KR101372730B1 - High carbon rolled steel sheet for electric resistance welded tubes having excellent uniformity and method for production thereof - Google Patents

Abstract

The present invention relates to a high carbon hot rolled steel sheet for electric resistance welded steel pipe having excellent material uniformity, and more particularly, to a high carbon hot rolled steel sheet having excellent material uniformity that can be used for steel pipe for automobile parts and the like and a method of manufacturing the same. .
The present invention can produce a high-carbon hot-rolled steel sheet for electrical resistance welded steel pipe excellent in uniformity between the hot-rolled structure by controlling the composition, microstructure and process conditions of the steel material. After forming the high carbon hot rolled steel sheet, not only the dimensional accuracy of the parts is excellent, but also no defects occur during machining, and excellent molding is also performed in the polarization test of the welded steel pipe, and even after the final heat treatment, uniform structure and hardness distribution are achieved. Can have

Description

HIGH CARBON ROLLED STEEL SHEET FOR ELECTRIC RESISTANCE WELDED TUBES HAVING EXCELLENT UNIFORMITY AND METHOD FOR PRODUCTION THEREOF}

The present invention relates to a high carbon hot rolled steel sheet for electric resistance welded steel pipe having excellent material uniformity, and more particularly, to a high carbon hot rolled steel sheet having excellent material uniformity that can be used for steel pipe for automobile parts and the like and a method of manufacturing the same. .

High carbon hot rolled steel sheet using high carbon steel has been used in various applications such as machine parts, tools, and automobile parts, and has obtained the desired shape through blanking, bending, and pressing after manufacturing hot rolled steel sheet having a desired thickness. After that, high hardness is finally given through heat treatment.

The characteristics required for high carbon hot rolled steel sheet are excellent material uniformity. If the material deviation in the high carbon hot rolled steel sheet is large, not only the dimensional accuracy of the part is degraded during the forming process, but it also causes defects during machining, and causes uneven structure distribution even in the final heat treatment process.

In particular, in the case of electric resistance welded high carbon steel pipe, if the material variation in the hot-rolled steel sheet becomes large, the steel pipe forming is not smoothly performed, which leads to non-uniformity of the outer diameter of the steel pipe, and easily cracks in the flat test.

Various inventions have been proposed to improve the formability of the high carbon hot rolled steel sheet, but most of the inventions focus only on the control of carbide size and distribution in the microstructure after cold rolling and annealing. It is not an invention on formability and heat treatment uniformity of formability and annealed steel sheet.

According to Patent Document 1 on the formability of a high carbon annealed steel sheet after cold rolling and annealing, when the carbide distribution having an average carbide particle diameter of 1 μm or less and a carbide fraction of 0.3 μm or less is 20% or less by controlling the annealing conditions, Although the properties are improved, there is no mention of the formability in the state of the hot rolled steel sheet, and there is no necessity that the carbide grain size should be formed to 1 μm or less after annealing the hot rolled steel sheet having excellent formability.

By controlling the annealing conditions to obtain the standard deviation of the carbide grain size divided by the average carbide diameter of 1.0 or less, the patent document 1 which improved press formability and the annealing conditions were appropriately controlled so that the ferrite grain size was 5 μm or more and the standard deviation of the carbide grain size was controlled. In Patent Literature 2, which specifies that is less than or equal to 0.5, there is no reference to the hot rolled structure, and there is no necessity to have a distribution as described above after the hot rolled steel sheet having excellent formability passes through the usual annealing conditions.

Patent Document 3 discloses that the fraction of the ferrite and cementite is 10% or less, and the fine blanking processability is increased when the grain size of the ferrite is in the range of 10 to 20 μm, but this is also limited to the microstructure control of the annealed steel sheet. As a result, there is a distance from the moldability of the hot rolled tissue, and in improving the moldability of the hot rolled tissue, it is possible to utilize means for minimizing material variation by suppressing the formation of ferrite and obtaining a uniform phase distribution.

