COLD-ROLLED STEEL SHEET, STEEL COMPONENTS, METHOD FOR PRODUCING COLD-ROLLED STEEL SHEET, AND METHOD FOR PRODUCING STEEL COMPONENTS

Abstract

 A cold-rolled steel sheet having excellent toughness is provided. The cold-rolled steel sheet has a defined chemical composition, an average particle size of 0.10 µm or more of carbides containing at least one of Nb, Ti, and V in ferrite grains, and a number density of 100/mm² or more of the carbides having a particle size of 0.10 µm or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a cold-rolled steel sheet, and in particular to a cold-rolled steel sheet for use in producing a steel component having excellent toughness. Further, the present disclosure relates to a steel component using the cold-rolled steel sheet, a method for producing the cold-rolled steel sheet, and a method for producing the steel component.

BACKGROUND

[0002] Cold-rolled steel sheets are widely used as a material for producing various steel components. Cold-rolled steel sheets made of high-carbon steel have high hardness and are used for applications requiring wear resistance, including components for textile machinery, bearing components, machine blades, and household knives.

[0003] On the other hand, steel components such as components for textile machinery, bearing components, machine blades, and household knives are repeatedly subjected to impacts from reciprocating motion when in use. Therefore, steel components are also required to have excellent toughness to help prevent damage due to impact caused by reciprocating motion.

[0004] However, achieving both hardness and toughness in metal materials is difficult, because the higher the hardness, the more brittle the material becomes. For example, quenching and tempering are commonly used to improve the toughness of steel components. However, hardness of steel material is reduced by quenching and tempering processes, and therefore conventional quenching and tempering processes do not achieve both hardness and toughness at a high level.

[0005] Therefore, various methods have been proposed to achieve both hardness and toughness.

[0006] For example, Patent Literature (PTL) 1 and PTL 2 describe technologies to improve the toughness of high-carbon cold-rolled steel sheets by utilizing a crystal grain refinement effect due to Nb addition.

[0007] Further, in PTL 3, a technology is proposed to improve the wear resistance of cold-rolled steel sheets by densely dispersing coarse Nb-containing carbides in a matrix consisting of ferrite phase, and to improve toughness by utilizing the crystal grain refinement effect due to Nb addition.

[0008] In PTL 4, a technology is proposed to improve the wear resistance and toughness of cold-rolled steel sheets by densely dispersing coarse Nb, Ti carbides in a matrix and reducing the number density of voids.

[0009] In PTL 5, a technology is proposed to improve the spheroidization ratio of carbides such as cementite by annealing before final quenching and tempering of steel sheets containing 0.5 mass% to 0.7 mass% carbon, thereby improving toughness.

[0010] In PTL 6, a technology is proposed to produce a soft high-carbon steel sheet having excellent blanking properties by increasing the number density of generated voids in the material by bringing the material to an annealing finishing state in a stage immediately before final quenching and tempering.

[0011] In PTL 7, a technology is proposed to improve impact toughness and wear resistance in high-carbon steel sheets by controlling the formation of cementite, not including niobium, titanium, or vanadium carbides, and by achieving desired values for the spheroidization rate and number density of cementite.

CITATION LIST

Patent Literature

[0012] 

PTL 1: JP H05-345952 A

PTL 2: JP 2017-036492 A

PTL 3: JP 2015-190036 A

PTL 4: JP 2017-190494 A

PTL 5: JP 2009-024233 A

PTL 6: JP 2011-012316 A

PTL 7: JP 6880245 B

SUMMARY

(Technical Problem)

[0013] According to the technologies proposed in PTL 1 and 2, the toughness of high-carbon cold-rolled steel sheets is improved by utilizing the crystal grain refinement effect due to Nb addition. However, the crystal grain refinement effect of Nb saturates at a Nb content of about 0.1 mass%, so the required toughness is not obtainable from the crystal grain refinement effect alone.

[0014] Further, according to the technology proposed in PTL 3, toughness is improved by utilizing the crystal grain refinement effect due to Nb addition. However, in PTL 3, Nb-containing carbides are utilized to improve wear resistance, and Nb-containing carbides are a factor that reduces toughness. Therefore, the effects of Nb addition and Nb-containing carbides cancel each other out, and the required toughness is not obtainable.

[0015] Similarly to the technology proposed in PTL 3, the technology proposed in PTL 4 also utilizes the effect of improving wear resistance by densely dispersing hard Nb, Ti carbides. However, when Nb, Ti carbides are densely dispersed, voids form between the matrix and the carbides during cold rolling, resulting in reduced toughness. Therefore, in PTL 4, the generation of voids is inhibited by limiting the rolling ratio in cold rolling. However, this method limits the rolling ratio, and therefore inevitably limits the thickness and mechanical properties of the cold-rolled steel sheet that may be produced, and is therefore not really a solution.

[0016] Further, according to the technologies proposed in PTL 5 to 7, toughness was still insufficient.

[0017] The present disclosure is made in view of the circumstances described above, as it would be helpful to achieve even better toughness in cold-rolled steel sheets having hardness increased by use of carbides such as Nb.

(Solution to Problem)

[0018] As a result of studies, the inventors arrived at the following discoveries.

(1) By appropriately controlling the size and density of Nb, Ti, V carbides in a cold-rolled steel sheet, the toughness of the cold-rolled steel sheet after quenching and tempering may be effectively improved. As a result, producing steel components that have a high level of both hardness and toughness becomes possible.

(2) The size and density of Nb, Ti, V carbides in a cold-rolled steel sheet may be appropriately controlled by appropriate control of the chemical composition of a steel slab used and the production conditions of the cold-rolled steel sheet.

[0019] The present disclosure is based on the discoveries described above, and primary features of the present disclosure are as described below.

