KR20110062903A - Ultra high strength steel plate for pipeline with high resistance to surface cracking and manufacturing metod of the same - Google Patents

Abstract

PURPOSE: An ultra high strength steel palate for a pipeline with excellent resistance to surface cracking and a manufacturing method thereof are provided to improve resistance to surface cracking by lowering the hardness of a surface part. CONSTITUTION: An ultra high strength steel palate for a pipeline with excellent resistance to surface cracking comprises C0.03~0.10 weight%, Si less than 0.6 weight%, Mn 1.6~2.1 weight%, Cu less than 1.0 weight%, Ni less than 1.0 weight%, Nb 0.02~0.06 weight%, V less than 0.1 weight%, Mo 0.1~0.5 weight%, Cr less than 1.0 weight%, Ti 0.005~0.03 weight%, Al less than 0.005 weight%, B 0.005~0.0025 weight%, N 0.001~0.006 weight%, Ca less than 0.006 weight%, P less than 0.02 weight%, S less than 0.005 weight%, and Fe and inevitable impurities.

Description

ULTRA HIGH STRENGTH STEEL PLATE FOR PIPELINE WITH HIGH RESISTANCE TO SURFACE CRACKING AND MANUFACTURING METOD OF THE SAME}

The present invention relates to a steel sheet for line pipes and a method of manufacturing the same, and more particularly, to a steel plate for ultra-high strength line pipe and a method of manufacturing the same, which has improved surface cracking resistance by lowering the hardness of the surface portion compared to the center of the steel sheet.

Line pipe means a steel pipe mainly used for the transportation of crude oil or natural gas, and because high pressure gas or crude oil flows in the line pipe, high pressure acts on the line pipe, so ultra-high strength of the steel plate for line pipe is required. It is becoming. In addition, in recent years, as the development of oil fields in cold regions, such as Siberia and Alaska, has become increasingly active, the low temperature toughness requirements of line pipe steel sheets have been strengthened.

In addition, in the case of ultra high strength steel, the surface hardness is very high, and the high surface hardness also causes the destruction through surface cracking, so that a large accident may occur, so surface hardness should be stabilized. In particular, since the outer diameter portion of the line pipe is subjected to tensile stress by the tubing, stable surface cracking resistance is required.

In general, increasing the strength of a material, in turn, reduces its toughness. This is because an alloying element added usually plays a contradictory role in inhibiting toughness even though it has a favorable effect on strength. In order to solve this problem, the method of improving the strength and toughness of the steel while suppressing the adjustment of the element as much as possible, the so-called TMCP (Thermo Mechanical Controlling Process), reduces the grain size inside the steel sheet to improve the toughness, Many methods have been used to form and improve strength.

One example of such a technique is as shown in curve 2 of FIG. 1, in which a hard tissue is secured by additionally tempering below Ac1 transformation temperature (temperature at which ferrite transforms into austenite when heated) after cooling after hot rolling. There is a technology, but there is a problem that the effect of reducing steel costs due to the cost increase due to the additional tempering process after cooling.

As another technique, as shown in curve 1 of Fig. 1, there is a technique of controlling the cooling rate and the cooling stop temperature without performing tempering. The first example of this technique is a technique to improve the toughness and strength, including a certain amount (0.15% by weight or 0.2% by weight) of Mo, but the tensile strength of 930MPa or more is not obtained and high by expensive Mo in steel manufacturing It requires cost. In addition, the technique employs a cooling method such as curve A of FIG. 2, and obtains the lower bainite structure as shown in FIG. 3, but as shown in FIG. 2, the temperature range in which phase transformation occurs is very narrow to produce steel. The cooling conditions are strict, the cooling rate is very high, the manufacturing equipment capacity conditions are very strict, and the manufacturing conditions, such as the need to solve the problem for shape control after steel sheet manufacturing has a problem. As a second example, by adding only 0.10% or more of an expensive alloy element Mo and adopting a cooling method such as curve B of FIG. 2, there is a technology that can secure tensile strength of 930 MPa or higher and high low temperature toughness. Since the central tissue is composed of bainitic ferrite and ash cura ferrite, there is a problem that the risk of brittle fracture is high because the surface crack sensitivity is too high due to high surface hardness.

The present invention is to control the microstructure of the steel plate surface portion by the first cooling after rolling to lower the hardness of the surface portion to improve the surface cracking resistance, and to improve the strength and toughness by controlling the microstructure of the center of the steel plate by the secondary cooling To provide a steel sheet for pipes and a method of manufacturing the same.

