JP2023116800A - Manufacturing method of steel component - Google Patents

Abstract

To provide a steel component excellent in wear resistance.SOLUTION: A steel slab having a predetermined component composition is subjected to heating, hot rolling, coiling, first annealing, second annealing, cold rolling, machining, and heat treatment under a predetermined condition so that the average grain size of the former austenite grains is 25 μm or less, having a carbide containing at least one of Nb, Ti, and V, and the average grain size of the particles of the carbide of 0.1 μm or more is 0.15-2.5 μm, and the average grain size of the particles of the carbide of less than 0.1 μm is 0.005-0.05 μm.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to steel parts, particularly to steel parts having excellent wear resistance. The present invention also relates to a method for manufacturing the steel component.

Carbon steel, which is a steel containing a high concentration of carbon, has high hardness, so it is used as a material for steel parts that require wear resistance, such as textile machine parts, bearing parts, and machine blades. Widely used.

In the manufacture of general steel parts, a cold-rolled steel sheet as a raw material is processed into a part shape, and then quenching and tempering are performed. If the hardness is increased by the quenching treatment, the toughness is lowered, but the toughness can be improved by performing the subsequent tempering treatment. However, there is a problem that the tempering treatment lowers the hardness.

Therefore, various techniques have been proposed to further increase the hardness of steel parts and achieve even better wear resistance.

For example, in Patent Document 1, in a steel sheet having a ferrite-cementite structure, by increasing the grain size of ferrite, spheroidizing carbide (mainly cementite) with an appropriate grain size, and reducing the pearlite structure, the formability and resistance are improved. Techniques for improving wearability have been disclosed.

Further, in Patent Document 2, the cold-rolled steel sheet is annealed under specific conditions to change the metal structure into a pearlite structure, which is a layered structure of hard cementite and soft ferrite. Techniques for improving wearability have been disclosed.

Patent Literature 3 discloses a technique for improving the wear resistance of a steel sheet by precipitating coarse Nb/Ti-based carbides having an equivalent circle diameter of 0.5 μm or more in a ferrite phase, which is a matrix structure.

Patent Document 4 discloses a technique for improving the wear resistance of steel materials by precipitating coarse carbides having a particle size of 2 μm or more in a matrix structure.

In Patent Document 5, a steel sheet containing 0.5 to 0.7% by mass of C is subjected to an annealing finish immediately before final quenching and tempering, so that cementite and the like are added. Techniques have been proposed to improve the spheroidization rate of carbides and improve toughness.

In Patent Document 6, in the stage immediately before final quenching and tempering, the material is annealed to increase the number density of generated voids contained in the material. Techniques for producing carbon steel sheets have been proposed.

In Patent Document 7, in a high-carbon steel sheet, impact toughness and resistance are improved by controlling the formation of cementite carbides that do not contain niobium, titanium, and vanadium carbides, and setting the spheroidization rate and number density of cementite carbides to desired values. Techniques for improving abrasion resistance have been proposed.

In Patent Document 8, the grain size of cementite carbides that do not contain niobium, titanium, and vanadium carbides, the retained austenite, and the old Technology has been proposed to improve toughness by adjusting the austenite grain size and obtaining a bainite structure instead of obtaining a tempered martensite structure obtained by quenching and tempering, which is a general refining heat treatment. there is

WO2016/204288 JP 2020-132953 A JP 2017-190494 A JP 2010-138453 A JP-A-2009-24233 JP 2011-12316 A Patent No. 6880245 JP 2018-48374 A

According to conventional techniques such as those proposed in Patent Documents 1 to 8, a certain improvement can be seen in the hardness and wear resistance of steel materials. However, according to the studies of the present inventors, it has been found that when steel parts manufactured using conventional steel materials are actually used, sufficient wear resistance may not be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to provide steel parts having excellent wear resistance.

The present inventors have obtained the following findings as a result of examining methods for solving the above problems.

(1) When steel parts are actually used, the temperature rises due to friction with other members. For example, when steel parts are used as parts for textile machinery, such as knitting needles, the steel parts are constantly exposed to friction with the fibers, resulting in an increase in temperature.

(2) Therefore, in order to exhibit excellent wear resistance during actual use, it is necessary not only to suppress static wear due to grinding between materials, but also to prevent the softening of the steel plate due to the temperature rise during friction. must be suppressed.

(3) In order to improve the wear resistance of steel parts, it is necessary to precipitate carbides containing at least one of Nb, Ti and V in the steel. At that time, among the carbides, coarse ones have the effect of suppressing static wear. For example, in the case of textile machinery parts, the presence of coarse carbides can reduce the amount of material to be cut by fibers and foreign matter such as pebbles adhering to the fibers.

(4) On the other hand, among the carbides, fine ones have the effect of suppressing the softening of the steel sheet due to the temperature rise during friction. That is, the presence of fine carbides suppresses the recovery of the dislocation structure and the decrease in hardness when the temperature rises due to friction. In addition, in steels containing fine carbides, prior austenite grains are refined during quenching and tempering, increasing the grain boundary strengthening effect, and as a result, further suppressing the decrease in hardness due to the recovery of the dislocation structure. be done.

