KR101267715B1 - Hot-rolled steel sheet, method of manufacturing the hot-rolled steel sheet and method of manufacturing oil tubular country goods using the hot-rolled steel sheet - Google Patents

Abstract

Disclosed are a hot rolled steel sheet having excellent effect of preventing grain boundary cracking and crack damage through controlling alloy components and controlling process conditions, and a method for manufacturing the same, and a method for manufacturing a steel pipe using the same.
Method for producing a hot rolled steel sheet according to the present invention (a) carbon (C): 0.17 ~ 0.23% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 0.50 ~ 0.70% by weight, chromium (Cr): Reheating the slab plate comprising 0.35 to 0.45% by weight, nickel (Ni): 0.015 to 0.025% by weight, copper (Cu): 0.015% by weight or less, and remaining iron (Fe) and unavoidable impurities; (b) finishing hot rolling the reheated sheet to a Finishing Delivery Temperature (FDT): 800 to 840 ° C .; And (c) winding the hot rolled sheet to CT (Coiling Temperature): 600 to 650 ° C. to wind up.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-rolled steel sheet, a method of manufacturing the same, and a steel pipe manufacturing method using the hot-rolled steel sheet,

The present invention relates to a hot rolled steel sheet and a steel pipe manufacturing technology, and more particularly, to a hot rolled steel sheet having excellent grain boundary characteristics through control of alloy components and process conditions, and a manufacturing method thereof, and a steel pipe manufacturing method using the same.

Nuclear power plant's reactor piping is used in a complex structure and harsh environments such as high temperature, high pressure, corrosion, and fatigue, causing thinning and defects in feeder piping of CANDU (CANada Deuterium Uranium) -type nuclear power plant, and leakage due to cracks in the bent portion. The identification of metallographic causes for securing safety, such as an accident, is urgently required.

The causes of cracks in reactor piping can be largely classified into stress corrosion cracking and low temperature creep cracking.

From the present investigation, residual stress is considered to be a major factor in cracking externally, and the residual stress below yield strength is likely to reduce the possibility of cracking during operation. Although the stresses generated during operation are not considered sufficient to initiate cracking by themselves, there seems to be some contribution to the cracking mechanism.

A related prior art is Korean Patent Laid-Open Publication No. 10-2001-0062875 (published on July, 2001).

It is an object of the present invention to provide a method for manufacturing a hot rolled steel sheet which can secure excellent physical properties by controlling alloy components and controlling process conditions.

It is another object of the present invention to provide a method for manufacturing a steel pipe having excellent excellent effect of preventing grain boundary cracking and crack damage by injecting a hot rolled steel sheet manufactured by the above method.

Another object of the present invention is prepared by the above method, has a tensile strength (TS): 810 MPa or more and yield strength (YS): 650 MPa or more, the final microstructure has a composite structure including ferrite and pearlite, It is to provide a hot rolled steel sheet having an average particle diameter of the ferrite is 5 ~ 10㎛.

Hot-rolled steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is (a) carbon (C): 0.17 ~ 0.23% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 0.50 ~ 0.70% by weight, chromium (Cr): 0.35 to 0.45% by weight, nickel (Ni): 0.015 to 0.025% by weight, copper (Cu): 0.015% by weight or less, and reheating the slab plate composed of the remaining iron (Fe) and unavoidable impurities Doing; (b) finishing hot rolling the reheated sheet to a Finishing Delivery Temperature (FDT): 800 to 840 ° C .; And (c) winding the hot rolled sheet to CT (Coiling Temperature): 600 to 650 ° C. to wind up.

Steel pipe manufacturing method according to an embodiment of the present invention for achieving the above another object is (a) carbon (C): 0.17 ~ 0.23% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 0.50 ~ 0.70% by weight, chromium (Cr): 0.35 to 0.45% by weight, nickel (Ni): 0.015 to 0.025% by weight, copper (Cu): 0.015% by weight or less, and reheating the slab plate composed of the remaining iron (Fe) and unavoidable impurities Doing; (b) finishing hot rolling the reheated sheet to a Finishing Delivery Temperature (FDT): 800 to 840 ° C .; (c) coiling the hot rolled sheet by cooling to CT (Coiling Temperature): 600 to 650 ° C .; (d) uncoiling the wound steel sheet to normalize at 880 to 920 ° C. for 5 to 20 minutes; (e) warming and normalizing the normalized steel sheet at 740 to 840 ° C. and air-cooling the steel sheet to form a steel pipe by cold banding; And (f) stress relaxation heat treatment of the processed steel pipe at 620 to 680 ° C for 10 to 40 minutes.

