KR101242692B1 - High carbon hot/cold rolled steel coil and manufactureing method thereof - Google Patents

Abstract

The present invention relates to a post-process omitted high carbon hot rolled steel sheet having excellent hardening ability capable of satisfying the quality of the final product even after partially omitting a process after hot rolling and a manufacturing method thereof.
In the present invention, in the present embodiment, i) by weight% C: 0.15 to 0.7%, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: than 0 Preparing a high carbon steel comprising greater than 0.05%, S: greater than 0 and less than 0.03%, consisting of balance Fe and other unavoidable impurities; Ii) reheating the steel and then performing hot rolling in an austenite region where the finishing temperature of hot rolling is equal to or higher than the Ar3 transformation temperature to produce a steel sheet; Iii) cooling the steel sheet to 520 ° C. to 670 ° C. before phase transformation starts in a run-out table (ROT); Iii) maintaining the cooling holding temperature uniformly such that the cooled steel sheet undergoes phase transformation at any one of the cooling temperatures; Iii) winding the steel sheet at the cooling holding temperature; provides a method for manufacturing a high carbon hot rolled steel sheet comprising a.

Description

High carbon hot rolled steel sheet, cold rolled steel sheet and manufacturing method thereof {HIGH CARBON HOT / COLD ROLLED STEEL COIL AND MANUFACTUREING METHOD THEREOF}

The present invention relates to a high carbon steel sheet and a method for manufacturing the same, and more particularly, a post-process omitted high carbon hot rolled steel sheet having excellent hardening ability capable of satisfying the quality of the final product without partially eliminating the process after hot rolling. It is about manufacturing method

The high carbon steel sheet refers to steel sheet containing 0.3 wt% or more of carbon and its crystal structure having a pearlite crystal phase. Or it may be classified as a high carbon steel sheet when it contains alloying elements, such as Mn and B, and contains 0.15 weight% or more of carbon.

The high carbon steel sheet has high strength and high hardness after the final process. As such, the high carbon steel sheet is used as tool steel, spring steel, or mechanical structural steel that requires high strength and hardness because of its high strength and high hardness.

A method for producing high carbon steel for spring is described.

High carbon steels for springs are first manufactured from high carbon steels, followed by hot rolling, pickling and nodular annealing. Then, the first cold rolling, heat treatment and pickling are repeated, followed by the second cold rolling to produce high carbon steel for spring.

The reason for pickling after hot rolling is to remove an oxide layer which is inevitably generated in the initial material manufactured by hot rolling. The reason for performing the spheroidizing annealing is to homogenize the uneven structure of the material by hot rolling and to lower the strength of the material so that the primary cold rolling is possible.

In addition, the primary cold rolling is subjected to the primary cold rolling in advance in order to optimize the reduction ratio of the secondary cold rolling. And the heat treatment process after the first cold rolling is carried out under appropriate heat treatment conditions to obtain the desired quality to determine the microstructure of the final product.

After the heat treatment, it is pickled again to remove the additional oxide layer formed on the surface of the steel, and finally, the secondary cold rolling to make the final product of the desired thickness.

However, the manufacturing method of the high carbon steel for spring as described above has to go through a variety of processes even after hot rolling, there is a problem that takes a very large cost and time due to the cost of each process and the logistics between processes.

It has a uniform pearlite structure and a thin carbide layer to provide an excellent high carbon hot rolled steel sheet that can secure both strength and ductility at the same time.

It provides a method for producing a high carbon hot rolled steel sheet that can form a uniform pearlite in the hot rolling process to omit the subsequent heat treatment process.

One embodiment of the present invention is i) by weight: C: 0.15 to 0.7%, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: greater than 0 and 0.05 Preparing a high carbon steel comprising less than%, S: greater than 0 and less than 0.03%, consisting of balance Fe and other unavoidable impurities; Ii) reheating the steel and then performing hot rolling in an austenite region where the finishing temperature of hot rolling is equal to or higher than the Ar3 transformation temperature to produce a steel sheet; Iii) cooling the steel sheet to a cooling temperature of 520 ° C. to 670 ° C. before phase transformation is started in a run-out table (ROT); Iii) maintaining the cooling holding temperature uniformly such that the cooled steel sheet undergoes phase transformation at any one of the cooling temperatures; Iii) winding the steel sheet at the cooling holding temperature; provides a method for manufacturing a high carbon hot rolled steel sheet comprising a.

