CN117396624A

CN117396624A - Cold-rolled steel sheet, steel member, method for producing cold-rolled steel sheet, and method for producing steel member

Info

Publication number: CN117396624A
Application number: CN202280038891.2A
Authority: CN
Inventors: 土屋荣司; 松村雄太; 太田裕树; 蛭田修平; 小岛真由美; 樱井康广; 船川义正; 木户章雅; 木村英之
Original assignee: JFE Steel Corp; TOKUSHU KINZOKU EXCEL CO Ltd
Current assignee: JFE Steel Corp; TOKUSHU KINZOKU EXCEL CO Ltd
Priority date: 2021-06-18
Filing date: 2022-06-10
Publication date: 2024-01-12
Also published as: EP4324952A1; JP2023116799A; JP7329780B2; WO2022264947A1; TW202338105A; TWI815504B; JPWO2022264947A1

Abstract

The invention provides a cold-rolled steel sheet with excellent toughness. A cold-rolled steel sheet having a predetermined composition, wherein carbides containing at least one of Nb, ti and V present in ferrite grains have an average grain size of 0.10 [ mu ] m or more, and wherein the number density of carbides having a grain size of 0.10 [ mu ] m or more among the carbides is 100 pieces/mm ² The above.

Description

Cold-rolled steel sheet, steel member, method for producing cold-rolled steel sheet, and method for producing steel member

Technical Field

The present invention relates to a cold-rolled steel sheet, and more particularly, to a cold-rolled steel sheet capable of producing a steel member having excellent toughness. The present invention also relates to a steel member using the cold-rolled steel sheet, a method for producing the cold-rolled steel sheet, and a method for producing the steel member.

Background

Cold-rolled steel sheets are widely used as materials for manufacturing various steel parts. Among them, cold-rolled steel sheets composed of high-carbon steel have high hardness, and are therefore used in applications requiring wear resistance, such as textile machine parts, bearing parts, and machine and household tools.

On the other hand, steel members such as textile machine members, bearing members, and machine and household tools repeatedly receive impacts of reciprocating motions during use. Therefore, in order to prevent breakage due to impact of reciprocating motion, steel members are also required to have excellent toughness.

However, since the higher the hardness, the more brittle the metal material becomes, it is difficult to achieve both hardness and toughness. For example, although quenching and tempering are generally performed to improve the toughness of steel members, the hardness of steel is reduced by quenching and tempering, and thus it is not possible to achieve both hardness and toughness at a high level in conventional quenching and tempering treatments.

Accordingly, various methods for achieving both hardness and toughness have been proposed.

For example, patent documents 1 and 2 disclose techniques for improving toughness of high-carbon cold-rolled steel sheets by utilizing the effect of grain refinement by Nb addition.

Patent document 3 proposes a technique for improving the wear resistance of a cold-rolled steel sheet by dispersing coarse Nb-containing carbide in a matrix composed of ferrite phase at a high density and improving toughness by utilizing the effect of fine grains added by Nb.

Patent document 4 proposes a technique for improving the wear resistance and toughness of a cold-rolled steel sheet by dispersing coarse nb—ti-based carbides in a matrix at a high density and reducing the number density of voids.

Patent document 5 proposes the following technique: by annealing a steel sheet containing 0.5 to 0.7 mass% of carbon before final quenching and tempering, the spheroidization rate of carbide such as cementite is improved, and as a result, the toughness is improved.

Patent document 6 proposes the following technique: by being in the final annealed state immediately before the final quenching and tempering, the number density of voids to be formed in the material is increased, thereby producing a soft high-carbon steel sheet excellent in punching properties.

Patent document 7 proposes the following technique: in a high-carbon steel sheet, the formation of cementite carbide free of niobium, titanium and vanadium carbide is controlled so that the spheroidization ratio and number density of the cementite carbide are set to desired values, thereby improving impact toughness and wear resistance.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 05-345952

Patent document 2: japanese patent laid-open No. 2017-036492

Patent document 3: japanese patent application laid-open No. 2015-190036

Patent document 4: japanese patent application laid-open No. 2017-190494

Patent document 5: japanese patent laid-open No. 2009-024333

Patent document 6: japanese patent application laid-open No. 2011-012316

Patent document 7: japanese patent No. 6880245

Disclosure of Invention

In the techniques proposed in patent documents 1 and 2, the toughness of the high-carbon cold-rolled steel sheet is improved by utilizing the effect of grain refinement by Nb addition. However, since the grain refining effect of Nb is saturated when the Nb content is about 0.1 mass%, the necessary toughness cannot be obtained only by the grain refining effect.

