CN112501491A

CN112501491A - Martensitic stainless steel alloy

Info

Publication number: CN112501491A
Application number: CN201910870222.1A
Authority: CN
Inventors: 萨拉·维克隆德; 约纳什·尼尔森; 安德斯·赫尔; 斯文-尹格·马特松; 柴国才
Original assignee: Sandvik Materials Technology AB
Current assignee: Alleima AB
Priority date: 2019-09-16
Filing date: 2019-09-16
Publication date: 2021-03-16

Abstract

The invention relates to a martensitic stainless steel alloy. In particular, the present invention relates to a martensitic stainless steel alloy comprising in weight percent (wt%): c is more than 0.50-0.60; 0.10-0.60% of Si; 0.40-0.80 Mn; 13.50-14.50% of Cr; 0 to 1.20 of Ni; mo is 0.80-2.50; n is 0.050 to 0.12; 0.10 to 1.50 of Cu; vmax 0.10; the maximum value of S is 0.03; pmax 0.03; the remainder being Fe and unavoidable impurities. The invention also relates to a stainless steel strip and a mechanical component comprising said martensitic stainless steel alloy.

Description

Martensitic stainless steel alloy

Technical Field

The invention relates to a martensitic stainless steel alloy. In particular, the present invention relates to a martensitic stainless steel alloy, a stainless steel strip comprising such a martensitic stainless steel alloy and different parts made thereof.

Background

The current martensitic stainless steels generally have high performance and good properties such as high strength and high ductility, making them suitable for use in different strip applications.

However, there is an increasing market demand for martensitic stainless steel alloys that can be used in demanding and high temperature applications. Therefore, there is a need for martensitic stainless steel alloys having a combination of high fatigue and mechanical properties, good wear resistance and temperature stability, i.e. good properties at high temperatures (temperatures of about 300 ℃).

It is therefore an aspect of the present invention to provide a solution or reduction to this problem.

Disclosure of Invention

Accordingly, the present invention relates to a martensitic stainless steel alloy that has a combination of improved fatigue strength, high hardness and good temperature stability at high temperatures (temperatures of about 300 ℃). In addition, the martensitic stainless steel of the present invention has improved wear resistance.

Accordingly, the present invention provides a martensitic stainless steel alloy having a composition in weight percent (wt.%):

the remainder being Fe and unavoidable impurities.

The present invention is based on the following surprising findings: martensitic stainless steel alloys with a C content of more than 0.50(>0.50) to 0.60 wt.%, having improved tensile strength and hardness and high ductility. Therefore, the fatigue resistance will be better. In addition, it has been shown that martensitic stainless steel alloys as defined above or below will provide good temperature stability in high temperature applications. The higher the C content, the higher the carbide density can be, which will have an influence on the wear resistance of the material.

Furthermore, strips and mechanical parts comprising or consisting of a martensitic stainless steel alloy as defined above or below will have improved fatigue strength and, depending on the conditions in which hardening and tempering have been carried out, will have better temperature stability and at the same time good hardness.

Detailed Description

The invention relates to a martensitic stainless steel alloy comprising, in weight percent (wt%):

the remainder being Fe and unavoidable impurities.

The proposed martensitic stainless steel alloy (hereinafter also referred to as "stainless steel alloy") has a matrix comprising martensite, retained austenite, carbides and carbonitrides after hardening and tempering. The microstructure of the hardened and tempered martensitic stainless steel alloy is further characterized in addition to M₂₃C₆And M₇C₃Besides carbides or other types of carbides, MCN type metal carbonitrides are present, wherein M represents one or more metal atoms.

The stainless steel alloy of the present invention will provide an increase in hardness without sacrificing temperature stability as compared to conventional martensitic stainless steels. High temperature stability is important because it means that the stainless steel alloy can be used in high temperature applications (about 300 ℃).

The martensitic stainless steel alloy of the invention has a suitable hardening temperature in the range of 980-1100 ℃, such as 1020-1060 ℃. Depending on the application, the tempering temperature may be in the range of 200 to 500 ℃. The hardening and tempering times will vary depending on the application and the size of the product. Hardening and tempering are carried out in an oven. High furnace temperatures may require varying processing times to avoid negatively impacting material properties.