In the cooling step of the method of manufacturing a high carbon hot rolled steel sheet, the steel sheet has a phase transformation rate of 10% or less during cooling, and the steel sheet is preferably kept uniformly in the range of ± 20 ° C. of the cooling holding temperature. A more preferable range of such cooling holding temperature is ± 5 ° C.

In addition, in the winding step, the phase transformation fraction of the steel sheet is preferably wound at 70% or more.

In the step of maintaining the cooling temperature, the steel sheet passing through the water cooling stand is preferably air cooled at the top and water cooled at the bottom.

In addition, in the hot rolling step, the steel sheet is preferably hot rolled to a thickness of 1.4mm ~ 4.0mm.

And the cooling rate of the steel sheet in the cooling step is preferably 5 ~ 300 ℃ / sec.

In addition, it is preferable to maintain the steel sheet for 5 seconds to 100 seconds in the step of maintaining the cooling temperature.

Another embodiment of the present invention provides a method for manufacturing a high carbon hot rolled steel sheet, which omits any one or more processes selected from a pickling process, a spheroidization annealing process, and a primary cold rolling process for the wound steel sheet.

Another embodiment of the present invention provides a method for manufacturing a high carbon hot rolled steel sheet further comprising the step of skipping the heat treatment process for the wound steel sheet and cold rolling at a reduction ratio of 20% or more.

In another embodiment of the present invention by weight% C: 0.15 to 0.7%, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: greater than 0 and 0.05 High carbon hot rolled steel containing% or less, S: greater than 0 and 0.03% or less, consisting of balance Fe and other unavoidable impurities. The content of Mn and B, which is a hardenability enhancing element, is Mn + 500B> 1.0% Provide the steel sheet.

Herein, the average colony size of such a uniform pearlite phase is preferably 1 to 10 µm.

In addition, it is preferable that the volume fraction of such a uniform pearlite phase is 70% or more, and more preferably, the sum of the volume fractions of the uniform pearlite phase and the bainite phase is 90% or more.

And the Vickers hardness of such a hot-rolled steel sheet is preferably 200 ~ 320 HV and elongation is 12% or more.

Another embodiment of the present invention provides a high carbon cold rolled steel sheet cold rolled high carbon hot rolled steel sheet as described above.

The method of manufacturing a high carbon hot rolled steel sheet according to an embodiment of the present invention has a technical effect of effectively controlling transformation heat generated during phase transformation in a hot rolling process of high carbon steel through a weak cooling pattern of upper air cooling and lower water cooling.

As such, there is a technical effect of effectively controlling the transformation heat generation to produce a uniform pearlite in the hot rolling step.

In addition, by controlling the cooling pattern in the hot rolling process, it is possible to prevent a shape defect or a local overcooling caused by the upper cooling to improve the product quality.

In addition, the effect of the hardenability improving element can suppress the phase transformation during cooling even at a slow cooling rate, thereby obtaining a uniform microstructure.

The high carbon hot rolled steel sheet manufactured according to an embodiment of the present invention can produce a pearlite and a thin carbide layer having a uniform interlayer spacing, thereby providing an excellent high carbon hot rolled steel sheet which can secure strength and ductility at the same time. There is a technical effect.

According to one embodiment of the present invention, a high-carbon hot-rolled steel sheet may produce pearlite having a uniform interlayer spacing, and thus may have a technical effect of omitting a heat treatment step in a subsequent manufacturing process.

In addition to the heat treatment process after hot rolling, there is a technical effect that can further omit the subsequent pickling process, the spheroidizing annealing process, and the primary cold rolling.

By making it possible to omit the subsequent manufacturing process step as described above, it is possible to reduce the cost of the subsequent process when producing the product, it is possible to shorten the manufacturing process time.

In addition, there is also an effect that can prevent the environmental pollution occurring in the pickling process and heat treatment process.

1 is a graph showing a phase transformation temperature and time according to a cooling rate of a phase transformation process of a high carbon hot rolled steel sheet according to an embodiment of the present invention.
Figure 2 is a graph showing the phase transformation temperature and time according to the cooling rate of the phase transformation process of a high carbon hot rolled steel sheet according to a comparative example of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite. As used herein, the term "comprising" embodies a particular characteristic, region, integer, step, operation, element, and / or component, and other specific characteristics, region, integer, step, operation, element, component, and / or group. It does not exclude the presence or addition of.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

In the present invention, the indication of the chemical composition of the constituent elements means weight percent unless otherwise specified.

