WO2014162661A1

WO2014162661A1 - Cold-rolled steel sheet and manufacturing method therefor

Info

Publication number: WO2014162661A1
Application number: PCT/JP2014/001265
Authority: WO
Inventors: 勇人齋藤; 崇小林; 船川　義正
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2013-04-02
Filing date: 2014-03-07
Publication date: 2014-10-09
Also published as: TWI551696B; MX2015013941A; TW201441383A; TW201621061A; KR20150119097A; KR101717002B1; TWI534273B; JP5633594B2; CN105074039A; JP2014201766A; CN105074039B

Abstract

Provided are a cold-rolled steel sheet with excellent blanking quality and heat strain resistance, and a manufacturing method therefor. A cold-rolled steel sheet with a Vickers hardness of at least Hv 170 comprising: a composition containing, in mass%, C: 0.01% to 0.08%, Si: 0.01% to 1.0%, Mn: 0.05% to 1.0%, P: not more than 0.03%, S: not more than 0.015%, Al: 0.005% to 0.10%, and N: not more than 0.01%, the remainder being obtained from Fe and unavoidable impurities; and a structure wherein the area ratio of ferrite phases is at least 80%, the total area ratio of hard phases obtained from pearlite phases and/or bainite phases is not more than 20%, the mean particle diameter for the hard phases is 1 µm to 10 µm, and the mean aspect ratio for the hard phases is not more than 10.0.

Description

Cold rolled steel sheet and method for producing the same

The present invention is suitable for materials such as plates, disks, and rings that are automotive drive-train parts, and is suitable for materials such as punchability and heat distortion characteristics (heat The present invention relates to a cold-rolled steel sheet having excellent strain resistance and a method for producing the same.

Plates, disks, rings, etc. (such as AT plates, etc.) used as automotive drive system parts have conventionally been quenched for the purpose of hardening after steel plates are punched into a predetermined shape. The heat treatment is performed to adjust the hardness to a predetermined level, and then a friction material is adhered. However, in the conventional manufacturing method, a great amount of heat energy is consumed for the heat treatment, and a dedicated heat treatment facility is required, so an increase in production cost is inevitable.

Under such circumstances, in recent years, cold rolled steel sheets adjusted to have a desired hardness using cold rolling instead of heat treatment after blanking have been applied. When a cold-rolled steel sheet is applied, a heat treatment such as quenching is not necessary, so that the manufacturing cost can be greatly reduced. However, when a cold-rolled steel sheet is applied, a large warp may occur in a part after punching. For this reason, there is a problem that shape correction (shape straitening) is required after punching. Furthermore, there is a problem that a predetermined shape cannot be obtained even if correction is performed. Even if a good flatness is obtained after punching, the residual stress (residual stress) when exposed to high temperatures in the bonding process with the friction material or the actual use environment (actual use environment) There is also a problem of thermal strain in which) is released and deforms.

Various technologies have been proposed so far for cold-rolled steel sheets for plates and other drive train components.
For example, in Patent Document 1, in mass%, C: 0.05 to 0.6%, Si: 2.0% or less, Mn: 0.2 to 2.0%, P: 0.03% or less, S: 0.03% or less, Sol.Al: 0.1% or less , N: Steel containing less than 0.01%, balance Fe and inevitable impurities, hot-rolled at a finish temperature (Ar ₃ transformation point –20 ° C) or higher, cooling rate over 120 ° C / second, cooling stop temperature Cooling is performed so that the difference in cooling stop temperature between the center part and the edge part in the width direction of the steel sheet is 650 ° C or lower, and the cooling temperature is 30 ° C or lower. By cold rolling at the above, or by cold rolling at an annealing temperature of 600 ° C. or more and an _AC1 transformation point or less and then rolling at a reduction rate of 40% or more, the surface hardness of the steel sheet Hv is reduced. A technique for manufacturing a thin steel sheet having a maximum hardness difference ΔHv of 20 or less at each position in the longitudinal direction and the width direction of the steel sheet between 170 and 300 has been proposed. According to the technique proposed in Patent Document 1, the difference in residual stress depending on the position in the longitudinal direction and width direction of the steel sheet is suppressed, and a cold-rolled thin steel sheet having excellent flatness after punching is obtained. Has been.

Patent Document 2 includes mass%, C: 0.05 to less than 0.10%, Si: 0.5% or less, Mn: 0.20 to 2.0%, P: 0.03% or less, S: 0.020% or less, Cr: 0.05 to 0.5% The steel material with the balance consisting of Fe and inevitable impurities is hot-rolled with the finish rolling finish temperature of finish rolling set to the Ar ₃ transformation point or higher, and cooled to 500-650 ° C within 8 s after finish rolling. , Having a base made of pro-eutectoid ferrite, pearlite, bainitic ferrite or bainite, after being wound at 500 to 650 ° C. Tensile strength having a structure in which cementite existing in the matrix is dispersed at an average of 1.0 × 10 ⁴ pieces / mm ² or more is made into a hot rolled steel sheet of 440 MPa or more. A technique for producing a cold-rolled steel sheet by performing cold rolling of up to 70% has been proposed. And according to the technique proposed by patent document 2, it is supposed that the cold-rolled steel plate excellent in the flatness after stamping and excellent in the end surface property will be obtained.

