EP4596736A1

EP4596736A1 - Martensitic free-cutting stainless steel bar material and method for producing same

Info

Publication number: EP4596736A1
Application number: EP23871654.2A
Authority: EP
Inventors: Yoshihiro Oka; Kohji Takano; Masayuki Tendo; Toshiharu Aiso
Original assignee: Nippon Steel Stainless Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-09-26
Filing date: 2023-08-29
Publication date: 2025-08-06
Also published as: EP4596736A4; TWI891056B; JPWO2024070413A1; JP7791491B2; TW202449187A; KR20250029956A; CN119654434A; WO2024070413A1

Abstract

A martensitic free-machining stainless bar-shaped steel product having a chemical composition including C: 0.10 to 0.60%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, and a balance consisting of Fe and impurities is provided, where an average equivalent circle diameter of Cr carbide is 0.80 µm or less and a number density of the Cr carbide is 0.10 pieces/µm² or more. More preferably, an average equivalent circle diameter of sulfide is 5.0 µm or less and a number density of the sulfide is 0.010 pieces /µm² or more. Accordingly, excellent tool's life span during a precision machining process and a machined surface texture with excellent flatness after machining process can be provided.

Description

TECHNICAL FIELD

The present invention relates to a material machinable into a small component that requires corrosion resistance, more specifically relates to a martensitic free-machining stainless bar-shaped steel product that exhibits reduced tool abrasion and built-up edge and is excellent in machinability, and a manufacturing method thereof.

BACKGROUND ART

Highly corrosion-resistant martensitic stainless steel with a high hardness of 400 HV or more is, in view of its wear resistance, fatigue strength, and corrosion resistance, used for components of industrial machinery and precision equipment. Especially, components of precision equipment produced by machining cold-drawn bar made of a bar steel, which are often used for rotors, are required to be highly accurately machined. Specifically, the components of precision equipment are required to have tool abrasion as small as 200 µm or less during the machining process and to have flat and smooth surface with minute trace of built-up edge sparsely observed after being machined. The tool abrasion is promoted by hard precipitates and inclusions present in the machined material. The built-up edge, which is a deposit of a base material generated/grown by adhering on an edge of a cutting tool, falls off from the edge of the tool in the process of machining to be pressed and attached on the machined surface, thereby deteriorating the texture of the machined surface.
A martensitic free-machining stainless steel, which is an S-containing highly corrosion-resistant martensitic stainless steel with high hardness, has so far been proposed, where the composition and size of sulfide and the size of carbide have been specifically defined to improve wear resistance and machinability (see Patent Literature 1). However, the proposal, which focuses only on carbide of a very large size that is 10 µm² or more, cannot achieve satisfactory machining tool's life span and satisfactory machined surface texture.
Moreover, in a cold work tool steel with high cold workability is proposed a steel with improved cold workability and machinability by specifying an average equivalent circle diameter of carbide and cleanliness of inclusions (see Patent Literature 2). However, according to the proposal, the average equivalent circle diameter of carbide is defined to be in a range from 0.25 µm to 0.8 µm, thereby improving machinability and cold workability with low hardness. It is not described that coarse carbide with the average equivalent circle diameter of more than 0.8 µm promotes tool abrasion to lower machinability. In contrast, it is taught that the average equivalent circle diameter of carbide of less than 0.25 µm results in high hardness and no improvement in machinability. Further, nothing is disclosed on the machined surface texture.
Further, in an S-containing martensitic stainless free-machining steel, there has been proposed a steel containing B and N and a specified size of a BN intermetallic compound or a steel having an excellent surface texture by decreasing an aspect ratio of inclusions in the steel to restrain formation of the built-up edge (see
Patent Literatures 3, 4). However, nothing is disclosed on the tool's life span.

CITATION LIST

PATENT LITERATURE(S)

Patent Literature 1: JP 2015-137381 A
Patent Literature 2: JP 2006-193790 A
Patent Literature 3: JP 2013-185195 A
Patent Literature 4: International Publication No. WO2019/240209

SUMMARY OF THE INVENTION

PROBLEM(S) TO BE SOLVED BY THE INVENTION

The present inventors have found that the known techniques disclosed in the above "BACKGROUND ART" section or a combination thereof cannot achieve a highly corrosion-resistant martensitic free-machining stainless steel with high hardness, which is provided with excellent machinability and can restrain the tool abrasion in a machining process at 200 µm or less to achieve excellent tool's life span, and, further, cannot simultaneously achieve the tool's life span and a flat and smooth surface texture with the trace of built-up edge being restrained.
An object to be solved by the invention is to provide a highly corrosion-resistant martensitic free-machining stainless bar-shaped steel product with high hardness, which is usable for precision components made of a martensitic stainless steel to be used under a severely corrosive environment, is excellent in tool's life span in a machining process, and can provide a machined surface texture suitably with excellent flatness by a machining process, and a manufacturing method thereof.

MEANS FOR SOLVING THE PROBLEM(S)

In order to solve the above problems, the inventors have made various studies on the S-containing martensitic free-machining stainless steel capable of achieving high hardness of 400 HV or more through a quenching process. As a result, it is found that an excellent tool's life span during a precision machining process can be achieved by controlling an average equivalent circle diameter and a number density of Cr carbide and, preferably, a surface texture with excellent flatness can be provided by controlling an average equivalent circle diameter and a number density of sulfide to restrain the trace of built-up edge.
A summary of the invention, which is made based on the above findings, is as follows.