In addition, in the technique proposed in patent document 3, the toughness is improved by utilizing the effect of grain refinement of Nb addition. However, in patent document 3, nb-containing carbide is used to improve wear resistance, and Nb-containing carbide is a factor of reducing toughness. Therefore, the effect of Nb addition and the effect of Nb-containing carbide cancel each other, and the necessary toughness cannot be obtained.

As in patent document 3, the technique proposed in patent document 4 also uses the effect of improving the wear resistance by dispersing hard nb—ti based carbide at a high density. However, when nb—ti based carbide is dispersed at a high density, voids are generated between the matrix and carbide during cold rolling, and as a result, toughness is lowered. Therefore, in patent document 4, the occurrence of voids is suppressed by limiting the rolling reduction in cold rolling. However, this method is not an essential solution because the rolling reduction is limited, and therefore the thickness and mechanical properties of the cold-rolled steel sheet that can be produced are inevitably limited.

In addition, the techniques proposed in patent documents 5 to 7 are still insufficient in toughness.

The present invention has been made in view of the above circumstances, and an object of the present invention is to achieve further excellent toughness in a cold-rolled steel sheet having increased hardness using a carbide such as Nb.

The present inventors have studied a method for solving the above problems and as a result, have obtained the following findings.

(1) By properly controlling the size and density of nb—ti—v carbides in the cold-rolled steel sheet, the toughness of the cold-rolled steel sheet after quenching and tempering can be effectively improved. As a result, a steel member having both hardness and toughness at a high level can be manufactured.

(2) The size and density of nb—ti—v carbide in the cold-rolled steel sheet can be appropriately controlled by appropriately controlling the composition of the steel slab to be used and the production conditions of the cold-rolled steel sheet.

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

1. A cold-rolled steel sheet having the following composition: comprises the following components in percentage by mass:

C：0.6～1.25％、

Si：0.10～0.55％、

Mn：0.20～2.0％、

P：0.0005～0.05％、

s:0.03 or less,

Al：0.001～0.1％、

N：0.001～0.009％、

Cr:0.1 to 1.0%

Ti:0.01 to 1.0 percent of Nb:0.05 to 0.5 percent and V: 1 or more than 2 of 0.01 to 1.0 percent,

the remainder is composed of Fe and unavoidable impurities;

the average grain size of carbide containing at least one of Nb, ti and V existing in ferrite grains is 0.1 μm or more,

and the number density of the carbide having a particle diameter of 0.1 μm or more in the carbide is 100 pieces/mm ² The above.

2. The cold-rolled steel sheet according to the above 1, wherein the component composition further comprises, in mass%, a component selected from the group consisting of:

sb: less than 0.1 percent,

Hf: less than 0.5 percent,

REM: less than 0.1 percent,

Cu: less than 0.5 percent,

Ni:3.0% or less,

Sn: less than 0.5 percent,

Mo: less than 1%,

Zr: less than 0.5 percent,

B:0.005% or less, and

w: 1 or 2 or more of 0.01% or less.

3. A steel member obtained by quenching and tempering the cold-rolled steel sheet according to 1 or 2.

4. The steel member according to the above 3, wherein the steel member is any one of a component for textile machinery, a bearing member and a cutter.

5. A method for producing a cold-rolled steel sheet, comprising heating a steel sheet blank having the composition of 1 or 2,

the heated steel slab is subjected to finish rolling inlet side temperature: hot-rolled steel sheet is produced by hot-rolling under the condition of Ac3 point or above,

and (c) the hot rolled steel sheet is cooled from the end of the hot rolling to the start of cooling: average cooling rate of 2 seconds or less: 25 ℃/s or more, cooling stop temperature: cooling is carried out under the condition of 720 ℃ or lower,

the cooled hot rolled steel sheet is wound,

annealing temperature is applied to the coiled hot rolled steel sheet: 650-780 ℃ and annealing time: a first annealing under conditions of 3 hours or more,

the rolling ratio of the hot rolled steel sheet after the first annealing is repeated twice or more: cold rolling of 15% or more and annealing temperature: the second annealing at 600-800 ℃ is performed, and then the final cold rolling with the rolling rate of more than 20% is further performed.

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

7. A method for producing a steel member, comprising the step of quenching a cold-rolled steel sheet produced by the production method described in 5 or 6 above at a quenching temperature: 700-800 ℃ and holding time: quenching is performed for 1 minute or more and less than 60 minutes, followed by tempering at a tempering temperature: 150-300 ℃ and holding time: tempering is carried out under the condition of 20 minutes to 3 hours.