In one embodiment, the present invention provides, in weight percent, carbon (C): 0.03 to 0.10%, silicon (Si): 0.6% or less, manganese (Mn): 1.6 to 2.1%, copper (Cu): 1.0% or less, Nickel (Ni): 1.0% or less, Niobium (Nb): 0.02 to 0.06%, Vanadium (V): 0.1% or less, Molybdenum (Mo): 0.1 to 0.5%, Chromium (Cr): 1.0% or less, Titanium (Ti) ): 0.005 to 0.03%, aluminum (Al): 0.01 to 0.06%, boron (B): 0.0005 to 0.0025%, nitrogen (N): 0.001 to 0.006%, calcium (Ca): 0.006% or less, phosphorus (P) : 0.02% or less, sulfur (S): 0.005% or less, including residual iron (Fe) and other unavoidable impurities, and the microstructure of the steel plate surface portion (within 2 mm from the surface) is composed of Polygonal Ferrite and It contains more than 75% of the area of Acicular Ferrite (Acicular Ferrite), and the microstructure of the center of the steel sheet (except the surface part of the steel plate) contains more than 75% of the area of Bainitic Ferrite and Ashcula Ferrite. It provides a steel sheet for line pipe containing.

The microstructure of the center of the steel sheet preferably contains 5% or less granular bainite.

The steel sheet may have a tensile strength of 930 MPa or more, -40 ° C. impact toughness of 230 Joules or more, and a surface hardness of 170 to 260 Hv.

As another embodiment of the present invention, in weight%, carbon (C): 0.03 to 0.10%, silicon (Si): 0.6% or less, manganese (Mn): 1.6 to 2.1%, copper (Cu): 1.0% or less, Nickel (Ni): 1.0% or less, Niobium (Nb): 0.02 to 0.06%, Vanadium (V): 0.1% or less, Molybdenum (Mo): 0.1 to 0.5%, Chromium (Cr): 1.0% or less, Titanium (Ti) ): 0.005 to 0.03%, aluminum (Al): 0.01 to 0.06%, boron (B): 0.0005 to 0.0025%, nitrogen (N): 0.001 to 0.006%, calcium (Ca): 0.006% or less, phosphorus (P) : 0.02% or less, sulfur (S): 0.005% or less, a heating step of heating a slab containing residual iron (Fe) and other unavoidable impurities to 1050 to 1150 ° C .; A recrystallization reverse rolling step of rolling the heated slab once or multi-stage at least twice in an austenite recrystallization temperature region; A non-recrystallization station rolling step of rolling the rolled slab once in austenite recrystallization temperature, at least Ar3, or at least two times in a multi-stage rolling step; A first cooling step of cooling the rolled steel sheet at a cooling rate of 5 to 15 ° C./s to stop cooling at Ar 3-20 ° C. to Ar 3-10 ° C .; And a second cooling step of cooling the first cooled steel sheet at a cooling rate of 20 to 50 ° C./s to stop cooling at 200 to 400 ° C.

The recrystallization rolling is preferably carried out with a cumulative reduction of 20 to 60%.

The unrecrystallized rolling is preferably carried out with a cumulative reduction of 40 to 50%.

After the second cooling step, the steel sheet may be cooled by air or by air.

According to the present invention, the hardness of the surface portion of the steel sheet can be lowered to 170 to 260 Hv, thereby providing a steel sheet having excellent surface cracking resistance, tensile strength of 930 MPa or more and Charpy impact toughness (-40 ° C) of 230J or more.

The present invention is to control the microstructure of the central portion and the surface portion of the steel sheet through the cooling step of the two stages after rolling, to ensure excellent strength and toughness, and to improve the surface cracking resistance. To this end, as shown in the graph C of Figure 2, by limiting the cooling rate and the cooling stop temperature during the first and second cooling, the microstructure of the center of the steel sheet is a bainitic ferrite (ashini) and ash curules having fine grains It is controlled by ferrite (Acicular Ferrite), and the microstructure of the surface portion of the steel sheet is controlled by Polygonal Ferrite and Ashicular Ferrite.

Hereinafter, the component system and the composition range of the steel sheet of the present invention will be described in detail.

Carbon (C): 0.03 to 0.10 wt%

Carbon is the most effective element for strengthening the matrix of metals and welds through solid solution strengthening, and it is possible to obtain the strengthening effect by precipitation hardening by the formation of small size cementite, V and Nb carbonitride and Mo carbide. In addition, Nb carbonitride can improve both strength and low temperature toughness through grain refinement by inhibiting austenite recrystallization and preventing grain growth during hot rolling. In addition, it also serves to improve the hardenability, which is the ability to form a strong microstructure inside the steel sheet during cooling. When the carbon content is less than 0.03% by weight, the above effect cannot be obtained. When the carbon content exceeds 0.10% by weight, the toughness of the base metal and the welded heat affected zone is reduced, including low temperature cracking after the spot welding.

Silicon (Si): 0.6 wt% or less (excluding 0)

Silicon assists aluminum in deoxidizing molten steel and acting as a solid solution strengthening element. When the silicon content exceeds 0.6% by weight, the development weldability and the toughness of the weld heat affected zone are greatly deteriorated. Since aluminum or titanium plays a role in deoxidation, it is not necessary to add silicon for deoxidation.