1. A cold-rolled steel sheet comprising a chemical composition containing (consisting of), in mass%,

C: 0.6 % to 1.25 %,

Si: 0.10 % to 0.55 %,

Mn: 0.20 % to 2.0 %,

P: 0.0005 % to 0.05 %,

S: 0.03 % or less,

Al: 0.001 % to 0.1 %,

N: 0.001 % to 0.009 %,

Cr: 0.1 % to 1.0 %, and

at least one of Ti: 0.01 % to 1.0 %, Nb: 0.05 % to 0.5 %, and V: 0.01 % to 1.0 %,

with the balance being Fe and inevitable impurities,

wherein the average particle size of carbides containing at least one of Nb, Ti, and V in ferrite grains is 0.10 µm or more, and

the number density of carbides having a particle size of 0.10 µm or more is 100/mm² or more.

2. The cold-rolled steel sheet according to aspect 1, wherein the chemical composition further contains, in mass%, at least one 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.

3. A steel component produced from the cold-rolled steel sheet according to aspect 1 or 2 after quenching and tempering.

4. The steel component according to aspect 3, wherein the steel component is any one of a textile machinery component, a bearing component, or a blade.

5. A method for producing a cold-rolled steel sheet, the method comprising:

heating a steel slab having the chemical composition according to aspect 1 or 2;

hot rolling the heated steel slab under a set of conditions including a finisher entry temperature of Ac3 or more to obtain a hot-rolled steel sheet;

cooling the hot-rolled steel sheet under a set of conditions including: a time from end of hot rolling to start of cooling of 2 s or less, an average cooling rate of 25 °C/s or more, and a cooling stop temperature of 720 °C or less;

coiling the cooled hot-rolled steel sheet;

applying, to the hot-rolled steel sheet after coiling, first annealing under a set of conditions including: an annealing temperature of 650 °C or more and 780 °C or less, and an annealing time of 3 h or more;

applying, to the hot-rolled steel sheet after the first annealing, a cycle applied twice or more of cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C to 800 °C; and

final cold rolling at a rolling ratio of 20 % or more.

6. The method for producing a cold-rolled steel sheet according to aspect 5, wherein a heating rate in the second annealing is 50 °C/h or more.

7. A method for producing a steel component, the method comprising: quenching a cold-rolled steel sheet produced by the method according to aspect 5 or 6 under a set of conditions including a quenching temperature of 700 °C or more and 800 °C or less and a holding time of 1 min or more to less than 60 min, followed by tempering under a set of conditions including a tempering temperature of 150 °C to 300 °C and a holding time of 20 min or more to 3 h or less.

(Advantageous Effect)

[0020] According to the present disclosure, even better toughness after quenching and tempering is obtainable for a cold-rolled steel sheet having a hardness improved by use of carbides such as Nb carbides. Therefore, the cold-rolled steel sheet is very well suited as a material for various steel components, including components for textile machinery, bearing components, machine blades, and household knives. Further, a steel component made using the cold-rolled steel sheet is also provided.

DETAILED DESCRIPTION

[0021] A detailed description is provided below. The present disclosure is not limited to the following embodiments. The present disclosure focuses on carbides present in ferrite grains and containing at least one of Nb, Ti, and V. Therefore, in the following description, "carbides present in ferrite grains and containing at least one of Nb, Ti, and V" may simply be referred to as "carbides".

[Chemical composition]

[0022] The cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.

C: 0.6 % to 1.25 %

[0023] C is an element necessary to improve hardness after quenching and tempering. Further, C is an element necessary to form cementite and carbides of elements such as Nb, Ti, V, and the like. To produce the required carbides and to obtain strength after quenching and tempering, C content needs to be 0.6 % or more. The C content is therefore 0.6 % or more. The C content is preferably 0.7 % or more. On the other hand, when the C content exceeds 1.25 %, hardness increases excessively and embrittlement occurs. Further, when the C content exceeds 1.25 %, surface scale becomes firm during heating, resulting in degradation of surface characteristics. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less.

Si: 0.10 % to 0.55 %

[0024] Si is an element having an effect of increasing strength by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.12 % or more. The Si content is more preferably 0.14 % or more. However, excessive Si content leads to Si oxide formation and a decrease in toughness. Further, excessive Si content promotes ferrite formation and grain growth, promotes carbide precipitation to grain boundaries, and inhibits intragranular carbide precipitation. Further, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. The Si content is therefore 0.55 % or less. The Si content is preferably 0.50 % or less. The Si content is more preferably 0.45 % or less.

Mn: 0.20 % to 2.0 %

[0025] Mn is an element having an effect of improving hardness by promoting quenching and inhibiting temper softening. In order to inhibit temper softening, inhibiting the formation of C as cementite or delaying dislocation recovery is necessary, and Mn has both of these effects. Accordingly, the addition of Mn may maintain a high dislocation density and high hardness microstructure even after tempering. To achieve these effects, Mn content is 0.20 % or more. The Mn content is preferably 0.25 % or more. On the other hand, when the Mn content exceeds 2.0%, a banded microstructure is formed due to Mn segregation. In particular, abnormal grain growth and microstructural nonuniformity are likely to occur at MnS segregations, and local precipitation to ferrite grain boundaries inhibits intragranular carbide formation. Further, this is a cause of cracking and shape defects during machining. The Mn content is therefore 2.0 % or less. The Mn content is preferably 1.95 % or less.

P: 0.0005 % to 0.05 %

[0026] The addition of a trace amount of P has a strength improving effect due to solid solution strengthening. To achieve this effect, P content is 0.0005 % or more. The P content is preferably 0.0008 % or more. On the other hand, when the P content exceeds 0.05 %, toughness is reduced due to grain boundary embrittlement. The P content is therefore 0.05 % or less. The P content is preferably 0.045 % or less.

S: 0.03 % or less

[0027] S causes a decrease in toughness by forming sulfides with Mn. S content is therefore 0.03 % or less. The S content is preferably 0.02 % or less. From the viewpoint of improving toughness, the lower the S content, the better, and therefore a lower limit of S content is not particularly limited and may be 0 %. However, excessive reduction leads to increased production costs, and therefore from the viewpoint of industrial production, the S content is preferably 0.0005 % or more. The S content is more preferably 0.001 % or more.