According to the present invention, further excellent toughness after quenching and tempering can be obtained in a cold-rolled steel sheet having increased hardness by using a carbide such as Nb. Therefore, the cold-rolled steel sheet of the present invention can be suitably used as a material for various steel members such as a fiber machine member, a bearing member, and a machine/household tool. Further, according to the present invention, a steel member using the cold-rolled steel sheet can be provided.

Detailed Description

The present invention will be described in detail below. The present invention is not limited to this embodiment. In the present invention, attention is paid to carbide containing at least one of Nb, ti, and V existing in ferrite grains. Therefore, in the following description, "carbide containing at least one of Nb, ti, and V existing in ferrite grains" is sometimes simply referred to as "carbide".

[ composition of ingredients ]

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

C：0.6～1.25％

C is an element required for improving the hardness after quenching and tempering. C is also an element necessary for forming cementite and carbide with Nb, ti, V, and other elements. In order to produce necessary carbide to obtain strength after quenching and tempering, the C content needs to 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 excessively increases and embrittles. In addition, if the C content exceeds 1.25%, the surface scale upon heating becomes hard, with the result that the surface properties deteriorate. Therefore, the C content is 1.25% or less, preferably 1.20% or less.

Si：0.10～0.55％

Si is an element having an effect of improving strength by solid solution strengthening. In order to obtain the above effect, the Si content is set to 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more. On the other hand, if the Si content is excessive, si oxide is formed, and toughness is lowered. On the other hand, if the Si content is excessive, ferrite generation and grain growth are promoted, carbide precipitation at grain boundaries is promoted, and carbide precipitation into grains is suppressed. In addition, if Si is excessive, the surface scale upon heating becomes hard, resulting in deterioration of surface properties. Therefore, the Si content is 0.55% or less, preferably 0.50% or less, and more preferably 0.45% or less.

Mn：0.20～2.0％

Mn is an element that has an effect of improving hardness by promoting quenching and suppressing temper softening. In order to suppress temper softening, it is necessary to suppress the formation of C as cementite or delay dislocation recovery, and Mn has both functions, so that Mn is added to maintain a high-hardness structure with a high dislocation density even after tempering. 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%, a band structure is generated due to Mn segregation. In particular, abnormal grain growth and uneven structure are likely to occur in the segregation portion of MnS, and local precipitation into ferrite grain boundaries occurs, so that carbide formation in grains is suppressed. In addition, the cracks and shape defects during processing are caused. Therefore, the Mn content is 2.0% or less, preferably 1.95% or less.

P：0.0005～0.05％

By adding a small amount of P, the effect of improving strength by solid solution strengthening can be obtained. 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%, toughness is lowered due to grain boundary embrittlement. Therefore, the P content is 0.05% or less, preferably 0.045% or less.

S: less than 0.03%

S reduces toughness by forming sulfides with Mn. Therefore, the S content is 0.03% or less, preferably 0.02% or less. On the other hand, from the viewpoint of improving toughness, the lower the S content, the better, and therefore, the lower limit of the S content is not particularly limited and may be 0%. However, since excessive reduction leads to an increase in manufacturing cost, the S content is preferably 0.0005% or more, more preferably 0.001% or more, from the viewpoint of industrial production.

Al：0.001～0.1％

Al is an element required for deoxidization in steel production. Therefore, the Al content is 0.001% or more. On the other hand, if Al is excessive, nitride is formed, and formation of cracks and voids starting from the nitride is promoted, resulting in a decrease in toughness. Therefore, the Al content is 0.1% or less, preferably 0.08% or less, and more preferably 0.06% or less.

N：0.001～0.009％

Nitrogen is an element that improves toughness by forming a fine nitride and making the grain size finer. Therefore, the N content is 0.001% or more. On the other hand, if N is excessive, it combines with Al to form nitride, and formation of cracks and voids starting from the nitride is promoted, resulting in a decrease in toughness. Therefore, the N content is 0.009% or less, preferably 0.008% or less.

Cr：0.1～1.0％

Cr is an element that improves the hardenability and strength of steel. In order to obtain the above effect, the Cr content is 0.1% or more, preferably 0.12% or more. On the other hand, if Cr is excessive, coarse Cr carbide and Cr nitride are formed, and voids are generated around the Cr carbide and Cr nitride, with the result that toughness is lowered. Therefore, the Cr content is 1.0% or less, preferably 0.95% or less.

The composition of the components comprises Ti:0.01 to 1.0 percent of Nb:0.05 to 0.5 percent and V:0.01 to 1.0% of 1 or more than 2 kinds. In order to obtain a desired number density of carbides, at least one of Ti, nb, and V in the above amounts needs to be added.

