CS216366B1 - Structural steel for cheaioco-heat treatment and heat treatment - Google Patents

Abstract

The invention relates to low-alloy structural steel, suitable for chemical-thermal processing such as annealing, in machine, highly stressed parts, e.g. gears. The essence of the invention lies in that the steel has a chemical composition according to the weight of 0.10 to 0.65% carbon, 0.40 to 1.50% manganese, traces of up to 0.35% silicon, 0.30 to 1.50% iron, impurities such as phosphorus not more than 0.035% and sulfur not more than 0.080%, further contains 0.012 to 0.070% nitrogen and 0.005 to 0.05% soluble aluminum, the rest iron. The properties of the steel can be improved by a weight content of 0.10 to 0.50% molybdenum and 0.04 to 0.30% lead. The steel according to the invention is advantageous for further processing by heat-treatment processes such as tempering, nitrocarburization, etc. The steel according to the invention is particularly suitable for the production of gears, springs and other machine parts, for which high hardness of functional surfaces and impact resistance are desired.

Description

The essence of the invention lies in the fact that the steel has a chemical composition according to the content of 0.10 to 0.65% carbon, 0.40 to 1.50% manganese, traces of up to 0.35% silicon, 0.30 to 1.50% iron, impurities such as phosphorus at most 0.035% and sulfur at most 0.080%, further contains 0.012 to 0.070% nitrogen and 0.005 to 0.05% soluble aluminum, the rest is iron. The properties of the steel can be improved by a weight content of 0.10 to 0.50% molybdenum and 0.04 to 0.30% lead.

The steel according to the invention is advantageous for further processing by heat-treatment processes such as tempering, nitrocarburization, etc. The steel according to the invention is particularly suitable for the production of gears, springs and other machine parts, for which high hardness of functional surfaces and impact resistance are desired.

216,366

The invention relates to structural steel for chemical-thermal treatment and refinement, such as carburizing, nitrocarburizing, and with the aim of achieving favorable properties of surface-hardened layers of highly stressed machine parts, as well as parts with higher strength, treated by hardening and tempering.

The type of steel plays a significant role in the final properties of machine parts and their efficient production, especially in the economical chemical-thermal processing. Most of these requirements are met primarily by steels alloyed with a combination of chromium, molybdenum, or nickel.

These steels are mostly characterized by a low chromium content, of the order of around 0.5% Cr by weight, which contributes to lower deformations during hardening for carburizing. The acceptable hardenability of the cemented layer with a higher carbon content is influenced by the addition of molybdenum in the range of 0.2 to 0.5% by weight. Steels with the addition of molybdenum are fine-grained with a grain size of 7/8 according to ASTU. They also resist grain coarsening well during ohmic-heat treatment. Steels intended for the most exposed parts tend to also have a nickel addition in the range of 1.5 to 4.0% by weight. The carbon content in these steels, mainly in the range of 0.10 to 0.25% by weight, influences the required strength and possibly also the toughness of the core of the case-hardened steel.

The constantly decreasing availability and rising price of molybdenum and nickel on world markets force the search for substitute steel types, mostly with manganese and chromium additives, or their combination. The most common mass contents of manganese are in the range of 0.75 to 1.40%, chromium 0.50 to 1.15%, and the mass content of carbon is usually in the range of 0.14 to 0.24%. Today, substitute steel types also include steels with boron additives, most often with mass contents in the range of 0.0005 to 0.0030%. The boron additive is used in a number of steel types alloyed with Mn, Cr, Mo, Ni, or their combinations. We mostly encounter reduced contents of the listed alloying elements in common steel types without boron. The disadvantage is that higher chromium contents and also the addition of boron only increase the hardenability of the starting steel. These elements practically do not increase the hardenability and the required structural homogeneity of the cemented layer.

In addition, for steels with boron addition, there is a relatively low production certainty to achieve reliable boron efficiency on hardenability, requiring special measures to ensure effective oxygen and nitrogen binding. The additive used for nitrogen binding is most often titanium. The resulting products such as titanium carbides and nitrides lead, on the other hand, to a deterioration in machinability,

In connection with the high demands on the machinability of steels for carburizing, great attention is paid to increasing the machinability of these steels. The simplest way to increase machinability is to increase the mass content of sulfur in the steel in the range of 0.020 to 0.080 56. We also encounter the addition of lead in the range of 0.04 to 0.30 56. Finally, there is also the issue of controlling the morphology and properties of inclusions by calcium-based ferroalloy additives while simultaneously limiting the proportion of aluminum.

