SPECIAL GRADES FOR ROLLS

INFORMATION FOR THE CORRECT STEEL GRADE SELECTION

The challenge coming from the mills for rolling always higher tensile strength materials, thinner gauges and mirror surface finished products has fostered an evolution of the work roll performance.
Further the cost saving purchasing procedures and the work roll efficiency are pushing for more and more competitive products.

To meet these customer requirements BSA has joint studies and tests on metallurgy and heat treatment methodes of several steel grades, thus allowing to develop a more efficient roll manufacturing technology and an advanced quality of production.


Chromium, Molybdenum, Vanadium benefits - image

INFLUENCE OF THE ELEMENTS IN STEEL CHEMISTRY

The elements present in the different grades can modify the characteristics of the alloy to improve some desired properties.
But the working performance of the rolls, like wear resistance, toughness, yeild strength and compressive strength, result not only from the chemical composition of the materials, but mainly driving and keeping under control the complete manufacturing process.
In fact will be the execution, the cleaness and the heat treatment cycle combined with an appropiate machining procedure to give to materials the desired final working configuration and at the end the performances of the rolls will be given from:


BSA cold working grades:

Material DIN Werkstoff N° Cr Mo V % Surface Hardness Tensile strength
51CrMoV4 1.7701 1.1 0.2 0.1 64->66 HRC 860->1000 N/mm2
86CrMoV7 1.2327 1.8 0.3 0.1 64->67 HRC 900->1050 N/mm2
X63CrMoV5.1 1.2362 5.3 1.2 0.3 62->64 HRC 860->1000 N/mm2
~X82CrMoV8.2 1.2390 7.8 1.6 2.5 60->64 HRC 800->1000 N/mm2
X155CrMoV12.1 1.2379 12 1.4 1.6 60->63 HRC 800->1000 N/mm2


Below some relevant elements related with the rolls production:

C: Carbon is the most relevant element to control hardness characteristics, it is necessary to guarantee the correct martensitic trasformation.
Caution: High C rate is dangerous reduceing toughness and is the principal cause of brittleness High C rate can give final undesired metallic structures leading to lack of wear resistance High C rate is responsible of thermal shock sensibility and heating cracks.

Mn: Manganese is always present in steel and makes carbides that increase wear resistance, increase toughness in presence of S and increase impact strength; it is considered as element of the alloy only in amounts higher than 1%.

Cr: Chromium carbides increase wear resistance, increase smootly the hardenability, breake corrosion phenomenous at high temperature, decarburization and oxides, increase mechanical characteristics at high temperature conditions, improve polishability.

Mo: Molybdenum is responsible for steel hardenability, increases surface hardening depth because it decreases the critical cooling rate, guarantees uniformity for cross-section hardness, helps to keep small the austenitic grain during heating leading to fine martensitic structure.

V: Vanadium is responsible for high yeild strength and elongation and increases compressive strength, giving steel the best toughness and schock resistance capabilities; further increases wear resistance and helps to guarantee low grain size after hardening.

Ni: Nickel increases the mechanical characteristic leading to high tensile strength materials, slightly helps hardenability and increases impact strength.

Si: Silicium improves thermal shock resistance in the case of skidding problems, increases wear resistance but decreases toughness and machinability.

Undesired elements often present as iclusions in the metallic matrix are as follows:
Ca, Cu, P, Pb, S, and Sn are elements that decrease the mechanical characteristics of the steel.
Caution: some of them can be introduced to enchance the material machinabilty, but with dangerous results for the final working conditions.


CHARACTERISTICS AND APPLICATIONS FOR COLD WORK GRADES

51CrMoV4
86CrMoV7
X63CrMoV5.1
~X82CrMoV8.2
X155CrMoV12.1