Abstract :

20µ

ALLOY CONSERVATION THROUGH PRECIPITATION ALLOY CONSERVATION THROUGH PRECIPITATION STRENGTHENING BY MICROALLOY ADDITION IN STEEL STRENGTHENING BY MICROALLOY ADDITION IN STEEL

FOR AUTOMOTIVE APPLICATIONFOR AUTOMOTIVE APPLICATION Niranjan MoreNiranjan More, Sunil Nair Sunil Nair && K R SrinivasanK R Srinivasan

Mukand Ltd, MumbaiMukand Ltd, Mumbai

The effectiveness of enhancing metallurgical features through microalloying practice using Vanadium, Niobium or Titanium is by now established universally, with

Vanadium topping the list. It has been possible to obtain a good combination of strength, ductility and machinability characteristics through careful microalloying

percentages. The precipitation mechanism of Vanadium as a microalloying constituent for achieving the desired mechanical properties has been outlined in this

paper. With control on base microstructure through optimized chemistry and hot working temperature, it was established by industrial research trials that the

Microalloy technique could replace some of the conventional alloy steel grades, thus leading to alloy conservation. Typical properties obtained and their applications

have been outlined for two microalloyed grades, with Vanadium as the principal microalloying constituent. There is a huge potential for use of these grades in

different applications, replacing the conventional alloy grades.

1.0 INTRODUCTION1.0 INTRODUCTION 3.0 RESULTS3.0 RESULTS The industrial experience gained in developing two microalloyed steel

grades, 38MnVS6 and 27MnSiVS6 for autocomponent application is described in this presentation.

The strength of microalloyed steels is obtained through precipitation of microalloy constituents such as carbides and nitrides in the matrix microstructure of ferrite and pearlite.

Vanadium is preferable as a microalloy addition because of its solubility at normal hot working temperatures.

Scope exists for using this steel for auto component applications such as connecting rods, crankshafts, power steering rack bar and shaft applications.

38MnVS627MnSiVS6

Microalloyed Steels

Normalising38MnVS6, 27MnSiVS6

Still Air & Forced Air Cooled

Resulting properties studied: Microstructure properties Hardenability Tensile Properties Hardness, BHN

CHEMICAL COMPOSITION:

Grade Type %C %Si %Mn %P %S %Cr %Mo %V %Ti %AlN2

(ppm)

38MnVS6Microalloyed

Steel0.38 0.59 1.26 0.013 0.034 0.13 0.007 0.155 0.015 0.015 158

42Cr4Mo2Conventional

Alloy Steel0.42 0.22 0.75 0.025 0.023 1.05 0.22 - - 0.025

27MnSiVS6Microalloyed

Steel0.27 0.60 1.48 0.020 0.034 0.17 0.007 0.11 0.014 0.015 178

41Cr4Conventional

Alloy Steel0.43 0.22 0.85 0.020 0.020 1.15 - - - 0.022

MICROSTRUCTURE OF HOT ROLLED BAR RELATED TO COOLING

1000°C Still Air Cooled

W –ABS-131

1000°C Forced Air Cooled

1200°C Fan Cooled

Rack Bar 27MnSiVS6

38MnVS6

250

255

260

265

270

275

280

285

1000°C 1100°C 1200°C

Temperature

Har

dnes

s (B

HN

)

820

840

860

880

900

920

940

Ten

sile

str

engt

h (

Mpa

)

Forced Air Cooled Hardness Still Air Cooled Tensile strength

Forced Air Cooled Tensile Strength Still Air cooled Hardness

Solubility limits of Vanadium & Nitrogen at various austenitising temperatures

From the above curve it is possible to determine the hot working temperature for a given concentration of Vanadium and Nitrogen. In the heat manufactured by us Vanadium level was kept at 0.15% and Nitrogen at 0.015%. From the above graph the hot rolling temperature was decided at 1220 deg C. This temperature ensured that the austenite was homogeneous prior to hot working.

38MnVS6

Axle shaft44MnVS6

Coarse Microstructure

Fine Microstructure

EXAMPLES OF REPLACEMENT OF CONVENTIONAL QUENCH AND TEMPERED ALLOY STEELS WITH MICROALLOY GRADES

FOR AUTOCOMPONENT APPLICATION

Effect of hot working temperature on Hardness & Tensile strength of 38MnVS6

4.0 BENEFITS4.0 BENEFITSPROCESS STEP REDUCTION BY USE OF MICROALLOYED STEEL

7 step process; 4

heating & 3 shaping

3 step process; 1 heating & 2 shaping

Designation 0.2% Proof stress ( MPa )

UTS

( MPa )

Elongation ( % )

Post forging heat treatment

Air Cooled Quench & Temper

42CrMo4 680 min 850 - 1000 13

38MnVS6 (C38mod )

580 min 850 - 1000 12

41Cr4 560 min 850 -1000 12 27MnSiVS6 450 min 700 – 900 14

1000°C

1100°C

1200°C

MECHANICAL PROPERTIES:

TYPE STRENGTHENING MECHANISM

GRADE COMPOSITION

APPLICATIONS

Basic Precipitation of Vanadium Carbide

Medium C micro alloyed steel

Connecting Rod

Crank Shaft

Wheel Hub

Improved Toughness

Intensify precipitation of V compounds

Lower Carbon

Microalloyed steel with lower C, and small addition of Cr

Front Axle

Steering Knuckle

Rear hub support

Connecting Rod

High Toughness High Strength

Lower Carbon

Forms Martensite / Banite

Low Carbon Direct quenching

Knuckle Spindle

APPLICATION BASED DEVELOPMENT OF MICROALLOYED STEEL GRADES

1200°C

1100°C215

225

235

245

255

265

1000°C 1100°C 1200°C

Temperature

Har

dnes

s (B

HN

)

700

740

780

820

860

900

Tens

ile S

treng

th (M

pa)

Still Air Cooled Hardness Forced Air Cooled HardnessStill Air Cooled tensile strength Forced Air Cooled Tensile strength

Effect of hot working temperature on Hardness & Tensile strength of 27MnSiVS6

MECHANICAL PROPERTIES RELATED TO HOT WORKING TEMPERATURE AND COOLING CONDITION

2.0 STUDY CARRIED OUT2.0 STUDY CARRIED OUT

5.0 CONCLUSIONS5.0 CONCLUSIONS

1. Microalloyed steels are finding greater acceptance in the autocomponent industry due to its cost effectiveness w.r.t to processing and alloy composition.

2. With proper hot working temperature Vanadium goes completely into solution and precipitates uniformly as V(C,N) or VN particles giving maximum strength in the direct cooled post forged component.

3. Mechanical properties obtained in as forged condition are equivalent to the conventional Quench & Tempered alloy steel grades made of costly alloying elements like Mo and Cr. Therefore , considerable alloy conservation could be obtained with the use of Microalloy steel technology.

4. It is recommended that the hot rolling temperature be maintained as per solubility curves. It is preferable to have forced air cooling to obtain relatively finer grain size and better mechanical properties.

5. 38MnVS6 achieved very good results in Pulsation test as reported by our customer.

6. Studies related to microstructure and mechanical properties of higher Vanadium and Nitrogen grades are being carried out.