Manganese-chromium steels are among the most widespread group of steels processed into machined parts intended for cementation. In these steels, the mass content ranges from 0.14 to 0.22% carbon, 1.00 to 1.40 56 manganese, 0.80 to 1.30% chromium, phosphorus and sulfur individually at most 0.035 56. MmCrTi steels with titanium addition are produced to a relatively limited extent.

0.04 to 0.10% by weight. We also encounter a smaller range of use in steels alloyed with nickel, namely the MnCrNi or CrNi type. This type of steel has a chromium content of around 1% by weight and a further graded nickel content in the range of 0.40 to 4.70% by weight. The carbon content in these steels is in the usual range from 0.10 to 0.24% by weight. So far, we encounter the use of MnCrB steel with a carbon content of 0.28 to 0.35% in a relatively limited range.

C and manganese and chromium contents characteristic of MnCr type steel. Increased hardenability is sometimes achieved by adding boron 0.001 to 0.005 by weight in chromium-manganese steels with a carbon content of 0.28 to 0.35%.

Generally, the chromium content of the steels used is around 1% by weight. This results in low hardenability of the cemented layer, which is also heterogeneous and consists of carbides and martensite. MnCr steels with nickel additions, or CrNi steels, have higher hardenability and homogeneity of the cemented layer.

The issue of uniform machinability, or increasing machinability, has so far been addressed to a limited extent by increased sulfur contents, i.e. 0.020 to 0.035% by weight, or even above this limit. To a limited extent, the issue of inclusion morphology is also addressed by calcium additives. Limited attention has also been paid to the issues of high-temperature annealing at temperatures of the order of 980°C with the aim of grain coarsening and thus embrittlement of the steel to increase machinability.

The actual austenitic grain size of commonly produced steels for case hardening is mostly in the range of 5 to 7 according to ASTM. In many steels produced by melts, we encounter austenitic grain coarsening even at common exposure temperatures during case hardening of 920 to 930 °C. The mostly low resistance to grain coarsening and higher chromium contents of the order of around 1% in most of the steels used practically exclude hardening from case hardening temperatures and by cooling, which is important both from an energy point of view and from the point of view of the final properties of machine parts.

From the above evaluation it is clear that the key issue for the effective solution of steels for chemical-thermal treatment is the fine grain of the steel and high resistance to grain coarsening at carburizing temperatures, suitable hardenability of the starting steel and the cemented layer, good chemical-thermal processability with the possibility of direct hardening from carburizing temperatures, the price of steel, influenced by the availability of alloying elements and good and uniform machinability of steels of individual types and brands. These issues are all the more serious because steels for carburizing are mostly processed in the field of mass and serial mechanical engineering production. From the price point of view and also from the point of view of the availability of alloying elements, steels alloyed with manganese, manganese, or a combination of both elements are generally very attractive today. The key element of steels for carburizing high-quality machine parts is undoubtedly molybdenum, or possibly nickel, especially in terms of hardenability, properties and structure of the cemented layers. Steels with molybdenum usually also have higher resistance to grain growth during carburizing. They usually allow direct hardening and generally have higher hardenability than cemented carbides.

In connection with steels for case hardening, the importance of nitrogen as a microalloying element has been neglected. On the other hand, we very often encounter simultaneous saturation of layers during case hardening with both carbon and nitrogen with the aim of improving the properties of the layers or making the process technology more efficient. Nitrogen in low-alloy steels is usually in the range of 0.005 to

0.02.2% by weight and its nitrided form are among the key issues of resistance to austenitic grain cracking. In current technical practice, it is mostly an aluminum additive, commonly used for the final deoxidation and denitrification of steel, which creates conditions for the formation of nitrides, necessary to prevent grain growth at exposure temperatures during carburizing. In the production of case-hardening steels, characterized by a low carbon content, mainly in the range of 0.14 to 0.24%, there is usually a significantly different aluminum recovery from 20 to 80%. At the same time, we encounter a different proportion of the active component of aluminum, bound to nitrogen and evaluated as soluble aluminum. Its value is a certain criterion for assessing resistance to grain growth, and therefore sometimes indicates the required minimum value of 0.020% soluble aluminum. It cannot be overlooked that the conditions for the formation of aluminum nitride are significantly influenced by the temperature history of cast and wrought steel. In addition, the mutual ratio of soluble aluminum and nitrogen content, or the value of their product, also significantly interact here.