[1] A martensitic free-machining stainless bar-shaped steel product having a chemical composition includes, in mass%: C: 0.10 to 0.60%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, and a balance consisting of Fe and impurities, in which an average equivalent circle diameter of Cr carbide is 0.80 µm or less and a number density of the Cr carbide is 0.10 pieces/µm² or more.
[2] A martensitic free-machining stainless bar-shaped steel product having a chemical composition includes, in mass%: C: 0.10 to 0.60%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, and a balance consisting of Fe and impurities, in which an average equivalent circle diameter of Cr carbide is 0.80 µm or less, a number density of the Cr carbide is 0.10 pieces/µm² or more, an average equivalent circle diameter of sulfide is 5.0 µm or less, and a number density of the sulfide is 0.010 pieces/µm² or more.
[3] The martensitic free-machining stainless bar-shaped steel product according to [1] or [2], in which the chemical composition of the steel further includes, in place of a part of the Fe, in mass%, one or more of elements selected from the group consisting of: B: 0.01% or less, N: 0.15% or less, Al: 0.008% or less, O: 0.015% or less, Ni: 1.5% or less, Cu: 1.5% or less, Co: 1.5% or less, Mo: 2.5% or less, W: 2.5% or less, Bi: 0.2% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ag: 0.3% or less, Te: 0.1% or less, V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, Ta: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, Hf: 0.01% or less, and REM: 0.05% or less.
[4] A manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to [1] or [2] includes: hot working the steel at a finish rolling temperature ranging from 800 to 1150 degrees C; after the hot working, subjecting the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel at a rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C or subjecting the steel to the batch annealing and strand annealing for retaining the steel at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
[5] The manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to [4] further includes: cold working the steel at a reduction rate ranging from 20.0 to 99.0%.
[6] A manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to [3] includes: hot working the steel at a finish rolling temperature ranging from 800 to 1150 degrees C; after the hot working, subjecting the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel at a rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C or subjecting the steel to the batch annealing and strand annealing for retaining the steel at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
[7] The manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to [6] further includes: cold working the steel at a reduction rate ranging from 20.0 to 99.0%.

According to the above aspects of the invention, there are provided a martensitic free-machining stainless bar-shaped steel product capable of providing excellent machining tool's life span and an excellently flat machined surface texture with restrained trace of built-up edge, which is usable as a material suitable for precision components with high hardness and excellent corrosion resistance; and a manufacturing method thereof.

DESCRIPTION OF EMBODIMENT(S)

Requirements of the invention will be described below. It should be noted that the percentage (%) in the description below is, unless otherwise specified, mass%.

Requisite Composition Elements

Components with 400 HV or more hardness, which are generally effective in ensuring wear resistance and are targets to be achieved by the invention, are based on a martensitic free-machining stainless steel exhibiting high hardness of at least 400 HV or more after being quenched.
A C content is 0.10% or more in order to achieve the hardness of 400 HV or more after a base material is quenched. However, when the C content exceeds 0.60%, an average equivalent circle diameter of Cr carbide exceeds 0.8 µm to lower a machining tool's life span. Accordingly, the C content is limited to 0.60% or less. Preferably, the C content is in a range from 0.20 to 0.50%.
Si, which is a deoxidizing element for restraining generation of coarse inclusions that decrease the machining tool's life span, is contained at a content of 0.1% or more. However, the Si content exceeding 2.0% hardens the steel and decreases the machining tool's life span and, in addition, promotes adhesion of the base material on the tool to assist generation of the trace of built-up edge. Accordingly, the Si content is limited to 2.0% or less. Preferably, the Si content is in a range from 0.2 to 1.0%.
Mn, which is a deoxidizing element for restraining generation of coarse inclusions that decrease the machining tool's life span and forms a sulfide to ensure an excellent machined surface texture, is contained at a content of 0.1% or more. However, the Mn content exceeding 3.0% hardens the steel and decreases the machining tool's life span and, in addition, exhibits an average sulfide equivalent circle diameter exceeding 5 µm to deteriorate the machined surface texture. Accordingly, the Mn content is limited to 3.0% or less. Preferably, the Mn content is in a range from 0.2 to 2.0%.
S, which forms a sulfide to ensure excellent machining tool's life span and machined surface texture, is contained at a content of 0.15% or more. However, the S content exceeding 0.40% hardens the steel and decreases the machining tool's life span and, in addition, exhibits an average equivalent circle diameter exceeding 5 µm to deteriorate the machined surface texture. Accordingly, the S content is limited to 0.40% or less. Preferably, the S content is in a range from 0.17 to 0.35%.
P is an element mixed in a form of inevitable impurities from a source material. The P content exceeding 0.10% not only lowers the corrosion resistance by grain boundary segregation but also significantly lowers the productivity. Accordingly, the P content is limited to 0.10% or less. Preferably, the P content is 0.05% or less.
Cr, which is a fundamental element for providing the corrosion resistance to a stainless steel, is contained at a content of 11.0% or more. However, the hardness of 400 HV or more cannot be achieved after quenching at a Cr content of more than 16.0%. Accordingly, the Cr content is limited to 16.0% or less. Preferably, the Cr content is in a range from 11.5 to 15.0%.