Hot-rolled steel sheet according to an embodiment of the present invention for achieving the above another object is carbon (C): 0.17 ~ 0.23% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 0.50 ~ 0.70% by weight , Chromium (Cr): 0.35 ~ 0.45% by weight, nickel (Ni): 0.015 ~ 0.025% by weight, copper (Cu): 0.015% by weight or less and the remaining iron (Fe) and inevitable impurities, tensile strength (TS) : 810 MPa or more and yield strength (YS): 650 MPa or more, the final microstructure has a composite structure containing ferrite and pearlite, characterized in that the average particle diameter of the ferrite has a 5 ~ 10㎛.

Hot rolled steel sheet according to the present invention and a method for manufacturing the same by adding the content of chromium (Cr) of 0.35% by weight or more, to reduce the carbon activity (carbon activity) to prevent decarburization by reducing the grain boundary mobility And, there is an effect that can prevent the coarsening of the microstructure.

In addition, the nuclear steel pipe manufactured using the hot-rolled steel sheet according to the present invention is excellent in the prevention of grain boundary cracking and crack damage through the adjustment of alloy components and process conditions control, it is suitable for use as reactor piping used in harsh environments Do.

1 is a flow chart showing a method for manufacturing a hot rolled steel sheet according to an embodiment of the present invention.
2 is a flowchart illustrating a method of manufacturing a steel pipe according to an embodiment of the present invention.
3 to 6 are photographs showing microstructures before and after decarburization of specimens prepared according to Example 1 and Comparative Example 1. FIG.
7 is a photograph showing the microstructure after 24 hours of decarburization for the specimens prepared according to Example 1 and Comparative Example 1, respectively.
Figure 8 is a photograph showing the microstructure after 24 hours of decarburization for the specimen prepared according to Example 1 and Comparative Example 1.
9 is a decarburization time-hardness graph for specimens prepared according to Example 1 and Comparative Example 1. FIG.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with 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.

Hereinafter, a hot rolled steel sheet according to a preferred embodiment of the present invention, a manufacturing method thereof, and a steel pipe manufacturing method using the same will be described in detail with reference to the accompanying drawings.

Hot-rolled steel sheet

Hot rolled steel sheet according to the present invention aims to secure excellent physical properties through the control of alloy components and process conditions control, the effect of preventing grain boundary cracking and crack damage.

To this end, the hot rolled steel sheet according to the present invention is carbon (C): 0.17 ~ 0.23% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 0.50 ~ 0.70% by weight, chromium (Cr): 0.35 ~ 0.45% by weight, nickel (Ni): 0.015 to 0.025% by weight, copper (Cu): 0.015% by weight or less, and the remaining iron (Fe) and inevitable impurities, the final microstructure is a composite structure containing ferrite and pearlite It has a mean particle diameter of 5 ~ 10㎛.

In this case, the steel sheet may include phosphorus (P): 0.008% by weight or less and sulfur (S): 0.025% by weight or less.

Hereinafter, the role and content of each component included in the hot-rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is added to secure the strength of the steel sheet.

Carbon is preferably added in 0.17 to 0.23% by weight of the total weight of the steel sheet according to the present invention, more preferably 0.20% by weight can be presented. If the carbon content is less than 0.17% by weight, it is difficult to secure sufficient strength. On the contrary, when the content of carbon exceeds 0.23% by weight, there is a problem of lowering toughness and lowering weldability.

Silicon (Si)

Silicon (Si) acts as a deoxidizer in the steel and contributes to securing strength.

The silicon is preferably added in 0.25 ~ 0.35% by weight of the total weight of the steel sheet according to the present invention, more preferably 0.27% by weight can be presented. If the content of silicon is less than 0.25% by weight, the addition effect is insufficient. On the contrary, when the content of silicon exceeds 0.35% by weight, the toughness and weldability of the steel sheet are deteriorated.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element that is effective in securing strength by improving the hardenability of steel.

The manganese is preferably added in an amount ratio of 0.50 to 0.70% by weight of the total weight of the steel sheet according to the present invention in consideration of the strength improving effect and the center segregation, and more preferably 0.64% by weight. If the content of manganese is less than 0.50% by weight, the effect of strengthening solid solution is insufficient. On the contrary, when the content of manganese exceeds 0.70% by weight, the weldability is greatly reduced, and there is a problem in that the ductility of the steel is greatly reduced by the generation of MnS inclusions and the generation of center segregation.