In Patent Document 3, C: 0.15 to 0.25 mass%, Si: 0.25 mass% or less, Mn: 0.3 to 0.9 mass%, P: 0.03 mass% or less, S: 0.015 mass% or less, Al: 0.01 to 0.08 mass% , N: 0.008% by mass or less, Ti: 0.01-0.05% by mass, B: 0.002-0.005% by mass, with the balance being substantially Fe composition, hot rolling finish temperature: Ar ₃ Hot rolling at a transformation point or higher, coiling temperature: 500-600 ° C, hot-rolled steel sheet pickled, then cold rolled at a reduction rate of 50% or higher without annealing, and diameter of 300mm or higher A technique for manufacturing a cold-rolled steel sheet has been proposed by performing light reduction rolling with a reduction ratio of 1% or less using this roll. According to the technique proposed in Patent Document 3, it is said that a cold rolled steel sheet for automatic transmission with reduced residual stress generated during cold rolling can be obtained.

Japanese Unexamined Patent Publication No. 2006-307281 Japanese Unexamined Patent Publication No. 2008-138237 Japanese Patent Laid-Open No. 2005-200712

However, in the technique proposed in Patent Document 1, rapid cooling is performed after hot rolling is completed, and winding is performed at a low temperature of 600 ° C. or lower, so that the internal residual stress is increased at the stage of the hot rolled steel sheet, resulting in cold rolling. Subsequent thin steel sheets are also in a state where a large residual stress is accumulated. Punching and forming a cold-rolled thin steel sheet with a large residual stress accumulated in this way gives good flatness in the state of punching, but heating in the friction material bonding process following punching There was a problem of thermal strain that the residual stress was released and the flatness deteriorated.

In the technique proposed in Patent Document 2, hot rolling steel sheet containing a larger amount of hard pearlite and bainite than ferrite is cold-rolled, so the vicinity of the phase boundary between ferrite and pearlite and bainite during cold rolling. Causes uneven deformation and increases the residual stress inside the steel sheet. Therefore, when the cold-rolled steel sheet obtained in this way is stamped and formed, a good flat shape and a punched end surface property (appearances of punched surface) can be obtained in the state of being punched and formed. As with the technology, there was a problem that heat strain occurred.

In the technique proposed in Patent Document 3, the C content is as high as 0.15 to 0.25% by mass and the coiling temperature in the hot rolling process is as low as 500 to 600 ° C. Generate. Therefore, as in the technique proposed in Patent Document 2, the residual stress inside the steel sheet increases during cold rolling. As a result, even if a flat shape is obtained in the state of punching, heat is generated by heating after punching. There was a problem that distortion occurred.

The object of the present invention is to solve the above-mentioned problems and to provide a cold-rolled steel sheet having hardness necessary for drive system parts such as a plate and a disk, flatness after punching, punched end face properties, and heat-resistant strain characteristics, and its It is to provide a manufacturing method. Here, the heat-resistant strain property means that when the friction material is heated to a temperature range of about 100 to 400 ° C in the adhesion process of the friction material or in an actual use environment, the thin steel sheet is less deformed and sufficient flatness is obtained. Means a characteristic.

In order to solve the above-mentioned problems, the present inventors conducted extensive research on various factors that affect the heat-resistant strain characteristics, flatness after punching, and punched end surface properties of cold-rolled steel sheets.

As a result, it has been found that the reduction of residual stress after cold rolling is effective for improving the thermal strain characteristics. In order to reduce the residual stress after cold rolling, the rolling reduction in cold rolling is set to a predetermined value or less, and the C content of the steel material is reduced to a predetermined value or less and hot rolling is performed. By defining the coiling temperature of the process, the hot-rolled sheet before cold rolling is the ferrite main phase, the volume fraction of the pearlite phase and bainite phase of the hot-rolled sheet is reduced, and after cold rolling It was found that it is important to control the average grain size and average aspect ratio of the pearlite and bainite phases of the steel sheet.

The hot-rolled sheet before cold rolling is a pearlite phase or a bainite phase that is a hard phase together with ferrite as a main phase (hereinafter, either one of the pearlite phase or the bainite phase may be referred to as a “hard phase”). When it contains, non-uniform deformation occurs near the ferrite phase-hard phase interface during cold rolling, and residual stress tends to accumulate. Accordingly, the hot-rolled sheet before cold rolling is used as a ferrite main phase, and the volume fraction of the hard phase contained in the hot-rolled sheet is reduced, so that the vicinity of the ferrite / pearlite interface during cold rolling, or ferrite / Residual stress accompanying non-uniform deformation near the interface of bainite can be reduced, and the thermal strain characteristics of the cold-rolled steel sheet can be improved. If such a cold-rolled steel plate is used to manufacture drive system parts such as plates, it will be obvious that after punching into a predetermined shape, even if it is exposed to high temperatures in the bonding process with friction materials or in the actual usage environment, The problem of a decrease in flatness due to strain can be solved.

