KR20200076045A - Manufacturing method of high-carbon steel with improved bendability and the method for manufacturing the same - Google Patents

Abstract

Disclosed are a high-carbon steel with improved bendability and a manufacturing method thereof. According to an embodiment of the present invention, a manufacturing method of a high-carbon steel with improved bendability comprises the steps of: reheating the slab whose composition includes 0.6 to 0.85 wt% of C, 0.01 to 0.5 wt% of Si, 0.3 to 2.0 wt% of Mn, 0.1 wt% or less of Al (excluding 0 wt%), 2.0 wt% or less of Cr (excluding 0 wt%), 0.01 wt% or less of N (excluding 0 wt%), and the remainder containing Fe and inevitable impurities; manufacturing a hot-rolled steel sheet by hot rolling the slab at the finishing temperature of 800 to 880°C; cooling the manufactured hot-rolled steel sheet to 600-650°C at a cooling rate of 30°C/s or higher; and winding the cooled hot-rolled steel sheet at 600 to 680°C.

Description

High carbon steel with improved bending processability and its manufacturing method{MANUFACTURING METHOD OF HIGH-CARBON STEEL WITH IMPROVED BENDABILITY AND THE METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high carbon steel having excellent bending processability and a method for manufacturing the same.

High-carbon hot-rolled steel sheet that can be used for various purposes such as construction, tools, automobile parts, etc. undergoes pickling and cold rolling in re-rolling, and is processed into parts suitable for heat treatment and purpose at end customers. In the re-rolled yarn, the hot-rolled coil is uncoiling and introduced to the pickling side, and the front end of the coil is bent to bend to facilitate entry of the coil. At this time, if the brittleness of the high carbon steel is large, cracks may occur in the strip or, in severe cases, fracture.

When the cooling is excessive or the coiling temperature is low, a low-temperature structure with high brittleness is formed in the surface or edge of the hot-rolled steel sheet, such as bainite, resulting in cracking. Therefore, it was intended to suppress cooling or to suppress the formation of low-temperature tissue by controlling the winding temperature upward.

However, recently, there has been a problem in that cracks are generated even in a material in which a low-temperature structure is not generated by raising the coiling temperature. In the case of Application No. 10-1995-0068432, the surface decarburization layer was used to improve the ductility of the surface to improve the bending processability. However, this method has a problem of lowering the hardness of the high carbon steel and causing grain boundary oxidation, and thus it is impossible to secure the quality of the final product made of the high carbon steel.

Embodiments of the present invention is to provide a high-strength steel wire with improved bending processability by optimizing the alloy composition and manufacturing conditions and a method for manufacturing the same.

The method of manufacturing the “high carbon” hot-rolled steel sheet with improved bending processability according to an embodiment of the present invention is weight%, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (Excluding 0), Cr: 2.0% or less (excluding 0), N: 0.01% or less (excluding 0), the rest reheating the slab containing Fe and unavoidable impurities; Hot rolling the slab by hot rolling at a finishing temperature of 800 to 880°C; Cooling the manufactured hot rolled steel sheet to 600 to 650°C at a cooling rate of 30°C/s or higher; And winding the cooled hot-rolled steel sheet at 600 to 680°C.

In addition, the reheating temperature may be 1100 to 1300°C.

In addition, after the reheating step is completed, a rough rolling step; may further include.

In addition, cooling can be performed within 3 seconds after hot rolling.

In addition, after winding, the block size may be 30 μm or less.

(Here, Blcok Size means the grain size of the final pearlite.)

The high-carbon steel with improved bending processability according to an embodiment of the present invention is weight%, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (excluding 0) , Cr: 2.0% or less (excluding 0), N: 0.01% or less (excluding 0), the rest contains Fe and unavoidable impurities, and the block size is 30 μm or less.

(Here, Blcok Size means the grain size of the final pearlite.)

In addition, the microstructure may include pearlite and residual ferrite of 90% or more in an area fraction.

