KR100363191B1 - The method of manufacturing linepipe steel with good formability - Google Patents

Abstract

The present invention relates to a method for manufacturing line pipe steel, which is easy to process, and its purpose is to maintain a yield strength of 50 kgf / mm ² and to maintain a yield ratio of 85% or less, so that the pipe is easy to be processed, and cracks due to H ₂ S gas The present invention provides a method for producing ultra low carbon bainite line pipe steel that is not produced (HIC).

The present invention having such an object is, in weight%, C: 0.02-0.03%, Mn: 1.6-2.0%, Si: 0.1-0.2%, Al: 0.02-0.05%, Ti: 0.01-0.02%, Nb: 0.04-0.06%, Mo: 0.15-0.25%, B: 0.001-0.002%, V: 0.01% or less, S: 0.002% or less, P: 0.02% or less, N: 20-60 ppm, Ca: 20-50 ppm, remaining A steel slab made of Fe and other unavoidable impurities is heated in the temperature range of 1150 to 1250 ° C., hot rolled under the condition that the finish rolling temperature is Ar ₃ + (100 to 200 ° C.), and then cooled before the Ar ₃ temperature. The technical gist of the present invention relates to a method for producing a line pipe steel having excellent workability by accelerating cooling to a temperature of 480 ± 30 ° C. at a cooling rate of 7 to 11 ° C./sec and air cooling to room temperature.

Description

Method for manufacturing line pipe steel with excellent workability {THE METHOD OF MANUFACTURING LINEPIPE STEEL WITH GOOD FORMABILITY}

The present invention relates to a method for manufacturing line pipe steel that is easy to process. More specifically, yield strength of 50kgf / mm ² grade API (American Petroleum Association) X70 grade steel has excellent overall properties compared to conventional line pipe yield strength. A method for producing a type bainite alloy steel.

Conventionally, high strength structural steels requiring a yield strength of 50kgf / mm ² or more contain a large amount of C, Mn and trace elements to increase the strength of the steel, or to improve weldability during welding, which is an essential subsequent process of the steel. The method of compensating the content of the alloying element as much as possible and thus reducing the strength through the control of the microstructure through accelerated cooling has been mainly used.

However, this technique basically has a ferrite + pearlite structure of the final microstructure, and further improvement in yield strength cannot be expected, and in order to secure a strength above the standard, it is subjected to low temperature rolling and accelerated cooling, but the rolling temperature is increased. Because the cooling start temperature, cooling end temperature, cooling rate, etc. must be strictly controlled, there is a disadvantage in that the productivity is expensive and the production cost is expensive because the defective rate of the final product is relatively large. In addition, the H ₂ S (sour) gas resistance, which is required for the line pipe, is also heat-resistant because hydrogen organic cracks propagate along the pearlite band. In particular, another disadvantage of ferritic + perlite steels is that the yield ratio (YR = yield strength / tensile strength) has a value of about 90%, which makes it difficult to process during the piping process, which is a subsequent process of line pipe material. to be. In recent years, the demand for surrendering costs is lower than 85% in the market, but there is a concern that the failure rate of tubing will increase in existing organizations with a surrendering cost of around 90%.

In sum, the line pipe steel manufactured by the existing accelerated cooling technology has a problem that it is difficult to comprehensively satisfy the strength, workability, H ₂ S gas resistance, etc. by having a ferrite + pearlite structure. Therefore, the present inventors have made a multifaceted study to improve the problems of the prior art, and it is confirmed through experiments that the microstructure of the steel can be solved by appropriately combining the control of the steel system and the accelerated cooling conditions to make the bainite. It came to propose the present invention.

The present invention maintains a yield strength of 50kgf / mm ² class, yield rate is 85% or less, easy pipe processing, and H ₂ S gas (HIC: Hydrogrn Induced Cracking) ultra-low carbon type bainite line It is an object of the present invention to provide a method for producing steel for pipes.

1 is a graph comparing the physical properties of the inventive steel and the comparative steel

Method for producing a line pipe steel of the present invention for achieving the above object, in weight%, C: 0.02-0.03%, Mn: 1.6-2.0%, Si: 0.1-0.2%, Al: 0.02-0.05%, Ti : 0.01-0.02%, Nb: 0.04-0.06%, Mo: 0.15-0.25%, B: 0.001-0.002%, V: 0.01% or less, S: 0.002% or less, P: 0.02% or less, N: 20-60 ppm Hot-rolled steel slab consisting of Ca: 20-50ppm, remaining Fe and other unavoidable impurity elements in the temperature range of 1150-1250 ℃ and finishing rolling temperature under Ar ₃ + (100-200) ℃ And cooling before the Ar ₃ temperature is accelerated to a temperature of 480 ± 30 ° C. at a cooling rate of 7 to 11 ° C./sec and air cooled to room temperature.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

According to the present invention, the steel component is made of ultra low carbon, and the other components are properly managed without adding the alloying element V, which has been managed as an essential component in the yield strength of 50kgf / mm ² steel, and the appropriate accelerated cooling conditions are controlled. By doing so, the final microstructure of the steel is made of bainite to improve the overall physical properties.