Equivalent Circle Diameter and Number Density of Cr Carbide and Equivalent Circle Diameter and Number Density of Sulfide

Cr carbide, which has high hardness, causes abrasive wear on the tool during a machining process to lower the tool's life span. The influence becomes more eminent as the Cr carbide increases in size and decreases in number density. Accordingly, the average equivalent circle diameter of the Cr carbide is 0.80 µm or less and the number density of the Cr carbide is 0.10 pieces/µm² or more. Preferably, the average equivalent circle diameter of the Cr carbide is 0.70 µm or less and the number density of the Cr carbide is 0.20 pieces/µm² or more.
Sulfide, which creates start points for separating chips generated during the machining process, is effective in restraining formation of the built-up edge to form an excellent surface texture. The influence becomes more eminent as the sulfide decreases in size and increases in number density. Accordingly, the average equivalent circle diameter of the sulfide is preferably limited to 5.0 µm or less and the number density of the sulfide is preferably limited to 0.010 pieces/µm² or more. More preferably, the average equivalent circle diameter of the sulfide is 3.0 µm or less and the number density of the sulfide is 0.050 pieces/µm² or more.

Optional Composition Elements

The stainless steel of the invention contains chemical components consisting of Fe and impurities in addition to the above-described elements. In addition, below-listed element(s) is optionally contained in addition to the above components in place of a part of Fe.
B, which forms minute BN when being contained together with N to restrain the base material from being attached on the surface of the tool and to prevent the trace of built-up edge on the machined surface, is optionally contained as necessary. However, at a B content exceeding 0.01%, coarse boride is generated to lower the machining tool's life span and promote generation of the built-up edge. Accordingly, the B content is limited to 0.01% or less. In order to securely achieve the above advantages, the B content is preferably in a range from 0.002% to 0.007%.
N, which not only forms minute BN to prevent the trace of built-up edge on the machined surface but also increases the hardness after the base material is quenched, is optionally contained as necessary. However, at the N content exceeding 0.15%, coarse nitride is formed to lower the machining tool's life span. Accordingly, the N content is 0.15% or less. In order to securely achieve the above advantages, the N content is preferably in a range from 0.03% to 0.10%.
Al is optionally contained for deoxidation. However, when the Al content exceeds 0.008%, coarse inclusions are formed to lower the machining tool's life span. Accordingly, the Al content is limited to 0.008% or less. Preferably, the Al content is 0.006% or less.
O, which is mixed in a form of inevitable impurities, creates coarse inclusions to lower the machining tool's life span at a content exceeding 0.015%. Accordingly, the O content is limited to 0.015% or less. Preferably, the O content is 0.012% or less.
Ni, Cu, and Co, which improve corrosion resistance and toughness of products, are optionally contained as necessary. However, when a content of each of the above elements exceeds 1.5%, the machining tool's life span is lowered. Accordingly, the content of each of the elements is limited to 1.5% or less. In order to securely achieve the above advantages, the content of each of the elements is preferably in a range from 0.01% to 1.0%.
Mo and W, which improve corrosion resistance of products, are optionally contained as necessary. However, when a content of each of the above elements exceeds 2.5%, the effect is saturated and the machining tool's life span is lowered. Accordingly, the content of each of the elements is limited to 2.5% or less. In order to securely achieve the above advantages, the content of each of the elements is preferably in a range from 0.01% to 2.0%.
Bi, which exhibits self-lubricity during a machining process to restrain generation of the built-up edge to improve the machined surface texture, is optionally contained as necessary. However, at a content exceeding 0.2%, hot workability is too lowered to produce any products. Accordingly, the Bi content is limited to 0.2% or less. In order to securely achieve the above advantages, the Bi content is preferably in a range from 0.005% to 0.10%.
Sn, Sb, and Ag, which exhibit self-lubricity during a machining process to restrain generation of the built-up edge to improve the machined surface texture, are optionally contained as necessary. However, when a content of each of the above elements exceeds 0.3%, hot workability is too lowered to produce any products. Accordingly, the content of each of the elements is limited to 0.3% or less. In order to securely achieve the above advantages, the content of each of the elements is preferably in a range from 0.005% to 0.20%.
Te, which generates spherical sulfide to restrain lamination and growth of the built-up edge to improve the machined surface texture, is optionally contained as necessary. However, at a content exceeding 0.1%, hot workability is too lowered to produce any products. Accordingly, the Te content is limited to 0.1% or less. In order to securely achieve the above advantages, the Te content is preferably in a range from 0.005% to 0.05%.
V, which improves corrosion resistance of products, are optionally contained as necessary. However, at the V content exceeding 0.8%, coarse carbonitride is formed to lower the machining tool's life span. Accordingly, the V content is limited to 0.8% or less. In order to securely achieve the above advantages, the V content is preferably in a range from 0.05% to 0.5%.
Nb, Ti, and Ta, which improve corrosion resistance of products, are optionally contained as necessary. However, when a content of each of the above elements exceeds 0.3%, coarse carbonitride is formed to lower the machining tool's life span. Accordingly, the content of each of the elements is limited to 0.3% or less. In order to securely achieve the above advantages, the content of each of the elements is preferably in a range from 0.01% to 0.2%.
Mg, Ca, and Hf, which are effective in improving hot workability, are optionally contained as necessary. However, when a content of each of the above elements exceeds 0.01%, its advantageous effect is saturated and coarse oxide is formed to lower the machining tool's life span. Accordingly, the content of each of the elements is limited to 0.01% or less. In order to securely achieve the above advantages, the content of each of the elements is preferably in a range from 0.001 % to 0.005%.
REM, which is effective in improving hot workability, is optionally contained as necessary. However, when a content of REM exceeds 0.05%, its advantageous effect is saturated and coarse oxide is formed to lower the machining tool's life span. Accordingly, the REM content is limited to 0.05% or less. In order to securely achieve the above advantages, the REM content is preferably in a range from 0.001 % to 0.005%. According to a normal definition, REM (Rare Earth Metal) is a generic term for two elements of scandium (Sc) and yttrium (Y) and fifteen elements (lanthanoids) from lanthanum (La) to lutetium (Lu). One of the REM may be solely contained or a mixture of two or more of the REM may be contained.
Typical examples of impurities include Zn, Pb, Ge, and Se, which are usually mixed in a range of about 0.1% as impurities in a steel manufacturing process.
Oxygen (one of the impurities), which is present in steel mainly in a form of inclusions, is contained in a stainless steel produced through a typical smelting process at a content ranging from 0.001 to 0.015%.
Typical optional composition elements are defined in the above [3]. However, element(s) not described herein is optionally contained within a range not impairing advantages of the invention.