According to the present invention, it is possible to provide a high-carbon hot-rolled steel sheet capable of preventing cracks and plate breakage. In the future, the pre-heating process can be omitted from the re-rolling process, thereby preventing additional costs. In addition, it is possible to prevent the occurrence of decarburization due to excessive transformation heating, thereby securing the hardness of the final product made of hot-rolled steel sheet.

1 is a graph showing the block size according to the finishing temperature of the finishing examples of the comparative examples and examples of the present invention.
Figure 2 is a picture taken using the EBSD block size of carbon steel according to Example 3 and Comparative Example 3.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the spirit of the present invention to those of ordinary skill in the art. The present invention is not limited to the embodiments presented herein, but may be embodied in other forms. To clarify the present invention, the illustration of parts irrelevant to the description may be omitted, and the size of components may be exaggerated to help understanding.

Throughout the specification, when a part “includes” a certain component, this means that other components may be further included rather than excluding other components unless specifically stated to the contrary.

Singular expressions include plural expressions, unless the context clearly has an exception.

The high-carbon steel with improved bending processability according to an aspect of the present invention is in weight%, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (excluding 0), Cr: 2.0% or less (excluding 0), N: 0.01% or less (excluding 0), the rest include Fe and unavoidable impurities.

Hereinafter, the reason for the numerical limitation of the content of the alloy component in the embodiment of the present invention will be described. In the following, unless otherwise specified, the unit is weight%.

The content of C is 0.6 to 0.85%.

Carbon (C) is an element added to improve strength, and it is preferable to add 0.6% or more to obtain a structure composed of pearlite and ferrite. However, when the content is excessive, there is a problem in that bending workability is lowered due to the precipitation of cementite on the grain boundary, and the upper limit can be limited to 0.85%.

The content of Si is 0.01 to 0.5%.

Silicon (Si) can be added to 0.01% or more for deoxidation. However, when the content is excessive, there is a problem in causing grain boundary oxidation on the surface of the hot-rolled steel sheet, and the upper limit may be limited to 0.5%.

The content of Mn is 0.3 to 2.0%.

Manganese (Mn) is an element added together with C to improve the strength of the steel, and it can be added in an amount of 0.3% or more to prevent hot embrittlement caused by the formation of FeS by completely precipitating sulfur in the steel to MnS. However, if the content is excessive, there is a problem of intergranular oxidation along with formation of central segregation and inclusions, and the upper limit may be limited to 2.0%.

The content of Al is 0.1% or less (excluding 0).

Aluminum (Al) is an element added for not only a deoxidizing effect but also a solid solution strengthening effect. However, if the content is excessive, there is a problem that not only causes slab cracks when playing, but also causes grain boundary oxidation in the final product, and the upper limit can be limited to 0.1%.

The content of Cr is 2.0% or less (excluding 0).

Chromium (Cr) is an element added to increase the hardenability of steel, and forms an passivation film in the air to suppress the rust of iron. However, if the content is excessive, there is a problem in that the scale is not easily removed during hot rolling, and there is a problem in that edge cracking of the hot rolled sheet occurs during cooling, and the upper limit thereof may be limited to 2.0%.

The content of N is 0.01% or less (excluding 0).

Nitrogen (N) has a solid solution strengthening effect, but if the content is excessive, there is a problem in that the surface element is inferior in quality due to the elongation of the yield point, and the workability is inferior due to precipitation of nitride, and the upper limit can be limited to 0.01%. have.

The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and therefore cannot be excluded. Examples of inevitable impurities include P (phosphorus) and S (sulfur). Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

The P content is 0.03% or less (excluding 0).

Phosphorus (P) is an element that is inevitably added during the steel manufacturing process, and there is a problem of causing brittleness due to segregation, and the upper limit thereof can be limited to 0.03%.

The content of S is 0.03% or less (excluding 0).

Sulfur (S) is an element that is inevitably added during the manufacturing process of steel, and there is a problem of forming grain boundaries during hot rolling by forming an inclusion or forming FeS sulfide with a low melting point, and the upper limit can be limited to 0.03%.

As described above, the hot-rolled steel sheet of the present invention that satisfies the alloy composition is preferably composed of a ferrite and pearlite composite structure as a microstructure.

More specifically, the pearlite contains an area fraction of 90% or more, and may be made of the remaining ferrite.

