JP7626165B2 - Manufacturing method for steel parts - Google Patents

Description

The present invention relates to a steel part, in particular a steel part with excellent wear resistance. The present invention also relates to a method for manufacturing the steel part.

Carbon steel, which is a steel that contains a high concentration of carbon, has high hardness and is therefore widely used as a material for steel parts that require wear resistance, such as parts for textile machinery, bearing parts, and machine blades.

In the manufacture of typical steel parts, cold-rolled steel sheets are processed into the shape of the part and then quenched and tempered. Increasing hardness through quenching reduces toughness, but toughness can be improved by performing tempering afterwards. However, there is an issue with tempering, in that it reduces hardness.

As a result, various technologies have been proposed to further increase the hardness of steel parts and achieve even better wear resistance.

For example, Patent Document 1 discloses a technology for improving formability and wear resistance in steel plate having a ferrite-cementite structure by increasing the grain size of ferrite, spheroidizing carbides (mainly cementite) to an appropriate grain size, and reducing the pearlite structure.

Patent Document 2 also discloses a technology in which cold-rolled steel sheet is annealed under specific conditions to change the metal structure to a pearlite structure, which is a layered structure of hard cementite and soft ferrite, thereby improving the wear resistance of the cold-rolled steel sheet.

Patent Document 3 discloses a technology for improving the wear resistance of steel plates by precipitating coarse Nb-Ti carbides with a circle equivalent diameter of 0.5 μm or more in the ferrite phase, which is the matrix structure.

Patent Document 4 discloses a technology that improves the wear resistance of steel materials by precipitating coarse carbides with a particle size of 2 μm or more in the matrix structure.

Patent Document 5 proposes a technology for improving the spheroidization rate of carbides such as cementite and improving toughness by subjecting steel plate containing 0.5 to 0.7 mass% C to an annealing finish immediately before the final quenching and tempering treatment.

Patent Document 6 proposes a technology that increases the density of voids generated in the material by annealing the material just before the final hardening and tempering treatment, thereby producing a soft high-carbon steel sheet with excellent punchability.

Patent Document 7 proposes a technology for improving impact toughness and wear resistance in high carbon steel plate by controlling the formation of cementite carbide, which does not contain niobium, titanium, or vanadium carbides, and by setting the spheroidization rate and number density of the cementite carbide to desired values.

Patent Document 8 proposes a technology that improves toughness by adjusting the grain size of cementite carbide that does not contain niobium, titanium, or vanadium carbides, the grain size of retained austenite, and prior austenite, by performing an annealing finish immediately before the final austempering heat treatment, and further by obtaining a bainite structure rather than a tempered martensite structure obtained by quenching and tempering, which is a typical heat treatment for refining.

International Publication No. 2016/204288 JP 2020-132953 A JP 2017-190494 A JP 2010-138453 A JP 2009-24233 A JP 2011-12316 A Patent No. 6880245 JP 2018-48374 A

Conventional techniques such as those proposed in Patent Documents 1 to 8 have shown a certain degree of improvement in the hardness and wear resistance of steel materials. However, the inventors' investigations have revealed that when steel parts manufactured using conventional steel materials are actually used, there are cases in which sufficient wear resistance is not obtained.

The present invention was made in consideration of the above circumstances, and aims to provide steel parts with excellent wear resistance.

As a result of investigating methods to solve the above problems, the inventors have come to the following conclusions.

(1) When steel parts are actually used, their temperature rises due to friction with other parts. For example, when steel parts are used as textile machine parts 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 caused by materials grinding against each other, but also to suppress the softening of the steel plate that accompanies the rise in temperature caused by friction.

(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. In this case, among the carbides, coarse carbides have the effect of suppressing static wear. For example, in the case of textile machine parts, the presence of coarse carbides can reduce the amount of wear caused by fibers or foreign objects such as pebbles attached to the fibers.

(4) On the other hand, fine carbides among the above carbides have the effect of suppressing the softening of the steel sheet due to the rise in temperature caused by friction. In other words, the presence of fine carbides suppresses the recovery of dislocation structure and the decrease in hardness when the temperature rises due to friction. Furthermore, in steel containing fine carbides, the prior austenite grains are refined when quenched and tempered, which enhances the grain boundary strengthening effect, and as a result, the decrease in hardness due to the recovery of dislocation structure is further suppressed.

