JP4765388B2 - Manufacturing method for cold rolled steel sheet with excellent flatness after punching - Google Patents

Description

The present invention relates to a manufacturing method of a preferred thin steel plate as a material for gears and plates such as transmission parts of automobiles, excellent dimensional accuracy after punching, especially while cold rolling, moreover, the hardness of the punched parts a heat treatment step for secured to a method for manufacturing a thin steel plate having excellent flatness after punching is not necessary.

  Gears and plates used as transmission parts for automobiles are used for hardening parts such as quenching and aging precipitation in order to adjust to the desired hardness after punching a steel sheet into a predetermined shape at a parts manufacturer. It is manufactured by applying the heat treatment.

  However, in recent years, for the purpose of reducing manufacturing costs, development of a steel sheet capable of securing hardness by cold rolling is required instead of the above heat treatment. However, when the hardness is ensured by such cold rolling, there is a case where a large warp occurs in a part after punching. Therefore, a heat treatment called press temper is required for the stamped part, and even if this press temper is performed, it may be difficult to correct the shape of the part.

  From such a current situation, there has been a strong demand for the development of a steel sheet that is excellent in the flatness of parts after being punched in the cold rolling state.

  In contrast to the above, as a prior art that omits heat treatment for securing the hardness after punching, such as gears and plates as transmission parts of automobiles, Patent Document 1 describes that a hot-rolled sheet structure has a hard bainitic structure. A method for producing a high-strength steel sheet for precision punching having ferrite or bainite as a main phase is disclosed.

  In addition, regarding the technology for improving the strength while ensuring the dimensional accuracy of the parts, Patent Document 2 discloses a high-carbon steel sheet and its manufacture that add Cu and V to increase the strength instead of the quenching process that increases thermal strain. A method is disclosed.

Furthermore, Patent Document 3 discloses a high-carbon steel plate with excellent flatness and a method for producing the same, by prescribing the maximum value of the hardness and the plate surface hardness difference in the width direction of the steel plate so that heat treatment can be omitted. .
JP-A-8-295927 JP-A-4-254546 JP 2004-285416 A

  However, Patent Document 1 has the following problems. First, Patent Document 1 relates to precision punchability after cold rolling, that is, control of the shape of the punched surface such as sagging and shear surface ratio, and does not mention anything about the flatness of the punched part. Furthermore, since the strength is secured by a low-temperature transformation phase such as bainite, if the coiling temperature varies during hot rolling, material fluctuations in the coil longitudinal direction or in the coil width direction increase in the wide width material. The dimensional accuracy of the punched parts after cold rolling varies.

  Patent Document 2 has a problem that heat treatment at a tempering treatment temperature for aging precipitation of Cu and V is necessary, and heat treatment for ensuring the hardness of the punched part cannot be omitted.

  Patent Document 3 is intended to obtain a steel sheet with excellent flatness by temperature control in the width direction during hot rolling, but the problem is that the material control in the coil longitudinal direction is insufficient and variation occurs. have.

  As described above, a steel sheet that is excellent in the flatness of a part after punching a thin steel sheet and that does not require a heat treatment step for securing the hardness of the punched part and a manufacturing method thereof have not been proposed yet. .

Therefore, in view of the above circumstances, the present invention is excellent in the flatness of a punched part as a material such as a gear or a plate as a transmission part of an automobile, and further requires a heat treatment process for ensuring the hardness of the punched part. and to provide a method for manufacturing a steel plate.

  The inventors of the present invention have made extensive studies in order to achieve the above object. As a result, the following knowledge was obtained.

The deterioration of flatness after punching is caused by the generation of internal residual stress due to the non-uniformity of the steel plate material. That is, when a large hardness difference occurs in the longitudinal direction and the width direction of the coil of the hot-rolled sheet, a difference in hardness occurs even after cold rolling, resulting in a difference in residual stress depending on the position in the longitudinal direction and the width direction. The flatness of parts deteriorates. Therefore, in order to prevent the flatness of the punched part from being deteriorated, the following points are important.
(1) Make the material in the longitudinal and width directions of the coil as cold-rolled uniform, especially reduce the hardness difference.
(2) To make the material in the longitudinal direction and the width direction of the hot-rolled steel sheet that is the base material for cold rolling uniform, in particular, to reduce the hardness difference between the longitudinal direction and the width direction of the hot-rolled steel sheet. An excellent flatness after punching can be obtained by rolling the steel sheet at an appropriate cold pressure ratio to give a uniform strain.
(3) Preferably, annealing is performed before cold rolling to reduce the residual stress and further homogenize the material of the hot-rolled sheet.
Thus, extremely excellent flatness after punching can be obtained.

