KR20150025948A - High carbon steel and method of manufacturing the carbon steel - Google Patents

Abstract

Disclosed are high carbon steel capable of increasing the yield of a final product by preventing an edge part from being broken, and preventing deformation by controlling a process condition and adjusting an alloy composition; and a method to manufacture the same. According to the present invention, the method to manufacture high carbon steel comprises: (a) step of reheating a steel slab comprising 0.82-0.88 wt% of carbon (C), 0.5 wt% or less of silicon (Si), 0.7-0.9 wt% of manganese (Mn), 0.35 wt% or less of phosphorus (P), 0.03 wt% or less of sulfur (S), 0.15 wt% or less of titanium (Ti), 0.01-0.05 wt% of niobium (Nb), 0.01-0.05 wt% of vanadium (V), and the remainder consisting of iron (Fe) and inevitable impurities at a slab reheating temperature (SRT) of 1100-1200°C; (b) step of finishing hot rolling the reheated steel at a finishing delivery temperature (FDT) of 750-800°C; and (c) step of cooling the finished hot rolled steel to a coiling temperature (CT) of 600-650°C, and winding the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to high carbon steels,

The present invention relates to a high carbon steel and a method of manufacturing the same. More particularly, the present invention relates to a high carbon steel and a method of manufacturing the same, which can improve the yield rate of the final product by preventing the cracks and edge cracks, ≪ / RTI >

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. In general production and processing of high carbon steel, the hot-rolled coil produced in the hot-rolling process is pickled, cold-rolled to a predetermined intermediate thickness, and subjected to spheroidizing annealing. Thereafter, the steel sheet is rolled to a final thickness through a secondary cold rolling, and the final object material is obtained by blanking and QT heat treatment (quenching & tempering) in the product shape.

The cooling process of such hot carbon manufacturing process of high carbon steel is very different from that of ordinary carbon steel. That is, since the high carbon steel exhibits an extremely exothermic reaction due to the pearlite transformation occurring in the cooling process, there is a great difficulty in controlling the coiling temperature. During the winding of the high carbon steel, the coiling temperature rapidly changes due to the transformation heat, which is also due to the characteristic shape of the coil of the high carbon steel.

In order to solve this problem, it is a general control pattern of the coiling temperature to perform the front end cooling on the cooling band (ROT) to complete the transformation before winding. However, when the shear cooling pattern is applied, the edge of the steel is suddenly cooled, resulting in bainite or martensite structure, which is a brittle structure in the edge portion, and cracks in the edge portion where fine cracks are generated in the edge portion in the process of winding in a coil shape .

A related prior art is Korean Patent Laid-Open Publication No. 10-2005-0094463 (published on September 27, 2005), which discloses a high strength, high-strength, high-carbon steel wire rod and a manufacturing method thereof.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing high carbon steel which can prevent the occurrence of cracks and cracks at the edges by controlling alloy components and controlling process conditions, thereby improving the yield rate of the final product.

Another object of the present invention is to provide a high carbon steel which is produced by the above method and which has excellent mechanical properties with a tensile strength (TS) of 650 to 750 MPa, a yield strength (YP) of 400 to 500 MPa and an elongation (EL) of 22% will be.

In order to accomplish the above object, the present invention provides a method of manufacturing high carbon steels, comprising the steps of: (a) providing 0.82 to 0.88 weight percent of carbon (C), 0.5 weight percent or less of silicon (Si) (S): 0.03 wt% or less, titanium (Ti): 0.15 wt% or less, niobium (Nb): 0.01 to 0.05 wt%, vanadium (V): 0.01 To about 0.05 wt.% And the balance iron (Fe) and inevitable impurities to a slab reheating temperature (SRT) of 1100 to 1200 DEG C; (b) subjecting the reheated steel to finishing hot rolling under finishing delivery temperature (FDT) conditions of 750 to 800 占 폚; And (c) cooling the finished hot-rolled steel to a CT (Coiling Temperature) of 600 to 650 ° C. and winding the finished steel.

According to another aspect of the present invention, there is provided a high carbon steel comprising 0.82 to 0.88% by weight of carbon (C), 0.5% by weight or less of silicon (Si), 0.7 to 0.9% by weight of manganese (Mn) (P): not more than 0.35 wt%, sulfur: not more than 0.03 wt%, titanium: not more than 0.15 wt%, niobium: 0.01 to 0.05 wt%, vanadium: (Fe) and inevitable impurities, and has tensile strength (TS) of 650 to 750 MPa and yield strength (YP) of 400 to 500 MPa.

