JP7229827B2 - Manufacturing method of high carbon steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-carbon steel sheet, and more particularly to a method for producing a high-carbon steel sheet that is soft and has good workability.

High-carbon steel is also called carbon tool steel, and is used for high-hardness parts such as automobile parts such as clutches and chains, blades such as saw blades, washers and springs. In recent years, these high-hardness parts are required to have high dimensional accuracy, and steel sheets before working are required to be uniformly soft.

A high-carbon steel sheet having a C (carbon) content of 0.70 to 1.10% by mass has a pearlite structure with high hardness in the hot-rolled state. However, by subjecting the hot-rolled steel sheet to appropriate annealing in a batch annealing furnace, cementite, which is a carbide, becomes a spheroidized structure (hereinafter referred to as a "spheroidized structure") and can be softened.

In a batch annealing furnace, it is common to insert the steel sheet as a coil into the furnace and perform annealing by heating in a mixed atmosphere containing hydrogen and nitrogen.

In batch annealing, the temperature of the part of the coil that is in contact with the atmosphere is rapidly raised by heat conduction from the atmosphere and radiation from the inner surface of the furnace, and the temperature of the inner part that is not in contact with the atmosphere is raised by heat conduction from the part that was heated earlier. Therefore, the temperature rises with a delay. Therefore, the coil during annealing will inevitably have an outer high-temperature portion and an inner low-temperature portion. Although this temperature difference varies depending on the type of furnace, atmosphere, temperature rising conditions, etc., the temperature difference between the highest temperature portion and the lowest temperature portion is considered to be about 20 to 60°C.

Appropriate annealing is required to obtain a soft spheroidized structure. If the annealing temperature is low or the annealing time is short, the annealing will be insufficient, the pearlite structure will remain, and the softening will be insufficient. On the other hand, when the annealing temperature is high or the annealing time is long, the annealing becomes excessive, cementite is completely dissolved and becomes austenite, and pearlite (recycled pearlite) is formed again during subsequent cooling, resulting in softening. I can't.

From the above, considering the temperature difference that occurs in the coil during batch annealing, it is desired that the softening temperature range be wide.

As a conventional annealing condition for obtaining a spheroidized structure, Patent Document 1 describes heating to Ac ₁ to Ac ₁ +50° C. and then holding for 5 hours or more. In Patent Document 2, the annealing temperature is Ac ₁ or more and 780° C. or less, the annealing time is 2 hours or more and 10 hours or less, and the average cooling rate in the temperature range up to 650° C. is 3° C./h to 20° C./h. It says to cool. In Patent Document 3, in order to promote spheroidization of carbide, annealing is performed at a temperature of Ac ₁ to Ac ₁ +50° C., and after annealing, slow cooling to Ac ₁ −30° C. at a cooling rate of 5° C./h or less is performed. Are listed.

JP 2015-34307 A JP 2012-172228 A JP 2011-168842 A

However, upon further investigation by the inventors of the present application, the annealing conditions described in Patent Document 1 have a slightly narrow range of annealing temperatures, and there is room for improvement in softening the entire coil.

In addition, Patent Documents 2 and 3 describe slow cooling after annealing in order to promote spheroidization of carbides, but slow cooling takes a long time, so there is a problem that productivity is remarkably low. be. In addition, Patent Documents 2 and 3 are directed to steel sheets with a C content of less than 0.70%, and in steel sheets with a C content of 0.70% or more, the entire coil can be softened. No technology was disclosed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a high-carbon steel sheet with a wide annealing temperature range capable of softening the high-carbon steel sheet and excellent productivity. That's what it is.

As a result of various studies, the inventors of the present invention have found that the above objects are achieved by the following inventions.

