JP3266902B2 - Manufacturing method of high carbon cold rolled steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for industrially and stably producing a high-carbon cold-rolled steel strip having a uniform shape and properties, a relatively soft and good working property.

[0002]

2. Description of the Related Art Generally, blades, springs, washers,
Spring, seat belt fittings and other high-hardness machine parts
A high carbon cold-rolled steel strip, a special steel strip specified in JIS G 3311, is used as a material for punching, bending, pressing,
It is manufactured through a processing step such as cutting, quenching, tempering, and other heat treatment steps. In order to improve the product quality, stabilize, and reduce the manufacturing cost, it is necessary that the high-carbon cold-rolled steel strip as the material is soft, has good workability, and has excellent structure uniformity.

[0003] The high-carbon cold-rolled steel strip is manufactured by pickling a hot-rolled steel strip, then cold rolling and annealing to adjust the hardness to a desired hardness. It is required to have a predetermined hardness by heat treatment performed after processing, and to have sufficient hardness and abrasion resistance for use as a product. Therefore, annealing after cold rolling must be performed at a high temperature, and as a result, seizure flaws are likely to occur due to close contact between steel strips. There is a problem that poor quality is liable to occur due to annealing seizure due to severe surface inspection.

Conventionally, as a method for forming a high-carbon cold-rolled steel strip into a soft and easy-to-work structure, in hot rolling, rapid transformation is performed on a run-out table to complete the phase transformation, and the steel having undergone the phase transformation is completed. Using a high-carbon hot-rolled steel strip obtained by winding the strip at 500 to 620 ° C as a base material, the base material is cold-rolled at a rolling reduction of 20% or more, and then, at a point of Ac 1 point or more and 770 ° C in a bell furnace.
A method of annealing at the following temperature (Japanese Patent Laid-Open No. 58-55532), C: 0.27 to 0.90%, Si: 0.15 to
0.30%, Mn: 0.60 to 0.90%, P: 0.0
30% or less, S: 0.035% or less, steel containing normal balance of Fe and unavoidable impurities is subjected to ordinary hot rolling, and then hot-rolled steel strip obtained by pickling is used as a starting material to start high-carbon cold rolling. In the method for producing a steel strip, primary annealing is performed by holding the hot-rolled steel strip as the starting material at a temperature of 680 to 720 ° C for 15 hours or more, and then cold rolling is performed at a rolling reduction of 20 to 45%, Thereafter, a method of performing finish annealing in which the temperature is maintained at a temperature of 630 to 720 ° C. for 10 hours or more (Japanese Patent Application Laid-Open No.
No. 19), in the cooling after hot rolling of high carbon steel, if the cooling rate is less than 0.6% carbon, the cooling rate is 30 to 45 ° C.
/ Sec, 15 to 45 ° C / sec when the carbon content is 0.6% or more.
and the subsequent winding is 46 if the carbon content is less than 0.6%.
0 to 600 ° C, 55% for carbon content of 0.6% or more and less than 0.8%
0 to 640 ° C, 620 to 680 when the carbon content is 0.8% or more
C., and cold rolling is performed at a rolling reduction of 5% if the carbon content is less than 0.6%.
0 to 85%, 30 for carbon content of 0.6% or more and less than 0.8%
After performing the spheroidizing annealing at a carbon content of 0.8% or more at 25 to 60%, the spheroidizing annealing is performed at a carbon content of less than 0.8% at 680 to 70%.
When the Ac point is 1 and the carbon content is 0.8% or more, the method is carried out at 680 to 750 ° C. (JP-A-3-122216).
Intermediate cold rolling of high-carbon hot-rolled steel sheets of 3% or more with a rolling reduction of 4
After performing at 5% or more, the intermediate annealing was performed by box annealing to obtain C: 0.
If it is less than 8%, it is 680 ° C. to Ac1 point, and if it is 0.8% or more, it is soaked at 680 to 750 ° C. for 20 to 40 hours, and the final cold rolling is performed with a rolling reduction of 13% or more. Moreover, when the sheet thickness before and after the intermediate cold rolling and the sheet thickness after the final cold rolling are defined as t ₀ , t ₁ and t ₂ , respectively, the logarithm of the reduction ratio ln (t ₀ / t ₁ ) of the intermediate cold rolling is obtained. On the other hand, the logarithm ln (t ₁ / t ₂ ) of the reduction ratio of the final cold rolling is set to be 0.5 times or less, and the final annealing is performed by box annealing to 550 to 550.
Heating at 700 ° C for 1 to 6 hours
No. 16519), C: 0.6 to 1.3%, Si:
0.5% or less, Mn: 1% or less, Cr: 1.6% or less,
A high-carbon hot-rolled steel strip having a chemical composition substantially composed of the balance of Fe is soaked in an atmosphere furnace containing 50% by volume or more of hydrogen and the balance being nitrogen in a temperature range of Ac1 point to 780 ° C for 1 hour or more. Thereafter, the first stage of soaking / slow cooling to cool directly below the Ar 1 point at a cooling rate of 60 ° C./Hr or less, and 3 to 3 immediately below the Ac 1 point.
After maintaining the soaking temperature for 20 hours, A was cooled at a cooling rate of 60 ° C / Hr or less.
After being subjected to a primary annealing treatment comprising a second stage of soaking and gradual cooling in which the temperature is lowered to r1 or lower, cold rolling is performed, and then 60
A method of performing a secondary annealing treatment in a temperature range from 0 ° C. to just below the Ac1 point (JP-A-4-202629) has been proposed.

[0005]

SUMMARY OF THE INVENTION The above-mentioned JP-A-58-55 is disclosed.
No. 532 discloses a method in which an annealing step before cold rolling is omitted, pickling is performed, then cold rolling is performed, and the subsequent annealing is performed using Ac1.
As described above, since the surface of the high-carbon cold-rolled steel strip has a bright skin as described above, as long as it is processed in a batch coil annealing furnace, it is for high-temperature annealing of more than Ac1 point.
Anneal seizure flaws occur, and it is difficult to stably produce high-carbon cold-rolled steel strip with actual production equipment. In addition, this method is based on the premise that a high-carbon hot-rolled steel strip obtained by winding a steel strip which has been rapidly cooled on a hot-rolling run-out table and phase transformation is completed at 500 to 620 ° C. is used as a base material. But 55
If the temperature is 0 ° C. or lower, the ductility of the base material is reduced, and breakage in the pickling line in the next step or breakage during cold rolling occurs, which has a disadvantage that the operation is significantly impaired.

