WO2021129836A1

WO2021129836A1 - Corrosion-resistant free-cutting steel

Info

Publication number: WO2021129836A1
Application number: PCT/CN2020/139612
Authority: WO
Inventors: 蒋肇谦; 杜冠宏; 林纪纲
Original assignee: 华新丽华股份有限公司
Priority date: 2019-12-26
Filing date: 2020-12-25
Publication date: 2021-07-01
Also published as: TW202124740A

Abstract

A ferrite type stainless steel having both corrosion resistance and free cutting properties. By controlling the specific weight percentages of carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen and boron in the ferrite type stainless steel and enabling same to satisfy a specific relationship, the steel has both good corrosion resistance and free cutting properties.

Description

Corrosion resistant free-cutting steel

Technical field

The invention relates to a free-cutting ferritic stainless steel steel material, and in particular to a free-cutting ferritic stainless steel steel material with good corrosion resistance.

Background technique

Nowadays, components installed in precision instruments, miniaturized devices, high-performance machinery or equipment have increasingly higher requirements for high dimensional accuracy, refined surface and good corrosion resistance. For example, electric or unmanned transportation vehicles, micro-drives, communication equipment, storage devices and components in electronic products.

Since most of these components are made of ferritic stainless steel and are formed by turning, the ferritic stainless steel used to manufacture the aforementioned components must have good free cutting properties to ensure stable dimensional accuracy and productivity.

Traditionally, ferritic stainless steels are made by adding sulfur to form manganese sulfide inclusions (the inclusion elements do not contain chromium, boron, nitrogen and calcium), thereby improving the free cutting performance. However, manganese sulfide is an inclusion with poor corrosion resistance. It will be corroded first on the surface and form a hole, which becomes the starting point of pitting corrosion. In addition, when the components are assembled into a product, this kind of manganese sulfide inclusions will generate hydrogen sulfide gas [H ₂ S _(g) ] after contacting the moisture in the environment, which is called Outgassing, and this gas will Corrosion of components and shorten the life of the product. The aforementioned pitting corrosion and outgassing corrosion greatly reduce the corrosion resistance of the free-cutting ferritic stainless steel material. In addition, although the addition of lead is also obviously beneficial to the improvement of rapid cutting properties, lead is environmentally toxic and harms the central nervous system and kidneys of the human body, and its use has been banned by national laws and regulations.

In view of this, it is urgent to develop a new free-cutting ferritic stainless steel with good corrosion resistance to improve the above-mentioned shortcomings of the known free-cutting ferritic stainless steel.

Summary of the invention

In view of the above-mentioned problems, one aspect of the present invention is to provide a ferritic stainless steel that has both corrosion resistance and free-cutting properties. With specific weight percentages of carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen and boron and satisfying specific relationships, this steel has both good corrosion resistance and free cutting properties.

According to one aspect of the present invention, a corrosion-resistant free-cutting steel is provided. This corrosion-resistant free-cutting steel contains carbon equal to or less than 0.06 weight percent, silicon from 0.01 weight percent to 1.3 weight percent, manganese from 0.1 weight percent to 0.5 weight percent, sulfur greater than 0.2 weight percent to 0.4 weight percent, 17 weight percent Percent to 22% by weight of chromium, 0.01% to 0.5% by weight of molybdenum, 0.001% to 0.035% by weight of nitrogen, 0.002% to 0.02% by weight of boron, balance of iron, and unavoidable impurities, and The corrosion-resistant free-cutting steel meets the following formulas (I) to (IV):

2[B]－[N]+[Mn]－[S]≦0 (I);

1.2≦[N][Mn]/[B][S]≦4 (II);

[Cr]+3.3[Mo]+16[N]≧18 (III);

[Mn]/[S]=0.5～1.5 (IV),

Wherein [B], [N], [Mn], [S], [Cr] and [Mo] represent the weight percentages of boron, nitrogen, manganese, sulfur, chromium and molybdenum, respectively.

According to an embodiment of the present invention, the corrosion-resistant free-cutting steel selectively satisfies the following formula (V):

[N]/[B]=1.5～3.0 (V),

Wherein [B] and [N] represent the weight percentages of boron and nitrogen, respectively.

