RU2238811C2 - Method for manufacture of seamless hot rolled tubes - Google Patents

Abstract

FIELD: tube rolling industry, in particular, production of seamless hot rolled tubes of average and large diameter on equipment with pilger mills.

SUBSTANCE: method involves heating bars-ingots; piercing bars on helical rolling mill; rolling bars-ingots on pilger mill with fixed wall thickness, with rolling process being provided on pilger mill in passes having sizes reducing from initial stage to final stage depending on rolling temperature, tube diameter, and steel linear expansion coefficient; determining difference between maximal and minimal pass values from expression: Δ=D_p-D_t(1+αt), where D_p is diameter of pass by rolling table, mm; D_t is diameter of tube in cold state, mm; t is rolling temperature current value, ⁰C; α is steel linear expansion coefficient, 10^-6 * 1/⁰C.

EFFECT: reduced difference in longitudinal tube wall thickness and enhanced reliability in operation of pilger mill during rolling of tubes from alloyed difficult-to-deform grades of steel and alloys.

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Description

The invention relates to pipe rolling production, and in particular to a method for the production of seamless hot-rolled pipes of medium and large diameters, and can be used in the production of them on pipe-rolling plants with pilgrim mills.

In pipe-rolling production, there is a known method for the production of seamless hot-rolled pipes of medium and large diameters in pipe-rolling plants with pilgrim mills, where the main deformation (work) is carried out on pilgrim mills (F.A. Danilov, A.Z. Gleiberg, V.B.Balakin, Goryachaya pipe rolling. - M., 1962, p. 280). The main disadvantage of this method is the longitudinal difference of the pipes with an average fixed wall thickness, increasing from the beginning of the steady-state process to the end due to an increase in metal pressure due to a decrease in the temperature of the sleeves. On pipe rolling plants with automatic mills, the average drawing coefficient for flashing is 4.0–4.5, and on an automatic mill it usually does not exceed 2.0. On pipe rolling plants with pilgrim mills, the drawing coefficient on piercing mills is 1.9-2.1, and on pilgrim mills it reaches 15-16. Therefore, the sleeve after the piercing mill is thick-walled and of relatively short length (2.5-3.5 m). The pilgrim rolling cycle consists of seed, steady-state process and rolling of the pilgrim head. When rolling thin-walled pipes (lash length 35-42 m), the steady-state process is from 88.0 to 90% of the total rolling time, and when rolling thick-walled pipes from 80.0 to 84.0% (pipe length from 4.0 to 7.0 m). The value of the total metal pressure on the roll during the rolling of one pipe (whip) varies significantly and depends on the temperature of the rolling, the diameter of the pipes and the coefficient of linear expansion of steel, since the machine time for rolling one pipe (whip) with a length of 36 m is 4, 2 to 5.5 minutes depending on the diameter, wall thickness and steel grade. With increasing pressure of the metal on the rolls from the beginning of rolling to the end, the system (pilgrim stand - bearings - rolls - compression screws) experiences increasing loads, i.e. deformation of the cage and rolls occurs, clearances in the bearings and pressure screws are selected, and therefore, the size of the gauge increases from the beginning of the steady rolling process to the end and, as a result, the pipe wall thickness increases by an amount equal to half the increase in caliber size.

The closest technical solution is a method for the production of seamless hot-rolled pipes of medium and large diameters on tube-rolling plants with pilgrim mills, including heating billets (ingots), piercing them in a cross-screw rolling mill into thick-walled sleeves and rolling on a pilgrim mill in pipes with a fixed wall size (F.A. Danilov, A.Z. Gleyberg, V.G. Balakin, Hot rolling of pipes. - M., 1962, p. 292-305).