The present inventors use a hot rolled sheet before cold rolling as a ferrite main phase, lower the volume fraction of the hard phase of the hot rolled sheet, and control the average grain size and average aspect ratio of the hard phase. Has been found to be extremely effective in improving the flatness after punching of the cold-rolled steel sheet and the properties of the punched end face.

By avoiding the generation of the hard phase as much as possible in the hot-rolled sheet before cold rolling, uniform characteristics can be obtained in the steel sheet after cold rolling, so that good flatness can be obtained after punching. In addition, by avoiding the generation of a hard phase as much as possible in the hot-rolled sheet before cold rolling, the number of out-of-phase interfaces (interfaces between ferrite and hard phases) that are prone to cracking during punching is reduced. Will improve.

In cold rolling, the hard phase is extended in the rolling direction, but the hardness difference between the main phase ferrite and the hard phase is large, so the hard phase extends in the rolling direction (that is, the aspect ratio of the hard phase). As the value increases, the strain accumulated at the interface between the ferrite phase and the hard phase increases, residual stress accumulates, and the heat-resistant strain characteristics of the cold-rolled steel sheet deteriorate. Therefore, in order to improve these characteristics, it is necessary to make the average aspect ratio of the hard phase composed of at least one of the pearlite and bainite phases below a predetermined value. In addition, by controlling the average particle size of the hard phase to a predetermined value or less, even if cracks occur from the boundary interface (boundary between different phases), it prevents the cracks from propagating greatly and punches without defects. An end face is obtained.

As described above, in order to use the hot rolled sheet before cold rolling as a ferrite main phase and reduce the volume fraction of the hard phase of the hot rolled sheet, the C content of the steel material is reduced to a predetermined value or less. In addition, it is necessary to increase the coiling temperature in the hot rolling process to a high temperature of more than 600 ° C. and 750 ° C. or less. In order to control the average grain size and average aspect ratio of the hard phase of the steel sheet after cold rolling, it is necessary to limit the rolling reduction of the cold rolling in addition to optimizing the rolling conditions of the hot rolled sheet.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] By mass%, C: 0.01% to 0.08%, Si: 0.01% to 1.0%, Mn: 0.05% to 1.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.005% 0.10% or less, N: 0.01% or less, with the balance consisting of Fe and inevitable impurities, the area ratio of the ferrite phase is 80% or more, and from one or more of pearlite phase and bainite phase The hard phase has a total area ratio of 20% or less, an average particle size of the hard phase of 1 μm to 10 μm, and an average aspect ratio of the hard phase of 10.0 or less, and a Vickers hard A cold-rolled steel sheet having a Vickers hardness of Hv170 or more and excellent in punchability and thermal strain characteristics.

[2] In the above [1], in addition to the composition, the punching property and heat resistance further include at least one of Cu: 0.01% to 0.20% and Ni: 0.01% to 0.50% in mass%. Cold rolled steel sheet with excellent strain characteristics.

[3] In the above [1] or [2], in addition to the above composition, by mass%, Ti: 0.005% to 0.10%, Nb: 0.005% to 0.10%, V: 0.005% to 0.50% , Zr: 0.005% or more and 0.10% or less, Mo: 0.02% or more and 0.50% or less, Cr: 0.03% or more and 0.50% or less, B: 0.0003% or more and 0.0050% or less Cold-rolled steel sheet with excellent characteristics.

[4] In any one of the above [1] to [3], in addition to the above composition, in addition to mass, Ca: 0.0003% to 0.0050%, REM: 0.0003% to 0.0100% or less A cold-rolled steel sheet excellent in punchability and heat distortion characteristics.

[5] In any one of the above [1] to [4], in addition to the above composition, by mass%, any one or more of Sb: 0.001% to 0.030%, Sn: 0.001% to 0.030% or less A cold-rolled steel sheet excellent in punchability and heat distortion characteristics.

[6] By mass%, C: 0.01% to 0.08%, Si: 0.01% to 1.0%, Mn: 0.05% to 1.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.005% More than 0.10%, N: 0.01% or less, and the steel material having the balance consisting of Fe and inevitable impurities is subjected to hot rolling at a finish rolling finish temperature of 800 ° C to 950 ° C, After hot rolling is completed, winding is performed at a winding temperature of more than 600 ° C and not more than 750 ° C, the scale is removed by pickling, and then cold rolling is performed at a reduction rate of 30% to 70%. A method for producing a cold-rolled steel sheet having excellent punchability and thermal strain characteristics.

[7] A method for producing a cold-rolled steel sheet having excellent punchability and heat-resistant strain characteristics after performing the cold rolling and then performing temper rolling in [6].

