KR20080085808A - A high carbon steel sheet superior in strenth and toughness and a manufacturing method thereof - Google Patents

Abstract

A high carbon steel sheet excellent in strength and toughness without using expensive alloy elements is provided, and a manufacturing method of the high carbon steel sheet is provided. A manufacturing method of a high carbon steel sheet excellent in strength and toughness comprises: a steel manufacturing step of manufacturing a steel for heat treatment comprising, by weight percent, 0.25 to 0.55% of C, 0.5 to 1.2% of Mn, more than 0 and not more than 0.4% of Si, more than 0 and not more than 2.0% of Cr, 0.01 to 0.1% of Al, more than 0 and not more than 0.012% of S, not less than 48/14x[N]% of Ti, 0.0005 to 0.0050% of B and not more than 0.006% of N with the balance being Fe and other inevitable impure elements, and satisfying a condition of 48/14x[N] to 0.03% of Ti in case of 0.0005<=B<=(11/14)x[N]%; a hot rolling step of heating the steel for heat treatment at a temperature of 1200 to 1250 deg.C to hot roll the steel for heat treatment; a coiling step of coiling the hot-rolled steel sheet in a temperature range of 640 to 700 deg.C; a cold rolling step of cold rolling the hot-rolled steel sheet to reduce thickness of the hot-rolled steel sheet; a heat treatment step of subjecting the cold-rolled steel sheet to an austenitizing heat treatment in a temperature range of Ac1 to Ac3 transformation temperature; and an austempering heat treatment step of subjecting the heat-treated steel sheet to an isothermal transformation treatment at 350 to 500 deg.C.

Description

High carbon steel sheet with high strength and toughness and its manufacturing method {A HIGH CARBON STEEL SHEET SUPERIOR IN STRENTH AND TOUGHNESS AND A MANUFACTURING METHOD THEREOF}

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.

Figure 3 is a graph showing the tensile strength and elongation in one embodiment and comparison of the present invention.

Figure 4 is a microstructure of an embodiment of the present invention.

5 is a microstructure of a comparative example.

6 is a microstructure of another comparative example.

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 strength and toughness and a method for producing the same.

In general, high carbon steel, which is used as a high strength, high toughness pavement steel, is produced as a final product through heat treatment of austempering. High carbon steel used as an ostempering heat treatment steel contains a large amount of alloying elements, such as Mn, Mo, Cr, in order to secure the properties after heat treatment. However, the recent surge in alloy element prices has increased the burden on the production cost of these high carbon alloy steels. Therefore, the development of a low cost heat treatment material that can replace the alloying elements has emerged.

The basic physical properties of the ostempering heat treatment material are hardenability. This is because the hardenability must be secured to ensure the required mechanical properties after heat treatment. Moreover, the steel sheet for packaging should be secured at the same time as the excellent strength and toughness after the osmosis heat treatment.

In order to secure such physical properties, ferrite and pearlite phases having low strength but high elongation or martensite phases having high strength and low elongation must be secured, and a bainite phase in which a combination of strength and elongation is properly maintained. After austempering heat treatment, an excellent combination of strength and elongation can be achieved by partially adjusting the fraction of the ferrite or martensite phase based on the bainite phase.

Conventional representative techniques for solving such a problem are the methods proposed in 2003-0016434, 2001-0047691, 1998-147816, 1998-251757, 1996-302445, but most of them refer only to the manufacturing conditions of hot rolled steel sheet, The heat treatment conditions of tempering are not specified.

First, in 2003-0016434, the steel having a carbon content of 0.35 to 0.55% was wound in a relatively low temperature range of 550 to 630 ° C. and subjected to spheroidizing annealing at an ac1 transformation temperature or less, thereby providing high press formability and high heat treatment property. It proposes a method of manufacturing high carbon steel strips with a beautiful cut surface after processing.

In addition, in 2001-0047691, hot-rolled steel sheet for which the carbon content of 0.3 ~ 0.8%, B, N, Ti is properly controlled to finish-rolled at 900 ℃ or less and manufactured to 650 ℃ or less in the temperature range of 650 ~ 710 ℃ Annealing is shown to produce a high carbon steel sheet excellent in ductility and heat treatment.

In addition, in 1998-147816 and 1998-251757, high carbon cold rolled steel sheet was produced by cold annealing before cold rolling of steel with carbon content of 0.25 ~ 0.65% and 0.25 ~ 0.45% by controlling B and Ti appropriately. It shows how to do it.

In addition, 1996-302445 improves the forging and hardenability by adding B to the carbon content of 0.15 to 0.35%, and prevents coarsening of austenite grains during quenching. The purpose of the river is to provide.

