How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel?

Concept and classification of inclusions

1. Endogenous inclusion

During the smelting process of steel, deoxidation reaction will produce oxides and other products.

If these products do not float out before solidification of molten steel, they will remain in the steel;

Mn+FeO → Fe+MnO

Si+2FeO → SiO2+2Fe

2Al+3FeO → 3Fe+Al2O3

Ti+2FeO → 2Fe+TiO2

The impurity elements such as oxygen, sulfur and nitrogen dissolved in the molten steel are precipitated in the solid solution during cooling and solidification, and finally remain in the ingot.

The distribution of endogenous inclusions is relatively uniform and the particles are small.

Correct operation and reasonable process measures can reduce the number of inclusions and change their composition, size and distribution, but it is generally inevitable.

2. Foreign inclusions

The slag suspended on the surface of molten steel during smelting and pouring, or the refractory materials or other inclusions peeled off from the inner walls of the steel-making furnace, tapping trough and ladle, etc. are not removed in time before the solidification of molten steel and remain in the steel.

It is the inclusion produced by the contact between metal and external substances during smelting.

The general characteristics of such inclusions are irregular shape, large size and irregular, also known as coarse inclusions.

Such inclusions can be avoided by proper operation.

Classification by chemical composition:

Nonmetallic inclusion

  • Sulfide: FeS, MnS
  • Oxides: FeO, Al2O3
  • Silicates: 2MnO·SiO2
  • Nitrides: TiN, ZrN

Classification by deformation capacity:

Nonmetallic inclusion:

  • Brittle inclusion: Al2O3
  • Plastic inclusion: FeS, MnS, 2MnO·SiO2
  • Invariant inclusion: SiO2

Classification by morphology and distribution:

Nonmetallic inclusion:

  • Class A – hydrophobic compounds
  • Class B – Alumina
  • Class C – Silicates
  • Class D – Spherical oxides
  • Class Ds – Single particle spheroid

Class A (sulfide): single gray inclusion with high ductility and wide range of morphology ratio, generally with rounded ends.

Class B (alumina): most of the particles are not deformed, angular, with small shape ratio (generally < 3), black or blue, and there are at least 3 particles in a row along the rolling direction.

Class C (silicate): single black or dark gray inclusion with high ductility and wide range of morphology ratio (generally ≥ 3), generally with acute angle at the end.

Class D (spherical oxide): undeformed, angular or circular, with small morphology ratio (generally < 3), black or bluish, irregularly distributed particles.

Class Ds (single particle spherical): round or nearly round, single particle inclusions with diameter ≥ 13μm.

Table 1 rating limits (minimum)

Rating chart level i

Inclusion category

A. Total length (um)

B total length (um)

C Total length (um)

 

D quantity

S diameter (um)

0.5

37

17

81

1

3

1

127

777

6

41

9

1.5

261

84

769

2

7

2

436

43

201

63

8

2.5

649

555

102

55

3

3

898(<1181)822(<1147)

46(<1029)3

6(<49)7

6(<107)

Note: the total length of the above class A, B and C inclusions is calculated according to the formula given in Appendix D, and the nearest integer is taken.

Table 2 inclusion width

Category

Fine system

Coarse system

Minimum width

(um)

Maximum width

 (um)

Minimum width

(um)

Maximum width

(um)

A

2

4

>4

12

B

2

9

>9

15

 

2

5

>5

12

D

3

8

>8

13

Note: the maximum size of class D inclusions is defined as the diameter.

Impact on service performance

1. Fatigue performance decreases;

2. The impact toughness and plasticity decrease;

3. The corrosion resistance decreases.

The inclusion with size less than 10μm promotes the nucleation of the structure, and the structure grain grows during welding.

(1) Due to the addition of Nb, V, Ti and other alloy elements, C and N compounds (a kind of micro inclusions) will be precipitated during continuous casting and heating;

(2) Calcified sulfides, silicates and fine ferrous oxide can refine the crystal nucleus.

It is beneficial to the toughness, plasticity and strength of the steel plate.

When the size of non-metallic inclusions is greater than 50μm, the plasticity, toughness and fatigue life of the steel are reduced, and the cold and hot working properties and even some physical properties of the steel are deteriorated.

Generally, the size of inclusions in our molten steel is greater than 50μm.

The toughness, plasticity and strength of steel plate are not used for large inclusions.

In addition to these properties, there are also reduced acid resistance, fatigue performance, surface finish and welding performance.

Influence on process performance

1. It is easy to crack during forging, cold working, quenching, heating and welding.

2. The surface quality after rolling and the surface roughness of the parts after grinding are reduced.

Influence on strength and elongation of steel plate

When the inclusion particles are relatively large (> 10μm), especially when the inclusion content is low.

The yield strength and tensile strength of the steel are obviously reduced;

When the inclusion particles are small to a certain size (< 10μm), the yield strength and tensile strength of the steel will be improved.

When the amount of small particles in the steel increases, the yield strength and tensile strength of the steel increase, but the elongation decreases slightly.

Influence on fatigue performance

It is generally believed that inclusions are the origin of fatigue failure of steel.

Brittle inclusions and spherical undeformed inclusions with weak binding force and large size have great influence on fatigue performance, and the higher the strength, the greater the hazard, as shown in Fig. 1.

