Automotive Steel - IF, BH, DP, TRIP, TWIN and other grades

Steel accounts for approximately 55% of a car's weight and is used primarily for the structural components of the body and chassis. Over the past several decades, steel production methods have improved significantly, resulting in steels that are both strong and ductile, or formable during processing and highly resistant after the paint-baking process. This article discusses the types of steel used in the automotive industry.

Automotive Steels – General Overview

Structural steels used in the automotive industry:

• Soft, ductile deep-drawing steels: 
  • Al-killed steels
  • Interstitial-free (IF) steels 
• High-strength deep-drawing steels or HSS :
  • IF-HS steels
  • Bake-hardenable (BH) steels
• AHSS (Advanced High Strength Steel)
  • Dual-phase (DP) steels
  • Complex-phase (CP) steels
• Manganese AHSS steels:
  • TRIP steels
  • TWIP steels

Al-Killed Steel

Description: Carbon is dissolved in the steel or occurs in the form of iron carbides, giving the material high ductility and softness. Susceptible to strain aging. 

Standards: These steels often meet the requirements of DC01 and DC03.

• DC01 steel:
  • carbon content below 0.12%; may contain Ti or B
  • yield strength Re: <280 MPa
  • tensile strength Rm: 270–410 MPa
  • elongation A: 28%
• DC03 steel:
  • carbon content below 0.10%; may contain Ti or B
  • yield strength Re: <240 MPa
  • tensile strength Rm: 270–370 MPa
  • elongation A: 34%

Interstitial-Free (IF) Steel

Description: Carbon and nitrogen are bound into stable compounds and are not present in the ferrite matrix, making this steel extremely ductile and very soft. Effective removal of C and N from ferrite requires alloying additions of Al, Ti, and Nb. Resistant to strain aging.

Typical properties:

• r_s value: 1.9–2.4
• Tensile strength Rm: 330–390 MPa

Standards: Only interstitial-free (IF) steels meet the requirements of DC06 and DC07.

• DC06 steel:
  • carbon content below 0.02%; Ti+Nb content below 0.3%;
  • yield strength Re: <170 MPa
  • tensile strength Rm: 270–330 MPa
  • elongation A: 41%
• DC07 steel:
  • carbon content below 0.01%; Ti+Nb content below 0.2%;
  • yield strength Re: <150 MPa
  • tensile strength Rm: 250–310 MPa
  • elongation A: 44%

High-Strength IF Steel

Description: Contains alloying additions of titanium, niobium, and vanadium. It provides high strength while retaining excellent deep-drawability. Resistant to strain aging.

Standards: Selected flat products according to PN-EN 10268:2008:

• HC180Y:
  • composition: C<0.01; Si<0.3; Mn<0.7; P<0.06; Ti<0.12
  • tensile strength Rm> 340 MPa
  • elongation A> 36%
  • aging coefficient r: >1.7 
  • strain hardening exponent n: >0.19
• HC220Y
  • composition: C<0.10; Si<0.3; Mn<0.9; P<0.08; Ti<0.12
  • tensile strength Rm> 350 MPa
  • elongation A> 34%
  • aging coefficient r: >1.6 
  • strain hardening exponent n: >0.18
• HC260Y
  • composition: C<0.01; Si<0.3; Mn<1.6; P<0.08; Ti<0.12
  • tensile strength Rm> 350 MPa
  • elongation A> 34%
  • aging coefficient r: >1.6 
  • strain hardening exponent n: >0.17

Steel strengthened during BH paint curing

Automotive application: car body.

Description: these steels are very soft and ductile in the as-delivered condition, which enables cold forming. A paint coating is then applied to the product, after which the entire component is placed in an oven for curing. During paint curing, a significant increase in the yield strength of the steel also occurs (by at least 30 MPa), along with a slight increase in tensile strength. Paint curing takes place at a temperature of 150–250°C for 15–20 minutes. Steel sheets strengthened during paint curing can be obtained by recrystallization annealing of low-carbon steel strip in a bell-type furnace or continuously, or by recrystallization annealing of interstitial-free steel strip in a continuous process.

Standards: selected flat products according to PN=EN 10268:2008:

• HC180B:
  • composition: C<0.05; Si<0.7; Mn<0.7; P<0.06;
  • tensile strength Rm> 300 MPa
  • elongation A> 34%
  • aging coefficient r: >1.6 
  • strain hardening exponent n: >0.17
• HC220B:
  • composition: C<0.06; Si<0.5; Mn<0.7; P<0.08
  • tensile strength Rm> 320 MPa
  • elongation A> 33%
  • aging coefficient r: >1.5 
  • strain hardening exponent n: >0.16
• HC260B:
  • composition: C<0.08; Si<0.5; Mn<0.7; P<0.1
  • tensile strength Rm> 360 MPa
  • elongation A> 29%
• HC300B:
  • composition: C<0.1; Si<0.5; Mn<0.7; P<0.12
  • tensile strength Rm> 400 MPa
  • elongation A> 26

Dual phase DP steel

Automotive applications: frames, side members, pillars, seats, side impact reinforcement beams, critical safety components.