As described above, only the manufacturing conditions of the hot rolled steel sheet are described, but not the heat treatment conditions of the osmosis.

In order to solve the above problems, a high carbon steel sheet excellent in strength and toughness without using expensive alloying elements and a method of manufacturing the same are provided.

18 high carbon steel sheet according to an embodiment of the present invention by weight% C: 0.25 ~ 0.55%, Mn: 0.5 ~ 1.2%, Si: 0.4% or less, Cr: 2.0% or less, Al: 0.01 ~ 0.1%, S : 0.012% or less, Ti: 48 / 14ⅹ [N]% or more, B: 0.0005 ~ 0.0050%, N: 0.006% or less, consisting of remaining Fe and other unavoidable impurities, 0.0005≤B≤ (11 / 14) When [N]%, Ti: 48/14 / [N]% ~ 0.03% is satisfied.

In addition, the manufacturing method of the high carbon steel sheet according to the embodiment of the present invention is i) by weight% C: 0.25 ~ 0.55%, Mn: 0.5 ~ 1.2%, Si: 0.4% or less, Cr: 2.0% or less, Al: 0.01 ~ 0.1%, S: 0.012% or less, Ti: 48 / 14ⅹ [N]% or more, B: 0.0005 ~ 0.0050%, N: 0.006% or less, consisting of the remaining Fe and other inevitable impurities, 0.0005 ≤ B ≤ (11/14) ⅹ [N]%, manufacturing a heat treatment steel that satisfies the conditions of Ti: 48/14 ⅹ [N]% ~ 0.03%, ii) the heat treatment steel 1250 ℃ Hot rolling after heating at the following temperature, and iii) winding the hot rolled steel sheet in a temperature range of 640 ° C to 700 ° C.

On the other hand, the method of manufacturing a high carbon steel sheet according to an embodiment of the present invention is iii) reducing the thickness of the manufactured hot rolled steel sheet by cold rolling, ii) the cold rolled steel sheet in the Ac1 to Ac3 transformation temperature range Aging (austenitizing) heat treatment, and iii) the step of constant temperature transformation of the heat-treated steel sheet at the martensite transformation temperature (Ms) to bainite transformation temperature (Bs).

When explaining the reason for limiting the chemical composition of the high carbon steel sheet according to the embodiments of the present invention as follows.

First, the content of carbon (C) is 0.25 to 0.55%. Thus, the reason for limiting the content of carbon (C) is that when the content of carbon is less than 0.25%, it is difficult to secure hardness, that is, excellent durability due to quenching. In addition, when the carbon (C) is more than 0.55%, the transformation delay effect of the boron (B) is canceled, which adversely affects the ostempering heat treatment.

The content of manganese (Mn) is 0.5 to 1.2%. The reason for limiting the content of manganese (Mn) is the formation of iron sulfide (FeS) in which sulfur (S) and iron (Fe) are inevitably contained during the steel manufacturing process when the content of manganese (Mn) is less than 0.1%. Redness brittleness is caused by. In addition, when manganese (Mn) exceeds 1.2%, segregation such as central segregation or micro segregation becomes severe. Since manganese (Mn) is a constituent element of cementite, the density and size of carbide in the segregation zone becomes large. Moldability is impaired.

The content of silicon (Si) is made 0.4% or less. 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 made 0.5% or less. Chromium (Cr), like boron (B) is known as an element that improves the hardenability of the steel can be effective in controlling the phase transformation when added in combination with boron (B). However, if chromium (Cr) exceeds 0.5%, the spheroidization rate can be delayed. Therefore, the content of chromium (Cr) is preferably 0.5% or less.

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 refines the grain size by fixing nitrogen (N) present in the steel with aluminum nitride (AlN). 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 to be 0.012% or less. 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 less than 0.012%.

Titanium (Ti) is made 48/14 kPa [N]% or more. 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 less than 48/14 × [N]%, the effect of scavenging nitrogen (N) from the matrix is small, and thus the formation of boron nitride (BN) cannot be effectively prevented. Further, when 0.0005 ≦ B ≦ (11/14) × [N]%, when the content of titanium (Ti) is 48/14 × [N]% or more, and 0.03% or less, titanium nitride of nitrogen (N) ( Removal of nitrogen (N) by TiN) precipitation is efficient. If 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 carbon (C), and also limited to 0.03% or less since the steelmaking unit increases.