Chromium (Cr)

Chromium (Cr) is a ferrite stabilizing element and contributes to strength improvement. In addition, chromium inhibits decarburization and microstructural coarsening by reducing carbon activity and boundary mobility to prevent conversion to spheroidization.

The chromium is preferably added in an amount ratio of 0.35 to 0.45% by weight of the total weight of the steel sheet according to the present invention, and more preferably 0.41% by weight. If the content of chromium is less than 0.35% by weight, the above effects cannot be exerted properly. On the contrary, when the content of chromium exceeds 0.45% by weight, not only the toughness of the welding heat affected zone (HAZ) is deteriorated but also the workability is inhibited.

Nickel (Ni)

Nickel (Ni) fine grains and solidify in the austenite and ferrite to strengthen the matrix. In particular, nickel is an effective element for improving low-temperature toughness.

The nickel is preferably added in an amount of 0.015 to 0.025% by weight of the total weight of the steel sheet according to the present invention. If the nickel content is added below 0.015% by weight, the above effects cannot be exerted properly. On the contrary, when the content of nickel exceeds 0.025% by weight, there is a problem of causing red hot brittleness.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability of the steel and the impact resistance at low temperatures.

However, when the content of copper exceeds 0.015% by weight of the total weight of the steel sheet according to the present invention, there is a problem of lowering the surface quality of the steel. Therefore, the copper content is preferably added in an amount ratio of 0.015% by weight or less of the total weight of the steel sheet according to the present invention.

Phosphorus (P)

Phosphorous (P) is added to inhibit cementite formation and increase strength.

However, phosphorus deteriorates the weldability and causes the final material deviation by slab center segregation. Therefore, in the present invention, the content of phosphorus (P) was limited to 0.008% by weight or less of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits the toughness and weldability of steel, and forms an MnS non-metallic inclusion by binding with manganese, thereby generating cracks during steel processing.

Therefore, in the present invention, the content of sulfur (S) was limited to 0.025% by weight or less of the total weight of the steel sheet.

Hot-rolled steel sheet manufacturing method

1 is a flowchart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated method for manufacturing a hot rolled steel sheet 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 perform the slab reheating step (S110) in order to obtain effects such as the reuse of the precipitate.

In the method for manufacturing a hot rolled steel sheet according to the present invention, the slab sheet material of the semi-finished state, which is the target of the hot rolling process, includes carbon (C): 0.17 to 0.23 wt%, silicon (Si): 0.25 to 0.35 wt%, and manganese (Mn): 0.50 to 0.70% by weight, chromium (Cr): 0.35 to 0.45% by weight, nickel (Ni): 0.015 to 0.025% by weight, copper (Cu): 0.015% by weight or less, and the remaining iron (Fe) and inevitable impurities.

At this time, the slab plate may include phosphorus (P): 0.008% by weight or less and sulfur (S): 0.025% by weight or less.

Reheat slab

In the slab reheating step S110, the slab plate having the above composition is reheated at a slab reheating temperature (SRT) of 1150 to 1250 ° C for 1 to 3 hours. Through the reheating of the slab plate, re-use of the segregated components and re-use of precipitates may occur during casting.

If the slab reheating temperature (SRT) is less than 1150 ° C. or less than 1 hour, the segregated components during casting may not be sufficiently reusable. On the contrary, when the SRT exceeds 1250 DEG C or exceeds 3 hours, the austenite crystal grain size is increased and the ferrite of the final microstructure is coarsened, so that it may be difficult to secure strength, and due to the excessive heating process The manufacturing cost of the steel sheet can be raised only.

Hot rolling

Hot rolling step (S120) is to finish roll the reheated steel slab in the austenite recrystallization area.

At this time, finishing rolling temperature (FDT) is preferably carried out at 800 ~ 840 ℃. If the finish rolling temperature FDT is performed at less than 800 ° C., problems such as a mixed structure generated by abnormal reverse rolling may occur. On the contrary, when the finish rolling temperature (FDT) exceeds 840 ° C, the austenite grains are coarsened so that the ferrite grains may not be sufficiently refined after transformation, thereby making it difficult to secure the strength.