Ti：0.01～1.0％

Ti is an element having an effect of forming carbide in grains and improving toughness. When Ti is added, the Ti content is set to 0.01% or more, preferably 0.015% or more, in order to obtain the above-mentioned effects. On the other hand, if Ti is excessively added, the austenitizing temperature becomes high, and therefore ferrite is easily generated on the surface of the steel sheet due to a decrease in temperature during hot rolling. The cold rolling and annealing after the surface-generated ferrite passes remain, and carbide is preferably generated at the grain boundary, with the result that carbide generation in the grains is suppressed. Therefore, the Ti content is 1.0% or less, preferably 0.9% or less.

Nb：0.05～0.5％

Nb is an element having an effect of forming carbide in grains and improving toughness. Nb is an element having a large effect on grain refinement. When Nb is added, the Nb content is set to 0.05% or more in order to obtain the above-described effect. On the other hand, if Nb is excessively added, carbides are formed at the grain boundaries, and the number density of carbides formed within the grains decreases. Since carbides generated at grain boundaries become starting points of voids and cracks, toughness is reduced. Therefore, the Nb content is 0.5% or less, preferably 0.45% or less.

V：0.01～1.0％

V is an element having an effect of forming carbide in grains and improving toughness. In addition, V also has an effect of improving hardenability, and improves the strength of steel. In addition, in order to suppress tempering and softening, it is necessary to suppress the formation of C as cementite or delay dislocation recovery, and V has both functions, and by adding V, the worked structure can be maintained even after tempering, and toughness is improved. When V is added, the V content is set to 0.01% or more in order to obtain the above-mentioned effects. On the other hand, if V is excessively added, carbides formed at the grain boundaries coarsen, and carbides formed at the grain boundaries become starting points of voids and cracks, so that toughness is lowered. Therefore, the V content is 1.0% or less, preferably 0.95% or less.

The cold-rolled steel sheet according to one embodiment of the present invention has a composition composed of the above components, and the balance of Fe and unavoidable impurities.

In another embodiment of the present invention, the above-described composition may optionally further contain a compound selected from the group consisting of Sb:0.1% or less, hf: below 0.5%, REM:0.1% or less, cu: less than 0.5%, ni:3.0% or less, sn: less than 0.5%, mo: less than 1% Zr:0.5% or less, B:0.005% below and W: 1 or 2 or more of 0.01% or less.

Sb: less than 0.1%

Sb is an effective element for improving corrosion resistance, but if it is excessively added, an Sb-rich layer is formed under the scale formed during hot rolling, and surface peeling (scratches) of the steel sheet occurs after hot rolling. Therefore, the Sb content is 0.1% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but from the viewpoint of improving the addition effect, the Sb content is preferably 0.0003% or more.

Hf: less than 0.5%

Hf is an effective element for improving corrosion resistance, but if it is excessively added, an Hf-rich layer is formed under the scale formed during hot rolling, and surface peeling (scratches) of the steel sheet occurs after hot rolling. Therefore, the Hf content is 0.5% or less. On the other hand, the lower limit of the Hf content is not particularly limited, but from the viewpoint of improving the addition effect, it is preferable to make the Hf content 0.001% or more.

REM: less than 0.1%

REM (rare earth metal) is an element that improves the strength of steel. However, if REM is excessively added, spheroidization of cementite may be delayed, and uneven deformation may be promoted during cold working, thereby deteriorating surface properties. Therefore, REM content is 0.1% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of improving the addition effect, the REM content is preferably 0.005% or more.

Cu: less than 0.5%

Cu is an effective element for improving corrosion resistance, but if it is excessively added, a Cu-rich layer is formed under the scale formed during hot rolling, and surface peeling (scratches) of the steel sheet occurs after hot rolling. Therefore, the Cu content is 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of improving the addition effect, the Cu content is preferably 0.01% or more.

Ni:3.0% or less

Ni is an element that improves the strength of steel. However, if the additive is excessively added, uneven deformation may be promoted during cold working, and the surface properties may be deteriorated. Therefore, the Ni content is 3.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of improving the addition effect, the Ni content is preferably 0.01% or more.

Sn: less than 0.5%

Sn is an effective element for improving corrosion resistance, but if it is excessively added, an Sn-rich layer is formed under the scale formed during hot rolling, and surface peeling (scratches) of the steel sheet occurs after hot rolling. Therefore, the Sn content is 0.5% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of improving the additive effect, the Sn content is preferably 0.0001% or more.