In the method disclosed in Japanese Patent Application Laid-Open No. 61-76619, the primary annealing temperature of a hot-rolled steel strip is 680 to 720.
° C and the Ac1 point or less, a sufficiently soft high carbon steel strip cannot be obtained.

Furthermore, the method disclosed in Japanese Patent Application Laid-Open No. 3-122216 is disadvantageous in terms of efficiency because the intermediate annealing temperature is lower than the Ac1 point, requiring an annealing time of 20 to 40 hours.

Furthermore, the method disclosed in Japanese Patent Publication No. Hei 7-116519 is disadvantageous in terms of efficiency because the intermediate annealing is performed at 680 ° C. to an Ac point or less for 20 to 40 hours, and wasteful in actual production equipment. . Also, Japanese Patent Application Laid-Open No. Hei 3-21235
In the gazette, "If the intermediate annealing temperature is higher than the Ac1 point, the steel having a C: 0.6% or less generates a ferrite phase, so that the spheroidization ratio is reduced. Because of the complete austenitization, coarse pearlite is generated during cooling after annealing, and the spheroidization rate is also lowered. "C: 0.3 to 0.8% high carbon hot rolled steel strip However, there is a problem that the spheroidization rate cannot be reduced. Moreover, the final annealing temperature is soaked by box annealing at 550 to 700 ° C. for 1 to 6 hours. However, it is necessary to increase the annealing temperature for softening. Since it occurs, it has to be annealed at a low temperature, and soft annealing of a thin material is difficult.

In the method disclosed in Japanese Patent Application Laid-Open No. Hei 4-202629, the soaking time when the annealing temperature is raised to a temperature between Ac 1 point and 780 ° C. is 1 hour or more.
No upper limit is specified. As described later, it is difficult to obtain spherical cementite after annealing for a long time at an Ac point or more, and it is rather deteriorated after working. Even after repeated cold rolling and annealing, the workability of a high carbon steel strip is improved. Improvement cannot be expected. Further, since no specific method for realizing short-time annealing at an Ac1 point or higher in coil annealing is not disclosed, it is impossible to stably obtain a good spheroidized structure with an actual machine.

An object of the present invention is to solve the above-mentioned problems of the prior art, to provide a high carbon cold rolled steel sheet having workability equal to or higher than that of a conventional high carbon cold rolled steel strip and having excellent crystal structure homogeneity. Steel strip,
An object of the present invention is to provide a method of manufacturing a high-carbon cold-rolled steel strip that can stably produce at low cost while preventing the occurrence of seizure flaws while shortening the annealing time.

[0011]

According to the first aspect of the present invention, there is provided a method for producing a high-carbon cold-rolled steel strip, comprising: C: 0.3 to 0.8%;
i: 0.03 to 0.35%, Mn: 0.20 to 1.50
%, The balance being substantially Fe and unavoidable impurities, after hot rolling, pickling and descaling, and then performing cold rolling at a rolling reduction of 25% or more and 80% or less. And a bell-type batch annealing furnace in a gas atmosphere consisting of at least 75% by volume of hydrogen and the balance substantially consisting of nitrogen and unavoidable impurities, and performing annealing under the following annealing conditions (1) to (7). (1) Heating rate up to the first stage soaking temperature: 20 ° C / Hr
１００100 ° C./Hr (2) First stage soaking temperature × time: 680 ° C. to just below Ac1 point × 6Hr or more (3) Heating rate up to second stage soaking temperature: 100 ° C./H
r or less (4) Second-stage soaking temperature x time: Ac1 point to Ac1 point +
(5) Cooling rate after completion of second-stage soaking: 50 ° C / Hr or less (6) Ultimate cooling temperature after completion of second-stage soaking: Ar 1 point or less (7) After that, 20 ° C / Cool down to room temperature at Hr-100 ° C / Hr

Thus, C: 0.3-0.8%, S
i: 0.03 to 0.35%, Mn: 0.20 to 1.50
%, The balance being substantially Fe and inevitable impurities, hot rolling, pickling, descaling, and then cold rolling at a rolling reduction of 25% or more and 80% or less. By this, the pearlite is refined,
By using a bell-type batch annealing furnace in a gas atmosphere consisting of 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen and unavoidable impurities, annealing is performed under the above-described annealing conditions (1) to (7), so that the cooling process is performed. Cementite can be spheroidized in a short time by preventing the formation of pearlite, and a soft high-carbon cold-rolled steel strip can be obtained.

The method for producing a high-carbon cold-rolled steel strip according to claim 2 of the present invention is characterized in that: C: 0.3 to 0.8%, Si: 0.03
０．３0.35%, Mn: 0.20 to 1.50%,
After the high-carbon steel slab substantially consisting of Fe and unavoidable impurities is subjected to hot rolling, pickling, and descaling, cold rolling is performed at a rolling reduction of 25% to 80%, and then 75% by volume or more. Using a bell-type batch annealing furnace in a gas atmosphere in which hydrogen and the balance substantially consist of nitrogen and unavoidable impurities, primary annealing is performed under the annealing conditions (1) to (7),
Cold rolling is performed at a rolling reduction of 20% or more and 80% or less, and finish annealing is performed in a bell-type batch annealing furnace at 600 ° C. to just below Ac1 point.

Thus, C: 0.3-0.8%, S
i: 0.03 to 0.35%, Mn: 0.20 to 1.50
%, The balance being substantially Fe and inevitable impurities, hot rolling, pickling, descaling, and then cold rolling at a rolling reduction of 25% or more and 80% or less. By this, the pearlite is refined,
By using a bell-type batch annealing furnace in a gas atmosphere consisting of 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen and unavoidable impurities, annealing is performed under the above-described annealing conditions (1) to (7), so that the cooling process is performed. Cementite can be spheroidized in a short time by preventing the formation of pearlite.
Cold rolling is performed at 0% or more and 80% or less, and finish annealing is performed in a bell-type batch annealing furnace at a temperature of 600 ° C. to just below the Ac1 point, thereby releasing the cold rolling strain to grow crystal grains and obtain a soft high A carbon cold rolled steel strip can be obtained.