According to another embodiment of the present invention, the corrosion-resistant free-cutting steel selectively contains 0.01 wt% to 0.5 wt% copper.

According to another embodiment of the present invention, the corrosion-resistant free-cutting steel selectively contains 0.001 wt% to 0.02 wt% calcium.

According to another embodiment of the present invention, the corrosion-resistant free-cutting steel includes a first inclusion and a second inclusion.

According to another embodiment of the present invention, the first inclusion is _{represented by MnSX 1} , X ₁ represents the first constituent element, and the first constituent element is chromium.

According to another embodiment of the present invention, the total weight based on the composition of the first inclusions is 100 weight percent, and the chromium content of the composition of the first inclusions is 10 weight percent to 60 weight percent.

According to another embodiment of the present invention, the first inclusion further includes a second constituent element (X ₂ ), represented by MnSX ₁ X ₂ (also referred to as the third inclusion), and the second constituent element is selected from At least one member of a group consisting of boron, nitrogen, and calcium.

According to another embodiment of the present invention, the second inclusion is represented by BN, the second inclusion further includes a third constituent element (Y ₁ ), _{represented by BNY 1} , and the third constituent element is calcium or chromium.

According to another embodiment of the present invention, the second inclusion further includes a fourth element (Y ₂ ), represented by BNY ₁ Y ₂ , and the fourth element is selected from the group consisting of manganese and sulfur At least one.

Application of the corrosion-resistant free-cutting steel material of the present invention, wherein by controlling the specific weight percentages of carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen, and boron of the corrosion-resistant free-cutting steel material and satisfying a specific relationship, the steel has both Good corrosion resistance and fast cutting properties.

Description of the drawings

In order to have a more complete understanding of the embodiments of the present invention and its advantages, please refer to the following description and the corresponding drawings. It must be emphasized that the various features are not drawn to scale and are for illustration purposes only. The contents of the relevant drawings are as follows:

Fig. 1 is an optical microscope photograph showing inclusions in a transverse section and a longitudinal section of a steel material of Example 1 of the present invention.

Detailed ways

The manufacture and use of embodiments of the present invention are discussed in detail below. However, it can be understood that the embodiments provide many applicable inventive concepts, which can be implemented in various specific contents. The specific embodiments discussed are for illustration only, and are not intended to limit the scope of the present invention.

The corrosion-resistant free-cutting steel material of the present invention belongs to the ferritic stainless steel material. The constituent elements of this corrosion-resistant free-cutting steel include carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen, boron, iron and unavoidable impurities. By controlling the specific weight percentages of carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen, and boron in the steel material and satisfying specific relationships, a steel material with specific inclusions can be obtained, so it has both corrosion resistance and rapidity. Cutting sex. The following describes the constituent elements of steel, the relationship between the constituent elements, and inclusions in order.

Carbon (C) is an element that improves the strength and hardness of steel. In addition, carbon easily combines with chromium to form carbides, which reduces the corrosion resistance of steel. Therefore, in steels that do not require strength, the carbon content should be controlled in a low range. Within (that is, below 0.06 weight percent). When the carbon content is greater than 0.06 weight percent, the strength and hardness of the steel are excessively increased, and its corrosion resistance becomes poor.

Silicon (Si) is an oxygen scavenger required for stainless steel smelting, which can remove oxygen in molten steel and reduce the formation of harmful oxide impurities. When the silicon content is less than 0.01% by weight, the oxygen content of the produced steel is relatively high, and oxide impurities increase, so the workability of the steel is reduced. When the silicon content is greater than 1.3% by weight, a large amount of _{hard oxides such as SiO 2} remain in the steel material, which reduces the free-cutting property of the steel material. Preferably, the silicon content can be 0.01 wt% to 0.6 wt%.

Manganese (Mn) is an important element that affects the surface roughness, the shape of turning chips and the corrosion resistance of steel after turning. In detail, manganese can combine with sulfur to form manganese sulfide inclusions, which can improve the free-cutting properties of steel. However, the corrosion resistance of simple manganese sulfide inclusions is poor. When the manganese content is less than 0.1% by weight, too little manganese cannot form sufficient manganese sulfide inclusions, which reduces the free cutting property of the steel. Conversely, when the manganese content is greater than 0.5% by weight, the corrosion resistance of the steel is poor, and pitting corrosion is prone to occur.