As described above, the hot-rolled steel sheet of the present invention that satisfies the alloy composition and microstructure configuration has excellent bending characteristics with a block size of 30 μm or less. In the present invention, Block Size means a grain size in which the final pearlite has a grain orientation of 8 degrees or less.

That is, the smaller the block size, the more the cracks that progress along the pearlite block are suppressed, and in the present invention, by forming the block size to 30 µm or less, it is possible to secure bending workability that can suppress crack generation during uncoiling of the material.

Hereinafter, the manufacturing method of the present invention will be described.

Method for producing a high strength steel wire with improved workability according to another aspect of the present invention, by weight, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less ( Re-heating the slab containing Fe and unavoidable impurities; Cr: 2.0% or less (excluding 0), N: 0.01% or less (excluding 0); Hot rolling the slab by hot rolling at a finishing temperature of 800 to 880°C; Cooling the manufactured hot rolled steel sheet to 600 to 650°C at a cooling rate of 30°C/s or higher; And winding the cooled hot-rolled steel sheet at 600 to 700°C. It includes.

The high carbon steel of the present invention can be manufactured by preparing a slab that satisfies the above-described alloy composition and then subjecting the slab to hot rolling-cooling-winding.

First, the slab having the above-described composition (Slab) is reheated at 1100 ~ 1300 ℃.

The reheating step is a process for homogenizing the slab, and when the temperature is less than 1100°C, there is a problem in that the rolling load increases rapidly during hot rolling. On the other hand, when the temperature exceeds 1300°C, the surface temperature of the slab rises when the hot rolling is finished, causing surface defects as the oxidation scale grows thick, or when the coil is unwound, the scale falls off the surface of the steel. There is a problem coming out.

After reheating, it is hot rolled to produce a hot rolled steel sheet. The hot rolling step may consist of rough rolling and finishing rolling.

It is possible to perform rough rolling of the reheated slab in a rough rolling mill composed of 2 to 4 rolling stands.

According to the disclosed embodiment, it is possible to control the end temperature of the finishing rolling of tandem rolling at regular intervals in a temperature range of 800 to 880°C.

If the finish rolling end temperature is less than 800°C, there is a problem in that the rolling load is greatly increased. In particular, in the case of both edge portions of the steel sheet where the temperature is severely reduced, a cornerstone ferrite phase is generated to ensure a uniform material in the width direction. none. On the other hand, when the finish rolling finish temperature exceeds 880°C, the structure of the high carbon steel becomes coarse, and the scale becomes thick, and thus there is a problem that the surface quality is deteriorated.

1 is a graph showing the block size according to the finishing temperature of the finishing examples of the comparative examples and examples of the present invention.

Referring to FIG. 1, it can be confirmed that the block size of the high carbon steel tends to increase as the finishing temperature of the finishing rolling increases.

Next, the hot rolled steel sheet is cooled. According to the disclosed embodiment, the hot-rolled steel sheet may be cooled by water cooling in a water-cooling table (ROT).

At this time, it is preferable to start cooling within 3 seconds in the cooling zone after finishing the finishing rolling. After finishing the finishing rolling, if the cooling start is delayed more than 3 seconds, the block size of pearlite becomes coarse, and a film type thin ferrite can be formed at the grain boundary.

Meanwhile, when cooling the hot-rolled steel piece, the cooling rate can be controlled to 30°C/s or more. When the cooling rate is less than 30°C/s, a film type thin ferrite may be formed at the grain boundary.

In the case of the present invention containing 0.6 to 0.85% carbon, when ferrite is formed at the grain boundary under conditions where cooling is not properly performed, a very thin ferrite in the form of a film is produced. Then, when bending deformation of the steel sheet, stress is concentrated on ferrite, which has relatively weak strength, and thus cracks are generated.

Cooling of the hot-rolled steel sheet is terminated in a temperature range of 600 to 650°C. When the cooling end temperature is less than 600°C, a low-temperature structure having high brittleness such as martensite and bainite is generated, and cracks may occur when bending deformation of the steel sheet.

