KR20040034982A - Method for hot rolled steel sheet having excellent coiling quality for line pipe - Google Patents

Abstract

PURPOSE: A manufacturing method of hot strip is provided which is capable of securing commercially required coiling quality as securing low temperature toughness and high strength by controlling manufacturing conditions per unit process of the hot rolling process. CONSTITUTION: In a method for manufacturing hot strip by hot rolling the reheated slab and coiling the hot steel bar after reheating a slab comprising 0.04 to 0.07 wt.% of C, 1.50 to 1.70 wt.% of Mn, 0.15 to 0.25 wt.% of Si, 0.010 wt.% or less of P, 0.003 wt.% or less of S, 0.040 to 0.060 wt.% of Nb, 0.040 to 0.060 wt.% of V, 0.010 to 0.025 wt.% of Ti, 0.10 to 0.30 wt.% of Mo, 0.10 to 0.30 wt.% of Ni and a balance of Fe and other inevitable impurities, the manufacturing method of hot strip for line pipes having superior coiling quality comprises the steps of rough rolling the slab by performing air cooling waiting on the rough rolled slab after performing two times of rough rolling before the final rough rolling of the reheated slab; finish rolling the rough rolled steel bar between stands of finishing mill as injecting cooling water onto the rough rolled steel bar; and injecting cooling water onto the outer side of the coil as coiling the finish rolled hot rolled strip to a temperature of 500 to 590 deg.C.

Description

Method for manufacturing hot rolled steel sheet for line pipe with excellent winding quality {Method for hot rolled steel sheet having excellent coiling quality for line pipe}

The present invention relates to a method for manufacturing hot rolled steel for high toughness-high strength line pipes used as materials for oilfield and natural gas transport pipes. The present invention relates to a method for manufacturing a hot rolled steel sheet for a line pipe that can secure winding quality by elimination.

Steel pipes for petroleum transportation have an important environmental resistance due to the inflow of impurities as the existing oil wells enter into the late stage. In addition, new wells are moving to the polar regions of poor climate, making low-temperature toughness important. In addition, in order to reduce construction costs, a thick pipe line is required in accordance with the promotion of large diameter-high strength.

As the usage environment moved to the extreme cold, the required thickness of the steel became thicker. For this purpose, in the case of thicker than 15mm, the plate was used. However, in the case of using a thick plate to produce a steel pipe made of a coil material increases the cost and decrease in productivity. Therefore, in order to secure competitiveness through productivity and cost reduction, it is required to supply a hot rolled coil even when the thickness is 15 mm or more. For hot rolled coils of 15 mm or more, the requirements for toughness as well as low temperature toughness are becoming more stringent. So-called cryogenic impact toughness and hot rolled steel for high-strength line pipes are required, and low-temperature rolling and low-temperature toughness methods are newly developed as a manufacturing method thereof.

The steel slab applied to low temperature rolling and low temperature toughness manufacturing method is% by weight of C: 0.04 ~ 0.07%, Mn: 1.50 ~ 1.70%, Si: 0.15 ~ 0.25%, P: 0.010% or less, S: 0.003% or less, Nb: 0.040 to 0.060%, V: 0.040 to 0.060%, Ti: 0.010 to 0.025%, Mo: 0.10 to 0.30%, Ni: 0.10 to 0.30%. The slab is reheated at 1150 ° C to 1200 ° C, rolled to 10-25% in the final pass of the rough rolling and rough rolled at 890-930 ° C to obtain a roughened bar of 45-55 mm, and then the finishing temperature is 790-830 ° C. Rolling is carried out under the conditions of and wound up at 560-600 degreeC. The wound hot rolled steel sheet is a fine acicular ferrite structure and has toughness and strength as a hot rolled steel sheet for line pipe. During this manufacturing process, the internal stress due to the volume change inside the tissue due to transformation and recrystallization in parts (a) and (b) of 2 is generated, and the internal stress due to low temperature transformation, as shown in FIG. When the winding as in the above, the reaction force due to the bending cannot be solved, and the winding failure due to the uneven stress in the longitudinal and width directions of the coil occurs.

That is, the conventional manufacturing method can satisfy the strength and toughness, but as shown in FIG. 1, there is a problem in that the winding coil is unwinded or excessive telescope occurs frequently. That is, the technology for producing low-temperature toughness-high strength steel through the microstructure of the cold rolling has shown a lot of progress, but the development of technology that can be easily used commercially by producing a coil rather than a thick plate is in a poor state. In particular, there is no technology that can secure low-temperature toughness and high strength while ensuring the winding quality.