As mentioned earlier, because sulfur (S) can form manganese sulfide inclusions with manganese, sulfur is also an important element that mainly affects the free-cutting and corrosion resistance of steel. When the sulfur content is less than or equal to 0.2% by weight, too little sulfur cannot form sufficient manganese sulfide inclusions, thus reducing the free-cutting properties of the steel. When the sulfur content is more than 0.4% by weight, the hot workability of the steel material deteriorates.

[Mn]/[S] will affect the final manganese content and sulfur content of manganese sulfide inclusions, which in turn affects the free-cutting and corrosion resistance of the steel. Generally speaking, the ratio of the weight percentage of manganese to sulfur ([Mn]/[S]) is 0.5 to 1.5, and preferably 0.5 to 1.1, so that the resulting steel material can have both free-cutting properties and corrosion resistance. When [Mn]/[S] is less than 0.5, although the corrosion resistance of the steel material becomes better, its free cutting property and hot workability become worse. Conversely, when [Mn]/[S] is greater than 1.5, although the free cutting property of the steel material becomes better, its corrosion resistance becomes worse.

Chromium (Cr) is an important element to improve the corrosion resistance of stainless steel, and can replace manganese in manganese sulfide inclusions to form manganese sulfide inclusions containing chromium to improve the corrosion resistance of steel. When the chromium content is less than 17% by weight, too little chromium is difficult to form sufficient chromium-containing manganese sulfide inclusions, which reduces the corrosion resistance of the steel. Conversely, when the chromium content is greater than 22% by weight, although the high-temperature ferrite phase structure can be stabilized, and its corrosion resistance can be increased, the workability of the steel will be reduced.

Molybdenum (Mo) can improve the corrosion resistance and strength of stainless steel. If molybdenum is not deliberately added to the steel, a small amount of molybdenum remains in the steel. When the molybdenum content is less than 0.01% by weight, the raw materials of the steel must be specially selected, which will increase the cost. Conversely, when the molybdenum content is greater than 0.5% by weight, the strength of the steel is too high, which reduces its free-cutting properties.

Nitrogen (N) can increase the strength of steel. When the nitrogen content is less than 0.001% by weight, too little nitrogen will increase the smelting cost and cause the strength of the steel to be too low, which is not conducive to the rapid cutting of the steel. When the nitrogen content is greater than 0.035 weight percent, the strength of the steel is excessively enhanced, and the toughness of the steel is reduced.

Boron (B) can combine with nitrogen to form boron nitride inclusions, and the inclusions can improve the free cutting performance of the steel. When the boron content is less than 0.002% by weight, too little boron cannot form sufficient boron nitride inclusions, thereby reducing the free-cutting properties of the steel. Conversely, when the boron content is greater than 0.02% by weight, the toughness of the steel decreases, which is not conducive to subsequent processing and forming.

In some embodiments, the ratio of the weight percentage of nitrogen to boron ([N]/[B]) is 1.5 to 3.0, and preferably 1.8 to 3.0. When [N]/[B] is less than 1.5, if the nitrogen content is too small, sufficient boron nitride inclusions will not be formed, which will reduce the free-cutting and corrosion resistance of the steel, and too much boron will remain in the steel. Reduce its toughness. Conversely, when [N]/[B] is greater than 3.0, less boron can not form sufficient boron nitride inclusions, which reduces the free cutting property of the steel. Or, too much nitrogen may increase the strength excessively, and reduce the toughness and free-cutting properties of the steel.

In addition, the unavoidable impurities referred to in the present invention refer to impurities that cannot be separated during the steelmaking process.

Further, the corrosion-resistant free-cutting steel material of the present invention satisfies the following formulas (I) to (III):

2[B]－[N]+[Mn]－[S]≦0 (I);

1.2≦[N][Mn]/[B][S]≦4 (II);

[Cr]+3.3[Mo]+16[N]≧18 (III),

It is said that the value of 2[B]-[N]+[Mn]-[S] is not more than 0, and preferably is -0.18 to 0. This relationship mainly affects the rapid cutting of steel. When the value is not greater than 0, the shape and composition of the boron nitride inclusions and manganese sulfide inclusions formed in the steel can improve the free cutting performance of the steel. When this value is greater than 0, it will shorten the life of the turning tool, increase the production cost, and reduce the precision processing performance of the steel, so that the processed product (such as the internal components of the hard disk (HDD)) will not easily reach the required Tolerance standards.

