The 22SiMn2TiB steel, also known as 616, is a special high-strength, martensitic, and heat-resistant steel with excellent comprehensive mechanical properties at extremely high temperatures.
This steel is primarily used for manufacturing high-temperature structural parts and protective devices and finds extensive applications in the electric power, shipbuilding, and weapons industries.
Our company’s product is made of 22SiMn2TiB special steel plate with a design specification that demands high hardness (420Hbw) and exceptional comprehensive properties, including a yield strength of 1200 N/mm2, tensile strength of 1500 N/mm2, elongation of 7%, and impact toughness of 20J.
However, due to the large size of the steel plate, which measures between 3 to 4.2 meters in length and 1.5 meters in width, it is challenging to maintain the uniformity and performance of the steel plate. Moreover, it is also challenging to ensure the flatness of the steel plate after heat treatment and prevent quenching deformation.
After reviewing the aforementioned issues, we conducted a study on the heat treatment process of 22SiMn2TiB special steel plate. Our findings offer valuable insights into resolving the technical challenges associated with quenching large-sized steel plates.
1. Formulation of process parameters
(1) Chemical composition (mass fraction,%) of 22SiMn2TiB:
C: 0.19 ~ 0.25, Si: 0.70 ~ 1.00, Mn: 1.50 ~ 1.85, P ≤ 0.025, S ≤ 0.020, B: 0.0005 ~ 0.0035, Ti: 0.01 ~ 0.06.
(2) The supercooled austenite curve of 22SiMn2TiB shows that the steel must reach a certain cooling rate if it is quenched into martensite.
The transformation point of steel is shown in Table 1.
Table 1 phase change point (unit: ℃)
| Ac1 | Ac3 | Ar1 | Ar3 |
| 730 | 874 | 625 | 728 |
(3) Grain size of steel
The addition of boron to the steel results in a coarse grain size.
To compare, 22SiMn2TiB samples with varying boron content underwent austenitizing treatment (water cooling) at different temperatures and holding times through rapid heating (warm charging). The actual austenite grain size was assessed according to the GB6394-1986 standard.
Inspection results depicted in Fig. 1 and Fig. 2 reveal that this steel is predominantly fine-grained. However, the austenite grain size increased significantly beyond 1000 ℃.
Within the specified composition range and time limit of the test, the holding time had no correlation with the grain size.
Fig. 1 Relationship between austenite grain size and temperature
Fig. 2 Relationship curve between austenite grain size and time
However, during slow heating, the grain size becomes coarse and mixed with grains measuring 7-7.5 (as shown in Table 2).
Therefore, when preparing for the heat treatment process, it is essential to strictly control the temperature rise speed and aim for a faster temperature rise to prevent the formation of coarse and mixed grains. This is necessary to ensure the steel’s hardenability, as well as its overall strength and durability.
Table 2 austenite grain size of different heating methods
|
B（T）（%） |
Austenite grain size (grade) |
||
|
Rapid heating |
Slow heating (1 ℃ / mm) |
Insulation steps are added in the two-phase zone |
|
|
0.0005 |
9.5 |
7.0 |
8.5 |
|
0.0020 |
9.0 |
7.5 |
8.0 |
|
0.003 |
9.5 |
7.5 |
8.0 |
A comparative test on the hardenability of steel was conducted during Steel Research (refer to Fig. 3).
The test shows that, when the steel composition is qualified, the hardenability of each furnace steel is basically the same.
The semi-martensite layer (calculated using HBW = 340) is between 24-39mm, and the full martensite layer (calculated using HBW = 420) is ≥ 22mm.
However, as shown in Fig. 3, when the chemical composition is not suitable, the hardenability is greatly affected.
Therefore, strict control of the chemical composition of the steel is necessary.
Fig. 3 hardenability curve of 22SiMn2TiB steel plate
Please note that the chemical composition of ID5244 exceeds the lower limit, with a carbon content of 0.18% and boron content of 0.0005%. Consequently, its hardenability is very poor, and the full martensite layer measures between 5 and 6mm.
To meet the hardenability requirements, the finished plate should be no more than 12mm thick.
(4) Quenching and tempering properties of 22SiMn2TiB steel plate
As depicted in Figure 4, steel exhibits the first type of temper brittleness between 270 and 400 ℃, whereas the second type occurs between 500 and 600 ℃.
Therefore, to ensure the workpiece’s good comprehensive mechanical properties, it should be kept away from the tempering brittle zone during tempering.