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

The present invention was completed based on the above findings, and its gist is as follows:

1. In mass percent,
C: 0.6-1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005-0.05%,
S: 0.01% or less,
Al: 0.001-0.1%,
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%,
A steel slab having a composition with the balance being Fe and unavoidable impurities is heated under the conditions of a slab heating temperature of 1100° C. or more and a holding time of 1.0 hour or more;
The heated steel slab is hot-rolled under a finish rolling start temperature of 1010°C or higher to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of a time from the end of the hot rolling to the start of cooling: 2.0 seconds or less, an average cooling rate: 25 ° C./s or more, and a cooling stop temperature: 640 ° C. to 720 ° C.,
The cooled hot-rolled steel sheet is coiled.
The coiled hot-rolled steel sheet is subjected to a first annealing under the conditions of an annealing temperature of 650° C. or more and 720° C. or less and an annealing time of 3 hours or more;
The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling ratio 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 and 190° C./h or less, which are repeated at least twice, and then subjected to final cold rolling at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
For the cold rolled steel sheet,
Processing into part shape and
The heat treatment includes quenching at a quenching temperature of 700°C to 950°C and a holding time of 1.0 minute to 60 minutes, and tempering at a tempering temperature of 100 to 400°C and a holding time of 20 minutes to 3 hours.
The average grain size of prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V; and
The average particle size of the carbide particles having a particle size of 0.1 μm or more is 0.15 to 2.5 μm;
The method for manufacturing a steel part includes the step of producing a steel part in which the average particle size of the carbide particles having a particle size of less than 0.1 μm is 0.005 to 0.05 μm.

2. The component composition is, 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 producing a steel part 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.

The main configuration of other embodiments of the present invention is as follows:

1. In mass percent,
C: 0.6-1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005-0.05%,
S: 0.01% or less,
Al: 0.001-0.1%,
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%,
The balance is Fe and unavoidable impurities,
The average grain size of prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V; and
The average particle size of the carbide particles having a particle size of 0.1 μm or more is 0.15 to 2.5 μm;
The steel part has an average grain size of 0.005 to 0.05 μm among the carbides having a grain size of less than 0.1 μm.

2. The component composition is, 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 steel part 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.

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 blade.

4. A steel slab having the 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 made into a hot-rolled steel sheet under the condition of a finish rolling start temperature of Ac3 point or more,
The hot-rolled steel sheet is cooled under the conditions of a time from the end of the hot rolling to the start of cooling: 2.0 seconds or less, an average cooling rate: 25 ° C./s or more, and a cooling stop temperature: 640 ° C. to 720 ° C.,
The cooled hot-rolled steel sheet is coiled.
The coiled hot-rolled steel sheet is subjected to a first annealing under the conditions of an annealing temperature of 650° C. or more and 720° C. or less and an annealing time of 3 hours or more;
The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling ratio 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, which are repeated at least twice, and then subjected to final cold rolling at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
For the cold rolled steel sheet,
Processing into part shape and
A manufacturing method for a steel part, comprising the steps of: quenching at a quenching temperature of 750°C or higher and 950°C or lower for a holding time of 1.0 minute or higher and 60 minutes or lower; and tempering at a tempering temperature of 100 to 320°C for a holding time of 20 minutes or higher and 3 hours or lower.

The present invention provides steel parts with excellent wear resistance. The steel parts of the present invention have excellent wear resistance not only under static conditions but also under conditions where the temperature is elevated due to friction, and therefore can be suitably used for a variety of applications including textile machine parts, bearing parts, and machine blades.

FIG. 2 is a schematic diagram showing the shape of a wear test piece. FIG. 1 is a schematic diagram of an abrasion test device. FIG. 2 is a schematic diagram showing the shape of an abrasion test device used in the examples.

The present invention will be described in detail below. Note that the present invention is not limited to this embodiment. As described above, the present invention focuses on carbides containing at least one of Nb, Ti, and V. Therefore, in the following description, "carbides containing at least one of Nb, Ti, and V" may be simply referred to as "carbides."

[Component composition]
The cold-rolled steel sheet of the present invention has the above-mentioned composition. The reasons for the limitations will be described below. In the following description, "%" as a unit of content refers to "mass%" unless otherwise specified.