On the other hand, Patent Document 4 is a ferrite by cooling the hot rolled plate at a rate of 120 ℃ or more per second with an annealing microstructure to control the grain size of ferrite after the annealing 6μm or less, carbide diameter 0.1 ~ 1.2μm for annealing to improve the elongation flange properties There is also proposed a method for specifying a hot rolled structure having a fraction of 10% or less. However, this invention is intended to improve the elongation flangeability of the annealing material, and a fast cooling rate of 120 ° C. per second is not necessary to make the ferrite fraction 10% or less in the hot rolled sheet.

In addition, Patent Document 5 obtains a high-carbon bainite structure in which the fractions of the cornerstone ferrite and the ferrite are each 5% or less, and the bainite fraction is 90% or more, and as a result, a structure in which fine cementite is distributed after annealing is obtained. A method of improving the formability of annealing plate is proposed. However, the present invention is only for improving the formability of the annealing material by finely controlling the average size of the carbide to 1 μm or less and the grain size to 5 μm or less, and is not an invention related to the formability of the hot rolled material for tube fabrication.

In particular, there is Patent Document 6 as an existing invention for improving the formability of high-carbon steel for electric resistance welded steel pipe, but the present invention proposes a method for improving the workability by controlling the content of Si and Mn, which is a ferrite hardening element. However, in actual piping, the problem of dimensional failure after piping is more serious than the lack of piping ability, and when the ferrite phase and the pearlite phase are mixed, the material deviation becomes large, so the steel tube formability is insufficient due to the dimensional defect after the piping and the presence of local cracks. Can lead to.

Japanese Laid-Open Patent Application No. 2005-344194 Japanese Patent Application Laid-Open No. 2005-344196 Japanese Patent Laid-Open No. 2001-140037 Japanese Patent Laid-Open No. 2006-063394 Korea Patent Publication No. 2007-0068289 Japanese Patent Laid-Open No. 2004-190086

The present invention provides a high carbon hot rolled steel sheet for electrical resistance welded steel pipe having a 95% or more perlite structure and excellent material uniformity by controlling the compositional components, microstructures and process conditions, in particular, cooling conditions, constituting the steel materials and manufacturing the same. To provide a way.

According to an aspect of the present invention,

By weight%, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (excluding 0) , S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe And it is made of other unavoidable impurities, it provides a high carbon hot rolled steel sheet for steel pipes having excellent material uniformity, characterized in that the area fraction of the pearlite phase is 95% or more.

According to another aspect of the present invention,

By weight%, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (excluding 0) , S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe And producing a high carbon slab made of other unavoidable impurities; Reheating the slab at 1100-1300 ° C .; After the reheating, performing hot rolling so that the finishing hot rolling temperature becomes 800 to 1000 ° C .; Cooling the hot rolled steel sheet to a cooling rate (CR1) satisfying the following formula (1) or formula (1 ') until reaching 580 ° C from the finish hot rolling temperature; Cooling the cooled steel sheet at a cooling rate CR2 that satisfies Equation 2 below until the coiling temperature CT is reached; And it provides a method for producing a high-carbon hot rolled steel sheet for steel pipe comprising the step of winding the steel sheet cooled to the coiling temperature to the coiling temperature (CT) satisfying the following formula (3).

≪ Formula (1) >

Cond1 ≤ CR1 (° C / sec) <120,

Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

<Equation (1 ')>

Cond1? CR1 (占 폚 / sec)? Cond1 + 20,

Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

<Equation (2)>

0 ≤ CR2 (° C / sec) ≤ Cond2,

Cond2 = 175-CT / 3.33

&Lt; Formula (3) >

Cond3 ≤ CT (℃) ≤ 580,

Cond3 = 640-237 × C (wt.%)-16.5 × Mn (wt.%)-8.5 × Cr (wt.%)

The present invention can produce a high-carbon hot-rolled steel sheet for electrical resistance welded steel pipe excellent in uniformity between the hot-rolled structure by controlling the composition, microstructure and process conditions of the steel material.

After forming the high carbon hot rolled steel sheet, not only the dimensional accuracy of the parts is excellent, but also no defects occur during machining, and excellent molding is also performed in the polarization test of the welded steel pipe, and even after the final heat treatment, uniform structure and hardness distribution are achieved. Can have

1 is a view showing a transformation curve of a hot rolled steel sheet according to the cooling rate control.
2 is a view showing the structure of the high carbon hot rolled steel sheet prepared according to the present invention.
3A is a micrograph of a high carbon hot rolled steel sheet manufactured according to a comparative example of the present invention, and FIG. 3B is a micrograph of a high carbon hot rolled steel sheet manufactured according to the inventive example.