A method for producing a high-carbon steel sheet according to one aspect of the present invention is a method for producing a high-carbon steel sheet having a C content of 0.70% by mass to 1.10% by mass, wherein the material steel sheet is made _of Ac A step of soaking and holding for 5 to 40 hours in a temperature range of ₁ +60 ° C. or less, and a step of cooling the steel plate after soaking and holding in a temperature range from the soaked temperature to Ac ₁ -50 ° C. In the step of cooling the steel sheet including the annealing step, at least the temperature range from Ac ₁ -5 ° C. to Ac ₁ -10 ° C. in the temperature range from the soaked temperature to Ac ₁ -50 ° C. is 3 ° C. /h or less, and the temperature range other than the slow cooling temperature range is cooled at a rate of 5 to 15°C/h.

According to the above configuration, the annealing temperature range in which the high-carbon steel sheet can be softened is as wide as 60 ° C. Therefore, even in batch annealing where a temperature difference inevitably occurs in the coil made of the high-carbon steel sheet, the entire coil can be made more uniform. can be softened to In addition, since the temperature range for slow cooling is narrow and there is no need for slow cooling over a long period of time, productivity is excellent.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the high carbon steel plate which has the wide annealing temperature range which can soften a high carbon steel plate, and was excellent in productivity can be provided.

The inventors of the present invention have completed the present invention after earnestly studying annealing conditions for widening the annealing temperature range in which high-carbon steel sheets can be softened. Hereinafter, a method for manufacturing a high-carbon steel sheet according to one embodiment of the present invention will be described.

(Component composition of high carbon steel sheet)
First, the chemical composition of the steel sheet manufactured by the method for manufacturing a high-carbon steel sheet according to the present embodiment will be described. Regarding elements other than C, the following component composition is merely an example in the present embodiment, and does not limit the present invention. All "%" in the following component compositions mean "% by mass".

[C: 0.70 to 1.10%]
C is an important element for ensuring the strength of the steel sheet. The lower limit of the C content is made 0.70% or more in order to secure the required steel plate strength. If the C content is less than 0.70%, the hardenability of the steel sheet deteriorates, and strength as a high-hardness part cannot be obtained. The lower limit of the C content is preferably 0.55% or more. However, if the C content becomes excessive, a long time is required for the heat treatment for ensuring the toughness and workability of the steel sheet. Therefore, the upper limit of the C content is made 1.10% or less. The upper limit of the C content is preferably 1.00% or less.

[Si: 0.001 to 1.5%]
Si is an effective element for improving the strength of the final product by solid solution strengthening and increasing resistance to temper softening. In order to obtain such effects, the lower limit of the Si content is preferably 0.001% or more. However, when the Si content is excessive, the ferrite is excessively hardened due to solid-solution strengthening, which causes cracks during working of the steel sheet. In addition, it promotes the generation of scale defects on the surface of the steel sheet during the manufacturing process of the steel sheet, and causes deterioration of the surface quality of the steel sheet. Therefore, the upper limit of the Si content is preferably 1.5% or less, more preferably 0.5% or less.

[Mn: 0.40 to 2.0%]
Mn is an element that acts as a deoxidizing agent for steel and is effective in improving hardenability. In order to obtain such effects, the lower limit of the Mn content is preferably 0.40% or more, more preferably 0.50% or more. However, if the Mn content is excessive, the hardness of the hot-rolled steel sheet becomes too high, making cold rolling difficult. Therefore, the upper limit of the Mn content is preferably 2.0% or less, more preferably 1.5% or less.

[P: 0.03% or less]
P is a solid-solution strengthening element and is an element effective for increasing the strength of the steel sheet. However, when the P content becomes excessive, the toughness of the steel sheet is lowered. Therefore, the upper limit of the P content is preferably 0.03% or less, more preferably 0.02% or less. There is no particular need to define the lower limit of the P content. However, excessively reducing the P content causes an increase in steel refining costs, so the lower limit of the P content may be 0.005% or more.

[S: 0.010% or less]
S forms non-metallic inclusions in steel and lowers the workability of the steel sheet and the toughness of the steel sheet after heat treatment. Therefore, the upper limit of the S content is preferably 0.010% or less, more preferably 0.004% or less. There is no particular need to define the lower limit of the S content. However, excessively reducing the S content causes a significant increase in steel refining costs, so the lower limit of the S content may be 0.0001% or more.