Further, the method for producing a high-carbon cold-rolled steel strip according to claim 3 of the present invention comprises the steps of:
Cold rolling at a rolling reduction of 20% or more and 80% or less and finish annealing in a bell-type batch annealing furnace at 600 ° C. to just below Ac1 point are performed in two steps.
Repeat more than once. As described above, after the primary annealing treatment, the cementite is obtained by repeating the cold rolling at a reduction ratio of 20% or more and 85% or less and the finish annealing at 600 ° C. to just below the Ac1 point in a bell-type batch annealing furnace at least twice. The spheroidization rate further improved and approached almost 100%,
It becomes possible to manufacture a softer hot-rolled steel strip, and if the desired sheet thickness cannot be obtained by a single cold rolling such as a thin sheet having a final sheet thickness of, for example, 0.3 mm, cold rolling is performed once. After finishing and softening by performing the finish annealing under the same conditions, the sheet thickness was adjusted to the desired thickness by performing cold rolling again, and the finish annealing was performed again under the same conditions to soften the sheet. A 0.3 mm thick soft high carbon cold rolled steel strip can be obtained.

Further, the method for producing a high-carbon cold-rolled steel strip according to claim 4 of the present invention is characterized in that: C: 0.3 to 0.8%;
A high-carbon steel slab containing 0.03 to 0.35%, Mn: 0.20 to 1.50%, and the balance substantially consisting of Fe and unavoidable impurities is subjected to hot rolling, pickling, and descaling. Then, after cold rolling at a rolling reduction of 25% or more and 80% or less, and a steel sheet roughness of 0.3 μm or more and 1.2 μm or less in center average roughness, hydrogen of 75% by volume or more and the balance Using a bell-type batch annealing furnace in a gas atmosphere composed of nitrogen and unavoidable impurities, first annealing under the above annealing conditions (1) to (7), the elongation is 0.6% or more and 4.0% or less. , The roughness of the steel sheet is treated to be 0.2 μm or less.

Thus, C: 0.3-0.8%, S
i: 0.03 to 0.35%, Mn: 0.20 to 1.50
%, And the balance is substantially hot-rolled, pickled and descaled from high-carbon steel slabs substantially composed of Fe and unavoidable impurities, and then subjected to cold rolling at a rolling reduction of 25% or more and 80% or less, and Steel plate roughness is 0.3μm or more and 1.2μm in center average roughness
After imparting the following, the primary annealing is performed under the annealing conditions (1) to (7) using a bell-type batch annealing furnace in a gas atmosphere consisting of 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen and unavoidable impurities. After treatment, elongation is 0.6% or more and 4.0
%, By treating the steel sheet with a roughness of 0.2 μm or less by temper rolling at not more than 0.2%, the pearlite is refined, the contact area of the steel sheet is reduced, and the annealing seizure flaw during annealing is prevented. A high-carbon cold-rolled steel strip that is soft, has excellent workability, and has excellent crystal structure homogeneity can be obtained.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of a high carbon steel strip used in the present invention, assuming plastic working such as punching properties and bending formability, are as follows.

C is an element having an effect of imparting strength, hardenability, wear resistance and the like to steel. The lower the C, the softer the workability and the better the workability.
A predetermined hardness cannot be obtained after heat treatment. On the other hand, if it exceeds 0.8%, the hardness at the stage of the hot-rolled steel strip is high, and cold rolling becomes difficult.

Si is an element to be added as a deoxidizing material, but if it is less than 0.03%, the deoxidizing effect is not sufficient, and if it exceeds 0.35%, the deoxidizing ability is saturated and solid solution 0.03 ~
0.35%.

Mn is an element which improves the strength and hardenability and has the effect of stabilizing cementite.
If it is less than 0.2%, stable hardenability cannot be secured, and if it exceeds 1.5%, the effect of improving the hardenability is saturated and the hardness of the material is increased.
2 to 1.5%.

The hot rolling of the high carbon steel slab having the above chemical composition is carried out by a usual method, and there is no particular limitation on the hot rolling conditions, but the winding of the hot rolled steel strip is carried out after the completion of the phase transformation. Is preferred.

The purpose of the reduction in the cold rolling in claim 1 of the present invention is to refine pearlite and promote spheroidization in the next annealing, but if it is less than 25%, pearlite cannot be sufficiently refined. The effect of cold rolling cannot be demonstrated,
If it exceeds 80%, problems such as edge cracking will occur.
5 to 80%.

In the case of batch coil annealing using a bell-type batch annealing furnace, the temperature difference between the outermost peripheral temperature and the coldest point is large, and the coldest point temperature is at least Ac1 point.
It is difficult to keep the soaking temperature beyond one point within 8 hours. Therefore, in the batch coil annealing in the bell-type batch annealing furnace according to the present invention, as shown in Table 1 and FIG. 1, at a temperature of 0 ° C. and a pressure of 0.1 MPa, the thermal conductivity is about 7 times that of nitrogen and the density is nitrogen. About 14 times as much hydrogen as
By setting it to 5% by volume or more, the coil width is 960m
m, coil outer diameter 1847 mm, coil inner diameter 610 mm,
As shown in FIG. 2, which shows the relationship between the soaking time and the temperature difference in the coil when the coil having a weight of 18 Ton was annealed at a soaking temperature of 770 ° C., the heat transfer inside the coil was improved, and the conventional nitrogen was mainly used. The temperature difference between the inner and outer peripheries of the coil can be reduced as compared with the case in which a bell-type batch annealing furnace can be applied.

[0025]

[Table 1]

As shown in FIG. 3, the annealing treatment in an atmosphere gas comprising 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen is as follows.
A first stage soaking step of soaking at a heating rate of 20 to 100 ° C./Hr directly below 680 ° C. to the Ac1 point for 6 hours or more, and a heating rate of 2
After soaking at 0 to 100 ° C. to Ac1 point to Ac1 point + 50 ° C. for 8 hours or less, a second stage soaking and slow cooling step of gradually cooling to 50 ° C./Hr or less to Ar 1 point or less, and thereafter to 20 to room temperature
A cooling step of cooling at a rate of 100 ° C./Hr.
FIG. 4 shows an example of a typical heat pattern.