[8] In the above [6] or [7], in addition to the composition, the composition further contains at least one of Cu: 0.01% or more and 0.20% or less, Ni: 0.01% or more and 0.50% or less. A method for producing a cold-rolled steel sheet having excellent punchability and thermal strain characteristics.

[9] In any one of the above [6] to [8], in addition to the composition, in addition to mass, Ti: 0.005% to 0.10%, Nb: 0.005% to 0.10%, V: 0.005% or more Punchability containing 0.5% or less, Zr: 0.005% or more and 0.10% or less, Mo: 0.02% or more and 0.50% or less, Cr: 0.03% or more and 0.50% or less, B: 0.0003% or more and 0.0050% or less And a method for producing a cold-rolled steel sheet having excellent heat distortion characteristics.

[10] In any one of the above [6] to [9], in addition to the above composition, by mass%, any one or more of Ca: 0.0003% to 0.0050%, REM: 0.0003% to 0.0100% The manufacturing method of the cold-rolled steel plate excellent in the punchability and heat-strain-resistant characteristic containing this.

[11] In any one of the above [6] to [10], in addition to the composition, in addition to mass%, any one or more of Sb: 0.001% or more and 0.030% or less, Sn: 0.001% or more and 0.030% or less The manufacturing method of the cold-rolled steel plate excellent in the punchability and heat-strain-resistant characteristic containing this.

According to the present invention, in addition to excellent flatness after punching and punched end face properties, a cold-rolled steel sheet having excellent heat-resistant strain characteristics can be easily produced, and a remarkable industrial effect can be achieved. The cold-rolled steel sheet according to the present invention is extremely suitable as a material for driving parts such as automobile parts, particularly plates and disks.

First, the reasons for limiting the component composition of the cold-rolled steel sheet of the present invention will be described. Unless otherwise specified,%, which is a unit of each component element content, means mass%.

C: 0.01% or more and 0.08% or less C is an element necessary for strengthening steel sheets. To obtain the hardness required for drive system parts such as plates and disks, the C content is 0.01% or more. There is a need to. On the other hand, if the C content exceeds 0.08%, the hardness becomes excessively high, the pearlite fraction increases, the flatness after punching deteriorates, and the punched end face properties deteriorate. Therefore, the upper limit of C content is 0.08%. In particular, from the viewpoint of punched end face properties, the C content is preferably less than 0.05%. More preferably, it is 0.04% or less.

Si: 0.01% or more and 1.0% or less Si is a deoxidizer for steel and has the effect of improving the cleanliness of the steel, so the content is made 0.01% or more. Si is also an element that increases the hardness of the steel sheet by solid solution hardening, and can be added to obtain a desired hardness. However, if the Si content exceeds 1.0%, the surface quality of the steel sheet deteriorates, so the upper limit of the Si content is 1.0%. In particular, a plate material used for a component that generates friction is required to have good surface properties, and therefore, it is preferably set to 0.6% or less.

Mn: 0.05% or more and 1.0% or less Mn is an element that increases the hardness of the steel sheet by solid solution strengthening, and the Mn content needs to be 0.05% or more in order to obtain a desired hardness. Preferably it is 0.1% or more. If the Mn content exceeds 1.0%, in addition to excessive generation of pearlite, bainite, etc., pearlite and the like are generated in layers due to segregation, and the punched end face properties deteriorate. Therefore, the Mn content is 1.0% or less. Preferably it is 0.5% or less.

P: 0.03% or less P is an element that is easily segregated in steel. When it is contained in a large amount, the formation of a layered structure is promoted by the segregation of P, and the punched end face properties deteriorate. Therefore, the P content needs to be 0.03% or less. Preferably it is 0.02% or less.

S: 0.015% or less S forms sulfide inclusions such as MnS and degrades the punched end face properties. Therefore, the S content needs to be 0.015% or less. Preferably it is 0.010% or less.

Al: 0.005% or more and 0.10% or less Since Al can improve the cleanliness of the steel sheet by adding it as a deoxidizing element, its content needs to be 0.005% or more. Preferably it is 0.03% or more. On the other hand, if the Al content exceeds 0.10%, oxides are excessively generated, the surface properties are deteriorated, and the adhesion to the friction material is deteriorated. Therefore, the Al content is 0.10% or less. Preferably it is 0.08% or less.

N: 0.01% or less N is a harmful element in the present invention, and if its content is excessive, the ductility of the steel sheet is lowered and the punched end face properties are deteriorated. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

The above are the basic components of the cold-rolled steel sheet of the present invention, but the cold-rolled steel sheet of the present invention can contain the following elements as necessary in addition to these basic components.