Description: multi-phase steels whose microstructure consists mainly of a ferritic matrix and martensite. They are strengthened during the material preparation stage. These steels are processed at temperatures close to the critical temperature, where the phases coexist. The low carbon content ensures good weldability. In automotive applications, they enable a 50% reduction in weight while maintaining the same component strength. In addition, they are formable, corrosion-resistant, and capable of absorbing significant amounts of energy. 

Standards: selected flat products according to PN=EN 10336:2007 (C denotes cold rolling, and D denotes hot rolling):

• HCT450X:
  • composition: C<0.14; Si<0.8; Mn<2.0; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 260-340 MPa
  • tensile strength Rm> 450 MPa
  • elongation A> 27%
  • strain hardening exponent n: >0.16
  • bake hardening index BH₂: >30 MPa
• HCT500X:
  • composition: C<0.14; Si<0.8; Mn<2.0; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 300-380 MPa
  • tensile strength Rm> 500 MPa
  • elongation A> 23%
  • strain hardening exponent n: >0.15
  • bake hardening index BH₂: >30 MPa
• HCT600X:
  • composition: C<0.17; Si<0.8; Mn<2.2; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 340-420 MPa
  • tensile strength Rm> 600 MPa
  • elongation A> 20%
  • strain hardening exponent n: >0.14
  • bake hardening index BH₂: >30 MPa
• HDT580X:
  • composition: C<0.17; Si<0.8; Mn<2.2; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 330-460 MPa
  • tensile strength Rm> 580 MPa
  • elongation A> 19%
  • strain hardening exponent n: >0.13
  • bake hardening index BH₂: >30 MPa
• HDT780X:
  • composition: C<0.18; Si<0.8; Mn<2.5; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 450-560 MPa
  • tensile strength Rm> 780 MPa
  • elongation A> 14%
  • bake hardening index BH₂: >30 MPa
• HDT980X:
  • composition: C<0.23; Si<0.8; Mn<2.5; P<0.08; Cr+Mo<1.0; Nb+Ti<1.15
  • yield strength R_p0.2 600-750 MPa
  • tensile strength Rm> 980 MPa
  • elongation A> 10%
  • bake hardening index BH₂: >30 MPa

Complex phase CP steel

Automotive applications: door beams, reinforcements, sills.

Description: steel containing upper and lower bainite, martensite, as well as a small amount of ferrite and less stable austenite than TRIP steel. The strip is produced in hot rolling mills with controlled cooling of the strip on run-out tables after the final rolling pass.

Standards: selected flat products according to PN=EN 10336:2007:

• HCT600C:
  • composition: C<0.18; Si<0.8; Mn<2.2; P<0.08; S<0.015; Al<2.0; Cr+Mo<1.0; Nb+Ti<0.15; V<0.2
  • yield strength R_p0.2 350-500 MPa
  • tensile strength Rm> 600 MPa
  • elongation A> 16%
• HCT980C:
  • composition: C<0.23; Si<0.8; Mn<2.2; P<0.08; S<0.015; Al<2.0; Cr+Mo<1.2; Nb+Ti<0.15; V<0.22
  • yield strength R_p0.2 700-900 MPa
  • tensile strength Rm> 980 MPa
  • elongation A> 7%

Transformation induced plasticity TRIP steel

Automotive applications: sills, door reinforcements, pillars, controlled crumple zones.

Description: steel with very high strength and good plastic properties. A characteristic feature is that phase transformations occur during mechanical deformation. This enables the absorption of significant amounts of energy, as the metal transforms impact energy into steel strengthening.

Standards: selected flat products according to PN=EN 10336:2007:

• HCT690T:
  • composition: C<0.32; Si<2.2; Mn<2.5; P<0.12; Cr+Mo<0.6; Nb+Ti<0.2
  • yield strength R_p0.2 430-550 MPa
  • tensile strength Rm> 690 MPa
  • elongation A> 23%
  • strain hardening exponent n: >0.18
  • bake hardening index BH₂: >40 MPa
• HCT500X:
  • composition: C<0.32; Si<2.2; Mn<2.5; P<0.12; Cr+Mo<0.6; Nb+Ti<0.2
  • yield strength R_p0.2 470-600 MPa
  • tensile strength Rm> 780 MPa
  • elongation A> 21%
  • strain hardening exponent n: >0.16
  • bake hardening index BH₂: >40 MPa

Twinning induced plasticity TWIP steel

Automotive applications: door beams and reinforcements, bumper beams.

Description: modern steels. Similarly to TRIP steels, impact causes steel strengthening, which gives these steels both high strength and ductility at the same time.