In order to promote spheroidization, it is ideal that the soaking in the first-stage soaking step is performed by annealing for a short time at a temperature equal to or higher than the Ac1 point. To reach the Ac1 point or more, the outer periphery of the coil must be Ac
At a temperature of one or more points, annealing is performed for a long time (exceeding 8 Hr), and the structure becomes austenite.

Therefore, the purpose of the first-stage soaking in the present invention is to raise the coldest point temperature of the coil from 680 ° C. to just below the Ac1 point to make the coil temperature uniform, and then heat to the second-stage soaking temperature. As a result, Ac1 in the second stage soaking
The point is to shorten the soaking time. In order for the coldest point temperature of the coil to reach the Ac1 point in a short time in the second-stage soaking, the coldest point temperature of the coil must be 68 during the first-stage soaking.
It is necessary to secure 0 ° C or more. In the first-stage soaking time, if the coldest point temperature of the coil reaches just below the Ac1 point, the intended purpose is achieved, but after extending the soaking time and promoting spheroidization in the ferrite region, Entering the second-stage soaking enables a better structure to be produced. For this reason, the first-stage soaking time was set to 6 Hr or more.

The heating rate up to the second stage soaking temperature is 100
If the temperature exceeds ℃ / Hr, the heating efficiency is poor and the coil temperature tends to be non-uniform.

If the second-stage soaking temperature is less than the Ac 1 point, it takes a long time to spheroidize the cementite. Since pearlite was formed and a good spheroidized structure could not be obtained even when the cooling conditions were controlled, the temperature was set to Ac1 point to Ac1 point + 50 ° C.

If the second-stage soaking time exceeds 8 hours, the structure becomes austenitized, so that pearlite is generated during the cooling process and a good spheroidized structure cannot be obtained.
8 Hr or less. The lower limit of the second-stage soaking time is not particularly limited as long as the coldest point of the coil exceeds the Ac1 point even for a moment, but the second-stage soaking time varies depending on the weight of the coil. In time, a large coil requires a soaking time close to 8Hr. However, when it exceeds 8 hours, the above-mentioned problem occurs.

The cooling rate after the completion of the second-stage soaking is 50 ° C. /
When the content exceeds Hr, the precipitation of solid solution C is suppressed, so that the generation of spheroidized carbide having undissolved carbide as a nucleus becomes insufficient,
Ar1 transformation occurs while a large amount of C is dissolved in austenite to form a lamellar pearlite structure, and the spheroidization rate of cementite decreases. In addition, when the cooling completion temperature at the time of completion of cooling exceeds the Ar1 point, the spheroidization rate of cementite decreases, so
Or less, but the lower limit temperature does not cause reduction in processing efficiency.

After the first and second stages of soaking and slow cooling, the cooling rate from the Ar 1 point or lower to room temperature is 20 ° C. /
If it is less than Hr, the operation efficiency is poor, and if it exceeds 100 ° C./Hr, the temperature difference between the inner and outer circumferences of the coil becomes large and annealing seizure frequently occurs.

In the second aspect of the present invention, after the first and second stages of soaking and slow cooling steps, the rolling reduction is 20 to 80%.
Perform finish cold rolling. It is preferable that the rolling reduction of the finish cold rolling is as large as the rolling mill capacity allows for the purpose of streamlining the process. However, as shown in FIG. 5, in the point of softening, the rolling reduction of the cold rolling peaks at around 20%. is there. 20% reduction
If it is less than 10, the crystal grains are less fragmented, and the grains do not soften due to insufficient grain growth energy during finish annealing. When the rolling reduction exceeds 80%, the crystal grains become too small,
Since the carbide at the time of spheroidization does not become large and is not sufficiently softened, breakage occurs. Therefore, the reduction rate of the finish cold rolling is 2
If the sheet thickness becomes a predetermined value in the vicinity of 0%, the softened high carbon cold rolled steel strip can be manufactured.

In the finish annealing treatment after the finish cold rolling, the spheroidization of the cementite is sufficiently advanced by the primary cold rolling and the intermediate annealing. Therefore, the finish annealing eliminates the strain after the cold rolling and the crystal grains grow. It is sufficient that the point is Ac1 point or less. For this reason,
As shown in FIG. 6, the effects of the soaking temperature and the soaking time are small, and the coldest temperature of the coil is 1H at 600 ° C. or higher.
r is enough. The finish annealing temperature is preferably as low as possible, since annealing seizure flaws occur when coil annealing of a thin material of 0.5 mm or less and this adversely affects the glossiness of the product. The recrystallization and grain growth of ferrite become insufficient and the softening effect becomes insufficient. However, if the coldest point temperature of the coil is maintained at 600 ° C. or more, softening annealing can be performed without generating the annealing seizure flaw. On the other hand, if it exceeds the Ac1 point, an austenite phase is formed, and layered pearlite appears during the cooling process to become hard, and there is a possibility that seizure flaws may occur.

Unlike the above-mentioned intermediate annealing, the finish annealing does not necessarily have to be performed in a high hydrogen atmosphere, but may be an atmosphere mainly containing nitrogen. In this finish annealing treatment, the soaking time is not so important, and a soaking time of about 1 hour is sufficient. Further, the control of the cooling rate is not particularly required, and for example, natural cooling can be performed.

After the intermediate annealing including the first and second stages of soaking and slow cooling steps, even if cold rolling is performed at a rolling reduction of 20 to 80%, the final sheet thickness is small, for example, 0.3 mm. In the following cases, if the desired thickness cannot be obtained by one cold rolling, the cold rolling is temporarily stopped, the finish annealing treatment is performed under the same conditions to soften, and then the cold rolling is performed again. To a desired plate thickness, and a finish annealing treatment is performed. The finish cold rolling and finish annealing can be performed a plurality of times according to the finish plate thickness.

According to a fourth aspect of the present invention, the contact area of the steel sheet is reduced by imparting the roughness of the steel sheet at a center line average roughness (Ra) of 0.3 μm or more and 1.2 μm or less during cold rolling. Prevention of seizure during annealing becomes possible. When the center line average roughness is less than 0.3 μm, there is no effect of preventing the seizure during annealing, and when the center line average roughness exceeds 1.2 μm, the temper rolling after annealing is performed with a bright roll. This is because the steel sheet roughness does not sufficiently decrease and cannot be applied to products requiring brightness.

