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.