The content of nitrogen (N) is made 0.006% or less. Nitrogen (N) forms boron nitride (BN) when only boron (B) is added without adding titanium (Ti), thereby suppressing the addition effect of boron (B), thereby minimizing the amount of addition.

Boron (B) prevents the transformation of austenite into ferrite or bainite by the effect of segregation at grain boundaries to lower grain boundary energy, or the fine precipitates of Fe23 (C, B) 6 segregate at grain boundaries and lower grain boundary area. do. In addition, it is an important alloy element for securing the hardenability during the heat treatment performed after the final processing. When boron (B) is added in less than 0.0005% it is difficult to expect the above effects. In addition, when the content of boron (B) exceeds 0.0050%, problems of deterioration of toughness and degradation of hardenability due to grain boundary precipitation of the boron (B) precipitate may occur.

Hereinafter, the boron (B) addition effect will be described in detail with reference to FIGS. 1 and 2.

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 v1, and ferrite, bainite, and martensite structures are cooled when cooling at a cooling rate of v2. Is obtained, and structures of ferrite, pearlite and bainite are obtained at the cooling rate of v3.

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 v1 and v2, and microstructures of bainite and martensite are obtained at the cooling rates of v3. 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 an embodiment of the present invention will be described.

First, C: 0.25 to 0.55%, Mn: 0.5 to 1.2%, Si: 0.4% or less, Cr: 2.0% or less, Al: 0.01 to 0.1%, S: 0.012% or less, Ti: 48/14 48 [ N]% or more, B: 0.0005 to 0.0050%, N: 0.006% or less, consisting of the remaining Fe and other unavoidable impurities, and when 0.0005 ≦ B ≦ (11/14) ⅹ [N]%, Ti : To manufacture heat treatment steel satisfying the condition of 48/14 / [N]% ~ 0.03%. The reason for limiting the chemical composition of such steel materials is as described above, so the description thereof is omitted here.

Next, the steel for heat treatment is heated at a temperature of 1250 ℃ or less and then hot rolled to produce a hot rolled steel sheet. Next, the hot rolled steel sheet is wound at a temperature range of 640 ° C. to 700 ° C. to produce a hot rolled coil having low hardness.

Next, after reducing the thickness of the manufactured hot rolled steel sheet through cold rolling, the cold rolled steel sheet is subjected to austenitizing heat treatment in the Ac1 to Ac3 transformation temperature range. When austenizing for a suitable time in the Ac1 to Ac3 transformation temperature range, it is possible to generate a microstructure of the ferrite and austenite phase, and to maintain a small grain size of austenite.

Next, the austenitizing heat-treated steel sheet is subjected to constant temperature transformation at the martensite transformation temperature (Ms) to bainite transformation temperature (Bs). Constant temperature transformation at a martensite transformation temperature (Ms) to bainite transformation temperature (Bs) for a relatively short time promotes the transformation rate of bainite, resulting in bainite and ferrite.

In this way, the final structure of the steel sheet to be bainite and ferrite, it is possible to obtain a high carbon steel sheet having excellent strength due to the bainite phase, excellent strength and toughness due to the ferrite phase.

Hereinafter, the present invention will be described in more detail through experimental examples. The following experimental examples are for illustrating the present invention, and the present invention is not limited thereto.

Experimental Example

First, a steel ingot of the composition shown in Table 1 was prepared by vacuum induction melting to a thickness of 60 mm and a width of 175 mm, and reheated at 1200 ° C. for 1 hour, followed by hot rolling to obtain a hot roll thickness of 2.0 mm. Next, after cooling to the target hot-rolling coil temperature, the hot-rolled coil is simulated by holding it for 1 hour in a furnace preheated to 640-700 ° C. and then cooling it. Next, after heating for 3 minutes at 880 ° C, 760 ° C for an ostempering heat treatment, it is quenched and subjected to constant temperature transformation at 350-500 ° C for 20 seconds.

Steel grade C Mn Si Cr Al S B N Ti Etc Example 1 0.37 0.70 0.17 0.15 0.032 0.0043 0.0023 0.0019 0.024 Balance Fe and impurities Comparative Example 1 0.83 0.42 0.21 0.16 0.011 0.005 - 0.0014 -

Example 1 shown in Table 1 is a steel grade which satisfy | fills the component conditions of this invention, and Comparative Example 1 is a steel grade which deviates from the component conditions of invention steel. Table 2 shows osstem heat treatment conditions and mechanical properties of Experimental Examples 2 to 4 and Example 2 of Table 1, and Experimental Example 5 of Comparative Example 1.

