TW202405199A - High-strength stainless steel wire and spring including C, Si, Mn, P, S, Ni, Cr, N, Al, O, and Ca - Google Patents

Abstract

The invention relates to a high-strength stainless steel wire, which contains: C, Si, Mn, P, S, Ni, Cr, N, Al, O, Ca, and the total amount of Ti, Nb, Ta and W is less than 0.50%. The remaining part of the high-strength stainless steel wire is composed of Fe and impurities. A value of Md30 shown in equation (1) is 0 to 30(DEG C). The strength of the high-strength stainless steel wire is 1800MPa or more. An amount of martensite iron induced by processing is 20 to 80vol.%. A tensile residual stress in a length direction of a surface layer of the high-strength stainless steel wire is 500MPa or less. The equation (1) is: Md30(DEG C)=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo in which element symbols in the equation (1) are the content (mass percentage) of each element composed of a steel. If the steel is free from a particular element in equation (1), 0 is inserted in the equation (1) for the particular element.

Description

High strength stainless steel wire and spring

Field of invention The present invention relates to high-strength stainless steel wires for springs and springs, and is a technology that combines the fatigue resistance characteristics and heat permanent deformation resistance of Worthfield iron-based stainless steel wires in the temperature range.

Background of the invention Stainless steel springs have been widely used in industrial machines, automobile parts, etc. because of their excellent corrosion resistance. In addition, in recent years, the demand for lightweight has increased, and high strength has been discussed. If the strength of the stainless steel wire, which is the raw material of the stainless steel spring, is increased, longitudinal cracks during wire drawing will become a problem. Therefore, some people have proposed technology to control the chemical composition, processing-induced loose iron content, crystal grain size, or hydrogen content. to prevent longitudinal cracks (Patent Documents 1 and 2).

On the other hand, in recent years, the use of high-strength stainless steel springs for various purposes has increased. With the increase in applications, the following requirements are also gradually increasing: improving fatigue resistance in the hot zone and heat permanent deformation resistance in the hot zone. For example, stainless steel springs are increasingly used for long periods of time in environments where the temperature rises to approximately 200°C, such as in a car engine room. Accordingly, there is a demand for springs that can cope with space saving, high strength and lightweight, and are also required to have fatigue strength and heat permanent deformation resistance in the temperature range (for example, around 200°C).

Patent Document 3 proposes a surface method. In order to improve the fatigue strength, the spring material composed of Worthfield iron-based stainless steel is surface carburized in a fluorine gas environment. However, the cost of surface treatment in a fluorine gas environment is high due to the long processing time and special treatment. Therefore, there is no idea about the mass production technology that can be improved through wire drawing materials, let alone the improvement in the temperature range. Technology that resists fatigue strength and heat-resistant permanent deformation.

The method proposed in Patent Document 4 is to stabilize the γ-stainless steel wire through a medium containing Mo and Al in order to improve the heat permanent deformation resistance, so that the processing-induced Asada loose iron is generated and NiAl is finely precipitated. However, it is difficult to ensure fatigue resistance through Al, Ti, and Nb-based inclusions.

As mentioned above, in the past, among high-strength Wirthfield iron-based stainless steel wires for springs, a technology that has both fatigue resistance and heat permanent deformation resistance in the normal temperature to high temperature range has not been proposed, and the normal temperature to high temperature range is to Around 200℃.

[Advanced technical documents] [Patent Document] [Patent Document 1] Japanese Patent No. 3542239 [Patent Document 2] Japanese Patent No. 4489928 [Patent document 3] Japanese Patent Application Publication No. 2005-200674 [Patent Document 4] Japanese Patent No. 6259579

Summary of the invention Invent the problem to be solved The present invention was made in view of the above-mentioned problems, and its object is to provide a high-strength stainless steel wire and spring for springs that are high in strength and have excellent fatigue resistance and heat permanent deformation resistance in a warm range.

means to solve problems In order to solve the above-mentioned problems, the inventors of the present case found out after review that for the dielectrically stable Worthfield iron-based stainless steel wire after strong wire drawing, the residual stress on the surface of the steel wire can be reduced by controlling the finishing wire drawing, and at the same time, It is intended to contain N. N is very effective in refining the crystal grains of strands and forming nitrogen clusters during aging. Furthermore, the amount of Al, the amount of O, and the solidification speed during casting can be further controlled to control the fineness near the surface layer. The composition of the deoxidation product enables high-strength spring materials to have both fatigue strength and permanent deformation in the temperature range. The present invention was completed based on this insight.