In some embodiments, the value of [N][Mn]/[B][S] is 1.2 to 4, and preferably 1.2 to 3.1. This relationship can determine the degree of galvanic corrosion that occurs when steel is in contact with other metals. When this value is less than 1.2 or greater than 4, the activity of steel is higher than other metals, and galvanic corrosion is prone to occur, and it is corroded first, so the product life is shortened.

In some embodiments, the value of [Cr]+3.3[Mo]+16[N] is not less than 18, and is preferably 18-23. This relationship mainly affects the corrosion resistance of steel. When this value is less than 18, the steel is prone to pitting corrosion, which accelerates the corrosion of the steel and shortens the service life of the product.

In some embodiments, the corrosion-resistant free-cutting steel material selectively contains copper (Cu) in an amount of 0.01 weight percent to 0.5 weight percent. Copper affects the corrosion resistance and strength of steel. When the copper content is in the aforementioned range, better corrosion resistance and appropriate strength of the steel can be provided.

In some embodiments, the corrosion-resistant free-cutting steel selectively contains 0.001 wt% to 0.02 wt% calcium (Ca). When the calcium content is in the aforementioned range, an appropriate amount of calcium can adjust the shape and distribution of manganese sulfide inclusions, and can form manganese sulfide inclusions containing chromium and calcium with chromium-containing manganese sulfide inclusions to improve the resistance of steel Corrosion and fast cutting.

In some embodiments, the corrosion-resistant free-cutting steel selectively contains phosphorus (P) equal to or less than 0.05 weight percent. Phosphorus is an impurity (that is, a harmful element) in the steel billet, and can cause embrittlement of the steel and reduce the workability. But in terms of free-cutting properties, phosphorus is one of the elements that can improve the free-cutting properties of steel. When the phosphorus content is in the aforementioned range, appropriate toughness and free-cutting properties of the steel can be provided.

In some embodiments, the corrosion-resistant free-cutting steel selectively contains nickel (Ni) equal to or less than 0.5 weight percent. Nickel is a stable element in the austenite phase, and ferritic stainless steel usually does not deliberately add nickel, but there are often traces of nickel remaining in the steel. When the nickel content is in the aforementioned range, it will not cause excessive formation of austenite phase at high temperature, and will not cause excessive increase in the strength of the steel at room temperature, which is beneficial to free cutting and workability.

In terms of the metallographic structure, the corrosion-resistant free-cutting steel material of the present invention contains manganese sulfide (MnS) inclusions and boron nitride (BN) inclusions. In some embodiments, the constituent elements of the manganese sulfide inclusions include sulfur, manganese, and the first constituent element (X ₁ ), where X ₁ may be, for example, chromium, which is generally _{represented by MnSX 1} (also referred to as the first inclusion). In addition, the aforementioned boron nitride (BN) inclusions are also referred to as second inclusions hereinafter.

In some embodiments of the foregoing first inclusions, the constituent elements of the first inclusions selectively include the first constituent element (X ₁ ) and the second constituent element (X ₂ ), generally represented by MnSX ₁ X ₂ (also known as Is the third inclusion). In some specific examples, the second constituent element (X ₂ ) is at least one member selected from a group consisting of boron, nitrogen, oxygen, and calcium. In other specific examples, the second constituent element (X ₂ ) is at least one member selected from a group consisting of boron, nitrogen, and calcium.

In some embodiments of the foregoing second inclusions, in addition to nitrogen and boron, the constituent elements of the boron nitride inclusions selectively include a third constituent element (Y ₁ ), generally _{represented by BNY 1} , where the third constituent element (Y ₁ ) is calcium or chromium. The constituent elements of the second inclusion optionally include the third constituent element (Y ₁ ) and the fourth constituent element (Y ₂ ), generally represented by BNY ₁ Y ₂ , where the fourth constituent element (Y ₂ ) is selected from At least one member of a group consisting of manganese, sulfur, and oxygen. In other specific examples, the fourth constituent element (Y ₂ ) is at least one member selected from a group consisting of manganese and sulfur. In other embodiments, the corrosion-resistant free-cutting steel may include both the first inclusion and the second inclusion, so that the steel has better corrosion resistance and free-cutting properties.