Hereinafter, embodiments of a high carbon hot rolled steel sheet and a method for manufacturing the same according to the present invention will be described in detail, but the present invention is not limited to the following examples. Therefore, those skilled in the art may implement the present invention in various other forms without departing from the technical spirit of the present invention.

Hereinafter, the present invention will be described in detail.

High-carbon hot-rolled steel sheet for steel pipe according to the present invention in weight%, C: 0.3 ~ 0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5 ~ 1.5%, Cr: 1.0% or less (excluding 0) , P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe and other unavoidable impurities.

The high carbon hot rolled steel sheet preferably contains 0.3 to 0.4% of carbon (C) by weight.

In addition, the high carbon hot rolled steel sheet preferably contains 0.4 to 0.6% of carbon (C) by weight.

Hereinafter, the reason for limiting the components in the high carbon hot rolled steel sheet of the present invention will be described in detail.

At this time, the content of the component element means all weight%.

C: 0.3 ~ 0.6%

Carbon (C) is an element necessary for securing the hardenability at the time of heat treatment and the hardness after the heat treatment, and is preferably added at least 0.3% to avoid the formation of the cornerstone ferrite phase. However, if the content exceeds 0.6%, it will have a very high hot-rolled hardness, the absolute value of the material deviation increases, and the moldability also worsens, so that the material uniformity characteristic of the present invention does not appear.

In particular, in the case of containing 0.3 to 0.4% of carbon (C), since the material is soft before the final heat treatment, various moldings such as drawing, forging, drawing, and the like are easy, so that it can be used for the production of complex mechanical parts.

In the case of containing 0.4 to 0.6% of carbon (C), it is relatively difficult to process during the molding process, but because of its high hardness after the final heat treatment, it is excellent in wear resistance and fatigue resistance and can be used for manufacturing a high mechanical load group. have.

Si: 0.5% or less (excluding 0)

Silicon (Si) is added together with Al for deoxidation. When Si is added, there is a reverse function that red scale occurs and there is a possibility of stabilizing ferrite to increase material variation, so the upper limit is limited to 0.5%. It is preferable.

Mn: 0.5 ~ 1.5%

Manganese (Mn) also contributes to increase hardenability and to secure hardness after heat treatment. If Mn is too low, below 0.5%, coarse FeS may be formed and the steel may be very fragile, and in particular, it is difficult to avoid the formation of a ferrite phase, thereby increasing hardness variation. On the other hand, when Mn is added in excess of 1.5%, there is a concern that the alloy cost increases and residual austenite is formed.

Cr: 1.0% or less (excluding 0)

Chromium (Cr) increases hardenability and contributes to hardness after heat treatment. Cr also contributes to the improvement of moldability by making the lamellar spacing of the pearlite fine. When the Cr content exceeds 1.0%, there is a fear that the alloy cost increases and the hot rolled material hardness is too high, and the upper limit thereof is preferably limited to 1.0%.

P: 0.03% or less (excluding 0)

Phosphorus (P) is an impurity element, and if its content exceeds 0.03%, the weldability is lowered and the risk of brittleness of steel is increased, so the upper limit is preferably limited to 0.03%.

S: 0.015% or less (excluding 0)

Sulfur (S) is an impurity element similar to phosphorus (P) and is an element that inhibits the ductility and weldability of the steel sheet. When the content exceeds 0.015%, the ductility and weldability of the steel sheet are highly likely to be impaired. Therefore, the upper limit thereof is preferably limited to 0.015%.

Al: 0.05% or less (excluding O)

Aluminum (Al) is an element added for deoxidation. It is not necessary to add more than 0.05% as a deoxidizer in the steelmaking process, and if the amount is too large, the nozzle may be clogged during playing, so the upper limit thereof is 0.05%. It is preferable to limit.