Microanalytically, we have demonstrated significant chemical inhomogeneities in the distribution of aluminum in steel, influenced by crystallization processes and solidification of steel. It has also been shown that the maximum and minimum aluminum contents in microregions depend on the value of the soluble aluminum content, determined by conventional chemical analysis. At aluminum contents up to 0.015%, it is necessary to count on microregions practically free of aluminum, which is reflected in coarser austenitic grains in these microregions. The consideration of the possibility of eliminating these chemical microinhomogeneities by diffusion annealing is very problematic. The size of the atomic radius of aluminum, which is larger than that of iron and is stated to be 0.128 µm, also has an unfavourable effect here. It has also been shown that increasing the soluble aluminum content, e.g. to 0.070%, leads to the formation of coarser nitrides, which are not very effective in terms of resistance to grain growth. With the usual nitrogen content of 0.007 to 0.012% and soluble aluminum of 0.910 to 0.030% in low-alloyed low-carbon steels, under normal production and processing conditions, only a partial binding of nitrogen to aluminum occurs, the value of which, 0.005%, ensures good resistance to grain growth, for example in steels of the LehlinCr type, which has a common grain size of 5 to 7 according to ASTM. The remaining free nitrogen in low-alloyed steels is bound to nitride-forming alloy elements such as chromium, manganese and molybdenum, and possibly also iron, i.e. to elements with a relatively high content, compared to the aluminum content. On the other hand, the mentioned alloy elements have a significantly lower negative value of the free enthalpy ÁG for nitride formation, compared to aluminum, and also less stability when heated to the austenitizing temperature. During austenitizing heating, a significant part of the nitrogen content is in unbound form in a solid solution of iron

The above disadvantages are eliminated by the low-alloy structural steel intended for chemical-thermal treatment and refining according to the invention. The essence of the invention lies in the fact that the steel contains in amounts by weight of 0.10 to 0.65% carbon, 0.40 to 1.50% manganese, traces of up to 0.35% silicon, 0.30 to 1.50% iron, impurities such as phosphorus from traces to 0.035% and zinc from traces to 0.080%, and also 0.012 to 0.070% nitrogen and 0.003 to 0.050% soluble aluminum, the rest iron. Further improvement of the properties of the steel can be achieved by adding molybdenum 0.10 to 0.50% by weight, or further by adding lead 0.04 to 0.30% by weight.

.4

The results of the orientation tests led to the microalloying of low-alloyed manganese-chromium steel intended for chemical-thermal processing. It was confirmed, in particular, that the addition of nitrogen contributes to a significant refinement of the austenitic grain, increases the hardenability of both the base steel and the cemented layer. No less significant is the increase in the hardness of the cemented layer. This is due primarily to the fact that the addition of nitrogen increases the activity of carbon in austenite. This issue is particularly serious for steels low-alloyed with chromium, when the occurrence of chromium carbides in the layer is suppressed, thereby increasing hardness and hardenability.

The addition of nitrogen also leads to a reduction in transformation temperatures, and thus the possible use of lower temperatures for chemical-thermal processing and hardening. Possible lower hardening temperatures clearly contribute to the formation of smaller deformations. In conjunction with favorable hardenability parameters, optimal conditions are created for lower cooling rates, even for larger components.

An equally significant effect resulting from the fine-grained structure during nitrogen microalloying is the favorable values of notch toughness in the hardened state. On the other hand, normalizing annealing with slow cooling leads to a significant decrease in toughness, primarily due to the formation of nitrides of alloying elements. The deteriorated notch toughness here creates suitable conditions for good machinability of the steel.

