Conventional low strength steels

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来自 Pohang University of Science and Technology 的课程

Ferrous Technology II

13 个评分

Pohang University of Science and Technology

Ferrous Technology II

13 个评分

Steel, ever-evolving material, has been the most preeminent of all materials since it can provide wide range of properties that can meet ever-changing requirements. In this course, we explore both fundamental and technical issues related to steels, including iron and steelmaking, microstructure and phase transformation, and their properties and applications.

从本节课中

Properties of steels and their applications I

In the following 3 courses, different types of steels will be introduced with their properties and applications. This course typically covers some of the first generation steels containing conventional low strength steels and HSLA (high strength low alloy) steels. Detailed explanation on the microstructures and related properties will be treated.

Introduction3:28

Effect of heat treatment paths: microstructure2:30

Conventional low strength steels3:34

Micro-alloyed ferrite-base steels9:26

与讲师见面

Sung-Mo Jung
Professor
Pohang University of Science and Technology
Kim, Sung-Joon
Professor
Graduate Institute ofFerrous Technology
Kang, Youn-Bae
Professor
Graduate Institute of Ferrous Technology
Suh, Dong Woo
Associate Professor
Graduate Institute ofFerrous Technology
Kim, Nack Joon
Professor
Graduate Institute of Ferrous Technology

Now let's look at the details of steels.

Starting with low strength grade steels called conventional low strength steels.

These include interstitial free steels, mild steels, bake hardenable steels, high strength

interstitial free steels and carbon manganese steels.

As you can see in this figure, their tensile strength ranges from about 200 MPa to about

600 MPa, which is on the low side.

However these steels show very good ductility up to 50 percent.

So these steels usually show very good formability which is ideal for deep drawing process.

One of them, the mild steel, is based on ferrite with a small amount of pearlite and have good

drawing capability with low strength.

So this is the typical microstructure of mild steel showing the ferrite matrix with some,

small amount of pearlite.

However to have better formability than mild steels, a new type of steels called interstitial

free steels have been developed.

This steel is called interstitial free because they contain very small amount of interstitial

atom such as carbon and nitrogen.

As I just mentioned, interstitial free steel shows better formability and also higher work

hardening rate, which means n value, than mild steels.

However, the steels have somewhat lower strength, therefore higher strength version has been

developed, which utilize solid solution strengthening, precipitation of carbide and nitride and grain

refinement to increase strength.

So in this case, the control of alloy composition is needed.

That is microalloy concentration must exceed the stoichiometric ratio required to form

carbide and nitride, for example for 0.003 wt% each of C and N, you need 0.01 % titanium

and 0.02 % niobium.

Another type of steel that belongs to conventional low steels is the bake hardenable steels.

As you can expect from its name, strength of steel is increased during bake hardening

of the formed part.

So when the steel is subject to the forming process, for example from a to b, the strength

is increased by work hardening.

During the baking process, carbon atoms diffuse into dislocations making dislocation difficult

to move, which is known as strain aging phenomena.

Hence the formed part will have higher strength after baking and this steel, we have much

better dent resistance, which are ideal for outer panel of automobiles.

The last example of low strength steel is carbon manganese steel.

It's basically similar to mild steel, but they try to obtain higher strength so they

use higher solute content and therefore they can have higher strength, but lower ductility.

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