C: 0.6-1.25%
C is an element necessary for improving the hardness after quenching and tempering. In addition, C is also an element necessary for generating cementite and carbides with elements such as Nb, Ti, and V. In order to generate the necessary carbides and obtain the hardness and wear resistance after quenching and tempering, the C content must be 0.6% or more. Therefore, the C content is 0.6% or more, preferably 0.7% or more. On the other hand, if the C content exceeds 1.25%, the hardness increases excessively and the material becomes embrittled. In addition, if the C content exceeds 1.25%, the surface scale during heating becomes strong, resulting in deterioration of the surface properties, and not only is it easy for cracks to appear on the surface during subsequent cold rolling, but cracks occur during quenching and the wear resistance decreases. Therefore, the C content is 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 is set to 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 steel sheet surface during hot working, and the formation of carbides necessary for improving wear resistance in the subsequent process is inhibited. Therefore, the Si content is set to 0.55% or less, preferably 0.50% or less, more preferably 0.45% or less.

Mn: 0.20-2.0%
Mn is an element that has the effect of promoting quenching and suppressing temper softening, thereby improving hardness. In order to suppress temper softening, it is necessary to suppress the formation of C as cementite or to delay dislocation recovery, and Mn has both of these effects. Mn also 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 effect, the Mn content is set to 0.20% or more, preferably 0.25% or more. On the other hand, if the Mn content exceeds 2.0%, band-shaped structures are formed due to the segregation of Mn. In particular, abnormal grain growth and heterogeneity of the structure are likely to occur in the segregated parts of MnS, which suppresses the formation of carbides and causes cracks and shape defects during part processing. Therefore, the Mn content is set to 2.0% or less, preferably 1.95% or less.

P: 0.0005-0.05%
By adding a small amount of P, the hardness can be increased by solid solution strengthening, and as a result, the wear resistance can be improved. In order to obtain the above effect, the P content is set to 0.0005% or more, preferably 0.0008% or more. On the other hand, if the P content exceeds 0.05%, the strength of the grain boundary decreases and the material becomes embrittled. Therefore, the P content is set to 0.05% or less, preferably 0.045% or less.

S: 0.01% or less S forms sulfides with Mn and consumes Mn, which reduces hardenability. When hardenability is reduced, the strength of the steel is reduced, and as a result, the wear resistance is reduced. 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, so the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in manufacturing costs, so from the viewpoint of industrial production, the S content is preferably set to 0.0005% or more, and more preferably set to 0.001% or more.

Al: 0.001-0.1%
Al is an element necessary for deoxidization during steelmaking. Therefore, the Al content is set to 0.001% or more. On the other hand, excessive Al causes the formation of coarse nitrides. The nitrides are often formed on the surface of the 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 is set to 0.1% or less, preferably 0.08% or less, and more preferably 0.06% or less.

N: 0.001-0.009%
Adding a small amount of N forms fine nitrides, which have finer grain sizes and improve toughness. To achieve this effect, the N content is set to 0.001% or more. On the other hand, if there is an excess of N, it bonds with Al to form coarse nitrides. The nitrides are often formed on the surface of the 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 is set to 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 hardness, and therefore, adding Cr can improve wear resistance. In order to obtain the above effect, the Cr content is set to 0.05% or more, preferably 0.12% or more. On the other hand, if Cr is excessive, coarse Cr carbides and Cr nitrides are formed, and voids are generated around the Cr carbides and Cr nitrides, resulting in a decrease in the performance of the steel parts. In addition, as a result of the generation of Cr carbides, the generation of carbides that are effective in improving wear resistance is hindered. Therefore, the Cr content is set to 0.55% or less, preferably 0.95% or less.

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

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 making the prior austenite grains fine during quenching and suppressing dislocation recovery. When Ti is added, in order to obtain the above effect, the Ti content is set to 0.05% or more, preferably 0.015% or more. On the other hand, if Ti is added in excess, the carbides become coarser than necessary, and the carbides become the starting points of voids and cracks, so that the workability when processing the steel plate into a part shape is reduced. Therefore, the Ti content is set to 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. In addition, Nb also has the effect of improving wear resistance by making the prior austenite grains fine during quenching and suppressing dislocation recovery. When Nb is added, the Nb content is set to 0.1% or more in order to obtain the above effect. On the other hand, if Nb is added in excess, the carbides become coarser than necessary, and the carbides become the starting points of voids and cracks, so that the workability when processing the steel plate into a part shape is reduced. Therefore, the Nb content is set to 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 making the prior austenite grains fine during quenching and suppressing dislocation recovery. When V is added, the V content is set to 0.01% or more in order to obtain the above effect. On the other hand, if V is added in excess, the carbides become coarser than necessary, and the carbides become the starting points of voids and cracks, so that the workability when processing the steel plate into a part shape is reduced. Therefore, the V content is set to 1.0% or less, preferably 0.95% or less.

