Multiphase Steel Cold Flat Product Strength Elongation
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Solution Overview
Problem
Existing methods for producing cold flat products from multi-phase steels do not adequately balance high strength with high elongation at break, which is essential for complex component formation in vehicle body construction.
Innovation Solution
A method involving the production of a multi-phase steel with specific composition and processing steps, including reheating, hot rolling, cold rolling, and heat treatment, to achieve a structure with high martensite content and retained austenite, resulting in a cold flat product with tensile strength of at least 980 MPa and elongation at break of at least 15%.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon content (0.14-0.25% C) and alloying elements are used to increase tensile strength to at least 980 MPa, then strength is improved, but elongation at break deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.14-0.25%, Si: 1.4-2.0%, Mn: 1.7-2.5%, Al: 0.01-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, V: 0.06-0.15%, B: 0.0005-0.002%) and processing parameters (reheating temperature: 1100-1300°C, hot rolling end temperature: 820-950°C, coiling temperature: 400-750°C, cold rolling degree: 30-80%, annealing temperature: Ac1+20°C to Ac3, overaging temperature: 350-500°C) to achieve a balanced microstructure with 12-40% martensite and 6-25% retained austenite, resolving the contradiction between high strength and adequate elongation
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite for strength, retained austenite for ductility and TRIP effect, and bainitic ferrite) within the steel. This multi-phase composite structure allows the material to simultaneously achieve tensile strength of at least 980 MPa and elongation at break of at least 15% by combining the advantageous properties of different microstructural phases
2Strength
If high martensite content is achieved through specific heat treatment to increase strength, then tensile strength is improved, but formability deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the heat treatment parameters (annealing temperature range of Ac1+20°C to Ac3 with time of 2-30 minutes, followed by overaging at 350-500°C for 2-180 minutes) to control the transformation kinetics and achieve a balanced microstructure that provides both high strength and adequate formability for complex component formation
Solution Approach 2:
The patent exploits phase transitions during heat treatment to create the desired microstructure. The annealing process induces austenite formation, followed by controlled cooling that produces a mix of martensite and retained austenite. The subsequent overaging treatment promotes additional phase transformations that stabilize the microstructure, enabling the steel to achieve both high strength and formability through controlled phase evolution
3Strength
If complex heat treatment processes are applied to achieve high strength and elongation, then mechanical properties are improved, but manufacturing complexity increases
Solution Approach 1:
The patent merges multiple heat treatment operations into a combined annealing and overaging process that can be performed in sequence or partially overlapping. The annealing step (Ac1+20°C to Ac3 for 2-30 minutes) is combined with controlled cooling and followed by overaging (350-500°C for 2-180 minutes), creating an integrated thermal processing route that achieves the target microstructure while reducing the number of separate process steps and equipment requirements
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method produces a cold flat product with enhanced strength and formability, ensuring high tensile strength and ductility, making it suitable for complex component formation in vehicle body construction.
Implementation Method 1
the structure of the steel consists of 12 - 40% by volume of martensite and 6 - 25% by volume of retained austenite and 5 - 60% by volume of bainitic ferrite
Implementation Method 2
the preliminary product is then reheated to a temperature of 1100 - 1300 °C
Implementation Method 3
the cold flat product obtained is then subjected to a heat treatment, which includes continuous annealing and overaging of the cold flat product
Data Source
AI summary
To produce a cold-cast flat product with TRIP properties and increased strength combined with high elongation at break, a pre-product is cast according to the invention from a multi-phase steel containing, in addition to iron and unavoidable impurities (in wt.%), C: 0.14 - 0.25%, Mn: 1.7 - 2.5%, Si: 1.4 - 2.0%, Al: < 0.1%, Cr: < 0.1%, Mo: < 0.05%, Nb: 0.02 - 0.06%, S: up to 0.01%, P: up to 0.02%, N: up to 0.01%, and optionally an element from the group "Ti, B, V" according to the following specification: Ti: ≤ 0.1%, B: ≤ 0.002%, V: ≤ 0.15%, starting from an initial temperature of 1100 - 1300 °C and The strip is hot-rolled to a final temperature of 820–950 °C. This hot-rolled strip is then coiled at 400–750 °C and, after coiling, cold-rolled to the final flat product at temperatures of 30–80%.This is subjected to a heat treatment comprising continuous annealing at an annealing temperature of 20°C + Ac1 - Ac3 and over-aging at an over-aging temperature of 350 - 500°C. The invention also provides a multiphase steel suitable for this purpose and a correspondingly manufactured flat product.