(5) In order to obtain the above effects, it is necessary to control the average grain size of each of coarse carbides and fine carbides within a specific range.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

1. in % by mass,
C: 0.6 to 1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005 to 0.05%,
S: 0.01% or less,
Al: 0.001 to 0.1%,
N: 0.001 to 0.009%,
Cr: 0.05 to 0.55%, and one or more of Ti: 0.05 to 1.0%, Nb: 0.1 to 0.5%, and V: 0.01 to 1.0% including
A steel slab having a chemical composition with the balance being Fe and unavoidable impurities is heated under the conditions of a slab heating temperature of 1100° C. or higher and a holding time of 1.0 hour or longer,
The heated steel slab is hot-rolled under the conditions of finish rolling start temperature: 1010 ° C. or higher to obtain a hot-rolled steel plate,
The hot-rolled steel sheet is cooled under the conditions of time from the end of hot rolling to the start of cooling: 2.0 seconds or less, average cooling rate: 25 ° C./s or more, cooling stop temperature: 640 ° C. to 720 ° C.,
Winding the cooled hot-rolled steel sheet,
The coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 650° C. or higher and 720° C. or lower and an annealing time of 3 hours or longer,
The hot-rolled steel sheet after the first annealing is cold-rolled at a rolling rate of 15% or more, and subjected to annealing at a temperature of 600 to 800 ° C. and a heating rate of 50 ° C./h or more and 190 ° C./h or less. The second annealing is repeated twice or more, and then final cold rolling is performed at a rolling reduction of 30% or more to make a cold rolled steel sheet,
For the cold-rolled steel sheet,
Machining into part shape and
Quenching temperature: 700°C to 950°C, holding time: 1.0 minute to 60 minutes, and tempering temperature: 100 to 400°C, holding time: 20 minutes to 3 hours. By applying heat treatment including tempering under conditions,
The average grain size of the prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V, and
The average particle size of particles of the carbide having a particle size of 0.1 μm or more is 0.15 to 2.5 μm,
A method of manufacturing a steel part, wherein the average particle size of the particles of the carbide having a particle size of less than 0.1 μm is 0.005 to 0.05 μm.

2. The component composition, in mass%,
Sb: 0.1% or less,
Hf: 0.5% or less,
REM: 0.1% or less,
Cu: 0.5% or less,
Ni: 3.0% or less,
Sn: 0.5% or less,
Mo: 1% or less,
Zr: 0.5% or less,
2. The method for manufacturing a steel component according to 1 above, further comprising one or more selected from the group consisting of B: 0.005% or less and W: 0.01% or less.

Further, the gist configuration of another embodiment of the present invention is as follows.

1. in % by mass,
C: 0.6 to 1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005 to 0.05%,
S: 0.01% or less,
Al: 0.001 to 0.1%,
N: 0.001 to 0.009%,
Cr: 0.05 to 0.55%, and one or more of Ti: 0.05 to 1.0%, Nb: 0.1 to 0.5%, and V: 0.01 to 1.0% including
Having a component composition in which the balance is Fe and unavoidable impurities,
The average grain size of the prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V, and
The average particle size of particles of the carbide having a particle size of 0.1 μm or more is 0.15 to 2.5 μm,
The steel part, wherein the average particle size of the particles of the carbide having a particle size of less than 0.1 μm is 0.005 to 0.05 μm.

2. The component composition, in mass%,
Sb: 0.1% or less,
Hf: 0.5% or less,
REM: 0.1% or less,
Cu: 0.5% or less,
Ni: 3.0% or less,
Sn: 0.5% or less,
Mo: 1% or less,
Zr: 0.5% or less,
2. The steel component according to 1 above, further comprising one or more selected from the group consisting of B: 0.005% or less and W: 0.01% or less.

3. 3. The steel part according to 1 or 2 above, wherein the steel part is any one of a textile machine part, a bearing part, and a machine cutting tool.

4. A steel slab having the chemical composition described in 1 or 2 above is heated under the conditions of a slab heating temperature of 1100° C. or higher and a holding time of 1.0 hour or longer,
The heated steel slab is used as a hot-rolled steel sheet under the conditions of finish rolling start temperature: Ac3 point or higher,
The hot-rolled steel sheet is cooled under the conditions of time from the end of hot rolling to the start of cooling: 2.0 seconds or less, average cooling rate: 25 ° C./s or more, cooling stop temperature: 640 ° C. to 720 ° C.,
Winding the cooled hot-rolled steel sheet,
The coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 650° C. or higher and 720° C. or lower and an annealing time of 3 hours or longer,
The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling rate of 15% or more, and second annealing at an annealing temperature of 600 to 800 ° C. and a heating rate of 50 ° C./h or more. , repeatedly applied twice or more, and then subjected to final cold rolling at a rolling rate of 30% or more to make a cold rolled steel sheet,
For the cold-rolled steel sheet,
Machining into part shape and
Quenching temperature: 750°C to 950°C, holding time: 1.0 minute to 60 minutes, and tempering temperature: 100 to 320°C, holding time: 20 minutes to 3 hours. A method of manufacturing a steel component by subjecting it to heat treatment, including tempering under conditions.

ADVANTAGE OF THE INVENTION According to this invention, the steel components excellent in wear resistance can be provided. The steel parts of the present invention exhibit excellent wear resistance not only under static conditions but also under conditions where the temperature is elevated due to friction. It can be suitably used for various uses.

It is a schematic diagram which shows the shape of a wear test piece. It is a schematic diagram of a wear test apparatus. It is a schematic diagram which shows the shape of the abrasion test apparatus used in the Example.