Cr Carbide Control Method

A Cr carbide control method of the invention will be described below.
In order to obtain the average equivalent circle diameter of Cr carbide to 0.80 µm or less and to finely disperse the Cr carbide so that the number density becomes 0.10 pieces/µm² or more, it is preferable to subject the steel to hot working (e.g. hot rolling) at a finish rolling temperature ranging from 800 to 1150 degrees C, and, after the hot working, subject the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel to 600 degrees C at a rate ranging from 20 to 200 degrees C/h or more. It is also preferable to, in addition to the batch annealing under the same conditions as the above, subject the steel to strand annealing for retaining the steel at a temperature in a range from 700 to 850 degrees for 30 to 1000 seconds. The above range of manufacture conditions will be herein referred to as a suitable range.
At the finish rolling temperature during hot working of less than 800 degrees C, rolling load becomes so high that the workability is lowered. On the other hand, when the finish rolling temperature exceeds 1150 degrees C, the Cr carbide is coarsened to fail to satisfy the above-described conditions for the average equivalent circle diameter of the Cr carbide. Accordingly, the finish rolling temperature during hot working is defined to range from 800 to 1150 degrees C.
During the batch annealing performed after the hot working, when the batch annealing temperature is lower than 700 degrees C and a retention time of the batch annealing is shorter than 60 minutes, productivity is lowered due to insufficient annealing. On the other hand, when the batch annealing temperature is higher than 900 degrees C, the retention time of the batch annealing is longer than 300 minutes, and the cooling rate until the temperature reaches 600 degrees C after the retention is lower than 20 degrees C/h, Cr carbide is coarsened to fail to satisfy the above-described conditions for the equivalent circle diameter. It should be noted that, at a cooling rate higher than 200 degrees C/h, martensitic transformation occurs to harden the steel, whereby delayed fracture and the like easily occur. The cooling rate in a temperature range of less than 600 degrees C is not necessary to be specifically defined.
When the steel is subjected to the strand annealing in addition to the batch annealing after the hot working, the conditions for the batch annealing are the same as the above-described conditions for subjecting the steel only to the batch annealing. With regard to the strand annealing, when the strand annealing temperature is lower than 700 degrees C and a retention time of the strand annealing is shorter than 30 seconds, productivity is lowered due to insufficient annealing. On the other hand, when the temperature for the strand annealing is higher than 850 degrees C and the retention time for the strand annealing is longer than 1000 seconds, the Cr carbide is coarsened to fail to satisfy the above-described conditions for the equivalent circle diameter and, in addition, martensitic transformation occurs to harden the steel when the steel is cooled at a temperature exceeding the Ac1 temperature, whereby delayed fracture and the like easily occur.
As described above, it is specified that the finish rolling temperature during the hot working ranges from 800 to 1150 degrees C, the batch annealing is performed under the conditions that the retention is performed at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and the cooling is performed at the cooling rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C, and the strand annealing is performed under the conditions that the retention is performed at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
The average equivalent circle diameter of the Cr carbide tends to decrease and the number density of the Cr carbide tends to increase with the decreasing finish rolling temperature, the decreasing temperature for the batch annealing, the decreasing retention time for the batch annealing, the increasing cooling rate for the batch annealing after the retention, the decreasing temperature for the strand annealing, and the decreasing retention time for the strand annealing, within the above suitable range. Accordingly, the average equivalent circle diameter of the Cr carbide can be reliably reduced to 0.80 µm or less and the Cr carbide can be reliably finely dispersed so that the number density of the Cr carbide is 0.10 pieces/µm² or more by controlling the manufacture conditions.
It should be noted that the finish rolling temperature is preferably less than 1000 degrees C, more preferably less than 900 degrees C in order to finely disperse the Cr carbide. Preferably, the finish rolling temperature is 850 degrees C or more and less than 900 degrees C, the batch annealing is performed under the conditions that the retention is performed at a temperature ranging from 750 to 850 degrees C for 90 to 200 minutes and the cooling is performed at the cooling rate ranging from 30 to 100 degrees C/h until the temperature reaches 600 degrees C after the retention, and the strand annealing is performed under the conditions that the retention is performed at a temperature ranging from 750 to 825 degrees C for 40 to 800 seconds. Under the above manufacture conditions, the Cr carbide can be more reliably finely dispersed so that the average equivalent circle diameter of the Cr carbide is 0.80 µm or less and the number density of the Cr carbide is 0.10 pieces/µm² or more.
It should be noted that the strand annealing refers to a process of annealing where wire rods or a coil of steel wire wound in a ring-shape are linearly stretched to provide a single linear wire rod /steel wire and the stretched single linear wire rod /steel wire are subjected to a short-time thermal treatment (in an atmosphere of nitrogen, Ar, ammonia decomposition gas, or the like) to be air-cooled or indirectly water-cooled. The cooling rate can be significantly increased as compared with the batch annealing on the entirety of the ring-shaped coil.