In the fourth aspect of the present invention, the temper rolling after imparting the roughness of the steel sheet with the center line average roughness of 0.3 μm or more and 1.2 μm or less is performed in order to prevent annealing seizure during cold rolling. Center line average roughness 0.3 μm or more and 1.2 μm
The main purpose is to adjust the following steel plate roughness, and the elongation is 0.6% or more and 4.0% or less so that the final product roughness is 0.20 μm or less in center line average roughness. If the elongation percentage during temper rolling is less than 0.6%, the final product roughness cannot secure a center line average roughness of 0.20 μm or less, and if the elongation percentage exceeds 4.0%, Although the final product roughness can secure a center line average roughness of 0.20 μm or less, the hardness of the steel sheet becomes high, and the original intended soft steel sheet cannot be manufactured.

[0040]

【Example】

Example 1 Steel No. 1 having the chemical composition shown in Table 2 was used. 1 to 4 high carbon steel slabs
Finish plate thickness 2mm, Finish temperature 840 ° C, Winding temperature 650 ° C
After hot rolling under hot rolling conditions to form a high carbon hot rolled steel strip, pickling and descaling, cold rolling is performed at a rolling reduction of 70%, and a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen Using a bell-type batch annealing furnace, test Nos. 1 to
A first annealing treatment comprising 25 first and second-stage soaking and slow cooling steps was performed to obtain a high-carbon cold-rolled steel strip. A test piece was cut out from each high-carbon cold-rolled steel strip, and Vickers hardness (HV) was measured according to the Vickers hardness test method specified in JIS Z2244. 3
Evaluation was made based on the softness (see Table 4) in comparison with the Vickers hardness after annealing specified for the special steel strip specified in 311.

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

As shown in Table 3, in test No. 1 in which the first-stage soaking time in the primary annealing treatment was less than 6 hours. The high carbon cold rolled steel strips of 4, 5 have Vickers hardness HV: 174, 194 and are not sufficiently softened. First in primary annealing
Test No. having a stage soaking temperature of less than 680 ° C. The high-carbon cold-rolled steel strip of No. 5 has a Vickers hardness HV of 194 and is not sufficiently softened. Test No. 2 in which the second-stage soaking temperature was less than Ac1.
The high-carbon cold-rolled steel strip No. 2 has a Vickers hardness HV: 178, and is not sufficiently softened. Test No. 2 in which the second-stage soaking temperature exceeded Ac1 point + 50 ° C. Ten high carbon cold rolled steel strips
Vickers hardness HV is as hard as 165. Test No. 2 in the second annealing in the primary annealing process exceeded 8 hours.
The high-carbon cold-rolled steel strip of No. 11 has a Vickers hardness HV: 178.
And hard. Test No. 1 in which the cooling rate after the second-stage soaking in the primary annealing treatment exceeded 50 ° C./Hr. The twelve high carbon cold rolled steel strips are also hard. Test No. in which the ultimate cooling temperature in the primary annealing treatment was Ar 1 point or more. Thirteen high-carbon cold-rolled steel strips are hard with Vickers hardness HV: 182.

On the other hand, in the first annealing treatment, the first-stage soaking temperature is 680 ° C. or more, just below the Ac1 point, the first-stage soaking time is 6 hours or more, the second-stage soaking time is 8 hours or less, and the cooling rate is 50%. ° C / Hr or less, and the cooling ultimate temperature was just below the Ar1 point. As shown in Table 3, the high carbon cold rolled steel strips of 1, 3, 6 to 8 and 9 all have a Vickers hardness (HV) that is softer than the standard. Also,
As shown in Table 3, Test No. As shown in FIG. 14, the heating rate and cooling rate of each stage in the primary annealing treatment were 5 ° C./Hr, 3
Even if it is ° C / Hr, it does not adversely affect the hardness of the high-carbon cold-rolled steel strip, but there is no merit in terms of the production efficiency of the annealing treatment and energy saving.

Example 2 Steel No. 1 having the chemical composition shown in Table 2 was used. The high carbon steel slabs of Nos. 1-4 are hot-rolled under hot rolling conditions of a finishing plate thickness: 2 mm, a finishing temperature: 840 ° C, and a winding temperature: 650 ° C to form a high-carbon hot-rolled steel strip, pickling, After descaling, intermediate cold rolling was performed at a rolling reduction of 50%. Then, using a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen, test Nos. Intermediate annealing treatment including 15 to 28 first and second stage soaking and slow cooling steps was performed. Next, after cold rolling at a rolling reduction of 20%, a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen was used, and a soaking temperature: 6
A finish annealing treatment at 00 ° C. and soaking time of 1 hour was performed to obtain a high carbon cold-rolled steel strip. A test piece was cut out from each high-carbon cold-rolled steel strip, and Vickers hardness (HV) was measured according to the Vickers hardness test method specified in JIS Z 2244. Evaluation was made based on the softness (see Table 4) in comparison with the Vickers hardness after annealing specified for the special steel strip specified in G 3311. Table 5 shows the results.

[0047]

[Table 5]

As shown in Table 5, in test No. 1 in which the first-stage soaking time in the intermediate annealing treatment was less than 6 hours. Vickers hardness HV: 169, 18
8, resulting in insufficient softening. Test No. 1 in which the first-stage soaking temperature in the intermediate annealing treatment was less than 680 ° C. The high carbon cold rolled steel strip of No. 19 has a Vickers hardness HV of 188 and is not sufficiently softened. Test No. 2 in which the second-stage soaking temperature was less than Ac1. The high carbon cold rolled steel strip of 16 has a Vickers hardness HV:
174, resulting in insufficient softening. Test No. 2 in which the second-stage soaking temperature exceeded Ac1 point + 50 ° C. The high carbon cold rolled steel strip of No. 24 has a Vickers hardness HV: 160 and is hard. Test No. 2 in the second annealing in the primary annealing process exceeded 8 hours. 25 high carbon cold rolled steel strip has a Vickers hardness H
V: Hard as 174. Test No. 1 in which the cooling rate after the second-stage soaking in the primary annealing treatment exceeded 50 ° C./Hr. 2
The high carbon cold rolled steel strip No. 6 is also hard. Test No. in which the ultimate cooling temperature in the primary annealing treatment was Ar 1 point or more. The 27 high-carbon cold-rolled steel strip has a Vickers hardness HV of 180 and is hard.

