RU2778650C1 - Method for manufacturing grinding balls from steel - Google Patents

Abstract

FIELD: heat treatment.

SUBSTANCE: invention relates to a method for heat treatment of grinding balls made of steel containing wt.%: C 0.75-0.80; Mn 0.80-0.90; Si 0.25-0.35; Al less than 0.020; P less than 0.015; S less than 0.020; Cr 0.35-0.40; Ni 0.15-0.25; Cu less than 0.030; V 0.10-0.15; Mo less than 0.05; N less than 0.015; H is less than 0.0003, Fe is the rest. The method includes the production of balls with a conditional diameter of 60-80 mm, while rolling the balls, subsequent cooling of the balls to a temperature of 750 to 830°C, hardening of the balls in a quenching medium with holding for 2.5 min to 3.5 min and subsequent tempering at a temperature of 140 to 250°C and a holding time of 160 to 240 minutes.

EFFECT: increasing the operational stability of the balls, obtaining grinding balls with a nominal diameter of 60-80 mm with a bulk hardness characterized by hardness group 5 according to GOST 7524-2015 on a production scale.

3 cl, 2 tbl, 1 dwg

Description

The invention relates to rolling production, in particular to the heat treatment of grinding balls.

Known inventions for obtaining steel for the production of grinding balls: patent RU No. 2415194 C1 "Steel" IPC ⁶ C22C 38/38; C22C 38/34; C22C 38/32, published 03/27/2011, bul. 9 [1], patent RU No. 2425168 C2 "Steel" IPC ⁶ C22C 38/26, published on July 27, 2011, bul. 21 [2], patent RU No. 2425169 "Steel" IPC ⁶ C22C 38/40, published on July 27, 2011, bul. 21 [3], as well as copyright certificate SU1497262 A1 "Steel" IPC ⁶ C22C 38/14, convention priority 01/13/1988 [4], copyright certificate SU1446189 A1 "Steel" IPC ⁶ C22C 38/16, published on December 23, 1988 [ 5].

A known method for the production of steel grinding balls patent RU No. 2596737 C1 IPC ⁶ C21D 9/36; B21H 1/14; C21D 1/02; B23P001500; C22C 38/40, published on September 10, 2016, bull. 25 [6], including heating of a continuously cast billet, rolling of round billets of the appropriate size on a section hot rolling mill, their subsequent heating in an induction device, rolling of balls from them at a helical rolling mill at a temperature of 950-1050°C, cooling the balls before hardening, hardening and self-tempering of the balls in containers, while a square continuously cast billet is made with a cross section of (100-150) × (100-150) mm. from steel with the following ratio of components, wt %: carbon 0.6-1.05; silicon 0.15-2.0; manganese 0.2-1.2; chromium 0.03-0.5; copper 0.03-0.40; iron and unavoidable impurities remain the rest, and heating of round blanks is carried out in an induction device to a temperature at the outlet of the inductors of 1070-1140°C, cooling of the balls to a hardening temperature of 840-900°C is carried out in a cooling drum with a speed of its rotation in the range of 6.0- 22.0 rpm with equalization of the temperature of the balls over the section due to the rotation of the balls in the drum for less than 2 minutes, and the hardening of the balls is carried out in the hardening drum with a speed of its rotation in the range of 0.4-2.5 rpm with running water at a temperature 25-42°C up to the temperature of the balls after hardening 125-160°C.

The disadvantage of this method is that this method does not allow to obtain the 5 hardness group with continuous hardenability, but only partial hardenability corresponding to the 4th group, and has a lower gradient.

A device and method for heat treatment of balls is known, patent RU No. 2455369 C1 MPK ⁶ C21D 9/36, published on 10.07.2012, bul.19 [7], including, after stamping or rolling heating, cooling the balls from the stamping or rolling temperature to the hardening temperature, by cooling the balls from the quenching temperature in water and tempering, while before quenching, the temperature of the balls is equalized, and then they are cooled with water at a cooling rate of not more than 12 deg. / s 150-200 ° C below the temperature of the end of rolling or stamping.

The disadvantage of this method is that it lacks the technology for obtaining balls of the 5th hardness group. Also, a significant difference in the technology is the temperature of the balls before hardening, which is 830-900°C, which implies the use of target steels below the 5th hardness group.

A known method of heat treatment of grinding balls patent No. 2113513 IPC ⁶ C21D 9/36; B21H 1/14, published on September 10, 2016, bul. 25 [8], including rolling, cooling to the hardening temperature for 2-12 minutes. and hardening.

A known method of heat treatment of grinding balls author's certificate No. 1344793 A1 IPC ⁶ C21D 9/36, published 10/15/1987 [9], including heating to austenitization temperature, holding, cooling, and quenching in water, while cooling is carried out at a speed of 20-50 °С/s until reaching the average temperature of 600-70°С.