Sulfide Control Method

A sulfide control method will be described below.
In order to finely disperse sulfide so that the average equivalent circle diameter of the sulfide is 5.0 µm or less and the number density of the sulfide is 0.010 pieces/µm² or more, the steel is subjected to cold working (e.g. cold drawing) at a reduction rate ranging from 20.0 to 99.0% after the hot working (e.g. hot rolling) and annealing as described in the above "Cr Carbide Control Method." The sulfide, which has been stretched through the hot rolling and annealing, is fragmented to be finely dispersed by the cold working (e.g. cold drawing). Accordingly, the reduction rate is defined to be 20.0% or more. However, at the reduction rate exceeding 99.0%, the base material is embrittled to impair the productivity. Accordingly, the reduction rate is defined to be 99.0% or less. Preferably, the reduction rate is in a range from 30.0 to 90.0%.
It should be noted that defects such as shrinkage cracks are sometimes caused due to work-hardening at a reduction rate of approximately 70% or more during the cold working (e.g. cold drawing). In order to prevent the occurrence of such defects, the cold working (e.g. cold drawing) and annealing are optionally repeated for plural times. In this case, the annealing performed between the cold working processes has to be done under the annealing conditions described in the above "Cr Carbide Control Method" in order to control the Cr carbide. It should also be noted that the reduction rate in the cold working (e.g. cold drawing) in the above case refers a sum of reduction rates of the cold working processes performed for plural times.
The above-described aspect of the invention can provide a martensitic free-machining stainless steel that provides excellent tool's life span during a machining process and excellent machined surface texture, as a stainless bar-shaped steel product for high-hardness components to be precisely machined.
It should be noted that the martensitic stainless steel of the invention refers to a steel hardened by martensitic transformation during a quenching process. In the invention, the martensitic stainless steel refers to, for instance, a steel whose 70% or more of micro structure exhibits a martensitic structure when being quenched by air-cooling from 1050 degrees C to be hardened to 400 HV or more.
The bar-shaped steel product herein includes "rod steel," "bar steel," "wire rod," "steel wire," "deformed wire," "deformed rod steel" and the like.

Examples

Example 1

In order to check the effects of the components, steels of chemical compositions shown in Tables 1 to 3 were melted in a vacuum smelting furnace of 45 kg at 1600 degrees C and then were each casted in a casting die. Subsequently, the steels were each heated to 1200 degrees C, followed by hot-rolling at a finish rolling temperature of 880 degrees C to be formed into a wire rod with a 5.5-mm diameter, and cooled to a room temperature. Subsequently, each of the steels was subjected to batch annealing under the conditions of the annealing temperature of 850 degrees C, the retention time of 180 minutes, and the cooling rate of 50 degrees C/h until the temperature reaches 600 degrees C after the retention. After the batch annealing, each of the steels was subjected to cold working (cold drawing) at a reduction rate of 66.1% to reduce the diameter to 3.2 mm. Subsequently, the wire rod was subjected to straightening and centerless polishing processes to prepare a cold-drawn bar (bar steel) with a 3.0-mm diameter, which was used as a material to be machined.

Table 1

Steel No.	Chemical Composition (mass%)
Steel No.	C	Si	Mn	P	S	Cr	Others
A	0.12	0.3	0.8	0.04	0.16	12.1
B	0.23	0.6	0.9	0.02	0.34	13.3
C	0.35	0.7	0.7	0.03	0.25	12.8
D	0.43	0.9	1.2	0.04	0.19	13.5
E	0.53	0.2	0.5	0.03	0.22	12.4
F	0.58	0.4	0.6	0.03	0.29	14.6
G	0.18	0.5	0.9	0.04	0.38	13.5
H	0.27	1.2	1.0	0.03	0.18	13.2
I	0.31	1.6	0.9	0.04	0.27	12.9
J	0.23	1.9	0.2	0.02	0.21	12.2
K	0.41	0.2	1.8	0.06	0.30	12.8
L	0.37	0.6	2.9	0.03	0.26	13.0
M	0.46	0.5	1.6	0.06	0.24	14.2
N	0.21	0.5	1.3	0.09	0.22	13.6
O	0.29	0.3	0.5	0.03	0.18	11.2
P	0.19	0.2	0.9	0.05	0.20	11.5
Q	0.33	0.2	0.7	0.07	0.27	11.9
R	0.40	0.3	0.7	0.05	0.28	14.6
S	0.36	0.4	0.5	0.04	0.16	15.7
T	0.48	0.7	0.6	0.03	0.17	14.2
U	0.35	0.8	1.0	0.04	0.18	12.8
V	0.25	0.9	0.4	0.04	0.38	13.6
W	0.27	0.7	1.3	0.06	0.36	13.0