On the other hand, in the first annealing treatment, the first stage soaking temperature was 680 ° C. or more, just below the Ac1 point, the first stage soaking time was 6 hours or more, the second stage soaking time was 8 hours or less, and the cooling rate was 50 hours. ° C / Hr or less, and the cooling ultimate temperature was just below the Ar1 point. 15, 17, 20-20
As shown in Table 3, the high-carbon cold-rolled steel strip No. 3 has a Vickers hardness (HV) that is softer than the standard. Also,
As shown in Table 3, Test No. As shown in FIG. 28, the heating rate and cooling rate of each stage in the primary annealing treatment were 5 ° C./Hr, 3
Even if it is ° C / Hr, it does not adversely affect the hardness of the high-carbon cold-rolled steel strip, but there is no merit in terms of the production efficiency of the annealing treatment and energy saving.

Example 3 In order to investigate the influence of chemical components in steel, steel N shown in Table 6 was used.
o. Each high carbon steel slab of 5 to 14 was hot-rolled under hot rolling conditions of a finishing plate thickness: 2 mm, a finishing temperature: 840 ° C, and a winding temperature of 650 ° C to form a high-carbon hot-rolled steel strip. After pickling and descaling the hot-rolled steel strip, cold rolling was performed at a rolling reduction of 70%, and a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen was used. ° C / H
r, first-stage soaking temperature: 710 ° C, first-stage soaking time: 20
Hr, heating rate after completion of first-stage soaking: 50 ° C / Hr, second-stage soaking temperature: Ac 1 point + 20 ° C, second-stage soaking time: 5Hr, after completion of second-stage soaking Cooling rate 30 ° C / H
r, Cooling temperature: Annealing treatment was performed by performing a second-stage soaking at an Ar point of −10 ° C. and slow cooling. A test piece was collected from each of the obtained high-carbon cold-rolled steel strips and was subjected to JIS Z in the same manner as in Example 1.
Vickers hardness was measured according to the Vickers hardness test method specified in 2244. Table 6 shows the results.

[0051]

[Table 6]

As shown in Table 6, the steel Nos. 10 and 14 high carbon cold rolled steel strips
Since the addition amounts of i and Mn exceed the upper limit of the range of the present invention, the steel strip is harder than other steel strips corresponding to the present invention. In addition, steel N
o. The high-carbon cold-rolled steel strips Nos. 7 and 11 are soft because they are below the lower limit of the present invention. Steel No. 7
In the high-carbon cold-rolled steel strips described above, since the Si content is low, the fatigue strength after quenching and tempering involves the risk of property deterioration due to an increase in oxides. In addition, steel No. In the high-carbon cold-rolled steel strip of No. 11, since the Mn content is low, the quenchability decreases, and there is a risk that sufficient hardness cannot be obtained after quenching and tempering.

Example 4 In order to investigate the effects of the rolling reduction in the finish cold rolling after the intermediate annealing treatment and the soaking temperature in the finish annealing after the finish cold rolling, the steel Nos. 12 high carbon steel slabs
Finish plate thickness: 2mm, Finish temperature: 840 ° C, Winding temperature 65
Hot-rolled under hot rolling conditions of 0 ° C to form a high-carbon hot-rolled steel strip,
After pickling and descaling, intermediate cold rolling was performed at a rolling reduction of 40%, and a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen was used.
° C / Hr, first stage soaking temperature: 710 ° C, first stage soaking time: 20Hr, heating rate after first stage soaking: 50 ° C / H
r, second-stage soaking temperature: Ac 1 point + 20 ° C., second-stage soaking time: 4 Hr, cooling rate after second-stage soaking: 10 ° C./Hr,
Ultimate cooling temperature after second-stage soaking: second-stage soaking at Ar1 point,
Slow cooling was performed to perform an intermediate annealing treatment. Then, after the finish cold-rolling with the rolling reduction changed in the range of 20 to 85% as shown in Table 7, a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen was used. 60
A high carbon cold-rolled steel strip was obtained by performing a finish annealing treatment at 0 ° C. to 720 ° C. and a finish annealing time of 1 hr and 5 hr. A test specimen was collected from each of the obtained high-carbon cold-rolled steel strips, and the Vickers hardness (HV) was measured in the same manner as in Example 1 according to the Vickers hardness test method specified in JIS Z2244. Table 7 shows the results.

[0054]

[Table 7]

As shown in Table 7, in test cold rolling with a rolling reduction of 20% in the finish cold rolling. As is clear from FIG. 29, when the rolling reduction is less than 20%, abnormal growth of crystal grains occurs, and thus the elongation is reduced. Test No. with a rolling reduction of 85% in the finish cold rolling. In No. 33, fracture occurred during cold rolling. For this reason, the rolling reduction in the finish cold rolling is limited to 20 to 80%.
Further, in Test No. in which the soaking temperature of the finish annealing was 590 ° C. 3
In No. 4, the hardness is high. For this reason, the soaking temperature of the finish annealing needs to be 600 ° C. or higher. On the other hand, Test No. 3 satisfying the conditions of the present invention. 30-3
In the case of 2, 35 to 37, those having satisfactory hardness HV were obtained. Test No. From the result of No. 38, it is understood that the softening is sufficiently softened even when the soaking time of the finish annealing is 1 hour.