However, the known method has the following disadvantages. This method does not provide a qualitative and quantitative assessment of the longitudinal difference of the pipes depending on the rolling temperature, the diameter of the pipes and the coefficient of linear expansion of the rolled metal. Reducing the longitudinal difference by reducing the feed rate, without taking into account the plastic (strength) characteristics of the metal and the geometric dimensions of the rolled profiles (pipes), will increase the machine time for rolling pipes (lashes), which in turn will lead to a decrease in the temperature of the end of rolling and, as consequence, to exceed critical loads on the mill line (breakage of safety bolts, rolls and spindles) when rolling pipes of large diameters with relatively thin walls and pipes made of alloyed hard to deform world-class grades of steel and alloys with thicker walls.

The aim of the proposed method is to reduce the longitudinal difference in the pipes, eliminating breakage of safety bolts, rolls and spindles of the pilgrim mill when rolling pipes from alloyed hard to deform steel grades and alloys and thin-walled pipes of large diameters from carbon steel grades, due to improperly selected sizes of calibers and feed rates .

This goal is achieved by the fact that in the known method for the production of seamless hot-rolled pipes, including heating the billets (ingots), piercing them in a cross-helical rolling mill and rolling on a pilgrim mill with a fixed wall thickness, rolling the pipes on a pilgrim mill is carried out in calibers, the dimensions of which decrease from beginning to end depending on rolling temperature, pipe diameters and coefficient of linear expansion, and the difference between the maximum and minimum values of calibers is determined from the expression

Δ = D _to -D _t (1 + αt),

where D _to - the diameter of the caliber according to the rolling table, mm;

D _t - the diameter of the pipes in the cold state, mm;

t is the current value of the rolling temperature, ° C;

α is the coefficient of linear expansion of steel, 10 ^-6 * 1 / ° С.

Using this expression to determine the size of the gauge during the rolling process, in which the geometric dimensions of the rolled profile (pipe) are taken into account by the values of D _t , the metal properties by the linear expansion coefficient α, and the current value of the rolling temperature is t, we can determine the maximum caliber at the beginning of rolling d _{to max} and minimum at the end of D _{to min} , and then carry out the rolling process in manual mode with values smoothly decreasing from d _{to max} to D _{to min} or in automatic mode if there is a device fixing the dimensions of the caliber in given program. Using this method will make it possible to significantly reduce or completely eliminate the longitudinal spacing of the pipes, reduce the likelihood of breakage of safety bolts, rolls and spindles, increase the performance of the pilgrim mill by conducting the rolling process at optimal feed rates from the beginning to the end of rolling. A comparative analysis of the proposed solution with the prototype shows that the claimed method differs from the known one in that the pipes are rolled in a pilgrim mill in calibers, the dimensions of which are reduced from beginning to end depending on the rolling temperature, pipe diameters, linear expansion coefficient of steel, and the difference between maximum and minimum values of calibers (Δ) are determined from the expression

Δ = D _to -D _t (1 + αt),

where D _to - the diameter of the caliber according to the rolling table, mm;

D _t - pipe diameter in the cold state, mm;

t is the current value of the rolling temperature, ΔС;

α is the coefficient of linear expansion of steel, 10 ^-6 * 1 / ° С.

Thus, the claimed method meets the criteria of the invention of "novelty."

Comparison of the proposed solution (method) not only with the prototype, but also with other technical solutions in the art did not allow them to identify signs that distinguish the claimed solution from the prototype, which allows us to conclude that the criterion of "significant differences".

The method has been tested and implemented on a pipe-rolling installation with pilgrim mills 8-16 of ChTPZ OJSC. Comparative rolling of 325 × 8 and 426 × 9 mm pipes from 20 steel ingots of 470-500 × 1700 and 540-585 × 1700 mm was carried out (thin-walled pipes according to GOST 8732-78).