The present invention will be described in detail below. However, the present invention is not limited to this embodiment. Further, in the present invention, as described above, attention is paid to carbides containing at least one of Nb, Ti and V. Therefore, in the following description, "carbide containing at least one of Nb, Ti and V" may be simply referred to as "carbide".

[Component composition]
The cold-rolled steel sheet of the present invention has the chemical composition described above. The reason for the limitation will be described below. In the following description, "%" as a unit of content indicates "% by mass" unless otherwise specified.

C: 0.6-1.25%
C is an element necessary for improving the hardness after quenching and tempering. C is also an element necessary for forming cementite, elements such as Nb, Ti and V and carbides. In order to generate necessary carbides and obtain hardness and wear resistance after quenching and tempering, the C content must be 0.6% or more. Therefore, the C content should be 0.6% or more, preferably 0.7% or more. On the other hand, when the C content exceeds 1.25%, the hardness excessively increases and embrittlement occurs. In addition, if the C content exceeds 1.25%, the surface scale becomes hard during heating, resulting in deterioration of the surface properties, which not only makes the surface more prone to cracking during subsequent cold rolling, but also during quenching. Cracks occur in the surface and wear resistance decreases. Therefore, the C content should be 1.25% or less, preferably 1.20% or less.

Si: 0.10-0.55%
Si is an element that has the effect of increasing strength by solid solution strengthening, and the increase in strength also improves wear resistance. In order to obtain the above effect, the Si content should be 0.10% or more, preferably 0.12% or more. On the other hand, if the Si content is excessive, coarse ferrite is formed on the surface of the steel sheet during hot working, which inhibits the formation of carbides necessary for improving wear resistance in subsequent steps. Therefore, the Si content should be 0.55% or less, preferably 0.50% or less, and more preferably 0.45% or less.

Mn: 0.20-2.0%
Mn is an element that has the effect of improving hardness by promoting quenching and suppressing temper softening. In order to suppress temper softening, it is necessary to suppress the generation of C as cementite or delay the recovery of dislocations, and Mn has both effects. In addition, Mn has the effect of suppressing dislocation recovery due to frictional heat not only during tempering but also during use of steel parts. In order to obtain the above effects, the Mn content should be 0.20% or more, preferably 0.25% or more. On the other hand, when the Mn content exceeds 2.0%, a band-like structure is generated due to segregation of Mn. In particular, MnS segregation tends to cause abnormal grain growth and structural inhomogeneity, which suppresses the formation of carbides and causes cracks and shape defects during part processing. Therefore, the Mn content should be 2.0% or less, preferably 1.95% or less.

P: 0.0005 to 0.05%
By adding a small amount of P, the hardness is increased by solid-solution strengthening, and as a result, the wear resistance can be improved. In order to obtain the above effects, the P content should be 0.0005% or more, preferably 0.0008% or more. On the other hand, when the P content exceeds 0.05%, the strength of the grain boundary is lowered, resulting in embrittlement. Therefore, the P content should be 0.05% or less, preferably 0.045% or less.

S: 0.01% or less S forms a sulfide with Mn and consumes Mn, so that it lowers the hardenability. Decreased hardenability reduces the strength of the steel, resulting in reduced wear resistance. Therefore, the S content is set to 0.01% or less. On the other hand, from the viewpoint of improving wear resistance, the lower the S content, the better. Therefore, the lower limit of the S content is not particularly limited, and may be 0%. However, excessive reduction causes an increase in manufacturing costs, so from the viewpoint of industrial production, the S content is preferably 0.0005% or more, more preferably 0.001% or more.

Al: 0.001-0.1%
Al is an element necessary for deoxidation during steelmaking. Therefore, the Al content is set to 0.001% or more. On the other hand, excessive Al results in the formation of coarse nitrides. The nitrides are often formed on the surface of steel, and the formation of cracks and voids originating from these nitrides is promoted, resulting in a decrease in wear resistance. Therefore, the Al content should be 0.1% or less, preferably 0.08% or less, more preferably 0.06% or less.

N: 0.001 to 0.009%
By adding a small amount of N, fine nitrides can be formed, and the grain size can be refined to improve the toughness. In order to obtain the above effects, the N content should be 0.001% or more. On the other hand, when N is excessive, it combines with Al to form coarse nitrides. The nitrides are often formed on the surface of steel, and the formation of cracks and voids originating from these nitrides is promoted, resulting in a decrease in wear resistance. Therefore, the N content should be 0.009% or less, preferably 0.008% or less.

Cr: 0.05-0.55%
Cr is an element that has the effect of increasing the hardenability of steel and improving the hardness. Therefore, the addition of Cr can improve the wear resistance. In order to obtain the above effect, the Cr content should be 0.05% or more, preferably 0.12% or more. On the other hand, excessive Cr results in the formation of coarse Cr-carbides and Cr-nitrides and the formation of voids around said Cr-carbides and Cr-nitrides, resulting in poor performance of steel parts. Moreover, as a result of the formation of Cr carbides, the formation of carbides effective for improving wear resistance is hindered. Therefore, the Cr content should be 0.55% or less, preferably 0.95% or less.

The above component composition contains one or more selected from Ti: 0.05 to 1.0%, Nb: 0.1 to 0.5%, and V: 0.01 to 1.0%.