Manganese (Mn): 1.6-2.1 wt%

Manganese is an effective element to solidify the steel. The content should contain at least 1.6% by weight to improve the hardenability and strength. However, when it exceeds 2.1% by weight, the center segregation is promoted during slab casting in the steelmaking process and the toughness is reduced. In addition, the hardenability is excessively improved, the field weldability is deteriorated, and the toughness of the weld heat affected zone is lowered.

Copper (Cu): 1.0 wt% or less (excluding 0)

Copper is an element that enhances the strength of the base metal and the weld heat affected zone. However, when the content exceeds 1.0% by weight, the copper content is excessively reduced to reduce the toughness and field weldability of the weld heat affected zone.

Nickel (Ni): 1.0 wt% or less (excluding 0)

Nickel is an element that improves mechanical properties in low carbon steel without deteriorating field weldability and low temperature toughness. Compared with manganese and molybdenum, nickel forms less hard phases, such as phase martensite, which lowers low-temperature toughness, and improves the toughness of the weld heat affected zone. In addition, it suppresses the occurrence of surface cracks in the copper-added steel during continuous casting and hot rolling. However, since Ni is a high element and addition of excessive Ni lowers the toughness of the weld heat affected zone, the upper limit of the nickel content is preferably limited to 1.0% by weight.

Niobium (Nb): 0.02 to 0.06 wt%

Niobium serves to simultaneously improve strength and toughness through grain refinement. Nb carbonitride produced during hot rolling suppresses austenite recrystallization and prevents grain growth, thereby making fine austenite grains. When added together with molybdenum, the austenite recrystallization is suppressed to increase the grain refining effect, and the effect of strengthening precipitation and improving hardenability is great. When boron is present, the effect of further increasing the hardenability can be obtained. It is preferable to contain 0.02 weight% or more in order to acquire such an effect. However, when it exceeds 0.06% by weight, it is difficult to expect the effect increase any more, rather it adversely affects the weldability and the weld heat affected zone toughness.

Vanadium (V): 0.1 wt% or less (excluding 0)

Vanadium plays a similar role to niobium, but the effect is somewhat weaker than niobium. However, when niobium and vanadium are added together, the effect is greatly improved. However, considering the toughness and weldability of the weld heat affected zone, the upper limit of the vanadium content is preferably limited to 0.1% by weight.

Molybdenum (Mo): 0.1-0.5 wt%

Molybdenum is an effective element to improve the hardenability, especially when added with boron shows a very large hardening effect. In addition, when added with niobium, austenite recrystallization is suppressed and contributes to grain refinement. However, since excessive addition of molybdenum lowers the toughness of the weld heat affected zone during spot welding, the upper limit is preferably limited to 0.5% by weight, more preferably 0.15% by weight.

Chromium (Cr): 1.0 wt% or less (excluding 0)

Chromium is an element that plays a role of improving hardenability. However, since the addition of excessive chromium causes low temperature cracking after welding in the field, it lowers the toughness of the base metal and the heat affected zone of the welded part, so the upper limit is preferably limited to 1.0% by weight.

Titanium (Ti): 0.005 to 0.03 wt%

Titanium refines grains by forming fine Ti nitride (TiN) to suppress coarsening of austenite grains upon slab heating. In addition, TiN serves to prevent grain coarsening of the weld heat affected zone and to remove the nitrogen in the molten steel to improve toughness.

In addition, titanium is a very useful element to enhance the strength and refine the grain of the base metal and the weld heat affected zone. The presence of TiN in the steel has the effect of inhibiting the growth of grains in the heating process for rolling. In addition, the titanium remaining after reacting with nitrogen is dissolved in the steel to combine with carbon to form TiC precipitates, and the formation of TiC is very fine, greatly improving the strength of the steel.

When the addition amount of aluminum is very small, Ti oxide is formed and acts as a nucleation site of intragranular acicular ferrite in the weld heat affected zone. Therefore, it is necessary to add 0.005% by weight or more in order to obtain the austenite grain growth inhibition effect by TiN precipitation and the strength increase by TiC formation. On the other hand, when it exceeds 0.03% by weight, the coarsening of Ti nitride and the hardening by Ti carbide are excessively harmful to low temperature toughness, and when TiN is re-used by rapid melting to the melting point when manufacturing steel pipe by welding steel sheet Since the toughness of the weld heat affected zone deteriorates, the upper limit of Ti addition is made 0.03% by weight. However, in order to sufficiently remove nitrogen, titanium is preferably added at least 3.4 times the amount of nitrogen added.