In one embodiment of the present invention, the cold-rolled steel sheet has a composition consisting of the above components, with the remainder being Fe and unavoidable impurities.

In another embodiment of the present invention, the above composition may further contain one or more selected from the group consisting of 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, B: 0.005% or less, and W: 0.01% or less.

Sb: 0.1% or less Sb is an element effective in improving corrosion resistance, but if added in excess, it generates an Sb-rich layer under the scale generated during hot rolling, which causes surface scuffs (scratches) on the steel sheet after hot rolling. Therefore, the Sb content is set to 0.1% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but from the viewpoint of enhancing the effect of addition, it is preferable that the Sb content be 0.0003% or more.

Hf: 0.5% or less Hf is an element effective in improving corrosion resistance, but if added in excess, it generates a Hf-rich layer under the scale generated during hot rolling, which causes surface scuffs (scratches) on the steel sheet after hot rolling. 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, it is preferable that the Hf content be 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 inhomogeneous deformation during cold working, which may deteriorate 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 enhancing the effect of addition, it is preferable that the REM content is set to 0.005% or more.

Cu: 0.5% or less Cu is an effective element for improving corrosion resistance, but if added in excess, a Cu-rich layer is formed under the scale formed during hot rolling, causing surface scuffs (scratches) on the steel sheet after hot rolling. Therefore, the Cu content 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 enhancing the effect of addition, it is preferable that the Cu content be 0.01% or more.

Ni: 3.0% or less Ni is an element that improves the strength of steel. However, excessive addition of Ni may promote inhomogeneous deformation during cold working and deteriorate 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 enhancing the effect of addition, the Ni content is preferably set to 0.01% or more.

Sn: 0.5% or less Sn is an effective element for improving corrosion resistance, but if added in excess, it will form a Sn-rich layer under the scale formed during hot rolling, causing surface scuffs (scratches) on the steel sheet after hot rolling. 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, it is preferable that the Sn content be 0.0001% or more.

Mo: 1% or less Mo is an element that improves the strength of steel. However, excessive addition of Mo may delay the spheroidization of carbides and promote inhomogeneous deformation during cold working, which may deteriorate 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 enhancing the effect of addition, it is preferable that the Mo content is set to 0.001% or more.

Zr: 0.5% or less Zr is an element effective in improving corrosion resistance, but if added in excess, a Zn-rich layer is formed under the scale formed during hot rolling, causing surface scuffs (scratches) on the steel sheet after hot rolling. Therefore, the Zr content is set to 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, it is preferable that the Zr content be 0.01% or more.

B: 0.005% or less B is an element that has the effect of improving hardenability and can be added as desired. However, if the B content exceeds 0.005%, cracks are likely to occur on the surface during hardening. 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 effect of addition, when B is added, the B content is preferably set to 0.0001% or more.

W: 0.01% or less W is an element that has the effect of improving hardenability and can be added as desired. However, if the W content exceeds 0.01%, cracks are likely to occur on the surface during hardening. 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 set to 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, dislocation recovery during frictional heat generation is suppressed, making it possible to maintain hardness even in a thermal environment and improving wear resistance. In order to achieve this effect, the average grain size of the prior austenite grains is set to 25 μm or less.

[carbide]
The steel part of the present invention contains carbides containing at least one of Nb, Ti, and V. Conventionally, cementite has been used to improve wear resistance, but since carbides containing at least one of Nb, Ti, and V are harder than cementite, by precipitating carbides containing at least one of Nb, Ti, and V, it is possible to improve wear resistance more than conventionally.

As mentioned above, the coarse carbides have the effect of suppressing static wear, while the fine carbides have the effect of suppressing wear when the temperature rises due to friction. Therefore, by appropriately controlling the particle size of each of the coarse and fine carbides, it is possible to effectively improve the wear resistance of steel parts when they are actually used.

In this invention, carbides containing at least one of Nb, Ti, and V with a grain size of 0.1 μm or more are defined as coarse carbides, and those with a grain size of less than 0.1 μm are defined as fine carbides.