Accordingly, the present invention provides a method for manufacturing a hot rolled steel sheet that can secure the winding quality commercially required while ensuring low temperature toughness and high strength by controlling the manufacturing conditions for each unit process of the hot rolling process, the object thereof.

1 is a coil with poor winding quality

2 is Fe-C state diagram

3 is a continuous cooling transformation curve

4 is a graph showing the change in stress caused by the track spray during winding

5 is a graph comparing the degree of generation of telescope in the present invention and the comparative example

Method for producing a hot-rolled steel sheet for line pipe of the present invention for achieving the above object, by weight% C: 0.04 ~ 0.07%, Mn: 1.50 ~ 1.70%, Si: 0.15 ~ 0.25%, P: 0.010% or less, S : Slab composed of less than 0.003%, Nb: 0.040 ~ 0.060%, V: 0.040 ~ 0.060%, Ti: 0.010 ~ 0.025%, Mo: 0.10 ~ 0.30%, Ni: 0.10 ~ 0.30%, remaining Fe and other unavoidable impurities In the method of reheating, hot rolling and winding,

After the two rough rolling before the last rough rolling of the reheated slab, the step of rough rolling by air-cooling, respectively,

Finishing rolling the rough rolling bar while spraying cooling water between the stands of the finishing mill;

It comprises a step of spraying the cooling water on the outside of the coiling coil while winding the hot-rolled hot rolled plate at a temperature of 500 ~ 590 ℃.

Hereinafter, the present invention will be described in detail.

In the present invention is characterized in the production of hot-rolled steel sheet for line pipe having a toughness required in the cryogenic environment of the yield strength of 530Mpa or more and -60 ℃. In the present invention, in order to solve the stress generated by the internal heat generated by the processing during the rough rolling process and the volume change of the tissue due to the phase change, air cooling is performed for a predetermined time after the rough rolling from the second rolling just before the last rough rolling. In the rolling, the cooling water is sprayed between the rolling stands, and in the winding process, the cooling water is sprayed on the outer side of the winding coil in order to eliminate reaction forces due to bending.

Steel slab, which is the object of the present invention, is a line pipe material, and representative examples thereof are C: 0.04 to 0.07%, Mn: 1.50 to 1.70%, Si: 0.15 to 0.25%, P: 0.010% or less, and S: 0.003%. Hereinafter, Nb: 0.040 to 0.060%, V: 0.040 to 0.060%, Ti: 0.010 to 0.025%, Mo: 0.10 to 0.30%, Ni: 0.10 to 0.30%, remaining Fe and other unavoidable impurities.

The steel slab of the Nb-V-Ti-Mo-Ni component system described above is subjected to rough rolling by heating for a long time at 1150 to 1200 ° C. for 200 to 300 minutes for sufficient solid solution of the alloying element (Nb) in a heating furnace.

In the present invention, in order to solve the stress caused by the internal heat generated by processing in the rough rolling process and the volume change of the tissue due to the phase change, the two rollings just before the last rough rolling (n times rough rolling) (n-2, Air cooling is performed after n-1 rough rolling).

The rough rolling process is reversible rolling. In general, a 200-250 mm slab is carried out six or ten times to form a 40-60 mm rough-rolled bar for finishing rolling. In each rough rolling pass, processing heat is generated by rolling. In addition, in the rough rolling, high-pressure descaling is sprayed on the surface layer to remove the scale layer by high temperature oxidation. In this process, the temperature distribution form is different from the surface layer. Only the surface layer first shows non-uniform thermal equilibrium which leads to precipitation region temperature by cooling.

The present invention focuses on the physical metallurgy inherent characteristics of the structure change and precipitation behavior change according to the thermal history to remove the non-uniformity of heat generation and stress in the rough rolling process. Specifically, the slab extracted from the furnace performs descaling every pass, so that the grains become finer as the steel sheet is cooled, but the temperature difference between the surface layer and the inside occurs due to internal heat generated by the processing. do. It can be seen that the temperature rise due to internal heat generation occurs when the atmosphere is performed after rolling in rough rolling. Such a temperature difference between the inside and the surface layer causes stress unevenness during the next rolling, and the internal heat source acts as a heat source during recrystallization even after the next pass rolling, causing tissue coarsening.