Aluminum (Al): 0.01 ~ 0.06 wt%

Aluminum is generally added for the purpose of deoxidation of steel. Further, the structure is refined and the toughness of the heat affected zone is improved by removing nitrogen from the coarse grain region of the weld heat affected zone. However, when it exceeds 0.06% by weight, Al oxides (Al ₂ O ₃ ) are formed to reduce the toughness of the base metal and the heat affected zone.

Boron (B): 0.0005 to 0.0025 wt%

Boron greatly improves hardenability in low carbon steels and increases weldability and low temperature crack resistance. In particular, it serves to increase the hardenability improvement effect of molybdenum and niobium and to increase the strength of the grain boundary to suppress intragranular cracking generated by hydrogen. However, excessive addition of boron causes Fe ₂₃ (C, B) ₆ precipitates to form and cause embrittlement. Therefore, the content of boron should be determined in consideration of the content of other hardenable elements. In the present invention, the content of boron is preferably limited to 0.0005 to 0.0025% by weight as described above.

Nitrogen (N): 0.001-0.006 wt%

Nitrogen inhibits austenite grain growth during slab heating and forms TiN precipitates to suppress austenite grain growth in the weld heat affected zone. However, excessive nitrogen addition promotes slab surface defects, reduces the hardenability effect of boron, and in the presence of solute nitrogen, lowers the toughness of the matrix and the weld heat affected zone, so the upper limit is preferably limited to 0.006% by weight.

Calcium (Ca): 0.006% by weight or less (excluding 0)

Calcium mainly serves to control the shape of MnS inclusions and to improve low temperature toughness. However, since excessive Ca addition forms and binds a large amount of CaO-CaS to form coarse inclusions, lowers the cleanliness of the steel, and inhibits field weldability, the upper limit is preferably limited to 0.006% by weight.

The remaining component of the present invention is iron (Fe). However, in the usual steel manufacturing process, impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

However, since phosphorus and sulfur are generally mentioned impurities, the following briefly describes them.

Phosphorus (P): 0.02 wt% or less

Phosphorus is an element that is inevitably contained in steel production, and phosphorus is preferably combined with manganese and the like to form a non-metallic inclusion to embrittle the steel. Therefore, it is preferable to control it as low as possible. Normally inevitably must be added. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.02% by weight.

Sulfur (S): 0.005% by weight or less

Sulfur is an element that is inevitably contained in steel production, and in combination with manganese or the like to form a non-metallic inclusion to embrittle the steel and cause red brittleness, it is preferable to suppress the content as much as possible. In theory, it is advantageous to limit the content of sulfur to 0%, but it is inevitably added in the manufacturing process. Therefore, it is important to manage the upper limit, and the upper limit of the sulfur content in the present invention is preferably limited to 0.005% by weight.

As a steel sheet having the above-described component system, it is necessary to limit the microstructure of the steel sheet to preferable conditions for becoming a steel sheet having excellent surface cracking resistance and excellent strength and toughness.

In the present invention, in order to improve the surface cracking resistance of the steel sheet, the microstructure of the surface portion (within 2mm from the surface) of the steel sheet is controlled differently from the central portion (range except the surface portion of the steel sheet). In order to increase the surface crack resistance, the hardness of the steel plate surface should be lowered. Therefore, the microstructure contains more than 75% of the area fraction of polygonal ferrite and ash cura ferrite. Here, the ratio of the tissue means an area fraction. Under the cooling conditions described below, both polygonal ferrite and ash ferrite are produced, and in the present invention, the sum is preferably 75% or more irrespective of each fraction of polygonal ferrite and ash ferrite. In addition, the balance is not necessarily limited, and may be pearlite, phase martensite, or the like.

In addition, since the toughness and strength are greatly influenced by the microstructure of the steel plate center, the microstructure of the steel plate center is controlled to include 75% or more of bainitic ferrite and ashcura ferrite in an area fraction. Under the cooling conditions described below, both bainitic ferrite and ashcury ferrite are produced, and in the present invention, the sum is preferably 75% or more irrespective of each fraction of bainitic ferrite and ashcury ferrite. Some granular bainite may be formed in addition to the microstructures as described above. Since the granular bainite is a cause of inhibiting low-temperature toughness, it is preferable to limit the content to 5% or less based on the area fraction. 4 shows a transmission electron microscope image of a typical bainitic ferrite tissue, and a scanning electron microscope and a transmission electron microscope photograph of a typical ash ferrite tissue are shown in FIGS. 5 and 6, respectively, and granular bainite tissue is shown in FIG. The transmission electron micrograph of the is shown.

The steel sheet having the above-described component system and satisfying the internal structure conditions is a steel sheet that satisfies all properties desired in the present invention as having a tensile strength of 930 MPa or more, -40 ° C impact toughness of 230 Joules or more, and a surface hardness of 170 to 260 Hv.

The most preferred method elicited by the present inventors for producing the steel which satisfies the object of the present invention as described above is described below.

