KR100851176B1 - Hot-rolled steel sheet for line pipe having low anisotropy of low temperature toughness and yield strength and the method for manufacturing the same - Google Patents

Abstract

Mainly used in construction, pipelines and offshore structures, there is provided a hot rolled steel sheet for a line pipe having low temperature toughness and yield strength anisotropy and a method of manufacturing the same.

The hot rolled steel sheet is in weight percent, including C: 0.2% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01% to 0.08%, Ni: 0.1% to 0.5%, and Ti: 0.01% to 0.1%. It is composed of Fe and other unavoidable impurities, and the structure of the steel sheet has matrix structures of ferrite and pearlite and less than 1% of MA (martensite austentie constituent), and the grain size of guarferrite is 5 µm or less.

According to the present invention, a hot-rolled steel sheet having a yield strength difference (rolling right angle direction-rolling 45 degree direction) is 40 MPa or less and Charpy impact energy difference (rolling right angle direction-rolling 45 degree direction) is 50 J or less. Can provide.

Hot rolled steel sheet, line pipe, low temperature toughness anisotropy, yield strength anisotropy, Nb, hot rolling

Description

Hot-rolled steel sheet for line pipe having low anisotropy of low temperature toughness and yield strength and the method for manufacturing the same}

Figure 1 shows the relationship between the pipe diameter and the rolling direction in the manufacture of spiral steel pipe.

2 shows the optical and SEM structure of the inventive material according to the present invention.

Figure 3 shows the texture of the invention and the comparative material according to the present invention.

Japanese Patent Laid-Open No. 52-107225

Japanese Patent Laid-Open No. 5-214486

Korean Unexamined Patent Publication No. 2000-0039479

The present invention relates to a hot rolled steel sheet for line pipe with a yield strength of 540 MPa or more used in construction, pipelines and offshore structures. More specifically, the present invention relates to a hot rolled steel sheet for a line pipe having low low temperature toughness and yield strength anisotropy by adding Nb, Ni and Ti and optimizing the rolling manufacturing process conditions and a method of manufacturing the same.

Many attempts have been made all over the world to manufacture high strength hot rolled steel sheets having excellent low temperature toughness, and most of these studies have been conducted to improve low temperature toughness by performing hot rolling at most low temperatures. Japanese Patent Application Laid-Open No. 52-107225, Japanese Patent Laid-Open No. 5-214486, and Korean Laid-Open Patent Publication No. 2000-0039479.

The technical gist of the Japanese Patent Application Laid-open No. 52-107225 proposes C: 0.03-0.8%, Si: 0.6%, Mn: 0.7-2.0%, P: 0.010%, S: 0.008%, and Nb: 0.01-0.10. Low temperature toughness by lowering the finish hot rolling temperature of steel containing%, V: 0.01-0.15%, Mo: 0.50%, Ni: 1.0%, Cr: 1.0% and Cu: 1.0% to a low temperature of 850-600 ° C. will be. However, this hot rolled steel sheet has a problem that the optimization of the rolling process according to the addition of alloying elements is somewhat insufficient because the tensile strength is only 60kgf / mm ² .

In addition, Japanese Patent Laid-Open No. 5-214486 proposes a method of hot rolling, cooling at a rate of 15 ° C./sec or more at an Ar ³ temperature or more, and winding at about 250 ° C. However, this method has the disadvantage of having a very large capacity of water cooling equipment because the alloy component system is relatively simple but the winding temperature is very low.

Although the above-described prior arts can obtain high-temperature toughness and high strength at the same time, there is a problem that material anisotropy, in particular, the difference in material anisotropy of yield strength is large.

Material anisotropy means the difference between the property in the direction perpendicular to the rolling direction and the property in the direction of 30 degrees to the rolling direction. Recently, there is an increasing demand for spiral pipes, which are inexpensive instead of relatively expensive UOE pipes and are capable of producing pipes of various diameters. 1 is a view showing the relationship between the pipe diameter and the rolling direction in the manufacture of spiral steel pipe.