When this is solved, the structure can be miniaturized and the stress can be resolved. As the method, the present invention is air-cooled after roughing n-2 and n-1 times. When air-cooled after rough rolling, sufficient heat will generate and reaction force will be reduced at the time of subsequent rolling, and internal stress can be solved. After n-2 times of rough rolling, the air cooling standby time is preferably 20 to 30 seconds, and the time of n-1 times of rough cooling is preferably 10 to 20 seconds. The waiting time after n-2 pass rolling is time to generate | occur | produce heat sufficiently, and when the waiting time after n-1 pass passes, the pass time of rough rolling process will be excessive, and the fall of productivity takes into consideration.

According to the present invention, if the air is cooled after rolling n-2 and n-1 times, rolling is carried out at about 900 to 930 ° C., which is the end temperature of niobium (Nb), and pinning occurs. Mapping rolling can be carried out in a micronized state by the effect of preventing the coarsening), and the micronized structure can be secured even after recrystallization. This rough rolling condition is advantageous to secure the rough rolling finish temperature of 870 ~ 910 ℃ after the last pass. If the rough rolling temperature is too high, it is difficult to secure the strength by coarsening the austenite grain size, too low is unfavorable because it imposes pressure on the facility during finishing rolling.

Rough rolling of the rough rolling bar according to the present invention.

In general, filament rolling causes the cumulative rolling rate to be 50% or more to induce the structure of the hot-rolled steel sheet into the basintic tissue (Bainitic Ferrite). In case of the same filament rolling temperature and winding temperature, if the cumulative rolling reduction rate of filament rolling is large, the density, dislocation density and deformation band of grain boundary increase, so that the amount of strain induced precipitation during deformation period increases. This is because the grains become finer. The finishing rolling finish temperature is usually performed at 750 to 830 ° C., which is near the transformation point of ferrite in austenite for finer grain size.

In the present invention, the cooling water is injected between the stands of the finishing mill under the finishing rolling conditions as described above. Conventionally, cooling water is sprayed only on a part of a finishing mill stand. According to the present invention, spraying the cooling water between all the stands of the finishing mill eliminates the processing heat generated in every rolling and allows the rolling to be completed just before the transformation point of the ferrite, thereby maximizing the effect of improving the strength and toughness by the microstructure. have. In addition, as the finishing rolling is performed in a stable phase, it is possible to reduce the load fluctuation in the finishing mill. This prevents the camber and ensures a stable shape, which is more advantageous for improving the winding quality, and eliminates thermal stress unevenness due to low temperature rolling while at the same time securing a finer structure. It is advantageous for securing strength. Cooling water is injected at different pressures depending on the rolling mill, and the largest pressure cooling water is installed in the first rolling mill having the largest reduction ratio.

Roll as described above and wind up.

Usually, it is wound up at low temperature of 500-590 degreeC. As shown in FIG. 3, the coiling temperature is a range in which a line showing the cooling method after finishing rolling is intended to go. In this winding temperature range, fine needle structure (Bainitic Ferrite) is formed, so it is possible to secure strength and low temperature impact toughness. If the coil is wound at a high temperature, the ferrite particles become coarse, and if it is too low, the ferrite particles are transformed into high-strength tissue, resulting in winding failure and loss of function to the product.

In performing cooling control to secure this winding temperature range, the winding temperature does not maintain a constant temperature but has a certain level of variation. Temperature deviations in the width direction and the length direction cause uneven distribution of the fractions of the basintic ferrite, ferrite and bainite, so that even if the strength and toughness are secured, it causes the winding failure. Should be removed.

Therefore, in the present invention, the cooling water is injected to the outside of the winding coil from the time point (about 3 to 4 wheels) of the coil (Coil) coming from the finishing mill is wound on the winder. Spraying of the cooling water is preferably further sprayed for 60 to 90 seconds to the outside of the winding coil after the end of the winding. Excessive injection time affects the overall working time, which may result in a decrease in productivity. In case of insufficient injection time, the stress of the outer space with a small radius of curvature may not be completely solved, causing the phenomenon to be released. According to the present invention, the spraying of the cooling water on the outer side of the coiling coil can be performed through a conventional track spray equipment. Track Spray is a facility that cools the outer wound part by spraying water on the outer wound part (conventionally 10-30 seconds) after the end of winding.