In general, the content of C in the steel alloy is low, so that the fraction of the second phase structure is lowered, the strength is lowered. In many cases, the strength is increased, but the impact toughness, especially low temperature toughness, is impaired and the weldability at the time of welding is lowered. Therefore, the present invention manages the content of C at an extremely low level of 0.03% or less, makes the base structure after rolling into bainite and suppresses the formation of the second phase, but when the content of C is less than 0.02%, the strength of the bainite matrix is lower than Since the content of C is limited to 0.02% to 0.03%.

The Si is added as a deoxidizer during steelmaking and also has a solid solution effect, but the impact transition is an element that increases the temperature. However, at least 0.1% of Si is added, so that weldability is lowered and an oxide film is severely formed on the surface of the steel sheet. Is limited to the range of 0.1 to 0.2%.

Since Mn forms MnS, which is an elongated non-metallic inclusion such as S, and thus lowers room temperature elongation and low temperature toughness, it is preferable to manage it to 2.0% or less. However, when Mn is 1.6% or less due to the component properties of the present invention, which is extremely low carbon, Since granularity cannot be secured, it is difficult to form bainite, so it is difficult to secure strength, so it is limited to 1.6 ~ 2.0%.

The Al is added at least 0.02% as a deoxidizer during steelmaking, but when 0.05% or more is added, Al ₂ O ₃ , which is a non-metal oxide, is formed to reduce impact toughness. Therefore, Al is limited to 0.02% to 0.05%.

Ti is a major element that forms a TiN precipitate during the solidification process of steel to suppress grain growth during heating of the ingot, and inhibits the growth of recrystallized grains during hot rolling, thereby playing a major role in grain refinement of steel. The amount of Ti added varies depending on the content of N. When the amount of Ti is relatively small compared to the amount of nitrogen, the amount of TiN formed is small, which is disadvantageous to refine the grains, whereas when excessively added, TiN becomes coarse during heating. In addition, the effect of inhibiting grain growth is reduced. Therefore, the amount of Ti added is usually limited to 0.01-0.02% in consideration of the content of N contained (20-60 ppm).

The Nb is an important element that increases the strength of the steel by solidifying the austenite to increase the hardenability of the austenite and to precipitate as carbonitrides (Nb (C, N)) matching with the matrix (Matrix). In addition, the characteristics of the present invention is to produce a steel having excellent strength and low temperature toughness by the formation of the bainite matrix structure, it is important to contain a large amount of Nb in order to enhance the hardenability, even if added at least 0.06% Is not increased, and is present in the solid solution in the ferrite, thereby reducing the impact toughness and reducing the weldability, so it is limited to 0.04% -0.06%.

The B is known to increase the hardenability as well as segregation to enhance the binding of the grain boundary when dissolved in the matrix structure. This means that B contributes to the enhancement of low temperature toughness in the steel showing toughness fracture properties that progress along the grain boundary. Therefore, it is necessary to contain B more than 0.001%. However, if it contains more than 0.002%, the grain boundary tends to be withdrawn, so it is limited to the range of 0.001% -0.002%.

Mo is an element essential for making the base structure into bainite structure by increasing the hardenability and suppressing ferrite transformation, but when it contains 0.25% or more, Mo-C precipitates are formed to drop grain boundaries, and the cooling ability by C is also lowered. The limit is 0.15 ~ 0.25%.

The P is a particularly bad element to impact toughness, the lower the content is better, but is inevitable in the steelmaking process, so the content is limited to 0.02% or less so as not to adversely affect the physical properties.

S is present as a non-metallic inclusion of MnS and is elongated by hot rolling to promote anisotropy of steel sheet properties and lower impact toughness, so that its content is controlled to 0.002% or less.

The N forms a TiN precipitate with Ti to refine the grains of the steel, and the amount of N added thereto is 20-60 ppm in consideration of the Ti content.

Ca is produced by CaS is added to suppress the non-metallic inclusions of MnS, for this purpose is added 20ppm or more. If the amount is large, it is reacted with O contained in the steel to produce CaO, which is a nonmetallic inclusion, so it is 50 ppm or less.

In addition to the alloying components, O may be contained as in the case of general structural steel, in which case the content is controlled to O: 0.01-0.05%.

Hereinafter, the manufacturing method of this invention is demonstrated.