At this time, the rolling may be carried out in a rolling stand having 5 to 10 rolling passes, and the average reduction ratio is preferably performed to be 5 to 15% so that sufficient rolling may be made in each pass. If the average rolling reduction per pass is less than 5%, strain can not be sufficiently applied to the center of the thickness, so that it may be difficult to secure fine crystal grains after cooling. On the other hand, when the average reduction rate per pass is more than 15%, there is a problem that the production becomes impossible due to the load of the rolling mill.

Cooling / Winding

In the cooling / winding step (S130), the hot rolled sheet is cooled by winding to CT (Coiling Temperature): 520 to 560 ° C.

In the present invention, the cooling process by cooling the rolled plate to 520 ~ 560 ℃ by forced cooling method such as water cooling, to suppress the grain growth of the steel sheet to form a matrix structure having fine ferrite grains and to form a pearlite structure high strength And for the purpose of securing high toughness.

If the winding temperature CT is less than 520 ° C., there is a problem in that low temperature transformation tissue is formed in a large amount and low temperature impact toughness is insufficient. On the contrary, when the coiling temperature exceeds 560 ° C, there is a problem in that strength is insufficient due to coarse microstructure formation.

On the other hand, it is preferable to perform cooling rate at 4-50 degreeC / sec. If the cooling rate is less than 4 ℃ / sec may be difficult to secure the strength. On the other hand, when the cooling rate exceeds 50 DEG C / sec, the structure becomes weak and it may be difficult to secure the target low temperature impact toughness.

The hot rolled steel sheet according to the present invention manufactured by the above process has a composite structure including the ferrite (Ferrite) and pearlite (Pearlite) the final microstructure, the average particle diameter of the ferrite may have a 5 ~ 10㎛.

In addition, the hot rolled steel sheet manufactured by the above process may satisfy the tensile strength (TS): 810 MPa or more, yield strength (YS): 650 MPa or more and CLR (Crack Length Ratio): 10% or less.

Steel pipe manufacturing method

2 is a flow chart showing a steel pipe manufacturing method according to an embodiment of the present invention.

Referring to Figure 2, the steel pipe manufacturing method shown is a reheating step (S210), hot rolling step (S220), cooling step (S230), normalizing step (S240), warm / cold banding step (S250) and stress relaxation heat treatment step (S260).

At this time, the reheating step 210, the hot rolling step (S220) and the cooling step (S230) are carried out in the same manner as the method for manufacturing a hot rolled steel sheet described in FIG. do.

Normalizing

In the normalizing step (S240), the coiled steel sheet is uncoiled and subjected to normalizing for heat treatment for 5 to 20 minutes at 880 to 920 ° C.

If the normalizing heat treatment temperature is lower than 880 ° C., it is difficult to re-use the solid solution solute elements, which may make it difficult to secure sufficient strength. On the contrary, when the normalizing heat treatment temperature exceeds 920 ° C., there is a problem in that crystal grains grow to inhibit low temperature toughness.

On the other hand, when the normalizing heat treatment time is carried out in less than 5 minutes it may be difficult to ensure a uniform structure. On the contrary, when the normalizing heat treatment time exceeds 20 minutes, there is a problem of only raising the production cost without any further synergistic effect.

Warm / Cold Banding

In the warm / cold banding step (S250), the normalized steel sheet is warm banded at 740 to 840 ° C, and then cooled to 425 to 500 ° C. At this time, cooling may be used as air cooling.

If the warm bending temperature is less than 740 ° C., there is a problem in that workability is sharply lowered. On the contrary, when the warm banding temperature exceeds 840 ° C., there is a problem that grains grow and inhibit low temperature toughness.

Next, cold banding is performed to process the cooled steel sheet into a desired shape. By such a warm / cold banding step (S250), the steel sheet can be produced as a nuclear steel pipe.

Stress Relaxation Heat Treatment

In the stress relaxation heat treatment step (S260), the steel pipe processed in a desired shape by the warm / cold bending step (S250) is subjected to stress relaxation heat treatment at 620 to 680 ° C. for 10 to 40 minutes.

If the stress relaxation heat treatment temperature is performed at less than 620 ° C., it is not easy to remove residual stress at the joint or the like. On the contrary, when the stress relaxation heat treatment temperature exceeds 680 ° C, it may be difficult to secure sufficient strength.

On the other hand, the stress relaxation heat treatment time is preferably carried out for 10 to 40 minutes per thickness of 20 ~ 27mm, because when the stress relaxation heat treatment time is out of the above range, it is not easy to remove the residual stress at the joint to be.

