HSLA Steels and Thermo-mechanical processing 50-67 Flashcards
- Where, when and by whom was the first Nb micro-alloyed steel presented? Explain which three prerequisites then paved the way towards the current industrial success of HSLA steels?
1958, Great Lakes Steel (count as whom?), USA
Prerequisites for industrial success
Availability and low cost of Nb
Increased understanding of the Nb addition through research
Enticed further research of alloying elements and rolling processes
- Briefly explain the alloying concept of HSLA steels: What are typical C and Mn contents, which elements are so-called ‘microalloying’ elements (and what are their typical contents), what other elements may be added to these steels?
Low concentration of C (0.02-0.04 wt%)
For improved welding capability and reduced cracking
Presence of corrosion resistant alloys (<1.5%)
Ni, Cr, Cu and Mo
Micro-alloying elements (0.008 wt% up to 0.1-0.2 wt%)
B, Ti, Nb, V
Also possible to add Zr, Ta but to a lesser extent
Provides a wide range of mechanical properties
HSLA steels (high strength low alloy) include the addition of a tiny amount of alloying material that affects the mechanical properties majorly. This can then decrease the cost of the steel and make it more widely used.
C: 0.02 – 0.2 wt. %
Mn: ~1.92 wt. %
Micro alloying elements: Nb (0.045wt %), Ti (0.016wt %) and V (0.008-0.2wt %) (Occasionally Zr, Ta and/or B)
(e.g. 0.02wt% Nb can raise yield strength by as much as 200 MPa)
- Name three drawbacks of conventional C-steels. Then briefly list how these challenges were overcome by the development of HSLA steels.
Conventional steels - decreased Formability, Toughness, Weldability
Strengthening is mainly achieved through the addition of carbon. Drawbacks:
- elongation at fracture, formability, toughness and weldability decreases
Charpy impact test (smash two bars and record energy absorbed at different temperatures). Measures if the metal is brittle or ductile.
HSLA steels can overcome these by (strengthening through)
- grain refinement
- solid solution hardening
- precipitation hardening (through carbides, nitrides and carbo-nitrides)
· high strength → lightweight structures
· high toughness → also at low temps
· weldability → low C contents
· low alloying costs
- Tell me three important facts about the significance of HSLA steels. Name at least two typical matrix microstructures found in modern HSLA steels (other than precipitates).
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- What range of yield strength is usually achieved in standard HSLA steels, what range of maximum yield strength is possible (with what kind of matrix microstructures/cooling)? Also, name at least three typical applications of these steels.
Applications: - semi finished parts: sheets (hot rolled strip, cold rolled strip), profiles, pipes – parts for e.g. frames for heavy good vehicles (more on slides)
- What is a ‘stoichiometric carbo-nitride’, what is the typical crystal structure of microalloy precipitates and what kind of precipitate would you most likely find in a HSLA steel with Ti, Nb and C?
What is? A: similar to NaCl crystal structure. 50:50 ratio of Nb and C. Lattice parameters 0.415-0.455 nm.
Nb-V, Nb-V-Ti, V-Ti with or without deliberate N additions as well as soluble Al. depends on temperature at which precipitates are stable and… something about solubility product. Log[X] [C or N] = A-B/T
Nitrites is almost always more stable than the carbide. (if you add V will it be solid solution? Carbide? Or Nitrate? Determine from curves). More negative = more stable. Precipitates in ferrite are most stable than austenite. Titanium nitrate are the most stable of all.
- What do we mean by ‘row’ precipitates, how are they formed and how can they be imaged (name at least one technique)?
Formation of interphase precipitates by ledge migration transverse to the direction of interface movement.
3D atom probe tomography
- What is atom probe tomography (in max. five sentences), and why is this technique important in HSLA steel characterisation and research besides electron microscopy (at least one reason)?
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- What are ‘strain induced precipitation’ and ‘pipe diffusion’? Give one example of such precipitation in HSLA steels.
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- What kind of microalloy precipitate with low solubility in the austenite is typically used to avoid extensive austenite grain growth? What are clusters (or ‘atomic molecules’), often formed in the ferrite region during HSLA steel processing?
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- How can we achieve a fine ferrite grain size in HSLA steels via thermo-mechanical processing (three stages)?
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- Explain a typical thermo-mechanical schedule for a HSLA steel with a final microstructure of fine ferrite + pearlite. Perhaps draw the temperature versus time schedule on a piece of paper.
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- Explain the concept of the so-called ‘non-recrystallisation temperature’ used during thermo-mechanical processing of HSLA steels. Perhaps you want to draw on a piece of paper.
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- Compare typical thermo-mechanical schedules of (a) conventional rolling, (b) controlled rolling and (c) controlled rolling with accelerated cooling. Perhaps it is useful to draw typical T-t schedules on a piece of paper.
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- Name all strengthening mechanisms that are contributing to the total yield strength of a HSLA steel. Explain the interplay between (a) grain size, (b) particle radius and (c) particle volume fraction and the total yield strength in your own words.
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