[Al: 0.001 to 0.060%]
Al is an element that acts as a deoxidizing agent for steel, fixes solid solution N present in the steel as AlN, and improves the cold workability of the steel sheet. In order to obtain such effects, the lower limit of the Al content is preferably 0.001% or more, more preferably 0.005% or more. However, when the Al content becomes excessive, Al ₂ O ₃ as inclusions in the steel is excessively generated, and the cold workability of the steel sheet may deteriorate. Therefore, the upper limit of the Al content is preferably 0.060% or less, more preferably 0.050% or less.

[Ni: 0.005 to 0.20%]
Ni is an element effective in improving the hardenability of steel and improving the low-temperature toughness. In order to obtain such effects, the lower limit of the Ni content is preferably 0.005% or more, more preferably 0.05% or more. Ni also has the effect of canceling out the adverse effects of molten metal embrittlement caused by Cu when Cu is contained in steel. When Cu is contained in steel, it is effective to contain about the same amount of Ni as the Cu content in order to suppress the occurrence of molten metal embrittlement caused by Cu. However, Ni is expensive as an alloying element of steel, and excessive inclusion causes an increase in the cost of the steel sheet. Therefore, the upper limit of the Ni content is preferably 0.2%, more preferably 0.15%. %.

[Cr: 0.005 to 1.0%]
Cr is an element effective in improving the hardenability and temper softening resistance of steel. In order to obtain such effects, the lower limit of the Cr content is preferably 0.005% or more, more preferably 0.05% or more. However, if the Cr content is excessive, when the steel sheet is annealed to obtain a structure in which cementite, which is a carbide, is spheroidized, the carbide becomes difficult to dissolve, making it difficult to soften the steel sheet. Therefore, the upper limit of the Cr content is preferably 1.0%, more preferably 0.8%.

[Mo: 0.005 to 0.5%]
Mo, even in small amounts, is an element effective in improving the hardenability and temper softening resistance of steel in the same way as Cr. In order to obtain such effects, the lower limit of the Mo content is preferably 0.005% or more, more preferably 0.05%. However, if the Mo content is excessive, it becomes difficult to soften the steel sheet by annealing, and the cold workability before quenching may rather deteriorate. Therefore, the upper limit of the Mo content is preferably 0.5% or less, more preferably 0.3%.

[Nb: 0.005 to 0.050%]
Nb is an element that forms carbonitrides in steel and is effective in preventing grain coarsening and improving toughness of steel. In order to stably obtain such effects, the lower limit of the Nb content is preferably 0.005% or more, more preferably 0.01% or more. However, when the Nb content exceeds a certain amount, the effect of Nb is saturated. Therefore, the upper limit of the Nb content is preferably 0.050% or less, more preferably 0.030%.

[Ti: 0.005 to 0.050%]
Ti is an element used for deoxidizing and adjusting molten steel, and also has a denitrifying effect. In addition, since N dissolved in the steel plate is fixed as a nitride, when B is included in the steel for the purpose of improving the hardenability of the steel, Ti is also included to improve the hardenability of the steel. An effective amount of B required for improvement can be secured. In order to stably obtain such effects, the lower limit of the Ti content is preferably 0.005% or more, more preferably 0.01% or more. However, when the Ti content exceeds a certain amount, the effect of Ti saturates. Therefore, the upper limit of the Ti content is preferably 0.050%, more preferably 0.030%.

[V: 0.001 to 0.3%]
V, like Nb, forms carbonitrides in steel and is an element effective in preventing grain coarsening and improving toughness of steel. In order to stably obtain such effects, the lower limit of the V content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.02% or more. However, when the V content exceeds a certain amount, the effect of V saturates. Therefore, the upper limit of the V content is preferably 0.3% or less, more preferably 0.2%.

[N: 0.01% or less]
N is an element that forms nitrides in steel. If the N content in the steel is excessive, nitrides may precipitate during bending straightening of the slab in a curved continuous casting machine, and cracks may occur in the slab. The upper limit of the N content is preferably 0.01% or less, more preferably 0.007%, in order to suppress the occurrence of cracks in the slab caused by such nitrides. However, excessively reducing the N content causes an increase in steel refining costs, so the lower limit of the N content is preferably 0.001% or more, more preferably 0.003% or more.