It has been found that the martensitic stainless steel of the invention is capable of being tempered at temperatures between 400 ℃ and 450 ℃ and that the material still has a hardness that is sufficiently high for use in the desired application. By performing tempering at these temperatures, the resulting material will be temperature stable at high temperatures (about 300 ℃).

According to one embodiment, the martensitic alloy contains less than or equal to 0.5 wt% of unavoidable impurities, preferably less than or equal to 0.3 wt%. Unavoidable impurities may naturally be present in the raw or recycled materials used for producing the stainless steel alloy. Examples of unavoidable impurities are elements and compounds which are not deliberately added but cannot be completely avoided, since they are usually present as impurities. Thus, the inevitable impurities are present in the alloy in concentrations at which the inevitable impurities have a very limited effect on the final properties. The inevitable impurities present in the stainless steel alloy may for example include Co, Sn, Ti, Nb, W, Zr, Ta, B, Ce and O.

Furthermore, small amounts of alloying elements may be added during the production process, e.g. during the deoxidation step or to improve other properties. Examples of such alloying elements include, but are not limited to, Al, Mg, and Ca. The skilled person will know how many amounts are required, depending on the components used. However, these elements may be added to the stainless steel alloy in an amount of ≦ 0.02 wt%.

The alloying elements of the proposed martensitic stainless steel alloy are discussed below. However, their effects mentioned below should not be considered as limiting:

carbon (C)

C is the formation of M₂₃C₆、M₇C₃And MC type carbides and MCN type carbonitrides. C is also important for the hardenability of steel. However, too high a C content may combine with other alloying elements, resulting in the formation of large and undesirable primary carbides in the initial production phase. In addition, a high C content makes the martensite more brittle and lowers the Ms temperature at which the martensite starts to form, and may also increase the amount of retained austenite to too high a level. Therefore, the amount of C is limited to>0.50 to 0.60 wt%, preferably 0.51 to 0.56 wt%.

Silicon (Si)

Si is a ferrite stabilizer and can act as a deoxidizer. Si also increases the activity of carbon and contributes to strength by solid solution strengthening. Too high a content may lead to the formation of unwanted inclusions. Therefore, the amount of Si is limited to 0.10 to 0.60 wt%, for example 0.20 to 0.55 wt%, such as 0.30 to 0.50 wt%.

Manganese (Mn)

Mn is an austenite stabilizer and acts as a deoxidizer. Mn increases the solubility of N and improves hot workability. Too high a content may result in the formation of MnS inclusions in combination with S. Therefore, the amount of Mn is limited to 0.40 to 0.80 wt%, for example, 0.50 to 0.80 wt%.

Chromium (Cr)

Cr is critical to the corrosion resistance of steel, depending on the Cr content of the steel matrix. Cr forms carbide (M)₂₃C₆、M₇C₃MCN) and increases the solubility of C and N. Cr is a ferrite stabilizer, and too high an amount may result in the formation of δ ferrite. Therefore, the amount of Cr is limited to 13.50 to 14.50 wt%.

Molybdenum (Mo)

Mo is a ferrite stabilizer and a strong carbide former. Mo has a positive influence on both the corrosion resistance and the hardenability of the steel. Mo also contributes to the increase in ductility. Since Mo is an expensive element, its content should not be higher than necessary for economic reasons. Therefore, the amount of Mo is limited to 0.80 to 2.50% by weight, preferably 0.80 to 2.00% by weight, more preferably 0.90 to 1.30% by weight.

Nitrogen (N)

N is an austenite stabilizer and improves the strength of the steel through interstitial solid solution strengthening. N helps to increase the hardness of the martensite, similar to C. N also forms nitrides and carbonitrides. An excessively high N content may lower hot workability. Therefore, the amount of N is limited to 0.050 to 0.12 wt%, preferably 0.050 to 0.10 wt%, such as 0.055 to 0.085 wt%.

Nickel (Ni)

Ni is an austenite stabilizer and reduces the solubility of C and N. Since Ni is an expensive element, its content should be kept low for economic reasons, and Ni is not usually intentionally added. The amount of Ni should be < 1.20% by weight, preferably < 0.40% by weight, more preferably < 0.35% by weight. According to one embodiment, Ni is 0.15 to 0.35 wt%.

Copper (Cu)

Cu is an austenite stabilizer and contributes to substitutional solid solution strengthening of the steel. Cu also improves the corrosion resistance of the stainless steel alloy. Cu is intentionally added to the martensitic alloy of the present invention.