Table 2

Steel No.	Chemical Composition (mass%)
Steel No.	C	Si	Mn	P	S	Cr	Others
X	0.21	0.2	1.2	0.04	0.32	12.1	B:0.009
Y	0.20	0.2	1.2	0.03	0.31	12.2	B:0.005
Z	0.20	0.5	0.9	0.03	0.28	12.8	N:0.14
AA	0.21	0.5	0.9	0.04	0.29	12.8	N:0.08
AB	0.30	0.5	0.8	0.03	0.22	14.1	Al:0.007
AC	0.30	0.5	0.8	0.02	0.23	14.0	Al:0.005
AD	0.25	0.3	1.1	0.02	0.18	13.1	O:0.014
AE	0.26	0.3	1.1	0.03	0.19	13.1	O:0.011
AF	0.36	0.3	0.6	0.04	0.19	13.3	Ni:1.3
AG	0.35	0.4	0.6	0.04	0.18	13.4	Ni:0.8
AH	0.33	0.5	0.5	0.03	0.25	13.2	Cu:1.4
AI	0.34	0.5	0.6	0.04	0.24	13.1	Cu:0.9
AJ	0.45	0.2	0.5	0.04	0.27	12.4	Co:1.3
AK	0.44	0.2	0.5	0.03	0.28	12.3	Co:0.9
AL	0.44	0.2	0.9	0.04	0.24	13.5	Mo:2.3
AM	0.43	0.2	0.8	0.04	0.25	13.6	Mo:1.8
AN	0.48	0.6	1.3	0.03	0.20	14.2	W:2.4
AO	0.47	0.5	1.3	0.03	0.21	14.1	W:1.9
AP	0.39	0.6	0.9	0.05	0.18	12.8	Bi:0.1
AQ	0.38	0.8	0.8	0.03	0.22	14.4	Sn:0.2
AR	0.34	0.9	1.4	0.03	0.28	12.8	Sb:0.2
AS	0.25	0.9	1.4	0.04	0.30	13.1	Ag:0.2
AT	0.26	0.7	1.2	0.04	0.31	13.5	Te:0.05
AU	0.24	0.7	1.8	0.03	0.34	13.4	V:0.7
AV	0.24	0.8	1.7	0.03	0.35	13.3	V:0.4
AW	0.23	0.5	0.6	0.02	0.27	13.6	Nb:0.2
AX	0.24	0.5	0.6	0.03	0.27	13.6	Nb:0.1
AY	0.22	0.4	0.9	0.04	0.36	12.8	Ti:0.2
AZ	0.22	0.4	0.9	0.05	0.36	12.7	Ti:0.1
BA	0.29	0.3	0.5	0.03	0.21	12.1	Ta:0.2
BB	0.34	0.4	0.4	0.02	0.20	13.9	Mg:0.004
BC	0.30	0.2	0.6	0.02	0.19	13.8	Ca:0.005
BD	0.38	0.3	0.8	0.03	0.18	12.4	Hf:0.005
BE	0.40	0.2	0.7	0.04	0.20	12.9	REM:0.005

Table 3

Steel No.	Chemical Composition (mass%)
Steel No.	C	Si	Mn	P	S	Cr	Others
BF	0.09	0.4	1.1	0.03	0.23	12.2
BG	0.63	0.1	0.7	0.03	0.16	12.7
BH	0.35	0.07	0.7	0.04	0.19	13.2
BI	0.25	2.3	1.3	0.02	0.24	13.8
BJ	0.27	0.5	0.06	0.02	0.27	14.1
BK	0.45	0.2	3.3	0.04	0.30	14.8
BL	0.42	0.2	0.5	0.02	0.11	15.2
BM	0.37	0.3	0.5	0.04	0.44	11.2
BN	0.23	0.3	0.3	0.12	0.35	11.7
BO	0.21	0.4	0.3	0.04	0.38	10.3
BP	0.26	0.6	0.6	0.05	0.18	16.3
BQ	0.34	0.7	0.6	0.05	0.20	12.0	B:0.02
BR	0.34	0.8	0.8	0.02	0.27	12.6	N:0.18
BS	0.30	0.7	0.7	0.03	0.25	12.6	Al:0.011
BT	0.33	0.1	0.8	0.03	0.30	13.2	O:0.019
BU	0.32	0.1	0.9	0.03	0.32	13.8	Ni:1.7
BV	0.21	0.2	0.9	0.04	0.35	14.9	Cu:1.8
BW	0.29	1.1	1.1	0.02	0.17	15.3	Co:1.9
BX	0.27	1.3	1.2	0.03	0.22	12.1	Mo:2.8
BY	0.25	1.5	1.3	0.04	0.23	11.3	W:2.6
BZ	0.31	1.7	1.5	0.03	0.25	11.7	Bi:0.3
CA	0.31	0.5	0.8	0.05	0.28	13.3	Sn:0.4
CB	0.39	0.4	0.8	0.04	0.29	13.2	Sb:0.5
CC	0.27	0.3	0.6	0.03	0.32	12.5	Ag:0.4
CD	0.28	0.4	0.6	0.03	0.24	12.8	Te:0.2
CE	0.38	0.3	0.4	0.03	0.36	12.4	V:0.9
CF	0.27	0.1	0.4	0.04	0.37	13.5	Nb:0.5
CG	0.26	0.1	0.5	0.02	0.16	14.2	Ti:0.4
CH	0.23	0.2	0.5	0.04	0.17	12.9	Ta:0.5
CI	0.27	0.2	0.7	0.02	0.19	14.7	Mg:0.04
CJ	0.26	0.6	0.8	0.04	0.20	13.5	Ca:0.03
CK	0.24	0.6	0.7	0.03	0.22	12.4	Hf:0.02
CL	0.29	0.4	0.9	0.03	0.22	13.2	REM:0.07

Underlined parts indicate that the values are out of the range defined by the invention.