B: 0.0005 to 0.005%

Boron (B) is an element that greatly contributes to the hardenability of the steel material, it is necessary to add more than 0.0005% in order to obtain a hardenability strengthening effect, but if the addition amount is too large, boron carbide is formed at the grain boundary to provide a nucleation site rather There exists a possibility of worsening hardenability. Therefore, it is preferable to limit the upper limit to 0.005%.

Ti: 0.005 to 0.05%

Titanium (Ti) is an element added for the protection of so-called boron that inhibits the formation of BN by reacting with N to form TiN, and when the added amount is less than 0.005%, there is a fear that nitrogen in the steel may not be effectively fixed. If the amount is excessively large, the steel may be vulnerable due to coarsening of TiN or the like. Therefore, it is preferable to limit the upper limit to 0.05% in a range capable of sufficiently fixing nitrogen in the steel.

N: 0.01% or less (excluding 0)

Nitrogen (N) contributes to the hardness of steel, but it is difficult to control, and if the content exceeds 0.01%, the risk of brittleness is greatly increased, and the excess N remaining after TiN is formed should contribute to the hardenability. Since it may be consumed in the form of BN, the upper limit thereof is preferably limited to 0.01%.

The high carbon hot rolled steel sheet according to the present invention contains residual Fe and other unavoidable impurities in addition to the above element components.

The microstructure of the high carbon hot rolled steel sheet according to the present invention is preferably composed of at least 95% of the pearlite structure in an area fraction.

When the fraction of the pearlite phase is 95% or less, that is, when the fraction of the saltpeter ferrite phase, the bainite phase, and the martensite phase is formed at 5% or more, the material variation of the steel sheet is increased, making it difficult to obtain a hot rolled steel sheet having a uniform material.

In addition, it is preferable that the pearlite phase of the high carbon hot rolled steel sheet is obtained at least 75% by area fraction before winding. This is to impart material uniformity characteristics to the hot-rolled steel sheet. By obtaining 75% or more of the pearlite phase before winding, the average size of the pearlite colony divided by the grain boundary of 15 degrees or more is 15 μm. It may be formed below, and the average thickness of the cementite layer in the lamellar structure of the pearlite structure may be formed to 20 nm or less so as to have a fine and uniform structure.

If the fraction of the perlite phase transformed before winding is insufficient to 75% or less, a large amount of latent latent heat is accumulated in the coil after winding, causing partial spheroidization of the pearlite tissue, causing high hardness deviation, and lamellar. The coarsening of the structure occurs, resulting in the formation of a tissue of low hardness. In addition, there is a fear that a ferrite or bainite phase is formed during transformation.

Hereinafter, a method of manufacturing the high carbon hot rolled steel sheet of the present invention will be described in detail.

The following manufacturing method shows a preferable example in which the high carbon hot rolled steel sheet of the present invention can be produced, but is not limited thereto.

First, in weight percent, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (0 is S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), A high carbon steel slab made of balance Fe and other unavoidable impurities is produced.

The steel slab prepared above is reheated at 1100 ~ 1300 ℃.

At this time, if the reheating temperature is less than 1100 ℃ causes a problem that the hot rolling load increases sharply, if the reheating temperature is higher than 1300 ℃ may increase the amount of surface scale leading to material loss, and also increase the heating cost.

The reheated steel slab is subjected to hot rolling so that the finishing hot rolling temperature is 800 to 1000 ° C. to produce a steel sheet.

In this case, if the finishing hot rolling temperature is less than 800 ℃, there is a problem that the rolling load is greatly increased, and if the finish hot rolling temperature exceeds 1000 ℃, the structure of the steel sheet is coarsened, the steel is vulnerable, the scale is thick, and the surface quality related to the scale is degraded. May occur.

The hot rolled steel sheet is cooled in a run out table (ROT) until it reaches 580 ° C. from the finishing hot rolling temperature.

At this time, the cooling rate (CR1) is preferably controlled to the cooling rate in the range of Cond1 or more from less than 120 ℃ per second as shown in the following formula (1). If the cooling rate (CR1) is slower than Cond1, which is the value calculated by Equation (1), the ferrite phase is formed during cooling, and the hardness difference becomes larger than 25 HV. It will be greatly bad.

