What is Bake Hardening Steel?

Bake hardening steel, also known as BH steel, is a low carbon steel designed for automotive applications. This steel is made by adding trace amounts of titanium, niobium or molybdenum to the steel during the manufacturing process. The result is a steel that is highly formable and easy to machine, yet capable of hardening when exposed to high temperatures.

What are the advantages of bake hardening steel?

One of the main advantages of bake-hardening steel is its ability to increase the strength of body components without adding weight. This is achieved through a process called bake hardening, in which the steel is heated to high temperatures and then cooled rapidly. This process hardens the steel, making it stronger and more durable.

Another benefit of bake-hardened steel is its ability to resist corrosion. Adding titanium, niobium or molybdenum to steel creates a protective oxide layer that prevents rust and other forms of corrosion from forming. This makes bake-hardening steel ideal for automotive applications, where exposure to moisture and road salt can cause severe damage to unprotected metal parts.

What are the Bake Hardening Steel Grades?

Bake hardening steel (BH steel) is a low carbon steel commonly used in the automotive industry to produce body parts that require high strength and good formability. There are several different grades of BH steel to choose from, each with its own unique properties and characteristics.

Some common bake-hardening steel grades include:

BH180: This grade of BH steel typically has a yield strength of 180 MPa after the bake hardening process.

BH220: The yield strength of BH220 steel after bake hardening is higher, reaching 220 MPa, which is suitable for body parts that require higher strength.

BH260: BH260 steel is a high-strength grade of BH steel, and its yield strength can reach 260MPa after baking hardening.

BH300: BH300 steel is another high-strength grade of BH steel, and the yield strength after bake hardening can reach 300 MPa. This makes it suitable for body parts that require very high strength.

BH340: BH340 steel is the steel with the highest strength among BH steels, and its yield strength can reach 340MPa after bake hardening. It is typically used in body parts that require the highest strength, such as crash parts.

These are just a few examples of the different grades of bake hardening steel that are available. The specific steel grade chosen will depend on the specific requirements of the body part being produced, such as the required strength and formability, and the manufacturing process that will be used.

What is the bake hardening effect?

The bake hardening effect is a phenomenon observed in certain types of mild steels, including bake hardening (BH) steels. When these steels are made into car parts and then subjected to a heat treatment process called "bake hardening," their strength and hardness increase.

The bake hardening process involves heating the steel to temperatures usually between 170-220°C (338-428°F) for a period of time, usually between 20-30 minutes, followed by rapid cooling. During this process, the steel is transformed, leading to an increase in the number of dislocations in the crystal structure, which in turn leads to an increase in strength.

The extent of the bake hardening effect depends on several factors, including the chemical composition of the steel, the amount of cold work performed on the steel during forming, and the conditions of the heat treatment process. In general, steels that are lower in carbon and higher in manganese and silicon are better suited for bake hardening because they deform more easily and can tolerate greater dislocation densities.

The bake hardening effect is particularly useful in the automotive industry, where BH steels are commonly used to produce body parts. By bake-hardening these parts, manufacturers can increase their strength and durability, allowing them to better withstand the stresses and strains of everyday use. Additionally, the added strength helps reduce the weight of the part, which improves fuel efficiency and reduces emissions.

Can bake hardening (BH) steel be used as dent resistant steel?

Yes, bake hardening (BH) steels are available as dent resistant steels. BH steel has high yield strength and excellent formability, making it ideal for automotive body components. When subjected to a bake-hardening process, the steel is transformed, increasing its strength and resistance to deformation, making it more suitable for dent-resistant applications.

The bake hardening process increases the dislocation density in the steel crystalline structure, which increases strength and hardness. This process also improves the steel's dent resistance, since the increased strength and hardness make it harder for the steel to deform under stress.

Therefore, BH steel is commonly used in the automotive industry to produce parts that require both good formability and dent resistance, such as body panels. The steel's ability to resist dents and deformation, combined with its light weight and formability, make it an attractive option for reducing vehicle weight and improving fuel efficiency.

Weldability of bake-hardened steels

Welding of bake-hardened steels (BH steels) requires careful consideration of the welding process and conditions in order to maintain the desired properties of the steel. BH steels have increased strength and hardness due to the bake hardening process, which affects the weldability of the material.

An important factor to consider is the welding heat input, which should be controlled to prevent overheating and softening of the steel. High heat input causes the steel to lose strength and hardness, reducing its resistance to dents and deformation.

Another consideration is the choice of welding method. In general, BH steels are best welded using low heat input methods, such as resistance spot welding or laser welding, to minimize the heat-affected zone and preserve the properties of the steel. However, other welding methods, such as gas metal arc welding (GMAW) or shielded metal arc welding (SMAW), can also be used with appropriate welding parameters and techniques.

In addition, preheating and post-weld heat treatment may be required to reduce weld stresses and maintain steel properties. Preheating slows the cooling rate of the weld, minimizing the potential for cracking, while post-weld heat treatment helps restore the steel's strength and hardness.

Aging of bake-hardened steel

Bake hardening steel (BH steel) is a high strength steel that has undergone a special heat treatment process to increase its strength and dent resistance. However, like other materials, BH steels age over time, which can affect their properties and performance.

The aging of BH steel occurs due to the slow diffusion of carbon and other elements in the steel, resulting in changes in its microstructure and properties. This aging process causes the steel to lose strength and hardness while increasing ductility. The degree of aging depends on factors such as steel composition, heat treatment process and environmental conditions.

To mitigate the effects of aging on BH steels, manufacturers typically use alloys with specific compositions and carefully control the heat treatment process to minimize the risk of aging. Additionally, proper storage and handling of steel after production can also help prevent premature aging.

There are four main methods of preventing aging in bake-hardened steel:

1. Cold working

2. High temperature heating

3. Add elements

4. Coating

What are the applications of bake hardening steel?

Bake-hardening steel is commonly used to produce automotive body components such as doors, hoods and trunks. These components require high strength and durability to withstand the stress of everyday use, and bake-hardened steel provides the perfect solution. Additionally, steel's ability to be easily formed means that complex shapes can be easily produced, allowing for more creative designs and lighter weight components.