Al: 0.001 % to 0.1 %

[0028] Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, an excess of Al causes nitrides to form and promotes the formation of cracks and voids initiating from the nitrides, resulting in a decrease in toughness. Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less. The Al content is more preferably 0.06 % or less.

N: 0.001 % to 0.009 %

[0029] Nitrogen is an element that refines grain size and improves toughness through the formation of fine nitrides. N content is therefore 0.001 % or more. On the other hand, an excess of N combines with Al to cause nitrides to form and promotes the formation of cracks and voids initiating from the nitrides, resulting in a decrease in toughness. The N content is therefore 0.009 % or less. The N content is preferably 0.008 % or less.

Cr: 0.1 % to 1.0 %

[0030] Cr is an element that increases hardenability and improves strength of steel. To achieve the effects, Cr content is 0.1 % or more. The Cr content is preferably 0.12 % or more. On the other hand, an excess of Cr causes formation of coarse Cr carbides and Cr nitrides, and voids forming around the Cr carbides and Cr nitrides results in reduced toughness. The Cr content is therefore 1.0 % or less. The Cr content is preferably 0.95 % or less.

[0031] The chemical composition described above contains at least one element selected from the group consisting of Ti: 0.01 % to 1.0 %, Nb: 0.05 % to 0.5 %, and V: 0.01 % to 1.0 %. To obtain the desired number density of carbides, at least one of Ti, Nb, and V needs to be added in the amount described above.

Ti: 0.01 % to 1.0 %

[0032] Ti is an element that has an effect of forming carbides in grains and improving toughness. When Ti is added, in order to obtain this effect, Ti content is 0.01 % or more. The Ti content is preferably 0.015 % or more. On the other hand, excessive addition of Ti increases the austenitization temperature, making ferrite more likely to form on the surface of the steel sheet due to the lower temperature during hot rolling. The ferrite formed on the surface remains after subsequent cold rolling and annealing, and carbide formation at grain boundaries is prioritized, resulting in inhibition of intragranular carbide formation. The Ti content is therefore 1.0 % or less. The Ti content is preferably 0.9 % or less.

Nb: 0.05 % to 0.5 %

[0033] Nb is an element that has an effect of forming carbides in grains and improving toughness. Further, Nb is also a highly effective element for crystal grain refinement. When Nb is added, in order to obtain these effects, Nb content is 0.05 % or more. On the other hand, excessive addition of Nb results in the formation of carbides at grain boundaries and a decrease in the number density of carbides formed in grains. Carbides formed at grain boundaries are initiation points for voids and cracking, reducing toughness. The Nb content is therefore 0.5 % or less. The Nb content is preferably 0.45 % or less.

V: 0.01 % to 1.0%

[0034] V is an element that has an effect of forming carbides in grains and improving toughness. Further, V has an effect of improving hardenability and improving strength of steel. Further, in order to inhibit temper softening, inhibiting the formation of C as cementite or delaying dislocation recovery is necessary, and V has both of these effects. The addition of V may maintain deformed microstructure even after tempering, improving toughness. When V is added, to obtain these effects, the V content is 0.01 % or more. On the other hand, excessive addition of V causes coarsening of carbides formed at grain boundaries, and carbides formed at grain boundaries become initiation points for voids and cracking, resulting in a decrease in toughness. The V content is therefore 1.0 % or less. The V content is preferably 0.95 % or less.

[0035] The cold-rolled steel sheet according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurities.

[0036] Further, according to another embodiment of the present disclosure, the chemical composition described above contains at least one 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

[0037] Sb is an effective element for improving corrosion resistance, but when added in excess, a rich Sb layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling. Sb content is therefore 0.1 % or less. A lower limit of the Sb content is not particularly limited. From the viewpoint of increasing the effect of Sb addition, the Sb content is preferably 0.0003 % or more.

Hf: 0.5 % or less

[0038] Hf is an effective element for improving corrosion resistance, but when added in excess, a rich Hf layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling. Hf content is therefore 0.5 % or less. A lower limit of the Hf content is not particularly limited. From the viewpoint of increasing the effect of Hf addition, the Hf content is preferably 0.001 % or more.

REM: 0.1 % or less

[0039] REM (rare earth metals) are elements that improve strength of steel. However, excessive addition of REM may retard the spheroidization of cementite, promote non-uniform deformation during cold working and degrade surface characteristics. REM content is therefore 0.1 % or less. A lower limit of the REM content is not particularly limited. From the viewpoint of increasing the effect of REM addition, the REM content is preferably 0.005 % or more.

Cu: 0.5 % or less

[0040] Cu is an effective element for improving corrosion resistance, but when added in excess, a rich Cu layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling. Cu content is therefore 0.5 % or less. A lower limit of the Cu content is not particularly limited. From the viewpoint of increasing the effect of Cu addition, the Cu content is preferably 0.01 % or more.

Ni: 3.0 % or less

[0041] Ni is an element that improves strength of steel. However, excessive addition may promote non-uniform deformation during cold working and degrade surface characteristics. Ni content is therefore 3.0 % or less. A lower limit of the Ni content is not particularly limited. From the viewpoint of increasing the effect of Ni addition, the Ni content is preferably 0.01 % or more.

Sn: 0.5 % or less

[0042] Sn is an effective element for improving corrosion resistance, but when added in excess, a rich Sn layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling. Sn content is therefore 0.5 % or less. A lower limit of the Sn content is not particularly limited. From the viewpoint of increasing the effect of Sn addition, the Sn content is preferably 0.0001 % or more.