That is, the gist of the present invention is as follows. [1] A high-strength stainless steel wire whose chemical composition of steel is composed of the following: In mass %, C: 0.08~0.13%, Si: 0.2~2.0%, Mn: 0.3~3.0%, P: 0.035% or less, S: 0.008% or less, Ni: 5.0% or more and less than 8.0%, Cr: 14.0~19.0%, N: 0.04~0.20%, Al: 0.10% or less, O: 0.012% or less, Ca: 0.0040% or less, The total of Ti, Nb, Ta and W: 0.50% or less, Remaining part: Fe and impurities, and The value of Md30 shown in (1) below is 0 (℃) ~ 30 (℃); The strength of the steel wire is above 1800MPa. The amount of loose iron induced by processing is 20~80vol.%. The tensile residual stress in the length direction of the surface layer of the steel wire is below 500MPa; Md30(℃)＝551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo…(1) Among them, the element symbols in formula (1) are the content (mass %) of each element in the chemical composition of steel; if not included, substitute 0. [2] The high-strength stainless steel wire as described in [1], wherein: Al, O and Ca in the aforementioned chemical composition are calculated in mass %: Al: 0.01~0.08%, O: 0.005% or less, Ca: 0.0005~0.0040%; In steel, the average composition of deoxidation products with a diameter of 1~2 μm is: Al: 10~35%, Ca: 5~30%, Cr: less than 10%, Mn: 5% or less. [3] The high-strength stainless steel wire as described in [1], wherein: Al, O and Ca in the aforementioned chemical composition are calculated in mass %: Al: less than 0.01%, O: 0.003~0.008%, Ca: 0.0010% or less; In steel, the average composition of deoxidation products with a diameter of 1~2 μm is: Al: less than 10%, Ca: less than 10%, Cr: 10~45%, Mn: 10~30%. [4] The high-strength stainless steel wire as described in any one of [1] to [3], which further contains one or more of the following to replace a part of Fe: In mass %, Mo: 0.1~2.0%, Cu: 0.8% or less, V: 0.5% or less. [5] A spring composed of the high-strength stainless steel wire according to any one of [1] to [3]. [6] A spring composed of high-strength stainless steel wire as described in [4].

Invention effect The high-strength stainless steel wire of the present invention has high strength and excellent fatigue resistance and heat permanent deformation resistance in the temperature range. Therefore, even when it is made into a spring, it can still have both fatigue strength and heat permanent deformation resistance in the temperature range. properties, and can achieve lightweight springs and high durability in high temperature areas. In addition, the spring of the present invention can have both fatigue strength and heat permanent deformation resistance in a high temperature range, and can achieve lightweight springs and high durability in a high temperature range. Furthermore, the spring of the present invention can be used as a coil spring for precision parts.

Embodiments of the present invention Form used to implement the invention Regarding the high-strength stainless steel wire according to the embodiment of the present invention, the chemical composition of the steel is as follows: in terms of mass %, C: 0.08~0.13%, Si: 0.2~2.0%, Mn: 0.3~3.0%, P: 0.035% or less, S: 0.008% or less, Ni: 5.0% or more and less than 8.0%, Cr: 14.0~19.0%, N: 0.04~0.20%, Al: 0.10% or less, O: 0.012% or less, Ca: 0.0040% Below, the total of Ti, Nb, Ta and W: 0.50% or less, the remainder: Fe and impurities, and the value of Md30 shown in the following (1) is 0 (℃) ~ 30 (℃); the strength of the steel wire It is above 1800MPa, the amount of loose iron induced by processing is 20~80vol.%, and the tensile residual stress in the length direction of the surface layer of the steel wire is below 500MPa. Md30(℃)＝551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo…(1) Among them, the element symbols in formula (1) are the content (mass %) of each element in the chemical composition of steel; if not included, substitute 0.

In addition, the spring according to the embodiment of the present invention is composed of the above-mentioned high-strength stainless steel wire. The spring in this embodiment is preferably a helical spring, a helical compression spring, a helical tension spring, or a helical torsion spring.

Below, the chemical composition of high-strength stainless steel wire is explained. "%" regarding the chemical composition of steel means: mass %. In addition, the numerical range represented by "~" means the range including the numerical values written before and after "~" as the lower limit and upper limit. In addition, when the numerical value written before and after "~" is marked as "greater than" or "less than", the numerical range means: a range that does not include these numerical values as the lower limit or upper limit.

Regarding C, in order to obtain fatigue strength and heat permanent deformation resistance through high strength (especially 1800MPa or more) after wire drawing and aging, it is contained in 0.08% or more in terms of mass % (the following % is all mass %). However, when C content exceeds 0.13%, Cr carbides will precipitate at the grain boundaries and fatigue resistance will deteriorate. Based on this point, the C content is limited to less than 0.13%. It should be 0.09~0.12%.

Si contains 0.2% or more in order to deoxidize and reduce coarse inclusions. However, if the Si content exceeds 2.0%, the deoxidation products will become coarse and the fatigue resistance will be deteriorated. Based on this point, the Si content is limited to 2.0% or less. It should be 0.3~1.5%.

Regarding Mn, Mn is contained in order to deoxidize or control the amount of processing-induced loose iron after wire drawing within the range of the present invention, so as to reduce the residual stress on the surface layer of the wire drawing rod, prevent longitudinal cracks, and obtain predetermined strength and fatigue resistance properties. More than 0.3%. However, if the Mn content exceeds 3.0%, the amount of loose iron induced by processing after drawing will decrease and the strength will decrease. In addition, the fatigue resistance and heat permanent deformation resistance will decrease. Based on this point, the Mn content is limited to 3.0%. the following. It should be 0.5~2.0%.