[O: 0.0025% or less]
O is an element that forms oxides in steel. Agglomeration and coarsening of oxides in steel reduces the ductility of the steel sheet. Therefore, the upper limit of the O content is preferably 0.0025% or less. It is preferable that the amount of O is small, but since it is technically difficult to reduce the O content excessively, an O content of 0.0001% or more is acceptable.

[Other elements]
When scrap is used as a raw material for melting a steel sheet applied to the method for manufacturing a high-carbon steel sheet according to the present embodiment, elements such as Sn, Sb, As, Zn, and Zr are mixed as unavoidable impurities. In the present embodiment, these elements are allowed to be mixed within a range that does not impair the properties of the high-carbon steel sheet manufactured by the manufacturing method according to the present embodiment. The same applies to elements other than Sn, Sb, As, Zn and Zr.

(Manufacturing method of high-carbon steel sheet)
A method for manufacturing a high-carbon steel sheet according to an embodiment of the present invention will be described. If the method for manufacturing a high-carbon steel sheet according to the present embodiment includes an annealing step having a step of soaking and holding the material steel sheet and a step of cooling the steel sheet after soaking and holding, the other steps are can be a common process in the manufacture of

The material steel sheet to be subjected to the annealing process may be any material as long as the C content after the annealing process is 0.70 to 1.10%. Hot-rolled steel can be used.

(Casting process and hot rolling process)
First, a steel material (slab) for rolling having the above composition is produced. The slab can be prepared by any known method. As a method for producing a slab, for example, a method of producing a slab by melting steel having the above composition and performing ingot casting or continuous casting can be applied. If necessary, a cast material obtained by ingot casting or continuous casting may be bloomed to obtain a slab.

The slab obtained is preferably heated to 1200-1350° C. and subjected to hot rolling. If the heating temperature exceeds 1350° C., decarburization (deC) of the surface of the slab and its vicinity becomes significant in the heating process, and the hardenability of the surface of the obtained steel sheet deteriorates. If the heating temperature is less than 1200° C., the deformation resistance during rolling of the slab increases and the load on the rolling equipment increases. The lower limit of the slab heating temperature is more preferably 1225°C or higher, and the upper limit is more preferably 1325°C.

The lower limit of the finish temperature of hot rolling (hereinafter referred to as “hot rolling finish temperature”) is preferably 830° C. or higher from the viewpoint of surface defects in addition to the productivity and thickness accuracy of the steel sheet. If the hot-rolling finish temperature is lower than 830°C, many defects on the surface of the steel sheet due to seizure occur. On the other hand, if the hot-rolling finish temperature exceeds 910° C., the frequency of defects due to scale formed by hot rolling increases, the product yield decreases, and the cost increases. Therefore, the upper limit of the hot rolling end temperature is preferably 910°C.

The coiling temperature of the hot rolled steel sheet after hot rolling is preferably 400 to 700°C. If the coiling temperature is lower than 400° C., martensite transformation occurs in a part of the steel sheet, excessively increasing the strength of the steel sheet, and increasing the risk of breaking the steel sheet in subsequent steps. On the other hand, if the coiling temperature exceeds 700° C., the scale becomes thicker and the pickling property of the steel sheet deteriorates. After hot rolling, scales are removed in a pickling process to obtain a material steel sheet. The material steel plate may be in any state, and may be wound into a coil.

(annealing process)
Among steel sheet manufacturing processes, an annealing process usually includes a process of heating a material steel sheet, a process of soaking and holding the material steel sheet, and a process of cooling the steel sheet after soaking and holding. In the method for manufacturing a high-carbon steel sheet according to the present embodiment, in the annealing process, the step of soaking and holding the material steel sheet, and the temperature range from the temperature at which the steel sheet after soaking and holding to Ac ₁ -50 ° C. It is characterized by the step of cooling in

In the annealing step, annealing is usually performed in an atmosphere of hydrogen, nitrogen, or a mixture thereof, but the composition of the atmosphere is not particularly limited.