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: 0.05-0.6%, Si: 2.0% or less, Mn: 0.2-2.0%, P: 0.03% or less, S: 0.03 % Or less, Sol.Al: 0.1% or less, N: 0.01% or less, with the balance being a steel composed of Fe and inevitable impurities at a finishing temperature (Ar ₃ transformation point -20 ° C.) or higher. After hot rolling, cooling is performed so that the cooling rate exceeds 120 ° C./second, the cooling stop temperature is 650 ° C. or less, and the cooling stop temperature difference between the steel plate width direction center portion and the steel plate edge portion is 30 ° C. or less. Winding at a coiling temperature of 600 ° C or lower, pickling, and cold rolling at a reduction rate of 40% or higher, or cold rolling at an annealing temperature of 600 ° C or higher and an Ac ₁ transformation point and after annealing at a rolling reduction of 40% or higher The steel plate surface hardness Hv is 170 to 300, and the flatness after punching is characterized in that the maximum value ΔHv of the plate surface hardness difference at each position in the longitudinal direction and the width direction of the steel plate is 20 or less. A method for producing cold rolled steel sheets that are excellent in temperature.

  In the present specification, “%” indicating the component of steel is “% by mass”.

  According to the present invention, a steel plate suitable for materials such as gears and plates as automobile transmission parts, excellent in punching dimensional accuracy, particularly flatness, and not requiring a heat treatment process for securing the hardness of the punched parts is obtained. It is done. Since these have sufficient characteristics when used as materials such as gears and plates, industrially useful effects are brought about.

Hereinafter, the present invention will be described in detail.

(1) Component composition First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: 0.05-0.6%
C gives necessary strength to gears, plates, and the like as transmission parts of automobiles. If the amount of C is too low, hot rolling finish rolling beyond the Ar3 point becomes difficult, and the structure cannot be made uniform. Therefore, the amount of C needs to be at least 0.05%. However, if added over 0.6%, winding becomes difficult due to hardening by rapid cooling. Therefore, in the present invention, the C content is limited to the range of 0.05% to 0.6%.

Si: 2.0% or less Since Si is an element that improves the hardenability and increases the strength of the material by solid solution strengthening, it is preferably contained in an amount of 0.005% or more. However, if the content exceeds 2.0%, pro-eutectoid ferrite is likely to be generated, and ferrite grains that do not substantially contain carbides increase, which hinders homogenization of the material. Further, the carbide tends to be graphitized and the hardenability is hindered. Therefore, the Si content is 2.0% or less.

Mn: 0.2-2.0%
Mn is an element that improves the hardenability and raises the strength of the material by solid solution strengthening, like Si. Moreover, it is an important element which fixes S as MnS and prevents the hot crack of a slab. However, if the Mn content is less than 0.2%, these effects are reduced, and the formation of pro-eutectoid ferrite is promoted, the ferrite grains are coarsened, and the homogenization of the material is inhibited. In addition, the hardenability is greatly reduced. On the other hand, if it exceeds 2.0%, the tensile strength can be obtained, but the formation of a manganese band, which is a segregation band, becomes remarkable and the ductility deteriorates. Therefore, the Mn content is 0.2% or more and 2.0% or less. Preferably, it is 1.0% or less from the viewpoint of ductile deterioration due to manganese band formation.

P: 0.03% or less P is an element that must be reduced in order to segregate at grain boundaries and reduce toughness. However, since the P content is acceptable up to 0.03%, the P content is set to 0.03% or less.

S: 0.03% or less S is an element that must be reduced in order to form Mn and MnS and degrade stretch flangeability. However, since the S content is acceptable up to 0.03%, the S content is set to 0.03% or less.

Sol.Al: 0.1% or less
Al is used as a deoxidizer and is added at the steel making stage to improve the cleanliness of the steel. Usually, the steel contains approximately 0.005% or more of Sol.Al. On the other hand, even if the content of Sol.Al exceeds 0.1%, the effect of improving the cleanliness is saturated and the cost increases. If added in excess, a large amount of AlN precipitates, reducing the hardenability. Therefore, the content of Sol.Al in the steel is 0.1% or less. Preferably it is 0.08% or less.

N: 0.01% or less N is added in excess, so that ductility is lowered.
The balance other than the above consists of Fe and inevitable impurities.

  In addition, various elements such as Sn and Pb may be mixed as impurities during the manufacturing process, but such impurities do not particularly affect the effects of the present invention.

(2) Steel plate hardness The reason for limiting the hardness of the thin steel plate of the present invention will be described.
Steel plate hardness is particularly important when applied to automobile transmission parts such as gears and plates. When the surface hardness Hv of a cold-rolled steel plate is less than 170, sufficient wear resistance is obtained. On the other hand, when Hv exceeds 300, conversely, the wear of the counterpart material is increased. Therefore, the steel plate surface hardness Hv is set to 170 or more and 300 or less. More preferably, it is 200 or more and 270 or less.

If the maximum value ΔHv of the sheet surface hardness difference at each position in the longitudinal direction and the width direction of the steel sheet exceeds 20, the residual stress of the steel sheet becomes non-uniform and the flatness after punching decreases, so ΔHv is set to 20 or less. In order to obtain excellent flatness, ΔHv is preferably 15 or less.
If the sheet surface hardness difference due to the longitudinal and width direction positions of the hot-rolled steel sheet is large, even if cold rolling is carried out at an appropriate cold pressure ratio, the hardness difference cannot be resolved and the hardness difference after cold rolling Therefore, a difference occurs in the residual stress, and an excellent flatness of the punched part cannot be obtained. Therefore, it is desirable that the maximum value ΔHv of the sheet surface hardness difference at each position in the longitudinal direction and the width direction of the hot-rolled steel sheet is 20 or less. In order to obtain extremely excellent dimensional accuracy, the maximum hardness difference ΔHv is preferably 15 or less.
The hardness Hv in the present invention is Vickers hardness.

(3) Manufacturing conditions The reasons for limiting the manufacturing conditions of the thin steel sheet of the present invention will be described.
The steel sheet of the present invention is obtained by (A) hot rolling-pickling-cold rolling-temper rolling, (B) hot rolling-pickling-spheroidizing annealing-cold rolling. It is manufactured through any process of rolling-temper rolling.

  When producing the steel sheet of the present invention, the hot rolling process may be either a method of rolling after slab heating, a method of performing a heat treatment for a short time after continuous casting or a method of rolling immediately after omitting the heating step. In order to impart surface quality, it is preferable to sufficiently remove not only the primary scale but also the secondary scale generated during hot rolling. In hot rolling, coarse bar heating may be performed by a bar heater between rough rolling and finish rolling. Furthermore, in order to make the structure in the width direction of the hot-rolled steel sheet uniform, it is desirable to heat the rough bar edge portion with an edge heater to suppress overcooling of the edge portion.

Finishing temperature: The finishing temperature of (Ar ₃ transformation point -20 ° C.) or higher hot rolling is less than (Ar ₃ transformation point -20 ° C.), ferrite grains not containing carbide because ferrite transformation proceeds in some increases The organization cannot be made uniform. Therefore, finish rolling is performed at a finishing temperature of (Ar ₃ transformation point −20 ° C.) or higher. If the finishing temperature is too high, it becomes difficult to homogenize the structure.

Cooling conditions after rolling: Cooling rate> 120 ° C./second In the present invention, in order to reduce the volume fraction of ferrite grains after transformation and to make the structure in the coil longitudinal direction and width direction uniform, rapid cooling after cooling (cooling) I do. When the cooling method is slow cooling, the degree of supercooling of austenite is small and proeutectoid ferrite is generated. When the cooling rate is 120 ° C./second or less, pro-eutectoid ferrite is remarkably generated and it is difficult to make the structure uniform. Therefore, the cooling rate of the cooling after rolling is over 120 ° C./second.

Cooling stop temperature: 650 ° C. or less When the cooling stop temperature after rolling is high, ferrite is generated during cooling up to winding, and the lamella spacing of pearlite becomes coarse. Therefore, fine carbide cannot be obtained after annealing, and it becomes difficult to make the structure uniform. When the cooling stop temperature is higher than 650 ° C., the ferrite grains not containing carbide exceed 10%, and the flatness after the final cold rolling deteriorates. Therefore, the cooling stop temperature for cooling after rolling is set to 650 ° C. or lower. On the other hand, if the temperature is lower than 450 ° C., equiaxed ferrite grains cannot be obtained, and it may be difficult to make the structure uniform. Therefore, the lower limit of the cooling stop temperature is preferably 450 ° C. or higher.

During the runout cooling where the cooling stop temperature difference between the steel plate width direction center portion and the steel plate edge portion is 30 ° C. or less, it is preferable to prevent the steel plate edge portion from getting on the water and suppress the overcooling of the edge portion. And in order to ensure the material uniformity of the width direction, the cooling stop temperature difference of a steel plate width direction center part and a steel plate edge part shall be 30 degrees C or less. In order to prevent water riding on the edge portion, the amount of water is adjusted by adjusting a water cooling nozzle at the edge portion or by a shielding plate.

Winding temperature: The steel sheet is wound after cooling below 600 ° C. The higher the winding temperature, the greater the pearlite lamella spacing. Therefore, the carbide after annealing becomes coarse, and when the coiling temperature exceeds 600 ° C., the flatness after the final cold rolling deteriorates. Accordingly, the coiling temperature is set to 600 ° C. or lower. When the coiling temperature is less than 500 ° C, the cooling on the runout is large, the shape of the hot-rolled sheet deteriorates, non-uniform residual stress occurs in the width direction, and the dimensional accuracy after punching tends to deteriorate. It is desirable that the temperature be 500 ° C.

Annealing temperature: 600 ° C. or more and Ac ₁ transformation point or less The hot-rolled sheet is pickled and then annealed to further improve the flatness of the punched part after cold rolling. Annealing is performed to spheroidize the carbide, but when the annealing temperature is less than 600 ° C., the spheroidization of the carbide becomes insufficient and the structure becomes non-uniform. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, a part is austenitized, and pearlite is generated again during cooling, which causes a problem in homogenizing the structure. If it exceeds 680 ° C, it becomes so soft that the cold rolling rate has to be increased, and conversely the dimensional accuracy after punching deteriorates. Further, to obtain this effect, annealing for 20 hours or more is required. Therefore, when particularly excellent flatness is required, it is desirable to perform annealing at a temperature of 600 to 680 ° C. for 20 hours or more after pickling.

By controlling the cooling rate and cooling stop temperature during cold rolling hot rolling, it is possible to achieve the production of hot rolled steel sheets with excellent longitudinal and width structural uniformity, but with a reduction rate of 40% or more. By applying an appropriate strain, a cold-rolled steel plate having excellent flatness can be obtained. When the rolling rate during cold rolling is such a high rolling rate that exceeds 70%, the rolling load becomes too high and the productivity is lowered. For this reason, it is desirable that the cold rolling rate be 70% or less. The cold rolling at this time may be either tandem rolling or reverse rolling.

  Steel having the chemical composition of steel plate E from steel plate A shown in Table 1 is melted, and then hot rolling-cold rolling or hot rolling-annealing-cold rolling is performed according to the manufacturing conditions shown in Table 2. A cold-rolled thin steel plate having a plate width of 1000 mm and a plate thickness of 1.80 mm was produced.

The sheet surface hardness Hv was measured at three positions in the longitudinal direction (T part, M part, B part) and three positions in the width direction (15 mm from the center and both edges) of the cold-rolled steel sheet thus manufactured. The hardness difference was determined. In addition, after correcting the shape of the cold-rolled steel sheet thus obtained with a leveler, a disk-shaped specimen having a diameter of 89 mm is punched from this thin steel sheet (clearance: 12%), and the flatness of the specimen is evaluated. did.
The flatness was evaluated by measuring a maximum warp height (maximum height in the width direction) shown in FIG. These results are shown in Table 3.

As is apparent from Table 3, in No. 1 to No. 7 as examples of the present invention, it is clear that the hardness difference ΔHv in the longitudinal direction and the width direction of the steel sheet is 20 or less, the warp height is small, and excellent quality is obtained. It was. It has also been clarified that further excellent quality can be obtained by annealing before cold rolling.
On the other hand, in the comparative example, the hardness difference ΔHv between the steel plate longitudinal direction and the width direction is larger than 20, so that the warp height increases and the flatness is inferior. In particular, even when the manufacturing conditions were the same, in No15 where C was low, the final hardness varied greatly and the warp height increased.

  The thin steel sheet of the present invention is excellent in punching dimensional accuracy, particularly flatness, and does not require a heat treatment step for ensuring the hardness of the punched part. Therefore, it is an industrially useful material mainly for materials such as gears and plates as automobile transmission parts.

It is a figure which shows the measuring method of flatness (maximum curvature height).

Claims (1)

In mass%, C: 0.05-0.6%, Si: 2.0% or less, Mn: 0.2-2.0%, P: 0.03% or less, S: 0.03% or less, Sol.Al: 0.1% or less, N: 0.01% or less, with the balance being hot-rolled at a finishing temperature (Ar ₃ transformation point -20 ° C.) or more at the finishing temperature (Ar ₃ transformation point −20 ° C.) Then, cooling is performed so that the cooling rate exceeds 120 ° C./second, the cooling stop temperature is 650 ° C. or less, and the cooling stop temperature difference between the steel plate width direction center portion and the steel plate edge portion is 30 ° C. or less. By rolling at 600 ° C. or lower, pickling, and cold rolling at a reduction rate of 40% or higher, or by annealing at an annealing temperature of 600 ° C. or higher and an Ac ₁ transformation point or lower and then cold rolling at a reduction rate of 40% or higher. The steel plate surface hardness Hv is 170 to 300, and the maximum value ΔHv of the plate surface hardness difference at each position in the longitudinal direction and the width direction of the steel plate is 20 or less. A method for producing a thin steel sheet as cold-rolled, which is excellent in flatness after punching.

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