Ti: 0.05-1.0%
Ti is an element that forms fine carbides and has the effect of suppressing both static wear and thermal wear. Ti also has the effect of improving wear resistance by refining prior austenite grains during quenching and suppressing recovery of dislocations. When Ti is added, the Ti content should be 0.05% or more, preferably 0.015% or more, in order to obtain the above effects. On the other hand, if Ti is added excessively, the carbide becomes coarser than necessary, and the carbide becomes the starting point of voids and cracks. Therefore, the Ti content should be 1.0% or less, preferably 0.9% or less.

Nb: 0.1-0.5%
Nb is an element that forms fine carbides and has the effect of suppressing both static wear and thermal wear. Nb also has the effect of improving wear resistance by refining prior austenite grains during quenching and suppressing recovery of dislocations. When Nb is added, the Nb content is made 0.1% or more in order to obtain the above effect. On the other hand, if Nb is excessively added, the carbide becomes coarser than necessary, and the carbide becomes a starting point of voids and cracks. Therefore, the Nb content should be 0.5% or less, preferably 0.45% or less.

V: 0.01-1.0%
V is an element that forms fine carbides and has the effect of suppressing both static wear and thermal wear. V also has the effect of improving wear resistance by refining prior austenite grains during quenching and suppressing dislocation recovery. When V is added, the V content is made 0.01% or more in order to obtain the above effect. On the other hand, if V is added excessively, the carbides become coarser than necessary, and the carbides become starting points for voids and cracks, which lowers the workability when processing the steel sheet into a component shape. Therefore, the V content should be 1.0% or less, preferably 0.95% or less.

A cold-rolled steel sheet in an embodiment of the present invention has a chemical composition consisting of the above components and the balance of Fe and unavoidable impurities.

In another embodiment of the present invention, the above component composition is optionally Sb: 0.1% or less, Hf: 0.5% or less, REM: 0.1% or less, Cu: 0.5% Below, Ni: 3.0% or less, Sn: 0.5% or less, Mo: 1% or less, Zr: 0.5% or less, B: 0.005% or less, and W: 0.01% or less It can further contain one or more selected from the group.

Sb: 0.1% or less Sb is an element effective in improving corrosion resistance. scratches). Therefore, the Sb content is set to 0.1% or less. On the other hand, although the lower limit of the Sb content is not particularly limited, the Sb content is preferably 0.0003% or more from the viewpoint of increasing the effect of addition.

Hf: 0.5% or less Hf is an element that is effective in improving corrosion resistance. scratches). Therefore, the Hf content is set to 0.5% or less. On the other hand, the lower limit of the Hf content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Hf content is preferably 0.001% or more.

REM: 0.1% or less REM (rare earth metal) is an element that improves the strength of steel. However, excessive addition of REM may delay the refinement of carbides and promote uneven deformation during cold working, degrading the surface properties. Therefore, the REM content is set to 0.1% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of increasing the effect of addition, the REM content is preferably 0.005% or more.

Cu: 0.5% or less Cu is an element effective in improving corrosion resistance. scratches). Therefore, the amount of Cu to be added is set to 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Cu content is preferably 0.01% or more.

Ni: 3.0% or less Ni is an element that improves the strength of steel. However, excessive addition may promote uneven deformation during cold working, degrading the surface properties. Therefore, the Ni content is set to 3.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Ni content is preferably 0.01% or more.

Sn: 0.5% or less Sn is an element that is effective in improving corrosion resistance. scratches). Therefore, the Sn content is set to 0.5% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Sn content is preferably 0.0001% or more.

Mo: 1% or less Mo is an element that improves the strength of steel. However, excessive addition delays the spheroidization of carbides, promotes uneven deformation during cold working, and sometimes deteriorates the surface properties. Therefore, the Mo content is set to 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Mo content is preferably 0.001% or more.

Zr: 0.5% or less Zr is an element that is effective in improving corrosion resistance. scratches). Therefore, the Zr content should be 0.5% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Zr content is preferably 0.01% or more.

B: 0.005% or less B is an element that has the effect of improving hardenability and can be optionally added. However, if the B content exceeds 0.005%, the surface tends to crack during quenching. Therefore, the B content is set to 0.005% or less. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of enhancing the addition effect, when B is added, the B content is preferably 0.0001% or more.

W: 0.01% or less W is an element that has the effect of improving hardenability, and can be optionally added. However, if the W content exceeds 0.01%, the surface tends to crack during quenching. Therefore, the W content is set to 0.01% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of enhancing the effect of addition, when W is added, the W content is preferably 0.001% or more.

Average grain size of prior austenite grains: 25 μm or less By refining the prior austenite grains, the grain boundary strengthening effect is improved. As a result, since dislocation recovery is suppressed when frictional heat is generated, hardness can be maintained even in a thermal environment, and wear resistance is improved. In order to obtain the above effect, the average grain size of the prior austenite grains is set to 25 μm or less.

[carbide]
The steel component of the present invention contains carbide containing at least one of Nb, Ti and V. Conventionally, cementite has been used to improve wear resistance, but carbide containing at least one of Nb, Ti, and V is harder than cementite, so it contains at least one of Nb, Ti, and V. By precipitating carbide, wear resistance can be improved more than before.

At that time, as described above, coarse carbides have the effect of suppressing static wear, while fine carbides have the effect of suppressing static wear when the temperature is raised by friction. It has the effect of suppressing wear. Therefore, by appropriately controlling the grain sizes of coarse carbides and fine carbides, it is possible to effectively improve wear resistance when steel parts are actually used.