However, by controlling the contents of the C, Mn, Cr and B components constituting the steel as in the present invention, the desired material uniformity effect can be obtained even at a normal cooling rate.

&Lt; Formula (1) >

Cond1 ≤ CR1 (° C / sec) <120,

Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

In addition, the cooling rate (CR1) can be controlled to satisfy the Cond1 or more Cond1 + 20 ℃ / sec range as shown in the following formula (1 ').

By controlling the cooling rate CR1 as in the following formula (1 '), it is possible to further promote the ferrite transformation in the next step by avoiding the formation of the ferrite phase but not far from the phase transformation nose temperature.

<Equation (1 ')>

Cond1? CR1 (占 폚 / sec)? Cond1 + 20,

Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

Thereafter, the cooling is completed at the cooling rate CR2 until the temperature of the completed steel sheet reaches the winding temperature CT from 580 ° C.

At this time, the cooling rate (CR2) is preferably controlled to the cooling rate in the range of 0 or more and Cond2 or less as shown in the following formula (2). During cooling, by slow cooling at the cooling rate CR2, the pearlite phase can be transformed by 75% or more by area fraction before winding, thus finally obtaining a fine and uniform lamellar structure of the pearlite phase.

When the cooling rate CR2 is faster than Cond2, which is a value calculated by the following equation (2), sufficient pearlite transformation is not performed before the winding step, and there is a concern that a bainite phase may be formed during cooling depending on conditions. In this case, it is difficult to manufacture a steel sheet exhibiting excellent material homogenization effect.

<Equation (2)>

0 ≤ CR2 (° C / sec) ≤ Cond2,

Cond2 = 175-CT / 3.33

Finally, the steel sheet cooled to the winding temperature CT by passing through the water cooling stand ROT is wound up in a rolled coil.

At this time, it is preferable to control winding temperature CT to the temperature of the range of Cond3 or more and 580 degrees C or less like following formula (3). This is to suppress the formation of ferrite and bainite phases upon winding.

If the coiling temperature CT exceeds 580 ° C., the ferrite phase may be formed in the holding step after the winding, even if the cooling conditions are satisfied. In particular, when Cr is added as a ferrite stabilizing element in the manufacture of the high carbon hot rolled steel sheet, the tendency of forming the ferrite phase in the constant temperature holding step increases the need for a low winding temperature. On the other hand, if the coiling temperature (CT) is less than Cond3, the value calculated by the following equation (3), the bainite phase is formed to increase the hardness difference.

&Lt; Formula (3) >

Cond3 ≤ CT (℃) ≤ 580,

Cond3 = 640-237 × C (wt.%)-16.5 × Mn (wt.%)-8.5 × Cr (wt.%)

In manufacturing the high carbon hot rolled steel sheet, by controlling the composition and controlling the cooling rate and the winding temperature as shown in FIG. 1, the area fraction of the pearlite phase can be transformed to 75% or more before the winding step. By forming the pearlite phase above 75%, it is possible to have 95% or more of the pearlite phase after winding.

In addition, by controlling the production conditions such as composition and cooling rate, the average size of the pearlite colony is formed to 15 μm or less, and the average thickness of the cementite layer in the lamellar tissue is formed to 20 nm or less to obtain a fine and uniform lamellar structure. The steel plate which has can be manufactured.

In addition, the high-carbon hot-rolled steel sheet manufactured according to the above can ensure a hardness difference between the microstructures of 25 HV or less, and has excellent material uniformity characteristics. In this case, the hardness difference is defined as the difference between the 95% level hardness and the 5% level hardness when the maximum value of the hardness measured in the hot-rolled steel sheet is set to 100% and the minimum value to 0%.

In this way, when the low material deviation is ERW made using the same high-carbon hot-rolled steel sheet, the roundness of the cross section, that is, the average outer diameter tolerance can be secured low, and even when the flat test is carried out, the late breakage is high after the high processing load. Get up.

The hot rolled steel sheet manufactured by the manufacturing method according to the present invention may be used as it is without further processing thereafter, or may be used after further undergoing an annealing process or the like.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

< Example >

After preparing a hot rolled steel sheet having the composition shown in Table 1, an ERW steel pipe having an outer diameter of 55 mm and a thickness of 3.8 mm was manufactured from the hot rolled steel sheet.

