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Summary on "Investigation of the influence of the initial state on the kinetics of austenite grain growth under heating of 10g2fb steel"

Contents

Contents

Introduction

The current level of development of the oil and gas industry determines the high requirements that consumers place on the quality and reliability of pipe products. The constant increase in production volumes, including due to the development of new regions of industrial production, severe climatic conditions, lowering the operating temperature to (-40 - (-60)), ensuring corrosion resistance and reliability of pipelines constantly increases the level of requirements for the quality of pipes [1]. As the initial billet for the production of pipes using sheets of low-alloy steel in the hot-rolled state, after heat-treated normalization, normalization with tempering or rolled with controlled mode with accelerated cooling or without it. According to the method of manufacturing pipes are divided into seamless, welded with a longitudinal seam and welded with a spiral seam. Steel must meet the requirements of SNip. The ratio of yield strength to tensile strength should not exceed 0.8 for low alloy steels. Metal pipes should not have cracks, bundles, sunsets and other defects [2]. To meet the high requirements of the gas industry for strength, viscosity and resistance to brittle fracture of steels intended for the manufacture of large diameter pipes, low-alloy low-perlite steels with a given set of properties are created. Such steels are steel 10Г2ФБ, 06Г2ФБ, 09Г2ФБ and others [2]. The purpose and relevance of the work performed is to obtain more data on the effect of heating parameters on the size of the original and actual grain of austenite steel 10G2FB.

1. ANALYTICAL OVERVIEW

1.1 General characteristics of the process of controlled rolling of leaves

Controlled rolling is an optimized rolling heating and process ensuring the production of fine ferritic grains through small recrystallization austenitic grains formed during hot rolling in the range of average temperatures, and through the deformation of austenite below the recrystallization temperature, increasing the nucleation of ferritic grains[3].

Unlike conventional high-strength steels, which acquire optimal properties after normalization after hot rolling, steels that are thermomechanical rolled acquire them during processing during heating, rolling and cooling. Since these processes can not be repeated, they must be strictly controlled, a deviation from the specified parameters within narrow boundaries is allowed, so as not to reduce the effect of thermomechanical processing and product quality. At the same time, they strive for the formation and homogenization of austenite in the metal, dissolution of micro-alloying elements and particles created by these elements is necessary, and excessive coarsening of the austenite grain is not observed [4].

Deformation at a temperature above 1000 ° C leads to the formation of large recrystallization grains of austenite, which in polymorphic transformation form a coarse ferrite structure and the structure of the upper bainite. When deformed in the intermediate temperature range (from 1000 ° C to 900 ° C), austenite is crushed by re-recrystallization, resulting in a fine-grained ferrite. Deformation below the recrystallization temperature (below 900 ° C) contributes to the fine-grained structure of ferrite [5].

Uer controlled rolling transformation occurs at the end of the deformation process. This is achieved at a certain rolling end temperature. In the process of controlled rolling varied parameters should be considered heating temperature, deformation temperature, rolling speed, the number of passes and the duration of pauses between them, modes of post-forming cooling [6].

1.2 effect of heating conditions on the kinetics of austenite grain growth in structural steels

austenite Nuclei are formed at the boundaries of the ferrite — carbide interface when heated above the AC1 temperature. With this heating, the number of embryos is always large enough and the initial grain of austenite is small. The higher the heating rate, the smaller the austenite grain, as the rate of embryo formation is higher than the rate of their growth. With a further increase in temperature or increase in the duration of exposure at a given temperature, a collective recrystallization occurs and the grain increases. The growth of austenite grain occurs spontaneously and is caused by the desire of the system to reduce the free energy due to the reduction of the grain surface. Grain grows as a result of increasing some grains due to other, smaller, and therefore thermodynamically less stable.

Grain Size, formed by heating to a given temperature, of course, does not change with subsequent cooling.in
, the ability of austenite grain to grow varies even in steels of the same grade composition due to the influence of their smelting conditions.in
on the tendency to grain growth, there are two limiting types of steels: hereditary fine-grained hereditary-coarse-grained and.in
hereditary-fine-grained steel when heated to high temperatures (1000-1050 °C), the grain increases slightly, but at higher heating, rapid grain growth occurs. In hereditary coarse-grained steel, on the contrary, a strong grain growth is observed even with a slight overheating above AC1. in
Different propensity to grain growth is determined by the conditions of steel deoxidation and its composition.