In the manufacturing method of the present invention, after heating a slab having a steel composition of the present invention, the heated slab is subjected to one or two or more steps of multi-stage rolling in an austenite recrystallization zone, and then lower than the austenite recrystallization temperature. After finishing rolling in one or two or more stages of multi-stage at a temperature higher than the temperature at which the nit transforms into ferrite (Ar3), it is cooled at a cooling rate of 5 to 15 ° C / s and cooled at Ar3-20 ° C to Ar3-10 ° C. After stopping, cool it at a cooling rate of 20 ~ 50 ℃ / s and stop cooling at 200 ~ 400 ℃.

Hereinafter, detailed conditions of each step will be described.

Slab heating stage: 1050 ~ 1150 ℃

The heating process of the slab is a process of smoothly performing the subsequent rolling process and heating the slab to sufficiently obtain the mechanical properties of the target steel sheet, so the heating process should be performed within an appropriate temperature range for the purpose. It is to be uniformly heated so that the precipitated elements inside the steel sheet are sufficiently dissolved, but it is necessary to prevent excessive grain coarsening. If the slab heating temperature is less than 1050 ° C, niobium or vanadium cannot be re-used in steel, so the strength improvement effect is small, partial recrystallization occurs, and austenite grains are formed unevenly, making it difficult to toughen. On the other hand, when the temperature exceeds 1150 ° C., the austenite grains are excessively coarse, so that the grain size of the steel sheet increases and the toughness of the steel sheet is extremely degraded.

Rolling stage

In order to improve the low temperature toughness of the steel sheet, it is preferable to control the austenite grains to a fine size. This is possible by controlling the rolling temperature and the reduction ratio. In the present invention, the rolling is preferably carried out in two temperature ranges, the recrystallization behavior is different in the two temperature ranges, it is preferable to set the conditions respectively. First, one roll or two or more multi-stage rollings are performed in the austenite recrystallization region at a total reduction ratio of 20 to 60% with respect to the initial slab thickness. Rolling in the austenitic recrystallization area as described above has the effect of reducing the grain size through austenite recrystallization, in the case of multi-stage rolling to reduce the reduction rate and time of each step so that grain growth does not occur after austenite recrystallization It is desirable to control. Fine austenite grains formed by the above-described process serves to improve the low temperature toughness of the final plate.

After this, one rolling or two or more multistage rollings are performed in the austenite unrecrystallized region between Tnr (temperature at which austenite recrystallization does not occur) and Ar3 temperature (temperature at which austenite is converted to ferrite). At this time, rolling is performed at a total reduction ratio of 40 to 80% with respect to the slab thickness which has been rolled in the recrystallization temperature range. This rolling between Tnr (temperature at which austenite recrystallization does not occur) and Ar3 temperature (temperature at which austenite is transformed into ferrite) distorts the grains and develops a dislocation due to deformation inside the grains, which is a low temperature during cooling after rolling. It acts as a nucleation site that forms metamorphic phases.

After rolling, the microstructure can be controlled according to the cooling method. As shown in Fig. 2, the cooling curve A relates to one example of a conventional cooling method, and a lower bainite structure is generated by one rapid cooling. The cooling curve B relates to another example of the conventional cooling method, and the cooling sheet is slightly slowed down to include the bainitic ferrite and the ash ferrite structure. Cooling curve C relates to the cooling method of the present invention, by slowing down the primary cooling rate to form polygonal ferrite and ash ferrite on the surface of the steel sheet, and increases the secondary cooling rate higher than the primary cooling rate. It can be controlled to form ash cura ferrite. Hereinafter, the conditions of the cooling step of the present invention will be described in detail.

1st cooling stage

The first cooling step is cooled at a cooling rate of 5 ~ 15 ℃ / s to stop the cooling at Ar3-10 ℃ ~ Ar3-20 ℃. In the first cooling step, the cooling rate is slower than the cooling rate in the second cooling step and the time until the cooling stop temperature is short, so that only the microstructure of the surface portion of the steel sheet can be controlled. If the cooling rate is less than 5 ℃ / s, only polygonal ferrite is formed, fatigue fatigue resistance is poor. On the other hand, when the cooling rate exceeds 15 ° C / s, only low-temperature transformation tissue such as bainite and ash cura ferrite is formed, the surface portion may not have a low hardness value. In addition, since the first cooling step is for controlling the hardness value of the surface portion, it is important to control the cooling stop temperature. Cooling stop temperature of the first cooling step is preferably limited to Ar3-10 ℃ ~ Ar3-20 ℃. At a temperature below Ar3-20 ° C., a structure having a low hardness value up to an internal structure may be formed, and thus the tensile strength of the steel sheet may be lowered. On the other hand, in the case of exceeding Ar 3-10 ° C., cooling of the surface part may not be performed properly, and thus, a desired microstructure may not be obtained.