In the present invention, the steel slab formed as described above is heated and hot rolled in the recrystallization region, and the cold rolling rate is performed using hot rolling to finish the hot rolling in the temperature range of Ar ₃ + 100 ° C to Ar ₃ + 200 ° C. After cooling to a certain temperature of 7 ℃ / sec or more, and then consists of a series of processes to air-cooled to room temperature.

First, heating before hot rolling is performed in the temperature range of 1150-1250 degreeC, for the following reason. In the present invention, Nb is present in a dissolved state in austenite, and the refinement of the ferrite to the drop in the transformation temperature caused by the increase in the hardenability by the Nb is a major point for improving the strength and low temperature toughness. In the slab state, Nb is combined with C to exist as carbide NbC. Therefore, before hot rolling, the slab must be heated to 1150 ° C or higher to allow NbC to dissolve and exist in the atomic state, provided that the heating temperature is 1250 ° C or higher. The slab heating temperature is in the range of 1150 to 1250 ° C because the austenite particles are too coarse and some delta ferrite is generated in the steel to degrade the properties of the steel sheet.

After heating as above, it is hot rolled. At this time, the hot rolling is performed in the recrystallization zone and the rolling finish temperature is set at Ar ₃ + 100 ° C to Ar ₃ + 200 ° C. This is described as follows.

The finish rolling temperature is closely related to low-temperature toughness as well as strength, which is an index to be strictly controlled. In the case of conventional ferritic and perlite steels, the rolling was finished at about 30 ° C based on Ar ₃ . If the rolling finish temperature is too high, the ductility and low-temperature toughness are excellent, but the strength is lowered. If the rolling finish temperature is too low, abnormal reverse rolling occurs, resulting in stretched ferrite and pearlite, and the pearlite band is formed. Since the low temperature toughness is greatly lowered, the prior art has been to control the finish rolling temperature within ± 30 ° C based on Ar ₃ of Equation (1) below.

Ar ₃ = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo (℃) ····· (1)

Here, the unit of C, Mn, Cu, Ni, Cr, Mo is weight percent.

It is a feature of the present invention to provide a method for producing steel having improved productivity by about 25% by finishing rolling at a relatively high temperature, which is possible because the microstructure after air cooling can be made of bainite. When the structure after air cooling becomes bainite, the dependence on the finish rolling temperature of the strength and toughness becomes very small, so that rolling can be terminated at a higher temperature than in the prior art. In the present invention, the temperature range is 100 ° C. to 200 ° C. higher than the Ar ₃ temperature. To finish rolling.

After rolling and accelerated cooling as described above, and then air-cooled to room temperature, accelerated cooling starts cooling before the Ar ₃ temperature and cooled to a temperature of 480 ± 30 ℃ at a cooling rate of 7 ~ 11 ℃ / sec. The cooling start temperature is limited to the temperature before Ar ₃ because the cooling must be started before the transformation starts to prevent precipitation of the cornerstone ferrite. In addition, the cooling rate is limited to 7 to 11 ° C./sec because the cooling rate is too slow, and even if bainite is formed, the interlayer spacing is so low that the low temperature toughness is worsened. Because this is difficult. In addition, when the cooling end temperature is lower than 450 ℃ unstable phase, when higher than 510 ℃ Mn, C is concentrated and the part which is not transformed into bainite becomes pearlite.

Hereinafter, the present invention will be described in more detail with reference to Examples.

In the following examples, the slab of the alloy steel, which is composed as shown in Table 1, was used as a target steel type, wherein the inventive steel A is a steel that satisfies the component range of the present invention, and the comparative steels B, C, D, and E are presently known in the art. It is steel which is commercialized by the steel outside the component range of this invention.

division Chemical composition (% by weight) C Mn Si Nb Ti V Mo B Cu Ni Inventive Steel A 0.020 1.81 0.16 0.045 0.013 - 0.20 0.0011 - - Comparative steel B 0.063 1.74 0.29 0.046 0.017 0.072 - - - - C 0.099 1.50 0.28 0.055 0.014 0.067 - - - - D 0.095 1.30 0.31 0.048 0.013 0.069 - - 0.24 0.12 E 0.077 1.2 0.25 0.045 0.014 0.066 - - 0.24 0.13 Both the invention and comparative steels were managed with Al: 30ppm, N: 50ppm, O: 20ppm, S: 20ppm, P150ppm.

Specifically, the compositional difference between the inventive steel and the comparative steel shown in Table 1 is as follows. Inventive steel A was added with a very small amount of boron and 0.2% of Mo in order to minimize precipitation of the second phase by reducing the carbon content to about 0.02% and to improve the hardenability. Vanadium is excluded. In comparison, B and C steels were commercialized and developed for low temperature in 50kgf / mm grade ² line pipe, and D and E steels contained Cu, Ni and Mn content to improve sour gas resistance. The steel grade is limited to 1.3% or less.