P is limited to 0.04% or less because it causes grain boundary segregation and reduces the material, promotes the formation of microvoids, causes longitudinal cracks, and reduces fatigue strength. It should be less than 0.03%. Although the lower the P content, the better, but the refining cost will increase, so it is allowed to contain more than 0.005%.

Regarding S, it forms sulfides and causes micropores to be generated during wire drawing, thereby reducing the fatigue strength, so it is limited to 0.008% or less. It should be less than 0.005%. Although the lower the S content, the better, but the refining cost will increase, so it is allowed to contain more than 0.0001%.

Ni contains 5.0% or more in order to ensure toughness after drawing, prevent longitudinal cracks in the drawing, and control processing-induced Asada loose iron within the range of the present invention to obtain predetermined strength and fatigue resistance. However, if the Ni content exceeds 8.0%, the target machining-induced loose iron content after drawing will be reduced and the strength and fatigue resistance will be reduced. Based on this point, the Ni content is limited to less than 8.0%. It should be 6.0~7.5%.

Regarding Cr, 14.0% or more is contained in order to make the amount of processing-induced loose iron appropriate to ensure corrosion resistance of precision spring products and the like. However, if the Cr content exceeds 19.0%, the target processing-induced loose iron content after drawing will be reduced, and the strength, fatigue strength, and heat permanent deformation resistance will be deteriorated. Based on this point, the Cr content is limited to 19.0% or less. It should be 15.0~18.0%.

Regarding N, in order to obtain fatigue strength and heat permanent deformation resistance through high strength (especially 1800 MPa or more) after wire drawing and aging, 0.04% or more is contained in mass %. N will form N-based clusters especially during the aging heat treatment at 250~550°C after wire drawing and spring processing, and is very effective in achieving all of the required strength, fatigue strength, and heat permanent deformation resistance. However, when N exceeds 0.20%, Cr carbonitride will precipitate at the grain boundaries, and fatigue resistance will deteriorate. Based on this point, the N content is limited to less than 0.20%. It should be 0.05~0.15%.

Ti, Nb, Ta, and W are powerful carbonitride-forming elements and are effective in miniaturizing crystal grains as particles that anchor crystal grain boundaries. Therefore, these elements may also contain a total of 0.05% or more. On the other hand, these elements will generate coarse carbonitrides and promote the formation of micropores, thereby reducing the fatigue strength. Therefore, the total content of Ti, Nb, Ta and W is limited to less than 0.50%, preferably less than 0.20%, according to needs. The appropriate total content is 0.15% or less. In addition, regarding Ti, Nb, Ta, and W, at least one type, two or more types, or all of them may be contained.

Moreover, the high-strength stainless steel wire of this embodiment contains Al: 0.10% or less, O: 0.012% or less, and Ca: 0.0040% or less. Al and Ca are very effective elements in order to promote deoxidation and reduce the amount of various inclusions to improve the strength of steel. In addition, O (oxygen) is contained not only as an impurity in steel, but also in deoxidation products described later. In addition, the lower limit of Al may be 0.001% or more, the lower limit of O may be 0.0001% or more, and the lower limit of Ca may be 0.0001% or more. The contents of Al, Ca and O will be described in detail in the description of the deoxidation products.

Regarding Md30 (℃) represented by the above formula (1), after investigating the influence of each element on the amount of processing-induced loose iron in the base material after strong wire drawing, which is about 70%, the results show that: The amount of Asada Powder Iron has effective elements and degree of influence. If the value of Md30 is less than 0 (℃), the amount of loose iron induced by strong wire drawing will be small, the tensile strength of the steel wire will be less than 1800MPa, and the effect of the present invention will be weakened. Based on this point, the value of Md30 The value is set above 0°C. In addition, when the value of Md30 exceeds 30°C, the possibility that the processing-induced loose iron content after strong wire drawing will exceed 80 vol.% increases, and longitudinal cracks in the wire become prone to occur, so the value of Md30 is set to 30 below ℃.

Furthermore, the high-strength stainless steel wire of this embodiment may also contain one or more of the following: Mo: 0.1~2.0%, Cu: 0.8% or less, and V: 0.5% or less. The lower limits of Mo, Cu, and V may be 0% or more.

Mo is very effective for corrosion resistance, so it can be contained at 0.1% or more according to needs. However, even if it contains more than 2.0% Mo, its effect is not only saturated, but also the amount of loose iron induced by processing will be reduced, resulting in a reduction in strength and fatigue strength. Therefore, the upper limit is limited to 2.0%.

Cu will inhibit the work hardening of Worthfield iron and reduce the strength of the steel wire after wire drawing, so it is limited to less than 0.8%.

V will generate fine carbonitrides to refine the crystal grain size of Worthfield iron, thereby improving the heat permanent deformation resistance, so it can be contained below 0.5% as needed. However, when the content of V exceeds 0.5%, the carbonitrides will become coarse, and the formation of micropores will be promoted and the fatigue resistance will be deteriorated, so it is limited to less than 0.5%. The lower limit of the V content may be 0.03% or more.