Nuclear steel pipe manufactured by the above process is added to the content of chromium (Cr) more than 0.35% by weight, decarburization and microstructure of the carbon activity (boundary mobility) by reducing the carbon activity (boundary mobility) It is effective to prevent coarsening.

In addition, the nuclear steel pipe manufactured by the above process is excellent in the prevention of grain boundary cracking and crack damage through the adjustment of alloy components and process conditions control, it is suitable for use as reactor piping used in harsh environments.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. 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.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of specimens

Hot rolled specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were prepared under the compositions shown in Table 1 and the process conditions described in Table 2.

[Table 1] (unit:% by weight)

 [Table 2]

2. Evaluation of mechanical properties

Table 3 shows the mechanical property evaluation results for the hot-rolled specimen prepared according to Examples 1-3 and Comparative Examples 1-3.

[Table 3]

Referring to Tables 1 to 3, in the case of specimens prepared according to Examples 1 to 3, the target tensile strength (TS): 810 MPa or more, yield strength (YS): 650 MPa or more and CLR (Crack Length Ratio) ): It can be seen that all 5% or less are satisfied.

On the other hand, compared to Example 1, most alloy components are added in a similar content, but in the case of specimens prepared according to Comparative Example 1 and Comparative Example 2 in which the content of chromium (Cr) is outside the scope of the present invention, It can be seen that the tensile strength (TS): 810 MPa or more and the yield strength (YS): 650 MPa or more. In addition, it can be seen that the specimens prepared according to Comparative Example 1 and Comparative Example 2 had 17.3% and 16.1%, respectively, higher than the target 5% CLR.

In addition, compared to Example 1, most of the alloying components are added in a similar content, but the content of chromium (Cr) is excessively added than the range suggested in the present invention, and the winding temperature in Comparative Example 3 carried out at 700 ℃ According to the specimen prepared according to the target tensile strength (TS) and yield strength (YS) was satisfied, but it can be seen that it has a significantly higher 25.6% than the target CLR.

3. Evaluation of decarburization heat treatment

Decarburization heat treatment was performed on the specimens prepared according to Example 1 and Comparative Example 1 to form a decarburization layer and to compare and analyze the effect of chromium components by comparing and analyzing the microstructure and P impurity grain boundary segregation behavior in the decarburization layer.

For this decarburization, a double quartz horizontal furnace was set at 700 ° C., the water in a closed Erlenmeyer flask was set at 50 ° C. using a hot plate, and an alumina plate (Alumina) was used. Plates prepared according to Example 1 and Comparative Example 1 were placed side by side on top of the plate), and then the specimens prepared according to Example 1 and Comparative Example 1 were loaded into a horizontal quartz tube.

Thereafter, N ₂ and H ₂ having a purity of 99.9% were allowed to pass into the quartz tube of the Erlenmeyer flask set at 50 ° C. at a rate of 2 l / min and 5 l / min, respectively, and the decarburized layer after 6 hours, 12 hours and 24 hours. Thickness and carbon diffusivity were measured respectively.

Table 4 shows the microstructure characteristics after the decarburization heat treatment of the specimen prepared according to Example 1 and Comparative Example 1, Table 5 and Table 6 shows the physical properties of the decarbonized heat treatment of the specimen prepared according to Example 1 and Comparative Example 1 Each result is shown.

[Table 4]

[Table 5]

TABLE 6

Referring to Table 4, in the case of the specimen prepared according to Example 1 and Comparative Example 1, the shape of the pearlite structure is made of lath type (Lath type), respectively. On the other hand, the specimen prepared according to Example 1 is a discontinuous array of pearlite (discontinuous), the specimen prepared according to Comparative Example 1 can be seen that the arrangement of the pearlite structure (continuous).

In addition, for the specimen prepared according to Example 1, the volume fraction of pearlite has a cross-sectional area ratio of 29%, Pearlite Lamellar Spacing: 59.23 nm and Pearlite Packets Size: 6.23 µm. Able to know. On the other hand, for the specimen prepared according to Comparative Example 1, the volume fraction of pearlite has a cross-sectional area ratio of 26%, pearlite lamellar spacing: 65.42 nm, and pearlite packet size: 6.81 μm. Able to know.