In Fig. 1, D denotes a pipe diameter, B denotes a coil width, and α denotes an angle between the rolling direction and the pipe circumferential direction. For example, when a 46 ＂spiral pipe is constructed using a coil having a width of 1550, α is about 30 degrees.

In line pipes, the material is required for the circumferential direction of the pipe, so unlike conventional ERW or UOE pipes, in spiral pipes, the yield strength and low temperature toughness in the diagonal direction, that is, 30 to 50 degrees in the rolling direction, are different. This is important.

However, due to the shape of aggregates and microstructures inherently developed in hot-rolled steels, the yield strength and low temperature toughness in the diagonal direction are more inferior to the strength and low temperature toughness in the direction perpendicular to the rolling direction (90 degree direction). Therefore, the strength and low temperature toughness in the 90 degree direction satisfy the API standard, but the properties in the diagonal direction may not satisfy the yield strength criteria defined in the API standard.

The present invention is to improve the above-mentioned conventional problems, by adding Nb, Ni, Ti and by optimizing the rolling manufacturing process conditions, the hot rolled steel sheet for line pipes with low temperature toughness and yield strength anisotropy due to precipitation and microstructure, and its The purpose is to provide a manufacturing method.

The present invention for achieving the above object, in weight%, C: 0.2% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01 to 0.08%, Ni: 0.1 to 0.5%, Ti: 0.01 to The present invention relates to a hot-rolled steel sheet for line pipe, which is composed of the remaining Fe and other unavoidable impurities, including 0.1%, and has a low grain toughness and yield strength anisotropy with a grain size of ferrite of 5 µm or less.

In addition, the present invention, by weight, including C: 0.2% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01 to 0.08%, Ni: 0.1 to 0.5%, Ti: 0.01 to 0.1% The steel slab composed of the remaining Fe and other unavoidable impurities is reheated at 1120 ° C or lower, and the average grain size of the austenitic structure is controlled to be 50 µm or less before finishing rolling, and then the temperature range is Ar ₃ to Ar ₃ + 100 ° C. Hot-rolling for line pipes with low anisotropy and low yield toughness, which includes hot rolling to maintain a total reduction ratio of 60% or more while maintaining a reduction ratio per pass in 30% or less, and water cooling to 540 ° C or lower. It relates to a method for producing a steel sheet.

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

Material anisotropy is known to be influenced not only by texture but also by the shape of grains. Under normal hot rolling conditions, many grains elongated in the rolling direction are formed to generate material anisotropy due to the grains. The inventors have tried to reduce the material anisotropy by making the matrix structure fine to remove the grain anisotropy. However, in order to refine the grains, the reduction ratio per pass was added as much as possible to secure the ultrafineness of the steel. However, in this case, the heat generation amount due to processing is so large that the temperature of the material increases so that the grain growth occurs. I figured out.

Accordingly, the present inventors have repeatedly conducted research and experiments to overcome these problems, and based on the results, the present invention proposes the present invention, in which the problem of grain growth caused by a rise in material temperature during the hot working process does not occur. The following method was found to optimize the addition of alloying elements and manufacturing process conditions for finer grains.

First, the steel of the austenitic structure is hot worked to form an ultrafine ferrite structure using strain organic dynamic transformation. The austenitic grain size is finely controlled to 50 μm or less so that the strain organic dynamic transformation can occur well even at a small reduction ratio.

Second, the niobium (Nb) carbonaceous material is hot-processed by low-pressure multi-stage processing so that the fine ferrite structure formed by the strained organic transformation does not grow, and further, by adding a predetermined amount of Nb, the grain growth of the ferrite formed by the dynamic transformation does not occur. Distribute the cargo.

Third, the hot working temperature is optimally controlled to secure a sufficient fraction of the dynamic transformation ferrite and to obtain fine ferrite.

As such, in the present invention, by maintaining the average austenite grain size below a certain level, the steel is thermally cut to form a large amount of strain-organic dynamic transformation ferrite efficiently and uniformly at a high speed. In addition, while maintaining the rolling reduction rate for each pass during a thermal cutting process, the cumulative reduction rate is limited to a predetermined value or more, minimizing the amount of heat generated, so that ultra-fine grains of ferrite are formed as deformation organic dynamic during processing. By suppressing the growth of ultra-fine ferrite using the precipitate, it is possible to control the average size of the ferrite grains after the final cooling to 5㎛ or less.

Hereinafter, the composition range of the steel component of the present invention will be described.

The content of C is preferably 0.2% or less.

C is the most economical and effective element for strengthening steel. However, if the content is more than 0.2%, the proportion of ferrite in the final microstructure is less than about 60% and can not be classified as a low carbon steel, the toughness of the heat affected zone during welding may be a big problem. Therefore, the content of C is preferably limited to 0.2% or less.

The content of Si is preferably 1.5% or less.

Si is a component element that needs to be added for deoxidation in the steelmaking process with a solid solution strengthening effect. However, if the content exceeds 1.5%, the weldability is lowered, and an oxide film that is difficult to remove is likely to be formed on the surface of the steel sheet, and in particular, may promote coarsening of ferrite grains. Therefore, the content of Si is preferably limited to 1.5% or less.

The content of Mn is preferably 2.0% or less.

The Mn needs to be added for deoxidation, but if the content exceeds 2.0%, the hardenability is unnecessarily increased, thereby reducing the transformation rate of ferrite during rolling and increasing the possibility of low temperature structure during welding. . Therefore, the content of Mn is preferably limited to 2.0% or less.

The content of Nb is preferably 0.01 to 0.08%.

The Nb is a very important element in the present invention as a component that forms an ultra fine carbonitride precipitate having a size of several tens of nanometers by combining with carbon or nitrogen in steel during reheating or hot rolling.

As described above, the dynamic metamorphic ferrite structure is very fine in nature and thus can be easily grown during hot rolling, particularly during the holding time between passes. Therefore, it is essential to suppress such growth, and in order to suppress the growth of the dynamic ferrite structure, the addition of titanium, aluminum, or vanadium capable of forming carbonitride can be considered.

However, according to the research of the present inventors, since carbonitrides of titanium or aluminum are inherently coarse in size compared to niobium carbonitrides, they cannot effectively suppress the movement of ultra-fine ferrite grain boundaries of several microns, but only several tens of microns in size. It was confirmed that it is effective in suppressing austenite grain growth up to.

In the case of vanadium, it is difficult to form carbonitride in the hot rolling process, and it can be precipitated as very fine carbonitride in the cooling process after rolling, so it is effective to increase the strength of steel, but it is effective to move the ultrafine ferrite grain boundary. Could not be suppressed.

In view of the above advantages, the present invention intends to use niobium, which can easily form very fine carbonitrides of several tens of nanometers in the hot rolling step, thereby achieving the micro-fine ferrite growth of several microns as a result of dynamic transformation. It can be effectively suppressed.

However, if the niobium content is less than 0.01%, the number of niobium precipitates is too small to effectively suppress the grain growth of the entire superfine ferrite, and if it exceeds 0.08%, the effect of the addition is not only saturated, but too much steel. Hardening does not readily result in sufficient dynamic transformation ferrite structure. Therefore, the content of Nb is preferably limited to 0.01 ~ 0.08%.

The content of Ni is preferably 0.1 to 0.5%.

Ni is an austenite stabilizing element that inhibits the formation of pearlite, and is an element that facilitates the formation of acicular ferrite, which is a low-temperature transformation structure, is added at least 0.1% and is an expensive element and limited to 0.5% or less because it inhibits the toughness of the weld zone. It is preferable.

The content of Ti is preferably 0.01 to 0.1%.

Ti is a very useful element for refining crystal grains. It exists as TiN in steel and has the effect of inhibiting the growth of crystal grains during the heating process for hot rolling. Also, Ti remaining after being reacted with nitrogen is dissolved in carbon to form TiC. Precipitates are formed and the formation of TiC is very fine, which can greatly improve the strength of the steel. Therefore, in order to obtain the effect of inhibiting austenite grain growth due to TiN precipitation and increasing the strength due to TiC formation, at least 0.01% or more of Ti should be added. Since the toughness of the weld heat-affected zone may deteriorate as it is reused, the upper limit of the Ti addition is made 0.1%.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

In the present invention, the grain size of the ferrite is controlled to 5 μm or less in terms of securing low temperature toughness and yield strength anisotropy.

When rolling, many grains drawn in the rolling direction are formed. Material anisotropy occurs due to the grains. For this purpose, the grain size of the ferrite is finely set to 5 μm or less to remove the grain anisotropy, thereby reducing the low temperature toughness and yield strength anisotropy. can do. When the ferrite grain size exceeds 5 μm, the above effects cannot be sufficiently secured. Therefore, it is preferable to control the grain size of the ferrite to 5 μm or less.

In addition, the tissue of the present invention is composed of a matrix of ferrite and a small amount of fine pearlite and MA (martensite austentie constituent) tissue.

The second phase, Perlite and MA, is a harder phase than the ferrite, which is a known structure, and when the ferrite alone is present, it is effective to improve the overall strength. However, it is important to disperse finely when the second phase, Perlite or MA, is coarse or has a large number of fractions. When the ferrite is finely formed by dynamic transformation as in the present invention, since the second phase is finely dispersed, the yield strength can be improved without lowering the low temperature toughness. In addition, the finely dispersed second phase has the effect of reducing the yield strength and low-temperature toughness anisotropy caused by the texture formed during ferrite transformation. Preferably the fraction of the MA tissue is within 1%.

In addition, the hot rolled steel sheet of the present invention has a yield strength difference (rolling right angle direction-rolling 45 degree direction) of 40 MPa or less, and Charpy impact energy difference (rolling right angle direction-rolling 45 degree direction) of 50J or less at -80 ° C. As a result, excellent low-temperature toughness and yield strength anisotropy can be ensured.

Hereinafter, the manufacturing method of the hot rolled steel sheet which has the steel comprised as mentioned above is demonstrated in detail.

In the present invention, after manufacturing the steel composition as described above, it can be reheated and subjected to rough rolling process as necessary. At this time, it is required to control the reheating temperature, the roughening ratio, and the like of the steel so that the austenite structure average grain size of the steel is 50 µm or less. In the present invention, the reheating temperature is preferably 1120 ° C or less. If the size of the austenite grain size of the steel immediately before finishing rolling exceeds 50 µm, the formation rate of the strain organic dynamic transformation ferrite becomes significantly uneven during the subsequent hot working, and the formation position of the strain organic dynamic transformation ferrite becomes very uneven and finally mixed. It is very difficult to obtain ultrafine grained ferritic steel because the ferrite is very likely to be formed.

Therefore, in order to effectively obtain fine ferrite grains in the final product state, it is necessary to keep the size of the subcooled austenite grains entering the subsequent finishing hot rolling at an average of 50 μm or less. If the austenite is supercooled in the unrecrystallized region, the average length of the major and minor axes of the austenite grains deformed elliptical in the austenite cross section should be kept below 50 µm.

In addition, the steel having the average grain size of the austenitic structure controlled as described above is cooled to a predetermined cooling rate, and then subjected to finishing hot-stage processing in a supercooled state. At this time, the finishing hot rolling start temperature is limited to Ar ₃ to Ar ₃ + 100 ° C. do. Ten thousand and one wherein the finish hot rolling starting temperature is developed a problem which is lower than Ar ₃ is pro-eutectoid ferrite coarse before hot rolling are formed along the austenite grain lowering various physical properties by being long stretch of rolling work, the Ar ₃ + 100 ℃ If exceeded, the fraction of dynamic transformation ferrite may not be sufficiently secured, and the tissue refinement itself may be impossible.