Hereinafter, embodiments of the present invention will be described in detail. These embodiments are only for illustrating the present invention, and the present invention is not limited thereto.

High-carbon hot-rolled steel sheet according to an embodiment of the present invention by weight% C: 0.15 to 0.7%, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: Greater than 0 and less than or equal to 0.05%, S: greater than 0 and less than or equal to 0.03%, consisting of balance Fe and other unavoidable impurities. At this time, it is preferable that the content structure of Mn and B which are hardenability improvement elements consists of Mn + 500B> 1.0%.

Hereinafter, the reason for limiting the chemical composition of the high carbon hot rolled steel sheet will be described.

First, carbon (C) is demonstrated. Carbon (C) is a component that determines the fraction of high carbon steel microstructure. If the content of carbon (C) is less than 0.15%, even if it contains a hardenability enhancing element, the ferrite structure is formed in the hot rolling process or the carbide layer of pearlite becomes thin, which causes the strength of the structure to be lowered. In the case of containing C in excess of 0.7%, cementite cementite is formed in the hot rolling process or the carbide layer of pearlite becomes too thick, resulting in excessively high strength, which causes deterioration of ductility or lower end product durability. Becomes Therefore, it is preferable to contain carbon (C) in 0.15 to 0.7% of range.

Next, silicon (Si) will be described. Silicon (Si) not only acts as a deoxidizer but also improves strength. However, as the silicon (Si) content increases, the strength may increase, but scale may be formed on the surface of the steel sheet during the hot rolling process or in a subsequent manufacturing process, thereby degrading the surface quality of the product. Therefore, it is preferable to contain silicon (Si) larger than 0 and 0.5% or less.

Next, manganese (Mn) is demonstrated. Manganese (Mn) can improve the hardenability, improve the strength and combine with sulfur (S) to form MnS can suppress the crack generation due to sulfur (S). Therefore, it is necessary to contain at least 0.1% manganese (Mn) to form MnS. However, excessively high content of manganese (Mn) of 2.5% or more causes a decrease in toughness or delayed phase transformation more than necessary. Therefore, it is preferable to contain manganese (Mn) in 0.1 to 2.5% of range.

Next, the boron B will be described. Boron (B) functions to improve the hardenability. However, as the content of boron (B) increases, the hardenability may be increased, but FeB may be generated to cause red brittleness or to form precipitates such as BN to reduce toughness. Therefore, it is preferable to contain boron (B) larger than 0 and 0.01% or less.

Next, phosphorus (P) is demonstrated. Phosphorus (P), when its content exceeds 0.05%, segregates at grain boundaries and causes a decrease in toughness. Therefore, it is preferable that phosphorus (P) is larger than 0 and controls the content to 0.05% or less.

Next, sulfur (S) will be described. When the content of sulfur (S) is more than 0.03%, it precipitates during the manufacturing process to cause embrittlement of the steel. Therefore, it is preferable that sulfur (S) is larger than 0 and controls the content to 0.03% or less.

High-carbon hot-rolled steel sheet according to an embodiment of the present invention is iron (Fe) in addition to the above element components and other inevitable impurities are contained.

Next, the content structure of Mn and B which are hardenability improvement elements is demonstrated. Mn and B are elements that improve the hardenability. If the content of the composition is Mn + 500B <1.0%, the hardenability decreases, causing phase transformation at a slow cooling rate, thereby making it impossible to obtain a uniform microstructure. Therefore, the content composition of Mn and B is preferably Mn + 500B> 1.0%.

Hereinafter, a method of manufacturing the above-described high carbon hot rolled steel sheet will be described.

First, C: 0.15 to 0.7% by weight, Si: greater than 0 and 0.5% or less, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: greater than 0 and 0.05% or less, S: greater than 0 A high carbon steel (such as slab form) is produced which is large and contains 0.03% or less and consists of the balance Fe and other unavoidable impurities.