The remainder other than the above is Fe and impurities. As for impurities, Pb, Bi, Sn, Co, etc. may contain up to about 0.1% each, and B, Mg, Zr, and REM may contain up to about 0.01% each. In addition, in this embodiment, in order to suppress the formation of microcracks after wire drawing and improve the fatigue resistance strength, it is appropriate to limit hydrogen to 4 ppm or less as necessary.

The tensile strength of the high-strength stainless steel wire of this embodiment is 1800 MPa or more, preferably 2000 MPa or more. When the tensile strength is less than 1800MPa, the fatigue resistance strength becomes insufficient. Accordingly, by adjusting the chemical composition of the steel and the wire drawing rate, the strength is made above 1800MPa. It should be above 2000MPa.

The processing-induced loose iron content in high-strength stainless steel wires is set in the range of 20 to 80 vol.%. In order to ensure the strength of more than 1800MPa, the fatigue strength after aging, and the heat permanent deformation resistance, the chemical composition and wire drawing processing conditions are adjusted to ensure that the processing-induced hemp field iron content is more than 20Vol.%. However, when the amount of machining-induced loose iron exceeds 80 vol.%, micropores caused by microscopic cracks will form around the deoxidation products and carbonitrides, and the fatigue strength will deteriorate. Based on this point, it is limited to 80 vol. %the following. It is suitable to be 30~70vol.%.

The residual stress in the length direction of the surface layer of the steel wire is set to 500MPa or less. If there is a tensile residual stress greater than 500MPa, the fatigue strength and heat permanent deformation resistance of the aged spring will deteriorate. Based on this point, the upper limit is limited to less than 500MPa. In addition, the residual stress is controlled by adjusting the area reduction rate of the final finishing wire drawing rate, the mold angle of the mold used for wire drawing processing, and the wire drawing speed. It should be below 400MPa.

Next, the deoxidation products contained in the stainless steel wire of this embodiment and the preferable content ranges of Al, Ca, and O will be described. In the following explanation, the case where the Al amount is 0.01 to 0.08% and the case where the Al amount is less than 0.01% will be explained.

(When the Al content is 0.01~0.08%) Regarding the stainless steel wire of this embodiment, the chemical composition of Al, O and Ca in mass % is preferably: Al: 0.01~0.08%, O: 0.005% or less, Ca: 0.0005~0.0040%; in steel, the diameter is 1~ The average composition of deoxidation products of 2 μm is preferably: Al: 10~35%, Ca: 5~30%, Cr: 10% or less, Mn: 5% or less. Cr is preferably 1% or more, and may be 2% or more. Furthermore, in order to refine the oxide, Al: 10 to 35%, Ca: 5 to 30%, Cr: 2 to 9%, and Mn: 3% or less are preferred.

Al reduces deoxidation products through deoxidation, and while containing Ca, it also controls the solidification rate of the surface layer during casting (10~500°C/s) early, thereby suppressing coarse deoxidation products. This will improve the fatigue resistance of high-strength stainless steel wires and springs. In addition, fine AlN is generated and the crystal grains of the high-strength stainless steel wire are refined to a range of 30 μm or less, thereby improving the heat permanent deformation resistance. Therefore, the Al content is 0.01% or more. However, when Al content exceeds 0.08%, it will become strongly deoxidized, and coarse deoxidation products and coarse AlN will be generated, resulting in a decrease in fatigue strength. Accordingly, the Al content is limited to 0.01~0.08%. It should be 0.015~0.05%.

Ca is contained together with Al, thereby suppressing coarse deoxidation products and improving fatigue strength. According to this, when the content of Al is 0.01% or more, the content of Ca is 0.0005% or more. However, when it contains more than 0.0040% Ca, it will become strongly deoxidized, and coarse deoxidation products will be generated, resulting in a decrease in fatigue strength. Therefore, the Ca content is limited to 0.0040% or less. It should be 0.0010~0.0030%.

O, together with Al and Ca, affects the amount, composition, and size of deoxidation products; in order to obtain deoxidation products, it is limited to a range of 0.005% or less. Preferably it is less than 0.004%. The lower limit of O can also be set to 0.0001% or more.

The high-strength stainless steel wire of this embodiment contains deoxidation products with a diameter of 1 to 2 μm. The composition of such fine deoxidation products is an effective indicator of the degree of uneven size distribution of the entire deoxidation products, which greatly affects fatigue. The average composition of fine deoxidation products with a diameter of 1 to 2 μm is determined as follows: Al: 10 to 35%, Ca: 30% or less, Cr: 2 to 10%, Mn: 5% or less. Furthermore, the deoxidation product may also contain O, Si, Ti, Fe, etc. as the remainder.