In this case, it can be seen that the pearlite lamellar spacing and the pearlite packet size of the specimen prepared according to Example 1 are fine compared to the specimen prepared according to Comparative Example 1, which is an Fe atomic site. It is believed that the presence of Cr, a substitutional element, reduced carbon activity, thereby miniaturizing the pearlite structure.

On the other hand, Figures 3 to 6 are photographs showing the microstructure before and after decarburization for the specimen prepared according to Example 1 and Comparative Example 1. At this time, Figure 3 is a decarburization time 0 hours, Figure 4 is a decarburization time 6 hours, Figure 5 is a decarburization time 12 hours, Figure 6 is a microstructure photograph after 24 hours of decarburization time, in each drawing (a) is Comparative Examples 1 and (b) correspond to Example 1.

3, Table 5 and Table 6, in the case of the specimen prepared according to Example 1, the thickness of the decarburized layer is 235 ~ 515㎛ according to the progress of decarburization time (6, 12, 24hr), carbon diffusion degree × 10 ^-13 m ² and the s 6.39 ^- it can be seen that represents the ^- ~ 7.67 × 10 ^-13 m ² · s.

On the other hand, in the case of the specimen prepared according to Comparative Example 1, the thickness of the decarburized layer was 379 ~ 704㎛ according to the progress of decarburization time (6, 12, 24hr), the carbon diffusion degree 1.66 × 10 ^-12 m ² · s It can be seen that ^- to 1.434 x 10 ^-12 m ² · s ^- is represented.

Therefore, in the case of the specimen prepared according to Comparative Example 1, it can be seen that the carbon diffusion degree is faster than the specimen prepared according to Example 1. Particularly, in the case of the specimen prepared according to Example 1, the change in columnar type was prevented due to the decrease in grain boundary mobility due to the decrease in carbon activity due to the sharp increase in chromium content compared to Comparative Example 1. Seems to be.

Through this, the specimen prepared according to Example 1 to which the chromium content is added at 0.41% by weight is considered to have made a significant contribution to preventing decarburization and coarsening of the microstructure.

On the other hand, Figure 7 is a photograph showing the microstructure after 24 hours of decarburization for the specimen prepared according to Example 1 and Comparative Example 1, Figure 8 is a specimen prepared according to Example 1 and Comparative Example 1 It is a photograph showing the microstructure after 24 hours of decarburization.

7 and 8 (a) and (b) correspond to the microstructure photograph of the specimen prepared according to Comparative Example 1, Figures 7 and 8 (c) and (d) of Example 1 Corresponds to the microstructure picture of the specimen prepared according to. 7 and 8 (a) and (c) are low magnification photographs, and (b) and (d) are high magnification photographs.

In the case of the specimen prepared according to Comparative Example 1 shown in (a) and (b) of Figure 7, it can be seen that the pearlite lamellar spacing (Pearlite Lamellar Spacing) is not dense after 24 hours of decarburization. On the contrary, in the case of the specimen prepared according to Example 1 shown in (c) and (d) of FIG. 7, the pearlite lamellar spacing after 24 hours of decarburization compared to the specimen prepared according to Comparative Example 1 You can see that this is dense.

On the other hand, in the case of the specimen prepared according to Comparative Example 1 shown in (a) and (b) of Figure 8, it can be seen that the spheroidization of the pearlite structure after 24 hours of decarburization.

On the contrary, in the case of the specimen prepared according to Example 1 shown in (c) and (d) of FIG. Can be.

This is compared with the specimen prepared according to Comparative Example 1 in the central portion of the specimen prepared according to Example 1 by the addition of chromium, the carbon diffusivity is reduced, the spheroidization is prevented due to the lower carbon activity and grain boundary mobility, microstructure We believe the deterioration has been reduced.

On the other hand, Figure 9 is a decarburization time-hardness graph for the specimen prepared according to Example 1 and Comparative Example 1.

Referring to FIG. 9, it can be seen that the hardness of the specimen prepared according to Example 1 is generally increased compared to the specimen prepared according to Comparative Example 1.

At this time, in the case of the specimen prepared according to Example 1, the pearlite lamellar spacing (Pearlite Lamellar Spacing) is narrower than the specimen prepared according to Comparative Example 1, it is understood that the pearlite discontinuous arrangement and the pearlite packet size is fine.

