KR100851805B1 - Manufacturing method of high carbon steel sheet superior in impact toughness - Google Patents

Abstract

By weight% C: 0.2-0.5%, Mn: 0.1-1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% or less Ti: 0.5 × 48/14 × [N] -0.03%, B: 0.0005-0.0080%, N: greater than 0 and less than 0.006%, consisting of the remaining Fe and other unavoidable impurities, or, by weight% C: 0.2 to 0.5%, Mn: 0.1 to 1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% or less, Ti: Greater than 0 and less than 0.5 × 48/14 × [N]%, B: 0.0005 to 0.0080%, N: greater than 0 and 0.006% or less, and the condition of B (atomic%) / N (atomic%)> 1 Made up of the remaining Fe and other unavoidable impurities, the fraction of the cornerstone ferrite free of carbides and the perlite having a layered carbide structure are each greater than 0 and less than 5%, and the average size of carbide is greater than 0 and 1 μm. High-carbon steel sheet excellent in impact toughness mainly composed of bainite or less It provides a process for producing the same.

High carbon steel plate, impact toughness, fine carbide

Description

Manufacturing method of high carbon steel sheet with excellent impact toughness {MANUFACTURING METHOD OF HIGH CARBON STEEL SHEET SUPERIOR IN IMPACT TOUGHNESS}

1 is a diagram of a continuous cooling state of steel without adding boron (B).

2 is a continuous cooling state diagram of steel to which boron (B) is added.

The present invention relates to a high carbon steel sheet and a manufacturing method thereof, and more particularly, to a high carbon steel sheet excellent in impact toughness and a manufacturing method thereof.

In general, high carbon steel used for heat treatment is manufactured as a hot rolled steel sheet and subjected to QT (Quenching and Tempering) heat treatment. At this time, the structure of the hot rolled steel sheet is a pearlite structure containing lamellar cementite. When the hot rolled steel sheet of the pearlite structure is quenched for a short time, the lamellar cementite does not completely dissolve into the austenite phase and some lamellar cementite remains. In the case of impact fracture of the hot rolled steel sheet in which cementite remains, the fracture starts at relatively light cementite, thereby deteriorating impact toughness.

As a typical representative technique for solving this problem, there is a method proposed in Japanese Patent Laid-Open No. 2003-13144 and Japanese Patent Laid-Open No. 2003-13145.

In Japanese Patent Laid-Open Nos. 2003-13144 and 2003-13145, a steel containing 0.2 to 0.7% of carbon (C) is hot-rolled at a temperature of (Ar3 transformation point -20 ° C) or higher, and the cooling rate exceeds 120 ° C / sec. After cooling at a speed, the cooling was stopped at 650 ° C or higher, followed by winding at 600 ° C or lower, and pickling them, followed by annealing between 640 ° C and Ac1 transformation point, whereby the average particle size of carbide was 0.1-1.2 µm, and there was no carbide. By manufacturing a hot rolled high carbon steel sheet having excellent elongation flangeability by controlling the structure with the volume fraction of ferrite less than 10%, or applying hot rolling of 30% or more after pickling the hot rolled sheet in the above manufacturing method, it is 600 ℃ ~ Ac1 By performing annealing between transformation points, a method of controlling the tissue with a carbide average particle diameter of 0.1 to 2.0 µm and a volume fraction of carbide without ferrite of 15% or less is proposed.

However, it is not possible to perform cooling at a cooling rate exceeding 120 ° C / sec after hot rolling in a normal hot rolling mill. For this purpose, a specially designed cooling device is required and high cost is required for its installation. .

In order to solve the above problems, there is provided a high carbon steel sheet having excellent impact toughness in which re-dissolution of cementite is completely performed even in a short quenching heat treatment, and a method of manufacturing the same.

High carbon steel sheet according to an embodiment of the present invention, by weight% C: 0.2 ~ 0.5%, Mn: 0.1 ~ 1.2%, Si: greater than 0 and 0.4% or less, Cr: greater than 0 and 0.5% or less, Al: 0.01 ~ 0.1%, S: greater than 0 and 0.012% or less, Ti: 0.5 × 48/14 × [N] ~ 0.03%, B: 0.0005 ~ 0.0080%, N: greater than 0 and less than 0.006%, including the remaining Fe And other unavoidable impurities, the fraction of which is formed of bainite having a fraction of cornerstone ferrite free of carbides and a pearlite having a layered carbide structure of greater than 0 and less than 5%, and an average size of carbides greater than 0 and less than 1 µm. It is done.