Thus prepared bar steels were evaluated in terms of the average equivalent circle diameter of Cr carbide, the number density (i.e. the number of pieces per unit area) of the Cr carbide, the average equivalent circle diameter of sulfide, the number density of the sulfide, the tool's life span after machining an outer circumferential portion, and a machined surface texture according to evaluation methods described below. The results are shown in Tables 4 to 6. Tables 4 and 5 each show evaluation results of Inventive Examples. Table 6 shows evaluation results of Comparative Examples.
Quench hardness was evaluated after quenching by air-cooling from 1050 degrees C, where 400 HV or more was evaluated to be satisfactory. Corrosion resistance was evaluated through an acetic acid salt spray test for 48 hours, where an instance without flow rust was evaluated to be satisfactory. When one of the quench hardness and corrosion resistance was out of the satisfactory range, a comment "poor quench hardness" or "poor corrosion resistance" was made in Remarks and no further evaluation was made. Further, when the steel was not producible for some reason(s), a comment "non-producible" was made and no evaluation was made on quality.

Average Equivalent Circle Diameter and Number Density of Cr Carbide

A test sample was embedded in a resin so that a vertical cross section of the bar steel was positioned to be an inspection surface. Then, after the inspection surface was mirror-polished and etched with aqua regia, five visual fields each with an area of 100 µm2 were observed using an SEM. Among the inclusions in each of the visual fields, the inclusions in which much Cr and C were detected by EDX analysis were identified as Cr carbide. The average equivalent circle diameter of Cr carbide was calculated by calculating the equivalent circle diameter of each of 100 or more Cr carbides and averaging the calculated equivalent circle diameters. The number density of Cr carbide was calculated by counting a total of the number of Cr carbide in the five visual fields and converting the number into the number of Cr carbide per 1 µm².

Average Equivalent Circle Diameter and Number Density of Sulfide

A test sample was embedded in a resin so that a vertical cross section of the bar steel was positioned to be an inspection surface. Then, after the inspection surface was mirror-polished, five visual fields each with an area of 3 mm² were observed using an SEM. Among the inclusions in each of the visual fields, the inclusions in which much S was detected by EDX analysis were identified as sulfide. The average equivalent circle diameter of the sulfide was calculated by calculating the equivalent circle diameters of 100 or more sulfides and averaging the calculated equivalent circle diameters. The number density of the sulfide was calculated by counting a total of the number of sulfides in the five visual fields and converting the number into the number of sulfide per 1 µm².

Machining Tool's Life Span

An outer circumferential portion of the bar steel was machined in a circumferential direction for an hour under the following precision machining conditions: Used tool: super hard P type, edge R: 0.03 mm, machining speed: 20 m/min, feed: 0.01 mm/rev, cutting depth: 0.1 mm, cutting oil (mineral oil): yes. The tool after the machining process was observed using an optical microscope of 200 times magnification. The tool's life span was evaluated to be "S" when a boundary wear width of the tool after the machining process was 100 µm or less, evaluated to be "G" when the boundary wear width was more than 100 µm and 200 µm or less, and evaluated to be "X" when the boundary wear width was more than 200 µm.

Machined Surface Texture

An outer circumferential portion of the bar steel was machined in a circumferential direction for an hour under the following precision machining conditions: Used tool: super hard P type, edge R: 0.03 mm, machining speed: 20 m/min, feed: 0.01 mm/rev, cutting depth: 0.1 mm, cutting oil (mineral oil): yes. A surface of the machined bar steel was observed using an optical microscope of 100 times magnification. The machined surface texture was evaluated to be "X" when a clear trace of built-up edge was observed on the surface, evaluated to be "G" when a minute trace of built-up edge was sparsely observed, and was evaluated to be "S" when no clear trace of built-up edge was observed.
All of Inventive Examples 1 to 57 in Tables 4 and 5, where the average equivalent circle diameter of Cr carbide was 0.80 µm or less, the number density of Cr carbide was 0.10 pieces/µm² or more, the average equivalent circle diameter of sulfide was 5.0 µm or less, and the number density of sulfide was 0.010 pieces /µm² or more, show excellent machining tool's life span and machined surface texture.
In contrast, Comparative Examples 1 to 33 shown in Table 6, whose compositions were out of the range defined by the invention, do not satisfy both of the excellent machining tool's life span and excellent machined surface texture. It should be noted that, even when the average equivalent circle diameter and the number density of sulfide were within the suitable range, the machined surface texture was deteriorated as the machining tool's life span decreases.