The metallographic structure of the corrosion-resistant free-cutting steel can be inspected by the inspection method commonly used by those with ordinary knowledge in the technical field of the present invention to observe the metallographic structure of the steel and measure the types, constituent elements and numbers of inclusions. For example, using an optical microscope to observe the cross-section of steel, you can evaluate the grain size of the metallographic structure and the types of inclusions, or use an electron microscope for elemental analysis to obtain the constituent elements and number of inclusions.

The chromium element in the composition of the first inclusion can further improve the corrosion resistance of the steel and reduce the occurrence of pitting corrosion, so there is no specific limit on the chromium content. However, in some specific examples, based on the total weight of the first inclusion composition is 100 weight percent, the chromium content of the first inclusion composition is 10 weight percent to 60 weight percent, preferably 25 weight percent to 57 weight percent , And more preferably 37 weight percent to 54 weight percent.

It is said that the calcium, boron and nitrogen elements in the first inclusion composition can further improve the corrosion resistance, free cutting performance, dimensional accuracy of the steel and reduce the surface roughness after turning.

Preferably, based on the number of corrosion-resistant free-cutting steels detected is 100, the number of first inclusions detected is at least 10, and the following can also be expressed as a percentage of the number. When the number of detected first inclusions satisfies the aforementioned range, the corrosion resistance of the prepared steel material is better.

Compared with simple (without other elements) manganese sulfide inclusions, the second inclusions have better corrosion resistance and have a graphite-like structure, which can further improve the speed of steel cutting and the dimensional precision after processing . In some specific examples, based on the number of corrosion-resistant free-cutting steels detected is 100, and the number of second inclusions detected is at least 15, the following can also be expressed as a percentage of the number. When the number of second inclusions detected in the corrosion-resistant free-cutting steel material satisfies the aforementioned range, the corrosion resistance and free-cutting performance of the steel material can be further improved.

In some embodiments, based on the total weight of the calcium-containing boron nitride inclusion composition is 100 weight percent, the calcium content of the calcium-containing boron nitride inclusion composition is 0.05 weight percent to 1.5 weight percent. When the calcium content of the calcium-containing boron nitride inclusions is in the aforementioned range, the corrosion resistance and/or free-cutting properties of the steel material can be further improved.

In some embodiments, based on the total weight of the chromium-containing boron nitride inclusion composition is 100 weight percent, the chromium content in the chromium-containing boron nitride inclusion composition is 10 weight percent to 60 weight percent. When the chromium content of the chromium-containing boron nitride inclusions is in the aforementioned range, the corrosion resistance and/or free-cutting properties of the steel material can be further improved.

The following examples are used to illustrate the application of the present invention, but they are not intended to limit the present invention. Anyone familiar with the art can make various changes and modifications without departing from the spirit and scope of the present invention.

Steel manufacturing

Example 1

The steel material of Example 1 is an ingot steel material obtained by melting and solidifying in a vacuum induction furnace according to the composition shown in Table 1. The steel of Example 1 was subjected to various tests according to the following evaluation methods, and its constituent elements, relationships, and test results are shown in Tables 1 to 4.

Examples 2 to 9 and Comparative Examples 1 to 35

Examples 2 to 9 and Comparative Examples 1 to 35 were all manufactured by the same method as Example 1. The difference is that Examples 2 to 9 and Comparative Examples 1 to 35 change the element composition, and the detailed composition elements, relationships and test results are shown in Tables 1 to 2 respectively.

Furthermore, the steel material of Example 1 was analyzed for the content and number of constituent elements of the following inclusions, and the results are shown in Tables 3 to 4, respectively. In addition, the following observation of the distribution of inclusions was performed on the steel material of Example 1, and the results are shown in FIG. 1.