Steel according to the composition example contains in mass concentration 0.16% carbon, 0.78% manganese, 0.35% silicon, 0.028% sulfur, 0.95% chromium, 0.018% soluble aluminum, 0.033% nitrogen, the rest iron. Steel is produced by conventional metallurgical processes. For alloying nitrogen, it is possible to use suitable organic substances, nitrogen-rich ferroalloys, or even a gaseous mixture with nitrogen or its compound.

Steel of this composition has the following basic parameters:

size according to ASTM 9/l0

Austenitic grain coarsening threshold ®C 1,050 to 1,080

Hardenability

Notch toughness KCV at 20 °C

J HV-ll mm 290 cemented layer

J 58 HRC - am 42 sample hardened 850 ®C/oil tempered 180 ®C/air 60

R g KV 1 245 MPa

What other example of composition would we give for steel with a mass content of 0.17% carbon, 1.06% manganese, 0.34% silicon, 0.022% phosphorus, 0.020% sulfur, 0.98% chromium, 0.010% soluble aluminum, 0.027% nitrogen, the rest iron.

Steel of this composition has the following basic parameters:

ύ 3&·

Austenitic grain ASTM size Roughing threshold ®C 8/9 1,030 to 1,080 J HV-11 mm 300 Hardenability cement layer 3 58 MRO - mm 30 Notch toughness sample hardened 850 ®C/oil METAL at 20 °0 - J.cm“ ² temp. 180 ®C/air 40 R from HV 1 245 MPa n

see other 2 examples /steel with Me, steel and Pb/

From the above it is clear that the low-alloy steel according to the invention, microalloyed with nitrogen, meets most of the requirements placed on steels for chemical-thermal treatment. It can successfully replace a number of steel brands alloyed with scarcely available molybdenum, or replace part of the molybdenum or nickel in the steel. The steel according to the invention can also be used for the production of hardened and tempered machine parts, such as springs.

The steel according to the invention is particularly suitable for the production of gears and other machine components which are subjected to chemical-thermal treatment with the aim of achieving high hardness of functional surfaces and with high demands on resistance to impact stress.

Steel according to another embodiment contains in a mass concentration of 0.20% carbon, 0.29% silicon, 0.023% phosphorus, 0.018% sulfur, 0.60% chromium, 0.017% soluble aluminum, ^0.1023 % nitrogen and 0.22% molybdenum, the remainder being iron.

-Steel of this composition has the following basic parameters: size according to ASTM

Austenitic grain _coarsening threshold C

Hardenability

J HV - 11 mm cemented layer J 58 - HRC - mm

9/10

070 to 1,100

315

Notch toughness sample hardened 850 °C/oil

METAL at 20 °C J.om“ ² tempered 180 °C/air 65

R _a z HV 1 320 MPa

The steel according to another embodiment contains a mass concentration of 0.22% carbon,

0.25% silicon, 0.019% phosphorus, 0.018% sulfur, 0.80% chromium, 0.019% soluble aluminum,

0.025% nitrogen and 0.08% lead, the rest iron.

Steel of this composition has the following basic parameters»

Austenitic grain size according to ASTM coarsening threshold *C

8/9

060 to 1,080

Forgiveness

Notch toughness of METAL at 20 °C J. cm“ ²

J HV - 11 srn 330 cemented layer

J 58 HRC - no 45 sample hardened 850 ®C/oil tempered 180 ®C/air 45

R _B from HV 1 290 MPa

Claims (3)

SUBJECT OF THE INVENTION A constructional ooel for chemical-heat treatment and refining, characterized in that it contains in amounts by weight of 0.10 to 0.65% carbon, 0.40 to 1.50% manganese, traces up to 0.35% silicon 0.30 to 1.50% of chromium, impurities such as phosphorus traces up to 0.035% and sulfur traces up to 0.080%, further 0.012 to 0.070% nitrogen and 0.005 to 0.050% aluminum soluble, the remainder iron. Structural steel according to claim 1, characterized in that it contains 0.10 to 0.50% by weight of molybdenum. 3. Structural steel according to claim 1, characterized in that it contains 0.04 to 0.30% by weight of lead.