Cu: 0.01% or more and 0.20% or less, Ni: 0.01% or more and 0.50% or less Any one or more of Cu and Ni are elements that contribute to an increase in the hardness of the steel sheet by solid solution strengthening. It can be contained in order to impart a thickness. In order to obtain such an effect, it is preferable that the Cu content is 0.01% or more and the Ni content is 0.01% or more. More preferably, the Cu content is 0.02% or more, and the Ni content is 0.02% or more. On the other hand, when the content of these elements is excessive, the surface properties are deteriorated and the adhesion to the friction material is deteriorated. Therefore, the Cu content is preferably 0.20% or less and the Ni content is preferably 0.50% or less. More preferably, the Cu content is 0.10% or less, and the Ni content is 0.30% or less.

Ti: 0.005% to 0.10%, Nb: 0.005% to 0.10%, V: 0.005% to 0.50%, Zr: 0.005% to 0.10%, Mo: 0.02% to 0.50%, Cr: 0.03% or more 0.50% or less, B: Any one or more of 0.0003% or more and 0.0050% or less Ti, Nb, V, Zr, Mo, Cr, and B are elements that contribute to an increase in the hardness of the steel sheet. It is preferably contained for imparting hardness. On the other hand, when the content of these elements becomes excessive, the residual stress increases, and flatness after punching or thermal strain occurs. Therefore, Ti content is 0.005% to 0.10%, Nb content is 0.005% to 0.10%, V content is 0.005% to 0.50%, Zr content is 0.005% to 0.10%, Mo content is It is preferable that the content is 0.02% to 0.50%, the Cr content is 0.03% to 0.50%, and the B content is 0.0003% to 0.0050%. Ti content 0.008% to 0.03%, Nb content 0.008% to 0.05%, V content 0.01% to 0.20%, Zr content 0.01% to 0.03%, Mo content 0.05% More preferably, the content is 0.20% or less, the Cr content is 0.05% or more and 0.20% or less, and the B content is 0.0005% or more and 0.0030% or less.

Ca: 0.0003% or more and 0.0050% or less, REM: 0.0003% or more and 0.0100% or less One or more of Ca and REM both control the shape of the sulfide to be spherical and improve the punched end face properties of the steel sheet. It can be contained if necessary. In order to obtain such effects, the Ca content is preferably 0.0003% or more and the REM content is preferably 0.0003% or more. More preferably, the Ca content is 0.0008% or more and the REM content is 0.0008% or more. On the other hand, when the content of these elements becomes excessive, inclusions increase and the punched end face properties of the steel sheet are deteriorated. Therefore, the Ca content is preferably 0.0050% or less and the REM content is preferably 0.0100% or less. . More preferably, the Ca content is 0.0030% or less and the REM content is 0.0050% or less.

Sb: 0.001% or more and 0.030% or less, Sn: 0.001% or more and 0.030% or less, any one or more Sb and Sn are elements that improve the surface properties of the steel sheet, and have the effect of improving the adhesion to the friction material. is there. In order to obtain such an effect, the Sb content is preferably 0.001% or more and the Sn content is preferably 0.001% or more. On the other hand, if the content of these elements is excessive, surface segregation becomes prominent, the surface properties of the steel sheet deteriorate and the adhesion to the friction material decreases, so the Sb content is 0.030% or less. The Sn content is preferably 0.030% or less. More preferably, the Sb content is 0.005% or more and 0.020% or less, and the Sn content is 0.005% or more and 0.015% or less.

The balance other than the above components is Fe and inevitable impurities. Examples of unavoidable impurities include O, Mg, Co, Zn, Ta, W, Pb, Bi, and the like. The content of these elements is acceptable if it is about 0.01% or less.

Next, the structure of the cold-rolled steel sheet of the present invention will be described.
In the cold-rolled steel sheet of the present invention, the area ratio of the ferrite phase is 80% or more, the area ratio of the hard phase composed of at least one of the pearlite phase and the bainite phase is 20% or less in total, and the hard phase The average particle size of the hard phase is 1 μm or more and 10 μm or less, and the average aspect ratio of the hard phase is 10.0 or less.

In the present invention, the residual stress of the cold-rolled steel sheet is reduced by making the deformation in the steel sheet uniform during cold rolling. From such a viewpoint, the cold-rolled steel sheet of the present invention needs to make the main phase a soft ferrite phase and suppress hard pearlite phase and bainite phase. When a hard phase composed of one or more of pearlite phase and bainite phase is contained in a large amount exceeding 20% in total area ratio, the residual stress after cold rolling increases, and the steel plate shape after punching Deteriorates, or cracks occur at the interface between the ferrite phase and the hard phase during punching, and the punched end face properties of the steel sheet deteriorate. Furthermore, since the thermal strain characteristics deteriorate as the residual stress increases, thermal strain occurs when such a cold-rolled steel sheet is punched and exposed to a high temperature environment.

For these reasons, in the present invention, the area ratio of the ferrite phase needs to be 80% or more, and the area ratio of the hard phase composed of at least one of the pearlite phase and the bainite phase needs to be 20% or less in total. The area ratio of the ferrite phase is preferably 85% or more, and more preferably 90% or more. On the other hand, the total area ratio of the hard phase is preferably 15% or less, and more preferably 10% or less.