Example 1

Five steel slabs of Table 1 were rolled as shown in Table 2 below, followed by accelerated cooling, followed by air cooling to obtain a thick plate. Tensile test and low temperature toughness test were performed on the thick plate, and the results are shown in Table 3.

Finish rolling temperature Cooling start temperature Cooling end temperature Cooling rate Inventive Steel A 950 ℃ 750 ℃ 550 ℃ 7.8 ℃ / sec Comparative steel B 740 ℃ 720 ℃ 480 ℃ 11.3 ℃ / sec C 740 ℃ 720 ℃ 475 ℃ 11.7 ℃ / sec D 700 ℃ 676 ℃ 506 ℃ 9.1 ℃ / sec E 763 ℃ 723 ℃ 432 ℃ 10.0 ° C / sec

Yield strength YP (Kgf / mm ² ) Tensile Strength TS (Kgf / mm ² ) Yield Ratio (YP / TS) Charpy impact value (-50 ℃) Inventive Steel (A) 52.0 63.7 0.816 225J / cm Comparative steel B 54.2 61.0 0.888 270J / cm C 53.9 62.7 0.860 293J / cm D 56.5 61.2 0.923 150 J / cm E 55.8 59.9 0.931 189 J / cm

As can be seen from Table 3, the yield strength is superior to the D and E steels, and the low temperature toughness is superior to the B and C steels, but the comparative steels have a yield ratio far higher than 0.82. Therefore, it can be seen that when the comparative steel is applied to the line pipe steel, the processing occurs unevenly in the subsequent pipe processing, and locally, the bursting phenomenon may occur during the processing. On the other hand, the invention steel has a yield ratio of 0.816, it can be seen that the workability is very excellent.

Example 2

One of the most important properties required for line pipe steel is H ₂ S (sour) gas resistance. Since line pipe steel is used to transport crude oil and natural gas, it is always exposed to H ₂ S gas contained in crude oil, so there is a danger of sudden brittle fracture by HIC. For this reason, the HIC resistance should be included in the evaluation of the line pipe properties, and Table 4 below shows the results of the HIC characteristics test of the inventive steel and the comparative steel, and the HIC test conditions were tested according to the NACE-77 standard.

Crack Length Ratio (%) Inventive Steel (A) 0 Comparative steel B 72.5 C 25.6 D 14.7 E 2.3

As can be seen from Table 4, the cracks of the comparative steels B and C observed more than 20% of the cracks after the HIC test, and HIC cracks of about 10% of the D and E strengths, which enhanced the HIC resistance, were observed in the existing ferrite matrix steels. It became. In contrast, the inventive steel A can be seen that no HIC crack was observed. This is because in the existing ferrite matrix, the HIC easily propagates along the perlite band, whereas the present invention steel has no path for HIC propagation because the entire tissue is bainite, so no HIC is generated.

Example 3

The experimental results were compared to compare the physical properties of the inventive and comparative steels in FIG. The workability is yield ratio, the H ₂ S gas resistance is HIC crack ratio, the low temperature toughness is -50 ° C, the chaffe impact energy at -50 ° C, the productivity is the finish rolling temperature, and the quality stability is accelerated cooling rate. Each was marked. If the accelerated cooling rate is increased, material variation and plate deformation in the plate are generated. Therefore, it is reasonable to use the accelerated cooling rate as an index of quality stability because the same physical properties should be obtained at the smallest possible cooling rate. As described above, the higher the rolling finish at a higher temperature, the more favorable in terms of productivity. Therefore, the finishing rolling temperature was evaluated as an index of productivity.

As shown in Figure 1, when considering the above-mentioned evaluation items as a whole it can be seen that the invention steel has significantly superior physical properties and manufacturing conditions than the comparative steel.

As described above, according to the present invention, steel products satisfying the yield strength of 150kgf / mm ² , excellent in workability, and excellent in HIC resistance can be manufactured by using high temperature rolling + accelerated cooling method, thereby improving productivity. It is effective in stably providing steel for line pipes having excellent physical properties.

Claims (1)

By weight, C: 0.02-0.03%, Mn: 1.6-2.0%, Si: 0.1-0.2%, Al: 0.02-0.05%, Ti: 0.01-0.02%, Nb: 0.04-0.06%, Mo: 0.15- Steel with 0.25%, B: 0.001-0.002%, V: 0.01% or less, S: 0.002% or less, P: 0.02% or less, N: 20-60ppm, Ca: 20-50ppm, remaining Fe and other unavoidable impurities The slab is heated in the temperature range of 1150 to 1250 ° C. and hot rolled under the condition that the finish rolling temperature is Ar ₃ + (100 to 200) ° C., and then cooling is started before the Ar ₃ temperature to be 7-11 ° C./sec. A method for producing line pipe steel with excellent workability by accelerating cooling to a temperature of 480 ± 30 ℃ at a cooling rate and air cooling to room temperature.