RU2391415C1 - Method of low-alloy steel strip production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to metal pressure processing, particularly to rolling with reversing plate mill. For large diametre tubes continuously cast stock of 240-315 mm thickness is obtained with stable mechanical properties and heated to 1180-1230°C, with further reverse rolling in plate mill frame for 17-23 sequential passes with 93-98% total relative cobbing in height through all passes. Single relative cobbing of stock height in first four and last three passes do not exceed 13%. Rolling is finished at strip temperature of 750-810°C, and hydraulic descaling is activated in the frame at least three times during rolling. Rolled sheets are cooled in air after stapling of obtained strips in a pile of at least five sheets. Continuously cast blank is received from steel, containing wt %: carbon 0.08-0.11, silicon 0.16-0.3, manganese 1.30-1.45, titanium 0.005-0.02, vanadium 0.04-0.06, niobium 0.03-0.05, chrome <0.10, nickel <0.20, copper <0.20, aluminium 0.02-0.05, the rest is iron and admixtures.

EFFECT: enhanced performance of strip rolling process.

2 cl, 3 tbl

Description

The invention relates to the field of metal forming, in particular to technology and equipment for sheet rolling on a reversible plate mill.

A known method for the production of a strip for low-carbon steel main pipes, including heating a continuously cast billet, roughing this billet in thickness, conditioning the obtained intermediate billet in air, finishing by controlled rolling on a reversible plate mill with a degree of relative deformation of 3-12% in each pass until the specified thickness of the finished strip is obtained, as well as the regulated accelerated cooling of the obtained strip by supplying water to the front surfaces springs followed by natural cooling [1].

The disadvantages of this method are that the quality of the finished product is largely determined by the conditions for accelerated cooling of the strip after rolling. However, not all mills are equipped with an accelerated cooling installation, and, accordingly, in some cases, the method cannot be implemented without a significant reconstruction of the equipment. This significantly narrows the possibilities of the production process implementation. In addition, accelerated cooling in itself is an extremely unstable element of the technology, since it depends on parameters such as the temperature of the cooling water, the speed of the strip through the cooling unit, the thickness and width of the strip, the height and pressure of the stream of cooling water. Accordingly, the optimal parameters of the installation can only be determined empirically, i.e. trial and error, which involves a large metal consumption for debugging technology. Thus, it seems appropriate to develop a technology for the production of strips for main pipes from low carbon steel, which avoids the use of accelerated cooling of the strip.

The closest in its technical essence and the achieved results to the proposed invention is a method for the production of strips of low alloy steel [2]. In accordance with this method, to obtain a tube strip of strength category K52 (X56) -K60 (X65) from low-alloy steel, it is provided to produce continuously cast billets with a thickness of 240-315 mm, their heating and multi-pass reversible controlled rolling in a stand of a plate mill to produce a strip and cool it on air. The use of a continuously cast billet with a thickness of 240-315 mm allows us to ensure, when rolling in a given temperature range, the degree of metal deformation sufficient to form a fine-grained ferrite-bainitic structure in the finished product with the required level of mechanical properties.

In this case, the continuously cast billets are heated to a temperature of 1150-1200 ° C, rolling is carried out in two stages (in the roughing and finishing stands) with intermediate undercoating to a temperature of 920-980 ° C and with a compression of at least 8% during the passage during rough rolling. Finishing rolling is carried out with a total compression of at least 70% in thickness and is completed at a temperature not exceeding 820 ° C. The method is implemented using steel, which is characterized by the following chemical composition, wt.%: Carbon 0,003-0,14; manganese 0.50-1.65; silicon 0.15-0.7; niobium 0.015-0.06; titanium 0.005-0.03; aluminum 0.02-0.05; vanadium 0.02-0.14%; molybdenum 0.15; chrome 0.3; nickel 0.3; copper <0.3; the rest is iron and impurities. Achieving the required level of the mechanical properties of the strip is achieved due to the low level of carbon content, as well as microalloying with niobium under the conditions of thermomechanical (controlled) rolling. In addition, to obtain the desired metal structure, it is used to reinforce the obtained intermediate billet (tackle) after rough rolling, carried out during a special inter-deformation pause between rough and finish rolling. In this case, the tackle is kept on the rolling table of the mill until it reaches a predetermined temperature of 920-980 ° C (undercooling in air) to avoid deformation in the unfavorable temperature range. At this time, other billets are rolled on the mill.