However, when the area ratio of the hard phase becomes extremely low, the desired steel sheet hardness cannot be obtained. Therefore, the area ratio of the hard phase is preferably 2% or more in total. The cold-rolled steel sheet structure of the present invention may contain cementite in addition to the ferrite phase, pearlite phase, and bainite phase. The area ratio of cementite is preferably 1% or less.

Further, the cold-rolled steel sheet of the present invention has a structure in which an average particle diameter of a hard phase composed of at least one of a pearlite phase and a bainite phase is 1 μm or more and 10 μm or less, and an average aspect ratio of the hard phase is 10.0 or less. Have. In addition, how to obtain | require the average particle diameter and average aspect-ratio of a hard phase was described in the Example mentioned later.

When the average particle size of the hard phase exceeds 10 μm, cracks generated at the interface between the hard phase and the ferrite phase are greatly propagated, causing cracks on the punched end surface of the steel sheet, and the end surface properties are deteriorated. For this reason, the average particle diameter of the hard phase is 10 μm or less. Preferably, it is 7 μm or less. The smaller the average particle size of the hard phase, the more preferable from the viewpoint of the punchability of the steel sheet. However, in order to reduce the average particle size of the hard phase, it is necessary to lower the finish rolling end temperature and the winding temperature of hot rolling during the production of the steel sheet. On the other hand, in order to obtain a structure having the ferrite phase of the present invention as a main phase, it is necessary to set the finish rolling end temperature and the coiling temperature of hot rolling to a predetermined temperature or more as described later. There is a limit to the refinement of hard phase grains within the range of hot rolling conditions. Therefore, the average particle size of the hard phase is 1 μm or more.

Also, if the average aspect ratio of the hard phase exceeds 10.0, the strain at the interface between the hard phase and the ferrite phase, which is the main phase, increases and the residual stress increases, so the heat-resistant strain characteristics of the cold-rolled steel sheet deteriorate. Therefore, the average aspect ratio of the hard phase is 10.0 or less. Preferably it is 8.0 or less.

The hardness of the cold-rolled steel sheet of the present invention is 170 or more in terms of Hv (Vickers hardness). When the hardness of the steel sheet is less than Hv170, the strength is not sufficient, and the steel plate cannot withstand use as a plate, a disk, or a ring that is a driving system part for an automobile. For this reason, the hardness of a cold-rolled steel sheet shall be Hv170 or more. Preferably, it is Hv 190 or more. On the other hand, since the hardness of the cold-rolled steel sheet of the present invention is mainly based on work hardening by cold rolling, if the hardness becomes too high (that is, if the rolling reduction of cold rolling becomes too high). The residual stress of the steel sheet also increases, and the thermal strain characteristics deteriorate. For this reason, it is preferable that the cold-rolled steel sheet has a hardness of Hv250 or less.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
The cold-rolled steel sheet of the present invention is a hot-rolled sheet having a ferrite phase as a main phase by subjecting a steel material having the above chemical composition to hot rolling, and the scale is removed from the hot-rolled sheet by pickling. Thereafter, it is obtained by cold rolling at a predetermined rolling reduction.

The manufacturing method of the steel material need not be particularly limited. For example, molten steel having the above-described composition is melted in a converter or electric furnace, preferably subjected to secondary smelting in a vacuum degassing furnace (vacuum degassing furnace), and slab is obtained by a production method such as continuous casting. Any conventional method using steel materials such as can be applied.

The steel material is then subjected to hot rolling consisting of rough rolling and finish rolling to form a hot rolled sheet. When performing hot rolling, the steel material may be subjected to direct rolling (direct rolling) in which hot rolling is performed immediately after casting or after heating for heat supplement after casting. When the steel material is heated before hot rolling, the heating temperature is not particularly limited, but is preferably in the range of 1000 ° C. or higher and 1300 ° C. or lower. When the heating temperature is less than 1000 ° C., the deformation resistance becomes high and a good shape may not be obtained. On the other hand, when the temperature is higher than 1300 ° C., scale growth is promoted, and the surface properties of the steel sheet may be deteriorated. The conditions for rough rolling are not particularly limited.

Finish rolling end temperature: 800 ° C. or higher and 950 ° C. or lower When the finish rolling end temperature exceeds 950 ° C., the structure of the hot rolled sheet becomes coarse. As a result, since the average grain size of the hard phase of the steel sheet after cold rolling becomes large, the punched end face properties deteriorate. On the other hand, when the finish rolling finish temperature is less than 800 ° C., the structure of the hot-rolled sheet becomes a structure composed of extremely stretched crystal grains, and the aspect ratio of the hard phase also increases. In this way, when the steel sheet including the hard phase that has been extended in the rolling direction at the stage of hot rolling is further cold-rolled, there is a large difference in hardness between the hard phase and the ferrite phase that is the main phase. At the same time, a large residual stress is generated at the interface between the hard phase and the ferrite phase as the main phase. For this reason, the residual stress accumulate | stored in the steel plate after cold rolling becomes large, and it becomes easy to generate | occur | produce a thermal strain. Therefore, the finish rolling end temperature is set to 800 ° C. or more and 950 ° C. or less. Preferably they are 850 degreeC or more and 920 degrees C or less.

