Metallurgy of Martensitic Steels

Martensitic steels are characterized by a microstructure that is mostly all martensite, but possibly also containing small amounts of ferrite and/or bainite (Figure 1 and 2). Steels with a fully martensitic microstructure are associated with the highest tensile strength – grades with a tensile strength of 2000 MPa is commercially available, and higher strength levels are under development.

Figure 1: Schematic of a martensitic steel microstructure. Ferrite and bainite may also be found in Small amounts.

Figure 1: Schematic of a martensitic steel microstructure. Ferrite and bainite may also be found in small amounts.

Figure 2: Microstructure of MS 950/1200

Figure 2: Microstructure of MS 950/1200

To create MS steels, the austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. Adding carbon to MS steels increases hardenability and strengthens the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability.

These steels are often subjected to post-quench tempering to improve ductility, so that extremely high strength levels can be achieved along with adequate ductility for certain forming processes like Roll Forming.

Figure 3 shows MS950/1200 compared to HSLA. Engineering and true stress-strain curves for MS steel grades are presented in Figures 4 and 5.

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and MS 950/1200

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and MS 950/1200.

 

Figure 4:  Engineering stress-strain curves for a series of MS steel grades.S-5 Sheet thicknesses: 1.8 mm to 2.0 mm.

Figure 4:  Engineering stress-strain curves for a series of MS steel grades.S-5  Sheet thicknesses: 1.8 mm to 2.0 mm.

 

Figure 5:  True stress-strain curves for a series of MS steel grades.S-5  Sheet thicknesses: 1.8 mm to 2.0mm.

Figure 5:  True stress-strain curves for a series of MS steel grades.S-5  Sheet thicknesses: 1.8 mm to 2.0mm.

 

In addition to being produced directly at the steel mill, a martensitic microstructure also can be developed during the hot stamping of press hardening steels.

Examples of current production grades of martensitic steels and typical automotive applications include:

MS 950/1200 Cross-members, side intrusion beams, bumper beams, bumper reinforcements
MS 1150/1400 Rocker outer, side intrusion beams, bumper beams, bumper reinforcements
MS 1250/1500 Side intrusion beams, bumper beams, bumper reinforcements

 

Some of the specifications describing uncoated cold rolled 1st Generation martensite steel (MS) are included below, with the grades typically listed in order of increasing minimum tensile strength. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.

  • ASTM A980M, with Grades 130 [900], 160 [1100], 190 [1300], and 220 [1500]A-23
  • VDA 239-100, with the terms CR860Y1100T-MS, CR1030Y1300T-MS, CR1220Y1500T-MS, and CR1350Y1700T-MSV-3
  • SAE J2745, with terms Martensite (MS) 900T/700Y, 1100T/860Y, 1300T/1030Y, and 1500T/1200YS-18

 

Case Study: Using Martensitic Steels

as an Alternative to Press Hardening Steel

Martensitic steel grades provide a cold formed alternative to hot formed press hardening steels. Not all product shapes can be cold formed. For those shapes where forming at ambient temperatures is possible, design and process strategies must address the springback which comes with the high strength levels, as well as eliminate the risk of delayed fracture. The potential benefits associated with cold forming are lower energy costs and improved cycle times compared with hot forming processes.

Both Toyota and Nissan are known to have transitioned some applications typically associated with press hardening steels to a cold stamped martensitic steel, CR1200Y1470T-MS. One of these examples is found in the 2020 Nissan B-segment hatchback, which used CR1200Y1470T-MS as the material for the Second Cross Member Reinforcement. K-45

Highlighting product forms achievable in cold stamping, an automotive steel Product Applications Laboratory formed a Roof Center Reinforcement from 1.4 mm CR1200Y1470T-MS using conventional cold stamping rather than roll forming, Figure 6. Using cold stamping allows for the flexibility of considering different strategies when die processing which may result in reduced springback or incorporating part features not achievable with roll forming.

Figure 4: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1

Figure 6: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1

 

Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature. Shown in Figure 7 is a Center Pillar Outer cold stamped using a tailor welded blank containing CR1200Y1470T-MS and CR320Y590T-DP as the upper and lower portion steels.U-1

Figure 5: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1

Figure 7: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1

 

Another characteristic of martensitic steels is their high yield strength, which is associated with improved crash performance. In a laboratory environment, crash behavior is assessed with 3-point bending moments. A studyS-8 determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat shaped parts. Based on a comparison of yield strength, Figure 8 shows that CR1200Y1470T-MS has similar performance to hot stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness, and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold stamping approach, providing appropriate press, process, and die designs are used.

Figure 6: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8

Figure 8: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8

 

 

Related Posts
Filter by
Post Page
Steel Grades AHSS 1stGen AHSS Press Hardened Steels Metallurgy Tailored Products Cutting Cutting-Blanking-Shearing-Trimming Testing and Characterization
Sort by

Defining Steels

There are different ways to classify automotive steels. One is a metallurgical designation providing some process

8

Improvement by Metallurgical Approaches

The Hole Expansion test (HET) quantifies the edge stretching capability of a sheet metal grade having a specific

8

Dual Phase

Dual Phase (DP) steels have a microstructure consisting of a ferritic matrix with martensitic islands as a hard

8