In the present invention, by controlling the alloy components and controlling the process conditions, it is possible to manufacture high carbon steel which can prevent the occurrence of cracks and cracks at the edges, thereby improving the yield rate of the final product.

Therefore, the high carbon steel produced by the above method exhibits a tensile strength (TS) of 650 to 750 MPa, a yield strength (YP) of 400 to 500 MPa and an elongation (EL) of 22% or more.

1 is a flow chart showing a method for manufacturing high carbon steel according to an embodiment of the present invention.
2 is a graph showing the hardness measurement results of the edges of the specimens according to Examples 1 and 2 and Comparative Examples 1 and 2.
3 is a photograph showing the microstructure of the edges of the specimens according to Comparative Examples 1 and 2.
4 is a photograph showing the microstructure of the edge portions of the specimens according to Examples 1 and 2.

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. Like reference numerals refer to like elements throughout the specification.

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

High carbon steel

The high carbon steel according to the present invention aims to exhibit a tensile strength (TS) of 650 to 750 MPa, a yield strength (YP) of 400 to 500 MPa and an elongation (EL) of 22% or more.

The high carbon steel according to the present invention preferably contains 0.82 to 0.88 wt% of carbon (C), 0.5 wt% or less of silicon (Si), 0.7 to 0.9 wt% of manganese (Mn) (S): 0.03 wt% or less, titanium: 0.15 wt% or less, niobium: 0.01 to 0.05 wt%, vanadium (V): 0.01 to 0.05 wt% It is made of unavoidable impurities.

At this time, the high carbon steel according to the present invention has a composite structure in which the final microstructure includes pearlite uniform in width and length direction.

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

Carbon (C)

In the present invention, carbon (C) is added to secure the strength of the high carbon steel to be produced. The carbon (C) is a main element that determines the strength and hardness of the steel. As the content is higher, the strength is increased. The carbon (C) is combined with sulfur (S) to form an emulsion.

The carbon (C) is preferably added in a content ratio of 0.82 to 0.88% by weight of the total weight of the high carbon steel according to the present invention. When the content of carbon (C) is less than 0.82% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.88% by weight, the impact toughness is drastically lowered.

silicon( Si )

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel, and also serves to improve the solid solution strengthening effect.

The silicon (Si) is preferably added in a content ratio of 0.5% by weight or less based on the total weight of the high carbon steel according to the present invention. When the content of silicon (Si) exceeds 0.5% by weight, the toughness of the produced high carbon steel is lowered.

manganese( Mn )

Manganese (Mn) is an element that improves strength. In addition, manganese (Mn) binds with sulfur (S) to form MnS, thereby preventing red-hot brittleness and improving cutting workability.

The manganese (Mn) is preferably added in a content ratio of 0.7 to 0.9% by weight based on the total weight of the high carbon steel according to the present invention. When the content of manganese (Mn) is less than 0.7% by weight, the effect of strengthening the solid solution and securing the strength due to the addition of manganese (Mn) is insufficient. On the contrary, when the content of manganese (Mn) exceeds 0.9% by weight, the toughness is lowered and the production cost of carbon steel is greatly increased.

In (P)

Phosphorus (P) is added for improving cutting ability.

However, if the content of phosphorus (P) in the high carbon steel according to the present invention exceeds 0.35% by weight, the toughness and fatigue resistance deteriorate. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.35% by weight or less based on the total weight of the high carbon steel.

Sulfur (S)

Sulfur (S) is added in high carbon steels to improve machinability and processability.

However, when the content of sulfur (S) in the high carbon steel according to the present invention is larger than 0.03 wt%, it may cause micro segregation as well as center segregation, thereby adversely affecting the material and deteriorating the weldability have. Therefore, in the present invention, the content of sulfur (S) is limited to 0.03 wt% or less of the total weight of the high carbon steel.

titanium( Ti )

Titanium (Ti) has the effect of improving the toughness and strength of hot-rolled steel sheet by making Ti (C, N) precipitates having high stability at high temperatures, thereby finishing the austenite grain growth and refining the texture of the welded portion.

The titanium (Ti) is preferably added in an amount of 0.15% by weight or less based on the total weight of the high carbon steel according to the present invention. If the content of titanium (Ti) exceeds 0.15% by weight, corrosion resistance of the steel can be lowered by forming a coarse TiN precipitate.