2nd cooling stage

The second cooling step is cooled at a cooling rate of 20 ~ 50 ℃ / s to stop the cooling at 200 ~ 400 ℃. The second cooling step is a step of cooling to a temperature capable of completing phase transformation of the microstructure of the central portion of the steel sheet at a sufficient cooling rate. If the cooling rate is less than 20 ° C / s, tissues such as polygonal ferrite or granular bainite having coarse grains are formed to lower the strength and toughness. On the other hand, if the cooling rate exceeds 50 ℃ / s may cause distortion of the steel sheet due to excessive cooling. If the cooling stop temperature exceeds 400 ℃ fine grains and bainitic ferrite and ash cured ferrite can not be formed sufficiently in the center of the steel sheet, tensile strength is not improved. On the other hand. If the temperature is less than 200 ° C., the effect is saturated, and distortion of the steel sheet may occur due to excessive cooling.

Air or air cooling stage

Thereafter, the method may further include cooling through air cooling or room cooling.

Hereinafter, the present invention will be described through examples.

Example 1

The slab having the composition shown in Table 1 was heat-rolled-cooled to prepare a steel plate having a thickness of 16 mm. The manufacturing conditions of each steel plate were set the same. That is, regardless of the type of slab, the heating temperature for the slab was 1120 ° C, and then multi-stage rolling was performed at 1050-1100 ° C (austenite recrystallization temperature), and then at 700-950 ° C (austenite recrystallization temperature). Multi-step rolling was performed. Immediately after rolling, cooling is carried out at a cooling rate of 8 to 11 ° C / sec to stop the first cooling at Ar3-20 ° C to Ar3-10 ° C, and cooling is performed at a cooling rate of 25 to 35 ° C / sec to 250 to 350 The second cooling was terminated at ° C and then left to stand in air to allow air cooling. The microstructures, tensile strength, Charpy energy (-40 ° C.), ductility-brittle transition temperature and surface hardness of the surface and center portions of the inventive steel and the comparative steel were measured and shown in Table 2 below.

division C Si Mn Mo Cr Ni Ti Nb V Al Cu Ca * B * N * P * S * Inventive Steel 1 0.053 0.15 1.90 0.12 0.48 0.48 0.014 0.038 0.038 0.024 0.2 13 21 42 52 11 Inventive Steel 2 0.046 0.16 1.79 0.34 0.60 0.61 0.021 0.048 0.051 0.023 0.2 10 11 38 61 10 Invention Steel 3 0.054 0.15 1.88 0.11 0.59 0.53 0.018 0.042 0.049 0.023 0.2 11 23 35 70 9 Inventive Steel 4 0.060 0.15 1.92 0.30 0.38 0.49 0.016 0.046 0.042 0.022 0.2 14 11 52 57 8 Comparative Steel 1 0.028 0.16 1.88 0.27 0.38 0.52 0.017 0.042 0.043 0.022 0.2 13 18 37 68 10 Comparative Steel 2 0.121 0.15 1.89 0.15 0.55 0.57 0.026 0.048 0.048 0.021 0.2 11 14 48 56 9 Comparative Steel 3 0.052 0.15 2.32 0.18 0.46 0.48 0.015 0.041 0.042 0.020 0.2 10 16 37 52 11 Comparative Steel 4 0.061 0.15 1.89 0.07 0.51 0.52 0.025 0.043 0.038 0.023 0.2 12 33 45 70 9 Comparative Steel 5 0.056 0.17 1.87 0.52 0.52 0.53 0.028 0.041 0.041 0.022 0.2 11 20 40 63 10 Comparative Steel 6 0.051 0.16 1.88 0.20 0.46 0.49 0.038 0.042 0.040 0.020 0.2 12 21 42 52 10 Comparative Steel 7 0.053 0.15 1.89 0.15 0.50 0.50 0.015 0.041 0.039 0.021 0.2 10 33 35 60 7 Comparative Steel 8 0.052 0.16 1.91 0.25 0.53 0.51 0.017 0.043 0.037 0.021 0.2 12 45 37 71 8

(However, the content unit of each component is weight percent, and the content unit of component * is ppm.)

division PF + AF fraction in surface microstructure (%) Central microstructure
BF + AF fraction (%) The tensile strength
(MPa) vE-40
(Joule) vTrs
(℃) Surface hardness
(Hv) Inventive Steel 1 85 89 968 256 -92 211 Inventive Steel 2 88 82 943 284 -104 198 Invention Steel 3 92 77 976 267 -83 203 Inventive Steel 4 79 87 995 247 -79 219 Comparative Steel 1 95 67 526 296 -94 158 Comparative Steel 2 77 88 1042 102 -38 258 Comparative Steel 3 84 86 972 156 -54 225 Comparative Steel 4 92 83 886 226 -68 186 Comparative Steel 5 62 80 1048 220 -52 275 Comparative Steel 6 86 67 992 176 -54 224 Comparative Steel 7 87 72 1023 194 -48 231 Comparative Steel 8 85 65 1031 145 -52 234