Also, the type of furnace used in the annealing process is not particularly limited. Batch annealing furnaces in the form of, for example, bell furnaces can be used.

(heating)
First, the material steel plate is heated. In the method for manufacturing a high-carbon steel sheet according to the present embodiment, the heating rate of the steel sheet before soaking and holding in the annealing step is not particularly specified. However, when heating the coil using a batch annealing furnace, if the heating rate is too high, the heat conduction from the coil surface to the inside of the coil cannot keep up with the heating rate, and the temperature difference between the high temperature part and the low temperature part of the coil increases. This may cause uneven annealing and the resulting uneven hardness (softening) of the steel sheet. Therefore, when heating the coil using a batch annealing furnace, the upper limit of the heating rate is preferably 100° C./h or less. On the other hand, if the heating rate is too low, the productivity will decrease significantly, so the lower limit of the heating rate is preferably 10° C./h or more.

(Keeping uniform heat)
In the method for manufacturing a high-carbon steel sheet according to the present embodiment, in the step of soaking and holding the material steel sheet, the material steel sheet after heating is held in a temperature range of Ac ₁ to Ac ₁ +60° C. for 5 to 40 hours. When the temperature for soaking (hereinafter referred to as "soaking temperature") is less than Ac ₁ , and when the soaking temperature is Ac ₁ + 60 ° C. or less and the time for soaking (hereinafter "soaking time") ) is less than 5 hours, the growth of the structure in which pearlite remains or the spheroidal carbide in which cementite is spheroidized is insufficient, resulting in a structure in which the spheroidal carbide is finely dispersed, so that the steel sheet is not softened. be enough. When the soaking temperature exceeds Ac ₁ +60 ° C., and when the soaking temperature is Ac ₁ or higher and the soaking time exceeds 40 hours, the dissolution of carbides in the steel progresses. However, the reverse transformation to the austenite phase proceeds excessively, pearlite (regenerated pearlite) is generated in subsequent cooling, and the softening of the steel sheet becomes insufficient. Moreover, when the soaking time is set to a time exceeding 40 hours, the productivity of the steel sheet is lowered.

Ac ₁ can be calculated from the following formula (1). In the formula (1), (% elemental symbol) is the content (% by mass) of the element corresponding to the elemental symbol in the steel.
Ac ₁ (° C.)=723−10.7(%Mn)−16.9(%Ni)+29.1(%Si)+16.9(%Cr) (1)

When a coil is annealed in a batch annealing furnace, the surface of the coil and the vicinity thereof (hereinafter referred to as "near the surface") are high in temperature and the inside of the coil is low in temperature, resulting in a temperature difference (temperature distribution). In this embodiment, it is desirable to control the temperature distribution inside the annealing furnace so that both the high temperature portion and the low temperature portion of the coil are included in the above temperature range. As a method of obtaining the temperature distribution in the coil, there are a method of actually measuring by inserting a thermocouple into each part of the coil, and a method of calculating by numerical simulation.

(cooling)
In the method for manufacturing a high-carbon steel sheet according to the present embodiment, the step of cooling the steel sheet after soaking and holding in a temperature range from the temperature at which the soaking and holding is performed to Ac ₁ -50 ° C. (hereinafter referred to as "initial cooling temperature range"). is performed under the following conditions. In the initial cooling temperature range, the temperature range from at least Ac ₁ -5°C to Ac ₁ -10°C is defined as the "slow cooling temperature range". In the slow cooling temperature range, the cooling rate is set to 3° C./h or less. As a result, the spheroidal carbides grow sufficiently to obtain a softened steel sheet.

If the cooling rate in the slow cooling temperature range exceeds 3°C/h, the steel sheet cannot be softened because pearlite is formed during cooling. The upper limit of the cooling rate in the slow cooling temperature range is preferably 2.0° C./h or less, more preferably 1.5° C./h or less. The lower limit of the cooling rate in the slow cooling temperature range is preferably 0.5° C./h or more, more preferably 0.8° C./h or more.