Next, the manufactured steel is reheated and then hot rolled. Hot rolling is preferably performed in the austenite region where the finishing temperature is equal to or higher than the Ar3 transformation temperature. The reason why the finishing temperature of hot rolling is set as follows is as follows.

If the finish temperature of hot rolling is hot rolled below the Ar3 transformation temperature, cornerstone ferrite or cornerstone cementite is formed, which causes the strength and durability of the final structure to be lowered.

Under such conditions, the steel is hot rolled to produce a thin plate having a thickness of 1.4 mm or more and 4.0 mm or less. The reason for limiting the thickness of the hot rolled steel sheet is that if the thickness of the sheet exceeds 4.0mm, sufficient cooling amount cannot be obtained in the subsequent cooling step and temperature holding step, and thus the phase transformation rate cannot be obtained before winding. During the lower cooling step, the temperature deviation in the thickness direction becomes large and a uniform structure cannot be obtained. In addition, when the thickness of the hot rolled steel sheet is less than 1.4mm, the hot rolling load increases, so that the rolling is not good, and when the final product is manufactured after hot rolling, the cold rolling amount decreases due to the reduction in thickness through cold rolling. The strength of the product becomes low.

Next, the thin plate is preferably cooled to 520 ° C or more and 670 ° C or less before the start of phase transformation by controlled cooling in a water-cooling table (ROT; Run-Out Table). The cooling rate at this time is preferably 5 ~ 300 ℃ / sec. And the reason for cooling the thin plate in such a temperature range is as follows.

If the cooling temperature of the sheet is lower than 520 ˚C, the transformation is not transformed into uniform pearlite, but a large amount is transformed into bainite, which causes the durability of the final product. In addition, when the cooling temperature exceeds 670 ° C coarse pearlite or ferrite is formed to increase the interlayer spacing between the layered carbide causes a decrease in strength.

In addition, in this cooling step, it is necessary to control so that the phase transformation during cooling does not exceed 10%. This is because the phase transformation in the cooling step is not possible to obtain a uniform pearlite structure because the phase transformation occurs at a higher temperature than the temperature holding step.

Next, the cooled thin plate is preferably kept uniformly in the range of ± 20 ° C at any one of the cooling temperature section, and more preferably maintained uniformly in the range of ± 5 ° C. For example, if the thin plate is cooled to 580˚C, which is included in the cooling temperature range of 520˚C ~ 670˚C by cooling, the temperature of the thin plate within the range of 560˚C ~ 600˚C which is ± 20 ℃ of this temperature. It is desirable to maintain.

In the case of high carbon steel, the carbon is contained in the steel so that the temperature of the steel increases due to the transformation heat generation during phase transformation. As described above, when the transformation heat is generated during the phase transformation of the steel sheet, the temperature of the steel sheet rises during air cooling, and thus a uniform structure cannot be obtained.

Therefore, in order to prevent the temperature rise due to transformation heat generation and to maintain the temperature of the steel sheet uniformly, it is necessary to cool the steel sheet. However, if the upper and lower parts of the steel sheet is rapidly cooled in the hot rolling equipment, the cooling rate is increased and the temperature is lowered, resulting in uneven structure. Therefore, in order to prevent the temperature of the steel sheet from becoming uneven in this way, it is preferable to cool the upper part of the steel sheet being sold by air cooling and the lower part by water cooling.

Thus, the upper part of the cooled steel plate is cooled by air and the lower part thereof is water cooled to suppress the temperature rise caused by the transformation heat generation of the steel sheet, thereby maintaining a constant temperature of the steel sheet to undergo a temperature maintaining step of causing a uniform phase transformation. .

When controlled cooling in this way, only the temperature rise corresponding to the transformation heat can be cooled to maintain the temperature in the range of ± 20 ° C. As such, by maintaining the temperature of the steel sheet in phase transformation uniformly, the structure of the steel sheet can be phase-transformed into a uniform pearlite structure.

In addition, by cooling the upper portion of the steel sheet, the steel sheet can prevent the temperature variation in the width direction due to the water cooling, and the local subcooling due to the remaining water. This can reduce the material variation of the steel sheet and prevent the shape from being deteriorated. Therefore, ultimately, the quality of the steel product manufactured using the hot rolled steel sheet can be improved.

As described above, the thin plate is maintained at a constant temperature to complete phase transformation, and then the steel sheet is wound in a coil state in a winding machine. At this time, the coiling temperature is preferably wound directly at the cooling holding temperature of the steel sheet.