Coarse carbides Coarse carbides have 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 caused by rubbing with fibers or foreign objects such as pebbles attached to the fibers. However, if the average particle size of the coarse carbides is less than 0.15 μm, resistance to static wear is not exhibited. Therefore, the average particle size of the coarse carbides is set to 0.15 μm or more. On the other hand, if the carbides become too coarse, the opportunity for the carbides to function as resistance decreases, so the resistance effect to wear is saturated. Therefore, the average particle size of the coarse carbides is set to 2.5 μm or less. The number density of the coarse carbides is not particularly limited, but the higher the density, the better, and it is preferably 250 pieces/mm2 or ^more . The coarse carbides may be present either at the grain boundaries or within the grains.

Fine carbides Fine carbides stabilize the dislocation structure and prevent dislocation recovery 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 thermal environment can be improved. The effect of suppressing dislocation recovery is higher as the average particle size of the fine carbides is smaller. Therefore, the average particle size of the fine carbides is set to 0.05 μm or less. On the other hand, if the carbides are excessively fine, the hardness increases excessively and embrittlement occurs. Therefore, the average particle size of the fine carbides is set to 0.005 μm or more. It is more effective for fine carbides to be generated within crystal grains than at grain boundaries. The number density of the fine carbides is not particularly limited, but the more the effect, the greater the effect, and it is preferable that the number density is 0.11 pieces/μm2 or ^more .

The structure of the steel part of the present invention is not particularly limited, and the desired characteristics can be obtained as long as the above conditions are satisfied. Typically, the structure of the steel part of the present invention may be a structure consisting of tempered martensite, cementite, and carbides containing at least one of Nb, Ti, and V. The cementite and carbides containing at least one of Nb, Ti, and V are preferably spheroidized. Specifically, the spheroidization rate of the carbides, defined by the following formula using the average major axis La and the average minor axis Lb of the carbides, is preferably 0.71 or more.
Spheroidization rate = Lb/La
La is calculated by dividing the sum of the major axes of all carbides present within a 100 ^μm2 area by the number of the carbides. Lb is calculated by dividing the sum of the minor axes of all carbides present within a 100 ^μm2 area 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, and more preferably 0.2 mm or more. The upper limit of the thickness is also not particularly limited, but 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 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.

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

The cold-rolled steel sheet can be produced by sequentially carrying out the following steps on a steel slab having the above-mentioned composition.
(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. The above steps (6) and (7) are repeated two or more times. Each step is explained below.

(1) Heating First, a steel slab having the above-mentioned composition is heated. The steel slab can be produced by any method without any particular limitation. For example, the composition of the steel slab can be adjusted by a blast furnace converter method or an electric furnace method. Moreover, casting of the molten steel into a slab can be performed by a continuous casting method or by blooming rolling.

Slab heating temperature: 1100°C or higher, holding time: 1.0 hour or longer In the heating, the steel 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 order to homogenize the steel structure and to dissolve some of the carbides contained in the steel and precipitate the rest.

In addition, it is necessary to precipitate coarse Nb, Ti, V-based carbides combined with some of the C at the slab heating stage, but other undissolved carbides are dissolved at the slab heating stage in order to precipitate them to the desired dimensions at the later annealing stage. If the slab heating temperature is lower than 1100°C, or if the holding time is shorter than 1 hour, coarse Nb, Ti, V-based carbides will not precipitate, and coarse Nb, Ti, V-based carbides for increasing static wear resistance later will not be obtained. On the other hand, if the slab heating temperature is too high, Nb, Ti, and V will dissolve and the amount of precipitation will decrease, so it is preferable to set the slab heating temperature at 1380°C or less.

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

Finish rolling start temperature: Ac3 point or higher If the finish rolling start temperature in the hot rolling is lower than Ac3 point, extended ferrite is generated in the hot rolled steel sheet, and this extended ferrite remains in the finally obtained cold rolled steel sheet. As a result, the generation of carbides in grains and grain boundaries, which are effective in improving wear resistance, is hindered. Therefore, the finish rolling start temperature in the hot rolling is set to Ac3 point or higher. On the other hand, the upper limit of the finish rolling entry temperature is not particularly limited, but it is preferably set to 1200°C or lower.

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

(3) Cooling Time from the end of 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 passes between the end of hot rolling and the start of cooling, coarse ferrite grains are generated in the surface layer of the steel sheet and remain until the subsequent process. As a result, the precipitation of carbides in the grain boundaries and inside the grains, which is effective for improving wear resistance, is hindered. 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 viewpoint, 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 more, or may be 0.8 seconds or more.