If the slow cooling temperature range includes the temperature range from Ac ₁ -5 ° C. to Ac ₁ -10 ° C. within the initial cooling temperature range, it is higher than the temperature range from Ac ₁ -5 ° C. to Ac ₁ -10 ° C. It can be wide. From the viewpoint of productivity, the upper limit of the slow cooling temperature range is preferably Ac ₁ +40°C, more preferably Ac ₁ +20°C, and even more preferably Ac ₁ +10°C. The lower limit of the slow cooling temperature range is preferably Ac ₁ -40°C, more preferably Ac ₁ -30°C, still more preferably Ac ₁ -20°C.

In the initial cooling temperature range, the cooling rate is set at 5 to 15° C./h except for the slow cooling temperature range. If the cooling rate in the initial cooling temperature range other than the slow cooling temperature range is less than 5 ° C./h, a long time is required for cooling, productivity decreases, and the cooling rate outside the slow cooling temperature range is 15 ° C./h. If h is exceeded, the growth of spheroidal carbide becomes insufficient and the steel sheet cannot be softened. The upper limit of the cooling rate in the initial cooling temperature range other than the slow cooling temperature range is preferably 13° C./h or less, and the lower limit is preferably 7° C./h or more.

The total cooling time required for cooling the steel sheet in the initial cooling temperature range (from the soaked temperature to Ac ₁ -50°C) is preferably within 40 hours from the viewpoint of productivity.

In a batch annealing furnace during coil cooling, the portion of the coil that is in direct contact with the atmosphere cools the fastest. The inside of the coil is at a lower temperature than the vicinity of the surface of the coil during soaking, but is cooled after the vicinity of the surface of the coil cools during cooling.

Even if a high-temperature portion and a low-temperature portion are generated in the coil during uniform heating, both the high-temperature portion and the low-temperature portion inevitably pass through the slow cooling temperature region during cooling. Therefore, the cooling rate is controlled to be 3° C./h or less when passing through the slow cooling temperature range, and the cooling rate is controlled to be 5 to 15° C./h in the initial cooling temperature range other than the slow cooling temperature range.

The slow cooling temperature range of the high temperature portion of the coil and the slow cooling temperature range of the low temperature portion may be different as long as they include Ac ₁ -5°C to Ac ₁ -10°C.

After cooling in the initial cooling temperature range, the coil is cooled to room temperature under arbitrary conditions without reheating to Ac ₁ −50° C. or higher.

According to the method for manufacturing a high-carbon steel sheet according to the present embodiment, a structure in which spherical carbides are sufficiently grown can be obtained in a wide soaking temperature range of 60°C. Therefore, a softened steel sheet can be obtained, and even when the coil is annealed using a batch annealing furnace, the whole can be uniformly softened. In addition, the slow cooling temperature range of the initial cooling temperature range is narrow, and there is no need to carry out slow cooling over a long period of time, so the productivity is excellent. By processing the high-carbon steel sheet obtained by the method for manufacturing a high-carbon steel sheet according to the present embodiment, automobile parts such as clutches and chains with high dimensional accuracy, cutlery such as saw blades, high-hardness parts such as washers and springs, etc. can be manufactured.

Hereinafter, examples of the present invention will be described, but the conditions of the examples are one example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is limited to this one example of conditions. not something. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

(Test condition)
After heating the slab obtained by melting the steel to 1250 ° C., hot rolling is performed, the finishing temperature is 890 ° C., the coiling temperature is 620 ° C., and the thickness and chemical composition shown in Table 1 are obtained. A hot-rolled steel sheet was obtained. Of the chemical compositions shown in Table 1, the elements marked with "-" mean that the elements were not added in the steel smelting process.

The obtained hot-rolled steel sheet was cut into a predetermined size and pickled to obtain a sample. The samples after pickling were annealed under the annealing conditions shown in Table 2 (Test Nos. 1 to 43) using an atmosphere annealing furnace. Table 2 shows the soaking conditions, the soaking temperature ("soaking temperature" in Table 2), the soaking time ("holding time" in Table 2), the soaking temperature, and the Ac ₁ of each steel type. difference (“soaking - Ac ₁ ” in Table 2).