In addition, regarding the average composition of deoxidation products, the deoxidation products are extracted from high-strength stainless steel wires through electrolytic extraction. In addition, among the extracted precipitates and inclusions, those containing oxygen and having a round particle size of 1 to 2 μ are designated as deoxidation products. The extracted deoxygenated products were subjected to elemental analysis using SEM and EDS, and the content of each element was defined as 100% when the amount of all detected elements was considered. Detailed measurement methods are described in examples. During the measurement, the oxygen-containing extract was defined as the deoxygenated product. In addition, the diameter of the deoxidized product is calculated as (major diameter + short diameter)/2.

We have learned that in the high-strength stainless steel wire of this embodiment, the formation temperature of the main deoxidation products is adjusted to near the solidification point of the material, thereby suppressing the formation of coarse deoxidation products and the micropores generated around them, and It can improve the fatigue resistance of high-strength stainless steel wires and springs. In addition, in order to set the composition range of the deoxidation product to the above range, it is important to control the contents of Al, Ca, Si, Mn, and Cr that contribute to deoxidation and the cooling rate during solidification in the casting process, and it is effective to adjust the composition And it is quenched and solidified to prevent the deoxidation products from becoming coarse. Therefore, in the present invention, the average composition of the deoxidized products with a diameter of 1 to 2 μm generated during solidification is limited to the above-mentioned value as necessary.

(When the Al content is less than 0.01%) Regarding the stainless steel wire of this embodiment, the chemical composition of Al, O and Ca in mass % can be: Al: less than 0.01%, O: 0.003~0.008%, Ca: 0.0010% or less; in steel, the diameter is 1~2 μm The average composition of the deoxidation products can be: Al: less than 10%, Ca: less than 10%, Cr: 10~45%, Mn: 10~30%. More preferably, Al: 5% or more and less than 10%, Ca: less than 5%, Cr: 10 to 45%, and Mn: 10 to 30%.

By setting Al to less than 0.01% and controlling the surface solidification speed during casting to 10~500°C/s, the main components of the deoxidation product will be Cr and Mn, and the deoxidation product will be refined. In this way, the crystal grain size in the steel is refined, the heat permanent deformation resistance is improved, and the fatigue resistance strength of the product is also improved. Therefore, the Al content is limited to less than 0.01%.

Regarding Ca, when the amount of Al is low, it forms coarse deoxidation products together with Si and reduces the fatigue strength. Accordingly, when the Al amount is less than 0.01%, the Ca content is limited to 0.0010% or less. Ca may be 0.0001% or more.

Regarding O, together with Al and Ca, it affects the amount, composition, and size of deoxidation products; in order to obtain the above-mentioned predetermined deoxidation products, it is limited to the range of 0.003~0.008%.

Even when Al is set to less than 0.01%, the composition of the fine deoxidation products is effective as an index showing the degree of uneven size distribution of the entire deoxidation products. That is, the average composition of fine deoxidation products with a diameter of 1 to 2 μm is determined as follows: Al: less than 10%, Ca: less than 10%, Cr: 10 to 45%, and Mn: 10 to 30%. This suppresses the formation of coarse deoxidation products and improves the fatigue strength of high-strength stainless steel wires and springs. In addition, the fine deoxidation products will pin the crystal grain boundaries to refine the crystal grains in the steel and improve the heat permanent deformation resistance. If the composition of the deoxidation product is to be within the above range, it is effective to adjust the amounts of Al, Ca, Si, Mn, and Cr that contribute to deoxidation and to quickly solidify the steel during casting. Therefore, the composition of the deoxygenated product with a diameter of 1 to 2 μm is limited to the above-mentioned value according to the requirements. In addition, the remainder of the composition of the deoxidation product may contain O, Si, Ti, Fe, etc. The outline of the method for measuring the average composition of deoxygenated products is as described above.

The high-strength stainless steel wire of this embodiment is useful as a stainless steel wire as a spring material. The wire diameter of high-strength stainless steel wire should be in the range of 0.1~3.0mm. This makes it easier to process, for example, precision springs.

Hereinafter, the manufacturing method of the high-strength stainless steel wire of this embodiment is demonstrated. Regarding the high-strength stainless steel wire of this embodiment, molten steel prepared with a predetermined composition is cast to form a cast piece, and the obtained cast piece is then hot-rolled into a wire rod with a diameter of 5.5 to 15.0 mm. Then, the obtained wire rod is repeatedly subjected to wire drawing processing, annealing treatment, and cooling to produce a high-strength stainless steel wire with a diameter of 0.1 to 3.0 mm.

When casting molten steel into cast slabs, in order to control the composition of deoxidized oxides, it is advisable to control the cooling rate during casting to 5~500°C/s. The cooling rate is defined as the average cooling rate during the period from the casting temperature (for example, the temperature of the molten steel in the tundish) to the time when the surface temperature of the cast piece is cooled to 1300°C. If the cooling rate is less than 5°C/s or more than 500°C/s, the desired deoxidized oxide cannot be obtained, which is undesirable. The cooling rate can be 10~500℃/s, 20~400℃/s, or 40~300℃/s.

The conditions for hot rolling of cast slabs need not be particularly limited. In addition to hot rolling, hot forging can also be performed according to needs.