Niobium ( Nb )

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium-based carbides or nitrides improve grain strength and low-temperature toughness by suppressing grain growth during rolling and making crystal grains finer.

The niobium (Nb) is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the high carbon steel according to the present invention. When the content of niobium (Nb) is less than 0.01% by weight, it is difficult to see the effect of addition of niobium (Nb). On the contrary, when the content of niobium (Nb) exceeds 0.05% by weight, coarse secondary phases including niobium (Nb) are formed and act as a starting point of hydrogen organic cracking.

Vanadium (V)

Vanadium (V) is a carbide-generating element, and is particularly effective in increasing the high-temperature strength.

The vanadium (V) is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the high carbon steel according to the present invention. If the addition amount of vanadium (V) is less than 0.01 wt%, the effect of improving the strength is insufficient. On the other hand, when the amount of vanadium (V) added exceeds 0.05 wt%, there is a problem that the susceptibility to reheat crack increases.

High carbon steel  Manufacturing method

1 is a flow chart showing a method for manufacturing high carbon steel according to an embodiment of the present invention.

Referring to FIG. 1, a method for manufacturing high carbon steel according to an embodiment of the present invention includes a slab reheating step S110, a hot rolling step S120, and a cooling / winding step S130. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the step to derive effects such as reuse of precipitates.

In the present invention, the semi-finished steel slabs to be subjected to hot rolling include 0.82 to 0.88 wt% of carbon (C), 0.5 wt% or less of silicon (Si), 0.7 to 0.9 wt% of manganese (Mn) ): 0.35 wt% or less, sulfur (S): 0.03 wt% or less, titanium (Ti): 0.15 wt% or less, niobium (Nb): 0.01 to 0.05 wt%, vanadium (V) Iron (Fe) and inevitable impurities.

Reheating slabs

In the slab reheating step S110, the steel slab having the above composition is reheated to a slab reheating temperature (SRT) of 1100 to 1200 ° C. Here, the steel slab can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the steel slabs obtained through the continuous casting process are reheated to reuse the segregated components during casting.

At this stage, when the slab reheating temperature (SRT) is less than 1100 ° C, there is a problem that the segregated components can not be reused in casting. On the contrary, when the slab reheating temperature (STR) exceeds 1200 ° C, the austenite grain size increases, the final ferrite grain size coarsens and the strength decreases, and the manufacturing cost of the steel can be increased due to the excessive heating process.

Hot rolling

In the hot rolling step (S120), the reheated steel is finely hot-rolled at a finishing delivery temperature (FDT) of 750 to 800 占 폚. In particular, the hot rolling in the present invention is performed by finishing hot rolling at 750 to 800 占 폚, which corresponds to a considerably low temperature as compared with the conventional method, thereby finely finishing the austenite grain boundary. Such hot rolling at a low temperature is intended to prevent cracking of the edge due to increase of the transformation fraction at the run-out-table (ROT) and generation of hot tissue at the edge of the steel.

If the final hot rolling temperature (FDT) is less than 750 占 폚 at this stage, there may occur problems such as the occurrence of blistering due to abnormal reverse rolling. On the other hand, when the final hot rolling temperature (FDT) is higher than 800 ° C, the pearlite nucleation due to the coarsened austenite grains is delayed and the deviation from the coiling temperature is increased, thereby deteriorating the temperature controllability.

Cooling/ Coiling

In the cooling / winding step (S130), the finished hot-rolled steel is cooled to a CT (Coiling Temperature) of 600 to 650 ° C and is wound.

The cooling process in the present invention can be carried out by a forced cooling method such as water cooling.

In this step, if the coiling temperature (CT) is less than 600 ° C, cracking may be caused due to generation of a low-temperature phase at the edge of the steel. On the contrary, when the coiling temperature exceeds 650 ° C, the pearlite layer structure interval increases, and it becomes difficult to act as an obstacle to dislocation movement, and the strength is decreased, and deformation at the interface between coarse cementite and ferrite There is a problem that defects such as voids are concentrated and acts as a crack growth site, thereby deteriorating workability.

The high carbon steels produced in the above steps S110 to S130 can control the alloy components and control the process conditions to prevent the occurrence of cracks and edge cracks, thereby improving the yield rate of the final product, It is possible to manufacture a high carbon steel which can maintain a uniform pearlite structure irrespective of the size of the pellet.

Therefore, the high carbon steel produced by the above method exhibits a tensile strength (TS) of 650 to 750 MPa, a yield strength (YP) of 400 to 500 MPa and an elongation (EL) of 22% or more.

In particular, the high carbon steel produced by the above method can be finely hot-rolled at 750 to 800 占 폚 to reduce the austenite grain size, thereby preventing the occurrence of cracks and edge cracking. This makes it possible to prevent accidental operations such as plate breakage caused by workability degradation caused by duck holes and breakage of edges, and to improve the rate of error of the final product.

As a result, the high carbon steel produced by the above method can have a Vickers hardness of 400 to 450 Hv at an edge portion within 14 mm from the end.

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. Specimen Manufacturing

The specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the compositions shown in Table 1 and the process conditions shown in Table 2. At this time, in the case of the specimens according to Examples 1 to 3 and Comparative Examples 1 and 2, the ingots having the respective compositions were prepared, and the ingots were subjected to the hot rolling process of heating, hot rolling and cooling using a rolling simulation tester, Respectively.

[Table 1] (unit:% by weight)

 [Table 2]

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2, and Fig. 2 shows the results of evaluation of mechanical properties of the specimens according to Examples 1 to 3 and Comparative Examples 1 and 2, Of the hardness measurement results. In this case, Vickers hardness (Vihardness) is a measurement result of a portion of 10 mm from the end of each specimen cut into 2 × 2 mm.

[Table 3]

With reference to Tables 1 to 3, the specimens prepared according to Examples 1 to 3 and the specimens prepared according to Comparative Examples 1 and 2 have large tensile strength (TS), yield point (YP) and elongation (EL) It can be seen that there is no difference.

However, in the case of the specimens prepared according to Examples 1 to 3, the Vickers hardness at the edge portion was measured to be 428 Hv, 441 Hv, and 432 Hv, respectively, whereas in the case of the specimens prepared according to Comparative Examples 1 and 2 It can be seen that the Vickers hardness was measured to be 513 Hv and 491 Hv, which are higher than those of Examples 1 to 3, respectively.

3 is a photograph showing the microstructure of the specimen according to Comparative Examples 1 and 2, and FIG. 4 is a photograph showing the microstructure of the specimen according to Examples 1 and 2 with respect to the edge portion.

As shown in Fig. 3, in the case of the specimens prepared according to Comparative Examples 1 and 2, a large amount of brittle (bainite or martensite) was distributed in the edge portion. On the other hand, as shown in FIG. 4, it can be confirmed that the specimens prepared according to Examples 1 and 2 were composed of composite structure of ferrite and pearlite without low temperature structure.

As can be seen from the above experimental results, in the case of the specimens produced according to Comparative Examples 1 and 2, a large amount of low temperature structure was generated at the edge portion due to the high finish hot rolling temperature as compared with Example 1 .

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.

S110: Slab reheating step
S120: Hot rolling step
S130: cooling / winding step

Claims (5)

(A): 0.82 to 0.88 wt% of carbon (C), 0.5 wt% or less of silicon (Si), 0.7 to 0.9 wt% of manganese (Mn) (Fe) and unavoidable impurities, the steel slab comprising 0.03 wt% or less of titanium, 0.15 wt% or less of titanium, 0.01 to 0.05 wt% of niobium, 0.01 to 0.05 wt% of vanadium, SRT (Slab Reheating Temperature): reheating to 1100 to 1200 占 폚;
(b) subjecting the reheated steel to finishing hot rolling under finishing delivery temperature (FDT) conditions of 750 to 800 占 폚; And
(c) cooling the finished hot-rolled steel to a CT (Coiling Temperature) of 600 to 650 占 폚 and winding.
(Si): not more than 0.5% by weight, manganese (Mn): not more than 0.7% by weight, phosphorus (P): not more than 0.35% (Ti): 0.15 wt% or less, niobium (Nb): 0.01 to 0.05 wt%, vanadium (V): 0.01 to 0.05 wt%, and the balance of iron (Fe) and unavoidable impurities,
A tensile strength (TS) of 650 to 750 MPa and a yield strength (YP) of 400 to 500 MPa.
3. The method of claim 2,
The steel
Elongation (EL): 22% or more.
3. The method of claim 2,
The steel
And the Vickers hardness at the edge portion within 14 mm from the end has 400 to 450 Hv.
3. The method of claim 2,
The steel
Wherein the final microstructure has a composite structure comprising pearlite uniform in width and length direction.

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