Winding temperature: More than 600 ° C. and 750 ° C. or less When the winding temperature is 600 ° C. or less, a pearlite phase or a bainite phase is excessively formed, and the target steel structure of the ferrite phase cannot be obtained. On the other hand, when the coiling temperature exceeds 750 ° C., the pearlite phase and cementite particles are coarsened to deteriorate the punched end surface properties of the steel plate or the surface properties of the steel plate. Therefore, the coiling temperature is set to more than 600 ° C. and 750 ° C. or less. Preferably they are 620 degreeC or more and 700 degrees C or less.

Although there is no particular limitation on the cooling rate when cooling to the coiling temperature after the finish rolling is finished, in order to obtain the desired structure of the hot-rolled sheet and the finally obtained cold-rolled sheet, from the finish rolling end temperature. The average cooling rate in the temperature range up to the coiling temperature is preferably 10 ° C./s or more and less than 120 ° C./s. More preferably, it is 15 ° C./s or more and 50 ° C./s or less.
The hot-rolled sheet obtained as described above is removed from the scale by pickling and then cold-rolled to obtain a cold-rolled steel sheet.

Cold rolling reduction ratio: 30% or more and 70% or less In order to obtain a steel sheet hardness necessary for a material for a drive system component such as a plate by cold rolling, the rolling reduction needs to be 30% or more. On the other hand, when the rolling reduction exceeds 70%, the residual stress increases, the average aspect ratio of the hard phase composed of pearlite and / or bainite phase exceeds a predetermined value, and thermal strain is likely to occur. Therefore, the rolling reduction of cold rolling is set to 30% or more and 70% or less. Preferably they are 40% or more and 60% or less.

In the present invention, from the viewpoint of correcting the shape of the steel sheet and adjusting the residual stress, after cold rolling, temper rolling may be applied, or a leveler may be passed. From the viewpoint of adjusting the residual stress, when temper rolling is performed, the elongation rate is preferably 0.3% or more. More preferably, it is 0.4% or more. However, from the viewpoint of the flatness of the steel sheet after temper rolling, it is preferably 1.0% or less.

Steels with chemical components shown in Table 1 were melted in a converter and made into slabs (steel materials) by a continuous casting method. Next, hot rolling, cooling and winding were performed on these steel materials under the conditions shown in Table 2 to obtain hot rolled sheets. Next, after removing the scale by pickling, cold rolling was performed at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet. About some cold-rolled steel plates, shape correction was performed by a leveler or temper rolling.

From the obtained cold-rolled steel sheet, a test piece is collected, and the structure is observed by the following method. The area ratio of the ferrite phase, the pearlite phase, the bainite phase, and the average grain of the hard phase (pearlite phase and / or bainite phase) Diameter and average aspect ratio were determined. Further, test pieces were collected from the obtained cold-rolled steel sheets, and the hardness, flatness after punching, punched end face properties, and heat distortion characteristics were evaluated by the following methods.

Microstructure observation A sample with a plate thickness cross section parallel to the rolling direction was taken, and the structure was revealed with a 3% nital solution (nital) for the plate thickness cross section, and a scanning electron microscope was used at the 1/4 thickness position. (SEM) was used to photograph 3 fields of view at 500 times, and the area ratio of each phase and the particle size and aspect ratio of the hard phase composed of at least one of pearlite and bainite phases were quantified by image processing. The grain size is a square root (√ (a × b)) of a value obtained by multiplying the major axis length a and minor axis length b of the crystal grains. The aspect ratio is a value (a / b) obtained by dividing the major axis length a of the crystal grains by the minor axis length b. The average grain size and average aspect ratio of the hard phase are the arithmetic average of the grain sizes and aspect ratios of all crystal grains recognized as either a pearlite phase or a bainite phase in image processing.

Hardness Take a sample with a plate thickness cross section parallel to the rolling direction, embed it in a resin, polish the plate thickness cross section, and then use a Vickers hardness tester in accordance with the provisions of JIS Z 2244 at a thickness of 1/4. Used, 5 points were measured with a load of 500 gf, and the average value was determined to be hardness.

Flatness after punching From the obtained cold-rolled steel sheet, a ring-shaped plate with an outer diameter of 100 mm and an inner diameter of 80 mm is punched,
Place the plate on the measurement table of the laser displacement meter, measure the height from the table to the top surface of the annular portion of the plate with the laser measurement device, and check the difference between the minimum and maximum values. Was calculated. When the difference between the minimum value and the maximum value is 0.2 mm or less, the flatness: good (◯). On the other hand, the case where the difference between the minimum value and the maximum value was more than 0.2 mm was defined as flatness: defective (x).