In the present invention, among carbides containing at least one of Nb, Ti, and V, those having a particle size of 0.1 μm or more are called coarse carbides, and those having a particle size of less than 0.1 μm are called fine carbides. Define.

- Coarse carbide Coarse carbide has the effect of suppressing static wear. For example, in the case of textile machine parts such as knitting needles, the presence of coarse carbides can reduce wear due to rubbing by fibers and foreign substances such as pebbles adhering to the fibers. However, if the average grain size of coarse carbides is less than 0.15 μm, resistance to static wear is not exhibited. Therefore, the average particle size of coarse carbides is set to 0.15 μm or more. On the other hand, if the carbides become too coarse, the chances of the carbides functioning as resistance are reduced, so the wear resistance effect is saturated. Therefore, the average grain size of coarse carbides is set to 2.5 μm or less. The number density of coarse carbides is not particularly limited, but the higher the density, the better, and it is preferably 250/mm ² or more. The coarse carbides may exist either at grain boundaries or inside grains.

・Fine carbides Fine carbides stabilize the dislocation structure and prevent the dislocations from recovering when the temperature rises due to frictional heat. Therefore, by precipitating fine carbides, softening due to frictional heat can be suppressed, and wear resistance in a hot environment can be improved. The effect of suppressing dislocation recovery increases as the average grain size of fine carbides decreases. Therefore, the average grain size of fine carbides is set to 0.05 μm or less. On the other hand, if the carbides are excessively fine, the hardness excessively increases, leading to embrittlement. Therefore, the average grain size of fine carbides is set to 0.005 μm or more. Fine carbides are more effective when formed inside grains than at grain boundaries. The number density of the fine carbides is not particularly limited, but the larger the number, the greater the effect, and it is preferably 0.11/μm ² or more.

Further, the structure of the steel part of the present invention is not particularly limited, and desired characteristics can be obtained as long as the above conditions are satisfied. Typically, the structure of the steel component of the present invention may be a structure consisting of tempered martensite, cementite, and carbide containing at least one of Nb, Ti, and V. The carbide containing cementite and at least one of Nb, Ti and V is preferably spheroidized. Specifically, it is preferable that the spheroidization rate of the carbide defined by the following formula using the average major axis La and average minor axis Lb of the carbide is 0.71 or more.
Spheroidization rate = Lb/La
La is obtained by dividing the sum of the major diameters of all carbides present in the range of 100 μm ² by the number of the carbides. Also, Lb is obtained by dividing the sum of the short diameters of all carbides present in the range of 100 μm ² by the number of the carbides.

[Thickness]
The thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness, but is preferably 0.1 mm or more, more preferably 0.2 mm or more. Also, the upper limit of the plate thickness is not particularly limited, but it is preferably 2.5 mm or less, more preferably 1.6 mm or less, and even more preferably 0.8 mm or less. When the plate thickness is 0.2 mm or more and 0.8 mm or less, it can be particularly suitably used as a material for knitting needles.

[Manufacturing method of cold-rolled steel sheet]
Next, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

The cold-rolled steel sheet can be produced by sequentially subjecting a steel slab having the chemical composition described above to the following steps.
(1) heating (2) hot rolling (3) cooling (4) coiling (5) first annealing (6) cold rolling (7) second annealing (8) final cold rolling (9) working and heat treatment, and the above steps (6) and (7) are repeated two or more times. Each step will be described below.

(1) Heating First, a steel slab having the chemical composition described above is heated. The steel slab can be manufactured by any method without particular limitation. For example, the composition adjustment of the steel slab may be performed by a blast furnace converter method or by an electric furnace method. Casting of molten steel into slabs may be performed by continuous casting or by blooming.

Slab heating temperature: 1100°C or higher Holding time: 1.0 hour or longer The slab is heated under the conditions of a slab heating temperature of 1100° C. or higher and a holding time of 1.0 hour or longer.

In the step of heating the slab, it is necessary to precipitate some coarse Nb, Ti, and V-based carbides bonded with C, but other undissolved carbides have desired dimensions in the subsequent annealing step. In order to precipitate in the slab, it should be dissolved in the slab heating stage. When the slab heating temperature is lower than 1100° C. and the holding time is shorter than 1 hour, coarse Nb, Ti, V-based carbides are not precipitated, and later, the slab is used for increasing the resistance to static wear. Coarse Nb-, Ti-, and V-based carbides cannot be obtained. On the other hand, if the slab heating temperature is too high, Nb, Ti, and V will form a solid solution and the amount of precipitation will decrease.

(2) Hot rolling Next, the heated steel slab is hot rolled to form a hot rolled steel sheet. In the hot rolling, rough rolling and finish rolling can be carried out according to a conventional method.

Finish rolling start temperature: Ac 3 point or more When the finish rolling start temperature in the hot rolling is less than Ac 3 point, expanded ferrite is generated in the steel sheet after hot rolling, and this expanded ferrite is finally obtained. It also remains in cold-rolled steel sheets. As a result, the formation of carbides in grains and grain boundaries, which is effective in improving wear resistance, is prevented. Therefore, the finish rolling start temperature in the hot rolling is set to Ac3 point or higher. On the other hand, although the upper limit of the finish rolling entry side temperature is not particularly limited, it is preferably 1200° C. or less.