The disadvantage of these methods is that the modes of heat treatment do not allow to obtain continuous hardenability of the balls.

A known method and device for heat treatment of balls RU No. 2634541 C1 IPC ⁶ C21D 9/36, published on 10/31/2017, bul. 31 [10], including equalizing the temperature of the balls from the stamping or rolling temperature to the quenching temperature in air when they are placed on the conveyor, cooling in water from the quenching temperature in a rotating quenching drum and tempering, while the temperature of the balls is equalized to the quenching temperature in air for more than 40 s when placed on a conveyor designed to accommodate one ball in each of its structural elements, and cooling balls in water are produced to a temperature below the start point of the martensitic transformation Mn, while the balls are evenly and one by one placed in the cells of the hardening drum, into which water is supplied to wash the balls, then the balls are tempered by heating and temperature control in the furnace and subsequent final cooling.

The disadvantage of this method is that, according to the heat treatment technology, residual stresses remain on the proposed steel grades after continuous hardenability of the balls, which contribute to their further destruction during operation.

The closest (prototype) in technical essence to the claimed device, in terms of the number of similar features, is patent RU No. 2756671 C1 IPC ⁵¹ B21H/14; C21D 9/36; С22С 38/24 published on 04.10.2021, bull. 28 [11], including rolling, hardening, tempering, characterized in that balls with a nominal diameter of 80-100 mm are produced, after rolling the balls are cooled to a temperature of 740-800 ° C, hardening balls are produced in a quenching medium with a holding time of 3.0 to 4.0 minutes, and the subsequent tempering is carried out at a temperature of 180-260 ° C and a holding time of 180 to 320 minutes, while after tempering, self-tempering is carried out with a holding time within 12 to 48 hours.

The disadvantage of this method is that the described technology limits the lower threshold for the diameters of balls produced from this steel grade with a diameter of 80 mm.

The technical result to which the proposed invention is directed is: increasing the operational stability of the balls, obtaining grinding balls with a bulk hardness characterized by hardness group 5 with a diameter of 60-80 mm according to GOST 7524-2015 [12] on a production scale.

The technical result is achieved by the fact that in the method of obtaining grinding balls with bulk hardness from steel containing wt. % : C 0.75-0.80; Mn 0.80-0.90; Si 0.25-0.35; Al less than 0.020; P less than 0.015; S less than 0.020; Cr 0.35-0.40; Ni 0.15-0.25; Cu less than 0.030; V 0.10-0.15; Mo less than 0.05; N less than 0.015; H less than 0.0003, Fe the rest, including rolling, quenching, tempering, according to the invention, balls with a nominal diameter of 60-80 mm are produced, after rolling the balls are cooled to a temperature of 750 to 830 ° C, and then subjected to quenching in a quenching medium with holding for 2.5 to 3.5 minutes, and subsequent tempering is carried out at a temperature of 140 to 250°C and a holding time of 160 to 240 minutes.

In addition, water is used as a quenching medium at a temperature of from 30 to 46°C.

In addition, a medium with specific heat capacity from 3800 to 4000 J/(kg⋅deg.), thermal conductivity coefficient from 0.4 to 0.7 W/(m⋅deg.) is used as a quenching medium.

The essence of the invention is as follows.

Using the proposed method provides a martensitic structure throughout the depth of the grinding balls. The martensitic structure in steels is formed at a high cooling rate, at a phase transition below the AC3 point (point of the beginning of austenization) before the end of the transformation, according to the transformation kinetics from the diagrams of supercooled austenite. Obtaining continuous hardenability is possible at a high cooling rate of both the surface and the core (center) of the balls. It is especially difficult to achieve such a transformation with large ball diameters.

The ball is a figure with the maximum mass of the object (the ratio of mass to volume), so the heat transfer mirror in the medium is as small as possible. According to the Fourier law, heat transfer starts from the surface of the ball and ends with the center of the ball, so the maximum effect can be obtained on those materials in which the kinematics of the transformation occurs with maximum thermal conductivity inside the object.

In addition, continuous hardenability creates internal stresses (an intensively cooling surface has a denser structure than the inner part of the material and further cooling creates a layer-by-layer completion of the transformation), which, due to the difference in state, cause the appearance of dislocations and then can lead to the appearance of cracks on the balls, which unacceptable according to GOST 7524-2015.

The chemical composition of steel for the production of grinding balls, containing wt. % : C 0.75-0.80; Mn 0.80-0.90; Si 0.25-0.35; Al less than 0.020; P less than 0.015; S less than 0.020; Cr 0.35-0.40; Ni 0.15-0.25; Cu less than 0.030; V 0.10-0.15; Mo less than 0.05; N less than 0.015; H less than 0.0003, Fe the rest, allows solving this contradiction, ensuring high hardness of both the surface and the inner zone of finished balls, eliminating the occurrence of internal stresses, and also determining heat treatment modes for balls of large diameters.