Average cooling rate: 25°C/s or more Similarly, if the average cooling rate during the cooling is less than 25°C/s, coarse ferrite grains are generated in the surface layer of the steel sheet, and the precipitation of carbides, which is effective in improving wear resistance, is hindered. Therefore, the average cooling rate during the cooling is set to 25°C/s or more. 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 coiling shape becomes poor due to volume expansion caused by transformation during the subsequent coiling. Therefore, from the viewpoint of improving the coiling shape, the average cooling rate is preferably set to 160°C/s or less, and more preferably set to 150°C/s or less.

Cooling stop temperature: 640℃~720℃
In addition, if the cooling stop temperature during the cooling is too high, a non-uniform structure consisting of abnormally coarse and fine parts is formed, and as a result, the subsequent generation of carbides is hindered. 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 coiled shape becomes poor due to volume expansion caused by transformation during coiling. As a result, non-uniform strain is introduced into the steel sheet in the subsequent cold rolling process, so that carbides of the desired particle size cannot be obtained and the wear resistance is not improved. Therefore, the cooling stop temperature is set to 640°C or higher.

(4) Coiling After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. The coiling temperature is not particularly limited, but is preferably 600 to 700°C.

(5) First annealing Annealing temperature: 650°C or more, 720°C or less Annealing time: 3 hours or more The hot-rolled steel sheet after coiling is subjected to first annealing under the conditions of annealing temperature: 650°C or more, 720°C or less, and annealing time: 3 hours or more. The structure of the hot-rolled steel sheet after coiling is a pearlite structure in which cementite and ferrite formed in a plate shape are arranged side by side. If the structure is pearlite, strain is not stably introduced in the subsequent cold rolling, and the cold-rolled steel sheet may have a defective shape. Therefore, it is necessary to break down the pearlite structure and spheroidize the carbides. However, since the pearlite structure is stable against heat, it is easy to maintain the plate shape. It is necessary to hold the steel sheet at a high temperature for a long time to divide the plate-shaped structure and increase the area of the interface. Therefore, the first annealing is performed at a temperature of at least 650°C or more for 3 hours or more. By further performing cold rolling and second annealing after the first annealing, the cementite in the plate shape is easily broken. On the other hand, if the annealing temperature is higher than 720°C, the structural change proceeds preferentially from one part, resulting in a mixed structure of coarse and fine structures, and ultimately, carbides of the desired size cannot be obtained, and the wear resistance is not improved. There is no particular upper limit to the annealing time, but if it is too long, the productivity will decrease and the effect will saturate. Therefore, it is preferable to set it 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-shaped carbides divided in the first annealing are further divided and dispersed throughout the steel sheet, and the second annealing process allows them to be dispersed throughout the sheet in the desired dimensions, making it an important process for improving wear resistance. The plate-shaped carbides formed after hot rolling and coiling are stable, so they tend to remain for a long time, and the formation of plate-shaped carbides can cause void formation and cracks. In addition, in the structure parts where carbides are not formed because they are not dispersed throughout the steel sheet, wear progresses unilaterally.

Therefore, in order to precipitate carbides with a particle size effective for improving wear resistance, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing at least twice. The plate-shaped carbides formed in the steel sheet are broken down by cold rolling, and carbides of the desired size can be dispersed throughout the steel sheet by further performing the second annealing. To obtain the above effect, the rolling ratio of the cold rolling is set to 15% or more, and the annealing temperature in the second annealing is set to 600°C or more. On the other hand, if the annealing temperature is higher than 800°C, the prior austenite grains in the final structure will become coarse, and the wear resistance when frictional heat is generated will decrease. Therefore, the annealing temperature is set to 800°C or less.

If the heating rate in the second annealing is too slow, the carbides will become locally coarse, and the fine carbides necessary for suppressing the softening of the steel sheet due to the temperature rise during friction will not be obtained. Therefore, the heating rate is set to 50°C/h or more. On the other hand, the upper limit of the heating rate is not particularly limited, but it is preferable to set it to 200°C/s or less.

The higher the reduction ratio in the cold rolling, the better. However, if the reduction ratio is 65% or more, the shape of the resulting cold-rolled steel sheet may become unstable. Therefore, it is preferable to set the reduction ratio to less than 65%.