Mo: 1 % or less

[0043] Mo is an element that improves strength of steel. However, excessive addition of Mo may retard the spheroidization of cementite, promote non-uniform deformation during cold working, and degrade surface characteristics. The Mo content is therefore 1 % or less. A lower limit of the Mo content is not particularly limited. From the viewpoint of increasing the effect of Mo addition, the Mo content is preferably 0.001 % or more.

Zr: 0.5 % or less

[0044] Zr is an effective element for improving corrosion resistance, but when added in excess, a rich Zr layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling. Zr content is therefore 0.5 % or less. A lower limit of the Zr content is not particularly limited. From the viewpoint of increasing the effect of Zr addition, the Zr content is preferably 0.01 % or more.

B: 0.005 % or less

[0045] B is an element that has an effect of improving hardenability and may be added. However, when B content exceeds 0.005 %, surface cracking is likely to occur during quenching. The B content is therefore 0.005 % or less. A lower limit of the B content is not particularly limited. From the viewpoint of increasing the effect of B addition, when B is added, the B content is preferably 0.0001 % or more.

W: 0.01 % or less

[0046] W is an element that has an effect of improving hardenability and may be added. However, when W content exceeds 0.01 %, surface cracking is likely to occur during quenching. The W content is therefore 0.01 % or less. A lower limit of the W content is not particularly limited. From the viewpoint of increasing the effect of W addition, when W is added, the W content is preferably 0.001 % or more.

[Carbides]

[0047] The following is a description of the carbides in the cold-rolled steel sheet.

Average particle diameter: 0.10 µm or more

Number density: 100/mm² or more

[0048] When a microstructure in which at least one of Nb, Ti, and V carbides are formed in grains is prepared in advance at a stage before cold rolling in pre-processing prior to starting component working, followed by a deformed microstructure formed when cold rolling is applied, followed by the application of quenching and tempering treatment, some fine Nb, Ti, V carbides re-precipitate at subgrain boundaries. This microstructure increases resistance to strain introduced by repeated deformation and improves the toughness of the final product. To achieve these effects, the average particle size of carbides containing at least one of Nb, Ti, and V in ferrite grains needs to be 0.10 µm or more. For the same reason, the number density of the carbides having a particle size of 0.10 µm or more needs to be 100/mm² or more.

[0049] When the average particle size of the carbides is less than 0.10 µm, the amount of fine Nb, TI, and V carbides precipitating after quenching and tempering treatment is insufficient to achieve a high toughness improvement effect. When the number density of carbides is less than 100/mm², as in the case of insufficient average particle size, the amount of fine Nb, TI, and V carbides precipitating after quenching and tempering treatment is insufficient to achieve a high toughness improvement effect.

[Sheet thickness]

[0050] The sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. The sheet thickness is preferably 0.1 mm or more. The sheet thickness is more preferably 0.2 mm or more. Further, an upper limit of the sheet thickness is not particularly limited. The sheet thickness is particularly 2.5 mm or less. The sheet thickness is more preferably 1.6 mm or less. The sheet thickness is even more preferably 0.8 mm or less. When the sheet thickness is 0.2 mm or more and 0.8 mm or less, the cold-rolled steel sheet is particularly suitable for use as a material for textile machinery components such as knitting needles and the like.

[Method for producing cold-rolled steel sheet]

[0051] The following describes a method for producing a cold-rolled steel sheet according to an embodiment.

[0052] The cold-rolled steel sheet may be produced by performing the following processes in sequence, starting with a steel slab having the chemical composition described above.

(1) Heating

(2) Hot rolling

(3) Cooling

(4) Coiling

(5) First annealing

(6) Cold rolling

(7) Second annealing

(8) Final cold rolling

[0053] The processes (6) and (7) above are applied two or more times. The following describes each of the processes.

(1) Heating

[0054] First, a steel slab having the chemical composition described above is heated. A method for producing the steel slab is not particularly limited, and any method may be used. For example, composition adjustment of the steel slab may be performed by a blast furnace converter steelmaking process or by an electric furnace steelmaking process. Further, for example, casting from molten steel into a slab may be done by continuous casting or by blooming.

[0055] The heating may be performed by any method, but use of a heating furnace is preferred.

[0056] When the heating is performed using a heating furnace, the furnace temperature is not particularly limited. From the viewpoint of homogenizing the steel composition and dissolving segregation and unsolved carbides in the steel slab, the temperature is preferably 1,100 °C or more.

[0057] The holding time in the heating is not particularly limited. From the viewpoint of sufficient dissolving of unsolved carbides, the holding time is preferably 1 h or more.

(2) Hot rolling

[0058] The heated slab is then hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, rough rolling and finishing rolling may be performed according to conventional methods.

Finisher entry temperature: Ac3 or more

[0059] When the finisher entry temperature of the hot rolling is less than Ac3, stretched ferrite is formed in the steel sheet after hot rolling, and this stretched ferrite remains in the finally obtainable cold-rolled steel sheet. As a result, the formation of grain boundary carbides is promoted and the formation of intragranular carbides is inhibited, resulting in reduced toughness. For this reason, the finisher entry temperature of the hot rolling is Ac3 or more. An upper limit of the finisher entry temperature is not particularly limited. The finisher entry temperature is preferably 1,200 °C or less.

[0060] The Ac3 temperature (°C) is obtained by the following Formula (1).

[0061] Here, the element symbols denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.

(3) Cooling

Time from end of hot rolling to start of cooling: 2 s or less

[0062] The hot-rolled steel sheet is then cooled. When a long time elapses between the end of hot rolling and the start of cooling, coarse ferrite is formed and carbides containing at least one of Ti, Nb, and V precipitate non-uniformly at grain boundaries. This non-uniform microstructure does not homogenize in subsequent cold rolling and annealing and hinders intragranular carbide formation. The time between the end of hot rolling and the start of cooling is therefore 2 s or less. In view of the above, the shorter the time between the end of hot rolling and the start of cooling, the better, and therefore a lower limit is not particularly limited. However, from an industrial production viewpoint, the time may be 0.5 s or more, or even 0.8 s or more.