Punched end face properties From the obtained cold-rolled steel sheet, a ring-shaped plate with an outer diameter of 100 mm and an inner diameter of 80 mm is punched, the punched end surfaces are observed, and those without cracks or secondary shear surfaces are punched. : It was set as favorable ((circle)). On the other hand, a punched end surface property: defective (x) was observed where cracks or secondary shear surfaces were observed.

Heat-resistant strain characteristics A ring-shaped plate having an outer diameter of 100 mm and an inner diameter of 80 mm was punched out from the obtained cold-rolled steel sheet, and the plate was held at 300 ° C. for 1 hour, and then heat-treated by air cooling to room temperature. After the heat treatment, the shape of the plate was measured with a laser measuring instrument in the same manner as in the above “measurement of flatness after punching”, and the difference between the minimum value and the maximum value of the height of the annular portion of the plate was calculated.
The case where the difference between the minimum value and the maximum value was 0.2 mm or less was regarded as the heat distortion resistance: good (◯). On the other hand, the case where the difference between the minimum value and the maximum value was more than 0.2 mm was regarded as the thermal strain characteristic: defective (x).
These results are shown in Table 3.

Each of the cold-rolled steel sheets of the invention examples has a sufficient Vickers hardness of Hv170 or more, excellent flatness after punching, punched end face properties, and excellent thermal strain characteristics. On the other hand, the cold-rolled steel sheet of the comparative example outside the scope of the present invention is inferior in any of the characteristics.
This application claims priority based on Japanese Patent Application No. 2013-076860 filed in Japan on April 2, 2013, the entire contents of which are incorporated herein.

Claims

% By mass
C: 0.01% to 0.08%, Si: 0.01% to 1.0%,
Mn: 0.05% or more and 1.0% or less, P: 0.03% or less,
S: 0.015% or less, Al: 0.005% or more and 0.10% or less,
N: A composition containing 0.01% or less, the balance being Fe and inevitable impurities, and the area ratio of the ferrite phase being 80% or more, and the area of the hard phase consisting of at least one of the pearlite phase and the bainite phase The ratio is 20% or less in total, the hard phase has an average particle diameter of 1 μm or more and 10 μm or less, and the hard phase has an average aspect ratio of 10.0 or less, and the Vickers hardness is Hv 170 or more. A cold-rolled steel sheet.
The cold-rolled steel sheet according to claim 1, further comprising at least one of Cu: 0.01% to 0.20% and Ni: 0.01% to 0.50% in addition to the composition.
In addition to the above composition, Ti: 0.005% to 0.10%, Nb: 0.005% to 0.10%, V: 0.005% to 0.50%, Zr: 0.005% to 0.10%, Mo: 0.02 The cold-rolled steel sheet according to claim 1 or 2, comprising at least one of Cr: 0.03% to 0.50%, B: 0.0003% to 0.0050%.
The cold rolling according to any one of claims 1 to 3, further comprising at least one of Ca: 0.0003% to 0.0050% and REM: 0.0003% to 0.0100% in addition to the composition. steel sheet.
The cold rolling according to any one of claims 1 to 4, further comprising at least one of Sb: 0.001% to 0.030% and Sn: 0.001% to 0.030% in addition to the composition. steel sheet.
% By mass
C: 0.01% to 0.08%, Si: 0.01% to 1.0%,
Mn: 0.05% or more and 1.0% or less, P: 0.03% or less,
S: 0.015% or less, Al: 0.005% or more and 0.10% or less,
N: A steel material containing 0.01% or less and the balance of Fe and inevitable impurities is subjected to hot rolling with a finish rolling finish temperature of 800 ° C. or more and 950 ° C. or less, and after hot rolling is finished, A method for producing a cold-rolled steel sheet, which is wound at a coiling temperature of 600 ° C. or higher and 750 ° C. or lower, removed the scale by pickling, and then cold-rolled at a rolling reduction of 30% to 70%.
The method for producing a cold-rolled steel sheet according to claim 6, wherein after the cold rolling, temper rolling is performed.
The manufacturing of the cold-rolled steel sheet according to claim 6 or 7, further comprising at least one of Cu: 0.01% to 0.20% and Ni: 0.01% to 0.50% in addition to the composition. Method.
In addition to the above composition, Ti: 0.005% to 0.10%, Nb: 0.005% to 0.10%, V: 0.005% to 0.50%, Zr: 0.005% to 0.10%, Mo: 0.02 % Or more and 0.50% or less, Cr: 0.03% or more and 0.50% or less, B: 0.0003% or more and 0.0050% or less, The manufacturing method of the cold-rolled steel plate in any one of Claim 6 thru | or 8.
The cold rolling according to any one of claims 6 to 9, further comprising at least one of Ca: 0.0003% to 0.0050% and REM: 0.0003% to 0.0100% in addition to the composition. A method of manufacturing a steel sheet.
The cold rolling according to any one of claims 6 to 10, further comprising at least one of Sb: 0.001% to 0.030% and Sn: 0.001% to 0.030% in addition to the composition. A method of manufacturing a steel sheet.