Cu may also form a cluster that increases strength. The solubility of Cu in the matrix is greater than 0.4 wt% at equilibrium. In the present invention, the inventors have found that it is important to have supersaturated Cu in order to ensure maximum solid solution strengthening of the martensite phase and the retained austenite phase after hardening and tempering.

Therefore, the Cu content is 0.10 to 1.50 wt%, for example, 0.55 to 1.30 wt%.

Vanadium (V)

V is a strong carbide former and limits grain growth. As carbide-forming elements, V may be present in the martensitic alloy and may be added purposefully. It may also be present as a result of recycled material, but is subsequently considered an impurity. The content also depends on the source of the chromium. However, too high a V content may reduce ductility and hardenability and may result in undesirable primary carbides. If present in the stainless steel alloy, the amount of V is therefore limited to 0.010 to 0.10 wt.%, for example 0.030 to 0.10 wt.%.

Phosphorus (P)

P causes embrittlement. P is not generally added and should be limited to 0.03 weight percent.

Sulfur (S)

S will adversely affect hot workability and an excessively high S content will lead to formation of MnS inclusions. S is not usually added and should be limited to 0.03% by weight.

According to one embodiment, the stainless steel alloy of the present invention comprises any of the above alloying elements within any of the above ranges. According to another embodiment, the stainless steel alloy of the present invention consists of any of the above alloying elements within any of the above ranges.

The martensitic stainless steel alloy may suitably be produced in the form of a stainless steel strip, but may also be produced in the form of a wire, rod, bar or the like.

The martensitic stainless steel alloy of the invention can be used for different mechanical parts, for example valve parts of compressors, such as flapper valves. The martensitic stainless steel of the invention is also suitable for other applications where high fatigue strength and/or wear resistance and edge properties are required.

According to one embodiment, the stainless steel alloy of the invention may be produced therefrom:

the melting-melting process can be carried out after the AOD process by using EAF (electric arc furnace) and the final adjustment of the chemical composition can be carried out in ladle furnace treatment;

-casting-continuous casting of billets of desired shape, for example 365-265 mm;

-heating the blank until the material reaches a temperature of 1200-1350 ℃;

rolling-rolling the billet into a strip, for example with a thickness of 100 to 200 mm;

-heating-optionally reheating the strip until the material reaches a temperature of 1200-1300 ℃;

rolling-hot rolling the billet to a thickness of, for example, 3-6 mm. Hot rolling may be carried out several times, depending on the roller mill used;

-winding the strip at a winding temperature after cooling of about 600 to 750 ℃;

-annealing the hot rolled strip at 800 to 900 ℃ for at least 4 hours;

-pickling (pickling) -removal of oxides;

-rolling-cold rolling to a final thickness of e.g. 0.040-3 mm;

-optionally annealing-for recrystallization it may be necessary to carry out an intermediate annealing at a temperature of about 700 to 800 ℃;

hardening-hardening can be carried out in a continuous hardening line using the following steps: austenitizing, quenching, additional cooling, tempering, cooling to room temperature, and polishing. The speed of the hardening line depends on the thickness or mass flow of the material and the size of the furnace and can be between 100 and 1000 m/h. The length of the austenitizing furnace and the length of the tempering furnace are substantially the same.

The austenitizing temperature is 950-1080 ℃.

Omicron quenching should be performed as follows: the material temperature is typically rapidly reduced to less than-500 ℃ in 2 minutes to avoid brittleness or reduce corrosion resistance.

Optionally, additional cooling is performed to pass the material below the Ms temperature and to obtain the desired level of retained austenite. The cooling temperature may be-100 to 100 ℃ depending on the final application, but room temperature is generally used.

Depending on the final tensile strength of the target, the tempering may be set to 250-500 ℃.

Drawings

Fig. 1 shows the results of a comparison of a reference material with the inventive material according to one embodiment in a fatigue test.

Detailed Description

The invention is further illustrated by the following non-limiting examples

Examples

Example 1

Some of the alloy in the form of 1.5kg molten steel was heated, which was produced by melting using a vacuum induction melting furnace (VIM). The elemental composition (in weight%) of the alloy is set forth in Table I. The remainder being Fe and unavoidable impurities. When no value is specified for a particular element, then the amount of that element is below the detection limit. Alloys 1, 2 and 3 were included as comparative examples, while the remaining alloys represent different embodiments of the stainless steel alloy according to the present invention. The alloy, stainless steel alloy, was produced as follows.

