KR20160063172A

KR20160063172A - High carbon steel sheet and method of manufacturing the same

Info

Publication number: KR20160063172A
Application number: KR1020140166862A
Authority: KR
Inventors: 정준호; 나만호; 허성열
Original assignee: 현대제철 주식회사
Priority date: 2014-11-26
Filing date: 2014-11-26
Publication date: 2016-06-03

Abstract

Disclosed are a high-carbon steel sheet and a method of manufacturing the same. The method of manufacturing a high-carbon steel sheet includes the steps of: hot rolling a slab sheet consisting, by weight %, of carbon (C): 0.35 to 1.2%, silicon (Si): 1.5% or less, manganese (Mn): 12.5% or less, phosphorus (P): 0.03% or less, sulfur (S): 0.02% or less, soluble aluminum (sol. Al): 0.05% or less, nitrogen (N): 0.01% or less and the balance being iron (Fe) and inevitable impurities at a temperature higher than or equal to Arcm; firstly cooling the hot rolled sheet to the ferrite region; secondly cooling the firstly cooled sheet for 2 to 10 seconds at an average cooling speed lower than or equal to 5°C/sec.; cooling the secondly cooled sheet to the pearlite region and rolling the same; and spheroidization annealing the rolled sheet at a temperature lower than or equal to Ac1.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon steel sheet,

More particularly, the present invention relates to a high carbon steel sheet having excellent strength and elongation, and a method of manufacturing the same.

High carbon steel is used in all industries such as automobiles and machine parts because of its low price and easy control of materials by heat treatment. High carbon steels typically undergo hot rolling, cold rolling and spheroidizing annealing. Thereafter, final rolling is performed through secondary cold rolling, blanking is performed on the product shape, and final material is formed by Q / T heat treatment (Quenching & Tempering). However, in the case of high carbon steel, it is very difficult to control the coiling temperature due to an extreme exothermic reaction due to the pearlite transformation occurring in the cooling process after the hot rolling.

As a background technique related to the present invention, there is a high carbon steel manufacturing method disclosed in Korean Patent Laid-Open Publication No. 10-2014-0041279 (published April 04, 2014).

An object of the present invention is to provide a high carbon steel sheet excellent in strength and elongation through control of alloy components and process control and a method for manufacturing the same.

In order to accomplish the above object, a method for manufacturing a high carbon steel sheet according to an embodiment of the present invention is characterized in that the carbon steel is 0.35 to 1.2% by weight, the silicon steel is 1.5% or less, the manganese (Mn) is 12.5% (P): not more than 0.03%, sulfur (S): not more than 0.02%, soluble aluminum (sol.Al): not more than 0.05%, nitrogen (N): not more than 0.01% Hot rolling the slab plate at a temperature equal to or greater than Arcm; Firstly cooling the hot-rolled plate to a ferrite region; Secondarily cooling the primary cooled plate at an average cooling rate of 5 DEG C / sec or less for 2 to 10 seconds; Cooling the secondary cooled plate to a pearlite region and winding it; And spheroidizing and annealing the rolled sheet material at a temperature equal to or less than Ac1.

At this time, the slab plate may further include not less than 2.0% by weight of at least one of chromium (Cr) and molybdenum (Mo).

Further, the primary cooling may be performed at 650 to 700 占 폚 at an average cooling rate of 50 占 폚 / sec or more.

Further, the secondary cooling may be performed by an air cooling method.

Also, the tertiary cooling may be performed at 550 to 640 ° C at an average cooling rate of 70 to 80 ° C / sec.

The spheroidizing annealing may be performed for 10 to 20 hours at a temperature range of Ac1-100 ° C to Ac1.

In order to achieve the above object, a high carbon steel sheet according to an embodiment of the present invention includes 0.35 to 1.2% of carbon (C), 1.5% or less of silicon (Si), 12.5% or less of manganese (Mn) (P): not more than 0.03%, sulfur (S): not more than 0.02%, soluble aluminum (sol.Al): not more than 0.05%, nitrogen (N): not more than 0.01% A tensile strength of 500 MPa or more, and an elongation of 12% or more.