Average cooling rate: 25 °C/s or more

[0063] When the average cooling rate in the cooling is less than 25 °C/s, ferrite grains become coarse and carbides formed become localized, and therefore when subsequent cold rolling and annealing are repeated, carbide formation is concentrated at grain boundaries and the formation of intragranular carbides is inhibited. The average cooling rate is therefore 25 °C/s or more. An upper limit of the average cooling rate is not particularly limited. When the cooling rate is excessively high, volume expansion caused by transformation during subsequent coiling results in a poor coiling shape. Therefore, from the viewpoint of achieving a good coiling shape, the average cooling rate is preferably 160 °C/s or less. The average cooling rate is more preferably 150 °C/s or less.

Cooling stop temperature: 720 °C or less

[0064] Further, when the cooling stop temperature in the cooling is too high, ferrite grains coarsen and carbide formation into grains is inhibited when cold rolling and annealing is repeated. The cooling stop temperature is therefore 720 °C or less. A lower limit of the cooling stop temperature is not particularly limited. When the cooling stop temperature is excessively low, volume expansion caused by transformation during subsequent coiling results in a poor coiling shape. The cooling stop temperature is therefore preferably 620 °C or more. The cooling stop temperature is more preferably 640 °C or more.

(4) Coiling

[0065] After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. At this time, the coiling temperature is not particularly limited. The coiling temperature is preferably 600 °C to 730 °C. This temperature stabilizes the coiling shape by precipitating plate-like cementite.

(5) First annealing

[0066] 

Annealing temperature: 650 °C or more and 780 °C or less

Annealing time: 3 h or more

[0067] The hot-rolled steel sheet after the coiling is subjected to the first annealing under a set of conditions including: an annealing temperature of 650 °C or more and 780 °C or less, and an annealing time of 3 h or more. The microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like carbides and ferrite. The pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature. In order to break up the pearlitic microstructure and allow the subsequent cold rolling and annealing process to produce the desired carbides in grains, the annealing temperature needs to be 650 °C or more and the annealing time needs to be 3 h or more. On the other hand, when the annealing temperature is more than 780 °C, phase transformation begins preferentially from one portion, resulting in a locally coarse and non-uniform microstructure, making obtaining intragranular carbides difficult, such that the desired carbide number density may not be obtained. An upper limit of the annealing time is not particularly limited. An excessively long annealing time reduces productivity and also saturates the effect. Therefore, the annealing time is preferably 20 h or less.

[0068] Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.

(6) Cold rolling

(7) Second annealing

[0069] Plate-like carbides are formed in steel sheets after hot rolling. Such plate-like carbides are stable, and therefore tend to remain until later stages. Plate-like carbides that finally remain may cause void formation and cracking, reducing toughness. Therefore, in order to re-dissolve plate-like carbides into particle shapes by annealing heating and to cause intragranular precipitation of the carbides, the hot-rolled steel sheet after the first annealing is subjected to two or more cycles of cold rolling and second annealing.

Rolling ratio: 15 % or more

[0070] When the rolling ratio in the cold rolling is less than 15 %, carbides at grain boundaries become coarser, and therefore the number density of intragranular carbides formed decreases, and the particle size of intragranular carbides becomes smaller. The rolling ratio is therefore 15 % or more. An upper limit of the rolling ratio is not particularly limited. The rolling ratio is preferably 70 % or less.

Annealing temperature: 600 °C to 800 °C

[0071] When the annealing temperature in the second annealing is more than 800 °C, carbides at grain boundaries become coarser, and therefore the number density of intragranular carbides formed decreases, and the particle size of intragranular carbides becomes smaller. The annealing temperature is therefore 800 °C or less. On the other hand, when the annealing temperature is less than 600 °C, the formation of intragranular carbides is inhibited and the desired particle size is not obtainable. The annealing temperature is therefore 600 °C or more.

[0072] The heating rate in the second annealing is not particularly limited. When the heating rate is too slow, carbides tend to form at ferrite grain boundaries, which inhibits intragranular carbide formation. Therefore, from the viewpoint of further increasing the toughness improvement effect, the heating rate in the second annealing is preferably 50 °C/h or more. An upper limit of the heating rate is also not particularly limited. The heating rate is preferably 200 °C/s or less.

[0073] The number of cycles of the cold rolling and the second annealing is two or more. Two or more cycles of the cold rolling and the annealing promotes carbide formation and leads finally to achieving the desired intragranular carbide size and number density. An upper limit of the number of cycles is not particularly limited. The number of cycles is preferably five or less, as the effect saturates when the number of cycles is more than five.

(8) Final cold rolling

Rolling ratio: 20 % or more

[0074] After the cycle of the cold rolling and the second annealing is performed two or more times as described above, final cold rolling at a rolling ratio of 20 % or more is further applied. According to the final cold rolling at a rolling ratio of 20 % or more, toughness improves due to precipitation of carbides at the desired number density into grains during quenching and tempering. The larger the rolling ratio in the final cold rolling, the better, but the shape of the steel sheet may become unstable when the rolling ratio is 65 % or more. The rolling ratio is therefore preferably less than 65 %.

[0075] By satisfying the above conditions, a cold-rolled steel sheet having excellent toughness after quenching and tempering may be produced. The final cold-rolled steel sheet may be subjected to further optional surface treatment.

[Method for producing steel component]

[0076] According to an embodiment of the present disclosure, a steel component may be produced by quenching and tempering a cold-rolled steel sheet produced according to the method described above. The quenching and tempering conditions are not particularly limited. In order to obtain higher toughness, quenching is preferably performed under a set of conditions including: a quenching temperature of 700 °C or more and 900 °C or less, a holding time of 1 min or more to less than 60 min, followed by tempering preferably performed under a set of conditions including: a tempering temperature of 150 °C to 400 °C and a holding time of 20 min or more to 3 h or less. The quenching temperature is more preferably 750 °C or more. The quenching temperature is more preferably 850 °C or less. Further, the tempering temperature is more preferably 200 °C to 300 °C.