Mo: less than 1%

Mo is an element that improves the strength of steel. However, if the amount is excessively increased, spheroidization of cementite may be delayed, and uneven deformation may be promoted during cold working, thereby deteriorating surface properties. Therefore, the Mo content is 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of improving the addition effect, the Mo content is preferably 0.001% or more.

Zr: less than 0.5%

Zr is an effective element for improving corrosion resistance, but if it is excessively added, a Zn-rich layer is formed under the scale formed during hot rolling, and surface peeling (scratches) of the steel sheet occurs after hot rolling. Therefore, the Zr content is 0.5% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of improving the addition effect, the Zr content is preferably 0.01% or more.

B: less than 0.005%

B is an element having an effect of improving hardenability, and may be added arbitrarily. However, if the B content exceeds 0.005%, cracks are likely to occur on the surface during quenching. Therefore, the B content is 0.005% or less. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of improving the addition effect, when B is added, the B content is preferably 0.0001% or more.

W: less than 0.01%

W is an element having an effect of improving hardenability, and may be added arbitrarily. However, if the W content exceeds 0.01%, cracks are likely to occur on the surface during quenching. Therefore, the W content is 0.01% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of improving the addition effect, when W is added, the W content is preferably 0.001% or more.

[ carbide ]

Next, the carbide contained in the cold-rolled steel sheet of the invention will be described.

Average particle diameter: 0.10 mu m or more

Number density: 100 pieces/mm ² Above mentioned

In the stage before cold rolling in the preceding step of starting the processing of the component, a structure in which Nb, ti, and V-based carbides are formed in the crystal grains is previously prepared, and then, when the processing structure formed in the cold rolling is subjected to the subsequent quenching and tempering treatment, a part of fine Nb, ti, and V-based carbides are re-precipitated in the subgrain boundaries. By this structure, resistance to strain induced by repeated deformation is increased, and toughness of the final product is improved. In order to obtain this effect, it is necessary to set the average grain size of carbide containing at least one of Nb, ti, and V existing in ferrite grains to 0.10 μm or more. For the same reason, it is necessary to set the number density of carbides having a particle diameter of 0.10 μm or more to 100 pieces/mm in the above-mentioned carbide ² The above.

If the average grain size of the carbide is less than 0.10. Mu.m, the amount of fine Nb, TI and V carbides precipitated after quenching and tempering is insufficient and a high toughness improving effect cannot be obtained. In addition, if the number density of carbides is less than 100 pieces/mm ² As in the case of the average grain size, the amounts of fine Nb, TI, V carbides precipitated after quenching and tempering are insufficient and a high toughness improving effect cannot be obtained.

[ plate thickness ]

The thickness of the cold-rolled steel sheet is not particularly limited, and may be any thickness, but is preferably 0.1mm or more, and more preferably 0.2mm or more. The upper limit of the plate thickness is not particularly limited, but is preferably 2.5mm or less, more preferably 1.6mm or less, and further preferably 0.8mm or less. When the plate thickness is 0.2mm to 0.8mm, the material is particularly suitable for use as a material for textile machine parts such as needle and the like.

[ method for producing Cold-rolled Steel sheet ]

Next, a method for manufacturing 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 performing the following steps on a steel slab having the above-described composition.

(1) Heating

(2) Hot rolling

(3) Cooling

(4) Winding up

(5) First annealing

(6) Cold rolling

(7) Second annealing

(8) Final cold rolling

The steps (6) and (7) are repeated two or more times. Hereinafter, each step will be described.

(1) Heating

First, a steel slab having the above-mentioned composition is heated. The steel slab may be manufactured by any method without particular limitation. For example, the composition of the slab may be adjusted by a blast furnace converter method or an electric furnace method. The casting of the slab from the molten steel may be performed by continuous casting or by steel slab rolling.

The heating may be performed by any method, but is preferably performed using a heating furnace.

When the heating is performed using a heating furnace, the temperature in the heating furnace is not particularly limited, but is preferably 1100 ℃ or higher from the viewpoint of homogenizing the steel component and dissolving the segregation and undissolved carbide in the steel slab.

The holding time during the heating is not particularly limited, but is preferably 1 hour or more from the viewpoint of sufficiently dissolving the undissolved carbide.

(2) Hot rolling

Subsequently, the heated slab is hot-rolled to produce a hot-rolled steel sheet. In the above hot rolling, rough rolling and finish rolling may be performed in accordance with a conventional method.