At this time, the high carbon steel sheet may further include not less than 2.0% by weight of at least one of chromium (Cr) and molybdenum (Mo).

According to the method for manufacturing a high carbon steel sheet according to the present invention, microstructure coarsening caused by transformation heat can be prevented through primary cooling, secondary cooling and tertiary cooling control in addition to the control of alloy components. It is possible to form a sufficient spheroidized structure even through the spheroidizing annealing of the carbon steel sheet, and thus a high carbon steel sheet having both excellent strength and elongation can be produced.

1 is a flowchart schematically showing a method of manufacturing a high carbon steel sheet according to an embodiment of the present invention.
2 shows the microstructure of specimen 1 (comparative steel).
Fig. 3 shows the microstructure of specimen 2 (invention steel).

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a high carbon steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

High carbon steel plate

The high carbon steel sheet according to the present invention contains 0.35 to 1.2% of carbon (C), 1.5% or less of silicon (Si), 12.5% or less of manganese (Mn), 0.03% or less of phosphorus (P) (S): not more than 0.02%, soluble aluminum (sol.Al): not more than 0.05%, and nitrogen (N): not more than 0.01%.

In addition, the high carbon steel sheet according to the present invention may further comprise at least one of chromium (Cr) and molybdenum (Mo) in an amount of 2.0 wt% or less.

In addition to the above components, the remainder is composed of iron (Fe) and impurities inevitably included in the steelmaking process.

Hereinafter, the role and content of each component contained in the high carbon steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is added to ensure strength and hardness.

Carbon is preferably added in an amount of 0.35 to 1.2% by weight based on the total weight of the steel sheet. When the addition amount of carbon is less than 0.35% by weight, it is difficult to secure sufficient strength. On the other hand, when the addition amount of carbon exceeds 1.2% by weight, it is difficult to secure an elongation of 12% or more.

Silicon (Si)

Silicon acts as a deoxidizer and is also effective in improving the toughness and ductility of steel by inducing ferrite formation as a ferrite stabilizing element.

The silicon is preferably added in an amount of 1.5% by weight or less, more preferably 0.5% by weight or less, and more preferably 0.3% by weight or less based on the total weight of the steel sheet. If the content of silicon exceeds 1.5% by weight, the weldability of the steel may be impaired and the surface quality may be lowered by generating a large amount of red scales during hot rolling.

Manganese (Mn)

Manganese (Mn) is a substitutional element having an atomic diameter similar to iron (Fe), and is an element highly effective for solid solution strengthening. Manganese also plays a role in improving the hardenability of the steel.

The manganese is preferably added in an amount of 12.5% by weight or less based on the total weight of the steel sheet. On the other hand, when the addition amount of manganese exceeds 12.5% by weight, the weldability and processability are lowered and the performance process may become difficult.

In (P)

The phosphorus (P) causes weldability to deteriorate and causes a final material deviation by slab center segregation.

Accordingly, in the present invention, the content of phosphorus is limited to 0.03% by weight or less based on the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits the toughness and weldability of steel, and forms an MnS non-metallic inclusion by binding with manganese, thereby generating cracks during steel processing.

Accordingly, in the present invention, the content of sulfur is limited to 0.02 wt% or less of the total weight of the steel sheet.

Soluble aluminum (sol.Al)

Soluble aluminum (sol.Al) acts as deoxidizer with silicon.

The soluble aluminum is preferably added in an amount of 0.05 wt% or less based on the total weight of the steel sheet. If the added amount of soluble aluminum exceeds 0.05 wt%, the surface quality of the steel sheet may be deteriorated.

Nitrogen (N)

Nitrogen (N) causes property degradation such as toughness of steel.

Therefore, in the present invention, the content of nitrogen is limited to 0.01% by weight or less based on the total weight of the steel sheet.