[0077] Cooling in the quenching is not particularly limited and may be performed by any method. The cooling may be, for example, air cooling, water quenching, or oil quenching.

[0078] Prior to the quenching and tempering, the cold-rolled steel sheet may be optionally worked to a desired shape.

EXAMPLES

[0079] In order to confirm the effects of the present disclosure, cold-rolled steel sheets were produced according to the procedures described below, and the toughness of the resulting cold-rolled steel sheets after quenching and tempering was evaluated.

[0080] First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then heated, hot rolled, cooled, coiled, first annealed, cold rolled, second annealed, and finally cold rolled in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Tables 2 and 3. The cycle of cold rolling and second annealing was applied a number of times listed in Tables 2 and 3.

(Methods of measuring carbides)

[0081] Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. For each test piece, after polishing a cross-section in the rolling direction (L-section) of the test piece for microstructure observation, the polished surface was corroded with 1 vol% to 3 vol% nital solution to reveal the microstructure. Next, the surface of the test piece for microstructure observation was imaged using a scanning electron microscope (SEM) at a magnification of 3,000× to obtain a microstructure image. From the obtained microstructure image, the particle size of Nb, Ti, and V carbides formed in grains was measured by a cutting method, and the number density was calculated by counting the carbides in the measurement field of view. The average of three fields of view was calculated and used as particle size and number density. The measurement results are listed in Tables 4 and 5. Nb, Ti, and V carbides were identified using SEM energy dispersive X-ray spectroscopy (EDS) analysis. Elemental mapping was performed with respect to the observed fields of view to separate cementite from other carbides, and the other carbides were considered to be Nb, Ti, V carbides.

(Toughness after quenching and tempering)

[0082] Next, to evaluate the toughness of the resulting cold-rolled steel sheets after quenching and tempering, the following procedure was used to conduct tests and measure impact values using the Charpy impact test. First, the obtained cold-rolled steel sheets were quenched and tempered. The quenching was performed by holding the cold-rolled steel sheet in a furnace preheated to 800 °C for 10 min, followed by oil quenching at 80 °C. The tempering was performed by holding the quenched cold-rolled steel sheet in a furnace preheated to 250 °C for 1 h and then air cooling.

[0083] Charpy impact tests were then performed to measure impact values. The measurement results are listed in Tables 4 and 5. For the Charpy impact test, test pieces were used having a notch depth of 2.5 mm and a notch radius of 0.1 mm (notch width 0.2 mm) taken from cold-rolled steel sheets after quenching and tempering. For each of the test pieces, a U-notch was formed by electric discharge machining. According to the present disclosure, the toughness after quenching and tempering was judged to be excellent when the impact value was 8 J/cm² or more.

[Table 1]

[0084] 

Table 1

Steel sample ID	Chemical composition (mass%) *												Ac3 (°C)	Remarks
	C	Si	Mn	P	S	Al	N	Cr	Ti	Nb	V	Other
A	1.00	0.25	0.71	0.018	0.0110	0.003	0.003	0.40	0.04	0.05	0.10	-	760	Conforming steel
B	0.95	0.25	0.50	0.010	0.0114	0.003	0.003	0.50	0.07	0.10	-	-	777	Conforming steel
C	0.98	0.21	0.65	0.015	0.0100	0.004	0.005	0.72	0.10	0.08	0.07	-	781	Conforming steel
D	0.96	0.22	0.90	0.010	0.0210	0.003	0.002	0.40	0.05	0.21	0.02	-	756	Conforming steel
E	1.00	0.25	0.50	0.031	0.0100	0.002	0.002	0.44	-	0.22	0.10	-	759	Conforming steel
F	0.88	0.24	0.77	0.028	0.0022	0.030	0.002	0.55	-	0.19	-	-	773	Conforming steel
G	0.95	0.44	0.61	0.042	0.0026	0.002	0.006	0.32	-	0.25	-	-	778	Conforming steel
H	0.65	0.4	0.70	0.017	0.0080	0.020	0.003	1.00	0.04	-	-	Mo:0.02	807	Conforming steel
I	1.11	0.3	0.86	0.018	0.0170	0.040	0.002	0.80	-	-	0.06	Ni0.02, Cu:0.01	744	Conforming steel
J	0.77	0.55	0.81	0.022	0.0180	0.003	0.001	0.20	-	0.18	-	Sb:0.005, Sn:0.002, Hf0.001, REM:0.001, Zr:0.003, B:0.001, W:0.001	785	Conforming steel
K	0.45	0.28	0.60	0.010	0.0090	0.003	0.003	0.48	0.80	0.05	0.10	-	1129	Comparative steel
L	1.8	0.31	0.22	0.010	0.0140	0.002	0.001	0.50	0.06	0.50	0.22	-	709	Comparative steel
M	0.9	0.05	0.48	0.022	0.0090	0.090	0.008	0.51	0.10	0.40	0.18	-	834	Comparative steel
N	0.78	0.85	0.54	0.019	0.0060	0.060	0.002	0.35	0.25	0.33	0.20	-	928	Comparative steel
O	0.91	0.28	0.15	0.010	0.0100	0.003	0.003	0.52	0.30	0.30	-	-	885	Comparative steel
P	0.95	0.11	3.00	0.041	0.0150	0.006	0.002	0.44	0.28	0.35	0.80	-	804	Comparative steel
Q	1.1	0.31	0.61	0.100	0.0220	0.100	0.006	0.40	0.35	0.25	0.99	-	982	Comparative steel
R	0.86	0.24	0.59	0.016	0.100	0.050	0.002	0.42	0.40	0.22	0.78	-	942	Comparative steel
S	0.77	0.4	1.29	0.050	0.0006	0.200	0.002	0.85	0.10	0.02	0.26	-	907	Comparative steel
T	0.75	0.41	1.76	0.016	0.0007	0.004	0.021	0.90	0.85	0.04	0.06	-	1081	Comparative steel
U	1.05	0.51	2.00	0.025	0.0100	0.070	0.004	1.45	0.75	0.38	0.03	-	1037	Comparative steel
V	0.95	0.25	0.58	0.010	0.0100	0.003	0.003	0.80	-	-	-	-	743	Comparative steel
W	1.15	0.37	1.34	0.041	0.0260	0.002	0.001	0.45	1.20	-	-	-	1211	Comparative steel
X	0.88	0.28	1.00	0.022	0.0110	0.003	0.004	0.60	-	-	1.80	-	750	Comparative steel
Y	0.98	0.18	1.60	0.020	0.0030	0.004	0.005	0.23	-	0.90	-	-	720	Comparative steel