Example 2

In order to evaluate the effects of the manufacture process, the steel of the chemical composition shown as "Steel C" in Table 1 was melted in a vacuum smelting furnace of 45 kg at 1600 degrees C and was casted in a casting die. The manufacture conditions are shown in Table 7. After being heated to 1200 degrees C, the steel was hot-worked (hot-rolled) at a finish rolling temperature ranging from 760 to 1170 degrees C to be formed into a wire rod whose diameter ranges from 5.5 to 40.0 mm and then cooled to a room temperature. Subsequently, the steel was subjected solely to batch annealing or subjected to strand annealing in addition to batch annealing. During the batch annealing, the annealing temperature was in a range from 650 to 950 degrees C, the retention time was in a range from 40 to 360 minutes, and the cooling rate after the retention until the temperature reaches 600 degrees C was in a range from 10 to 250 degrees C/h. During the strand annealing (in an ammonia decomposition gas), the annealing temperature was in a range from 680 to 870 degrees C and the retention time was in a range from 20 to 1200 seconds. After completion of annealing, in Inventive Examples 59 to 67 and Comparative Examples 34 to 45 in Table 7, a cold-drawn bar (bar steel) with a diameter of 3.0 mm was produced by subjecting the steel to cold drawing at a reduction rate ranging from 19.0 to 99.4% and then subjecting the steel to straightening and centerless polishing processes, thereby preparing a material to be machined. When the steel showed great work-hardening due to high reduction rate, the material was prepared through repetition of annealing and drawing. In Inventive Example 58, the wire rod with a 5.5-diameter was not subjected to cold drawing but was subjected to centerless polishing into a cold-drawn bar (bar steel) with a 3.0-mm diameter, which was used as the material to be machined. Subsequently, the average equivalent circle diameter of Cr carbide, the number density of the Cr carbide, the average equivalent circle diameter of sulfide, the number density of the sulfide, the tool's life span after machining an outer circumferential portion, and a machined surface texture were evaluated according to the same evaluation methods as in the above Example 1. The results are shown in Table 7.
Inventive Examples 58, 59 shown in Table 7, which exhibit the average equivalent circle diameter of Cr carbide of 0.80 µm or less and the number density of the Cr carbide of 0.10 pieces/µm² or more, show excellent machining tool's life span. It should be noted that Inventive Example 58 in which no cold drawing was performed and Inventive Example 59 in which the reduction rate in cold drawing fell below the lower limit of the suitable range of the invention show that the average equivalent circle diameter of sulfide and the number density of the sulfide fell out of the suitable range, thus failing to provide satisfactory machined surface texture.
All of Inventive Examples 60 to 67 in Table 7, where the average equivalent circle diameter of Cr carbide was 0.80 µm or less, the number density of the Cr carbide was 0.10 pieces/µm² or more, the average equivalent circle diameter of sulfide was 5.0 µm or less, and the number density of the sulfide was 0.010 pieces /µm² or more, show excellent machining tool's life span and excellent machined surface texture.
In contrast, Comparative Examples 34 to 45 shown in Table 7, whose manufacture conditions fell out of the suitable conditions of the invention, were incapable of producing steel or, even when the steel was producible, showed the average equivalent circle diameter of Cr carbide and the number density of the Cr carbide out of the range defined by the invention. As a result, both of excellent machining tool's life span and excellent machined surface texture were not exhibited. Further, Comparative Example 46, in which the reduction rate during cold drawing exceeded the upper limit of the suitable range of the invention, did not produce a steel.

INDUSTRIAL APPLICABILITY

As is clear from the above Examples, the invention can provide a martensitic free-machining stainless steel with excellent tool's life span and, preferably, an excellent machined surface texture during a precision machining process, which can significantly improve durability of high-hardness components to be used in a highly corrosive environment requiring fatigue resistance and wear resistance, and thus is extremely industrially useful.

Claims

A martensitic free-machining stainless bar-shaped steel product having a chemical composition comprising, in mass%:
C: 0.10 to 0.60%, Si: 0.1 to 2.0%, Mn:0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, and a balance consisting of Fe and impurities, wherein
an average equivalent circle diameter of Cr carbide is 0.80 µm or less and a number density of the Cr carbide is 0.10 pieces/µm² or more.
A martensitic free-machining stainless bar-shaped steel product having a chemical composition comprising, in mass%:
C: 0.10 to 0.60%, Si: 0.1 to 2.0%, Mn:0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, and a balance consisting of Fe and impurities, wherein
an average equivalent circle diameter of Cr carbide is 0.80 µm or less, a number density of the Cr carbide is 0.10 pieces/µm² or more, an average equivalent circle diameter of sulfide is 5.0 µm or less, and a number density of the sulfide is 0.010 pieces/µm² or more.
The martensitic free-machining stainless bar-shaped steel product according to claim 1 or 2, wherein the chemical composition of the steel further comprises, in place of a part of the Fe, in mass%, one or more of elements selected from the group consisting of: B: 0.01% or less, N: 0.15% or less, Al: 0.008% or less, O: 0.015% or less, Ni: 1.5% or less, Cu: 1.5% or less, Co: 1.5% or less, Mo: 2.5% or less, W: 2.5% or less, Bi: 0.2% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ag: 0.3% or less, Te: 0.1% or less, V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, Ta: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, Hf: 0.01% or less, and REM: 0.05% or less.
A manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to claim 1 or 2, the method comprising:
hot working the steel at a finish rolling temperature ranging from 800 to 1150 degrees C;

after the hot working,
subjecting the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel at a rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C or

subjecting the steel to the batch annealing and strand annealing for retaining the steel at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
The manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to claim 4, further comprising: cold working the steel at a reduction rate ranging from 20.0 to 99.0%.
A manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to claim 3, the method comprising:
hot working the steel at a finish rolling temperature ranging from 800 to 1150 degrees C;

after the hot working,
subjecting the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel at a rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C or

subjecting the steel to the batch annealing and strand annealing for retaining the steel at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
The manufacturing method of the martensitic free-machining stainless bar-shaped steel product according to claim 6, further comprising: cold working the steel at a reduction rate ranging from 20.0 to 99.0%.