Chromium (Cr), molybdenum (Mo)

Chromium (Cr) and molybdenum (Mo) contribute to austenite stabilization.

When at least one of chromium and molybdenum is added, the addition amount thereof is preferably 2% by weight or less based on the total weight of the steel sheet. When the addition amount of chromium or the like exceeds 0.20% by weight, generation of center segregation, toughness, deterioration of ductility and the like may be problematic.

The high carbon steel sheet according to the present invention can exhibit a tensile strength of 500 MPa or more and an elongation of 12% or more through the process control described below together with the alloy composition described above.

In terms of microstructure, the high carbon steel sheet according to the present invention comprises 40 to 60 vol% of spheroidizing structure and the remainder contains low temperature transformation structure such as ferrite and pearlite, or ferrite and pearlite, .

High carbon steel plate manufacturing method

1 is a flowchart schematically showing a method of manufacturing a high carbon steel sheet according to an embodiment of the present invention.

1, a method of manufacturing a high carbon steel sheet according to the present invention includes a hot rolling step S110, a first cooling step S120, a second cooling step S130, a third cooling and winding step S140, And an annealing step (S150).

In the hot rolling step (S110), the slab plate having the above-described alloy composition is hot-rolled.

Before the hot rolling, a process of reheating the slab at about 1200 캜 may be performed for reusing the segregated components and dissolving the precipitates during casting.

Hot rolling may be carried out by rough rolling in the austenite recrystallization region and then finishing in an austenite non-recrystallized region.

At this time, the finish rolling temperature (FDT) of the hot rolling is preferably at least Arcm, more specifically 800 to 900 占 폚. Here, the Arcm temperature refers to the temperature at which cementite starts to precipitate in the austenite at the time of cooling in the quartz. If the finishing rolling temperature is lower than the Arcm temperature, rolling load may increase due to precipitation of cementite during rolling and physical properties of steel may be lowered.

Next, in the first cooling step (S120), the hot-rolled plate is primarily cooled to a ferrite region, more specifically, 650 to 700 占 폚. At this time, the primary cooling is preferably performed at an average cooling rate of 50 DEG C / sec or more, more preferably at an average cooling rate of 50 to 100 DEG C / sec. If the average cooling rate of the primary cooling is less than 50 占 폚 / sec, grain coarsening may occur.

Next, in the secondary cooling step (S130), the primary cooled plate is secondarily cooled for 2 to 10 seconds at an average cooling rate of 5 DEG C / sec or less. The secondary cooling is preferably carried out in an air cooling manner.

In the present invention, it is possible to minimize transformation heat due to abrupt pearlite transformation through the secondary cooling, thereby suppressing crystal grain coarsening. However, when the secondary cooling time is less than 2 seconds, it is difficult to suppress the heat generation of the transformation. In addition, when the secondary cooling time exceeds 10 seconds, it is difficult to secure strength due to excessive ferrite transformation.

Next, in the third cooling and winding step (S140), the secondary cooled plate is thirdly cooled to a pearlite area, more specifically, to 550 to 640 占 폚 and wound. The tertiary cooling is preferably carried out at an average cooling rate of 70 to 80 DEG C / sec. When the average cooling rate of the tertiary cooling is less than 70 DEG C / sec, sufficient pearlitic transformation does not occur. When the average cooling rate exceeds 80 DEG C / sec, toughness deterioration may be a problem.

Next, in the spheroidizing annealing step (S150), the rolled sheet is spheroidized and annealed at a temperature equal to or less than Ac1. Here, the Ac1 temperature is the temperature at which the transformation from ferrite to cementite to austenite begins upon heating.