On the other hand, when the cooling end temperature exceeds 650°C, coarse pearlite is formed, and thus the spheroidization speed is reduced during re-rolling, so that the hardness of the final product cannot be secured.

In addition, when the cooling end temperature exceeds 650°C, transformation does not occur sufficiently in the cooling step, and the temperature rise of the coil due to transformation heat after winding is excessive, causing surface problems such as decarburization or internal oxidation.

Next, the winding process of the cooled hot-rolled steel sheet may be performed in a temperature range of 600 to 680°C, which can prevent low-temperature structure, decarburization and internal oxidation.

As described above, the disclosed embodiment can improve the bending processability of the high carbon steel by appropriately controlling the finish rolling end temperature, the cooling start time, the cooling end temperature, and the coiling temperature. In more detail, the disclosed embodiment can suppress decarburization without cracking when uncoiling the hot rolled coil by optimizing the cooling process after finishing rolling.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

The steel slab having the alloy composition shown in Table 1 was reheated in a temperature range of 1,100 to 1,300°C.

Steel C Mn Si Al Cr N One 0.85 0.4 0.2 0.01 0.2 0.004 2 0.7 0.5 0.2 0.01 0.3 0.004 3 0.6 0.4 0.2 0.01 1.1 0.004

Then, the hot-rolled steel sheet was prepared by hot rolling and cooling the heated slab.

At this time, the hot rolling and cooling processes were performed according to the finishing temperature at the end of rolling, the cooling start time, the cooling speed, and the cooling end temperature at the end of rolling. Thereafter, coiling was performed at a coiling temperature of 600 to 700°C.

The block size of each manufactured hot rolled steel sheet is measured and shown in Table 2.

When the block size is measured at 1/4 point of the material thickness using Backscatter Diffraction (EBSD), the grain tolerance angle value is set to 8 degrees, and the one within 8 degrees is measured as a block. Did.

The depth of the ferrite decarburization layer was measured by a microscope according to the method for measuring the depth of the decarburization layer of steel (KS D 0216) as defined in the Korean Industrial Standards for the hot rolled steel sheet. Did.

The low-temperature tissue (martensite, bainite) was analyzed for occurrence using an optical microscope or a scanning electron microscope (SEM).

In addition, it was confirmed whether or not the occurrence of grain boundary ferrite and whether cracks occurred during uncoiling, and the results are shown in Table 2 below.

division Applied steel type Sasang Rolling
End temperature
(℃) After rolling
Cooling start
Time (seconds) Cooling
speed
(℃/S) Cooling down
Temperature (℃) Winding
Temperature
(℃) Block size (㎛) Decarburization
Good or not
(≤5㎛)
Cold tissue
Occurrence Whether grain boundary ferrite occurs When uncoiling
Cracking
Whether Example 1 Steel type 1 831 3 35 645 673 24 Good Not occurring Not occurring Not occurring Example 2 Steel type 1 878 3 36 608 632 29 Good Not occurring Not occurring Not occurring Example 3 Steel type 1 850 2 33 617 651 25 Good Not occurring Not occurring Not occurring Example 4 Steel type 2 844 3 37 622 654 24 Good Not occurring Not occurring Not occurring Example 5 Steel type 3 816 3 31 628 637 20 Good Not occurring Not occurring Not occurring Comparative Example 1 Steel type 1 786 3 32 632 654 16 Good Occur Not occurring Occur Comparative Example 2 Steel type 1 910 3 33 649 671 36 Good Not occurring Not occurring Occur Comparative Example 3 Steel type 1 931 2 36 633 669 43 Good Not occurring Not occurring Occur Comparative Example 4 Steel type 1 862 2 11 624 662 26 Good Not occurring Occur Occur Comparative Example 5 Steel type 1 852 5 34 638 662 25 Good Not occurring Occur Occur Comparative Example 6 Steel type 1 866 3 35 669 699 27 Bad Not occurring Not occurring Not occurring

In the present invention, in order to apply high carbon steel as a material for construction, tools, and automobile parts, it was intended to suppress the occurrence of decarburization without cracking when uncoiling the coil in re-rolling.

As shown in Table 2, Examples 1 to 5 prepared with components and conditions consistent with the present invention not only satisfy the decarburization criteria, but also do not generate low-temperature structure, and that cracking did not occur when the coil was released. Can be confirmed. Therefore, according to the disclosed embodiment, it is possible to improve the bending processability without lowering the strength.

On the other hand, in Comparative Example 1, the final rolling end temperature is 786°C, which is less than 800°C, so that the block size can be secured small. Cracking occurred when released.

In Comparative Examples 2 and 3, the end rolling temperatures were 910°C and 931°C, respectively, exceeding 880°C, resulting in coarse blocks, which increased brittleness. Did.

2 is a photograph taken using EBSD of the block size of carbon steel according to Example 3 and Comparative Example 3. Referring to FIG. 2, it can be seen that in Comparative Example 3 compared to Example 3, the block size of pearlite is coarse.

In Comparative Example 4, the cooling rate was 11°C/s, which was less than 30°C/s, and in Comparative Example 5, after the hot rolling was completed, the cooling start time was 5 seconds and exceeded 3 seconds. As in Comparative Examples 4 and 5, if the cooling conditions were not met, a grain boundary ferrite was formed, and stress was concentrated at the grain boundary, resulting in cracks when releasing the hot rolled coil.

On the other hand, although the finish rolling end temperature condition was satisfied, in the comparative example 6 in which the cooling end temperature exceeded 650°C to 669°C, the coiling temperature rose to 699°C due to transformation heat, thereby causing surface decarburization. In this case, the hardness of the final product manufactured by processing the hot rolled steel sheet cannot be sufficiently secured, and there is a risk of product damage.

In conclusion, in weight percent according to the disclosed embodiment, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (excluding 0), Cr: 2.0% or less ( 0 is excluded), N: 0.01% or less (excluding 0), and the rest, Fe and high carbon steel containing unavoidable impurities can improve alloy composition and manufacturing conditions to improve bending workability while securing 　 strength.

Therefore, it is possible to omit the pre-heating process in a subsequent re-rolling process, thereby preventing the occurrence of additional costs, and preventing cracking and fracture of the hot-rolled steel sheet. In addition, by preventing the occurrence of decarburization due to excessive transformation heating, it is possible to secure the hardness of the final product made of hot-rolled steel sheet.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and a person having ordinary skill in the art does not depart from the concept and scope of the following claims. It will be understood that various modifications and variations are possible.

Claims (7)

In weight percent, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (excluding 0), Cr: 2.0% or less (excluding 0), N: 0.01 % Or less (excluding 0), the rest reheating the slab containing Fe and unavoidable impurities;
Hot rolling the slab by hot rolling at a finishing temperature of 800 to 880°C;
Cooling the prepared hot-rolled steel sheet to 600 to 650°C at a cooling rate of 30°C/s or more; And
Winding the cooled hot-rolled steel sheet at 600 ~ 680 ℃; Method of manufacturing a high-carbon hot-rolled steel sheet having improved bending processability. According to claim 1,
The reheating temperature is 1100 ~ 1300 ℃ method of manufacturing a high-carbon hot-rolled steel sheet with improved bending processability. According to claim 1,
After the reheating step is completed, a rough rolling step; a method of manufacturing a high-carbon hot-rolled steel sheet further improved bending workability. According to claim 1,
A method of manufacturing a high-carbon hot-rolled steel sheet with improved bending processability that performs cooling within 3 seconds after hot rolling. According to claim 1,
After winding, a method for manufacturing a high-carbon hot-rolled steel sheet with improved bendability with a block size of 30 µm or less.
(Here, Blcok Size means the grain size of the final pearlite.) In weight percent, C: 0.6 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, Al: 0.1% or less (excluding 0), Cr: 2.0% or less (excluding 0), N: 0.01 % Or less (excluding 0), the rest contains Fe and unavoidable impurities,
High-carbon steel with improved bendability with a block size of 30 μm or less.
(Here, Blcok Size means the grain size of the final pearlite.) The method of claim 6,
The microstructure is an area fraction, high-carbon steel with improved bendability, including more than 90% of pearlite and residual ferrite.

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