RU2002849C1 - Steel - Google Patents

Description

th

3.20-4.00

0.20-0.50

0.005-0.05

0.005-0.05

Rest

This steel, heat-treated by tempering from 920-960 ° C and tempering at a temperature of 520-560 ° C, is intended for the manufacture of tools for hot pressing and extrusion of carbon and low alloy structural steels. The high content of carbon and chromium in the steel contributes to hardening after improvement. In addition, the intended purpose of introducing titanium into steel results in the formation of simple finely dispersed carbides of the TiC type, which significantly grind the primary grain, thereby increasing the toughness. Calcium enhances properties by influencing the form of inclusions. At the same time, obtaining high strength properties does not provide a complex that increases resistance to crack development in the low-cycle loading region. since a significant fraction of the carbide phase and hardening nonmetallic inclusions are formed in the structure, the amount of which is not regulated by ratios that quantitatively bind the content of elements.

Steel is also known in the metallurgical industry for the production of cast parts operating under high shock and fatigue loads at low temperatures. Steel has the following chemical composition, wt.%:

Carbon 0.35-0.45

Manganese1.20-1.60

Silicon 0.30-0.60

Titanium 0.005-0.015

Bor0,0005-0.003

Vanadium 0.01-0.06

Yttrium 0.0001-0.01

Calcium 0.0001-0.02

IronOther

. The brought steel - structural, possesses the increased physical and mechanical properties. It is ductile, has increased impact strength (especially at freezing temperatures), higher fluidity and crack resistance. The properties are ensured by the fine-grained structure and structural components and dispersed precipitates of the hardening solid phases. However, a large total amount of strong carbonitride-forming elements in steel with a relatively low

the amount of carbon leads to a decrease in the concentration of carbon 9 in the solid solution and the formation of an excess amount of ferrite in the molten state. After hardening heat treatment, a rather coarse batch martensite is formed in the hardened structure, which decomposes upon tempering to form large ferrite spots. The phase boundary is the initiator in crack formation, reduces the crack nucleation work and, consequently, the fracture resistance under alternating loads.

Steel in order to increase the impact strength and ductility contains components in the following ratio, May%

Carbon055-0.65

Silicon 0.60-0.90

Manganese 0.50-0.80

Aluminum 0.04-0.10

Gitan 0.05-0.15

IronOther

Steel is used for the manufacture of products operating under conditions of high wear, shock and bending loads, such as plow dumps. Silicon contained in steel s is introduced to increase hardenability, hardened ™, elasticity, wear resistance and toughness. Manganese prevents the release of carbon with a high content of silicon in the form of graphite. However, at this ratio of components, steel is prone to decarburization and coarsening of the ferrite structure, which does not provide high resistance to fracture under alternating loads on aggressive media due to an increase in the resistance of the material to crack development in the low-cycle loading region.

Closest to the proposed technical essence and the achieved effect is steel composition, wt.%:

Carbon 0.3-0.5

Manganese 0.5-0.8

Titanium 0.01-0.04

Silicon1.15-1.50

Bor0.001-0.008

Aluminum 0.01-0.08

As impurities, steel contains sulfur 0.005-0.045% and phosphorus 0.005-0.045%. The ratio of AI / B 10, Ti / B 5-10.

This steel contains elements in the given ratio in order to increase strength properties and is used for the manufacture of high-strength reinforcement. After heat treatment according to the hardening mode from 870 ° C and tempering at 400 ° C, the steel has the following properties: g / in 1501-2010 MPa, at 1452-1965 MPa. L 8-12.2% (dl

fittings f 10 mm). The formula stipulates the relationship between the amount of aluminum and titanium and boron. The limits of the ratio between these elements are narrowed compared to those obtained by calculating this ratio by the percentage given in the formula. However, the limits of AI / / B 5-10 do not reflect the dependence of the aluminum and titanium contents on the degree of deoxidation of the steel. With such a range of ratios, the elements can be completely connected in the inclusions, and boron, distributed unevenly over the grain boundaries, also forms compounds, which will lead to a drop in properties. The structure obtained in the steel of reduced composition provides enhanced strength properties, but does not contribute to increased fracture resistance of the material under alternating loads in aggressive environments in the area of low-cycle loading. Thus, the steel prototype does not have properties that give reason to recommend it for the manufacture of working bodies

tillage machines, in particular chisels and plowshares of plows, as well as cultivator knives.