* The balance being Fe and inevitable impurity

[Table 2]

[0085] 

Table 2

No.	Steel sample ID	Ac3	Hot rolling	Cooling			First annealing		Cold rolling and second annealing			Second annealing	Final cold rolling	Remarks
			Finisher entry temp.	Time until cooling start *	Cooling rate	Cooling stop temp.	Annealing temp.	Annealing time	Rolling ratio	Annealing temp.	No. of cycles	Heating rate	Rolling ratio
		°C	°C	s	°C/s	°C	°C	h	%	°C	Times	°C/h	%
1	A	760	1100	1.8	55	705	700	6	30	680	3	150	30	Example
2	B	777	1110	1.5	28	700	705	4	50	690	3	140	30	Example
3	C	781	1080	1.5	35	690	695	8	55	680	2	150	25	Example
4	D	756	1080	1.6	35	700	680	6	20	700	3	130	35	Example
5	E	759	1090	1.5	35	685	710	10	45	710	3	110	30	Example
6	F	773	1000	2.0	30	690	750	15	35	690	3	100	25	Example
7	G	778	1110	1.0	40	695	710	6	50	730	2	100	20	Example
8	H	807	1150	0.9	45	700	700	6	30	780	3	120	30	Example
9	I	744	1080	1.5	50	685	730	10	45	710	4	50	35	Example
10	J	785	1120	1.8	50	680	730	12	35	700	2	80	40	Example
11	K	1129	1210	1.8	35	680	710	5	20	700	3	130	35	Comparative Example
12	L	709	1030	2.0	45	700	690	6	15	720	5	150	40	Comparative Example
13	M	834	1080	1.6	55	680	680	8	25	750	4	170	30	Comparative Example
14	N	928	1070	1.5	50	700	710	10	15	750	3	110	25	Comparative Example
15	O	885	1100	1.8	50	680	720	12	30	600	2	130	45	Comparative Example
16	P	804	1050	2.0	30	690	700	6	40	650	3	150	50	Comparative Example
17	Q	982	1020	1.8	85	700	690	4	45	710	2	120	30	Comparative Example
18	R	942	1020	1.6	35	705	700	5	30	730	2	150	40	Comparative Example
19	S	907	1010	1.8	45	700	720	3	30	700	3	130	25	Comparative Example
20	T	1081	1100	2.0	90	280	700	4	35	720	3	90	20	Comparative Example
21	U	1037	1070	1.6	55	710	680	5	20	760	5	80	45	Comparative Example

* Time from end of hot rolling to start of cooling

[Table 3]

[0086] 

Table 3

No.	Steel sample ID	Ac3	Hot rolling	Cooling			First annealing		Cold rolling and second annealing			Second annealing	Final cold rolling	Remarks
			Finisher entry temp.	Time until cooling start *	Cooling rate	Cooling stop temp.	Annealing temp.	Annealing time	Rolling ratio	Annealing temp.	No. of cycles	Heating rate	Rolling ratio
		°C	°C	s	°C/s	°C	°C	h	%	°C	Times	°C/h	%
22	V	743	1000	2.0	80	710	700	8	30	680	3	100	30	Comparative Example
23	W	1211	1280	1.8	70	720	705	10	25	780	4	120	40	Comparative Example
24	X	750	1010	1.6	100	690	700	8	15	690	4	110	35	Comparative Example
25	Y	720	1000	1.5	120	680	710	6	20	800	3	60	30	Comparative Example
26	H	807	790	1.5	80	700	740	10	50	750	3	150	45	Comparative Example
27	A	760	1040	5.0	75	630	740	12	55	680	4	90	40	Comparative Example
28	B	777	900	2.0	15	680	715	15	60	780	4	180	33	Comparative Example
29	B	777	1040	1.5	160	685	705	15	50	780	3	130	30	Example
30	B	777	1040	1.8	55	760	690	10	45	610	3	80	35	Comparative Example
31	B	777	1050	1.9	55	625	670	12	30	700	3	120	30	Example
32	A	760	1040	1.9	55	700	620	12	40	680	5	120	35	Comparative Example
33	A	760	1000	2.0	60	710	790	10	45	620	4	150	50	Comparative Example
34	C	781	1050	1.3	120	685	700	2	50	660	4	70	33	Comparative Example
35	C	781	1050	0.9	60	720	680	10	10	680	5	160	25	Comparative Example
36	C	781	1040	1.1	65	700	705	18	68	680	3	100	30	Example
37	F	773	1060	1.1	65	695	720	8	45	550	4	160	20	Comparative Example
38	F	773	1070	1.3	50	690	770	8	55	810	4	100	20	Comparative Example
39	F	773	1040	1.8	80	705	690	12	55	710	1	190	25	Comparative Example
40	F	773	1050	2.0	100	695	680	10	35	680	6	100	35	Example
41	A	760	1060	1.3	90	680	700	6	30	700	2	140	10	Comparative Example
42	D	756	1090	1.6	35	700	700	6	20	700	3	30	40	Example

