Usually, cold rolled special steel strip is used during processing like cutting, punching, bending and drawing. According to the final usage of end products, we use different materials. Please let us know your requirement based upon the standard processing and mechanical property.
■ Heat treatmentMost of cold rolled special steel strip need to have quenching and tempering heat treatment depending on the use. The most important factors on heat treatment are:
- Equally heated and cooling by suitable condition.
- Avoid decarburization, scale and high temperature corrosion.
- Avoid quenching deformation.
■ Furnace temperature and material temperature
On heat treatment, normally furnace temperature is treated as material heating temperature. Practically, the actual material temperature has delta and stability concern versus the furnace temperature. Therefore, we need to check the temperature characteristic and do suitable adjustment on heat treatment.
■ Advanced treatment and handling
The higher carbon the steel contains, the higher chance the decarburization happens. The risk is increased while doing the high temperature heat treatment, especially on process like quenching. At the mean time, if material is heated with alien substance or pollution, high heating seizure and high heating corrosion would occur. Therefore, it is necessary to do material surface cleaning and atmosphere adjustment before heat treatment. Under the standard atmosphere, quenching uses RX gas, tempering uses NX gas and the others use N2、H2、AX gas.
Sometimes we use neutron salt-bath oven, metal bath oven, core and case to cut off outside air.
Usually we select the standard quenching temperature’s middle value as above. Depending on material’s quantity, size, shape, required performance, quenching processing will hold up to 10 seconds to several minutes.
Quenching processing has big influence over product performance .If quenching processing is over temperature or over time, grain growth or toughness reduction would occur, and the risk of decarburization would be increased. Vice versa, hardening would not be completed and soft spot might occur. Therefore, it is very important to pick a suitable solution for different materials’ quenching processing..
Oil or water is used for cooling within the industry. Water cooling is much harder than oil cooling, but it also has the risk of strain happening and quenching crack. Except some special matters, oil cooling is adopted on cold rolled steel strip.
To avoid strain on quenching processing, increase oil temperature for Martempering. Or, at some special conditions, we do quenching treatment with salt bath or metal bath (Austempering treatment).
| Type of steel | Quenching temperature ℃ |
|
Type of steel | Quenching temperature ℃ |
|---|---|---|---|---|
| TE 2 | 790～850 oil quenching （760～820 water quenching） |
SK S 2 | 830～880 oil quenching | |
| SK S 7 | 830～880 oil quenching | |||
| SK 4 | 790～850 oil quenching （760～820 water quenching） |
|||
| SK 5 | 790～850 oil quenching （760～820 water quenching） |
S70C | 790～850 oil quenching | |
| SK 6 | 790～850 oil quenching （760～820 water quenching） |
S60C | 800～860 water quenching | |
| SK 7 | 790～850 oil quenching （760～820 water quenching） |
S55C | 800～860 water quenching | |
| S50C | 800～860 water quenching | |||
| TNS42 | 790～850 oil quenching （760～820 water quenching） |
S45C | 800～860 water quenching | |
| M2 | 790～850 oil quenching （760～820 water quenching） |
S15C | 790～850 water quenching | |
| SK S 51 | 790～850 oil quenching （760～820 water quenching） |
SCM 415 | first stage 850～900 water quenching second stage 800～850 oil quenching |
■
Tempering
Tempering processing is always after quenching processing. Base on the finished products’ requirements, the tempering condition will depend on different material specification and quenching-tempering co-relation curve.
On cold rolled special steel strip, since the weight is low and continuous tempering processing is required. Also, a long time tempering processing is done to increase the toughness. However, fast tempering, by increasing the temperature, is mostly adopted within the industry for practical reason.
Generally, toughness would be increased better at LOW temperature LONG tempering processing, versus HIGH temperature SHORT tempering processing.
Tempering at oil bath, metal bath or salt bath can save more time air bath.
■
Annealing
In order to soften material and release stress, annealing will be applied. The suitable temperature is between 600 and 700℃. If the temperature is set too high, structure will be altered and the decarburization or scale might be occurred. Therefore, it is better to go through low temperature. Normally, 10~ 30 minutes cooling down to 200℃ and then to naturally cooling processing.
| Ferrite Series: | Its roll-up processing and mechanical property is similar to the middle carbon steel .To avoid stretcher stain, tempering roll-up material must go through the skin pass. |
| Martensite Series: | For low carbon type, the production processing is easier than Ferrite Series. However, the processing can be complex on higher carbon type after going through quenching for hardness increase. |
| Austenite Series: | On tensile strength, its proof stress is less but extension is higher. It is a suitable material for complex processing. Many factors like processing machine, dies, lubricate oil as well as operation flow need to be taken into consideration. |
■Heat Treatment
Quenching/Tempering: Martensite Series and Precipitation Hardening Series is the type of heat treatment hardness stainless steel. By quenching and tempering, the hardness of martensite series can be adjusted. Compared with carbon steel strip, it has such features as follow.
- High quenching temperature( Standard 1000-1070℃)
- Excellent quenching performance (Air cooling is also used, but it needs higher temperature, plus the hardness is lower.)
- Hardness would be decreased caused by tempering.
| Type of steel | Highest quenching hardness(Same as subzero treatment hardness) |
|---|---|
| TKK1 | 620～680 HV(650～720 HV) |
| SUS420J2 | 580～640 HV(590～670 HV) |
■ Annealing
- Soften material caused by air cooling processing
- Avoid grain boundary corrosion
- Precipitation or necessary element for solution
- Eliminate the magnetism caused by Austenite Series processing
| Standard annealing condition is as follow | ||
| Austenite Series | 1010℃-1150℃ | quick cooling |
| Martensite Series | 750℃-850℃ | slow cooling |
| Ferrite Series | 780℃-850℃ | quick cooling or slow cooling |