To produce a high-strength stainless steel wire from a wire rod made of stainless steel, in terms of conditions, an intermediate annealed material is obtained through an intermediate stage of cold drawing processing, annealing treatment and cooling, and the final cold drawing is performed on the obtained intermediate annealed material. Line processing. Regarding the final cold drawing process, the area reduction rate is set to 55~85%. The area reduction rate of the final mold is set to 3~25%, preferably 5~20%, and more preferably 8~15%. The area reduction rate of the final mold The approach angle is set in the range of 3 to 15° in terms of half angle, preferably in the range of 4 to 10°, and more preferably in the range of 5 to 8°. After wire drawing processing under such conditions, a high-strength stainless steel wire can be obtained, with a strength of more than 1800MPa. The processing-induced loose iron content in the steel is 20~80vol.%, and the tensile residual stress in the length direction of the surface layer of the steel wire is is below 500MPa.

In addition, if you want to manufacture a coil spring from a high-strength stainless steel wire, a coiling step is performed to wind the high-strength stainless steel wire into a spiral shape, and then perform an aging treatment. The aging treatment should be heated in the range of 250℃~550℃ for 10~300 minutes. The treatment temperature of aging treatment can be in the range of 300~450℃, or in the range of 350~400℃. The processing time can be in the range of 20 to 200 minutes, or in the range of 30 to 60 minutes.

The high-strength stainless steel wire of this embodiment has high strength and excellent fatigue resistance and heat permanent deformation resistance in the temperature range. Thereby, even when it is formed into a spring, it is possible to achieve both fatigue strength and heat permanent deformation resistance in a high temperature range, and it is possible to achieve lightweight springs and high durability in a high temperature range. In addition, the spring of this embodiment can have both fatigue strength and heat permanent deformation resistance in a high temperature range, and can achieve lightweight springs and high durability in a high temperature range. [Example]

(Experimental example 1) The steel with the chemical composition shown in Table 1A and Table 1B is melted in a 150kg vacuum melting furnace at about 1600°C, and then cast in a mold with a diameter of 170mm. In addition, the amount of O changes through the content of deoxidizing elements such as Al, Si, Mn, and the time from when the deoxidizing elements are added to the molten steel until the casting mold is tapped. The average cooling rate during the period from the casting temperature (the temperature of the molten steel in the feeding tank) to the cooling of the cast piece surface temperature to 1300°C is set to 40°C/second. After that, each cast piece is heat-processed into a wire rod with a diameter of 6 mm through hot rolling.

After that, the wire drawing process, annealing at 1100°C, and rapid cooling are further repeated to produce an annealed material of stainless steel wire with a diameter of 3.0~4.5mm. Next, the annealed material is subjected to final wire drawing processing. Specifically, a cold wire drawing process with an area reduction rate of 55~80% is performed, and the final mold is finished with a surface reduction rate of 7% and an advance angle of 7° in half angle terms, into a steel with a diameter of 2.0mm. String. High-strength stainless steel wire is produced in this way.

Next, for high-strength stainless steel wires, N-clustering treatment was explored by imagining it as a coil spring and aging it at 400°C for 30 minutes.

[Table 1A]

[Table 1B]

Then measure: the crystal grain size of the φ3.0~4.5mm annealed material before the final wire drawing, the amount of loose iron produced by the processing of the high-strength stainless steel wire after the final wire drawing, the tensile strength, and the length of the surface layer. directional residual stress, average composition of fine deoxidation products on the surface, heat resistance to permanent deformation, and fatigue strength. The results are shown in Table 2A and Table 2B.

[Table 2A]

[Table 2B]

Regarding the crystal grain size of the Worthfield iron in the annealed material before wire drawing, the cross section of the annealed material was electrolytically etched in 10% nitric acid solution, and then the metal structure was observed with an optical microscope, and the results were determined in accordance with JIS G 0551:2013 The specified intercept method is used to calculate the particle size number, which is used as the crystal particle size.

The tensile strength of the high-strength stainless steel wire after final drawing is measured in accordance with JIS Z 2241:2011.

Regarding the processing-induced loose iron content of the high-strength stainless steel wire after final drawing, the saturation magnetization value is measured with a DC magnetic measuring instrument (BH Tracer) and calculated from the saturation magnetization value.

The residual stress in the length direction of the surface layer of the high-strength stainless steel wire after final wire drawing is measured using an X-ray stress measuring device.

The method for measuring the fine deoxidation products on the surface of the steel wire is as follows. After the final wire drawing, the surface layer of the high-strength stainless steel wire is polished with #500, and then electrolyzed in a non-aqueous solution to dissolve the matrix. The non-aqueous solution after electrolysis is filtered through filter paper to extract the oxides. The non-aqueous solution is defined as the following methanol solution, which contains: 3% maleic acid and 1% tetramethylammonium chloride (Tetramethylammonium Chloride). The electrolysis conditions are set at a constant voltage of 100mV. After that, 20 random composition analyzes were performed on the oxides with a diameter of 1 to 2 μm remaining on the filter paper through SEM and EDS, and the average composition was calculated. The average composition is defined as the content rate of each element when the total element amount detected by SEM and EDS is 100%. In addition, oxides are defined as non-metallic inclusions containing O and containing Al, Ca, Mn, Si, Fe, Cr, Ti, etc. according to EDS analysis. The oxide diameter is calculated as (major diameter + minor diameter)/2. The average composition obtained in this way is determined as the average composition of the deoxygenated product.