The object of the present invention is to increase the fracture resistance under alternating loads in aggressive environments by increasing the resistance of the material to crack propagation in the low-cycle loading region.

The problem is solved due to the fact that the steel containing carbon, manganese, silicon, aluminum, boron, titanium and iron contains components taken in the following ratio, wt.%:

Carbon 0.30-0.37

Manganese1.10-1.63

Silicon0.40-0.60

Aluminum 0.02-0.04

Titanium 0.02-0.04

Bor0.002-0.004

IronOther

As impurities, steel contains sulfur not more than 0.03% and phosphorus not more than 0.03%, and the ratio

% Т-уо ° -% В 1.07-2.67х

% TI +% A1 + 10% B x% N

vo + mp

The composition of these steels contains components providing high properties under static loading, determined by impact strength and tensile strength. However, their composition does not provide resistance to steel fracture under alternating loads, especially in aggressive environments. As a result of the analysis, it was found that only the proposed set of components in the indicated quantity ensures the achievement of the goal. Based on this, we conclude that the proposed composition has significant

0 differences.

The rational carbon content in the proposed steel is 0.30-0.37 wt.% And provides strength characteristics and ductility, as well as resistance to fracture under the influence of alternating loads in aggressive environments. A decrease in carbon content of less than 0.30 wt.% Leads to a decrease in the strength of ze by increasing the amount of

0 residual austenite and ferritic component in the structure of heat-treated steel. An increase in% C of more than 0.37 wt.% Entails an increase in brittleness, which, combined with the presence in the structure of ultralight phases, contributes to a decrease in fracture resistance under cyclic loading.

Manganese is present in the proposed steel in an amount of 1.1-1.3 wt.% And contributes to an increase in hardenability, hardening of doped cementite and solid carbon solution in a-iron.

The proposed steel contains silicon in an amount of 0.4-0.6 wt.%. By

5 compared with the prototype is more than 2 times less. Silicon contributes to the hardening of ferrite and in such an amount does not adversely affect the plastic properties. With increasing content

0 silicas above 0.6% decrease the plastic and toughness of the steel. The introduction of less than 0.4% Si on the properties is negligible.

The final deoxidation of steel is almost always carried out by aluminum. The aluminum content influences not only the impact strength due to the release of AIN nitrides and their location along grain boundaries, but also due to the effect on the hardening mechanism of steel. When the AI content is above 0.04%, in spite of the high values of impact strength, a significant decrease in strength properties is noted, since a significant fraction of nitrogen, which binds to AIN nitride during austenitic heating, does not take part in the formation of the secondary structure and dispersion hardening. In the range of 0.02-0.04%, aluminum contributes to the complete deoxidation of steel, binds nitrogen, and suppresses the tendency of steel to age.

The purpose of introducing aluminum in a given amount is deoxidation and preparation for modification.

The necessity of introducing titanium into the composition of steel is explained by its activity, which leads to the formation of compounds with oxygen, nitrogen, and sulfur. Titanium acts as a strong deoxidizer and steel degasser. At etrm, its compounds — for example, carbonitrides — do not reduce the concentration of properties to certain limits and have a beneficial effect on the structure and mechanical properties. With a content of 0.02-0.04% in the proposed steel, titanium crushes the primary and secondary austenite grains, binds nitrogen into stable highly dispersed compounds, and favors an increase in ductility and toughness, as well as eliminating the tendency to aging. The complex effect of titanium on the structure increases the resistance to crack nucleation and development under loading, especially caused by alternating loads, since crack nucleation is more difficult in the fine-grained structure, and grain boundaries and special carbides are obstacles to crack propagation. The introduction of a steel of less than 0.02% Ti is impractical, since such a residual content does not significantly affect the structure.

At a content of more than 0.04%, the carbonitride phase coarsens, while large inclusions can be centers of crack initiation upon failure under the action of alternating loads. In the case of microcracks, already in the low-amplitude loading region, the inclusion – matrix interface is filled with an aggressive medium, which leads to a decrease in the fracture resistance.

