prepare a plate material having a thickness of 12 mm. The steel sheet thus obtained was subjected to solution treatment (ST) uniformly under the condition of water cooling at 1040 ° C for 30 minutes. Then, the first aging treatment by heating at 450 ℃ × 1 hour, and then 420 ℃
A second aging treatment by heating for 4 hours was performed to prepare a stainless steel specimen. Then, the prepared specimens were used as test pieces, and a hardness test, an impact test and an abrasion resistance test consisting of the following were carried out for each of them.

The hardness test was performed on the rolled L cross section according to the Vickers hardness test method specified in JIS Z 2244.
The adjustment of the test surface was performed up to the No. 600 sandpaper. The load was 98.07 N, and the average value measured by making 5 indentations was HV (10).

For the impact test, a No. 4 test piece (V notch, full size 10 × 10 mm cross section) specified in JIS Z 2202 was used.
The test was performed according to the impact test method specified in JIS Z 2242. The test piece was sampled in the rolling L direction, each temperature was tested with the number of repetitions of 2, and the Charpy fracture surface transition temperature vTrs (° C) was obtained.

For the abrasion test, the test apparatus shown in FIG. 2 and the test piece 1 shown in FIG. 3 (cross section B × total length C = 10φ × 60
mm), the test piece 1 was mounted at a distance A of 150 mm from the center of rotation, and a test piece rotation type wear test was performed. In order to evaluate the abrasion resistance in a water environment, a liquid 2 in which natural silica sand and pure water were mixed at a weight ratio of 2: 1 as an abrasion material was used by filling the device 3 with the liquid. The temperature was room temperature, the rotational speed was 4 m / sec at the test piece position, and the test time was 4 hours. JIS G 31
A test piece made of general structural steel SS400 specified in 01 was mounted as a standard sample, and the wear resistance was evaluated by the relative ratio (Rw) of the amount of wear to this and the test piece of the specimen. .

Table 2 collectively shows various characteristics. Impact characteristics
The VTrs and the Rw value showing the wear resistance are evaluated by the marks ○ and ×, respectively. The evaluation standard of the fracture surface transition temperature vTrs was 0 ° C. That is, the brittle fracture was evaluated as ◯ when the temperature was lower than the lowest temperature of 0 ° C. (0 ° C. or less) in the water environment, and as x when the temperature was high (over 0 ° C.). The wear resistance was evaluated as x when the wear resistance was inferior to that of the conventional 17-4PH steel aged heat treatment material (Rw = 0.65), and as o when the characteristics were equal or higher.

[0030]

[Table 2]

As shown in Table 2, at a low Cu content as in Comparative Steel No. 8, the hardness after aging is not sufficiently improved and the wear resistance is poor. However, as in Comparative Steel No. 12, when Cu is excessive, the toughness is reduced and vTrs ≦ 0 ° C. cannot be satisfied.

Regarding the amount of Nb, as is clear from Comparative Steel No. 9, if the content is excessive, the toughness deteriorates, and in the case of Comparative Steel No. 14, conversely the addition is not sufficient.
The amount of C that cannot be fixed as NbC becomes large, and the Cr carbide segregated at the grain boundaries easily becomes a base for brittle fracture, so that the brittleness also decreases. Similarly, in Comparative Steel No. 10, the C content was too large and the toughness was reduced.

In Comparative Steel No. 11, a typical example is shown in which the amount of ferrite and the amount of retained austenite are excessively increased because the Cr content is excessive, and the hardness and wear resistance after aging are not improved and the toughness is poor. Has been done. Insufficient hardness and wear resistance due to excess retained austenite amount are also undesired when the Ni content is excessive, as in Comparative Steel No. 13. On the contrary, when the Ni content is insufficient like Comparative Steel No. 15, the toughness is poor because the ferrite content is excessive.

On the other hand, in the steel Nos. 1 to 7 of the present invention, due to the proper balance of the components, the above problems do not occur, and the optimized Cu content causes the aging to greatly exceed that of the conventional 17-4PH steel. A cure amount is achieved and remarkably excellent wear resistance can be obtained.

[Embodiment 2] An embodiment of the second invention will be described. Table 3 shows the results of measuring the wear resistance by extracting characteristic ones from the heat treatment conditions examined. As the test steel, the steel No. 4 of the present invention shown in Table 1 was used. The values obtained by using Comparative Steel No. 8 shown in Table 1 are also shown in parentheses in Table 3.