In addition, regarding the cooling conditions in the initial cooling temperature range from the soaking temperature to Ac ₁ -50 ° C, cooling in a temperature range higher than the slow cooling temperature range is "cooling 1", cooling in the slow cooling temperature range "Slow cooling" and cooling in a temperature range lower than the slow cooling temperature range as "Cooling 3" are described as the start temperature, end temperature and cooling rate, respectively. For "slow cooling", the difference from Ac ₁ was noted for each of the start and end temperatures. In addition, the time required for cooling in the initial cooling temperature range from the soaking temperature to Ac ₁ -50° C. (“total cooling time” in Table 2) is shown.

When "slow cooling" was performed from the soaking temperature without performing "cooling 1", "-" was written in each column of "cooling 1". In addition, for comparative examples in which slow cooling was not performed, the start temperature, end temperature and cooling rate were collectively described in "Cooling 1", and "-" was described in each column of "Cooling 3".

Among the comparative examples, Test No. Regarding No. 16, it was described as "furnace cooling (220°C/h)" because it was cooled to room temperature at a cooling rate of 220°C/h in an atmosphere annealing furnace as "cooling 3".

The annealing atmosphere was 100% nitrogen. The samples were temperature-measured during annealing. Temperature measurements were made by attaching a thermocouple directly to the sample.

(evaluation)
The samples after annealing were evaluated using the Vickers hardness as an evaluation index.

(Method for measuring Vickers hardness and evaluation conditions)
The Vickers hardness was measured according to the Vickers hardness test method specified in JIS Z 2244:2009. Specifically, the sample after annealing was embedded in resin, and the cross section in the thickness direction was measured with a load of 5 kg. The measurement position is the depth t/2 of the cross section in the thickness direction of each sample (the center in the thickness direction) for the thickness t of each sample, and the Vickers hardness where n is 2 for each measurement position was measured. The average value of the two points is shown in Table 2 as the Vickers hardness (HV) of the sample.

When the Vickers hardness was 170 or less, the softening was sufficient and the sample was accepted. In addition, as described above, assuming the temperature difference between the high temperature part and the low temperature part in the coil when the coil is batch annealed, in the range of the soaking holding temperature difference of 30 ° C., the Vickers hardness HV of 170 or less It was also confirmed whether the hardness can be secured.

(Discussion)
From the values of the Vickers hardness HV shown together with the test conditions in Table 2, the following considerations can be made.

Test no. 1 to 8, 19 to 25, 34 to 37, and 39 to 43 are examples of the present invention that satisfy the requirements defined in the present invention. From these results, it can be seen that the Vickers hardness of both steel type 1 and steel type 2 can be made 170 or less according to the method for producing a high carbon steel sheet of the present invention.

Among the examples of the present invention, Test No. In each group of 1 to 5, 19 to 22, 23 to 25, 34 to 37, and 39 to 43, only the soaking temperature is changed within the range of Ac ₁ to Ac ₁ +50 ° C. within the range of at least 30 ° C. gone. This assumes a temperature difference between a high-temperature portion and a low-temperature portion that occurs in the coil when the coil is batch annealed. Of these, Test No. In each of groups 34 to 37 and 39 to 43, cooling 1 was not performed after soaking, and slow cooling was performed from the soaking holding temperature. From the results of each of these groups, according to the high-carbon steel sheet manufacturing method of the present invention, the soaking holding temperature difference is 170 or less in the entire range of 30 ° C., that is, the entire coil in both steel grade 1 and steel grade 2 It can be seen that Vickers hardness can be ensured.

Also, test no. Nos. 7 and 8 are examples in which the start temperature and end temperature of slow cooling were increased by 10° C. or 20° C. compared with other examples of the present invention. From these results, even if the slow cooling temperature range is shifted, a softened steel sheet with a Vickers hardness of 170 or less can be obtained by including the temperature range from Ac ₁ -5 ° C. to Ac ₁ -10 ° C. I understand.