Cooling the grinding balls to a temperature of 750 to 830°C after rolling makes it possible to provide the required temperature for the start of hardening. Deviation from the specified temperature range both above 830°C and below 750°C does not allow starting the hardening of balls from temperatures that ensure complete hardening; with incomplete hardening, a mixed structure of troostite and martensite or bainite and martensite is formed, which reduces the hardness of the balls below the established standard .

Increasing the residence time of the grinding balls in the quenching medium above the established limits is not advisable, because this time is sufficient for bulk hardenability to pass.

Reducing the residence time of the grinding balls in the quenching medium below the established limits will lead to insufficient hardenability of the ball and the lack of bulk hardness.

The subsequent tempering of grinding balls with a nominal diameter of 60-80 mm is carried out at a temperature of 140 to 250°C, which allows the removal of surface stresses that occur during the hardening process.

Deviation from the specified temperature range below the set temperature value will lead to the absence of surface heating and further to the possibility of cracking, and exceeding the set temperature value will lead to the onset of transformations in the grains (characteristic of tempering brittleness of the first kind) and the appearance of surface embrittlement.

The duration of the holding process for grinding balls with a nominal diameter of 60-80 mm is from 160 to 240 minutes. in the furnace, which ensures the sufficiency of the tempering process.

The method of obtaining grinding balls with bulk hardness works as follows.

Obtaining grinding balls with nominal diameters from 60 mm to 80 mm that meet the requirements of the 5th hardness group is possible using the material of the balls from the steel grade 77KhGFMN, containing, wt %: C 0.75-0.80; Mn 0.80-0.90; Si 0.25-0.35; Al less than 0.020; P less than 0.015; S less than 0.020; Cr 0.35-0.40; Ni 0.15-0.25; Cu less than 0.030; V 0.10-0.15; Mo less than 0.05; N less than 0.015; H less than 0.0003, Fe the rest, when carrying out the appropriate heat treatment.

The heat treatment technology consists of the following steps:

Rolling grinding balls at a temperature of 950-1050°C.

Cooling grinding balls with a nominal diameter of 60-80 mm to a temperature of 750 to 830 ° C, after rolling to the specified temperatures,

Hardening of grinding balls in water at a temperature of 30°C to 46°C: with a nominal diameter of 60-80 with a holding time of 2.5 minutes to 3.5 minutes, or in an environment with a specific heat capacity of 3800 J/(kg⋅deg ) up to 4400 J/(kg⋅deg) and thermal conductivity from 0.4 W/(m⋅deg) to 0.7 W/(m⋅deg), subject to the same temperature and time parameters.

Subsequent low-temperature tempering of balls with a nominal diameter of 60-80 at a temperature of 140 to 250°C with a holding time of 160 to 240 minutes.

An example of a specific implementation of the method.

Testing according to the technology of the proposed invention was carried out at the section of the ball-rolling mill of the rail and beam shop of EVRAZ NTMK JSC in the production of balls with a diameter of ∅60-80 mm. steel grades 77HGFMN.

Grinding balls with nominal diameters from 60 mm to 80 mm were rolled on a helical rolling mill at a temperature of 950–1050°C. Then the grinding balls were cooled to a temperature of 790°C. After that, grinding balls were quenched in water in a revolver-type hardening drum with a holding time of 2.5 min to 3.5 min.

Next, low-temperature tempering was carried out at a temperature from 140 to 250°C with a holding time of up to 200 min.

Determination of the hardness of grinding balls on the surface, depth of ¼, ½, ¾ radius and in the center of the ball was carried out in accordance with GOST 9013-59 [13].

The chemical composition of steel is given in table 1.

Table 1
Chemical composition of steel grade 77KhGFMN FROM, % Mn, % Si, % Al, % P, % S, % Cr, % Ni, % Сu, % V, % Mo, % N, % Hppm 0.75-0.80 0.80-0.90 0.25-0.35 <0.02 <0.015 <0.02 0.35-0.40 0.15-0.25 <0.30 0.10-0.15 <0.05 <0.015 <3.0

The test results showed that the proposed method for the production of grinding balls according to the chemical composition from table 1 ensures the production of finished balls with a hardness corresponding to group 5 according to GOST 7524-2015 and are presented in table 2.

table 2
Regulated and actual hardness values of grinding balls of the 5th group Nominal ball diameter, mm Hardness, HRC according to GOST, not less than Hardness, HRC actual, not less than ball surface Volumetric ball surface Volumetric 60 60 53 62.3-64.9 59.48-63.88 80 58 48 57.6-60.2 60.47-60.57

The bulk hardness indicated in Table 2 was determined by the method of finding the hardness gradient over the depth of calcination in the ball. On FIG. 1 shows the results of tests for surface and bulk hardness, HRC of balls from an experimental heat, while bulk hardness (HT) is calculated by the formula:

OT \u003d _0.289T sov +0.436T _0.25 +0.203T _0.5 +0.63T _0.75 +0.009T _c ,

where T _pov - the value of the hardness of the surface of the ball,

Tc - the value of the hardness of the center of the ball,

T _0.25 , T _0.5 , T _0.75 - the value of hardness at a distance from the surface of the ball.