And the phase transformation fraction of the steel sheet at the time of winding the steel sheet should be more than 70%. At this time, if the phase transformation fraction is less than 70%, the phase transformation occurs after winding, and the transformation heat is generated, and the phase transformation temperature is continuously increased to obtain a uniform pearlite structure. Moreover, it becomes a cause of winding shape fall by temperature rise and phase transformation. Thus, in order for the phase transformation fraction of a steel plate to be 70% or more, it is preferable to control the cooling temperature holding time of a steel plate to 5 second or more and 60 second or less.

The hot rolled steel sheet manufactured according to the above process can omit all of the following processes, or optionally any one process. Subsequent steps that can be omitted are pickling after hot rolling, spherical annealing, primary cold rolling and heat treatment.

Therefore, in the method of manufacturing high carbon steel according to one embodiment of the present invention, it is preferable to omit the heat treatment step and immediately perform cold rolling on the hot rolled steel sheet manufactured according to the above process.

At this time, the cold rolling of the steel sheet is preferably cold rolling at a reduction ratio of 20% or more. Cold rolling can adjust the reduction ratio according to the required characteristics of the final product to match the thickness of the product and ensure optimal strength and durability.

In the conventional hot rolling process, since a uniform pearlite structure could not be obtained, a uniform pearlite structure had to be made through a post-heat treatment process. However, since the method for manufacturing high carbon steel according to an embodiment of the present invention may form a uniform pearlite structure in the hot rolling process step, it is possible to omit a subsequent process and a heat treatment process for forming a fine pearlite structure.

The cold rolled steel sheet thus prepared is processed into a desired product through a blanking or burring process, and then manufactured into a final product through QT heat treatment (quenching and tempering).

Hereinafter, the structure of the post-process omitted high carbon hot rolled steel sheet manufactured through the above process will be described.

The average colony size of the pearlite uniform in the post-process omitted high carbon hot rolled steel sheet is preferably formed to 1μm to 10μm. At this time, when the colony size is smaller than 1 μm, the fatigue crack retardation effect is lowered. When the colony size is larger than 10 μm, the transformation rate is slowed to prevent the phase transformation fraction before winding.

In the microstructure of the post-process abbreviated high-carbon hot-rolled steel sheet, such a uniform pearlite phase occupies a volume fraction of 70% or more, and it is preferable that the sum of the uniform pearlite phase and the bainite phase is 90% or more.

Since the pearlite in the microstructure plays a role of improving the strength and durability, it is preferable that the pearlite phase has a volume fraction of 70% or more, and the bainite phase plays a role of maintaining high strength, so the sum with the pearlite phase is 90% or more. It is desirable to secure.

In addition, in the microstructure of the post-process omitted high-carbon hot-rolled steel sheet, it is preferable that the ferrite phase deteriorating strength and the martensite structure deteriorating durability do not exceed 10%.

In addition, the post-process omitted high-carbon hot-rolled steel sheet having excellent ductility preferably has a Vickers hardness of 200 HV to 320 HV. A hot rolled steel sheet having such a hardness range can secure an initial strength value necessary for obtaining the strength of the final product after subsequent cold rolling. Moreover, it is preferable that elongation is 12% or more. If the elongation is less than 12%, the ductility is low, which causes a decrease in workability and rolling property in a subsequent step.

Hereinafter, the present invention will be described in more detail with reference to experimental examples. These experimental examples are only for illustrating the present invention, and the present invention is not limited thereto.

<Experimental Example>

In order to investigate the phase transformation rate and physical properties of the post-process omitted high carbon hot rolled steel sheet, a high carbon steel having a composition as shown in Table 1 below was prepared.

division C (wt%) Si (wt%) Mn (wt%) B (wt%) P (wt%) S (wt%) Mn + 500B Example 1 0.2 0.2 One 0.002 0.015 0.005 2.00 Example 2 0.4 0.2 One 0.002 0.015 0.005 2.00 Example 3 0.6 0.2 One 0.002 0.015 0.005 2.00 Comparative Example 4 0.35 0.2 0.7 0.0003 0.015 0.005 0.85 Comparative Example 5 0.55 0.2 0.7 0.0003 0.015 0.005 0.85

After the slabs having the composition shown in Table 1 were prepared, a phase transformation heat treatment process was performed to compare phase transformation rates. Phase transformation velocity measurement was performed by measuring the change in the length of the test piece with temperature.