As shown in (a) of FIG. 4, the coil has a compressive and tensile stress to return to a flat state. When spraying the cooling water using the track spray on the outer side of the winding coil, the stress of the plate is solved by the thermal stress as shown in FIG. 4 (b) and maintains the equilibrium state of the stress as shown in FIG. 4 (c). In the case of using the track spray, even when the winding temperature is low and the winding failure is expected to be used, the winding failure can be prevented.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The slabs of Table 1 were heated at 1250 to 1300 ° C. for 200 to 300 minutes, and rolled and wound under the conditions of Table 2 to prepare hot rolled steel sheets. Inventive examples sprayed cooling water between the finishing mill stands. The mechanical properties, impact toughness and telescope of this hot rolled steel sheet were measured and shown in Table 3. The measurement results of the telescope are also shown in the form of a graph in FIG. 5.

Steel grade C Mn Si P S Nb V Ti Mo Ni A 0.045 1.58 0.24 0.010 0.003 0.056 0.057 0.020 0.13 0.14 B 0.051 1.55 0.21 0.008 0.001 0.052 0.053 0.023 0.17 0.18 C 0.053 1.63 0.21 0.010 0.001 0.049 0.050 0.016 0.25 0.20 D 0.059 1.48 0.17 0.009 0.003 0.059 0.048 0.017 0.29 0.28

division Steel grade 4-pass inlet temperature (℃) after rough rolling Air cooling time (seconds) after 3 pass after rough rolling 4-pass after cold rolling (seconds) Roughing Finishing Temperature (℃) Filament Rolling Coolant (pcs) Finishing temperature (℃) Winding temperature (℃) How to use track spray Inventive Example 1 A 913 25 10 909 6 785 585 Winding up Inventive Example 2 B 911 27 12 871 6 784 571 Winding up Inventive Example 3 C 917 30 15 895 6 800 545 Winding up Inventive Example 4 D 915 26 15 890 6 802 556 Winding up Comparative Example 1 A 957 - - 910 3 791 580 After winding Comparative Example 2 C 960 - - 900 3 789 573 After winding Comparative Example 3 C 961 - - 891 2 810 563 After winding Comparative Example 4 D 965 - - 895 2 811 559 After winding

division Steel grade Mechanical property (30 ℃ direction) Impact test (30 ° C direction) Telescope (mm) Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Yield Ratio (%) -60 ℃ (CVN, Joule) Inventive Example 1 A 542 668 35 81 310 21 Inventive Example 2 B 549 685 36 80 309 15 Inventive Example 3 C 552 674 35 82 315 17 Inventive Example 4 D 543 679 37 80 325 10 Comparative Example 1 A 539 666 35 81 305 13 Comparative Example 2 B 550 685 36 80 285 22 Comparative Example 3 C 555 694 35 80 293 25 Comparative Example 4 D 540 685 37 79 308 35

As shown in the table, the invention material was superior in strength and toughness and the winding shape was also higher than that of the comparative material. As can be seen from FIG. 5, the measurement result of the telescope, which is a measure of winding quality, was excellent in the winding shape of the invention material.

As described above, according to the present invention, there is a useful effect of eliminating the winding failure while maintaining the toughness and strength above the equivalent level.

Claims (3)

By weight% C: 0.04 ~ 0.07%, Mn: 1.50 ~ 1.70%, Si: 0.15 ~ 0.25%, P: 0.010% or less, S: 0.003% or less, Nb: 0.040 ~ 0.060%, V: 0.040 ~ 0.060%, In the method of reheating, hot rolling and winding a slab composed of Ti: 0.010 to 0.025%, Mo: 0.10 to 0.30%, Ni: 0.10 to 0.30%, and the remaining Fe and other unavoidable impurities, After the two rough rolling before the last rough rolling of the reheated slab, the step of rough rolling by air-cooling, respectively, Finishing rolling the rough rolling bar while spraying cooling water between the stands of the finishing mill; A method of manufacturing a hot rolled steel sheet for excellent line pipe, comprising the step of spraying cooling water on the outside of the winding coil while winding the hot rolled sheet at a temperature of 500 to 590 ° C. The method of claim 1, wherein the rough rolling is 20-30 seconds air-cooled after n-2 rough rolling in n rough rolling, 10-20 seconds air-cooling after n-1 rough rolling, and rough finishing Method for producing a hot rolled steel sheet for line pipe having excellent winding quality, characterized in that the temperature is 870 ~ 910 ℃. The method according to claim 1, wherein the winding further comprises spraying cooling water for 60 to 90 seconds to the outer winding portion of the winding coil after finishing the winding.