The cold rolling and second annealing are repeated two or more times. By repeating cold rolling and annealing two or more times, the structure is refined and the carbides are dispersed throughout the steel sheet, ultimately resulting in the desired carbide size. In order to stably obtain good steel sheet shape and thickness accuracy, it is preferable to repeat the process many times, and there is no particular upper limit to the number of repetitions. However, since the effect saturates if the number of repetitions exceeds five, it is preferable to keep the number of repetitions to five or less.

(8) Final cold rolling: Reduction ratio: 30% or more After repeating cold rolling and second annealing two or more times as described above, final cold rolling is further performed at a rolling ratio of 30% or more. By performing final cold rolling at a rolling ratio of 30% or more, fine carbides are generated during final quenching and tempering, and wear resistance during frictional heat generation is improved. In addition, by performing final cold rolling at a rolling ratio of 30% or more, the prior austenite grain size is refined, and wear resistance is further improved. It is better to have a large rolling ratio in the final cold rolling, but if it is 65% or more, the shape of the steel sheet may become unstable. Therefore, it is preferable that the rolling ratio is less than 65%.

The final cold-rolled steel sheet may then be further subjected to any surface treatment.

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

The heat treatment includes quenching at a quenching temperature of 700°C to 950°C and a holding time of 1.0 to 60 minutes, and tempering at a tempering temperature of 100 to 400°C and a holding time of 20 to 3 hours. The quenching and tempering conditions are important for controlling the grain size of the carbides and the grain size of the prior austenite to obtain excellent wear resistance.

To produce fine carbides, the quenching temperature (heating temperature during quenching) must be high. Therefore, the quenching temperature is set to 700°C or higher, and preferably 720°C or higher. On the other hand, if the quenching temperature is too high, the prior austenite grain size becomes large and wear resistance decreases. Therefore, the quenching temperature is set to 950°C or lower, and preferably 920°C or lower.

In order to generate carbides of the desired dimensions during the quenching, the heating temperature must be held for 1.0 minute or more. Therefore, the holding time is set to 1.0 minute or more. On the other hand, if the holding time exceeds 60 minutes, the prior austenite grains will become coarse and wear resistance will decrease. Therefore, the holding time is set to 60 minutes or less. For cooling during the quenching, it is preferable to cool to room temperature using oil or other cooling medium.

To improve hardness and obtain high wear resistance, it is necessary to lower the tempering temperature. Therefore, the tempering temperature is set to 400°C or less, and preferably 380°C or less. On the other hand, if the tempering temperature is too low, fine carbides will not grow and the desired dimensions will not be obtained. Furthermore, the hardness will become too high and the material will become embrittled. Therefore, the tempering temperature is set to 100°C or more, and preferably 130°C or more.

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

The heat treatment may be carried out after the processing or during the processing.

The above method allows the production of steel parts with excellent wear resistance. The steel parts can be used for any purpose, but are particularly suitable for use in applications requiring wear resistance, such as textile machine parts, bearing parts, and machine blades.

A steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by continuous casting. The steel slab was then heated, hot rolled, cooled, coiled, first annealed, cold rolled, second annealed, and final cold rolled in sequence to produce a cold-rolled steel sheet with a final thickness of approximately 0.4 mm. Each process was carried out 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. The cold-rolled steel sheet was then subjected to a heat treatment consisting of quenching and tempering under the conditions shown in Table 3 to obtain a sample. Note that in this example, the processing process into a part shape was omitted.

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

(Prior austenite grain size)
A test piece for carbide observation was taken from the obtained sample. The rolling direction cross section (L cross section) of the test piece for microstructure observation was polished, and then final polishing was performed with colloidal silica, and EBSD measurement was performed to identify the prior austenite grain boundaries. After identifying the prior austenite grain boundaries, the grain size and number of grains were obtained, and the circle equivalent diameter was calculated to obtain the average grain size. The evaluation results are shown in Table 4.

(coarse carbides)
A test piece for carbide observation was taken from the obtained sample. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, the polished surface was corroded with 1 to 3 vol% nital solution to reveal the structure. Next, the surface of the test piece for structure observation was photographed at a magnification of 3000 times using an SEM to obtain a structure 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 size of the carbide was calculated. Identification of Nb, Ti, and V-based carbides was performed using SEM-EDS analysis. Element mapping was performed on the observation field of view, and the cementite was separated from other carbides, and the other carbides were determined to be Nb, Ti, and V-based carbides. The evaluation results are shown in Table 4. Note that the items in which no coarse carbides were observed were left blank (-).