More specifically, the spheroidizing annealing is preferably performed for 10 to 20 hours at a temperature range of Ac1-100 ° C to Ac1. If the spheroidizing annealing temperature exceeds the Ac1 temperature, undesired phase transformation may occur after spheroidizing annealing due to austenite transformation. On the contrary, if the spheroidizing annealing temperature is lower than Ac1-100 deg. C, the formation of spheroidizing structure may be insufficient. When the spheroidizing annealing time is less than 10 hours, the spheroidizing structure formation may be insufficient, and even when the spheroidizing annealing time exceeds 20 hours, the spheroidizing efficiency is not improved any more.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Manufacture of hot-rolled steel sheet

The slab plate having the alloy composition shown in Table 1 was reheated at 1200 ° C for 2 hours, rough-rolled in the austenite recrystallization region, and hot-rolled at a finishing rolling temperature (FDT) condition shown in Table 2 at the austenite non- . Thereafter, the steel sheet was cooled and rolled under the conditions shown in Table 2, and then rolled at 250 DEG C into a spheroidizing annealing furnace, and spheroidizing annealing was performed under the conditions shown in Table 2. [

[Table 1] (unit:% by weight)

[Table 2]

2. Property evaluation

Table 3 shows the tensile test results of the specimens 1 to 6.

[Table 3]

Referring to Table 3, the specimens 2, 3, 4, and 6 satisfying the alloy composition and process conditions of the present invention all exhibited a tensile strength of 500 MPa or more and an elongation of 12% or more.

On the other hand, for specimen 1, the tensile strength was less than 500 MPa. In case of specimen 1, it is judged that the result is that the heat generated during the cooling process after hot rolling is excessive and the microstructure is coarsened and the pearlite structure is transformed into ferrite. Referring to FIG. 2, it can be seen that in the case of the microstructure of specimen 1 (comparative steel), spheroidized structure is formed in the coarse ferrite structure. On the other hand, referring to FIG. 3, in the case of the microstructure of specimen 2 (invention steel), it can be seen that spheroidized structure is formed in fine pearlite and ferrite structure.

In the case of specimen 5, the strength was very high, but the elongation was less than 12%. It is considered that this is because the low temperature transformation structure such as bainite is formed in the cooling process after the hot rolling so that the spheroidization structure is not sufficiently formed during the spheroidizing annealing.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims

(Si): not more than 1.5%, manganese (Mn): not more than 12.5%, phosphorus (P): not more than 0.03%, sulfur (S): not more than 0.02% Hot rolled at a temperature of Arcm or higher at a temperature of Arcm or higher at a temperature of not less than Arcm; solubilized aluminum (sol.Al): not more than 0.05%, nitrogen (N): not more than 0.01%, and the balance of Fe and unavoidable impurities;
Firstly cooling the hot-rolled plate to a ferrite region;
Secondarily cooling the primary cooled plate at an average cooling rate of 5 DEG C / sec or less for 2 to 10 seconds;
Cooling the secondary cooled plate to a pearlite region and winding it; And
And sintering the rolled sheet material at a temperature equal to or less than Ac1.

The method according to claim 1,
Wherein the slab plate further comprises at least 2.0 wt% of at least one of chromium (Cr) and molybdenum (Mo).

The method according to claim 1,
Wherein the primary cooling is performed at 650 to 700 占 폚 at an average cooling rate of 50 占 폚 / sec or more.

The method according to claim 1,
Wherein the secondary cooling is performed in an air cooling manner.

The method according to claim 1,
Wherein the tertiary cooling is performed at 550 to 640 占 폚 at an average cooling rate of 70 to 80 占 폚 / sec.

The method according to claim 1,
Wherein the spheroidizing annealing is performed for 10 to 20 hours at a temperature range of Ac1-100 ° C to Ac1.

(Si): not more than 1.5%, manganese (Mn): not more than 12.5%, phosphorus (P): not more than 0.03%, sulfur (S): not more than 0.02% (Sol. Al): not more than 0.05%, nitrogen (N): not more than 0.01%, and the balance of iron (Fe) and unavoidable impurities,
A tensile strength of 500 MPa or more, and an elongation of 12% or more.

8. The method of claim 7,
Wherein the high carbon steel sheet further comprises not less than 2.0% by weight of at least one of chromium (Cr) and molybdenum (Mo).