(However, PF: polygonal ferrite, BF: bainitic ferrite, AF: ashcula ferrite)

As shown in Table 2, Comparative steel 1 is low in the tensile strength of the carbon content of 0.028% by weight, which is the most effective element to increase the strength, Comparative steel 2 is excessively high in the carbon content of 0.121% by weight embrittle the steel sheet The low temperature toughness is low, and the comparative steel 3 has an excessively high manganese content of 2.32% by weight, so that the center segregation is encouraged during playing, and thus the low temperature toughness is low. Comparative steel 4 is 0.07% by weight of molybdenum, which is a hardenable element, so that the low-temperature transformation structure is poorly developed, so that both strength and toughness are bad. Hardness is excessively high, Comparative steel 6 is 0.038% by weight of titanium, excessively high coarse Ti carbonitride is produced, it can be seen that the low-temperature toughness. Comparative steels 7 and 8 were found to have low low-temperature toughness due to excessive content of boron, an element that can effectively increase the hardenability, and compared to comparative steels 7 and 8, the lower the low-temperature toughness as the boron increases It can be seen that it represents.

On the other hand, all of the inventive steels 1 to 4 satisfying the component system of the present invention have tensile strength of 930 MPa or more, impact energy (-40 ° C.) of 230J or more, and surface hardness of 170 to 260 Hv, all of tensile strength, low temperature toughness and surface crack resistance. It can be confirmed that excellent.

Example 2

Among the steels satisfying the conditions of the present invention, a slab having a composition of Inventive Steel 1 was selected and rolled and cooled under the conditions of Table 3 below. The microstructure, tensile strength, Charpy energy (-40 ° C.), ductility-brittle transition temperature and surface hardness of the surface and center portions of the invention and comparative examples were measured and shown in Table 4 below.

division Slab
Reheating Temperature (℃) Unresolved station
Rolling reduction (%) 1st cooling speed
(℃ / s) First cooling
Stop temperature (℃) 2nd cooling speed
(℃ / s) 2nd cooling
Stop temperature (℃) Inventive Example 1 1120 78 12 Ar3-15 23 327 Inventive Example 2 1132 76 8 Ar3-12 32 286 Inventive Example 3 1123 77 10 Ar3-18 39 363 Honorable 4 1095 74 9 Ar3-13 42 284 Comparative Example 1 1108 74 21 Ar3-17 26 336 Comparative Example 2 1111 75 13 Ar3-6 34 364 Comparative Example 3 1126 75 11 Ar3-27 41 296 Comparative Example 4 1112 74 12 Ar3-7 26 428 Comparative Example 5 1168 74 8 Ar3-13 34 252 Comparative Example 6 1176 76 12 Ar3-16 27 462 Comparative Example 7 1123 75 15 Ar3-14 16 278 Comparative Example 8 1132 78 17 Ar3-17 14 446 Comparative Example 9 1113 38 25 Ar3-8 28 265

division Surface texture
PF + AF fraction
(%) Internal organization
BF + AF fraction
(%) The tensile strength
(MPa) vE-40
(Joule) vTrs
(℃) Surface hardness
(Hv) Inventive Example 1 83 84 974 267 -123 215 Inventive Example 2 87 87 985 244 -98 204 Inventive Example 3 89 89 976 272 -96 220 Honorable 4 93 87 959 258 -82 196 Comparative Example 1 61 94 965 265 -95 295 Comparative Example 2 57 88 976 286 -104 287 Comparative Example 3 78 58 875 275 -96 245 Comparative Example 4 81 64 865 268 -84 224 Comparative Example 5 94 85 934 197 -64 195 Comparative Example 6 86 87 823 189 -50 211 Comparative Example 7 84 64 834 187 -48 224 Comparative Example 8 85 65 852 218 -52 216 Comparative Example 9 82 68 879 143 -42 224

(However, PF: polygonal ferrite, BF: bainitic ferrite, AF: ashcula ferrite)

As shown in Table 4, in Comparative Example 1, the first stage cooling rate is excessively high, so that the low temperature transformation phase is excessively formed on the surface, and the surface hardness is high. Hardness is not high enough to form a microstructure that can be lowered, and Comparative Example 3 has a low surface hardness but excessively low one-step cooling end temperature, it can be seen that the tensile strength of the steel is also low. Comparative Example 4 has a low tensile strength because the two-stage cooling end temperature is too high to form fine low-temperature phases properly, and Comparative Example 5 has a low low-temperature toughness due to coarse austenite grains because the slab reheating temperature is excessively high. In Example 6, the slab reheating temperature is excessively high and the cooling end temperature is too high, indicating that the low-temperature toughness is low for the same reason as in Comparative Example 5. In Comparative Example 7, the cooling rate is too low to form a low temperature transformation phase, and polygonal ferrite or granular bainite is mixed, so that both tensile strength and low temperature toughness are low. In Comparative Example 8, the cooling rate is too low and the cooling end temperature is low. It is excessively high, so both the tensile strength and low temperature toughness are low because of the above reason. In Comparative Example 9, the austenite unrecrystallized zone rolling reduction rate is too low. The low temperature toughness is very low because the low temperature transformation phase is not properly formed.