To this end, the specimen was reheated to 1200 ° C. to make a uniform structure, cooled to 880 ° C., and then the phase transformation start and end time were measured by changing the length of the specimen while cooling to room temperature at a constant cooling rate.

1 and 2 show the phase transformation temperature and time according to the cooling rate of the phase transformation process of Examples and Comparative Examples.

In FIG. 1, reference numeral 1 is a line indicating various cooling rates. The leftmost line represents the cooling rate of 100 ° C / sec and the rightmost line represents the cooling rate of 0.1 ° C / sec. Reference numeral 2 denotes a phase transformation start point, reference numeral 3 denotes a 50% phase transformation point, and reference numeral 4 denotes a phase transformation end point.

As shown in Figure 1 it can be seen that the phase transformation rate of the embodiment is slow compared to the comparative example of Figure 2, it can be confirmed that this is a hardenability effect by Mn and B. Therefore, even in the case of a slow cooling rate of 5 ℃ / sec in the case of the excellent curing ability, the phase transformation did not occur up to 650 ℃.

Meanwhile, in order to evaluate the physical properties of each thin plate, the slabs having the composition shown in Table 1 were reheated to 1170 ° C. and hot rolled to prepare thin plates.

The sheet thickness of the hot rolled steel sheet by hot rolling was 2.01 mm in both the comparative examples and the examples.

As described above, the thin hot rolled sheet was quenched in a water cooling stand, cooled to 550 ° C., and then uniformly maintained in a range of ± 5 ° C., and then wound at the cooling temperature.

As described above, the microstructure, hardness, and elongation of each thin plate manufactured by varying the transformation temperature were measured. The results are shown in Table 2 below.

division
Cooling rate
(℃ / sec) Transformation temperature
(℃) Hardness
(HV) Elongation
(%) Perlite fraction + bainite fraction
(%) Comparative Example 1-1 5 500 ± 5 251 10.7 99 Examples 1-2 5 550 ± 5 220 24 95 Example 1-3 5 600 ± 5 213 24.4 96 Example 1-4 5 650 ± 5 201 26.6 91 Comparative Example 2-1 5 500 ± 5 281 11.6 100 Example 2-2 5 550 ± 5 263 20.3 96 Example 2-3 5 600 ± 5 231 21.3 100 Examples 2-4 5 650 ± 5 210 23.7 98 Comparative Example 3-1 5 500 ± 5 341 13.7 100 Example 3-2 5 550 ± 5 314 18.1 100 Example 3-3 5 600 ± 5 281 19.2 100 Example 3-4 5 650 ± 5 258 20.9 99 Comparative Example 4-1 5 500 ± 5 258 11.3 99 Comparative Example 4-2 5 550 ± 5 238 13.3 87 Comparative Example 4-3 5 600 ± 5 220 15.9 85 Comparative Example 4-4 5 650 ± 5 204 18.5 84 Comparative Example 5-1 5 500 ± 5 322 15.4 99 Comparative Example 5-2 5 550 ± 5 303 18.8 88 Comparative Example 5-3 5 600 ± 5 263 20.6 86 Comparative Example 5-4 5 650 ± 5 238 24.9 86

As can be seen from Table 2, hardness showed an inverse relationship with transformation temperature, and elongation showed a proportional relationship with transformation temperature as opposed to hardness.

On the other hand, when Mn + 500B is 1.0 or more, the hardenability is high, and even at a cooling rate of 5 ° C./sec, a uniform pearlite and bainite structure of 90% or more is formed as in Examples 1 to 3, and both hardness and elongation are excellent. . However, when the transformation temperature is 500 ° C., as in Comparative Examples 1-1, 2-1, and 3-1, the transformation of the bainite structure is increased due to the transformation at too low temperature. As a result, the hardness increases and the elongation decreases. Results in. On the other hand, when Mn + 500B is less than 1.0, as in Comparative Examples 4 to 5, transformation occurs during cooling due to low hardenability. It may cause breakage or lower durability.

While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the following claims. Those who do it will easily understand.