Steel grade Bs (℃) Ms (℃) Austenitising temperature (℃) Constant temperature transformation temperature (℃) Tensile strength (kg / mm2) Elongation (%) Comparative Example 2 To 600 370 880 450 116 5.6 Example 2 To 600 370 760 450 98 11.8 Comparative Example 3 To 600 370 760 350 137 3.9 Comparative Example 4 To 600 370 760 650 67 14.6 Comparative Example 5 To 500 200 760 350 154 3.1

In Comparative Example 2, austenitic temperature is higher than the temperature of the present invention, and coarser austenite single phase is obtained. Therefore, the bainite transformation rate is delayed during constant temperature transformation, and the martensite structure is obtained from the bainite phase and unmodified austenite during a short time constant temperature transformation.

 In Comparative Example 3, the temperature was maintained in the ferrite and austenite two-phase regions during the austenitizing heat treatment, but the martensite structure was formed with a very high strength and a low elongation due to the constant temperature transformation temperature of Ms or less. Therefore, it shows a low elongation compared with Example 2.

In Comparative Example 4, the constant temperature transformation temperature was higher than the bainite formation temperature, thereby forming ferrite and pearlite structures having low strength and high elongation. Therefore, the strength is low compared to the elongation.

In Comparative Example 5, the strength of the bainite structure is rapidly increased due to the high carbon concentration, and the elongation is not good because the unmodified residual austenite is transformed to martensite upon cooling to room temperature.

 On the other hand, in the case of Example 2, austenite of fine grains and ferrite structure is improved by maintaining the temperature during austenizing heat treatment in the two-phase region of ferrite and austenite to improve elongation. In addition, when quenching steel materials after quenching, the austenite is transformed into bainite of appropriate strength and toughness, and thus the final structure becomes a composite structure of bainite and ferrite. Therefore, the combination of strength and elongation is excellent.

3 shows changes in tensile strength and elongation of Examples and Comparative Examples. In FIG. 1, the straight line represents the relationship between the approximate strength of the comparative examples and the change in elongation, and in the case of the example, a combination of the strength and the elongation improved compared to the comparative example.

4 to 6 show the microstructure of the steel sheet. Figure 4 shows the microstructure of the high carbon steel sheet according to an embodiment of the present invention, Figure 5 and Figure 6 shows the microstructure of the comparative example. Each picture is enlarged 3000 times, and in the case of the example, finer structure is distributed than the comparative example.

High carbon steel sheet according to an embodiment of the present invention includes boron (B) in 0.0005 ~ 0.0050% by weight is excellent in hardenability.

In addition, the temperature during the austenitizing heat treatment is maintained in the two-phase region of ferrite and austenite, thereby producing austenite of ferrite structure and fine grains which improves elongation.

In addition, by carrying out an ostempering heat treatment, the combination of strength and toughness is excellent.

Claims (2)

By weight% C: 0.25 ~ 0.55%, Mn: 0.5 ~ 1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 2.0%, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% , Ti: 48 / 14ⅹ [N]% or more, B: 0.0005 to 0.0050%, N: greater than 0 and less than 0.006%, consisting of the remaining Fe and other unavoidable impurities, A high carbon steel sheet excellent in strength and toughness in the case of 0.0005 ≦ B ≦ (11/14) ⅹ [N]%, satisfying the condition of Ti: 48 / 14≤ [N]% to 0.03%. By weight% C: 0.25 ~ 0.55%, Mn: 0.5 ~ 1.2%, Si: greater than 0 and less than 0.4%, Cr: greater than 0 and less than 2.0%, Al: 0.01 to 0.1%, S: greater than 0 and 0.012% , Ti: 48 / 14ⅹ [N]% or more, B: 0.0005 ~ 0.0050%, N: 0.006% or less, consisting of the remaining Fe and other unavoidable impurities, and 0.0005 ≦ B ≦ (11/14) ⅹ [ N]%, the steel manufacturing step of manufacturing a heat treatment steel that satisfies the conditions of Ti: 48 / 14ⅹ [N]% ~ 0.03%, Hot rolling step of hot rolling the heat treatment steel at a temperature of 1200 ℃ to 1250 ℃, Winding step of winding the hot rolled steel sheet in a temperature range of 640 ℃ to 700 ℃, Cold rolling step of reducing the thickness by cold rolling the hot rolled steel sheet, A heat treatment step of austenitizing heat treatment of the cold rolled steel sheet in the Ac1 to Ac3 transformation temperature range, and A method of manufacturing a high carbon steel sheet having excellent strength and toughness, including an austempering heat treatment step of constant temperature transformation of the heat-treated steel sheet at 350 to 500 ° C.

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