TABLE I

And (3) molten steel produced. The molten steels 1, 2 and 3 are out of the scope of the present invention. The remainder being Fe and unavoidable impurities.

Samples in the form of cylindrical test bars were produced from the molten steel for testing.

Therefore, the process flow is as follows: melting the feedstock in a vacuum induction melting furnace (VIM); casting; pre-heated at 700 ℃ before hot working (30 minutes) and then heat treated at 1150 ℃ (30 minutes); annealing (825-875 ℃ for 6 hours); and machining the sample, followed by hardening and tempering.

Hardness (HV1) measurements were made according to SS-EN ISO 6507. The value is the average of 5 measurements. The test specimens were hardened at 1030 ℃ and 1050 ℃, then quenched (to room temperature), and then tempered for 2 hours at 400 ℃ (hardened at 1050 ℃) and at 250 ℃ and 450 ℃ (hardened at 1030 ℃ and 1050 ℃), the results of which can be seen in table II.

TABLE II

The results show higher hardness for both sets of data hardened at 1030 ℃ and 1050 ℃ respectively. For each data point, the average of 5 measurements was taken. The data show that even with high tempering temperatures, the hardness increases significantly. Further high temperature tempering will lead to an increased risk of temper embrittlement. The data show that for tempers above 400 ℃, the hardness increases due to the addition of Cu. The results show high temperature stability compared to tempering at about 250 c, even though the tensile strength and hardness are the same.

Fatigue measurement

Large scale materials with a final thickness of 0.305mm were produced. By operating at-80 Hz resonanceThe material was tested for fatigue properties by the step method using a fluctuating tensile tester AMSLER with a 10% preload. The endpoint of the test was defined as 5X 10⁶And (4) one period. The sample consisted of a waist circumference of 10mm and a length of 15 mm. The method implies that the entire cross section is exposed to applied stress conditions, whereby the material properties are tested over a larger volume to determine the limiting coefficient. The sample was turned over to ensure proper edge and high surface residual stress. The probability of failure to perform the fatigue test was 50%.

The results of the fatigue test are shown in fig. 1. The ratio R represents the ratio of fatigue limit to tensile strength. The obtained standard deviations are represented by the size of each rectangle, respectively. As can be seen from the figure, the material of the invention exhibits a fatigue limit of 1505MPa, whereas the reference material (according to EN 1.4031) exhibits a fatigue limit of 1390 MPa.

Claims

1. A martensitic stainless steel alloy comprising, in weight percent (wt%):

the remainder being Fe and unavoidable impurities.

2. The martensitic stainless steel alloy according to claim 1, wherein the content of Si is between 0.20 and 0.55 wt.%, such as between 0.30 and 0.50 wt.%.

3. The martensitic stainless steel alloy according to claim 1 or claim 2, wherein the content of Mn is between 0.50 and 0.80 wt.%, such as between 0.60 and 0.80 wt.%.

4. The martensitic stainless steel alloy according to any one of claims 1 to 3, wherein the content of Mo is between 0.80 and 2.00 wt.%, more preferably between 0.80 and 1.30 wt.%, or still more preferably between 0.90 and 1.30 wt.%.

5. The martensitic stainless steel alloy according to any one of claims 1 to 4, wherein the content of Ni is ≦ 0.80 wt%, such as less than 0.40 wt%.

6. The martensitic stainless steel alloy according to any one of claims 1 to 5, wherein the content of N is between 0.050 and 0.10 wt.%, such as between 0.050 and 0.090 wt.%.

7. The martensitic stainless steel alloy according to any one of claims 1 to 6, wherein the content of V is between 0.030 and 0.10 wt%.

8. The martensitic stainless steel alloy according to any of the preceding claims, wherein the content of C is between 0.51 and 0.60 wt.%, or more preferably between 0.51 and 0.56 wt.%.

9. The martensitic stainless steel alloy according to any of the preceding claims, wherein said stainless steel alloy comprises 0.10-1.5 wt.% Cu, preferably 0.50-1.5 wt.% Cu.

10. A stainless steel strip comprising the martensitic stainless steel alloy according to any one of the preceding claims.

11. A mechanical component comprising the martensitic stainless steel alloy according to any one of claims 1 to 9.