Claims (20)

C: 0.15 to 0.7% by weight, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and less than 0.01% P: greater than 0 and less than 0.05%, S: greater than 0 and 0.03% Preparing a high carbon steel material comprising the following, the balance Fe and other unavoidable impurities;
Reheating the steel and then performing hot rolling in an austenite region where the finishing temperature of hot rolling is equal to or greater than the Ar3 transformation temperature to produce a steel sheet;
Cooling the steel sheet to a cooling temperature of 520 ° C. to 670 ° C. before phase transformation starts in a run-out table (ROT);
Maintaining a cooling holding temperature uniformly such that the cooled steel sheet undergoes a phase transformation at any one of the cooling temperatures; And
Winding the steel sheet at the cooling holding temperature;
The high carbon steel is Mn and B content of the steel is Mn + 500B> 1.0% manufacturing method of high carbon hot rolled steel sheet. delete The method of claim 1,
In the cooling step, the steel sheet is a method of manufacturing a high carbon hot rolled steel sheet having a phase transformation rate of 10% or less during cooling. The method of claim 3,
The method of manufacturing a high carbon hot rolled steel sheet at the cooling holding temperature the steel sheet is uniformly maintained in the range ± 20 ℃ of the cooling holding temperature. The method of claim 3,
The method of manufacturing a high carbon hot rolled steel sheet at the cooling holding temperature the steel sheet is uniformly maintained in the range ± 5 ℃ of the cooling holding temperature. 5. The method of claim 4,
The method of manufacturing a high carbon hot rolled steel sheet in which the phase transformation fraction of the steel sheet is wound at 70% or more in the winding step. The method according to claim 6,
The method of manufacturing a high-carbon hot rolled steel sheet in which the steel sheet passing through the water cooling stand at the step of maintaining the cooling temperature, the upper portion is cooled by air and the lower portion is cooled by water. The method according to any one of claims 1 and 3 to 7,
In the hot rolling step, the steel sheet is hot rolled to a thickness of 1.4mm ~ 4.0mm hot carbon steel sheet manufacturing method. 8. The method according to any one of claims 1 to 7,
Cooling rate of the steel sheet in the cooling step is a manufacturing method of high carbon hot rolled steel sheet 5 ~ 300 ℃ / sec. 8. The method according to any one of claims 1 to 7,
The method of manufacturing a high carbon hot rolled steel sheet for maintaining the steel sheet for 5 seconds to 100 seconds in the step of maintaining the cooling temperature. 8. The method according to any one of claims 1 to 7,
The wound steel sheet is a method of manufacturing a high carbon hot rolled steel sheet omitting any one or more processes selected from a pickling process, a spheroidizing annealing process and a primary cold rolling process. 8. The method according to any one of claims 1 to 7,
The rolled steel sheet is a method of manufacturing a high carbon hot rolled steel sheet further comprising the step of cold rolling at a reduction ratio of 20% or more omitting the heat treatment process. 9. The method of claim 8,
The rolled steel sheet is a method of manufacturing a high carbon hot rolled steel sheet further comprising the step of cold rolling at a reduction ratio of 20% or more omitting the heat treatment process. C: 0.15 to 0.7% by weight, Si: greater than 0 and less than 0.5%, Mn: 0.1 to 2.5%, B: greater than 0 and 0.01% or less, P: greater than 0 and 0.05% or less, S: greater than 0, 0.03 High-carbon hot-rolled steel sheet containing not more than% and consisting of the balance Fe and other unavoidable impurities, and the content of Mn and B, which is a hardenability improving element, of Mn + 500B> 1.0% 15. The method of claim 14,
The high-carbon hot-rolled steel sheet having an average colony size of 1 to 10 μm on the pearlite phase. 16. The method of claim 15,
A high carbon hot rolled steel sheet having a volume fraction of the pearlite phase of 70% or more. 16. The method of claim 15,
A high carbon hot rolled steel sheet in which the sum of the volume fractions of the pearlite phase and the bainite phase is 90% or more. 18. The method according to any one of claims 14 to 17,
Vickers hardness of the hot rolled steel sheet is 200 ~ 320 HV and elongation is 12% or more high carbon hot rolled steel sheet. The high carbon cold rolled steel sheet cold-rolled using the hot-rolled steel sheet of any one of Claims 14-17. A high carbon cold rolled steel sheet manufactured by the manufacturing method of claim 13.

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