Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis

22SiMn2TiB steel (also known as 616) is a martensitic heat-resistant high-strength special steel, which has extremely high temperature comprehensive mechanical properties.

It is used to manufacture high-temperature structural parts and protective devices, and is widely used in the fields of electric power, ships and weapons industry.

A product of our company uses 22SiMn2TiB special steel plate.

The design requires high hardness (420hbw) and high comprehensive properties: yield strength is 1200N/mm2, tensile strength is 1500N/mm2, elongation is 7%, and impact toughness is 20J.

Due to the large size of the steel plate (3-4.2m long and 1.5m wide), it is difficult to ensure the performance and uniformity of the steel plate, and it is also difficult to ensure the flatness of the steel plate after heat treatment and prevent quenching deformation.

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 1

In view of the above problems, we have conducted a study on the heat treatment process of 22SiMn2TiB special steel plate, which provides a reference for solving the technical problems of quenching large-sized steel plate.

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: ℃)

Ac1Ac3Ar1Ar3
730874625728

(3) Grain size of steel

Due to the addition of boron in the steel, the grain size of the steel is easy to be coarse.

The comparative analysis is as follows: the austenitizing treatment (water cooling) at different temperatures and holding times was carried out by rapid heating (warm charging) of 22SiMn2TiB samples with different boron contents.

The actual austenite grain size grade was evaluated according to GB6394-1986 standard.

The inspection results shown in Fig. 1 and Fig. 2 show that this steel is an essentially fine grain steel, and the austenite grain grew obviously after > 1000 ℃.

Within the specified time and composition range of the test, the grain size has nothing to do with the holding time.

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 2

Fig. 1 Relationship between austenite grain size and temperature

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 3

Fig. 2 Relationship curve between austenite grain size and time

However, during slow heating, the grain size is coarse and mixed with the grain size of 7-7.5 (see Table 2).

Therefore, when preparing the heat treatment process, the temperature rise speed during the heat treatment shall be strictly controlled, and the temperature rise shall be as fast as possible to avoid the occurrence of coarse grains and mixed grains, so as to ensure the hardenability, hardenability and good comprehensive properties of the steel.

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 the Steel Research (see Fig. 3).

It can be seen from the test that when the composition of steel is qualified, the hardenability of each furnace steel is basically the same.

The semi-martensite layer (calculated by HBW = 340) is 24-39mm, and the full martensite layer (calculated by HBW = 420) is ≥ 22mm.

However, it can also be seen from Fig. 3 that when the chemical composition is not suitable, the hardenability is greatly affected.

Therefore, the chemical composition of the steel should be strictly controlled.

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 4

Fig. 3 hardenability curve of 22SiMn2TiB steel plate

Note: ID5244 chemical composition exceeds the lower limit.

(C: 0.18%, B: 0.0005%) its hardenability is very poor.

The full martensite layer is between 5 and 6mm.

Only when the finished plate is ≤ 12mm, the hardenability can meet the requirements.

(4) Quenching and tempering properties of 22SiMn2TiB steel plate

As shown in Fig. 4, the first type of temper brittleness of steel occurs between 270 and 400 ℃, and the second type of temper brittleness occurs between 500 and 600 ℃.

That is, the workpiece should avoid the tempering brittle zone during tempering to ensure good comprehensive mechanical properties of the workpiece.

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 5

Fig. 4 change curve of properties with tempering temperature (quenching at 920 ℃)

(5) Develop process plan

The steel plate shall be heated quickly to avoid the reduction of the comprehensive properties of the steel plate due to the influence of the grain size of the steel plate due to the slow heating speed.

That is to say, the furnace is charged to the temperature to quickly rise to the quenching temperature.

The heating temperature shall not exceed 1000 ℃, and the quenching heating temperature is determined to be 920 ℃.

The tempering temperature is set at 180 ℃ by considering the properties of the steel and the temper brittle zone.

Considering the alloy element content and carbon content, the carbon equivalent of 22SiMn2TiB steel plate is calculated according to the calculation formula of carbon equivalent as follows:

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 6

That is, [C] is less than 0.75%, and the carbon content in the positive segregation zone of the 22SiMn2TiB steel plate is less than 0.31%.

The workpiece can be cooled by water quenching.

Therefore, in order to ensure the hardenability of the steel plate to the maximum extent and meet the high comprehensive mechanical performance requirements, it is decided to adopt water quenching and control the water temperature between 15 and 30 ℃.

According to the hardenability of the steel, 22SiMn2TiB steel plate is selected with the thickness specifications of 10mm, 15mm and 20mm respectively.

When the chemical composition is qualified, the steel plate shall be able to be transformed into full martensite after quenching, and the hardness shall be more than 420HBW.

The process is formulated as follows: the quenching temperature is 920 ℃, the tempering temperature is 180 ℃, the cooling medium is water, the quenching heating and holding time is determined according to the thickness of the steel plate as 3min / mm, and the Brinell hardness indentation diameter dB10 = 2.8 ~ 3.0mm, equivalent to 420 ~ 480HBW, for hardness testing after tempering.

2. Implementation of process plan

(1) Quenching roller hearth furnace production line ensures performance and uniformity

We introduced the 22SiMn2TiB steel plate quenching roller hearth furnace production line, which consists of three parts: quenching furnace, quenching press and high-pressure water storage facilities.

The quenching heating furnace enables the steel plate to be automatically heated back and forth in the furnace to reduce deformation during the heating process.

At the same time, the advanced temperature control system is adopted to ensure the uniformity of the furnace temperature.

Keep heating the steel plate evenly.

In the quenching press, the upper and lower indenters can simultaneously press the steel plate to prevent deformation.

At the same time, spray holes are set on the upper and lower indenters.

The quenching is carried out by cooling the upper and lower quench water at the same time, which ensures the uniformity of cooling of the steel plate during quenching and reduces the phenomenon of large deformation due to uneven cooling on both sides.

For the high-pressure water storage facilities, we set the water storage tank at a height of 20m above the ground.

The water is ejected at a high speed with great pressure to ensure rapid cooling.

(2) Self-made tempering fixture to prevent deformation

Self-made tempering groove steel frame is used for tempering of steel plate, and 2T square iron static pressure is placed on the steel plate to reduce the deformation generated during tempering.

(3) Three roll straightener ensures the flatness of correction

Due to the extremely high hardness of the 22SiMn2TiB steel plate after quenching, we have introduced the three-roll straightening machine, which can realize the flatness correction of the steel plate after quenching to reach the tolerance range.

(4) Production route arrangement

After the steel plate is put into storage, the steel plate shall be cut into heatable sizes according to the production capacity of the equipment for heating and quenching, then rough correction shall be carried out, and then the steel plate shall be cut into a single piece and then corrected.

Finally, all steel plates meet the design requirements and are assembled on the 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.

Heat Treatment of Special Steel Plate (22SiMn2TiB): Process Analysis 7

Fig. 5 hardness testing position of 22SiMn2TiB steel plate

4. Conclusion

After hundreds of heats of production, the hardness of the steel plate is tested after heat treatment.

The hardness of all parts on the plane is within the tolerance range dB10 = 2.8 ~ 3.0, and all properties meet the requirements.

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