Fig. 4 change curve of properties with tempering temperature (quenching at 920 ℃)
(5) Develop process plan
In order to prevent the reduction of the comprehensive properties of the steel plate caused by the influence of its grain size, the steel plate should be heated rapidly. This means that the furnace should be charged to the required temperature to quickly reach the quenching temperature.
The heating temperature should not exceed 1000 ℃, and the quenching heating temperature is determined to be 920 ℃.
The tempering temperature of 180 ℃ is selected based on the properties of the steel and the temper brittle zone.
To calculate the carbon equivalent of 22SiMn2TiB steel plate, the alloy element content and carbon content are taken into consideration, and the carbon equivalent is determined using the calculation formula for carbon equivalent.
The carbon content in the positive segregation zone of the 22SiMn2TiB steel plate is less than 0.31%, and the total carbon content is less than 0.75%.
Water quenching can be used to cool the workpiece.
To maximize the hardenability of the steel plate and meet the high comprehensive mechanical performance requirements, water quenching will be adopted, and the water temperature will be controlled between 15 and 30 ℃.
The 22SiMn2TiB steel plate will be selected with thickness specifications of 10mm, 15mm, and 20mm based on the hardenability of the steel.
Assuming the chemical composition is qualified, the steel plate should be able to transform into full martensite after quenching, with a hardness greater than 420HBW.
The process will be as follows: quenching temperature of 920 ℃, tempering temperature of 180 ℃, cooling medium of water, and quenching heating and holding time determined by the thickness of the steel plate at 3 minutes per millimeter.
The Brinell hardness indentation diameter dB10 will be between 2.8mm to 3.0mm, which is equivalent to a hardness of 420 to 480HBW. After tempering, hardness testing will be conducted.
2. Implementation of process plan
(1) Quenching roller hearth furnace production line ensures performance and uniformity
We would like to introduce our production line for quenching roller hearth furnace of 22SiMn2TiB steel plate, which comprises three main components: quenching furnace, quenching press, and high-pressure water storage facilities.
Our quenching heating furnace has an automatic feature that moves the steel plate back and forth to prevent deformation during the heating process. Additionally, we have implemented an advanced temperature control system to ensure uniformity in furnace temperature, which facilitates the even heating of the steel plate.
In our quenching press, the upper and lower indenters are capable of pressing the steel plate simultaneously, preventing any deformation. The press also features spray holes, which ensure uniform cooling during quenching. This is achieved by cooling both the upper and lower quench water simultaneously, which eliminates any large deformation due to uneven cooling on either side.
Regarding our high-pressure water storage facilities, we have installed a water storage tank at a height of 20m above the ground. By doing so, water can be ejected at high speeds and great pressure, ensuring rapid cooling.
(2) Self-made tempering fixture to prevent deformation
A self-made tempering groove with a steel frame is used for tempering steel plates. Additionally, a 2-ton square iron static pressure is applied to the steel plate to minimize the deformation that may occur during the tempering process.
(3) Three roll straightener ensures the flatness of correction
To address the challenge posed by the high hardness of the 22SiMn2TiB steel plate after quenching, we have implemented the use of a three-roll straightening machine. This machine enables us to correct the flatness of the steel plate after quenching and achieve the required tolerance range.
(4) Production route arrangement
After the steel plate is placed into storage, it should be cut into sizes suitable for heating and quenching based on the equipment’s production capacity. Rough correction should then be conducted before the steel plate is cut into single pieces and further corrected.
Ultimately, all steel plates must meet the design requirements and be assembled into the final product.
Route: blanking → quenching → tempering → correction → single piece blanking → correction.
3. Process test
(1) Blanking
According to the three specifications and heat number of plate thickness, one steel plate is put into each for process test, and the size is 1500mm × 4200mm.
(2) Heat treatment
Quenching and tempering process shall be carried out according to the above parameters.
(3) Check
After heat treatment, hardness test is carried out on the steel plate.
The test parts are shown in Fig. 5.
After testing, the indentation diameters of Brinell hardness are 2.8mm, 2.9mm, 2.85mm and 3.0mm, which meet the requirements.
Fig. 5 hardness testing position of 22SiMn2TiB steel plate
4. Conclusion
After hundreds of rounds of production, the hardness of the steel plate is tested following heat treatment.
The hardness of all parts on the plane falls within the tolerance range of dB10 = 2.8~3.0, and all properties meet the necessary requirements.