More specifically, after preparing a steel slab having the composition of Table 1, the slab was reheated at 1200 ℃ for 2 hours. Thereafter, the reheated slabs were hot rolled at 900 ° C. to prepare hot rolled steel sheets having a thickness of 3.8 mm.

After finishing rolling, the steel sheets were cooled at a cooling rate of CR1 to 580 ° C. in a water cooling zone (ROT), and then cooled to a cooling rate of CR2 up to the coiling temperature thereafter. Then, the hot rolled steel sheet which completed cooling to the coiling temperature was wound up and manufactured at each target coiling temperature. Cooling rates (CR1, CR2) and winding temperatures applied to the respective steel sheets are shown in Table 2 below.

The microstructure of the final hot rolled steel sheet obtained by completing the coiling was analyzed and the Vickers hardness was measured. At this time, the hardness was measured by Vickers hardness of 500 g load, and when the maximum value is set to 100% and the minimum value to 0% in the results measured more than 30 times, the difference between the hardness of 95% level and 5% level was defined as the hardness difference. In addition, the roundness (mean outside diameter tolerance) was measured from the ERW steel pipe, and the results are shown in Table 2. In addition, the thickness of the cementite layer in the lamellar structure of each steel material was measured by a scanning electron microscope and represented as an average value.

C Si Mn Cr B Ti Al P S N division A 0.311 0.254 1.35 0.105 0.0019 0.021 0.025 0.015 0.007 0.0044 Honor B 0.305 0.255 1.45 0.32 0.0002 0.002 0.033 0.014 0.0052 0.004 Comparative Example C 0.347 0.254 1.35 0.33 0.0017 0.017 0.035 0.013 0.0047 0.0052 Honor D 0.385 0.205 1.42 0.35 0.0010 0.015 0.041 0.016 0.0035 0.0057 Honor E 0.452 0.202 0.95 0.55 0.0015 0.025 0.017 0.017 0.0055 0.0035 Honor F 0.555 0.355 0.72 0.25 0.0021 0.018 0.025 0.01 0.0027 0.0047 Honor

Cond1 CR1 Cond2 CR2 Cond3 CT Pearlite
Fraction Hardness
Deviation Colony
size Sementite
thickness Roundness Flat test
Height division A 41 100 -5.2 0 543 600 87% 62 13 μm 23 nm -0.30mm 22mm Comparative Example B 38 50 2.3 1.1 541 575 75% 100 11㎛ 19 nm -0.26mm 28 mm Comparative Example C 25 50 2.3 0.9 533 575 97% 22 10 탆 15 nm -0.05mm 37 mm Honor D 10 50 9.8 5.8 522 550 97% 17 8㎛ 13 nm -0.15mm 40mm Honor E 10 50 39.9 25.8 513 450 83% 45 7 탆 10 nm + 0.32mm 20mm Comparative Example F 10 50 9.8 6.5 494 550 97% 13 9㎛ 11 nm -0.14mm 38 mm Honor

As a result of the measurement, in the case of Comparative Example B in which the boron addition amount does not satisfy the condition provided by the present invention, even if the cooling condition and the winding temperature condition provided by the present invention are satisfied, the fraction of perlite is 95% or less, and the hardness deviation is 100 HV. Was measured.

In addition, in the case of Comparative Examples A and E, which satisfied all the component conditions of the steel but did not satisfy the coiling temperature conditions, the pearlite fraction was 95% or less, and the hardness variation was also measured to be 25 HV or more. In particular, in the case of Comparative Example A, as shown in FIG. 3A, in addition to the ferrite phase, the ferrite phase was formed by about 13%, resulting in inferior material uniformity, and the thickness of the cementite layer in the tissue was 23 nm. Can be.

On the other hand, in the case of Inventive Example D, which satisfies both the component conditions and the preparation conditions provided by the present invention, as shown in FIG. 3B, the fraction of pearlite was 97%, and the hardness variation was measured at 17 HV. In addition, the cementite layer was found to have a very fine structure of 13 nm thickness.