The use of the proposed method for manufacturing grinding balls ensures the production of grinding balls with a bulk hardness characterized by hardness group 5 according to GOST 7524-2015 [11] on a production scale for balls from 60 to 80 mm. Thus, this technical solution meets the criterion of "novelty".

The analysis of patents and scientific and technical information did not reveal the use of new essential features used in the proposed solution. Therefore, the present invention meets the criterion of "inventive step".

It has been experimentally confirmed that the use of the proposed invention allows:

- to obtain grinding balls of the 5th group of hardness with diameters from 60 to 80 mm.

- in 2020-2021, 3001.913 tons of balls of the 5th hardness group from this steel grade were shipped to the consumer.

The test results of the experimental melting of balls obtained in the proposed stable modes showed a positive result in obtaining the required values of volumetric hardenability:

a. The values of the obtained surface and bulk hardness are presented in FIG 1 - these values meet the requirements of the 5 hardness group, according to table 2.

b. The total energy after the test on the hammer according to MI 102-142-355-2019, during the operation of the ball for 6 minutes, was ΣЕ = 285 kJ.

Sources of information

[1] patent RU No. 2415194 C1 "Steel" IPC ⁶ C22C 38/38; C22C 38/34; C22C38/32, published 03/27/2011, bul. 9;

[2] Patent RU No. 2425168 C2 "Steel" IPC ⁶ C22C 38/26, published on July 27, 2011, bul. 21;

[3] Patent RU No. 2425169 "Steel" IPC ⁶ C22C 38/40, published on July 27, 2011, bul. 21;

[4] copyright certificate SU No. 1497262 A1 "Steel" IPC ⁶ C22C 38/14, convention priority 01/13/1988;

[5] copyright certificate SU No. 1446189 A1 "Steel" IPC ⁶ C22C 38/16, published on 12/23/1988;

[6] patent RU No. 2596737 C1 C1 "Method of production of steel grinding balls" IPC ⁶ C21D 9/36; B21H 1/14; C21D 1/02; B23P 15/00; C22C 38/40, published 09/10/2016, bul. 25;

[7] patent RU No. 2455369 C1 "Device and method for heat treatment of balls" IPC ⁶ C21D 9/36, published on 10.07.2012, bul. 19;

[8] patent RU No. 2113513 C1 "Method of heat treatment of grinding balls" IPC ⁶ C21D 9/36; B21H 1/14, published 09/10/2016, bul. 25;

[9] copyright certificate SU No. 1344793 A1 “Method of heat treatment of grinding balls copyright certificate IPC ⁶ C21D 9/36, published on 10/15/1987;

[10] patent RU No. 2634541 C1 "Method and device for heat treatment of balls" IPC ⁶ C21D 9/36, published on 10/31/2017, bul. 31;

[11] patent RU No. 2756671 C1 "Method of production of grinding balls (options)" IPC ⁵¹ B21H/14; C21D 9/36; С22С 38/24 published on 04.10.2021, bul. 28;

[12] GOST 7524-2015;

[13] GOST 9013-59.

Claims (4)

1. Method for the production of bulk hardness grinding balls from steel, containing, wt.%: C 0.75-0.80; Mn 0.90-0.90; Si 0.25-0.35; Al less than 0.020; P less than 0.015; S less than 0.020; Cr 0.35-0.40; Ni 0.15-0.25; Cu less than 0.030; V 0.10-0.15; Mo less than 0.05; N less than 0.015; H less than 0.0003, Fe - the rest, including rolling, hardening, tempering, characterized in that balls with a nominal diameter of 60-80 mm are produced, after rolling the balls are cooled to a temperature of 750 to 830 ° C, the balls are hardened in a hardening medium with a holding time of 2.5 minutes to 3.5 minutes, and subsequent tempering is carried out at a temperature of 140 to 250°C and a holding time of 160 to 240 minutes. 2. The method according to claim 1, characterized in that water is used as a quenching medium at a temperature of from 30°C to 46°C. 3. The method according to claim 1, characterized in that a medium with a specific heat capacity of 3800 to 4000 J/(kg⋅deg) and a thermal conductivity coefficient of 0.4 to 0.7 W/(m⋅deg) is used as a quenching medium .

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