Example 5 Steel No. 3 in Table 6 of Example 3 was used. 9 high carbon steel slab, finishing plate thickness: 2 mm, finishing temperature: 840 ° C, winding temperature 640
After hot-rolling under hot-rolling conditions of 0 ° C. to form a high-carbon hot-rolled steel strip, pickling and descaling, the steel sheet roughness shown in Table 8 at a rolling reduction of 60% (finished sheet thickness 0.8 mm) was obtained. Cold-rolled while applying, using a bell-type batch annealing furnace in a gas atmosphere consisting of 99% by volume of hydrogen and the balance of nitrogen, heating rate up to first-stage soaking temperature: 50 ° C / Hr, first-stage soaking Temperature: 710 ° C,
First stage soaking time: 20Hr, heating rate to second stage soaking temperature: 50 ° C / Hr, second stage soaking temperature: Ac1 + 20
° C, second-stage soaking time: 5Hr, cooling rate after completion of second-stage soaking: 30 ° C / Hr, ultimate cooling temperature after completion of soaking: Ar
A finish annealing treatment at 1-10 ° C was performed. Next, using a work roll having a roll roughness shown in Table 8, temper rolling was performed at an elongation percentage shown in Table 8 to obtain a high-carbon cold-rolled steel strip. Table 8
The roughness in the middle is indicated by the center line average roughness (Ra), and the roughness evaluation was evaluated as acceptable (○) when roughness ≦ 0.20 μm from the viewpoint of ensuring brightness.

[0057]

[Table 8]

As shown in Table 8, Test No. In 39, when the roughness of the steel sheet after cold rolling is 0.2 μm, the final product roughness is satisfied, but seizure occurs during the annealing treatment, and the product does not become a product. Therefore, the test No. As shown after 40, the steel plate after cold rolling must have a roughness of 0.3 μm or more. Test No. The elongation during temper rolling of samples 42 and 44 is 0.
If it is less than 6%, the final product roughness will not be satisfied even if the steel sheet roughness during cold rolling is adjusted. Test No. If the roll roughness during temper rolling of Nos. 45 and 46 exceeds 0.1 μm, the final product roughness will not be satisfied even if other conditions are adjusted. Test No. 4
As shown in 0, 41 and 43, the steel plate roughness after cold rolling is 0.3 to 1.2 μm, and the roll roughness during temper rolling is 0.1.
When the elongation at the time of temper rolling satisfies 0.6 to 2.0%, seizure does not occur during the annealing treatment, and a product having a final product roughness of 0.20 μm or less can be obtained.

Example 6 In order to clarify the rolling reduction in cold rolling, steel No. 1 shown in Table 6 was used. 12 (S55CM) using a high-carbon hot-rolled steel strip, cold-rolled at a rolling reduction shown in Table 9, and then tested in Table 3
o. Annealing was performed under the annealing conditions of No. 13 to obtain a high-carbon cold-rolled steel strip. A test piece was cut out from each of the obtained high-carbon cold-rolled steel strips, and Vickers hardness (HV) was measured according to the Vickers hardness test method specified in JIS Z 2244. The evaluation was made based on the softness (see Table 4) in comparison with the Vickers hardness after annealing specified in JIS G 3311 as a special steel strip. Table 9 shows the results.

[0060]

[Table 9]

As shown in Table 9, Test No. As for 71, the rolling reduction in the cold rolling is an appropriate value of 80% or less.

[0062]

According to the method for producing a high-carbon cold-rolled steel strip according to the first aspect of the present invention, the high-carbon hot-rolled steel strip is subjected to cold rolling at a rolling reduction of 25% or more and 80% or less, and 75% by volume or more. Using a bell-type batch annealing furnace in a gas atmosphere in which hydrogen and the remainder substantially consist of nitrogen and unavoidable impurities, heat at a heating rate of 20 to 100 ° C./Hr to 680 ° C. to just below the Ac1 point to maintain a soaking temperature of 6 hours or more. Then, after heating at a heating rate of 100 ° C./Hr or less from Ac1 point to Ac1 point + 50 ° C. and maintaining a soaking temperature of 8 Hr or less,
By performing an annealing treatment of cooling to Ar 1 point or less at a cooling rate of 50 ° C./Hr or less, cementite can be spheroidized in a short time by preventing the generation of pearlite in the cooling process, and a soft high-carbon cold-rolled steel strip can be obtained. Obtainable.

In the method for producing a high-carbon cold-rolled steel strip according to claim 2 of the present invention, the high-carbon hot-rolled steel strip is subjected to cold rolling at a rolling reduction of 25% to 80%, and then 75% by volume or more of hydrogen. And a balance of substantially nitrogen and unavoidable impurities, using a bell-type batch annealing furnace in a gas atmosphere, heating at a heating rate of 20 to 100 ° C./Hr to 680 ° C. to just below the Ac1 point, and maintaining a soaking temperature of 6 hours or more. After heating at a heating rate of 100 ° C./Hr or less from Ac1 point to Ac1 point + 50 ° C. and maintaining a soaking temperature of 8 Hr or less,
After cooling to Ar 1 point or less at a cooling rate of 50 ° C./Hr or less and performing primary annealing, cold rolling is performed at a rolling reduction of 20% or more and 80% or less, and 600 ° C. to A in a bell-type batch annealing furnace.
By performing the finish annealing just below the point c1, the spheroidized soft cold-rolled steel strip is formed to a predetermined thickness, the cold rolling strain is released, crystal grains grow, and the soft high-carbon cold-rolled steel strip is released. You can get a belt.

In the method for producing a high-carbon cold-rolled steel strip according to claim 3 of the present invention, the high-carbon hot-rolled steel strip is subjected to cold rolling at a rolling reduction of 25% or more and 80% or less, and then 75% by volume or more of hydrogen. And a balance of substantially nitrogen and unavoidable impurities, using a bell-type batch annealing furnace in a gas atmosphere, heating at a heating rate of 20 to 100 ° C./Hr to 680 ° C. to just below the Ac1 point, and maintaining a soaking temperature of 6 hours or more. After heating at a heating rate of 100 ° C./Hr or less from Ac1 point to Ac1 point + 50 ° C. and maintaining a soaking temperature of 8 Hr or less,
After cooling to Ar 1 point or less at a cooling rate of 50 ° C./Hr or less and performing primary annealing, cold rolling at a reduction ratio of 20% or more and 80% or less, and 600 ° C. to Ac in a bell-type batch annealing furnace
By repeating the finish annealing just below one point twice or more, a soft ultra-thin high carbon cold rolled steel strip having a thickness of 0.3 mm or less can be obtained.