However, in practice, the considered technology does not always provide high strength and plastic properties of the finished strip, corresponding to modern requirements for the material of large diameter pipes for main pipelines. This is largely due to the difficulty of maintaining the stability of the temperature-speed regime of two-stage rolling. In addition, the pause used for reinforcing can reach 10-20 minutes, which complicates the work of the mill operator (it is necessary to control several blanks on the roller table at once) and leads to a significant decrease in equipment productivity (many additional operations for the reverse transportation of rolling stock along the roller table).

Obviously, the need to master the production of new types of high-strength strip for main pipes from low-carbon steels determines the feasibility of developing technical solutions that provide the required level of mechanical properties of the finished product while increasing the productivity of the rolling process. This determines the relevance of developing a more technologically advanced, but quite effective in quality and productivity, method of producing a strip of strength class K52-K60 for main pipes on a reversible plate mill.

The technical problem solved by the invention consists in increasing the productivity of the strip rolling process for large diameter pipes on a plate reversing mill by reducing the technological cycle of processing a continuously cast billet (rolling time) while ensuring a stable level of mechanical properties corresponding to strength classes K52-K60 given in table 1 [3].

Table 1 Normative mechanical characteristics of main pipes [3] Strength class Temporary tensile strength, MPa (kgf / mm ² ), not less Yield strength, MPa (kgf / mm ² ), not less Relative elongation,%, not less Impact strength KCU _-40 J / cm ² (kgf / cm ² ), not less K52 510 (52) 355 (36) twenty 49.0 (5.0) K54 530 (54) 380 (39) twenty 49.0 (5.0) K55 540 (55) 390 (40) twenty 49.0 (5.0) K56 550 (56) 410 (42) twenty 49.0 (5.0) K60 590 (60) 460 (47) twenty 49.0 (5.0)

The stated technical problem is solved in that in the known method for the production of strips for main pipes from low carbon steel of strength category K52 (X56) -K60 (X65), which includes the production of continuously cast billets with a thickness of 240-315 mm, their heating and multi-pass reversible controlled rolling in a plate mill the mill with the receipt of the strip and cooling it in air, according to the proposed technical solution, the billets are heated to a temperature of 1180-1230 ° C, reverse rolling in the stand of the plate mill for 17-23 consecutive passes with a total relative compression in height in all passes of 93-98%, while in each of the first four and last three passes the single relative compression of the workpiece in height does not exceed 13%, and complete rolling at a strip temperature of 750 -810 ° C, and during rolling, the scale is scaled down at the mill at least three times, and the strip is cooled in air after stacking the obtained strips in a stack consisting of at least five strips.

In addition, continuously cast billets are obtained from steel, with the following ratio of components in it: 0.08-0.11% carbon, 0.16-03% silicon; 1.30-1.45% manganese, 0.005-0.02% titanium, 0.04-0.06% vanadium, 0.03-0.05% niobium, chromium <0.10%, nickel <0.20 %, copper <0.20%, 0.02-0.05% aluminum, iron and impurities - the rest.

A method of manufacturing a strip for main pipes of low alloy steel is implemented as follows. When continuously cast billets with a thickness of 240-315 mm are heated to a temperature of 1180-1230 ° C, austenization of low alloy steel of the used chemical composition takes place, and dispersed carbonitride reinforcing particles dissolve. Reversible rolling in a stand of a plate mill for 17-23 consecutive passes with reduction of reductions in each of the first and last passes, with a total relative reduction in height in all passes of 93-98%, with completion of rolling at a strip temperature of 750-810 ° С, to study the microstructure of the metal to its entire thickness, eliminate the axial segregation present in the continuously cast billet, and form a uniform fine-grained pearlite-ferrite microstructure with enhanced viscosity and strength properties and. Natural cooling of rolled strips in the air after stacking them in a stack of at least five pieces ensures the removal of internal stresses in the metal and the absence of warpage.

The application of the proposed rolling method provides the desired technical effect - increasing the productivity of strip rolling for large diameter pipes on a plate reversing mill by reducing the rolling time while ensuring a stable level of mechanical properties corresponding to strength classes K52-K60. This is due to the transition from two-stage rolling with reinforcement to single-stage rolling, which takes significantly less machine time. In addition, it is possible to avoid accelerated cooling of the strip after rolling.