1.3 Conclusions and objectives of the study

Considered in work steel 10Г2ФБ find wide application, as a steel pipe. At the moment, there is insufficient data in the literature on the influence of heating parameters on the size of the initial and actual austenite grain of many pipe steel grades, including the dependence on their initial state.

in order to perform the qualification work is to perform comparative studies of changes in the structure of austenite at high temperature heating of continuously cast steel 10G2FB and this steel after controlled rolling.

the Objectives:
1) search methods to identify original grains of austenite steinernematid and after kontroliruemoi rolling;
2) study of changes in the structure of austenitic continuous cast steel 10G2FB in the case of heating to temperatures 900-1100°С with exposures 15, 30, 60, 90 minutes;
3) study of changes in the structure of the steel after controlled rolling 10G2FB in the case of heating to temperatures 900-1100°С with exposures 15, 30, 60, 90 minutes;
and

2. The MATERIALS AND METhODS of RESEARCh

2.1 research Materials

Studies were performed on 10g2fb steel in the cast state and after controlled rolling.The chemical composition of which is given in table 2.1.

table 2.1 - chemical composition of steel 10G2FB, in % [14].

C Si Mn Ni S p Cr V N Nb Ti Al Cu
0.09-
0.12 		0.15-
0.35 		1.55-
1.75 		0.3 0.006 0.02 0.3 0.09-
0.12 		0.012 0.02-
0.04 		0.01-
0.035 		0.02-
0.05 		0.3

Before the start of the research were pre-cut using noevci equal samples. Sample sizes - 10g2fb steel in cast condition-10×10 & times; 9;10g2fb steel after controlled rolling -16×13× 2. A total of 24 samples were examined, 12 for each initial state. Each sample was heated to temperatures of 900°С , 1000°С , 1100°С , the exposure time was 15, 30, 60 and 90 minutes. At the end of heating they were cooled in water. As a result, austenite grains of various sizes were obtained.

2.2 Methods of detecting and assessing the size of the original austenitic grains

After finishing the heat treatment, the micro-sections were made in several stages: the samples were ground on an abrasive wheel to measure the hardness. After measuring the hardness, micro-grinding was prepared by grinding the samples on abrasive skins of different grit,sequentially grinding the surface layer until the removal of traces from the previous skin with a change in the direction of grinding by 90°. Polishing of the surface of the samples was carried out in two stages: first of all on the polishing wheel with the use of cloth and chromium oxide suspension. Next, the sample was washed with water and polished on cloth with the addition of a suspension of chromium oxide, with a change of polishing direction by 90°. After polishing, the samples were washed with water, then degreased with a swab soaked in alcohol, and dried with filter paper

After viewing the non-etched plume for a more complete study of the alloy structure the plume is etched. There are several methods of etching, which differ in their effect on the metal surface [15].

to identify the structure of steels, the following etchants have been tested:
- Etchant №1: saturated aqueous solution of picric acid with the addition of surfactants (liquid soap);
- Etchant # 2: the "Aqua Regia" (3 volume HNO3/sub>+HCl 1 volume);
- Etchant №3: saturated aqueous solution of picric acid with the addition of surfactants (shampoo); - Etchant №4: 50% H2O+50%HNO3.

the following etchants were selected as a result of the experiment:
- for continuously cast steel 10G2FB-saturated aqueous solution of picric acid with the addition of surfactants-shampoo.
- for steel 10G2FB after controlled rolling-saturated aqueous solution of picric acid with the addition of surfactants - liquid soap.