The Ac3 point (° C.) is obtained by the following formula (1).
Ac3 (°C) = 910 - (203 x C1 ^/2 ) + (44.7 x Si) - (30 x Mn) - (11 x Cr) + (400 x Ti) + (460 x Al) + (700 x P ) + (104 × V) + 38 … (1)
Here, the element symbol in the above formula (1) indicates the content (% by mass) of each element, and is zero when the element is not contained.

(3) Time from the end of cooling hot rolling to the start of cooling: 2.0 seconds or less Next, the hot rolled steel sheet is cooled. At that time, if a long time elapses from the end of hot rolling to the start of cooling, coarse ferrite grains are formed on the surface layer of the steel sheet and remain until the subsequent steps. As a result, precipitation of carbides in grain boundaries and grains, which is effective in improving wear resistance, is prevented. Therefore, the time from the end of hot rolling to the start of cooling is set to 2.0 seconds or less. On the other hand, from the above point of view, the shorter the time from the end of hot rolling to the start of cooling, the better, so the lower limit is not particularly limited. However, from the viewpoint of industrial production, it may be 0.5 seconds or longer, or 0.8 seconds or longer.

Average cooling rate: 25° C./s or more Similarly, when the average cooling rate during cooling is less than 25° C./s, coarse ferrite grains are formed on the surface layer of the steel sheet, which is effective for improving wear resistance. Carbide precipitation is prevented. Therefore, the average cooling rate in the cooling is set to 25° C./s or higher. On the other hand, the upper limit of the average cooling rate is not particularly limited, but if the cooling rate is excessively high, the coiled shape becomes defective due to volume expansion due to transformation during subsequent coiling. Therefore, from the viewpoint of improving the winding shape, the average cooling rate is preferably 160° C./s or less, more preferably 150° C./s or less.

Cooling stop temperature: 640°C to 720°C
Also, if the cooling stop temperature during the cooling is too high, a non-uniform structure consisting of extraordinarily coarse portions and fine portions is formed, and as a result, subsequent formation of carbides is inhibited. Therefore, the cooling stop temperature is set to 720° C. or lower. On the other hand, if the cooling stop temperature is too low, the winding shape becomes defective due to volume expansion due to transformation during winding. As a result, non-uniform strain is introduced into the steel sheet in the subsequent cold rolling process, so that carbide having a desired grain size cannot be obtained and the wear resistance is not improved. Therefore, the cooling stop temperature is set to 640° C. or higher.

(4) Winding After stopping the cooling, the cooled hot-rolled steel sheet is wound into a coil. At that time, the winding temperature is not particularly limited, but it is preferably 600 to 700°C.

(5) First annealing Annealing temperature: 650°C or higher and 720°C or lower Annealing time: 3 hours or longer Annealing temperature: 650°C or higher and 720°C or lower Annealing time: 3 hours or longer A first annealing is performed under the conditions of The structure of the hot-rolled steel sheet after coiling is a pearlite structure in which cementite and ferrite formed in a plate form are arranged side by side. If the structure is pearlite, strain may not be stably introduced in subsequent cold rolling, resulting in shape defects in the cold-rolled steel sheet. Therefore, it is necessary to break the pearlite structure and spheroidize the carbide. However, since the pearlite structure is stable against heat, it is easy to maintain the plate shape. It is necessary to divide the plate-like structure by keeping it at a high temperature for a long time and increase the area of the interface. Therefore, the first annealing is performed at a temperature of at least 650° C. for 3 hours or longer. By further performing cold rolling and second annealing after performing the first annealing, the plate-shaped cementite is easily split. On the other hand, if the annealing temperature is higher than 720° C., the structural change progresses preferentially from one part, resulting in a mixed structure of a coarse structure and a fine structure. performance does not improve. The upper limit of the annealing time is not particularly limited. Therefore, it is preferable to set the time to 20 hours or less.

It is also preferable to pickle the hot-rolled steel sheet prior to the first annealing.

(6) Cold rolling (7) Second annealing In cold rolling, the plate-like carbides divided by the first annealing are further divided to disperse throughout the steel sheet and undergo the second annealing process. This is an important process for improving wear resistance because it becomes dispersed throughout the plate in desired dimensions. Since the plate-like carbides formed after hot rolling and coiling are stable, they tend to remain until later, and the formation of plate-like carbides can cause voids and cracks. In addition, since it does not disperse throughout the steel sheet, wear progresses unilaterally in structural parts where carbide formation does not occur.

Therefore, in order to precipitate carbides having a particle size effective for improving wear resistance, the hot-rolled steel sheet after the first annealing is repeatedly subjected to cold rolling and second annealing two or more times. By performing cold rolling, the plate-like carbides generated in the steel sheet are divided, and by further performing the second annealing, carbides of a desired size can be dispersed throughout the steel sheet. In order to obtain the above effects, the rolling reduction of the cold rolling is set to 15% or higher, and the annealing temperature in the second annealing is set to 600° C. or higher. On the other hand, if the annealing temperature is higher than 800° C., the prior austenite grains in the structure finally obtained are coarsened, and the wear resistance at the time of frictional heat generation is lowered. Therefore, the annealing temperature is set to 800° C. or lower.

In addition, if the temperature rise rate in the second annealing is too slow, local coarsening of carbides proceeds, and fine carbides necessary for suppressing softening of the steel sheet due to temperature rise during friction are not obtained. do not have. Therefore, the rate of temperature increase is set to 50° C./h or more. On the other hand, although the upper limit of the temperature increase rate is not particularly limited, it is preferably 200° C./s or less.