It was experimentally established that an increase in the heating temperature of a continuously cast billet of more than 1230 ° C leads to an excessive growth of austenite grains, and also does not allow to provide the required temperature for the end of rolling. This affects the uniformity of the microstructure and strip properties. At the same time, a decrease in the heating temperature below 1180 ° C prevents the complete dissolution of the strengthening dispersed carbonitride particles, which impairs the homogeneity of the microstructure and the mechanical properties of steel.

With a total relative reduction of less than 93% in height under conditions of one-stage deformation, it is not possible to ensure a deformation in the axial zone sufficient to eliminate the axial segregation strip characteristic of continuously cast billets. This leads to the appearance of defects in the macrostructure and lower quality of the finished product. At the same time, an increase in the reduction of the workpiece over 98% is accompanied by a significant increase in the rolling force, which can reach critical values and lead to the emergence of a threat to the mill failure. Thus, based on considerations of the technical feasibility of the process and the need to eliminate defects in the microstructure on the finished strip, the total relative compression should be 93-98%.

It has been established that if less than 17 passes are used to compress a workpiece with a thickness of 240-315 mm, then the values of the compressions in individual passes can exceed a value determined from the conditions of the maximum allowable rolling forces. This may lead to an emergency. At the same time, when using more than 23 passes, there is an unjustified increase in the rolling cycle and, accordingly, a decrease in productivity. Thus, to compress the workpiece on a reversing plate mill, it is necessary to use 17-23 passes for reasons of technical feasibility of the process and ensuring sufficient performance.

In each of the first four passes, the unit relative compressions of the workpiece should not exceed 13% in height, in contrast to higher compressions during the main deformation. This is due to the need to ensure the most favorable conditions for the capture of the initial continuously cast billet with a thickness of 240-315 mm by the mill rolls at the initial stage of rolling. In each of the last three passes, the relative reductions should also not exceed 13%, since otherwise the rolling forces sharply increase, associated with high values of deformation resistance at low temperatures (810-840 ° C). Under certain conditions, this can lead to the failure of the mill. In other words, for the first and last passes, single reductions should not exceed 13% in order to avoid an emergency on the mill and to provide the possibility of technical implementation of the proposed method.

In the case of completion of rolling at a strip temperature above 810 ° C, it is not possible to achieve a degree of elaboration of the cast structure of the billet sufficient to obtain optimal grain sizes of the strip microstructure that provide the required set of mechanical properties on the finished product. The completion of rolling at temperatures below 750 ° C can lead to a decrease in the number of ferrite nuclei and a violation of the uniformity of the finely divided carbide phase, i.e. a reduction in the level of mechanical properties is possible.

In the course of rolling, hydroscale is included in the mill at least three times for each billet. This not only allows us to ensure good surface quality of the finished strip, but also helps to reduce the temperature of the end of rolling to the required level (less than 810 ° C) due to more intensive cooling during water breakdown. In other words, if in the production cycle of rolling, the hydraulic sludge scale will be turned on less than three times, then the workpiece will not have time to cool down to the desired temperature during deformation, which will lead to disruption of metal structure formation processes and will impede the achievement of the goal of the technical solution. Thus, the number of inclusions of descaling is actually a condition for adjusting the temperature of the end of rolling, ensuring the operability of the proposed technical solution.

Slow cooling of the strips after stacking helps to relieve internal thermal stresses. When stacking rolled strips into a stack of less than five pieces, due to the small total thickness of the stack, a sufficiently low rate of their natural cooling in air cannot be ensured. This leads to the precipitation of a finely dispersed carbide phase along the grain boundaries and, accordingly, to the predominance of hardening processes in the metal, which are accompanied by a decrease in the plastic characteristics of the strip below values acceptable for the strength category K52-K60, i.e. deterioration in the quality of finished products.

Carbon in low alloy steel of the proposed composition determines its strength. A decrease in carbon content of less than 0.08% leads to a drop in its strength properties below an acceptable level. An increase in carbon content of more than 0.11% affects the plastic properties of the metal and leads to their uneven distribution over the cross section.