Through the above results, it is possible to obtain a steel sheet excellent in material uniformity only if all the component conditions and manufacturing conditions provided by the present invention are satisfied.

Claims (8)

By weight%, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (excluding 0) , S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe And other unavoidable impurities,
A high carbon hot rolled steel sheet for steel pipes having excellent material uniformity, wherein an area fraction of the pearlite phase is 95% or more, and an average thickness of the cementite layer in the lamellar tissue is 20 nm or less.
The method of claim 1,
A high carbon hot rolled steel sheet for steel pipe having excellent material uniformity, characterized in that the size of the colony on the ferrite is 15 μm or less.
The method of claim 1,
The high carbon hot rolled steel sheet has a high uniformity of material, characterized in that the difference between 95% hardness and 5% hardness when the maximum value of hardness is 100% and the minimum value is 0% is 25 HV or less. Carbon hot rolled steel sheet.
The method of claim 1,
High carbon hot rolled steel sheet for steel pipes having excellent material uniformity, characterized in that more than 75% of the phase on the perlite is transformed before winding.
The method of claim 1,
High carbon hot rolled steel sheet for steel pipes having excellent material uniformity, characterized in that the content of the carbon (C) is 0.3 ~ 0.4%.
The method of claim 1,
High carbon hot rolled steel sheet for excellent steel material uniformity, characterized in that the content of the carbon (C) is 0.4 ~ 0.6%.
By weight%, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (excluding 0) , S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe And producing a high carbon steel slab made of other unavoidable impurities;
Reheating the slab at 1100-1300 ° C .;
After the reheating, performing hot rolling so that the finishing hot rolling temperature becomes 800 to 1000 ° C .;
Cooling the hot rolled steel sheet to a cooling rate (CR1) satisfying the following equation (1) until reaching 580 ° C. from the finishing hot rolling temperature;
<Equation (1)>
Cond1 ≤ CR1 (° C / sec) <120,
Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

Cooling the cooled steel sheet at a cooling rate CR2 that satisfies the following expression (2) until the winding temperature CT is reached; And
<Equation (2)>
0 ≤ CR2 (° C / sec) ≤ Cond2,
Cond2 = 175-CT / 3.33

Winding the steel sheet cooled to the coiling temperature at a coiling temperature CT satisfying the following formula (3);
<Equation (3)>
Cond3 ≤ CT (℃) ≤ 580,
Cond3 = 640-237 × C (wt.%)-16.5 × Mn (wt.%)-8.5 × Cr (wt.%)

Method for producing a high carbon hot rolled steel sheet for steel pipe comprising a.
By weight%, C: 0.3-0.6%, Si: 0.5% or less (excluding 0), Mn: 0.5-1.5%, Cr: 1.0% or less (excluding 0), P: 0.03% or less (excluding 0) , S: 0.015% or less (excluding 0), Al: 0.05% or less (excluding 0), B: 0.0005 to 0.005%, Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), balance Fe And producing a high carbon steel slab made of other unavoidable impurities;
Reheating the slab at 1100-1300 ° C .;
After the reheating, performing hot rolling so that the finishing hot rolling temperature becomes 800 to 1000 ° C .;
Cooling the hot rolled steel sheet to a cooling rate (CR1) satisfying the following formula (1 ') until reaching 580 ° C from the finish hot rolling temperature;
<Equation (1 ')>
Cond1? CR1 (占 폚 / sec)? Cond1 + 20,
Cond1 = 180-355 × C (wt.%)-20 × Mn (wt.%)-15 × Cr (wt.%) Or 10

Cooling the cooled steel sheet at a cooling rate CR2 that satisfies the following expression (2) until the winding temperature CT is reached; And
<Equation (2)>
0 ≤ CR2 (° C / sec) ≤ Cond2,
Cond2 = 175-CT / 3.33

Winding the steel sheet cooled to the coiling temperature at a coiling temperature CT satisfying the following formula (3);
<Equation (3)>
Cond3 ≤ CT (℃) ≤ 580,
Cond3 = 640-237 × C (wt.%)-16.5 × Mn (wt.%)-8.5 × Cr (wt.%)

Method for producing a high carbon hot rolled steel sheet for steel pipe comprising a.

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