Cold-Rolled Steel Sheet Composition for High Spheroidization Stamping
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Solution Overview
Problem
Existing automotive steel materials face challenges in achieving high spheroidization rates, good plasticity, and low strength, while also requiring complex heat treatment processes that increase production costs and impose high cooling rate requirements, leading to issues like insufficient hardenability and dimensional deformation.
Innovation Solution
A cold-rolled steel sheet with specific chemical compositions (Fe, C, Si, Mn, Al, Cr, Mo, Ti, Nb, B, N, S, P) and a manufacturing process involving low-temperature coiling and bell-type annealing to achieve a spheroidization rate of ≥90% with uniform cementite distribution, allowing for lower cooling rates during heat treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the number of spheroidizing annealing cycles is increased and annealing time is extended, then the spheroidization rate is improved, but production costs significantly increase
Solution Approach 1:
The invention changes the chemical composition parameters of the steel, specifically controlling C (0.10-0.20 wt%), Si (0.01-0.50 wt%), Mn (1.50-2.50 wt%), Cr (0.50-1.00 wt%), Mo (0.10-0.30 wt%), B (0.0001-0.0050 wt%), and other elements within specified ranges. This compositional optimization enables high spheroidization rate (≥90%) to be achieved with reduced annealing cycles and shorter annealing time, thereby resolving the contradiction between improving spheroidization and reducing production cost
2Strength
If a fast cooling rate (≥30 °C/s) is used to form martensite, then the strength of parts is enhanced, but high requirements are imposed on material thickness and cooling medium, and dimensional deformation occurs
Solution Approach 1:
The invention modifies the chemical composition parameters by adding Cr (0.50-1.00 wt%), Mo (0.10-0.30 wt%), and B (0.0001-0.0050 wt%) along with controlled C and Mn content, which significantly improves the hardenability of the steel. This enables martensite formation at slower cooling rates, reducing the cooling rate requirement from ≥30 °C/s to lower values, thereby eliminating dimensional deformation while maintaining high strength
Solution Approach 2:
The invention uses small amounts of alloying elements (particularly B at 0.0001-0.0050 wt%, Cr at 0.50-1.00 wt%, and Mo at 0.10-0.30 wt%) to achieve substantial improvement in hardenability. These minor additions act as efficient modifiers that enable slower cooling rates, effectively replacing the need for expensive thick materials and complex cooling systems while maintaining part strength
3Reliability
If medium-carbon steel, high-carbon steel, or alloy steel is used to achieve good hardenability, then full martensite can be formed through quenching, but the material requires fast cooling rates that cause dimensional deformation
Solution Approach 1:
The invention optimizes the chemical composition parameters by precisely controlling C (0.10-0.20 wt%), Si (0.01-0.50 wt%), Mn (1.50-2.50 wt%), Cr (0.50-1.00 wt%), Mo (0.10-0.30 wt%), and B (0.0001-0.0050 wt%) within specific ranges. This balanced compositional design achieves excellent hardenability that enables full martensite formation at moderate cooling rates, avoiding the dimensional deformation problems associated with fast cooling while ensuring reliable heat treatment performance
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 results in a steel sheet with excellent hardenability, good plasticity, and low strength, meeting complex stamping requirements while reducing production costs by minimizing annealing cycles and times.
Implementation Method 1
hot-rolled steel must undergo subsequent cold rolling and spheroidizing annealing processes
Implementation Method 2
The structure and properties of automotive steel for stamping are basic conditions for realizing the stamping of complex parts. To achieve good stamping performance, it is desirable for a material to have high plasticity, low strength, and uniform properties, which is expected to be achieved by increasing the spheroidization rate.
Implementation Method 3
for some materials or parts that require heat treatment, especially steel grades such as medium-carbon steel, high-carbon steel, and alloy steel, heat treatment is necessary after stamping. This necessitates a material to have good hardenability; only by quenching in a quenching medium such as oil can full martensite be formed, thereby enhancing the strength of the parts.
Data Source
Figure 1~2
Figure 3
AI summary
Disclosed in the present invention is a cold-rolled steel sheet, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: C: 0.06-0.10wt%, Si: 0.01-0.5wt%, Mn: 1.6-2.5wt%, Al: 0.01-0.07wt%, Cr: 0.5-1.0wt%, Mo: 0.1-0.3wt%, Ti: 0.01-0.05wt%, Nb: 0.001-0.03wt%, B: 0.0001-0.005wt%, and 0<N≤0.006wt%. A microstructure of the cold-rolled steel sheet is that spherical cementite particles are evenly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of spherical cementite is greater than or equal to 90%. Correspondingly, further disclosed in the present invention are a manufacturing method for the cold-rolled steel sheet, and a stamping member manufactured by using the cold-rolled steel sheet.