Cold-Rolled Special Steels

The Cold-Rolled Special Steels Family Tree

Introduction to the Cold-Rolled Special Steels.
Carbon content greatly affects hardenability, wear resistance, and workability.
Click on any particular grade to see the details. Please refer to the table of contents below to see the properties of each steel.

Features

Features and Main Properties

1.1 Features

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1.2 Mechanical Properties

Approximate rolling reduction rate of each temper / finish is as follows.

Approximate strength of each steel grade by temper/finish is shown in the table below. (Steel grades not supplied by our company are also listed for reference.)

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1.3 Chemical Compositions

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Workability

2.1 Bending

1. In the case of bending processes, cold-rolled materials may sometimes be used, but normally annealed finish or skin-pass finish materials are used. 
2. Since strip steel is generally directional, especially when cold-rolled. it is necessary to avoid bending parallel to the rolling direction of a material when working.
3. If you perform a bending process in which the burr appears on the outside of the bend after cutting, shearing, or processing, cracks may form from the burr. It is necessary to make the burr a non-bending surface or to perform bending after removing the burr.
4. During bending, the spring-back of the material varies depending on the degree of processing and the finishing condition of the material. Still, appropriate compensation is required depending on the mold and processing method.
5. Bending workability varies depending on the material and processing conditions, but the following processing is possible depending on the finish state.（Mainly targets low carbon steels below SK5）

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Temper/Finish	Thickness:below 1mm	Thickness:over 1mm
Annealed
Skin Passed
（Lightly）As-Rolled

[Note] t…Thickness　R…Internal Bend Radius 　
[Reference] Approximation of Bending Power

[Note] t…Thckness　b…Width　L…Die Groove Width　σB…Tensile Strength of Material 　

2.2 Drawing

Apart from light drawing, an annealed or skin-passed finish is usually used because uniformity of drawing in all directions is required.

Cold-rolled special steel strip does not have the significant stretcher strain phenomenon of soft steel, but its drawing limit is lower.For deep drawing, intermediate annealing is applied and re-drawing is performed.

In the case of normal flat drawaing : Drawing Ratio=0.4
In the case of redrawing 　　　　　: Drawing Ratio=0.6　

Drawing Ratio= d / D
d…Diameter of Product to be Drawn　　　D…Diameter before Drawing

Heat Treatment

3.1 Precautions for Heat Treatment　

Almost all cold-rolled special steels are heat-treated by quenching and tempering according to their applications.
The following points should be noted during this heat treatment process.

(1) Heat treatment and cooling should be performed uniformly under appropriate conditions.
(2) Avoid decarburization, scale formation, high-temperature corrosion, etc as much as possible.
(3) Use a quenching method that minimizes quenching distortion.

3.2 Temperature of the Furnace and That of Materials

Generally, when performing heat treatment, the furnace temperature of the heat treatment furnace is measured and used as the material heating temperature.
However large discrepancies or irregularities may occur between the true material temperature and the measured furnace temperature,
it is necessary to examine the temperature characteristics and make appropriate temperature compensation carefully or change the heat treatment method.

3.3 Pretreatment and Atmosphere

The higher the carbon content of a cold-rolled special steel strip, the more likely it is to be decarburized, and the risk increases when heating at high temperatures.
High-temperature corrosion will occur if the material is heated with dirt or foreign objects adhering to it.
Therefore, pretreatment to clean the material surface and adjustment of the furnace atmosphere is necessary before heat treatment.

The standard furnace atmosphere is RX gas for quenching and NX gas for annealing, but other than these, N2 gas, H2 gas, AX gas, etc. are also used,
Neutral salt bath furnaces, metal bath furnaces, furnace tubes, and cases are also used to prevent direct exposure to the outside air.

3.4 Quenching

The material is typically heated to the approximate midpoint of the specified quenching temperature range, and then held for several seconds to several minutes, depending on the material, dimensions, shape, required properties, and quenching method.Quenching conditions have a significant impact on product performance. If the quenching temperature is too high or the holding time is too long, it can lead to grain coarsening and a reduction in toughness, as well as an increased risk of decarburization. Conversely, if the temperature is too low or the holding time too short, the material may not harden completely, or soft spots may appear. Therefore, selecting the appropriate conditions is crucial.

For cooling, oil or water is generally used. Water cooling results in harder quenching than oil cooling, but it carries a higher risk of quenching distortion and cracking. Therefore, in the case of cold-rolled special steels, oil cooling is typically used, with some exceptions.

To prevent quenching distortion, martempering is performed by raising the oil temperature. In special cases, quenching is done in salt baths or metal baths (austempering). For ribbon-shaped materials or those with simple shapes, methods such as flat quenching or press quenching are also used.

3.5 Tempering

Materials that have undergone quenching become harder but lose ductility, becoming brittle. Therefore, tempering is essential to achieve a material with both toughness and strength. 
Tempering conditions are determined according to the performance requirements of each product by referring to tests and the quenching and tempering performance curves of the steel grade.

Since the cold rolled special steel strip is small and tempering is often performed continuously, tempering may be performed for a long time when particularly high tenacity is required, but in general, tempering is performed for a short time, within a few minutes.
In cases where a short time is required for continuous operation, the tempering temperature may be raised slightly or the tempering may be performed repeatedly.

However, as a general rule, tempering at a lower temperature for a longer time will result in greater toughness than tempering at a higher temperature for a shorter time,
Therefore, it is necessary to avoid using tempering temperatures that are too high or time that is too short.
Tempering in an oil bath, metal bath, or salt bath can also reduce the time required for tempering compared to tempering in air.