35MnB steel has been widely used in crawler chassis parts of construction machinery due to its good hardenability and heat treatment process performance, such as roller wheel body, crawler chain rail link, tooth block and other wear-resistant parts.
1. Effect of main elements in 35MnB steel
Due to the poor working conditions of its products, it is required to be used in the quenched and tempered state.
Hardenability and hardenability, as an important index of quenched and tempered steel, are usually ensured by strictly controlling the elements such as C, Si, Mn, B, Cr (the main influence factors of hardenability) in the steel.
1.1 Effect of C content
The carbon content in the steel determines the hardness that can be achieved after quenching. The higher the carbon content, the higher the quenching hardness, but it is easy to crack, and the plasticity and impact toughness are reduced.
For important parts such as crawler chassis, in order to reduce the influence of carbon content fluctuation on surface hardness and quenching layer depth, it is necessary to put forward the requirement of selecting carbon content, and generally control the upper and lower limits of carbon content within the range of 0.05%.
1.2 Effect of Si content
In addition to improving the strength and hardenability, the silicon in steel can also remove the gas in steel and play a calming role during steelmaking.
However, with the increase of Si content, the plasticity and toughness of the steel decrease, and it is easy to form banded structure.
1.3 Effect of Mn content
Mn, the main alloy element of 35MnB steel, can effectively improve the hardenability of the steel and reduce the critical cooling rate.
Mn forms solid solution in ferrite during heating, which can improve the strength of the steel.
Mn steel is often used when the depth of the hardened layer is greater than 4mm.
Because it reduces the critical cooling rate, it can obtain more uniform quenching hardness when the cooling specification is not stable.
It can be seen from the figure below that when the content of Mn in the steel is 1.10%, the strength of the steel is greatly improved, the plasticity is reduced very little, and the toughness is also slightly improved, as shown in Fig. 1 and 2.
If the content of Mn exceeds the above amount, the hardenability will be improved and the strength will be improved, while the toughness will be sharply decreased.
Fig. 1 Effect of alloying elements on solid solution strengthening
Fig. 2 Effect of alloying elements on impact energy of ferrite
1.4 Effect of B content
In the quenched and tempered high-strength steel, the addition of alloying element B can improve the hardenability.
The mechanism is that a small amount of B is dissolved in high-temperature austenite.
During the cooling process, it will segregate at the austenite grain boundary and inhibit the ferrite nucleation at the grain boundary, so as to improve the hardenability of the steel, especially under the condition of low cooling speed.
However, B in steel is an active element, which is easy to combine with N to form stable BN. These BNS are insoluble at quenching temperature, and the effective B content of solid solution in austenite is reduced, which reduces the effect of B on improving hardenability.
Therefore, it is necessary to add nitride forming elements, fix N elements, and maintain the solid solution amount of B in austenite, so as to improve the hardenability.
It is also pointed out that when the content of B exceeds 30ppm, the plasticity and toughness of the material will drop sharply.
Alloy elements V, Ti, Al and B are strong nitride forming elements in steel, and their binding ability with n is tin, BN, AlN and VN from strong to weak.
When the element is added to the steel containing B, the N in the steel will preferentially precipitate tin or Ti (C, N), and the precipitation start temperature is generally higher than 1400 ℃, which is much higher than the precipitation start temperature of BN.
With the decrease of temperature, the proportion of solid n in tin will increase, thus greatly fixing the N in steel, preventing the formation of BN, increasing the effective B Solid Solution Content in austenite, and improving the hardenability.
Therefore, it is necessary to reasonably control the tin ratio in the steel to increase the effective B content.
The data shows that the ideal value of the tin ratio is 3.42.
When the ratio is less than 3.42, there will be more residual N, and BN precipitation will increase. The effective B content will be greatly reduced, the hardenability of the steel will be reduced, and the brittleness will be increased.
Considering the harmfulness of tin, the residual N content in steel must be strictly controlled.
1.5 Effect of Cr content
Cr is an element that strongly increases the hardenability of steel.
Because Cr element of medium carbon chromium steel can increase the phase transformation incubation period, the isothermal transformation curve moves to the right;
At the same time, with the increase of Cr content, pearlite transformation moves to high temperature and bainite transformation moves to low temperature.
Therefore, when a proper amount of chromium is added to the steel, even if the cooling speed is slow during the quenching process, the undercooled austenite will not produce pearlite transformation and bainite transformation before reaching the martensite transformation temperature, thus significantly improving the hardenability of the steel.