Finish rolling inlet side temperature: ac3 point or higher

If the finish rolling inlet side temperature during the hot rolling is less than the Ac3 point, stretched ferrite is generated in the steel sheet after the hot rolling, and the stretched ferrite remains in the finally obtained cold-rolled steel sheet. As a result, the formation of grain boundary carbides is promoted, and the formation of intragranular carbides is suppressed, so that the toughness is reduced. Therefore, the finish rolling inlet side temperature in the hot rolling is not less than Ac3 point. On the other hand, the upper limit of the finish rolling inlet side temperature is not particularly limited, but is preferably 1200 ℃ or lower.

The Ac3 point (. Degree. C.) is determined 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 symbol in the above formula (1) refers to the content (mass%) of each element, and zero is set when the element is not contained.

(3) Cooling

Time from end of hot rolling to start of cooling: for less than 2 seconds

Next, the hot rolled steel sheet is cooled. At this time, if a long period of time passes from the end of hot rolling to the start of cooling, coarse ferrite is generated, and carbides containing at least one of Ti, nb, and V are unevenly precipitated in grain boundaries. The uneven structure is not homogenized during the subsequent cold rolling and annealing, and it becomes an obstacle to the formation of carbide in the crystal grains. Therefore, the time from the end of the hot rolling to the start of cooling is set to 2 seconds or less. On the other hand, from the above point of view, the shorter the time from the end of the hot rolling to the start of cooling is, the better, and therefore the lower limit is not particularly limited. However, from the viewpoint of industrial production, the time may be 0.5 seconds or more, or 0.8 seconds or more.

Average cooling rate: 25 ℃/s or more

If the average cooling rate during the cooling is less than 25 ℃/s, ferrite grains coarsen and the generated carbide localizes, so that the carbide generation at the grain boundary is concentrated and the generation of carbide in the grains is suppressed when the subsequent cold rolling and annealing are repeated. Therefore, the average cooling rate is 25 ℃/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 too high, the winding shape becomes poor due to volume expansion caused by phase transition at the time of subsequent winding. Therefore, from the viewpoint of improving the winding shape, the average cooling rate is preferably 160 ℃/s or less, more preferably 150 ℃/s or less.

Cooling stop temperature: 720 DEG C

In addition, since ferrite grains coarsen similarly even when the cooling stop temperature during the above cooling is too high, carbide formation in the grains is suppressed during the repetition of cold rolling and annealing. Therefore, the cooling stop temperature is 720 ℃ or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but if the cooling stop temperature is too low, the winding shape becomes poor due to volume expansion caused by phase transition at the time of subsequent winding. Therefore, the cooling stop temperature is preferably 620 ℃ or higher, more preferably 640 ℃ or higher.

(4) Winding up

After stopping the cooling, the cooled hot-rolled steel sheet is wound into a coil shape. In this case, the winding temperature is not particularly limited, but is preferably 600 to 730 ℃. By this temperature, plate-like cementite is precipitated, and the coil shape of the steel coil is stabilized.

(5) First annealing

Annealing temperature: 650-780 DEG C

Annealing time: for more than 3 hours

Annealing temperature is applied to the coiled hot rolled steel sheet: 650-780 ℃ and annealing time: and (3) first annealing under the condition of more than 3 hours. The structure of the hot rolled steel sheet after coiling is a pearlite structure in which plate-like carbide and ferrite are arranged. Since the pearlite structure is stable, it is not homogenized without being maintained at high temperature for a long time. In order to destroy the pearlite structure and to generate a desired carbide in the crystal grains in the subsequent cold rolling and annealing steps, it is necessary to set the annealing temperature to 650 ℃ or higher and the annealing time to 3 hours or longer. On the other hand, if the annealing temperature is higher than 780 ℃, the transformation is preferentially started from a part, and therefore, the locally coarse structure becomes an uneven structure, and thus, it is difficult to obtain intragranular carbide, and a desired carbide number density cannot be obtained. The upper limit of the annealing time is not particularly limited, but if it is too long, not only the productivity is lowered but also the effect is saturated. Therefore, it is preferably 20 hours or less.

It is preferable that the hot-rolled steel sheet is also subjected to acid washing before the first annealing.

(6) Cold rolling

(7) Second annealing

The steel sheet after hot rolling forms plate-like carbide. Since the plate-like carbide is stable, the plate-like carbide remaining after the end is likely to remain, and this eventually causes void formation and cracking, thereby reducing toughness. Therefore, in order to form plate-like carbides into particle shapes, redissolve the plate-like carbides by heating by annealing, and precipitate carbides in crystal grains, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing repeatedly twice or more.

Rolling rate: 15% or more

If the rolling reduction in the above cold rolling is less than 15%, carbides at the grain boundaries coarsen, so that the number density of carbides generated in the grains decreases, and the grain size of the carbides in the grains becomes smaller. Therefore, the rolling reduction is set to 15% or more. On the other hand, the upper limit of the reduction ratio is not particularly limited, but is preferably 70% or less.