A saturated aqueous solution of picric acid with the addition of surfactants was heated to make the etching reaction faster. The solution, which contained samples OF steel 10g2fb periodically changed, there was a reaction of oxidation of the surface layers, it became dark.

the holding time of samples from continuously cast steel 10G2FB in an aqueous solution of picric acid is 15...20 minutes.

the holding time of samples from steel 10G2FB after controlled rolling in an aqueous solution of picric acid is 13...18 minutes.

microstructure Studies were performed using metallographic microscopes "Neophot-21" and MIM-7. The average grain size was determined using photos of microstructures taken at magnification of 500 and 200 times using the secant method.

the Measurements were performed in three typical fields of view, each held by three intersecting.

hardness Testing was performed using a Rockwell TK – 2M hardness tester on a "C"scale.

for the scale "C" used a diamond cone at a load of 150kgs. At least 3 measurements were made on each sample and the average value was calculated.

3. The OBTAINED RESULTS AND THEIR ANALYSIS

3.1 the Study of the kinetics of grain growth of austenite during heating of the continuous cast steel

the study of the kinetics of grain growth of austenite was performed on 12 samples of continuous cast steel 10Г2ФБ. The results of changes in the intervals and the average size of the original austenitic steel grain 10G2FB listed in table 3.1

table 3.1-change Intervals and average values of the size of the initial austenitic grain of continuously cast steel 10G2FB depending on the heating temperature and holding time

heating Temperature duration of exposure, min Average grain size, mkm № grains
900 15 22,04-33,63/33,38 7
900 30 22,91-35,75/27,96 8
900 60 23,56-35,75/32,03 7
900 90 20,77-30,28/25,26 8
1000 15 26,97-34,87/29,91 7-8
1000 30 22,91-45,2/30,59 7
1000 60 39,11-44,35/41,73 6
1000 90 74,99-96,25/82,62 4
1100 15 25,82-38,96/32,52 7
1100 30 27,59-37,04/27,96 7
1100 60 79,31-109,73/97,63 4
1100 90 58,71-118,65/91,63 4

3.2 the Study of the kinetics of grain growth of austenite during heating of steel controlled rolling

investigation of austenite grain growth kinetics was performed on 12 samples of controlled rolling steel. The results of changes in the intervals and the average size of the initial austenitic steel grain 10G2FB listed in table 3.2

table 3.2-change Intervals and average values of the initial austenitic grain size of controlled rolling steel 10G2FB depending on the heating temperature and holding time

heating Temperature duration of exposure, min Average grain size, mkm № grain
900 15 11,16-17,44/13,9 9
900 30 12,24-17,26/14,67 9
900 60 16,37-20,71/17,27 8
900 90 19,87-23,93/21,67 7
1000 15 15,5-20,08/17,63 8
1000 30 17,55-18,81/18,04 8
1000 60 21,08-24,52/23,1 7
1000 90 16,49-20,86/18,35 8
1100 15 19,37-23,6/21,52 7
1100 30 24,59-29,29/26,85 7
1100 60 27,13-34,87/31,27 6
1100 90 40,52-53,87/49,96 6