The rolling reduction in the cold rolling is preferably as high as possible, but if the rolling reduction is 65% or more, the shape of the resulting cold-rolled steel sheet may become unstable. Therefore, the rolling reduction is preferably less than 65%.

The number of repetitions of the cold rolling and the second annealing is two or more. By repeating cold rolling and annealing two or more times, the structure can be refined and the carbides can be dispersed throughout the steel sheet to finally obtain a desired carbide size. In order to stably obtain a good steel plate shape and plate thickness accuracy, the number of repetitions is preferably as large as possible, and the upper limit of the number of repetitions is not particularly limited. However, if the number of repetitions exceeds 5, the effect saturates, so the number of repetitions is preferably 5 or less.

(8) Final cold rolling reduction: 30% or more After repeating cold rolling and second annealing twice or more as described above, final cold rolling is further performed at a rolling reduction of 30% or more. By performing final cold rolling at a rolling reduction of 30% or more, fine carbides are formed during final quenching and tempering, and wear resistance at the time of frictional heat generation is improved. Further, by performing the final cold rolling at a rolling reduction of 30% or more, the grain size of the prior austenite is refined, so that the wear resistance is further improved. The rolling reduction in the final cold rolling is preferably as large as possible, but if it is 65% or more, the shape of the steel sheet may become unstable. Therefore, the rolling reduction is preferably less than 65%.

Further, the finally obtained cold-rolled steel sheet may be subjected to arbitrary surface treatment.

(9) Working and Heat Treatment Next, the obtained cold-rolled steel sheet is worked into a part shape and heat treated to obtain a final steel part. The processing method is not particularly limited, and any method can be used. The processing may be, for example, at least one of punching, cutting, wire drawing, bending, and polishing.

The heat treatment includes quenching under conditions of a quenching temperature of 700° C. or higher and 950° C. or lower and a holding time of 1.0 minute or longer and 60 minutes or shorter, and a tempering temperature of 100 to 400° C. and a holding time of 20 minutes or longer. Tempering under conditions of 3 hours or less. The quenching and tempering conditions are important for controlling the grain size of carbides and prior austenite grains to obtain excellent wear resistance.

In order to generate fine carbides, it is necessary to raise the quenching temperature (heating temperature during quenching). Therefore, the quenching temperature should be 700° C. or higher, preferably 720° C. or higher. On the other hand, if the quenching temperature is too high, the grain size of the prior austenite will increase and the wear resistance will decrease. Therefore, the quenching temperature should be 950° C. or lower, preferably 920° C. or lower.

During the quenching, it is necessary to maintain the heating temperature for 1.0 minutes or more in order to form carbides of desired dimensions. Therefore, the retention time is set to 1.0 minute or longer. On the other hand, if the holding time exceeds 60 minutes, the prior austenite grains become coarse and the wear resistance decreases. Therefore, the retention time is set to 60 minutes or less. Cooling in the quenching is preferably carried out to room temperature with oil or other cooling medium.

In order to improve hardness and obtain high wear resistance, it is necessary to lower the tempering temperature. Therefore, the tempering temperature should be 400° C. or lower, preferably 380° C. or lower. On the other hand, if the tempering temperature is too low, fine carbides will not grow and the desired dimensions will not be obtained. In addition, the hardness becomes too high and the material becomes brittle. Therefore, the tempering temperature should be 100° C. or higher, preferably 130° C. or higher.

If the holding time during tempering is less than 20 minutes, not only will fine carbides not grow and the desired dimensions will not be obtained, but also the hardness will become too high and embrittlement will occur. do. On the other hand, if the holding time exceeds 3 hours, the fine carbides become too coarse and do not have the desired dimensions. Therefore, the retention time is set to 3 hours or less.

The heat treatment may be performed after the processing, or may be performed during the processing.

By the above method, a steel part having excellent wear resistance can be manufactured. The steel parts are not particularly limited and can be used for any purpose, but are particularly suitable for applications requiring abrasion resistance, such as parts for textile machines, bearing parts, and cutting tools for machines. can be done.

A steel having the chemical composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. Next, the steel slab is sequentially subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling to obtain a final thickness of about 0. .4 mm cold-rolled steel sheet. Each step was performed under the conditions shown in Tables 2 and 3, and cold rolling and second annealing were repeated the number of times shown in Table 3. Thereafter, the cold-rolled steel sheets were subjected to heat treatment consisting of quenching and tempering under the conditions shown in Table 3 to obtain samples. In addition, in the present embodiment, the process of processing into the shape of the component is omitted.

The average grain size of prior austenite grains, the average grain size of coarse carbides, and the average grain size of fine carbides in the obtained samples were measured by the following procedure.

(Previous austenite grain size)
A test piece for observing carbide was taken from the obtained sample. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, final polishing was performed with colloidal silica, and EBSD measurement was performed to identify prior austenite grain boundaries. After identifying the prior austenite grain boundaries, the individual grain sizes and the number of grains were obtained, and the circle-equivalent diameter was calculated as the average grain size. Table 4 shows the evaluation results.