The effect of boron introduced in steel in an amount of 0.002-0.004% as a microalloying element is due to the small size of boron atoms, as well as its strong carbide and nitride forming effect. The introduction of boron provides high hardenability in the proposed steel containing titanium. Grinding grain in steels with titanium is accompanied by a decrease in hardenability due to an increase in length, which are regions with an increased number of defects in the crystal structure and, consequently, a reduced stability of supercooled austenite. Horophilia of boron helps to reduce defective boundaries and, therefore.

increase the stability of austenite. In addition, boron additives alter the conditions for the formation and growth of martensitic crystals; boron, concentrating on austenite grain boundaries displaces carbon into the central regions of austenitic grains, where it contributes to the dispersion of the arising martensitic structure. A complex effect on the structure of microadditives, and boron, leads to a complex increase in viscous and strength properties. This affects the increase in low-cycle fatigue, which is associated with the formation of such a structural state, when along with hardening, due to the increase in the barrier strength of the subboundaries, prerequisites are created for the relaxation of microlocal stresses. The presence of boron in an amount of less than 0.002% is inefficient, since even if its amount is sufficient to reduce the defective boundaries, such a concentration is not enough to disperse the martensitic structure. An increase in boron content of more than 0.004% is impractical, since it leads to a decrease in viscous and plastic properties and, therefore, low-cycle durability,

Thus, in the proposed steel, in order to solve the problem of increasing the fracture resistance under alternating loads in aggressive environments by increasing the resistance of the material to crack propagation, carbon, manganese, silicon, aluminum, titanium and boron in a certain ratio are contained in the region of mapocycle loading. In addition, to obtain the desired structure, balances are established between deoxidizers and oxygen and nitride forming elements and nitrogen, and the ratio of the concentration of carbide forming elements to carbon is regulated.

The use of nitrogen as a modifier can only be carried out in combination with nitride-forming elements, which ensure the release of nitrogen from the solid solution into the nitride phase or decrease its activity to iron. In the proposed steel, such a function is performed by aluminum and titanium. Along with the modifying effect, nitrogen can manifest itself as an alloying element, i.e., it can noticeably change the primary structure of castings and affect the properties of the solid solution. In low-gelling steels, which include the proposed stan- dard, nitrogen has surface activity. Refractory inoculators, decreasing the initiation of nucleation of ausnite centers shift the beginning and completion of nonequilibrium transformation to a higher temperature region and inhibit the migration of austenitic grain boundaries. Nitrogen increases the nonequilibrium subcooling of the 5-phase and shifts the 5th process to the region of lower temperature and also inhibits the migration of boundaries. 8, a consistent reduction in the size of the eustenmt grain is achieved. The dispersion of the elements of the primary and secondary structures achieved by nitrogen modification, the reduction of physical, chemical, and structural heterogeneity, as well as the direct dispersion hardening of the metal by the nitride or carbonitride phase, provide the formation of a complex of physicomechanical properties not inferior to the level of complex alloyed steels.

The introduction of strong kzrboid-forming elements into steel promotes an increase in cyclic properties, but their effect is positive within narrow concentration limits. Therefore, it is necessary to specify the lower and upper limits of their joint content. An empirical relationship is proposed for this.

50 ..5.

The fulfillment of this ratio will ensure dispersion of the structure and the production of small inclusions, which serve as an obstacle to the propagation of cracks during failure under the influence of cyclic loading. The upper limit of the ratio stipulates narrower limits than the maximum formula titanium and manganese content and the minimum amount of carbon, and the lower one specifies the total ratio of titanium and manganese at the maximum percentage of carbon. This is related to that. that with an excess of titanium and manganese with respect to% C, grain refinement is not ensured by a sufficient number of carbon atoms to position it along the grain boundaries, which leads to carbon depletion of the solid solution and the appearance of excess ferrite. Thus, the resulting structure is inhomogeneous and does not have a high fracture resistance in aggressive media under low-cycle loading. With an excess of carbon compared to the total titanium and manganese content, the opportunity arises for the formation of large carbide inclusions and coarsening of the hardened structure, which significantly impairs the steel's resistance to fracture.

Consequently, only when the concentration of Ti, Ml, C in the range of 0.63-10.5 is ensured in the steel, is the resistance of the heat-treated steel to fracture under alternating loads in aggressive environments.