However, Cr also significantly promotes the temper brittleness of nickel and manganese steels. Therefore, Cr content in 35MnB steel is strictly controlled.
The influence of Trace Cr on the hardenability of 35MnB crawler link steel points out that even the change of Cr content (Cr ≤ 0.20%) in the residual range has a great influence on the hardenability, especially when the Cr content is greater than 0.10%, the hardenability of the steel can be greatly improved, especially the hardness value of the point far from the water-cooled end.
As shown in the figure below, the quenching hardness can be increased by 2 ~ 3HRC on average within the range of 1.5 ~ 20.0m from the water-cooled end;
When the distance from the water-cooled end is more than 20.0m, the hardness increases more, about 6HRC.
The diameter of quenchable round bar of 35mnb steel containing Cr0.18% is about 20mm larger than that of steel containing Cr0.02%.
Fig. 3 Effect of Cr content on Hardenability
Because Cr can form carbides, it needs to increase the heating temperature and prolong the heating time, which is unfavorable to induction hardening.
2. Harm of Tin in 35MnB steel
During the steel-making process, due to the high melting point of tin, the molten steel can precipitate tin in liquid phase before casting and solidification, that is, the liquid precipitated tin particles are generally 2-10 μ m.
The shape is mostly square, rhombus and triangle (different from BN, as shown in Fig. 6), and the hardness is extremely high (greater than 1000V).
As shown in Fig. 4 and Fig. 5, it can not be deformed by any processing, nor can it be solved by high-temperature solid solution, and the impact energy dispersion is large.
Fig. 4 observation under tin light microscope
Fig. 5 observation of tin under electron microscope
Fig. 6 observation of BN under electron microscope
Fig. 7 is a solubility product curve in liquid iron at 1400 ℃, 1450 ℃ and 1500 ℃;
It can be seen from the figure that when the temperature of molten steel at the beginning of solidification is 1500 ℃, there will be liquid precipitation and tin precipitation when the content of N in the steel is 80ppm and the content of Ti exceeds 0.043%, and there will be liquid precipitation and tin precipitation when the content of N in the steel is 40ppm and the content of Ti exceeds 0.086%.
When the final solidification temperature of dendrite molten steel is 1400 ℃, when the content of N in the steel is 80ppm and the content of Ti exceeds 0.012%, there will be liquid precipitation and tin precipitation;
When the content of N in the steel is 40ppm and the content of Ti exceeds 0.024%, there will be liquid precipitation and tin precipitation.
Fig. 7 tin solubility product curve
Therefore, in order to completely avoid the occurrence of liquidus tin, it is necessary to reasonably adjust the content of Ti and N in the steel to suppress the precipitation of liquidus tin when the molten steel starts to solidify, and at the same time, appropriately increase the cooling speed during casting to suppress the precipitation in the last solidified molten steel (accelerate the cooling speed so that there is not enough time to start precipitation in dynamics).
According to the calculation of the solubility product of tin in liquid iron, the final solidification temperature during smelting and pouring is about 1495 ℃, and the equilibrium solubility product of tin is 0.00302 at this time.
If the content of n is controlled at 80ppm, the maximum amount that can be dissolved in liquid iron at the final solidification temperature is 0.0413%.
In order to avoid liquid precipitation of tin, the chemical composition is designed to have a content of Ti ≤ 0.0413%.
If the nitrogen content is controlled at 60ppm, the maximum T content that can be dissolved in the liquid iron at the final solidification temperature is 0.05%.
In order not to produce liquidus tin, the design Ti content of the steel chemical composition is ≤ 0.05%.
In order to increase the effective B content in 35MnB steel, the N content in the steel should be reduced to below 60ppm.
If the liquid phase precipitation tin exceeds 6μm, the fatigue life and impact toughness of the material will be greatly reduced.
If the liquid phase precipitation tin exceeds 6μm, it shall be judged as Al2O3 brittle inclusion.
Such hard and brittle inclusions as tin and Al2O3, MgO · Al2O3, Cao · Al2O3, etc. do not have plasticity under the deformation temperature.
It is very easy to separate from the body structure during deformation, thus damaging the continuity of the body.
In serious cases, cracks or cavities appear at the edge of the undeformed inclusion!
In the service process, the stress concentration is easy to occur under the action of alternating stress, which becomes the source of metal fatigue.
Good material composition control is the first step to ensure material performance.
35MnB material composition (melting composition w%) is recommended: