JP4319948B2 - High carbon cold-rolled steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a high carbon cold-rolled steel sheet having excellent stretch flangeability used for machine structural parts such as a drive system of an automobile.

  High carbon steel sheet is usually processed by cutting, punching, bending, drawing, etc. to a predetermined size and shape, then subjected to heat treatment such as quenching and tempering, and tempered to the required strength. It is often applied to the intended use. In addition, a method of integrally forming parts produced by joining a plurality of parts by welding or the like by rolling or FCF processing is becoming widespread. Some of these parts require stretch flangeability. The high carbon steel sheet is tempered with necessary characteristics such as hardness by heat treatment such as quenching and tempering, and therefore the amount of C added is inevitably high. In order to improve the workability, a method of spheroidizing carbide is generally used. However, due to high carbon, a large number of hard carbides are present in the steel, so that it is not suitable for stretch flange processing. For this reason, there exists a request | requirement of the high carbon steel plate excellent in stretch flangeability. At the same time, since it is used after quenching and tempering, it is also required that the toughness after quenching and the dimensional change due to quenching and tempering be small.

Patent Document 1, Patent Document 2, and Patent Document 3 are disclosed as techniques for improving stretch flange workability of a high-carbon cold-rolled steel sheet. Patent Document 1 defines the initial cold rolling conditions and intermediate annealing conditions in the steps of cold rolling, annealing, cold rolling, and annealing a hot-rolled sheet, and it is necessary to repeat cold rolling and annealing, and the manufacturing cost is low. Get higher. Patent Document 2 is a technique for improving stretch flangeability by adopting a special annealing cycle. Although this technique has an effect of slightly improving the stretch flange, it cannot be applied to a product that is subjected to severe stretch flange processing or further processing such as increasing the thickness of the stretch flange processed portion. Patent Document 3 specifies the degree of dispersion of carbide and the average particle size of carbide, and its technical idea is to increase the particle size of the carbide, disperse it uniformly, and to reduce macro cracks caused by the carbide during processing. However, it is in the category of the prior art, and cannot be applied to products that are subjected to severe stretch flange processing or further processing such as increasing the thickness of stretch flange processed portions. As described above, the conventional disclosed technique is a technique for controlling the size of carbide and improving stretch flangeability, and is not a technique for drastically improving stretch flangeability. This is not applicable to products that are subjected to secondary processing such as thickening, and the processing process has not been drastically omitted. Therefore, a new technology for high-carbon steel sheets with excellent stretch flangeability Development is desired.
JP-A-7-166243 Japanese Patent Laid-Open No. 11-140544 JP 2001-220463 A

  The problem to be solved by the present invention is to provide a high-carbon cold-rolled steel sheet that has excellent stretch flangeability before quenching, can obtain a predetermined hardness and toughness after quenching, and has low quenching strain. is there.

As a result of intensive studies on the stretch flangeability and hardenability of the high carbon steel sheet, the inventors of the present invention have excellent stretch flangeability and hardenability by controlling the steel composition, ferrite grain size, and inclusion shape. It was also found that an excellent high carbon cold rolled steel sheet can be obtained.
The gist is ( 1 ) "mass% C: 0.20-0.60%, S: 0.005% or less, Ti: 0.001-0.050 %, B: 0.0003-0.0050% , Si: 0.50% or less, Mn: 2.0% or less, P: 0.03% or less, Al: 0.08% or less, N: 0.010% or less, Cr: within 1.0%, balance The composition of Fe and inevitable impurities, the ferrite average crystal grain size is 5 μm or more and 18.3 μm or less, and the value obtained by dividing the standard deviation of the ferrite crystal grain size by the average grain size is 0.14 or more and 0.50 or less. High carbon cold-rolled steel sheet with excellent stretch flangeability, characterized by
( 2 ) "C: 0.20 to 0.60% by mass, S: 0.005% or less, Ti: 0.001 to 0.050%, B: 0.0003 to 0.0050%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.03% or less, Al: 0.08% or less, N: 0.010% or less, Cr: within 1.0%, remaining Fe and inevitable The average grain size of the ferrite is 5 μm or more and 18.3 μm or less, the value obtained by dividing the standard deviation of the ferrite crystal grain size by the average grain size is 0.14 or more and 0.50 or less, and the shape of the inclusion A high carbon cold-rolled steel sheet with excellent stretch flangeability, characterized in that the ratio is 2.0 or less. "

First, it is possible to obtain excellent stretch flangeability by setting the ferrite average crystal grain size, which is an important constituent factor of the present invention, to 5.0 μm or more and the standard deviation degree of ferrite crystal grains to 0.50 or less. Explain the findings.
S45C (C: 0.45%, Si: 0.17%, Mn: 0.78%, P: 0.012%, S: 0.003%, Cr: 0.08%, Al: 0.032% , N: 0.0035%, and all the% indications are mass%, and the same shall apply hereinafter.) After heating the slab to 1250 ° C, the reduction schedule to 6.0 mm thickness, finishing temperature, and cooling conditions after hot rolling It was hot rolled with various changes. After this hot-rolled sheet was pickled, it was annealed at 600 to 720 ° C. for 24 hours, and after cold-rolling, a steel plate ground from the surface to a sheet thickness suitable for the cold rolling rate was 2.0 mm. Cold-rolled to a thickness of 0.0 mm. The cold rolled sheet was annealed at 680 ° C. × 12 hours. From this steel plate, a 150-square hole-expanding test piece and a cross-sectional microstructure observation specimen were collected. The hole-expanded test piece was punched with a 10φ hole at the center of the plate with a clearance of 12%, and the hole was pushed up with a conical punch of 60 degrees, and the test was stopped when a crack penetrating the plate thickness occurred on the punched surface. The difference between the hole diameter and the initial hole diameter was divided by the initial hole diameter, and the ratio expressed as 100 minutes was defined as the hole expansion ratio. The average grain size of ferrite was determined by obtaining the grain size number defined by JIS, assuming that the ferrite grains were spheres, and the diameter was defined as the average ferrite grain size (dm). The standard deviation of the ferrite crystal grain size is the same observation plane where the ferrite crystal grain size was measured, the area of each ferrite crystal grain was obtained, the ferrite was assumed to be a circle, the diameter was multiplied by 4 / π, and the standard deviation ( σ) was determined, and the standard deviation obtained by dividing the standard deviation by the average particle diameter was defined as the standard deviation degree (σ / dm). FIG. 1 shows the relationship between the standard deviation degree of the ferrite average crystal grain size of 7 to 9 μm and 16 to 18 μm and the hole expansion rate.

  When the standard deviation degree of the ferrite crystal grain size becomes small, the hole expansion rate becomes large even with the same ferrite crystal grain size. If the standard deviation degree is the same, the hole expansion rate of a steel plate having a large ferrite average crystal grain size is increased. FIG. 2 shows the relationship between the standard deviation degree of the ferrite crystal grain size and the hole expansion rate when the ferrite crystal average grain size is 5.0 μm or more. When the variation in the ferrite crystal grain size becomes small, the hole expansion ratio becomes good, and a steel sheet having an excellent hole expansion ratio when σ / dm is 0.50 or less is obtained. From this, the standard deviation degree (σ / dm) was specified to be 0.50 or less for the purpose of obtaining good stretch flangeability. A preferable range of the standard deviation is 0.30 or less for the same reason.

Hereinafter, requirements constituting the present invention will be described.
C is an element that deteriorates workability, but is 0.20% at a minimum for the purpose of quenching and tempering. On the other hand, if the amount of C exceeds 0.60%, it becomes hard, and the amount of carbide becomes too large, and sufficient stretch flangeability cannot be obtained.
Since S becomes an inclusion such as MnS and deteriorates stretch flangeability, it is necessary to make it 0.005% or less.

It is well known that B is an element that enhances hardenability. Also in the present invention, the steel is added in a range of 0.0003 to 0.0050% when steel with a low amount of C, or particularly when hardenability is required. If it is less than 0.0003%, the effect of improving the hardenability cannot be stably exhibited. On the other hand, addition over 0.0050% causes defects and flaws in the steel sheet when it is produced, which lowers the product yield and increases the production cost. A preferable range is 0.0005 to 0.0030% for the same reason.
Ti is added in the range of 0.01 to 0.050% in order to exhibit the effect of adding B when B is added.

  Inclusions are factors that affect stretch flangeability as well as the steel sheet structure. The influence of the inclusion on the stretch flangeability is the projected length of the inclusion, and the longer this length, the worse the stretch flangeability. Therefore, even with the same inclusion, the greater the degree of extension, the worse the stretch flange. For this reason, it is necessary to make the shape ratio represented by the ratio of the major axis and minor axis of inclusions 2.0 or less with an average value of 100 or more inclusions.

  The reason why the stretch flangeability is improved by controlling the standard deviation of the ferrite crystal grain size to be small is not clear, but the present inventors consider as follows. The punched surface before stretch flange processing is sheared and hardened, and microcracks are already generated before stretch flange processing. When this is stretch-flange processed, a large crack is generated starting from a microcrack, which is the stretch flange processing limit. Therefore, firstly, the stretch flangeability depends on the degree of macro cracks on the shear surface. Second, when there is no macro crack on the shear surface, the stretch flange depends on how uniformly the processing strain is shared, and when the strain distribution becomes uneven, the stretch flangeability deteriorates and the strain is uniformly strained. If this is shared, stretch flangeability becomes good. The macro cracks on the punched surface are generated from a portion having a large deformation amount and a portion having a poor deformability. The deformation unit is a ferrite particle size. If the ferrite particle size is not uniform, the deformation concentrates on a large ferrite grain portion that deforms with a small stress, and if the deformation exceeds the deformability, a crack occurs. In a uniform structure, each crystal grain equally distributes strain and suppresses the generation of cracks. A similar behavior also occurs during stretch flange molding, and in a steel sheet having a uniform ferrite grain structure, each crystal grain uniformly bears deformation and increases the strain causing cracks.

  In this invention, in obtaining a high carbon cold-rolled steel sheet having excellent stretch flangeability, the ferrite crystal grain size and the standard deviation degree are controlled, and at the same time, the shape ratio of inclusions is specified, so that the hole expanding process can be performed. Generation of cracks can be suppressed. It is possible to provide a high-carbon cold-rolled steel sheet that does not crack even if it is subjected to upsetting after hole expansion, has excellent stretch flangeability, and has excellent impact properties after quenching and high-frequency quenchability. By using such a high-carbon cold-rolled steel sheet, it is possible to obtain a high degree of processing in the processing of automobile transmission parts and the like, and as a result, the manufacturing process can be omitted and parts and the like can be manufactured at low cost. This is an industrially extremely useful invention.

  Steel used in the present invention includes C: 0.20 to 0.60%, S: 0.005% or less, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0050%, The ferrite crystal grain size, its dispersion, and the inclusion shape ratio may be 2.0 or less, and other chemical components such as Si, Mn, P, Al, N, and Cr should be added within a normal range. There is no need to specify it. The amount of these elements may be determined in relation to other characteristics, and need not be specified. However, preferably, it may be added as follows.

  Since Si hardens the steel sheet when the addition amount is large, it is preferable to make it 0.50% or less. Mn is well known as an element that enhances hardenability, but if added excessively, ductility is impaired, so it is preferable to add it at 2.0% or less. P not only impairs workability, but also deteriorates toughness after quenching and tempering, so 0.03% or less is preferable. Al is a deoxidizing element, but if it is added in an increased amount, it tends to cause surface defects. Therefore, it is preferably 0.08% or less. When N is added excessively, not only ductility deteriorates, but also toughness after quenching and tempering deteriorates. Therefore, N is preferably made 0.010% or less. It is known that Cr is an element that enhances hardenability. When hardenability is particularly required, or when carburizing or nitriding is performed, it is preferable to add within 1.0%. Furthermore, elements such as Cu, Ni, Mo, Nb, V, Zr, Ca, and Mg may be added in a range that is usually added according to the purpose. Even if other impurities are mixed in the manufacturing process, the characteristics of the present invention are not impaired.

Controlling the standard deviation degree of the ferrite grains to 0.50 or less is, in other words, controlling the structure uniformly. Even if manufactured under normal conditions, the steel sheet of the present invention cannot be obtained. Therefore, it is necessary to select manufacturing conditions with great care in the manufacturing process of the steel sheet.
The steel of the present invention is melted by adjusting the components using a converter or electric furnace, and if necessary, vacuum degassing. In this way, the component-adjusted high carbon steel is made into a slab by ingot-making, ingot rolling or continuous casting. If the segregation of components in the slab is large, even if the manufacturing conditions are carefully selected, the structure becomes uneven and it is difficult to obtain a steel sheet that satisfies the requirements of the present invention. For this reason, when manufacturing a slab, it is preferable to employ a method of reducing segregation such as electromagnetic stirring and light pressure in an unsolidified region. This slab is subjected to hot rolling. However, even if normal hot rolling is performed, it is difficult to reduce the dispersion of the ferrite crystal grain size, which is a feature of the present invention. Since the hot rolling heating temperature does not particularly affect the characteristics of the present invention, it is performed at 1050 ° C. or higher. In order to reduce the variation in the ferrite crystal grain size after cold rolling and annealing, it is important to make the structure of the hot rolled sheet uniform. The structure of the hot-rolled sheet is made uniform by, for example, selecting as fine a rolling and temperature history as possible for the austenite grains before finish hot rolling. In finish hot rolling, the cumulative reduction amount is in the temperature range immediately above the Ar3 point temperature. In order to reduce the difference in structure between the steel plate interior and the surface layer, the hot rolling finish is lubricated and rolled. Moreover, the cooling conditions after hot rolling are also an important factor for making the hot rolled structure uniform and thus reducing the variation of the structure after cold rolling annealing. For example, a method of cooling the temperature history in which the ferrite transformation temperature region is rapidly cooled, the pearlite transformation temperature region balances the amount of heat generated by transformation heat generation and heat removal by water injection, and the pearlite transformation is performed at a constant temperature is selected.

Hot-rolled coil is spheroidized and annealed after descaling, cold-rolled and annealed, or hot-rolled coil after descaling is cold-rolled and annealed, or cold-rolled after spheroidizing and annealed, recrystallized and cooled. It may be manufactured in the process of performing the final annealing.
However, unless careful attention is paid to the combination of the cold rolling rate and the annealing conditions, the variation in the ferrite crystal grain size cannot be reduced. The optimum conditions for the cold rolling rate and the annealing conditions differ depending on the steel components, the structure of the hot-rolled sheet, the presence or absence of spheroidizing annealing, and it is preferable to select the optimum conditions according to the respective conditions.
The steel sheet produced in this way is subjected to temper rolling as needed and provided to the product.

  Using a slab of S45C, it was hot-rolled under various conditions, cold-rolled and annealed to produce a steel plate having a thickness of 1.6 mm. Some steels were spheroidized and then cold rolled and annealed to produce steel sheets. A sample of 150 squares was taken from this steel plate, and a hole of 10.0φ was punched in the center with a tool with a clearance of 12%, pushed up with a 60-degree conical punch, and when a crack penetrating the plate thickness occurred on the punched surface Then, the test was stopped, the hole diameter at that time was measured, the difference from the initial hole diameter was divided by the initial hole diameter, and the hole expansion rate was expressed in%. Moreover, the rolling direction cross section of the steel plate was observed with a microscope, the ratio of the long axis to the short axis of 100 inclusions was taken, and the average value thereof was measured as the inclusion shape ratio. From the same observation plane, ferrite crystal grains were observed, and the standard deviation of the ferrite average crystal grain diameter was measured. The ferrite average crystal grain size was determined by measuring the ferrite crystal grain size No defined by JIS and determining the diameter when the crystal grains were assumed to be spheres. The standard deviation of the crystal grain is measured by measuring the area of the ferrite crystal grain on the observation surface, assuming that the ferrite crystal grain on the observation surface is a circle, and multiplying the diameter by 4 / π to obtain the standard deviation. Was divided by the average ferrite grain size to obtain the standard deviation. These values are shown in Table 1.

  Steel No. No. 1 has an ferrite grain size of 12.5 μm, a grain size deviation of 0.17, and an inclusion shape ratio of 1.15, which is an example within the scope of the present invention. Moreover, it has excellent stretch flangeability with a hole expansion ratio of 93.4%. Steel No. No. 2 is an example within the scope of the present invention, with a ferrite grain size of 9.8 μm, a grain size deviation of 0.18, and an inclusion shape ratio of 1.21. It has excellent stretch flangeability with a hole expansion ratio of 89.3%. Steel No. 3 is an example within the scope of the present invention, with a ferrite grain size of 18 μm, a grain size deviation of 0.19, and an inclusion shape ratio of 1.05. It has excellent stretch flangeability with a hole expansion ratio of 103.2%. Steel No. No. 4 is a comparative steel having a ferrite grain size of 14.5 μm and an inclusion shape ratio of 1.23, which is within the scope of the present invention, but having a deviation degree of ferrite grain size of 0.57 and deviating from the scope of the present invention. This steel plate has a significantly low hole expansion rate of 38.5%. Steel No. 5 is a comparative example in which the ferrite crystal grain size is 2.5 μm, which is out of the scope of the present invention. If the ferrite grains become too fine, the hole expansion rate will deteriorate and the stretch flangeability will deteriorate. There are various methods for producing the steel sheet of the present invention. 1 will be described. The cooling rate of continuous casting was increased to 1.5 times that of a normal one, and a slab having an equiaxed crystal ratio of 90% was made by electromagnetic stirring. Hot rolling was performed at a finishing temperature of 815 ° C. while lubrication was performed at the lower three stands of the finishing stand at 850 ° C. or less with equal strain distribution. Cooling after hot rolling was performed at 20 ° C./second immediately after finish rolling, cooling was controlled so that the pearlite transformation temperature range was within 15 ° C., and the winding temperature was 605 ° C. The hot-rolled sheet was pickled, annealed at 680 ° C. for 14 hours, cold-rolled at a cold rolling rate of 50%, and cooled after holding at 700 ° C. for 3 minutes. During the cooling, holding was performed at 400 ° C. for 5 minutes. Steel No. In No. 2, slabs were made by continuous casting under light pressure in the unsolidified region. Hot-rolling heating temperature: 1200 ° C, finishing temperature: 800 ° C, winding temperature: 580 ° C was hot-rolled to produce a 3.2 mm-thick hot-rolled steel strip. The three stands after the hot-rolling finish rolling were hot-rolled with equal strain reduction distribution. Cooling after hot rolling was performed while controlling water injection so that the pearlite transformation progressed between 590 and 600 ° C. Cold rolling was performed at a cold rolling rate of 50%, and annealing was performed at 630 ° C. for 18 hours. Steel No. 3 is steel No. 3. The hot rolling was performed in the same manner as in No. 1, the cold rolling rate was 30%, and the final annealing was produced at 640 ° C. × 12 hours.

  Steels having the compositions shown in Table 2 were melted in a converter and slabs were made by continuous casting. This slab was annealed by hot rolling under various conditions, cold rolling after pickling, and annealing. Some of the pickled coils before cold rolling were subjected to cold rolling after annealing at 690 ° C. × 18 hours. A sample of 150 squares was taken from these steel plates, and a 10.0φ hole was punched in the center with a 12% clearance tool, pushed up with a 60 ° conical punch, and a crack that penetrated the thickness of the punched surface occurred. Then, the test was stopped, the hole diameter at that time was measured, the difference from the initial hole diameter was divided by the initial hole diameter, and the hole expansion rate was expressed in%. In the upsetting test, a hole of 15 mmφ was punched out with a clearance of 12% at the center of 100 corners, and it was pushed up with a flat bottom punch with a diameter of 20 mmφ until the hole diameter became the same as the punch diameter. The head of the hole expansion was compressed with a compression tester, and it was evaluated whether or not a crack occurred in the head when the height reached 50%. A case where no crack was observed was evaluated as “◯”, a case where a crack was slightly observed as “Δ”, and a case where a crack was present as “×”. Moreover, the ratio of the major axis and the minor axis of 100 inclusions measured in the rolling direction cross section of the steel sheet was taken, and the average value thereof was measured as the shape ratio. From the same observation plane, ferrite crystal grains were observed, and the standard deviation of the ferrite average crystal grain diameter was measured. The ferrite average grain size was measured by measuring the ferrite grain size No. stipulated in JIS, and the diameter when the crystal grain was assumed to be a sphere was obtained. The standard deviation of the crystal grain was the area of the ferrite crystal grain on the observation surface. Assuming that the ferrite crystal grains on the observation surface are circles, the diameter is multiplied by 4 / π to obtain the standard deviation, and the standard deviation is obtained by dividing by the average ferrite grain diameter. These measured values are shown in Table 3.

  Steel A is a steel having a composition equivalent to S35C. A-1 has an average ferrite grain size of 10.5 μm, a deviation degree of crystal grain size of 0.18, and an inclusion shape ratio of 1.10. All of these are the steel plates of the examples within the scope of the present invention. The hole expansion rate of this steel plate is as high as 98.5%, and no cracks are generated even after upsetting after the hole expansion. On the other hand, A-2 is an average ferrite crystal grain size: 13.5 μm and inclusion shape ratio: 1.25, but the deviation degree of the crystal grain size is 0.55, which is out of the scope of the present invention. . It can be seen that when the crystal grain size varies greatly, the hole area ratio is significantly inferior to that of the steel within the scope of the present invention, cracks are generated in the upsetting test after hole expansion, and the stretch flangeability is inferior.

  Steel B is a steel plate in which the amount of Mn is reduced from the S35C component and Cr is added. B-1 is an example in which the ferrite average crystal grain size: 8.9 μm, the degree of deviation of crystal grain size: 0.19, and the shape ratio of inclusions: 1.15, all within the scope of the present invention. The hole expansion ratio of this steel sheet is 105.2%, no cracks are observed in the upsetting test, and it has excellent stretch flangeability. B-2 is a steel having the same composition as B-1, but it can be seen that when the deviation degree of the crystal grain size is as large as 0.83, the hole expansion rate is greatly deteriorated.

  Steels C-1 and C-2 are examples in which Ti and B were added to C: 0.28%, Cr 0.25%, and Mn: 0.78%. These two steel plates have almost the same average ferrite grain size and inclusion shape ratio, and the hardness of C-2 is slightly softer, but the hole expansion rate is significantly larger than C-1. Inferior to The difference between the two steel plates depends on the degree of deviation of the crystal grain size, and it can be seen that the stretch flangeability deteriorates if the crystal grain size varies greatly.

  Steels D-1 and D2 are steel plates having a composition corresponding to SCM435. D-1 is a steel sheet within the scope of the present invention in terms of crystal grain size, degree of deviation thereof, and inclusion shape ratio. It can be seen that the hole expansion rate of this steel sheet is good and has excellent stretch flangeability. On the other hand, among the crystal grain size, the degree of deviation thereof, and the shape ratio of inclusions, D-2, which is a steel sheet having a ferrite grain size outside the scope of the present invention, has a stretch flangeability significantly inferior to the examples within the scope of the present invention. I understand that.

  Steels E-1 and E-2 are steel plates with a C content of 0.50%. This steel also shows that E-1 with a small variation in ferrite grains has excellent stretch flangeability, and E-2 with a large variation in ferrite grains has a poor hole expansion rate.

  Steel with the chemical composition shown in Table 2 is melted in a converter, slabs are made by continuous casting, and hot rolling, pickling, cold rolling and annealing are performed under various conditions to produce a steel strip with a thickness of 1.6 mm did. Some hot rolled coils were cold rolled and annealed after annealing at 690 ° C. for 18 hours after pickling. The ferrite average crystal grain size, the standard deviation degree of the ferrite crystal grain size, and the shape ratio of inclusions were measured. These measurement methods are the same as those in Example 1. The 100φ steel plate subjected to the structure investigation was heated at 880 ° C. for 50 minutes and then quenched in oil at 60 ° C., tempered at various temperatures, and the hardness was adjusted to Hv: 420 or Hv: 440. The dimensional difference before and after heat treatment of this steel sheet was measured. From this quenched / tempered material, a JIS No. 4 impact test piece was sampled and measured for an impact value at 20 ° C. Further, in order to investigate the induction hardenability, the sample was heated at 850 ° C. for 1 second and then cooled with water to measure the hardness. These measurement results are shown in Table 4.