(coarse carbide)
A test piece for observing carbide was taken from the obtained sample. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, the structure was exposed by corroding the polished surface with a 1 to 3 vol % nital solution. Next, the surface of the test piece for tissue observation was imaged using an SEM at a magnification of 3000 to obtain a tissue image. The particle size of each carbide containing at least one of Nb, Ti, and V contained in the obtained structure image was measured by a cutting method, and the average value of the particle sizes of the carbides was calculated. Identification of Nb-, Ti-, and V-based carbides was performed using SEM-EDS analysis. Elemental mapping was performed on the observation field to separate cementite from other carbides, and the other carbides were defined as Nb, Ti, and V-based carbides. Table 4 shows the evaluation results. In addition, blanks (-) were given for those in which coarse carbides were not observed.

(Fine carbide)
A test piece for observing carbide was taken from the obtained sample, and after thinning to a thickness of about 70 μm, an observation sample was produced by electropolishing. Observe at a magnification of 150,000 to 250,000 times with a TEM, analyze carbides containing at least one of Nb, Ti, and V by TEM-EDS, determine the diameter of each carbide by a cutting method, and calculate the obtained grain size. It was defined as the average particle size of fine carbides on average. Table 4 shows the evaluation results. In addition, blanks (-) were given for those in which fine carbides were not observed.

(wear resistance)
The wear resistance of the obtained steel sheets after quenching and tempering was evaluated under the following two conditions.

First, the wear resistance under static conditions, in which temperature rise due to friction hardly occurs, was evaluated by the following procedure.

A wear test piece 10 having the shape shown in FIG. 1 was taken from each test piece. One abrasion test piece 10 is provided with four holes 11 for threading.

A wear test was carried out using the obtained wear test piece 10 and the wear test apparatus 20 shown in FIG. Specifically, the yarn S fed from the yarn unwinding device 21 was run 100,000 m per hole while contacting the side surface of the hole 11 of the abrasion test piece 10, and the amount of wear was measured. As for the running speed of the yarn S, full dull polyester knitting yarn was used, and the running speed of the yarn S was 5 m/min. Also, the tension of the thread was adjusted to 10±2 N/cm using the tension adjusting device 22 .

As shown in FIG. 3, a groove 12 is formed by abrasion at the portion of the hole 11 that was in contact with the thread. Therefore, when the thread was run for 100,000 m, the running was stopped, and the depth d (wear depth) of the groove 12 was measured with an optical microscope.

A similar test was performed on four holes 11, and the average value of the obtained four wear depths was taken as the wear depth of the wear test piece. If the abrasion depth was less than 490 μm, the abrasion resistance was judged as good (◯), and if it was 490 μm or more, the abrasion resistance was judged as poor (x). Table 4 shows the evaluation results.

Next, in order to evaluate wear resistance under conditions where the temperature rises due to friction, a test was conducted in the same procedure as the test under static conditions except that the yarn running speed was 180 m / min. The wear resistance was evaluated according to the criteria of Table 4 shows the evaluation results.

10 Abrasion test piece 11 Hole 12 Groove 20 Abrasion test device 21 Yarn unwinding device 22 Tension adjusting device 23 Yarn winding device S Yarn d Wear depth

Claims (2)

in % by mass,
C: 0.6 to 1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005 to 0.05%,
S: 0.01% or less,
Al: 0.001 to 0.1%,
N: 0.001 to 0.009%,
Cr: 0.05 to 0.55%, and one or more of Ti: 0.05 to 1.0%, Nb: 0.1 to 0.5%, and V: 0.01 to 1.0% including
A steel slab having a chemical composition with the balance being Fe and unavoidable impurities is heated under the conditions of a slab heating temperature of 1100° C. or higher and a holding time of 1.0 hour or longer,
The heated steel slab is hot-rolled under the conditions of finish rolling start temperature: 1010 ° C. or higher to obtain a hot-rolled steel plate,
The hot-rolled steel sheet is cooled under the conditions of time from the end of hot rolling to the start of cooling: 2.0 seconds or less, average cooling rate: 25 ° C./s or more, cooling stop temperature: 640 ° C. to 720 ° C.,
Winding the cooled hot-rolled steel sheet,
The coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 650° C. or higher and 720° C. or lower and an annealing time of 3 hours or longer,
The hot-rolled steel sheet after the first annealing is cold-rolled at a rolling rate of 15% or more, and subjected to annealing at a temperature of 600 to 800 ° C. and a heating rate of 50 ° C./h or more and 190 ° C./h or less. The second annealing is repeated twice or more, and then final cold rolling is performed at a rolling reduction of 30% or more to make a cold rolled steel sheet,
For the cold-rolled steel sheet,
Machining into part shape and
Quenching temperature: 700°C to 950°C, holding time: 1.0 minute to 60 minutes, and tempering temperature: 100 to 400°C, holding time: 20 minutes to 3 hours. By applying heat treatment including tempering under conditions,
The average grain size of the prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V, and
The average particle size of particles of the carbide having a particle size of 0.1 μm or more is 0.15 to 2.5 μm,
A method of manufacturing a steel part, wherein the average particle size of the particles of the carbide having a particle size of less than 0.1 μm is 0.005 to 0.05 μm. The component composition, in mass%,
Sb: 0.1% or less,
Hf: 0.5% or less,
REM: 0.1% or less,
Cu: 0.5% or less,
Ni: 3.0% or less,
Sn: 0.5% or less,
Mo: 1% or less,
Zr: 0.5% or less,
The method for manufacturing a steel component according to claim 1, further comprising one or more selected from the group consisting of B: 0.005% or less and W: 0.01% or less.

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