For high-strength steel, if the surface of the component is well processed, the crack initiation and inclusion become the main fatigue cracking mode.

Small size inclusions may have little effect on crack nucleation, but they are beneficial to fatigue crack propagation.

Fig. 2 is a schematic diagram of void formation and growth around small inclusions.

It is believed that dimples are related to inclusions smaller than 0.5 mm.

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 1

Fig. 1 inclusion size and fatigue life under the same stress level

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 2

Fig. 2 Schematic diagram of formation of micro voids between non adjacent inclusions

Failure examples:

The elastic shaft of an equipment motor breaks after running for a period of time.

Fig. 3 shows the macroscopic morphology of the fracture.

It can be seen from the direction of the macroscopic fatigue stripes on the fracture surface and the direction of the radial stripes that the crack originates from the surface of the elastic shaft and corresponds to a longitudinal stripe on the shaft surface.

Because the fracture surface at the crack initiation is severely worn, the morphological characteristics of the original fracture surface are not clear.

According to the macroscopic and microscopic observation of the elastic shaft that has not been broken for a period of time, as shown in Fig. 4, there are longitudinal cracks of different degrees on the surface of the elastic shaft, and there are non-metallic inclusions in the crack occurrence part.

The energy spectrum analysis results show that the non-metallic inclusions in the cracks are aluminum oxide.

The spherical oxide inclusions and single particle spherical inclusions of the elastic shaft of the motor are grade 2.0.

The main reason for the premature fracture of elastic shaft is the fatigue fracture caused by the inclusion as the core forming fatigue source under the action of alternating stress.

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 3

Fig. 3 macroscopic appearance of fracture of elastic shaft of fractured motor

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 4

Fig. 4 SEM analysis of inclusions in elastic shaft

Influence on corrosion resistance

The non-metallic inclusion in steel is an important reason to reduce the corrosion resistance of steel.

There are different chemical positions between the non-metallic inclusions and the base steel, and it is easy to form a micro cell with the base steel.

Once there is an environmental corrosion medium, electrochemical corrosion will occur, forming corrosion pits and cracks, and serious cases will lead to fracture failure.

Failure example: the heating water pipe leaks prematurely, and the material is Q235B carbon structural steel.

Fig. 5 (a) shows the macroscopic appearance of the leaking water pipe.

The surface of the water pipe near the leakage point has been corroded.

After the oxidation and corrosion products are removed, it can be seen that there are obvious grooves in the welds where the leakage point is located, as shown in Fig. 5 (b).

After comprehensive analysis of metallography, inclusions, energy spectrum and simulated accelerated corrosion test on the leaked water pipe and the original water pipe, it is concluded that the oxide inclusions or composite oxide inclusions penetrating the inner surface at the weld joint are the main reasons for local corrosion, formation of corrosion grooves and premature leakage of the water pipe.

Under the action of corrosive media such as O2, S, Cl, etc. contained in the pipe, the non-metallic inclusions will form a corrosion cell with adjacent metal iron to cause electrochemical corrosion, resulting in the final water pipe leakage.

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 5

Fig. 5 macroscopic appearance of leaking water pipe

Influence on hydrogen induced delayed fracture

The hydrogen invading into the material or the hydrogen generated by the electrochemical interaction between the medium and the surface of the material will continue to diffuse under certain conditions, and it is easy to aggregate and combine into hydrogen molecules at the traps such as inclusions.

When the pressure of hydrogen molecules at the traps exceeds the strength limit of the material, crack nuclei will be formed.

With the continuous diffusion and aggregation of hydrogen, the macro fracture of the material will eventually be caused.

There are many factors affecting hydrogen induced cracking, but for a specific steel, the influence of inclusions is the most important except for the influence of process factors.

Inclusions are strong traps of hydrogen.

The hydrogen pressure around non-metallic inclusions (especially long MNS) is very high, and the bonding strength between inclusions and matrix is relatively weak.

With the increase of hydrogen pressure, cracks will be generated at the interface between inclusions and matrix.

The nucleation probability of hydrogen induced cracks at inclusions is high.

The higher the level and quantity of inclusions, the greater the susceptibility to hydrogen induced cracking.

Failure example: the 200 m3 LPG storage tank of a LPG company is made of 16Mn, with a plate thickness of 24mm and a working pressure of 1.18 MPa.

After many years of use, 54 bulges were cracked on the surface of the spherical tank, of which 20 had been cracked.

Through metallographic examination, SEM and energy spectrum analysis, it is found that there are serious MNS inclusions in and around the drum, and hydrogen is contained in the drum.

The reason for the bulging is that the hydrogen infiltrated into the steel accumulates and forms the bulging at the inclusion matrix interface defect due to the cathodic hydrogen evolution reaction.

The surface crack of the bulging is the hydrogen induced delayed fracture under the action of tensile stress.

Fig. 6 shows the macroscopic appearance of bulge on the inner and outer surfaces of the storage tank.

Fig. 7 is the micro morphology of the inner wall surface of the drum and the surface distribution of Mn and S elements.

The serious non-metallic inclusion is the material factor of forming hydrogen blister and cracking of blister.

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 6

Fig. 6 macroscopic appearance of tank drum

How Do the Non-metallic Inclusions in Steel Affect the Quality of Steel? 7

Fig. 7 micro morphology of inner wall surface of drum and distribution diagram of Mn and S elements

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