  Steel A-3 and Steel A-4 are S35C equivalent components, A-3 is an example within the scope of the present invention, and A-4 is a comparative example. It can be seen that A-3 of the example in the present invention has better impact value and high frequency hardenability than A-4 of the comparative example. B-3 and B-4 are steels of Mn: 0.45% and Cr: 0.30%. It can be seen that B-3, in which the ferrite crystal grain size, the standard deviation, and the inclusion shape ratio are both within the scope of the present invention, has excellent impact resistance and high-frequency hardenability. C-3 and C-4 are steels to which Ti and B are added, C-3 is an example within the scope of the present invention, and C-4 is a comparative example in which the standard deviation degree of the ferrite crystal grain size is out of the scope of the present invention. However, compared with C-3, the impact value and hardness during induction hardening are low. Regardless of the type of steel, the steels of the examples within the scope of the present invention having a small standard deviation degree of the crystal grain size are smaller in the dimensional change due to quenching and tempering than the comparative examples having large crystal grain size variations. I understand.

  As described in the above examples, it is not always possible to obtain good stretch flangeability with the same components, and it is not possible to obtain desired properties for the first time by reducing the variation in ferrite grain size when the ferrite grain size is 5 μm or more. Is obtained. In the description of the examples, a part of the description of the individual manufacturing conditions is omitted, but the means for reducing the segregation described in the above-mentioned [Best Mode for Carrying Out the Invention] and the hot-rolled structure are uniform. The variation in ferrite grain size can be reduced for the first time only by combining the means, the component-cold rolling-annealing conditions, and the optimization of the cold rolling annealing conditions determined by the combination. The inventive examples of Examples 2 and 3 were manufactured by combining these, and the comparative material was manufactured under conditions that did not include the above conditions at all or only some of the conditions.

  In this invention, in obtaining a high carbon cold-rolled steel sheet with excellent stretch flangeability, the ferrite crystal grain size and its standard deviation degree are controlled, and by specifying the shape ratio of inclusions, Generation of cracks can be suppressed. It is possible to provide a high-carbon cold-rolled steel sheet that does not crack even if it is subjected to upsetting after hole expansion, has excellent stretch flangeability, and has excellent impact properties after quenching and high-frequency quenchability. By using such a high-carbon cold-rolled steel sheet, it is possible to obtain a high degree of processing in the processing of automobile transmission parts and the like, and as a result, the manufacturing process can be omitted and parts and the like can be manufactured at low cost. This is an industrially extremely useful invention.

The figure which shows the relationship between the standard deviation of a ferrite particle size, and a hole expansion rate. The figure which shows the relationship between the deviation degree of a ferrite particle size, and a hole expansion rate.

Claims (2)

C: 0.20 to 0.60% by mass%,
S: 0.005% or less,
Ti: 0.001 to 0.050%,
B: 0.0003 to 0.0050%,
Si: 0.50% or less,
Mn: 2.0% or less,
P: 0.03% or less,
Al: 0.08% or less,
N: 0.010% or less,
Cr: within 1.0%,
In the composition of the balance Fe and inevitable impurities,
The ferrite average crystal grain size is 5 μm or more and 18.3 μm or less,
A high carbon cold-rolled steel sheet having excellent stretch flangeability, wherein a value obtained by dividing the standard deviation of the ferrite crystal grain size by the average grain size is 0.14 or more and 0.50 or less. C: 0.20 to 0.60% by mass%,
S: 0.005% or less,
Ti: 0.001 to 0.050%,
B: 0.0003 to 0.0050%,
Si: 0.50% or less,
Mn: 2.0% or less,
P: 0.03% or less,
Al: 0.08% or less,
N: 0.010% or less,
Cr: within 1.0%,
In the composition of the balance Fe and inevitable impurities,
The ferrite average crystal grain size is 5 μm or more and 18.3 μm or less,
The value obtained by dividing the standard deviation of the ferrite crystal grain size by the average grain size is 0.14 or more and 0.50 or less,
A high carbon cold-rolled steel sheet having excellent stretch flangeability, wherein the inclusion has a shape ratio of 2.0 or less.

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