Test no. 9 to 18, 26 to 33, and 38 are comparative examples that do not satisfy the requirements defined in the present invention.

Of these, Test No. 11 to 15 and 26 to 30 are examples in which slow cooling was not performed in cooling in the initial cooling temperature range.

Test no. In Nos. 11 to 15, slow cooling was not performed on steel grade 1 steel sheets. Of these, Test No. 15 had a soak temperature relatively close to Ac ₁ . However, a soft steel sheet with a Vickers hardness of 170 or less could not be obtained. This is probably because steel type 1 has a high Cr content, so carbides are difficult to dissolve, and slow cooling was not performed and the cooling rate was high, so that spherical carbides did not grow sufficiently. Test no. The reason why it was not possible to obtain a soft steel sheet with a Vickers hardness of 170 or less in Test Nos. 11 to 14 was It is considered that the recycled pearlite was generated because the soaking temperature was higher than that of No. 15 and the cooling rate was faster.

Test no. In Nos. 26 to 30, slow cooling was not performed on steel grade 2 steel sheets. Of these, Test No. In No. 29, a soft steel sheet with a Vickers hardness of 170 or less could be obtained by selecting an appropriate soaking temperature without slow cooling. However, the soaking holding temperature is the test No. Test no. 26-28 and test no. Test no. With 30, a soft steel sheet with a Vickers hardness of 170 or less could not be obtained.

Test no. 9, 10, 16 to 18, and 31 are examples in which slow cooling was performed in the cooling in the initial cooling temperature range.

Test no. Test No. 9 has a slow cooling end temperature of Ac ₁ -3.0°C. In No. 10, the slow cooling end temperature was Ac ₁ +7.0° C., the slow cooling temperature range did not satisfy the stipulations of the present invention, and a soft steel sheet with a Vickers hardness of 170 or less could not be obtained. This is considered to be due to the generation of recycled perlite.

Test no. In No. 16, in cooling in the initial cooling temperature range, the cooling rate outside the slow cooling temperature range exceeds 15 ° C./h, does not satisfy the provisions of the present invention, and a soft steel sheet with a Vickers hardness of 170 or less can be obtained. I didn't. This is considered to be due to the generation of recycled perlite.

Test no. Test No. 17 has a soaking holding time of less than 5 hours. In Nos. 18 and 31, the soaking temperature was lower than Ac ₁ , neither satisfied the requirements of the present invention, and a soft steel sheet with a Vickers hardness of 170 or less could not be obtained. This is probably because the spheroidal carbide did not grow sufficiently, resulting in a structure in which the spheroidal carbide was finely dispersed.

Test no. 32 and 33 are test No. 1 steel plates. Reference numerals 38 and 39 are examples in which the steel plate of steel grade 2 was slowly cooled from the soaking holding temperature without performing cooling 1 after soaking. Test no. All of Nos. 32, 33, 38 and 39 have soaking holding temperatures lower than Ac ₁ and do not meet the requirements of the present invention. In Nos. 32, 33 and 38, a soft steel sheet having a Vickers hardness of 170 or less could not be obtained.

Claims (1)

A method for producing a high-carbon steel sheet having a C content of 0.70% by mass to 1.10% by mass,
A step of soaking and holding the material steel plate in a temperature range of Ac ₁ or more and Ac ₁ +60 ° C. or less for 5 to 40 hours;
cooling the steel plate after soaking in a temperature range from the soaking temperature to Ac ₁ -50 ° C.,
In the step of cooling the steel sheet, in the temperature range from the soaked temperature to Ac ₁ -50 ° C., at least the temperature range from Ac ₁ -5 ° C. to Ac ₁ -10 ° C. at a rate of 3 ° C./h or less and slowly cooling the temperature range other than the temperature range to be slowly cooled at a rate of 5 to 15 ° C./h,
A method for producing a high-carbon steel sheet , wherein the temperature range other than the slow cooling temperature range includes at least a temperature range from Ac ₁ -40°C to Ac ₁ -50°C.