* Time from end of hot rolling to start of cooling

[Table 4]

[0087] 

Table 4

No.	Steel sample ID	Carbides*1		Toughness after quenching and tempering	Remarks
		Average particle size	Number density *2	Impact value
		µm	particles/mm2	J/cm2
1	A	0.50	330	19.0	Example
2	B	0.32	350	17.8	Example
3	C	0.45	300	18.0	Example
4	D	0.50	410	17.8	Example
5	E	0.30	320	17.8	Example
6	F	0.70	400	20.1	Example
7	G	0.69	330	17.0	Example
8	H	1.05	270	13.0	Example
9	I	0.39	420	18.5	Example
10	J	0.22	315	17.1	Example
11	K	0.02	65	4.0	Comparative Example
12	L	0.12	130	3.0	Comparative Example
13	M	0.22	240	4.5	Comparative Example
14	N	0.28	95	5.0	Comparative Example
15	O	0.29	290	5.5	Comparative Example
16	P	0.32	90	4.0	Comparative Example
17	Q	0.20	210	4.5	Comparative Example
18	R	0.39	300	4.5	Comparative Example
19	S	0.12	180	3.3	Comparative Example
20	T	0.19	220	4.5	Comparative Example
21	U	0.42	220	6.4	Comparative Example

* 1 Carbides containing at least one of Nb, Ti, and V in ferrite grains *2 Number density of carbides having a particle size of 0.10 µm or more

[Table 5]

[0088] 

Table 5

No.	Steel sample ID	Carbides *1		Toughness after quenching and tempering	Remarks
		Average particle size	Number density *2	Impact value
		µm	particles/mm2	J/cm2
22	V	-	0	3.0	Comparative Example
23	W	0.02	75	3.5	Comparative Example
24	X	0.42	190	6.4	Comparative Example
25	Y	0.03	85	3.5	Comparative Example
26	H	0.13	65	2.8	Comparative Example
27	A	1.20	15	4.5	Comparative Example
28	B	1.50	23	4.0	Comparative Example
29	B	0.16	120	8.0	Example
30	B	0.80	38	3.5	Comparative Example
31	B	0.50	190	8.5	Example
32	A	0.55	90	3.0	Comparative Example
33	A	0.80	82	4.0	Comparative Example
34	C	0.90	85	4.0	Comparative Example
35	C	1.30	55	3.5	Comparative Example
36	C	0.80	210	9.0	Example
37	F	0.06	100	4.5	Comparative Example
38	F	0.05	78	3.0	Comparative Example
39	F	0.80	30	3.0	Comparative Example
40	F	0.35	140	8.5	Example
41	A	0.45	90	6.5	Comparative Example
42	D	0.45	100	8.0	Example

*1 Carbides containing at least one of Nb, Ti, and V in ferrite grains *2 Number density of carbides having a particle size of 0.10 µm or more

[0089] As indicated in Tables 1 to 5, cold-rolled steel sheets meeting the conditions of the present disclosure have excellent toughness after quenching and tempering. According to the present disclosure, both high hardness and excellent toughness is obtainable due to Nb, Ti, V carbides, and therefore the cold-rolled steel sheet according to the present disclosure may be used to produce a steel component that has a high level of both hardness and toughness. Therefore, the cold-rolled steel sheet according to the present disclosure is very well suited as a material for various steel components, including components for textile machinery, bearing components, blades, and the like.

Claims

1. A cold-rolled steel sheet comprising a chemical composition containing, in mass%,

C: 0.6 % to 1.25 %,

Si: 0.10 % to 0.55 %,

Mn: 0.20 % to 2.0 %,

P: 0.0005 % to 0.05 %,

S: 0.03 % or less,

Al: 0.001 % to 0.1 %,

N: 0.001 % to 0.009 %,

Cr: 0.1 % to 1.0 %, and

at least one of Ti: 0.01 % to 1.0 %, Nb: 0.05 % to 0.5 %, and V: 0.01 % to 1.0 %,

with the balance being Fe and inevitable impurities,

wherein the average particle size of carbides containing at least one of Nb, Ti, and V in ferrite grains is 0.10 µm or more, and

the number density of carbides having a particle size of 0.10 µm or more is 100/mm² or more.

2. The cold-rolled steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, at least one 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.

3. A steel component produced from the cold-rolled steel sheet according to claim 1 or 2 after quenching and tempering.

4. The steel component according to claim 3, wherein the steel component is any one of a textile machinery component, a bearing component, or a blade.

5. A method for producing a cold-rolled steel sheet, the method comprising:

heating a steel slab having the chemical composition according to claim 1 or 2;

hot rolling the heated steel slab under a set of conditions including a finisher entry temperature of Ac3 or more to obtain a hot-rolled steel sheet;

cooling the hot-rolled steel sheet under a set of conditions including: a time from end of hot rolling to start of cooling of 2 s or less, an average cooling rate of 25 °C/s or more, and a cooling stop temperature of 720 °C or less;

coiling the cooled hot-rolled steel sheet;

applying, to the hot-rolled steel sheet after coiling, first annealing under a set of conditions including: an annealing temperature of 650 °C or more and 780 °C or less, and an annealing time of 3 h or more;

applying, to the hot-rolled steel sheet after the first annealing, a cycle applied twice or more of cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C to 800 °C; and

final cold rolling at a rolling ratio of 20 % or more.

6. The method for producing a cold-rolled steel sheet according to claim 5, wherein a heating rate in the second annealing is 50 °C/h or more.

7. A method for producing a steel component, the method comprising: quenching a cold-rolled steel sheet produced by the method according to claim 5 or 6 under a set of conditions including a quenching temperature of 700 °C or more and 900 °C or less and a holding time of 1 min or more to less than 60 min, followed by tempering under a set of conditions including a tempering temperature of 150 °C to 400 °C and a holding time of 20 min or more to 3 h or less.