On the other hand, in the present invention, it is required to perform hot rolling in such a manner that the cumulative rolling rate is 60% or more while maintaining the reduction rate per pass in 30% or less. If the reduction ratio per pass of the hot rolled rolling is set to 30% or more, the temperature of the hot work material is excessively increased due to the processing heat, thereby causing a problem of halving the miniaturization effect of the grains. Therefore, in the present invention, it is essential to keep the rolling reduction per pass of the hot rolled rolling at 30% or less.

In addition, when the cumulative reduction ratio of the hot rolled rolling becomes less than 60%, it is difficult to obtain ultrafine grain structure because the amount of formation of dynamic transformation ferrite effective for miniaturization is insufficient. Such hot rolling is preferably carried out so that the strain organic dynamic transformation ferrite fraction at the end of rolling is 40% or more in terms of securing the final fine ferrite microstructure. If the fraction is less than 40%, the size of the static ferrite formed during cooling after processing becomes coarse, so that a structure of sufficiently fine and uniform final product cannot be secured.

Next, in the present invention, the hot rolled steel is water cooled to a cooling end temperature of 540 ° C. or less. When it exceeds 540 ℃, the ferrite grains generated after the dynamic transformation becomes large, and the second phase, the perlite fraction not only increases, but also increases the average size, which adversely affects low temperature toughness.

As described above, in the present invention, by appropriately controlling the particle size of the austenite structure and the finish hot rolling conditions before finishing hot rolling, steel materials having a ferrite structure having an average grain size of 5 µm or less can be effectively produced.

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

EXAMPLE

Inventive steels (A, B) and Comparative steel C, which are formed as shown in Table 1, were manufactured as slabs by a continuous casting method, and then hot-rolled under the conditions as shown in Table 2 to prepare a plate.

In order to determine the mechanical properties of the steel from the hot-rolled sheets as described above, tensile test pieces and impact test pieces were taken at 0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees clockwise with respect to the rolling direction.

Tensile test pieces were used for API 5L standard test pieces, and the tensile test was conducted at a cross head speed of 10 mm / min. In addition, the ferrite fraction after cooling was measured using an optical microscope and an image analyzer, and the low temperature toughness was evaluated by Charpy impact test at 25 ~ 100 ℃ by processing the impact specimen in the center of the base material.

In addition, in order to compare the effect of the texture on the material anisotropy, the texture of the invention material A and the comparative material C was measured by X-ray.

Steel grade C Si Mn P S Nb Ni Mo Ti Inventive Steel A 0.060 0.25 1.6 0.02 0.02 0.055 0.15 - 0.02 Inventive Steel B 0.055 0.24 1.5 0.02 0.02 0.054 0.16 - 0.019 Comparative Material C 0.062 0.22 1.6 0.015 0.01 0.060 0.25 0.3 0.15

Steel grade Steel Reheating Temperature (℃) Rolling end temperature (℃) Rolling Rate Per Pass (%) Total Pressure Drop (%) Cooling end temperature (℃) CT (℃) A Invention 1 1113 820 20 62 511 511 A Invention 2 1100 805 20 62 510 510 B Invention 3 1108 801 20 62 519 519 B Comparative Material 1 1111 790 20 50 542 542 C Comparative Material 2 1180 790 17 45 575 575 C Comparative Material 3 1170 810 15 30 567 567 C Comparative Material 4 1183 801 15 30 542 542