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, S210: Slab reheating step
S120, S220: Hot Rolling Step
S130, S230: cooling / winding step
S240: Normalizing Step
S250: Warm / Cold Banding Step
S260: stress relaxation heat treatment step

Claims (9)

(a) Carbon (C): 0.17 to 0.23 wt%, Silicon (Si): 0.25 to 0.35 wt%, Manganese (Mn): 0.50 to 0.70 wt%, Chromium (Cr): 0.35 to 0.45 wt%, Nickel (Ni ): Reheating the slab plate consisting of 0.015 to 0.025% by weight, copper (Cu): more than 0% by weight to 0.015% by weight and remaining iron (Fe) and unavoidable impurities;
(b) finishing hot rolling the reheated sheet to a Finishing Delivery Temperature (FDT): 800 to 840 ° C .; And
(c) coiling the hot rolled sheet (CT): coiling by cooling to 600 ~ 650 ℃; includes,
After the step (c), by the effect of the addition of the chromium element, the plate prevents decarburization, reduces the carbon activity (carbon activity) to refine the pearlite structure, the final microstructure comprises ferrite and pearlite Hot-rolled steel sheet manufacturing method characterized in that it has a composite structure, the average particle diameter of the ferrite has a 5 ~ 10㎛.
The method of claim 1,
The step (a)
SRT (Slab Reheating Temperature): Hot rolled steel sheet manufacturing method characterized in that carried out for 1 to 3 hours at 1150 ~ 1250 ℃.
The method of claim 1,
The slab plate
Phosphorus (P): more than 0% by weight to 0.008% by weight or less and sulfur (S): more than 0% by weight to 0.025% by weight or less, characterized in that it comprises a hot rolled steel sheet manufacturing method.
(a) Carbon (C): 0.17 to 0.23 wt%, Silicon (Si): 0.25 to 0.35 wt%, Manganese (Mn): 0.50 to 0.70 wt%, Chromium (Cr): 0.35 to 0.45 wt%, Nickel (Ni ): Reheating the slab plate consisting of 0.015 to 0.025% by weight, copper (Cu): more than 0% by weight to 0.015% by weight and remaining iron (Fe) and unavoidable impurities;
(b) finishing hot rolling the reheated sheet to a Finishing Delivery Temperature (FDT): 800 to 840 ° C .;
(c) coiling the hot rolled sheet by cooling to CT (Coiling Temperature): 600 to 650 ° C .;
(d) uncoiling the wound steel sheet to normalize at 880 to 920 ° C. for 5 to 20 minutes;
(e) warming and normalizing the normalized steel sheet at 740 to 840 ° C. and air-cooling the steel sheet to form a steel pipe by cold banding; And
(f) stress relaxation heat treatment of the processed steel pipe at 620 ~ 680 ℃ 10 ~ 40 minutes; includes;
Between the steps (c) and (d), the plate prevents decarburization and decreases carbon activity by refining the chromium element, thereby refining the pearlite structure, thereby producing a final microstructure. It has a composite structure containing ferrite and pearlite, the steel pipe manufacturing method, characterized in that the average particle diameter of the ferrite has a 5 ~ 10㎛.
5. The method of claim 4,
The slab plate
Phosphorus (P): more than 0% by weight to 0.008% by weight or less and sulfur (S): more than 0% by weight to 0.025% by weight or less of the steel pipe manufacturing method characterized in that it further comprises.
5. The method of claim 4,
In the step (e)
The cooling is a steel pipe manufacturing method, characterized in that for performing air cooling to 370 ~ 420 ℃.
Carbon (C): 0.17 to 0.23 wt%, Silicon (Si): 0.25 to 0.35 wt%, Manganese (Mn): 0.50 to 0.70 wt%, Chromium (Cr): 0.35 to 0.45 wt%, Nickel (Ni): 0.015 ~ 0.025% by weight, copper (Cu): more than 0% by weight ~ 0.015% by weight and consists of the remaining iron (Fe) and inevitable impurities,
Tensile strength (TS): 810 MPa or more and yield strength (YS): 650 MPa or more,
The final microstructure has a composite structure containing ferrite and pearlite, the average particle diameter of the ferrite has a 5 ~ 10㎛,
Hot-rolled steel sheet, characterized in that by the effect of the addition of the chromium element, to prevent decarburization, to reduce the carbon activity (fine carbon) to refine the pearlite structure.
The method of claim 7, wherein
The steel sheet
Phosphorus (P): more than 0 wt% to 0.008 wt% or less and sulfur (S): more than 0 wt% to 0.025 wt% or less. delete

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