The method for producing a high-carbon cold-rolled steel strip according to claim 4 of the present invention is characterized in that the high-carbon hot-rolled steel strip is provided with a steel sheet roughness of 0.3 to 1.2 μm at a rolling reduction of 20% to 85%. After performing cold rolling while performing cold rolling, a bell-type batch annealing furnace in a gas atmosphere consisting of 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen and inevitable impurities is used at a heating rate of 20 to 100 ° C./Hr for 68 hours.
After heating from 0 ° C. to just below the Ac1 point and maintaining a soaking temperature of 6 hours or more,
Ac1 point to Ac1 point + at a heating rate of 100 ° C./Hr or less
After heating to 50 ° C and holding soak for 8 hours or less, 50 ° C / Hr
After cooling to the Ar 1 point or lower at the following cooling rate and annealing, and then temper rolling at an elongation of 0.6 to 2.0%, the final product is free from annealing seizure even during intermediate annealing. A soft high-carbon cold-rolled steel strip having a roughness of 0.20 μm or less can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between hydrogen concentration and thermal conductivity at temperatures of 400 ° C., 600 ° C., and 800 ° C.

FIG. 2 is a graph showing a relationship between a soaking time at an atmospheric temperature of an atmosphere gas consisting of 75% of hydrogen and the balance of nitrogen and an atmosphere gas consisting of 5% of hydrogen and a balance of nitrogen, and a temperature difference in a coil.

FIG. 3 is a graph showing a heat pattern of an annealing treatment including a first-stage soaking, a second-stage soaking, and a slow cooling process according to the present invention.

FIG. 4 is a graph showing an example of a typical heat pattern of an annealing process including a first-stage soaking, a second-stage soaking, and a slow cooling process of the present invention.

FIG. 5 is a graph showing a relationship between a rolling reduction and hardness (HV) in cold rolling.

FIG. 6 is a graph showing a relationship between a soaking temperature, a soaking time, and a hardness (HV) during a finish annealing treatment.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-287390 (JP, A) JP-A-5-171288 (JP, A) JP-A-4-28823 (JP, A) 87736 (JP, A) JP-A-9-87805 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/02 C21D 9/52 C21D 9/67 C21D 1/26

Claims (4)

(57) [Claims] 1. C: 0.3-0.8%, Si: 0.03
０．３0.35%, Mn: 0.20 to 1.50%,
After the high carbon steel slab substantially consisting of Fe and unavoidable impurities is subjected to hot rolling, pickling, and descaling, and then cold rolling at a rolling reduction of 25% or more and 80% or less, then 75% by volume or more. High-carbon cold-rolled steel characterized by being annealed using a bell-type batch annealing furnace in a gas atmosphere consisting essentially of nitrogen and unavoidable impurities with hydrogen and the balance being the following (1) to (7): The production method of the belt. (1) Heating rate up to the first stage soaking temperature: 20 ° C / Hr
１００100 ° C./Hr (2) First stage soaking temperature × time: 680 ° C. to just below Ac1 point × 6Hr or more (3) Heating rate up to second stage soaking temperature: 100 ° C./H
r or less (4) Second-stage soaking temperature x time: Ac1 point to Ac1 point +
(5) Cooling rate after completion of second-stage soaking: 50 ° C / Hr or less (6) Ultimate cooling temperature after completion of second-stage soaking: Ar 1 point or less (7) After that, 20 ° C / Cool down to room temperature at Hr-100 ° C / Hr 2. C: 0.3-0.8%, Si: 0.03
０．３0.35%, Mn: 0.20 to 1.50%,
After the high-carbon steel slab substantially consisting of Fe and unavoidable impurities is subjected to hot rolling, pickling, and descaling, cold rolling is performed at a rolling reduction of 25% to 80%, and then 75% by volume or more. Using a bell-type batch annealing furnace in a gas atmosphere in which hydrogen and the balance substantially consist of nitrogen and inevitable impurities, primary annealing is performed under the following annealing conditions (1) to (7),
A method for producing a high-carbon cold-rolled steel strip, comprising: performing cold rolling at a rolling reduction of 20% or more and 80% or less, and performing finish annealing in a bell-type batch annealing furnace at a temperature of 600 ° C. to just below Ac1 point. (1) Heating rate up to the first stage soaking temperature: 20 ° C / Hr
１００100 ° C./Hr (2) First stage soaking temperature × time: 680 ° C. to just below Ac1 point × 6Hr or more (3) Heating rate up to second stage soaking temperature: 100 ° C./H
r or less (4) Second-stage soaking temperature x time: Ac1 point to Ac1 point +
(5) Cooling rate after completion of second-stage soaking: 50 ° C / Hr or less (6) Ultimate cooling temperature after completion of second-stage soaking: Ar 1 point or less (7) After that, 20 ° C / Cool down to room temperature at Hr-100 ° C / Hr 3. After the primary annealing treatment, cold rolling at a rolling reduction of 20% or more and 80% or less and 60% in a bell-type batch annealing furnace.
The method for producing a high-carbon cold-rolled steel strip according to claim 1 or 2, wherein the finish annealing at 0 ° C to just below the Ac1 point is repeated at least twice. 4. C: 0.3-0.8%, Si: 0.03
０．３0.35%, Mn: 0.20 to 1.50%,
The high-carbon steel slab substantially consisting of Fe and unavoidable impurities is hot-rolled, pickled, and descaled, and then cold-rolled at a rolling reduction of 25% or more and 80% or less, and the center of the steel sheet roughness is measured. After imparting an average roughness of 0.3 μm or more and 1.2 μm or less, using a bell-type batch annealing furnace in a gas atmosphere consisting of 75% by volume or more of hydrogen and the balance substantially consisting of nitrogen and unavoidable impurities, the following (1) )-Primary annealing under the annealing conditions of (7), followed by temper rolling at an elongation of 0.6% or more and 4.0% or less, to make the steel sheet roughness 0.2 μm or less. Manufacturing method of carbon cold rolled steel strip. (1) Heating rate up to the first stage soaking temperature: 20 ° C / Hr
１００100 ° C./Hr (2) First stage soaking temperature × time: 680 ° C. to just below Ac1 point × 6Hr or more (3) Heating rate up to second stage soaking temperature: 100 ° C./H
r or less (4) Second-stage soaking temperature x time: Ac1 point to Ac1 point +
(5) Cooling rate after completion of second-stage soaking: 50 ° C / Hr or less (6) Ultimate cooling temperature after completion of second-stage soaking: Ar 1 point or less (7) After that, 20 ° C / Cool down to room temperature at Hr-100 ° C / Hr