Evaluation method

1. Corrosion resistance test

This evaluation method is to closely fit the round bar test piece (about 30mm in diameter) made by processing the steel materials of Examples 1 to 9 and Comparative Examples 1 to 35 and a copper sheet [about 10mm×15mm (length×width)] After that, it was placed in a closed container with 2ml of pure water in it, and the temperature was maintained at 85°C. The above-mentioned bonded test piece is placed in the water in the container with the copper piece facing up, and the height of the water surface is based on the principle that it does not touch the copper piece (that is, the water does not touch the copper piece). After 20 hours, use it Energy Dispersive Spectroscopy (EDS) measures any five positions (not repeated) on the copper sheet of the bonded test piece, and evaluates the corrosion resistance of the steel according to the number of positions that contain sulfur. Corrosion includes consideration of pitting corrosion, galvanic corrosion and outgassing corrosion, and the specific evaluation criteria are as follows:

◎: The number of positions where sulfur is measured = 0,

△: The number of positions where sulfur is measured = 1,

×: The number of positions where sulfur is measured ≧2.

2. Free cutting test

This evaluation method is to perform outer peripheral surface turning on round bars (about 30 mm in diameter) prepared after processing the steel materials of Examples 1 to 9 and Comparative Examples 1 to 35. The material of the tool used for turning is tungsten carbide, and the turning is performed under the conditions of a rotation speed of 1800 rpm, a feed rate of 0.25 mm/rev, and a turning speed of 187/m/min. According to the degree of wear of the tool, the fast cutting performance of steel is evaluated, and the specific evaluation criteria are as follows:

◎: Under the optical microscope, no tool wear is observed.

△: Slight tool wear is observed under an optical microscope,

×: Under an optical microscope, severe tool wear is observed.

3. Analysis of the element content and number of inclusions

An energy scattering spectrometer was used to perform elemental analysis of the inclusion composition of the steel in Example 1. The conditions used were those commonly used by those with ordinary knowledge in the technical field of the present invention. The content and number of the elements of the inclusion composition were measured, and the results showed In Tables 3 and 4.

4. Observation of the distribution of inclusions

Use an optical microscope to observe the transverse and longitudinal cross-sections of the steel material of Example 1. Observe the distribution of inclusions on the surface, the radius of 1/2 from the surface and the observation position of the center point, and take pictures. The photo is shown in Figure 1. Show.

Table 1

Table 1 (continued)

Table 2

Table 2 (continued)

table 3

"Wt%" means weight percentage.

"-" means that it is below the detection limit and this element has not been detected.

"N" means that this element has not been measured.

Table 4

Please refer to Table 1 and Table 2. According to the results of corrosion resistance and free-cutting properties, compared with each comparative example, the steel of each example has specific carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen and boron The weight percentage satisfies the aforementioned formulas (I) to (V), and has both better corrosion resistance and free-cutting properties.

Please refer to Table 3. According to the results of element content, the steel of Example 1 contains manganese sulfide inclusions and boron nitride inclusions. The manganese sulfide inclusions at the first and second detection positions belong to MnSX ₁ X ₂ inclusions, and their constituent elements include nitrogen and calcium. The manganese sulfide inclusions at the third and fourth detection positions belong to MnS inclusions, and their constituent elements include at least one of boron, nitrogen, and calcium. _{Furthermore, the chromium content of the MnSX 1} X ₂ inclusions at the first and second detection positions is 38.9 wt% and 53.6 wt%, respectively. Due to the inclusion of the aforementioned constituent elements, the steel can have both better corrosion resistance and free-cutting properties, and the aforementioned content of chromium can further improve the corrosion resistance and free-cutting properties of the steel.

The boron nitride inclusions at the fifth to eighth and eleventh detection positions belong to BNY ₁ Y ₂ inclusions, and their constituent elements include at least one of manganese, sulfur, calcium and chromium, and the calcium content of the composition is 0.1 Weight percentage to 1.2 weight percentage. In addition, the chromium content of the boron nitride inclusions at the fifth and sixth detection positions is 57.7 weight percent and 56.3 weight percent, respectively. In addition, the boron nitride inclusions at the ninth to tenth detection positions are BN inclusions, and their constituent elements include manganese and sulfur. Due to the inclusion of the aforementioned constituent elements, the steel can have both better corrosion resistance and free-cutting properties, and the aforementioned content of calcium and chromium elements can further improve the corrosion resistance and free-cutting properties of the steel.