When the silicon content is less than 0.16%, deoxidation of steel deteriorates, strength properties decrease. An increase in the silicon content of more than 0.3% leads to an increase in the number of silicate inclusions and is accompanied by a decrease in the impact strength of the strip.

The addition of manganese within the claimed limits provides solid solution hardening of the metal. A decrease in manganese content of less than 1.3% increases the oxidation of steel, accompanied by a decrease in strength properties. An increase in the manganese content above 1.45% can lead to an increase in the ratio of yield strength to temporary tensile strength above the allowable limit.

Nickel, copper and chromium also contribute to solid-solution hardening of the metal, as well as improving the cold resistance and corrosion resistance of strips. Being impurity elements in this case, at concentrations above 0.2%, 0.2% and 0.1%, respectively, they have a detrimental effect on the weldability of steel in the production of pipes. At the same time, remaining within the proposed boundaries, they expand the possibilities of using scrap metal for smelting, which is accompanied by a reduction in the cost of finished products.

Aluminum deoxidizes and modifies steel. It provides grain grinding due to the formation of finely dispersed carbides, which impede the growth of austenite grain during heating, which increases the yield strength and cold resistance of strip steel. At a concentration of less than 0.02%, its effect is weak and usually impairs the mechanical properties of the strips. An increase in its content of more than 0.05% leads to graphitization of carbon, which also negatively affects the quality of the finished product.

Vanadium grinds the grain of the microstructure, increases the strength and viscosity of the strips rolled according to the proposed modes. When the vanadium content is less than 0.04%, the strips have insufficient viscosity at subzero temperatures. An increase in the content of vanadium in excess of 0.06% is economically disadvantageous, since it is not accompanied by an improvement in mechanical properties, but it leads to a higher cost of hire.

Niobium in steel at a rolling temperature above 810 ° C and a total reduction of more than 93% contributes to the production of a cellular dislocation microstructure of steel, which provides a combination of strength and plastic properties of strips without additional accelerated cooling. At a niobium concentration of less than 0.03%, the mechanical properties of the strips in the hot-rolled state are not high enough. An increase in the concentration of niobium of more than 0.05% does not lead to a further increase in the level of the mechanical properties of the metal, however, it leads to an increase in the consumption of expensive ligatures and therefore seems inappropriate.

Titanium is present in the steel in question in very small quantities and actually acts as an impurity with a normalized content. It should also be noted that the steel of the proposed composition may include in the form of impurities not more than 0.018% phosphorus, not more than 0.007% sulfur and not more than 0.010% nitrogen. At the indicated maximum concentrations, these elements in the steel of the proposed composition do not have a noticeable negative effect on the quality of the strips, while their complete removal from the steel melt at the current level of development of steelmaking technology is practically impossible. In general, the proposed steel is more economically alloyed in comparison with the prototype, which ensures its lower cost.

The application of the method is illustrated by an example of its implementation. In an electric arc furnace with a capacity of 100 tons, low-alloy steel of strength category K56 was smelted of various compositions (table 2). The smelted steel of each of the six chemical compositions was cast on a continuous casting machine in billets with a cross section of 315 × 1750 mm. The finished continuously cast billets were heated in a methodical furnace to a temperature T _n and rolled on a plate reversing mill 5000 for 19 consecutive passes to a thickness of 15.75 mm, with a total relative height reduction in all passes of 95%. Moreover, in the first four and last three passes, the unit relative reduction of the workpiece in height was 5-12%. The rolling was completed at a strip temperature T _kp . To obtain a given temperature at the end of rolling, T _kp , during the deformation, hydroscale of the scale was switched on 4 times in the mill. The laminated strip was cooled in vivo in air after stacking the obtained stripes in a stack consisting of N sheets.

The options for rolling strips in various modes (for parameters that affect the level of mechanical properties) from steels of various compositions, as well as the obtained values of mechanical properties are shown in table 3. It should be noted that table 3 does not consider technological parameters that determine the technical feasibility of the proposed way.

As follows from tables 2 and 3, when implementing the proposed method (options 2-4), the required quality of strips corresponding to strength category K56 is achieved. In the event that the variable technological parameters go beyond the boundaries established for this method (options 1 and 5) and when implementing the parameters of the prototype method (option 6), the strips do not always meet the requirements of the specified strength category by their mechanical characteristics. Varying the parameters of the chemical composition and technology within the boundaries established for this technical solution allows us to obtain mechanical properties corresponding to strength classes from K52 to K60.