3.3 Comparative analysis of structural changes occurring during heating of steels with different initial state

The study of the austenite grain growth kinetics of tubular steels is not only scientific, but also of practical interest. Steel grade 10G2FB is one of the most used steels used for the manufacture of large diameter pipes for main pipelines.
A thick sheet for the manufacture of pipes is rolled from continuously cast slabs heated under rolling to 1100-1200oC. In the process of heating, there is a change in the structure of austenite, primarily the growth of its grain, which affects the change in its structure during subsequent rolling, as well as the final structure and properties of steel in the finished sheets. Therefore, it is of interest to study the changes in the structure of austenite by heating samples cut from a continuously cast slab.
Sheets of continuously cast slabs are rolled by the technology of controlled rolling, and the pipes are made of them by molding with subsequent connection of the edges by electric arc welding. During the welding process, the weld zone is heated to high temperatures to obtain an austenitic structure in the zone of a certain width. The temperature of the steel in this zone gradually decreases as it moves away from the weld. Therefore, it may also be of practical interest to obtain information on changes in the austenite structure resulting from the heating of controlled rolling steel, depending on the heating conditions.
In addition, various connecting elements for pipelines (couplings, corners, etc.) are made of sheets of the same steel using welding and machining, and then subjected to heat treatment – normalization or hardening with high-temperature tempering. The same types of heat treatment and subjected to controlled rolling sheets, which according to the results of mechanical tests do not meet the conditions of delivery. When performing such heat treatment, the heating duration of the products is within the limits of the extracts studied in this work. Therefore, the study of the influence of heat treatment parameters on the change in the structure of austenite of 10g2fb steels in the state after controlled rolling is also of practical interest.
A comparative analysis of the results of changes in the size of austenitic grains depending on the heating parameters of continuous cast steel and steel after controlled rolling reveals a number of features.
The first feature is that the austenite grain of steel of controlled rolling at all investigated modes of the subsequent heat treatment remains considerably smaller, than in continuously cast steel. Given THAT 10g2fb steel is hereditary-fine – grained due to microalloying of its vanadium and niobium, it can be assumed that the preservation of a finer grain of austenite in the steel of controlled rolling is due to the presence of more dispersed particles of carbides of these elements (NbC, VC) in comparison with the continuous-cast steel, in which these carbides are larger, and their quantity is smaller.Carbides, carbonitrides and nitrides, located along the borders of the granularity, are barriers to the growth of its grain [7].
Hot plastic deformation, especially in controlled rolling mode, stimulates the release of these particles from austenite in defective places, thereby increasing their number.
The second feature of the behavior of steel with different initial state is the difference in the nature of the change in the average size of the initial austenitic grain depending on the heating parameters.
In steel controlled rolling observed mainly known from the literature the nature of changes in the size of austenitic grains: increasing the heating temperature and increasing the duration of exposure leads to an increase in the average grain size of austenite as a result of the development of collective recrystallization.
the differences in grain size depending on the duration of exposure at a temperature of 900 ° C and 1000 ° C can be considered relatively small: the average grain size varied from 13.9 to 18.35 µm at 900 ° C and from 18 to 23.1 µm at 1000 ° C. At a heating temperature of 1100 ° C, these differences were more significant: from 21.5 to 51 µm. In the case of the initial continuous cast steel, the intensive growth of austenitic grain with an increase in the heating temperature was observed only in the case of the longest exposures – 60 and 90 minutes.
In the case of shorter exposures-15 and 30 minutes-the average size of the initial austenitic grain with an increase in the heating temperature varied in a relatively small range: from 30 to 33.4 microns with exposure time of 15 minutes and from 28 to 33 microns with exposure time of 30 minutes.
Therefore, at such exposures can not be afraid of overheating steel 10G2FB.
Thus, when assigning the heat treatment mode, the initial state of the steel should be taken into account. Heating of both cast and deformed steel up to 100°С in the case of holding time up to 30 minutes provides a fine-grained austenite structure: the average size of the initial austenite grain does not exceed 15-20 µm for the initial deformed state and 30-35 µm for initial cast state.

CONCLUSIONS

the effect of the initial state on the kinetics of austenite grain growth during heating of 10g2fb steel at a change in the heating temperature in the range 900-1100 was studied?C and the duration of exposure within 15-90 minutes.

It was found that in continuously cast steel10g2fb austenite grain begins to grow rapidly at a temperature of 1000?Since, since 60 minutes of endurance.

in steel 10G2FB after controlled rolling a noticeable growth of austenitic grains was also observed at a temperature of 1100?C, starting with a shutter speed of 60 minutes. However, continuous cast steel compared with steel after controlled rolling showed a more noticeable tendency to the growth of solid solution grain.

consequently, continuous cast steel may Be considered to be more prone to austenite grain growth than steel after controlled rolling.

the following features in the kinetics of austenite grain growth depending on the initial state of steel were Revealed:
- a smaller tendency of steel to a controlled rolling of grain growth of austenite during heating compared to the continuous cast steel;
- in controlled rolling steel, an increase in the average grain size of austenite is observed naturally both with an increase in the heating temperature and with an increase in the holding time;
- in continuously cast steel, the intensive growth of austenite grain with an increase in the heating temperature was observed only at a long exposure time – 60 and 90 minutes: with the duration of exposure to 30 minutes, the average size of the initial austenite grain with an increase in the heating temperature from 900 to 1100 ° C varied within relatively small limits (3-5 µm).

activities have been developed to ensure safe operation when using heat treatment equipment, as well as measures to protect the environment. The feasibility study of using a more efficient steel grade for the manufacture of products operating at high temperatures was carried out.

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