Please refer to Table 4. According to the analysis results of the number of _{inclusions, the percentage of the number of MnSX 1} X ₂ inclusions in the first and second detection positions of the steel of Example 1 are 59.8% and 15.6%, respectively, and the fifth detection The percentage of the number of BNY ₁ Y _{2 inclusions in the measured position is 18.2%.} Due to the inclusion of these percentages of the aforementioned inclusions, the steel can have both better corrosion resistance and free-cutting properties.

Please refer to FIG. 1, which is an optical microscope photograph showing the inclusions in the transverse and longitudinal cross-sections of the steel material of Example 1 of the present invention. The inclusions in the steel of Example 1 are evenly distributed, so the steel has better corrosion resistance and free cutting properties.

To sum up, the corrosion-resistant free-cutting steel of the present invention is controlled by the specific carbon, silicon, manganese, sulfur, chromium, molybdenum, nitrogen and boron of ferritic stainless steel with both corrosion resistance and free-cutting properties. The weight percentage and meet the specific relationship, so that the steel has both good corrosion resistance and fast cutting properties.

Although the present invention has been disclosed in the above embodiments, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field of the present invention can make various modifications without departing from the spirit and scope of the present invention. Movement and modification, therefore, the protection scope of the present invention should be subject to the scope defined by the appended claims.

Claims

A corrosion-resistant free-cutting steel, which is characterized in that it contains:

Carbon equal to or less than 0.06 weight percent;

0.01 weight percent to 1.3 weight percent of silicon;

0.1% to 0.5% by weight of manganese;

More than 0.2 weight percent to 0.4 weight percent of sulfur;

17% to 22% by weight of chromium;

0.01 wt% to 0.5 wt% of molybdenum;

0.001 weight percent to 0.035 weight percent nitrogen;

0.002 weight percent to 0.02 weight percent of boron;

Excess iron; and

Inevitable impurities; and

The corrosion-resistant free-cutting steel meets the following formulas (I) to (IV):

2[B]－[N]+[Mn]－[S]≦0 (I);

1.2≦[N][Mn]/[B][S]≦4 (II);

[Cr]+3.3[Mo]+16[N]≧18 (III); and

Wherein [B], [N], [Mn], [S], [Cr] and [Mo] represent the weight percentages of boron, nitrogen, manganese, sulfur, chromium and molybdenum, respectively.
The corrosion-resistant free-cutting steel according to claim 1, wherein the corrosion-resistant free-cutting steel also satisfies the following formula (V):

Wherein [B] and [N] represent the weight percentages of boron and nitrogen, respectively.
The corrosion-resistant free-cutting steel material according to claim 1, wherein the corrosion-resistant free-cutting steel material further comprises 0.01 wt% to 0.5 wt% copper.
The corrosion-resistant free-cutting steel material according to claim 1, wherein the corrosion-resistant free-cutting steel material further contains 0.001 wt% to 0.02 wt% calcium.
4. The corrosion-resistant free-cutting steel material of claim 4, wherein the corrosion-resistant free-cutting steel material comprises a first inclusion and a second inclusion.
The corrosion-resistant free-cutting steel according to claim 5, wherein the first inclusion is represented by MnSX 1 , the X 1 represents a first constituent element, and the first constituent element is chromium.
The corrosion-resistant free-cutting steel according to claim 5, wherein a total weight based on a composition of the first inclusion is 100 weight percent, and a chromium content of the composition of the first inclusion is 10 weight. Percent to 60% by weight.
The corrosion-resistant free-cutting steel according to claim 6, wherein the first inclusion further contains a second constituent element (X 2 ), represented by MnSX 1 X 2 (also called the third inclusion), And the second constituent element is at least one selected from a group consisting of boron, nitrogen and calcium.
The corrosion-resistant free-cutting steel according to claim 5, wherein the second inclusion is represented by BN, the second inclusion further includes a third constituent element (Y 1 ), represented by BNY 1 , and the The third constituent element is calcium or chromium.
The corrosion-resistant free-cutting steel according to claim 9, wherein the second inclusion further contains a fourth element (Y 2 ), represented by BNY 1 Y 2 , and the fourth element is selected from At least one of a group consisting of manganese and sulfur.