On the contrary, in the case of Inventive Examples 1 to 4, which satisfies all the conditions of the present invention, the tensile strength of 930 MPa or more, the impact energy (-40 ° C.) of 230J or more, and the surface hardness of 170 to 260 Hv are high. It can be seen that the strength and impact toughness have excellent characteristics.

1 is a heat treatment curve of a method of manufacturing a tempering and a method of manufacturing a tempering of the conventional method for manufacturing a steel sheet for line pipe.

Figure 2 is a curve showing the cooling pattern of the steel sheet after rolling in producing the steel sheet for line pipes according to the conventional method and the present invention.

It is a graph which shows typically the cooling method of the case where the surface hardness is high (A, B) and the case where the surface hardness is low (C) among the manufacturing methods of the steel plate for line pipes.

Figure 3 is a transmission electron microscope observation of the lower bainite tissue.

4 is a transmission electron microscope observation photograph of the bainitic ferrite tissue.

5 is a scanning electron microscope photograph of the Ashcula ferrite tissue.

Figure 6 is a transmission electron microscope observation picture of the Ashcula ferrite tissue.

7 is a transmission electron microscope observation photograph of granular bainite tissue.

Claims (7)

By weight%, carbon (C): 0.03 to 0.10%, silicon (Si): 0.6% or less, manganese (Mn): 1.6 to 2.1%, copper (Cu): 1.0% or less, nickel (Ni): 1.0% or less , Niobium (Nb): 0.02 to 0.06%, vanadium (V): 0.1% or less, molybdenum (Mo): 0.1 to 0.5%, chromium (Cr): 1.0% or less, titanium (Ti): 0.005 to 0.03%, aluminum (Al): 0.01 to 0.06%, boron (B): 0.0005 to 0.0025%, nitrogen (N): 0.001 to 0.006%, calcium (Ca): 0.006% or less, phosphorus (P): 0.02% or less, sulfur (S ): 0.005% or less, including residual iron (Fe) and other unavoidable impurities, The microstructure of the steel plate surface (within 2mm from the surface) contains more than 75% of the area of polygonal ferrite and ashcura ferrite, The microstructure of the center of the steel sheet (part of the steel sheet except for the surface portion of the steel sheet) is a line pipe steel sheet containing at least 75% of bainitic ferrite and ash cura ferrite as an area fraction. The steel sheet for line pipes according to claim 1, wherein the microstructure of the central portion of the steel sheet comprises 5% or less of granular bainite. The steel sheet for line pipe of claim 1, wherein the steel sheet has a tensile strength of 930 MPa or more, -40 ° C impact toughness of 230 Joules or more, and a surface hardness of 170 to 260 Hv. By weight%, carbon (C): 0.03 to 0.10%, silicon (Si): 0.6% or less, manganese (Mn): 1.6 to 2.1%, copper (Cu): 1.0% or less, nickel (Ni): 1.0% or less , Niobium (Nb): 0.02 to 0.06%, vanadium (V): 0.1% or less, molybdenum (Mo): 0.1 to 0.5%, chromium (Cr): 1.0% or less, titanium (Ti): 0.005 to 0.03%, aluminum (Al): 0.01 to 0.06%, boron (B): 0.0005 to 0.0025%, nitrogen (N): 0.001 to 0.006%, calcium (Ca): 0.006% or less, phosphorus (P): 0.02% or less, sulfur (S ): A heating step of heating the slab containing 0.005% or less, balance iron (Fe) and other unavoidable impurities to 1050-1150 ° C .; A recrystallization reverse rolling step of rolling the heated slab once or multi-stage at least twice in an austenite recrystallization temperature region; A non-recrystallization station rolling step of rolling the rolled slab once in austenite recrystallization temperature, at least Ar3, or at least two times in a multi-stage rolling step; A first cooling step of cooling the rolled steel sheet at a cooling rate of 5 to 15 ° C./s to stop cooling at Ar 3-20 ° C. to Ar 3-10 ° C .; And And a second cooling step of cooling the first cooled steel sheet at a cooling rate of 20 to 50 ° C./s to stop cooling at 200 to 400 ° C. 2. 5. The method of claim 4, wherein the recrystallization rolling step is performed with a cumulative reduction of 20 to 60%. The method of claim 4, wherein the non-recrystallization rolling step is performed with a cumulative reduction of 40 to 80%. 5. The method of claim 4, wherein the steel sheet is cooled by air or by air after the second cooling step.

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