Annealing temperature: 600-800 DEG C

If the annealing temperature in the second annealing is higher than 800 ℃, carbides at the grain boundaries coarsen, so that the number density of carbides generated in the grains decreases and the grain size of the carbides in the grains becomes smaller. Therefore, the annealing temperature is set to 800 ℃ or lower. On the other hand, if the annealing temperature is less than 600 ℃, the formation of carbide in the grains is suppressed, and the desired grain size cannot be obtained. Therefore, the annealing temperature is set to 600 ℃ or higher.

The heating rate in the second annealing is not particularly limited, but if the heating rate is too low, carbide is easily formed in the ferrite grain boundary, and thus carbide formation in the crystal grains is suppressed. Therefore, from the viewpoint of further improving the toughness improvement effect, the temperature rise rate in the second annealing is preferably 50 ℃/hr or more. On the other hand, the upper limit of the temperature increase rate is not particularly limited, but is preferably 200℃per second or less.

The number of repetitions of the cold rolling and the second annealing is two or more. By repeating the cold rolling and annealing twice or more, carbide formation can be promoted, and the desired size and number density of carbides in the crystal grains can be finally obtained. The upper limit of the number of repetitions is not particularly limited, but even if the repetition exceeds 5 times, the effect is saturated, and therefore the number of repetitions is preferably 5 times or less.

(8) Final cold rolling

The rolling rate is more than 20 percent

After the cold rolling and the second annealing are repeated twice or more as described above, final cold rolling is further performed at a rolling reduction of 20% or more. By performing the rolling ratio: and 20% or more of final cold rolling, in which carbides of a desired number density are precipitated in grains during quenching and tempering, and toughness is improved. The rolling reduction in the final cold rolling is preferably larger, but if it is 65% or more, the shape of the steel sheet may become unstable. Therefore, the rolling reduction is preferably less than 65%.

By satisfying the above conditions, a cold-rolled steel sheet excellent in toughness after quenching and tempering can be produced. The finally obtained cold-rolled steel sheet may be further subjected to any surface treatment.

[ method for producing Steel Member ]

In one embodiment of the present invention, the steel member may be manufactured by quenching and tempering a cold-rolled steel sheet manufactured by the above manufacturing method. The conditions of the quenching and tempering are not particularly limited, but in order to obtain higher toughness, it is preferable that the quenching temperature is: 700-900 ℃ and the holding time is as follows: quenching is performed for 1 minute or more and less than 60 minutes, followed by tempering at a tempering temperature: 150-400 ℃ and holding time: tempering is carried out under the condition of 20 minutes to 3 hours. The quenching temperature is more preferably 750 to 850 ℃. The tempering temperature is more preferably 200 to 300 ℃.

The cooling in the quenching is not particularly limited, and may be performed by any method. The cooling may be, for example, any of air cooling, water quenching, and oil quenching.

The cold-rolled steel sheet may be optionally processed to have a desired shape before the quenching and tempering.

Examples

In order to confirm the effects of the present invention, a cold-rolled steel sheet was produced in accordance with the procedure described below, and the toughness of the cold-rolled steel sheet after quenching and tempering was evaluated.

First, steel having the composition shown in table 1 was melted in a converter, and a steel slab was produced by a continuous casting method. Subsequently, the steel slab is subjected to heating, hot rolling, cooling, winding, first annealing, cold rolling, second annealing, and final cold rolling in this order to obtain a final plate thickness: about 0.4 mm. Each step was carried out under the conditions shown in tables 2 and 3, and the cold rolling and the second annealing were repeated the times shown in tables 2 and 3.

(method for measuring carbide)

From the obtained cold-rolled steel sheet, a test piece for tissue observation was collected. After polishing the rolling direction cross section (L section) of the test piece for observing the structure, the polished surface was etched with 1 to 3vol% of an ethanol nitrate etching solution, thereby developing the structure. Next, the surface of the test piece for tissue observation was photographed at 3000 times magnification using SEM (scanning electron microscope ), and a tissue image was obtained. Based on the obtained tissue image, the grain size of Nb, ti, V-based carbide produced in the crystal grains was measured by a cutting method, and the carbide in the measurement field was counted, thereby calculating the number density. The average of the three fields was calculated as particle size and number density. The measurement results are shown in tables 4 and 5. Identification of Nb, ti, V-based carbides was performed using SEM-EDS (energy dispersive X-ray Spectroscopy ) analysis. The observation field was subjected to element mapping, and cementite and other carbides were separated, and the other carbides were Nb, ti, and V-based carbides.