High carbon steel sheet according to another embodiment of the present invention by weight% C: 0.2 ~ 0.5%, Mn: 0.1 ~ 1.2%, Si: greater than 0 and 0.4% or less, Cr: greater than 0 and less than 0.5%, Al: 0.01 ~ 0.1%, S: greater than 0 and less than 0.012%, Ti: greater than 0 and less than 0.5 × 48/14 × [N]%, B: 0.0005 to 0.0080%, N: greater than 0 and less than 0.006%, including B (Atomic%) / N (atomic%)> 1, the content of the remaining Fe and other unavoidable impurities, and the fraction of the cornerstone ferrite and layered carbide structure without carbides is greater than 0, respectively The main phase is bainite which is 5% or less and whose average size of carbide is larger than 0 and 1 µm or less.

Method for producing a high carbon steel sheet according to an embodiment of the present invention, i) by weight% C: 0.2 ~ 0.5%, Mn: 0.1 ~ 1.2%, Si: greater than 0, 0.4% or less, Cr: greater than 0 and 0.5 % Or less, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% or less, Ti: 0.5 x 48/14 x [N]-0.03%, B: 0.0005 to 0.0080%, N: greater than 0 and 0.006% or less And, ii) manufacturing a steel material consisting of the remaining Fe and other unavoidable impurities, ii) reheating the manufactured steel and hot rolling the finishing temperature at an Ar ₃ transformation temperature or higher, to produce a hot rolled steel sheet, iii) the hot rolled steel sheet Cooling at a cooling rate in the range of 20 ° C./sec to 100 ° C./sec, and iii) winding the cooled hot rolled steel sheet at a temperature in the range of Ms to 530 ° C. to produce a hot rolled coil. The prepared high carbon steel sheet has a fraction of the cornerstone ferrite free of carbide and the pearlite having a layered carbide structure of greater than 0 and 5%, respectively. And a, the average size of carbides is more than 0 and less than or equal to the 1㎛ bainite as the main phase.

In the method for producing a high carbon steel sheet according to another embodiment of the present invention, i) C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2%, Si: greater than 0 and 0.4% or less, Cr: greater than 0 and 0.5 % Or less, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% or less, Ti: greater than 0 and less than 0.5 × 48/14 × [N]%, B: 0.0005 to 0.0080%, N: greater than 0 and 0.006% A method of manufacturing a steel material comprising the following Fe and satisfying a condition of B (atomic%) / N (atomic%)> 1, and remaining Fe and other unavoidable impurities, ii) reheating the steel and changing the finishing temperature to Ar ₃ Manufacturing hot rolled steel sheet by hot rolling above a temperature, i) cooling the hot rolled steel sheet at a cooling rate ranging from 20 ° C./sec to 100 ° C./sec, and iii) cooling the cold rolled steel sheet in the range of Ms˜530 ° C. Winding up at a temperature to produce a hot rolled coil, wherein the high carbon steel sheet produced by this method is characterized by the formation of a cemented ferrite and layered carbide structure free of carbides. The fraction of pearlite having is larger than 0 and 5% or less, respectively, and bainite having an average size of carbides greater than 0 and 1 µm or less is the main phase.

As described above, the reason for limiting the chemical composition of the high carbon steel sheet according to the embodiments of the present invention is as follows.

First, the content of carbon (C) is 0.2 to 0.5%. Thus, the reason for limiting the content of carbon (C) is that when the carbon content is less than 0.2%, it is difficult to secure hardness increase, that is, excellent durability due to quenching. In addition, when the carbon (C) exceeds 0.5%, workability such as elongation flangeability deteriorates after spheroidizing annealing due to an increase in the absolute amount of cementite as the second phase.

The content of manganese (Mn) is 0.1 to 1.2%. Manganese (Mn) is added in order to prevent red brittleness due to FeS formation in which S and Fe are inevitably contained in the steel manufacturing process. When the content of manganese (Mn) is less than 0.1%, red light brittleness occurs, and when manganese (Mn) is more than 1.2%, segregation such as central segregation or micro segregation becomes severe.

The content of silicon (Si) is greater than 0 and less than 0.4%. If the content of silicon (Si) exceeds 0.4%, an increase in scale defects results in a decrease in surface quality.

The content of chromium (Cr) is greater than 0 and less than 0.5%. Chromium (Cr), like boron (B), is known as an element that improves the hardenability of steel, and when added in combination with boron (B), the hardenability of steel can be significantly improved. However, since it is known as an element which delays spheroidization, when a large amount is added, it may cause an adverse effect.

The content of aluminum (Al) is 0.01 to 0.1%. Aluminum (Al) removes the oxygen present in the steel to prevent the formation of non-metallic inclusions during solidification, and the nitrogen (N) present in the steel is fixed with aluminum nitride (AlN) to refine the grain size. However, if the content of aluminum (Al) is less than 0.01% can not achieve the purpose of such addition. In addition, when the content of aluminum (Al) is more than 0.1%, there is a problem of increasing the strength of the steel and raising of the steelmaking unit.

The content of sulfur (S) is greater than 0 and less than 0.012%. When the content of sulfur (S) exceeds 0.012%, manganese sulfide (MnS) is precipitated and the formability of the cold rolled steel sheet is deteriorated. Therefore, the content of sulfur (S) is preferably greater than 0 and 0.012% or less.

Titanium (Ti) removes nitrogen (N) by depositing titanium nitride (TiN). Therefore, boron nitride (BN) is formed by nitrogen (N) to prevent boron (B) from being consumed. Thereby, the addition effect of boron (B) can be made to appear. The addition effect of boron (B) will be described later. However, when the content of titanium (Ti) is greater than 0 and less than 0.5 × 48/14 × [N]%, the effect of scavenging nitrogen (N) from the matrix is small, thus forming boron nitride (BN). It cannot be prevented effectively. Therefore, in this case, the condition of B (atomic%) / N (atomic%)> 1 must be satisfied. However, when the content of titanium (Ti) is 0.5 × 48/14 × [N]% or more, nitrogen (N) can be efficiently removed by precipitation of titanium nitride (TiN) of nitrogen (N), so that B (atomic%) It is not necessary to satisfy the condition / N (atomic%)> 1.

However, when the content of titanium (Ti) is more than 0.03%, titanium carbide (TiC) is formed, the heat treatment property is reduced by the effect of reducing the amount of carbon (C), and the steelmaking unit increases.

The content of nitrogen (N) is greater than 0 and less than 0.006%. Nitrogen (N) forms boron nitride (BN) when only boron (B) is added without addition of titanium (Ti), thereby suppressing the addition effect of boron (B), and therefore it is preferable to minimize the addition amount. However, if the content of nitrogen (N) is more than 0.006% in the range where nitrogen (N) is satisfied with B (atomic%) / N (atomic%)> 1, boron ( Counteract the addition of B). Therefore, the content of nitrogen (N) is preferably greater than 0 and less than 0.006%. However, when titanium (Ti) is added, boron nitride (BN) is not formed by the precipitation of titanium nitride (TiN), and therefore, when titanium (Ti) is added at 0.5 × 48/14 × [N]% or more. It is not necessary to satisfy the condition of B (atomic%) / N (atomic%)> 1.

Boron (B) is segregated at the grain boundary to lower the grain boundary energy, or Fe ₂₃ (C, B) ₆ to precipitate the austenite transformed to ferrite or bainite by the effect of segregating the grain boundary to lower the grain area Suppress In addition, it is a very important alloy element for securing the hardenability during the heat treatment performed after the final processing. When boron (B) is added at less than 0.0005%, it is difficult to expect the above effects. In addition, when the content of boron (B) is more than 0.0080%, problems of deterioration of toughness and degradation of hardenability due to grain boundary precipitation of the boron (B) precipitate may occur.

1 and 2 are schematic diagrams showing phase transformation control by addition of boron (B). In the figure, Ms represents martensite formation start temperature and Mf represents martensite formation end temperature.

FIG. 1 is a schematic continuous cooling state diagram of a microstructure obtained by cooling a steel without boron (B) from a high temperature, for example, finishing finishing temperature to room temperature at different cooling rates.

As shown in FIG. 1, when boron (B) is not added to the steel, martensite single phase is obtained when cooling at a cooling rate of v ₁ , and ferrite, bainite and martensite are cooled when cooling at a cooling rate of v ₂ . Site structures are obtained, and upon cooling at a cooling rate of v ₃ , structures of ferrite, pearlite and bainite are obtained.

As shown in FIG. 2, when boron (B) is added to the steel, ferrite and pearlite bainite transformation curves move to the right along the time axis as compared to FIG.

That is, the addition of boron (B) results in a different microstructure than in the steel without boron (B) for the same cooling rate. That is, martensite is obtained at the cooling rates of v ₁ and v ₂ , and microstructures of bainite and martensite are obtained at the cooling rates of v ₃ . In this way, the effect of increasing the cooling rate is obtained by the addition of boron (B).

Hereinafter, a method of manufacturing a high carbon steel sheet according to embodiments of the present invention will be described.

First, by weight% C: 0.2-0.5%, Mn: 0.1-1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01-0.1%, S: greater than 0.012 % Or less, Ti: greater than 0 and less than 0.5ⅹ48 / 14ⅹ [N]%, B: 0.0005 to 0.0080%, N: greater than 0 and less than 0.006%, B (atomic%) / N (atomic%)> 1 Satisfies the condition, and the rest to produce a steel material consisting of Fe and other unavoidable impurities.

Or in weight% C: 0.2-0.5%, Mn: 0.1-1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01-0.1%, S: greater than 0.012 % Or less, Ti: 0.5 x 48/14 k [N]-0.03%, B: 0.0005-0.0080%, N: greater than 0 and less than 0.006%, the remainder is made of steel slab consisting of Fe and other unavoidable impurities. The reason for limiting the chemical composition of the steel slab is as described above, so the description thereof will be omitted.

Next, the steel is reheated and hot rolled at a finishing temperature of Ar3 transformation temperature or more to produce a hot rolled steel sheet. At this time, the reason why the hot rolling finish temperature is more than the Ar3 transformation temperature is to prevent the two-phase reverse rolling. That is, when two-phase rolling is performed, a large amount of cornerstone ferrite without carbides is generated, so that a uniform distribution of carbides over the entire structure cannot be obtained.

Next, the hot rolled steel sheet is cooled at a cooling rate in the range of 20 ° C / sec to 100 ° C / sec. If the cooling rate after the hot rolling is less than 20 ℃ / sec is a large amount of precipitation of ferrite and pearlite is made it becomes impossible to obtain a mixed structure of hot rolled bainite, bainite and martensite or martensite structure. In addition, in order to obtain a cooling rate of more than 100 ℃ / sec requires a new equipment, such as a pressurized rapid cooling equipment rather than the conventional method is a cause of cost increase.

Next, the hot rolled steel sheet is wound at a temperature in the range of Ms to 530 ° C. If the coiling temperature is higher than 530 ℃, the low temperature structure can not be obtained due to perlite transformation, so the coiling temperature should be below 530 ℃. If the odor is less than Ms, martensite transformation may occur during odor and cracks may occur. In practice, the winding temperature largely depends on the performance of the winder.

Next, the prepared hot rolled steel sheet is subjected to Q / T (Quenching and Tempering) heat treatment. Since the high carbon steel sheet according to the embodiment of the present invention has a fine bainite structure, carbides are sufficiently dissolved only by heat treatment at 900 ° C. for 10 seconds to 30 minutes.

Experimental Example

Ingots of the composition shown in Table 1 were prepared by vacuum induction melting to a thickness of 60 mm and a width of 175 mm, and re-heated at 1200 ° C. for 1 hour, followed by hot rolling to a hot rolled thickness of 4.3 mm. The hot rolling finish temperature was above the Ar3 transformation point, and the ROT cooling rate was 10 ° C / sec and 30 ° C / sec. After cooling to the target hot-rolling winding temperature, the furnace was preheated to a furnace heated to 450 to 600 ° C for 1 hour and then cooled. The hot rolling was simulated. Annealing heat treatment was performed at 900 degreeC for 10 second, and annealing heat treatment was performed at 600 degreeC for 3 minutes.

Steel grade C Mn Si Cr Al S B N Ti Etc One 0.34 0.73 0.21 0.09 0.030 0.0027 0.0058 0.0010 0.024 Balance Fe and impurities

Table 2 shows the presence of undissolved-cementite and ductile-brittle transition temperature according to the process conditions using steel grade 1 having the composition of Table 1.

Remarks ROT cooling rate (℃ / sec) Coiling temperature (℃) Pearlite With / Without Cementite with or without after quenching Ductile-brittle transition temperature after annealing Comparative Example 1 10 450 U U 23 Example 1 30 450 radish radish -12 Comparative Example 2 30 600 U U 48

The pearlite structure containing lamellar cementite is formed during cooling when the ROT cooling rate is slow after hot rolling, and when the winding temperature is high even though the cooling rate is high. Therefore, bainite structures having carbides are formed only when the conditions of cooling rate and winding temperature are satisfied. The coarse cementite of lamellar tissue has a slow dissolution rate into austenite during annealing heat treatment, and thus, after 10 seconds of heat treatment, complete dissolution does not occur so that the cementite structure remains after the final annealing heat treatment.

On the other hand, the bainite structure containing fine cementite is rapidly dissolved in the cementite to austenite during the hardening heat treatment, and thus complete dissolution occurs even in the heat treatment of about 10 seconds. Therefore, after the final annealing heat treatment, an annealing heat treatment structure in which fine carbides are present is generated. The coarse cementite remaining after hardening or annealing heat treatment from the pearlite hot-rolled tissue serves as a source of fracture cracking when the steel is impacted. Therefore, the impact toughness of steel is reduced. On the other hand, the fine cementite produced after quenching or annealing heat treatment from bainite hot-rolled tissue is too small to act as a source of fracture cracking upon impact. Therefore, cementite does not act as a source of fracture cracking, so the impact toughness of the steel is improved.

In Example 1 according to an embodiment of the present invention, a pearlite structure containing coarse cementite is not produced from the hot rolled tissue. Therefore, no coarse cementite remains after the hardening heat treatment. Therefore, compared with Comparative Example 1 and Comparative Example 2 in which coarse cementite remains, the ductility-brittle transition temperature is significantly improved to -12 ° C.

As mentioned above, although preferred embodiment of this invention was described, this invention is not limited to this, It can be variously modified and implemented in a claim, a detailed description of an invention, and the range of an accompanying drawing. And of course, this also belongs to the scope of the present invention.

High carbon steel sheet according to an embodiment of the present invention, in the manufacturing process, the cooling rate after finishing rolling is lower than the conventional 20 ℃ / sec ~ 100 ℃ / second, it is wound at a low temperature. Therefore, carbides are finely and uniformly distributed in the steel sheet.

In addition, since the carbide is fine, the hardening heat treatment property is excellent, and the impact toughness after annealing is excellent.

Claims (1)

By weight% C: 0.2-0.5%, Mn: 0.1-1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% or less Ti: 0.5 × 48/14 × [N] ~ 0.03%, B: 0.0005 ~ 0.0080%, N: greater than 0 and less than 0.006%, manufacturing a steel material consisting of the remaining Fe and other unavoidable impurities, Or in weight% C: 0.2-0.5%, Mn: 0.1-1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 0.5%, Al: 0.01-0.1%, S: greater than 0.012 % Or less, Ti: greater than 0 and less than 0.5 × 48/14 × [N]%, B: 0.0005 to 0.0080%, N: greater than 0 and 0.006% or less, and B (atomic%) / N (atomic%) Manufacturing a steel material satisfying the condition of> 1 and consisting of the remaining Fe and other unavoidable impurities, Reheating the steel and hot rolling a finish temperature above the Ar ₃ transformation temperature to produce a hot rolled steel sheet, Cooling the hot rolled steel sheet at a cooling rate in a range of 20 ° C./sec to 100 ° C./sec, and And winding the cooled hot rolled steel sheet at a temperature in the range of Ms˜530 ° C. to produce a hot rolled coil. A steel sheet composed mainly of bainite having a primary phase ferrite free of carbides and a pearlite having a layered carbide structure of greater than 0 and less than 5%, and an average size of carbide greater than 0 and less than 1 µm at 900 ° C. A method of manufacturing a high carbon steel sheet having improved impact toughness, which further includes a reheating process for about 2 to 30 minutes.

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