40 ingots with a diameter of 14 "(470-500 mm) and 40 ingots with a diameter of 16" (540-585 mm) of grade 20 steel were set into production. Half of the ingots were rolled into pipes of 325 × 8, 426 × 9 in accordance with existing technology, and the second half - according to the proposed method with determining the difference between the maximum and minimum values of calibers according to the proposed formula and with a gradual increase in this difference from zero to maximum, i.e. with a decrease in caliber from D _{to max} to d _{to min by the} rollers in manual mode due to the reduction of the rolls by pressure screws.

Data on the results of rolling and delivery of pipes of 325 × 8, 426 × 9 in accordance with GOST 8732 are given in the table. The table shows that the average length of pipes (lashes) of 325 × 8 mm in size, rolled according to the existing technology, was 40.95 m, which were cut into four pipes (krata). The average length of pipes at delivery was 35.65 m: of these, the first three pipes were put on the wall of 8 mm with a positive tolerance, and the fourth pipes were put on the wall of 9 mm. The average weight of the pipes was 2.302 tons, and the expenditure coefficient of the metal was 1.266. The average length of pipes (lashes) measuring 325 × 8 mm at the rental, rolled according to the proposed technology, was 42.15 m, and at delivery 36.9 m. All pipes were put on the wall 8.0 mm. The average weight of pipes at delivery amounted to 2.308 tons, and the expenditure coefficient of the metal was 1.259. A similar picture was obtained when rolling pipes with a size of 426 × 9 mm. The expenditure coefficient of the metal pipes rolled by the existing technology amounted to 1.273, and according to the proposed method 1.265. Thin-walled pipes according to GOST 8732, rolled according to the existing technology, had a longitudinal difference, as a result of which the third pipe edges 325 × 8 mm in size were put on an 8 mm wall with a positive tolerance field, and the last (fourth edges) on a 9.0 mm wall. A similar picture when rolling pipes with a size of 426 × 9 mm. The second krat (pipes) surrendered with a positive tolerance field, and the third krat on the wall 10 mm. The delivery of pipes to the wall of 9 and 10 mm is not custom-made products that go to the warehouse and wait for new customers. According to the proposed method for the production of pipes in accordance with GOST 8732, a decrease in the expenditure coefficient of the metal from 7.0 to 8.0 kg per ton was obtained, depending on the assortment.

Thus, the table shows that when rolling thin-walled pipes according to GOST 8732 according to the proposed method, a reduction in the expenditure coefficient of the metal from 7.0 to 8.0 kg was obtained, the production of non-custom parts of the pipes, i.e. pipe rental to the warehouse.

Using the proposed method for the production of seamless hot-rolled pipes of medium and large diameters in tube-rolling plants with pilgrim mills will reduce metal consumption in pipe production by reducing the longitudinal difference, i.e. increase the average wall thickness from the beginning to the end of rolling, significantly reduce or completely eliminate the non-custom part when rolling thin-walled pipes by transferring the last lash pipes to thicker walls, eliminate the breakage of safety bolts, rolls and spindles of the pilgrim mill when rolling pipes from alloyed hard to deform steel grades and alloys and thin-walled large-diameter pipes made of carbon steel grades due to excess loads from incorrectly selected feed rates, increase productivity l of the mill due to an unreasonable reduction in feed rates when rolling pipes of medium diameter from carbon steel grades, and therefore, reduce the cost of pipes.

Claims (1)

A method for the production of seamless hot-rolled pipes, including heating ingot billets, piercing them in a cross-helical rolling mill and rolling them in a pilgrim mill with a fixed wall thickness, characterized in that the pipes are rolled in a pilgrim mill in calibers, the dimensions of which are reduced from the beginning to the end depending on the rolling temperature, pipe diameters and coefficient of linear expansion of steel, and the difference between the maximum and minimum values of calibers is determined from the expression Δ = D _to -D _t (1 + α t), where D _to - the diameter of the caliber according to the rolling table, mm; D _t - the diameter of the pipes in the cold state, mm; t is the current value of the rolling temperature, ° C; α is the coefficient of linear expansion of steel, 10 ^-6 · 1 / ° C.