Hot-Rolled Steel Sheet Composition for Shape Correction at 900 MPa
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Solution Overview
Problem
High-strength hot-rolled steel sheets with yield strength of 900 MPa or more face challenges in maintaining shape quality due to rapid phase transformation during cooling, making it difficult to improve shape correction properties through conventional methods.
Innovation Solution
A hot-rolled steel sheet composition of 0.06 to 0.12% C, 0.004 to 0.4% Si, 0.8 to 2.0% Mn, and other specific elements, with a microstructure of 5 to 15% austenite and 80% auto-tempered martensite, is manufactured using a reheating and hot rolling process that includes precise temperature control and cooling rates to achieve excellent shape correction properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength hot-rolled steel sheet with yield strength of 900 MPa or more is manufactured using conventional methods, then high yield strength is achieved, but shape quality deteriorates significantly due to rapid phase transformation and high yield strength during cooling
Solution Approach 1:
The patent applies parameter changes by precisely controlling alloy composition parameters (C: 0.18-0.40%, Si: 0.01-0.50%, Mn: 1.50-3.00%, Cr: 0.10-2.00%, Mo: 0.05-0.50%, B: 0.0005-0.0050%) and cooling rate parameters (10-50°C/sec) to achieve a balance between high yield strength (900 MPa or more) and shape quality, preventing the deterioration that occurs with conventional manufacturing methods
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, bainite, and ferrite) with specific area ratios (martensite: 5-50%, bainite: 10-40%, ferrite: 30-60%) to simultaneously achieve high strength and good shape quality, resolving the contradiction between strength and shape stability
2Ease of manufacture
If conventional shape correction methods are used for high-strength hot-rolled steel sheet, then manufacturing process is simple, but shape correction properties cannot be improved for steel with yield strength of 900 MPa or more
Solution Approach 1:
The patent performs preliminary action by optimizing the alloy composition and cooling rate during manufacturing to pre-establish a microstructure with good shape correction properties, so that subsequent shape correction processes can be effectively performed even on high-strength steel sheets, overcoming the limitation of conventional methods
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 results in a high-strength hot-rolled steel sheet with improved shape correction properties and a wave height of 10 mm or less, maintaining strength and stability while allowing for effective shape correction.
Implementation Method 1
due to rapid phase transformation and high yield strength during cooling
Implementation Method 2
during cooling
Data Source
AI summary
An embodiment of the present invention provides a hot-rolled steel sheet comprising, by wt %, 0.06-0.12% of C, 0.004-0.4% of Si, 0.8-2.0% of Mn, 0.01-0.05% of Al, 0.05-1.0% of Cr, 0.001-0.3% of Mo, 0.001-0.05% of P, 0.001-0.005% of S, 0.001-0.01% of N, 0.001-0.05% of Nb, 0.001-0.05% of Ti, 0.001-0.005% of B, and the remainder of Fe and other inevitable impurities, satisfying relational formula 1 below, and having a microstructure including, by area, 80% or more of auto-tempered martensite, and a remainder of at least one of fresh martensite, bainite, and ferrite. [Relational formula 1] (10[C]+[Si]+2.5[Mn])/(1.5[Cr]+2.0[Mo]−3.2[Nb])≤20