High-Strength Steel Sheet Weldability Microstructure
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Solution Overview
Problem
Existing steel sheets with high strength often suffer from inadequate weldability, leading to cracking issues during spot welding, and previous techniques have not sufficiently addressed this problem while also achieving the desired tensile strength and elongation.
Innovation Solution
A steel sheet composition with specific elements like V, Ti, Nb, Cr, Mo, Ni, B, and Sb, along with a controlled microstructure and plating layer, is developed to enhance weldability and strength, involving a production process that includes hot rolling, cold rolling, and annealing to achieve a balanced ferrite and martensite structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the strength of steel sheet is increased to reduce automobile weight, then weight reduction is achieved, but weldability deteriorates causing cracking during welding
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting C content to 0.05% or less and Si content to 0.03% or less, while controlling Mn content within 1.5-3.5%. These parameter changes enable the steel to achieve both high strength (980-1500 MPa) and good weldability by preventing excessive carbide formation and maintaining a balanced microstructure with controlled martensite and retained austenite phases.
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (martensite, retained austenite, and ferrite) with specific area ratios. This composite structure at the micro level provides both the strength needed for weight reduction and the ductility required for weldability, resolving the contradiction between strength and weldability.
2Strength
If high strength steel sheet is used to achieve tensile strength of 980 MPa or more, then strength is improved, but elongation and weldability become insufficient
Solution Approach 1:
The invention achieves tensile strength of 980 MPa or more while maintaining good weldability and elongation by precisely controlling chemical composition parameters (C: 0.05% or less, Si: 0.03% or less, Mn: 1.5-3.5%) and processing parameters (cooling rate: 10-100°C/s, finishing temperature: 800-950°C). These parameter changes enable the formation of a microstructure with controlled phase distribution that simultaneously provides strength, ductility, and weldability.
Solution Approach 2:
The invention creates local quality variations in the microstructure by controlling the distribution and area ratios of different phases (martensite: 70-90%, retained austenite: 5-20%, ferrite: 0-10%). This local phase distribution provides hard martensite regions for strength while maintaining softer retained austenite and ferrite regions for ductility and weldability, resolving the contradiction between strength and formability.
3Strength
If conventional high strength steel composition is used to achieve high tensile strength, then strength is improved, but spot weldability becomes insufficient due to cracking
Solution Approach 1:
The invention improves spot weldability while maintaining high strength by changing the chemical composition parameters to C: 0.05% or less and Si: 0.03% or less. These parameter changes prevent excessive carbide precipitation and reduce the hardness difference between the base metal and heat-affected zone, thereby preventing cracking during spot welding while achieving tensile strength of 980 MPa or more.
Solution Approach 2:
The invention applies beforehand cushioning by pre-controlling the chemical composition and microstructure to prevent welding cracks. By limiting C and Si content and controlling the microstructure before welding, the steel sheet is prepared in advance to resist crack formation during spot welding, ensuring both high strength and good weldability.
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 a tensile strength of 900 MPa or more, excellent weldability, and improved elongation, enabling further weight reduction in automobile parts while preventing cracking during welding.
Implementation Method 1
a finishing temperature of the finish rolling being 820°C or higher, and coiling the steel material at 350°C or higher and 680°C or lower; cold rolling the hot-rolled steel material; and after cold rolling, heating and retaining the cold-rolled steel material at 840°C or higher for 20 seconds or more and 180 seconds or less, then cooling, retaining at 770°C or higher and 820°C or lower for 10 seconds or more, and cooling under conditions that an average cooling rate is 35°C/s or more between cooling start temperature and 450°C
Implementation Method 2
heating a steel material at 1150°C or higher and 1350°C or lower, hot rolling including rough rolling and finish rolling the steel material at a finishing temperature of the finish rolling being 820°C or higher
Implementation Method 3
cold rolling the hot-rolled steel material; and after cold rolling, heating and retaining the cold-rolled steel material
Data Source
AI summary
Provided are a steel sheet with excellent weldability, and a production method therefor. The steel sheet is characterized by having a specific composition and a metallographic structure containing, in terms of an area ratio, ferrite of 25% or more and 65% or less, martensite having iron-based carbides precipitated in the grains of 35% or more and 75% or less, and the balance structure other than the ferrite and the martensite of 20% or less (including 0%) in total, the average grain diameters of the ferrite and the martensite being respectively 5 µm or lower, the total of concentration of Si and Mn at interface between the ferrite and the martensite being, in terms of an atomic concentration, 5% or more, and having a tensile strength of 900 MPa or higher.