Regarding the heat-resistant permanent deformation of high-strength stainless steel wire after aging treatment, it is assumed that the coil spring undergoes stress relaxation due to torsional stress, and the steel wire torsion test is used to evaluate it. Regarding the evaluation conditions, the distance between chucks is set to 150mm, heated to 200°C and fixed at a predetermined torsion position with a predetermined initial torsional stress τ0, and then the torsional stress τ reduced due to stress relaxation after maintaining for 24 hours is measured. And it is determined by the stress relaxation rate S=(1-τ/τ0)×100(%).

If the stress relaxation rate is more than 10%, the heat permanent deformation resistance will be rated as "×"; if it is more than 5% and less than 10%, it will be rated as "△"; if it is more than 3% and less than 5%, it will be rated as "0"; if it is less than 3% The one is designated as "◎". △, 〇 and ◎ are regarded as qualified.

Regarding the fatigue resistance characteristics of the high-strength stainless steel wire after aging treatment, it is assumed that the coil spring will fatigue due to repeated torsional stress, and the steel wire torsion test is used to evaluate it. Regarding the evaluation conditions, the distance between the chucks is set to 150mm, heated to 200°C, and the set stress τ is changed from 200 to 700MPa. The amplitude stress τa is τa/τ=0.3 and the speed is 25Hz. Repeated torsional stress is applied until 10 ⁶ The stress at which failure will not occur for several times is determined as the fatigue limit, and is evaluated in this way.

If the fatigue limit is less than 200MPa, the thermal fatigue resistance characteristics are rated as "×"; if it is greater than 200MPa and less than 250MPa, it is rated as "△"; if it is greater than 250MPa and less than 350MPa, it is rated as "○"; if it is greater than 350MPa, it is rated as "◎" ; Use this to evaluate. △, 〇 and ◎ are regarded as qualified.

As shown in Tables 1A to 2B, regarding the examples of the present invention, No. 1a to 19a, their chemical compositions are within the scope of the invention, and the strength, processing-induced hemp field iron content, and residual stress of the surface layer are within the scope of the invention, and the manufacturing conditions are also within the scope of the invention. Within the appropriate range, therefore, the heat permanent deformation resistance and heat fatigue resistance strength are within acceptable levels. In addition, the average crystal grain size of the annealed material is often 30 μm or less, which is good.

Furthermore, regarding No. 7a, 8a, 13a~15a and 19a, they set the Al amount to 0.01~0.08% and the Ca amount to 0.005 or less, and mainly controlled the O amount to 0.005~0.0040% through Al deoxidation, etc. The composition of each element in the deoxidation product reaches an appropriate range, the heat permanent deformation resistance and fatigue strength reach "○" or "◎", and the heat permanent deformation resistance and heat fatigue resistance reach a higher level.

What's more, regarding Nos. 9a to 12a and 16a to 18a, they set the Al content to less than 0.01% and the Ca content to 0.0010% or less, and controlled the O content to 0.003 to 0.008% mainly through Si deoxidation, etc. The composition of each element in the deoxidation product reaches an appropriate range, the heat permanent deformation resistance and fatigue strength reach "◎", and the heat permanent deformation resistance and heat fatigue resistance reach a higher level.

On the other hand, in Comparative Examples Nos. 1b to 20b, the steel composition and Md30 are outside the appropriate range, and the strength, processing-induced hemp field iron content, and residual stress in the surface layer are outside the appropriate range, and the thermal fatigue resistance strength thereof is outside the appropriate range. Difference. Furthermore, regarding Nos. 3b, 7b, 8b, 11b to 15b, 18b, and 19b, their heat permanent deformation resistance was also poor.

(Experimental example 2) Next, in order to investigate the influence of residual stress on the surface layer of the steel wire, final wire drawing processing was performed on the 4 mm diameter annealed material composed of steel A and steel H produced under the conditions shown in Table 3. That is, the annealed material with a diameter of 4.0 mm is subjected to cold wire drawing processing with an area reduction rate of 75%, and the area reduction rate of the final mold is set to 1~26%, so that the advancing half angle changes from 2 to 16°. In this way, it is finished into a high-strength stainless steel wire with a diameter of 2.0mm. Then, it is assumed that the coil spring product is subjected to aging treatment at 400° C. for 30 minutes. In this way, high-strength stainless steel wires No. 20a to 24a and 21b to 27b are produced. Then, in the same manner as shown in Table 2, the processing-induced hemp iron content, strength, and residual stress on the surface layer were measured, and the heat permanent deformation resistance and heat fatigue resistance strength were evaluated. The results are shown in Table 3.