Titanium, boron and aluminum are strong nitride-forming elements and form compounds with nitrogen both during crystallization and during cooling during heat treatment. As an uncontrolled impurity, nitrogen is contained in all steels and its percentage varies depending on the technological conditions of production. In the proposed steel, the nitrogen content must be regulated within certain limits. However, depending on the content of the above nitride-forming elements in the steel, the nitrogen content must correlate with their total percentage and balance with the ratio between titanium, boron and oxygen as deoxidizing agents. The formula has the following c. d:

% T1 +10% B% 0

When the nitrogen content is excessive relative to the sum of the nitride forming elements, the tendency of the steel to age and embrittlement increases due to the blocking of dislocations by nitrogen. An insufficient nitrogen content in relation to (% T1 +% A1 + 10-% B) leads to the formation of carbonitride inclusions mainly due to carbon, which entails a decrease in% C in solid solution and softening of steel, especially after improving heat treatment. In addition, after quenching, the amount of residual austenite increases, which coarsens the structure after tempering. This effect on the structure of the proposed steel elements in the ratio

% Ti +% AI + 10% V

% N

in an amount of less and more than the specified limit reduces the resistance of the material to crack development under alternating loads in aggressive environments.

The degree of deoxidation of steel is determined primarily by the amount of oxygen. Therefore, in the proposed steel, the oxygen content should be stipulated by the ratio between the elements working as a deoxidizer and modifiers, i.e. titanium and boron and oxygen, which is shown on the left side of the balance as an expression

% T1 + 10% B% 0

In this ratio, the concentration limits should be balanced in comparison with the ratio obtained with maximum oxygen and minimum titanium and boron and vice versa. This measure is dictated by the fact that with maximum titanium and boron and minimum oxygen there is a risk of obtaining an excessive amount of modifiers that form inclusions, linking other elements, e.g. nitrogen, and reduce cyclic properties. An increase in the oxygen content indicates insufficient deoxidation of the steel, and in this case, the added microadditives will be consumed for deoxidation, which reduces the modification effect and does not lead to an increase in fracture resistance under alternating loads.

Thus, the proposed steel contains components in such an amount and the ratio between them and natural impurities that, after heat treatment, the dispersion of the structure and structural components is ensured, the grain boundaries are defective and the formation of the hardening phase, which in the complex leads to an increase resistance to fracture under alternating loads in aggressive environments by increasing the resistance of the material to crack development in the low-cycle loading region.

Example. The experimental steels were smelted in an induction furnace with a capacity of 60 kg with magnesite lining. The deoxidation was performed with aluminum. The casting of OCJF-c was carried out in cast-iron molds. Additional alloying was carried out to the bucket when introducing ligatures of ferrotitanium and ferroboron under a stream of metal discharged from the furnace

Table 1 shows the compositions of the steels proposed (1-4), for the proposed content of components (5 6) as well as the compositions of the prototype (7-9).

Ingots weighing 20 kg were rolled in a laboratory mill onto a strip 10 mm thick. Flat strips 2.5 mm thick were made from the strip for low cycle fatigue testing. The tests were carried out according to a rigid loading scheme with pure bending in air (A), as well as in air after exposure to pre-chemicals (B) and complex mineral fertilizers (C), in a 3.5% NaCl solution with cathodic polarization (G) and 25% aqueous solution of formic acid (D) Values of properties (average results of 3-5 tests) of steels

in the state of quenching from 850-870 ° С and tempering at ЗШ ° С are given in table 2

As can be seen from the table, the highest rates of low-cycle durability are of the steel of the proposed composition at

air tests As the activity of the low-cycle medium increases, the durability of both the proposed and the steels outside the claimed composition decreases. The steel x 7-9 prototype offered

the ratios are only partially fulfilled; therefore, the structure does not fully meet the requirements ensuring a high level of low-cyclic durability

The increased properties of the steels of the proposed composition make it possible to increase the durability of the working bodies of tillage machines operated in aggressive environments (56) USSR Copyright Certificate

No. 557120. cl. C 22 C 38/14 1977

Table

Note The number of test cycles is given before the destruction of the sample.

Claims (1)

17 Claims STEEL containing carbon, manganese, silicon and aluminum. titanium, boron, iron, characterized in that it contains components in the following ratio, wt.%: Carbon.ZO - 0.37 Manganese1.10-1.63 2002849 eighteen Silicon 0.40 - 0.60 Aluminum 0.02 - 0.04 Titanium 0.02 - 0.04 Bor0.002 - 0.004 . IronOther when the ratio of 50x titanium + manganese 10x carbon 0.63-10.5.