Psalter Yield strength (MPa) 90YS-45YS (MPa) Impact Energy (-80 ℃) 90E-45E (J) Average FGS (μm) 45 degree direction (45YS) 90 degree direction (90YS) 45 degree direction (45E) 90 degree direction (90E) Invention 1 570 600 30 120 80 -40 4 Invention 2 568 606 38 115 79 -36 5 Invention 3 565 598 33 118 85 -33 4 Comparative Material 1 510 565 55 50 280 230 7 Comparative Material 2 500 640 140 63 274 211 12 Comparative Material 3 510 630 120 58 261 203 15 Comparative Material 4 523 632 109 49 268 219 16

As shown in Table 3, in the case of the invention materials (1 to 3) manufactured according to the production method of the present invention using the invention steel (A, B) satisfying the component range of the present invention, the average grain size of ferrite The yield strength (FGS) of 5 µm was satisfied and the yield strength difference of 565 MPa or more, the yield strength difference of 90 and 45 degrees in the direction of 40 MPa or less, and the impact energy difference of -40 J or less were obtained. Furthermore, in the case of the invention materials 1 to 3, the impact toughness of the 45 degree direction shows a better value than the 90 degree direction, and it is understood that it is advantageous to the spiral pipe steel.

However, in the case of the comparative materials (1 to 4) manufactured out of the manufacturing method of the present invention using the comparative material C, which does not satisfy the component range of the present invention, the average grain size of the ferrite is 7 µm, which is the target in the present invention. The size was not satisfied. In addition, the comparative materials (1 to 4) can be expected to be carried out at a temperature higher than the reheating temperature of the present invention so that the average grain size of the austenitic structure is larger than the target range of the present invention, from which the final ferrite structure It will affect coarsening.

In addition, in the case of the comparative materials (1 to 4), the yield strength difference in the 90 degree and 45 degree directions was 55 to 140 MPa, and the impact energy difference was more than 200J.

2 shows that a small amount of fine pearlite and M / A are mixed in the ferrite matrix as the optical and SEM structures of the inventive material.

3 is an optical and SEM texture of the invention material A and comparative material C. In general, it is known that the material anisotropy of steel is mainly determined by the texture and grain shape. However, it can be seen from FIG. 2 that Inventive Material A and Comparative Material C of the present invention have similar texture components of similar strength. It can be seen that.

As described above, according to the present invention, the yield strength difference (rolling right angle direction-rolling 45 degree direction) is 40 MPa or less, and Charpy impact energy difference (rolling right angle direction-rolling 45 degree direction) at -80 ° C. A hot rolled steel sheet for line pipe having low temperature toughness and yield strength anisotropy satisfying 50 J or less can be provided.

Claims (4)

% By weight, including C: 0.2% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01-0.08%, Ni: 0.1-0.5%, Ti: 0.01-0.1% It is composed of impurities, and the structure of the steel sheet has matrix structures of ferrite and pearlite and less than 1% of MA (martensite austentie constituent) structure. Hot rolled steel sheet. delete The method of claim 1, wherein the hot-rolled steel sheet has a yield strength difference (rolling right angle direction-rolling 45 degree direction) is 40MPa or less, Charpy impact energy difference (rolling right angle direction-rolling 45 degree direction) at -80 ℃ Hot rolled steel sheet for line pipe with low temperature toughness and yield strength anisotropy satisfying 50J or less. % By weight, including C: 0.2% or less, Si: 1.5% or less, Mn: 2.0% or less, Nb: 0.01-0.08%, Ni: 0.1-0.5%, Ti: 0.01-0.1% Before re-heating the steel slab composed of impurities at 1120 ° C or less, the average grain size of the austenitic structure is controlled to be 50 µm or less, and then the reduction ratio per pass in the temperature range of Ar ₃ to Ar ₃ + 100 ° C is obtained. The method of manufacturing a hot rolled steel sheet for line pipe having low anisotropy and low temperature toughness and yield strength anisotropy, which is thermally end-rolled and water cooled to 540 ° C. or lower so that the total reduction ratio is 60% or more while maintaining the temperature at 30% or less.

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