Ferrite-Carbide Steel Sheet for Hardenability and Cold Workability
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Solution Overview
Problem
Existing steel sheets for automotive parts with larger sheet thicknesses require enhanced quenching up to the central portion, but existing technologies are unsuitable due to limited alloying elements like Ni, Cr, and Mo, and lack adequate post-quenching surface layer hardness.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including ferrite and carbides, with controlled ratios and concentrations of elements such as C, Si, Mn, Cr, and Ni, Mo, and a manufacturing process involving hot rolling, cooling, coiling, and annealing to achieve excellent cold workability, hardenability, and post-quenching surface layer hardness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If alloying elements like Ni, Cr, and Mo are increased to enhance hardenability for thicker sheets, then hardenability improves, but manufacturing cost and material complexity increase
Solution Approach 1:
The patent changes the parameters of alloying element composition by strictly limiting Ni, Cr, and Mo to 0.50 mass% or less in total, while optimizing other elements like C (0.20-0.40%), Si (0.10-0.50%), and Mn (0.50-1.50%) to achieve the required hardenability through a balanced compositional approach rather than relying on high concentrations of traditional hardenability-enhancing elements
Solution Approach 2:
The patent creates a composite alloy system that combines multiple elements in specific proportions, where the synergistic effect of C, Si, Mn, and small amounts of Ni/Cr/Mo produces the desired hardenability and mechanical properties, replacing the need for high concentrations of any single alloying element
2Strength
If carbon content is increased to improve strength and hardness, then post-quenching hardness improves, but cold workability deteriorates
Solution Approach 1:
The patent optimizes the carbon content parameter to a specific range of 0.20-0.40%, which is higher than ultra-low carbon steels to ensure sufficient post-quenching hardness, but controlled below 0.40% to maintain adequate cold workability, representing a balanced parameter selection to satisfy conflicting requirements
Solution Approach 2:
The patent creates local quality differences through phase distribution in the microstructure, where proeutectoid ferrite (20-80% by area) provides soft regions for cold workability, while the remaining microstructure containing carbides and eutectoid ferrite provides hard regions for post-quenching strength, achieving both properties through spatial distribution of phases
3Ease of manufacture
If alloying elements are limited to reduce cost and complexity, then manufacturing simplicity improves, but hardenability and post-quenching surface layer hardness deteriorate
Solution Approach 1:
The patent changes the compositional parameters by setting specific ranges for multiple elements: C (0.20-0.40%), Si (0.10-0.50%), Mn (0.50-1.50%), Cr (0.05-1.50%), and limiting Ni+Mo to 0.50 mass% or less, creating a optimized composition that achieves both manufacturing simplicity and excellent post-quenching surface layer hardness
Solution Approach 2:
The patent performs preliminary action during the steelmaking and hot rolling processes by controlling the chemical composition and microstructure formation in advance, ensuring that the steel sheet has the required hardenability and surface layer hardness potential before cold working and quenching, thereby simplifying subsequent manufacturing steps
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 steel sheet exhibits improved cold workability, hardenability, and post-quenching surface layer hardness, making it suitable for automotive parts like gears and transmissions, with enhanced wear resistance and tool life during blanking.
Implementation Method 1
performing rough hot rolling on a steel raw material having the chemical composition according to any one of [1] to5], subsequently performing finish rolling at a finishing temperature of 920°C or less
Implementation Method 2
performing cooling such that an average cooling rate from the finishing temperature to 700°C is 50°C/s or less
Implementation Method 3
causing a ratio of a volume of proeutectoid ferrite grains with grain diameters of 3 μm or more to a volume of an entire microstructure to be 20% or more and 80% or less
Implementation Method 4
subsequently performing annealing at an annealing temperature of 700°C or more and less than an Ac1 transformation temperature
Data Source
AI summary
An issue of the present invention is to provide a steel sheet and a member excellent in cold workability, hardenability, and post-quenching surface layer hardness, and methods for manufacturing the steel sheet and the member. The steel sheet of the present invention has a predetermined chemical composition and a microstructure containing ferrite and carbides; in the steel sheet of the present invention, the ratio of the volume of ferrite and carbides to the volume of the entire microstructure is 90% or more, the ratio of the volume of proeutectoid ferrite to the volume of the entire microstructure is 20% or more and 80% or less, the Mn concentration in the carbides is 0.10 mass% or more and 0.50 mass% or less, and the ratio of the number of carbides with particle diameters of 1 µm or more to the total number of carbides is 30% or more and 60% or less.