[0036]

[Table 3]

In Table 3, the trial heat treatment conditions are shown as a combination of the first aging heat treatment conditions on the vertical axis and the second aging heat treatment conditions on the horizontal axis, and the obtained Rw value ( Rw = R / R _SS400 × 100%) is shown. In Table 3, 17
-4PH steel heat treatment by the conventional method, namely, H900 (480 ℃ heat treatment), H1025 (550 ℃ heat treatment), H1075 (570 to 590 ℃ heat treatment), H1150 (620 ℃) specified in JIS G 4303.
The results of heat treatment) are also compared. Both are single heat treatments. In Table 3, "JIS" is shown to the right of the value.
It is shown as a supplementary note. Among JIS methods, H900 gives the highest wear resistance, but its value remains at the level of Rw = 0.65, and the wear resistance according to the method of the present invention shown in the thick frame is 10 to 15%. Is also excellent.

FIG. 1 shows that the first aging treatment condition was 450 ° C. × 1 hour within the range of the present invention or 450 ° C. × 8 hours outside the range of the present invention, and the second aging heat treatment condition was 400 ° C. × 0.5 to 16 hours. Change in wear resistance in the case of being subjected to
No. 8 is shown. When heat-treated under the conditions within the scope of the claims of the present invention, the wear resistance is remarkably improved, which greatly exceeds the level of Rw = 0.65 (similar to the conventional material) shown by the dotted line in FIG. Even when the comparative steel No. 8 is subjected to the heat treatment within the scope of the claims of the present invention, such improvement in wear resistance is not observed. The heat treatment condition range of the present invention is a range shown by a thick frame in Table 3, and all show excellent wear resistance exceeding Rw = 0.65. If the second aging heat treatment is carried out for a long time, the toughness deteriorates as shown in Table 3, which is not desirable in actual use. When vTrs ≧ 0 ° C., “black triangle mark” is added beside the value shown in Table 3.
It can be understood that the material of the present invention has no problem in terms of toughness.

[0039]

As described above, according to the present invention, the corrosion resistance, the ductility and the toughness can be maintained without being a component system far from the precipitation hardening type stainless steel of 17-4PH or 15-5PH. Compared with the existing precipitation hardening stainless steels, we can provide stainless steels that show significantly higher performance in hardness and wear resistance, and can significantly improve the life of wear resistant materials in water environments. At the same time, in terms of hardness and strength as well, performance superior to that of conventional precipitation hardening stainless steel materials is obtained, thus providing industrially useful effects.

[Brief description of drawings]

[Fig. 1] First aging heat treatment condition is 450 ℃ × 1 hour or 8
3 is a graph showing changes in wear resistance when the second aging heat treatment condition is 400 ° C. × 0.5 to 16 hours for the steel of the present invention and the comparative steel.

FIG. 2 is an explanatory diagram showing a test device used for evaluation of wear resistance in a water environment.

FIG. 3 is an explanatory diagram showing a shape of a test piece used for evaluation of wear resistance in a water environment.

[Explanation of symbols]

 1 Test piece 2 Liquid 3 Test device

Claims (2)

[Claims] 1. C: 0.05 wt.% Or less, Si: 1.00 wt.% Or less, Mn: 1.0 wt.% Or less, P: 0.04 wt.% Or less, S: 0.01 wt.% Or less, Cu: 3.5 to 5.5. wt.%, Ni: 3.0 to 5.5 wt.%, Cr: 14.0 to 17.5 wt.%, Nb: 0.15 to 0.35 wt.%, with Nb ≦ C × 7.8, and the rest: Fe and inevitable impurities Precipitation hardening type stainless steel with excellent wear resistance. 2. C: 0.05 wt.% Or less, Si: 1.0 wt.% Or less, Mn: 1.0 wt.% Or less, P: 0.04 wt.% Or less, S: 0.01 wt.% Or less, Cu: 3.5 to 5.5. wt.%, Ni: 3.0 to 5.5 wt.%, Cr: 14.0 to 17.5 wt.%, Nb: 0.15 to 0.35 wt.%, with Nb ≦ C × 7.8, and the rest: Fe and inevitable impurities A steel ingot having a chemical composition of
Hot rolling the steel ingot into a hot rolled steel material, then subjecting the hot rolled steel material to solution treatment, and then heating at a temperature in the range of 430 to 500 ° C. for 30 minutes to 2 hours; Precipitation hardening with excellent wear resistance, characterized by being subjected to a first aging treatment, and then a second aging treatment in which the heating is carried out at a temperature in the range of 380 to 450 ° C. for 1 to 10 hours. For manufacturing type stainless steel.