(fine carbide)
A test piece for carbide observation was taken from the obtained sample, thinned to a thickness of about 70 μm, and then electrolytically polished to prepare an observation sample. The specimen was observed at 150,000 to 250,000 times with a TEM, and carbides containing at least one of Nb, Ti, and V were analyzed by TEM-EDS, and the diameter of each carbide was determined by a cut-off method, and the obtained particle diameters were arithmetically averaged to obtain the average particle diameter of the fine carbides. The evaluation results are shown in Table 4. Note that the items in which fine carbides were not observed were left blank (-).

(Wear resistance)
The wear resistance of the resulting quenched and tempered steel plates was evaluated under the following two conditions.

First, we evaluated the wear resistance under static conditions, where there is almost no temperature rise due to friction, using the following procedure.

Abrasion test pieces 10 with the shape shown in Figure 1 were taken from each test piece. Each abrasion test piece 10 has four holes 11 for threading the thread.

A wear test was carried out using the obtained wear test piece 10 and the wear test device 20 shown in FIG. 2. Specifically, the yarn S fed from the yarn unwinding device 21 was run 100,000 m per hole while in contact with the side of the hole 11 of the wear test piece 10, and the amount of wear was measured. A full-dull polyester knitting yarn was used as the running speed of the yarn S, and the running speed of the yarn S was 5 m/min. The tension of the yarn was adjusted to 10±2 N/cm using the tension adjustment device 22.

As shown in Figure 3, a groove 12 is formed due to wear at the location of hole 11 that was in contact with the thread. Therefore, the thread was stopped when it had run for 100,000 m, and the depth d of groove 12 (wear depth) was measured using an optical microscope.

A similar test was performed on four holes 11, and the average of the four obtained wear depths was regarded as the wear depth of the wear test piece. If the wear depth was less than 490 μm, the wear resistance was rated as good (◯), and if it was 490 μm or more, the wear resistance was rated as poor (×). The evaluation results are shown in Table 4.

Next, to evaluate the abrasion resistance under conditions involving a temperature rise due to friction, the test was carried out in the same manner as the test under static conditions described above, except that the yarn running speed was set to 180 m/min, and the abrasion resistance was evaluated using the same criteria. The evaluation results are shown in Table 4.

10 wear test piece 11 hole 12 groove 20 wear test device 21 yarn unwinding device 22 tension adjusting device 23 yarn winding device S yarn d wear depth

Claims (2)

In mass percent,
C: 0.6-1.25%,
Si: 0.10 to 0.55%,
Mn: 0.20-2.0%,
P: 0.0005-0.05%,
S: 0.01% or less,
Al: 0.001-0.1%,
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%,
A steel slab having a composition with the balance being Fe and unavoidable impurities is heated under the conditions of a slab heating temperature of 1100° C. or more and a holding time of 1.0 hour or more;
The heated steel slab is hot-rolled under a finish rolling start temperature of 1010°C or higher to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of a time from the end of the hot rolling to the start of cooling: 2.0 seconds or less, an average cooling rate: 25 ° C./s or more, and a cooling stop temperature: 640 ° C. to 720 ° C.,
The cooled hot-rolled steel sheet is coiled.
The coiled hot-rolled steel sheet is subjected to a first annealing under the conditions of an annealing temperature of 650° C. or more and 720° C. or less and an annealing time of 3 hours or more;
The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling ratio 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 and 190° C./h or less, which are repeated at least twice, and then subjected to final cold rolling at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
For the cold rolled steel sheet,
Processing into part shape and
The heat treatment includes quenching at a quenching temperature of 700°C to 950°C and a holding time of 1.0 minute to 60 minutes, and tempering at a tempering temperature of 100 to 400°C and a holding time of 20 minutes to 3 hours.
The average grain size of prior austenite grains is 25 μm or less,
Carbide containing at least one of Nb, Ti, and V; and
The carbide particles have an average particle size of 0.1 μm or more and an average particle density of 250 particles/mm2 ^or more.
The method for manufacturing a steel part includes producing a steel part in which the average particle size of particles of the carbide having a particle size of less than 0.1 μm is 0.005 to 0.05 μm and the number density is 0.11 particles/μm2 or more ^. The composition of the components is, 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 producing a steel part 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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