Flat Steel Microstructure for Deep Drawing and Edge Crack Resistance
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Solution Overview
Problem
High-strength flat steel products with good deep-drawing ability and low edge crack sensitivity are challenging to achieve simultaneously, as high strength typically reduces formability and increases edge crack risk during processing.
Innovation Solution
A cold-rolled flat steel product with a specific microstructure comprising at least 75% tempered martensite, 5-25% untempered martensite, 5% retained austenite, 0.5-10% ferrite, and minimal bainite, along with a Mn-poor ferrite fringe at phase boundaries, and controlled carbon content to optimize mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel is used to reduce vehicle weight, then tensile strength is improved, but formability and deep-drawability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.23-0.38%, Si: 1.00-2.50%, Mn: 1.50-3.50%, Al: 0.015-2.000%, Ti: 0.005-0.050%, B: 0.0005-0.0050%) and heat treatment parameters (austenitizing temperature, cooling rate, holding time) to achieve a microstructure with 20-50% retained austenite and 50-80% tempered martensite, thereby simultaneously achieving high tensile strength (≥1200 MPa) and good formability (elongation 10-30%, hole expansion 50-120%).
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (retained austenite, tempered martensite, and optionally bainite and ferrite) within the steel matrix. This composite microstructure combines the high strength of martensite with the ductility and formability of retained austenite, resolving the contradiction between strength and formability.
2Strength
If high-strength steel is used, then tensile strength is improved, but edge crack sensitivity increases
Solution Approach 1:
The patent controls specific parameter ranges including lower carbon content (0.23-0.38% vs. higher C in conventional steels) combined with optimized Si (1.00-2.50%) and Mn (1.50-3.50%) contents, along with controlled Ti (0.005-0.050%) for carbide precipitation, to achieve a microstructure that provides both high strength and reduced edge crack sensitivity during shear cutting processes.
Solution Approach 2:
The patent creates local quality differences through the distribution of retained austenite (20-50%) throughout the microstructure, which acts as a ductile phase that can locally accommodate stress concentrations at cut edges, thereby reducing edge crack sensitivity while maintaining overall high strength.
3Strength
If high-strength steel is used, then tensile strength is improved, but deep-drawability deteriorates
Solution Approach 1:
The patent achieves the contradiction resolution by controlling the retained austenite content at 20-50% (higher than conventional steels) through specific compositional parameters (particularly Si: 1.00-2.50%, Mn: 1.50-3.50%, and controlled cooling rates during heat treatment), which provides sufficient ductility for deep-drawing operations while maintaining tensile strength of at least 1200 MPa.
Solution Approach 2:
The patent applies preliminary action through the heat treatment process that pre-forms the desired microstructure (with 20-50% retained austenite and 50-80% tempered martensite) before the deep-drawing operation. This pre-established microstructure with ductile retained austenite distributed throughout prepares the material to accommodate the high deformation demands of deep-drawing processes.
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 solution achieves a balance of high tensile strength (900-1500 MPa), good deep-drawing ability, and reduced edge crack sensitivity, with a bending angle greater than 80° and hole expansion greater than 25%, while maintaining weldability and surface quality.
Implementation Method 1
after austenitizing, they are cooled to a cooling stop temperature, held at that temperature, and then reheated
Implementation Method 2
The flat steel products are subjected to a heat treatment in which, after austenitizing, they are cooled
Data Source
Figure 1~2
AI summary
The present invention relates to a flat steel product which has good deep-drawing ability, low edge-crack sensitivity and good bending behaviour. To this end, the flat steel product contains a steel which consists of (in wt%) 0.1-0.5% C, 1.0-3.0% Mn, 0.9-1.5% Si, up to 1.5% AI, up to 0.008% N, up to 0.020% P, up to 0.005% S, 0.01-1% Cr and optionally one or more of the following elements: up to 0.2% Mo, up to 0.01% B, up to 0.5% Cu, up to 0.5% Ni and optionally a total of 0.005-0.2% microalloying elements, the remainder being iron and unavoidable impurities, wherein 75 < (Mn2 + 55*Cr)/Cr < 3000 where Mn is the Mn content of the steel in wt% and Cr is the Cr content of the steel in wt%. The steel has a structure which consists of at least 80 area % martensite, of which at least 75 area % is tempered martensite and at most 25 area % is non-tempered martensite, at least 5 volume % residual austenite, 0.5 to 10 area % ferrite and at most 5 area % bainite, wherein in the region of the phase boundary between tempered martensite and residual austenite there is a low-Mn ferrite seam which has a width of at least 4 nm and at most 12 nm and the Mn content of which is at most 50% of the average Mn content of the flat steel product. The flat steel product contains carbides with a length of less than or equal to 250 nm.The invention also relates to a method for producing a flat steel product according to the invention, in which method the structural characteristics of the flat steel product according to the invention are set by suitable heat treatment.