(toughness after quenching and tempering)

Next, in order to evaluate toughness after quenching and tempering of the obtained cold-rolled steel sheet, a test was performed in accordance with the following procedure, and an impact value in the charpy impact test was measured. First, the obtained cold-rolled steel sheet is quenched and tempered. The quenching is performed by oil quenching at 80 ℃ after the cold-rolled steel sheet is held in a furnace preheated to 800 ℃ for 10 minutes. The tempering is performed by air-cooling the quenched cold-rolled steel sheet after being held in a furnace preheated to 250 ℃ for 1 hour.

Then, a Charpy impact test was performed to measure an impact value. The measurement results are shown in tables 4 and 5. The Charpy impact test was performed using a test piece having a notch depth of 2.5mm and a notch radius of 0.1mm (notch width of 0.2 mm) obtained from a quenched and tempered cold-rolled steel sheet. The U-cut of the test piece was formed by electric discharge machining. In the present invention, when the impact value is 8J/cm ² In the above cases, it was determined that the toughness after quenching and tempering was excellent.

/>

TABLE 4

*1 carbide containing at least one of Nb, ti, V existing in ferrite grains

*2 the number density of the carbide having a particle diameter of 0.10 μm or more in the carbide

TABLE 5

*1 carbide containing at least one of Nb, ti, V existing in ferrite grains

As is clear from the results shown in tables 1 to 5, the cold-rolled steel sheet satisfying the conditions of the present invention is excellent in toughness after quenching and tempering. According to the present invention, since both high hardness and excellent toughness due to nb—ti—v carbide can be achieved, steel parts having both hardness and toughness at high levels can be produced by using the cold-rolled steel sheet of the present invention. Therefore, the cold-rolled steel sheet of the present invention can be suitably used as a material for various steel parts such as textile machine parts, bearing parts, and tools.

Claims

1. A cold-rolled steel sheet having the following composition:

comprises the following components in percentage by mass:

C：0.6～1.25％、

Si：0.10～0.55％、

Mn：0.20～2.0％、

P：0.0005～0.05％、

s: less than 0.03 percent,

Al：0.001～0.1％、

N：0.001～0.009％、

Cr:0.1 to 1.0%

the remainder is composed of Fe and unavoidable impurities;

and the average grain size of carbide containing at least one of Nb, ti and V in ferrite grains is 0.10 μm or more,

and the number density of the carbide with the grain diameter of more than 0.10 mu m in the carbide is 100 pieces/mm ² The above.

2. The cold-rolled steel sheet as claimed in claim 1, wherein the component composition further comprises, in mass%, a composition selected from the group consisting of:

sb: less than 0.1 percent,

Hf: less than 0.5 percent,

REM: less than 0.1 percent,

Cu: less than 0.5 percent,

Ni:3.0% or less,

Sn: less than 0.5 percent,

Mo: less than 1%,

Zr: less than 0.5 percent,

B:0.005% or less, and

w: 1 or 2 or more of 0.01% or less.

3. A steel member obtained by quenching and tempering the cold-rolled steel sheet according to claim 1 or 2.

4. A steel part according to claim 3, wherein the steel part is any one of a part for a textile machine, a bearing part and a cutter.

5. A method for producing a cold-rolled steel sheet, comprising heating a steel sheet blank having the composition of claim 1 or 2,

-bringing the heated steel slab at finish rolling inlet side temperature: hot-rolled steel sheet is produced by hot-rolling under the condition of Ac3 point or above,

and (c) heating the hot rolled steel sheet at a time from the end of the hot rolling to the start of cooling: average cooling rate of 2 seconds or less: 25 ℃/s or more, cooling stop temperature: cooling is carried out under the condition of 720 ℃ or lower,

winding the cooled hot rolled steel sheet,

and (3) carrying out annealing temperature on the coiled hot rolled steel plate: 650-780 ℃ and annealing time: a first annealing under conditions of 3 hours or more,

and repeating the rolling ratio of the hot rolled steel sheet after the first annealing twice or more: cold rolling of 15% or more and annealing temperature: the second annealing at 600-800 ℃ is performed, and then the final cold rolling with the rolling rate of more than 20% is further performed.

7. A method for manufacturing a steel part, comprising the step of quenching a cold-rolled steel sheet manufactured by the manufacturing method according to claim 5 or 6 at a quenching temperature: 700-900 ℃ and the holding time is as follows: quenching is performed for 1 minute or more and less than 60 minutes, followed by tempering at a tempering temperature: 150-400 ℃ and holding time: tempering is carried out under the condition of 20 minutes to 3 hours.