The technical and economic advantages of the proposed method are that hot rolling of steel strips of a given composition according to the established conditions provides a significant increase in the production process productivity due to the absence of a pause in the rolling to reinforce the rolled stock and the formation of the required complex of mechanical properties corresponding to strength category K52-K60.

As a basic object in determining the technical and economic advantages of the proposed method adopted the prototype method [2]. Using the proposed method for the production of strips of strength category K52-K60 from low alloy steel will increase the profitability of their production by 5-10%.

Thus, the obtained data confirm the correctness of the recommendations for choosing the value of the technological parameters of the proposed method for the production of strips from low alloy steel. Using this method allows to increase the productivity of the process of rolling the strip for large diameter pipes on a plate reversing mill by reducing the technological cycle of processing the workpiece (rolling time) while ensuring a stable level of mechanical properties corresponding to the strength category K52-K60.

Information sources

1. Technology of rolling production. Reference, vol. 2 / Ed. V.I. Zyuzina, A.V. Tretyakova. - M.: Metallurgy, 1991, p. 544-561.

2. RF patent No. 2201972, IPC C21D 8/02, C22C 38/58, B21B 1/26, 04/10/2003.

3. Nesterov G.V. and other pipes for the construction of the BTS-2 oil pipeline. Technical requirements. Territory NEFTEGAZ, 2007, No. 10, pp. 62-64, Tables 3, 5.

table 2 The chemical composition of low alloy steels Composition number The content of chemical elements, wt.% FROM Si Mn Ti V Nb Cr Ni Si Al Mo one 0.06 0.1 1,1 0.002 0,03 0.02 - - - 0.01 - 2 0.08 0.16 1.3 0.005 0.04 0,03 - - - 0.02 - 3 0.09 0.22 1.35 0.01 0.05 0.04 0.005 0.015 0.015 0,03 - four 0.11 0.3 1.45 0.02 0.06 0.05 0.01 0.02 0.02 0.05 - 5 0.13 0.35 1,5 0,025 0,07 0,060 0.012 0,022 0,022 0,055 - 6 prototype 0,07 0.43 1.13 0.018 0.08 0,038 0.2 0.2 0.2 0.04 0.1 Note: The composition of steels 1-6 in the form of impurities includes: 0.015% P; 0.004% S; 0.008% Nz

Table 3 Modes of hot rolling of strips of strength category K56 and indicators of their quality No. p / p Steel composition number T _n , ° C T _CP , ° C N sheets in a stack σ _in , MPa σ _t , MPa δ,% KCU _-40 J / cm ² The proportion of viscous component in IPG,% one one 1160 750 2 540 400 twenty 45 85 2 2 1180 760 5 580 420 22 fifty 90 3 3 1200 790 6 580 440 25 80 90 four four 1230 810 9 550 435 24 75 90 5 5 1250 830 12 530 405 eighteen 65 90 6 6 1175 800 - 570 399 38 70 82 prototype

Claims (2)

1. A method of manufacturing a strip of low-alloy steel, including the production of continuously cast billets with a thickness of 240-315 mm, their heating and multi-pass reversible controlled rolling in the stand of a plate mill to produce a strip and its cooling in air, characterized in that the billets are heated to a temperature of 1180- 1230 ° C, reverse rolling in the stands of the plate mill is carried out for 17-23 consecutive passes with a total relative compression in height in all passes of 93-98%, while in each of the first four and three in the last passes, the unit relative reduction of the workpiece in height does not exceed 13%, and they complete rolling at a strip temperature of 750-810 ° С, and during rolling, the scale is hydrated in the mill at least three times, and the strip is naturally cooled in air after stacking sheets in the foot, consisting of not less than five sheets. 2. The method according to claim 1, characterized in that the continuously cast billets are obtained from steel in the following ratio of components in it, wt.%:
carbon 0.08-0.11 silicon 0.16-0.3 manganese 1.30-1.45 titanium 0.005-0.02 vanadium 0.04-0.06 niobium 0.03-0.05 chromium <0.10 nickel <0.20 copper <0.20 aluminum 0.02-0.05 iron and impurities rest