[table 3]

As shown in Table 3, it is understood that the residual stress on the surface layer will change depending on the final wire drawing processing conditions. Regarding Comparative Examples Nos. 21b to 27b, the final wire drawing process was performed under conditions outside of the appropriate conditions, resulting in residual stress outside the scope of the present invention, and the heat permanent deformation resistance and heat fatigue resistance were inferior.

On the other hand, it was found that in Invention Examples Nos. 20a to 24a, excellent heat permanent deformation resistance and thermal fatigue resistance were obtained by setting the residual stress of the surface layer to 500 MPa or less; by setting the residual stress of the surface layer to Below 400MPa, the effect will become particularly significant.

(Experimental example 3) Next, in order to investigate the influence of the cooling rate of the surface layer of the cast piece, steel G and steel J were melted in a 150kg vacuum melting furnace at about 1600°C, and then cast in a mold with a diameter of 100~250mm. The material of the casting mold is determined to be iron-based, magnesia-based, or silicon oxide-based. Then, for steel G and steel J, the average cooling rate during solidification is changed depending on the presence or absence of refractory material (kaowool).

A rolling material (billet) is prepared from the obtained cast piece by grinding the surface layer so that the internal structure of the cast piece becomes a surface layer structure. The cooling rate of the surface layer of the rolling material during casting is slower than the cooling rate of the surface layer of the cast piece before grinding. In addition, the average cooling rate during solidification is determined by measuring the secondary dendrite arm spacing (λ) near the surface layer of the cast slab cross section and using the formula of average cooling rate (℃/s) = (110/λ) ^2.2 , The average cooling rate during solidification is estimated from the average value of λ.

Thereafter, an annealed material of a stainless steel wire with a diameter of 4.0 mm was produced in the same manner as in Experimental Example 1, and a final wire drawing process was performed in the same manner as in Experimental Example 1 to produce a high-strength stainless steel wire with a diameter of 2.0mm. Aging treatment was performed in the same manner as in Experimental Example 1. Then, in the same manner as in Table 2, the amount of processing-induced Asada loose iron, strength, residual stress in the surface layer, and the average composition of deoxidation products were measured, and the heat permanent deformation resistance and heat fatigue resistance strength were evaluated. The results are shown in Table 4.

[Table 4]

As shown in Table 4, it is understood that in Invention Examples Nos. 25a to 28a, the components such as Al, Ca, and O are appropriately adjusted, and the solidification rate is controlled toward rapid cooling to appropriately adjust the composition of the fine deoxidation products. This will improve the heat permanent deformation resistance and heat fatigue resistance.

Availability in business It is clear from the above embodiments that according to the present invention, a high-strength stainless steel wire for springs with excellent heat permanent deformation resistance and heat fatigue resistance can be stably provided, and precision springs used in high-temperature regions can be lightweight while improving durability. Sex is extremely useful in industry.

(without)

Claims (6)

A high-strength stainless steel wire, which is composed of the following: C: 0.08~0.13%, Si: 0.2~2.0%, Mn: 0.3~3.0%, P: 0.035% or less, S: 0.008% or less, Ni: 5.0% or more and less than 8.0%, Cr: 14.0~19.0%, N: 0.04~0.20%, Al: 0.10% or less, O: 0.012% or less, Ca: 0.0040% or less, The total of Ti, Nb, Ta and W: 0.50% or less, Remaining part: Fe and impurities, and The value of Md30 shown in (1) below is 0 (℃) ~ 30 (℃); The strength of the steel wire is above 1800MPa. The amount of loose iron induced by processing is 20~80vol.%. The tensile residual stress in the length direction of the surface layer of the steel wire is below 500MPa; Md30(℃)＝551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo・・・(1) Among them, the element symbols in formula (1) are the content (mass %) of each element in the chemical composition of steel; if not included, substitute 0. For example, the high-strength stainless steel wire of claim 1, wherein, Al, O and Ca in the aforementioned chemical composition are calculated in mass %: Al: 0.01~0.08%, O: 0.005% or less, Ca: 0.0005~0.0040%; In steel, the average composition of deoxidation products with a diameter of 1~2 μm is: Al: 10~35%, Ca: 5~30%, Cr: less than 10%, Mn: 5% or less. For example, the high-strength stainless steel wire of claim 1, wherein, Al, O and Ca in the aforementioned chemical composition are calculated in mass %: Al: less than 0.01%, O: 0.003~0.008%, Ca: 0.0010% or less; In steel, the average composition of deoxidation products with a diameter of 1~2 μm is: Al: less than 10%, Ca: less than 10%, Cr: 10~45%, Mn: 10~30%. For example, the high-strength stainless steel wire of any one of claims 1 to 3 further contains one or more of the following to replace a part of Fe: In mass %, Mo: 0.1~2.0%, Cu: 0.8% or less, V: 0.5% or less. A spring is composed of a high-strength stainless steel wire according to any one of claims 1 to 3. A spring is composed of high-strength stainless steel wire as claimed in claim 4.