Press Hardening Steel Alloy Control via Press Hardness Number
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Solution Overview
Problem
The press hardening process faces challenges in controlling and optimizing process parameters, leading to undesired diffusion-controlled conversions and reduced cooling rates, which affect the formation of martensite in steel, resulting in inconsistent properties and increased scrap rates.
Innovation Solution
A method utilizing a press hardness number (PHZ) is introduced, calculated from the chemical composition and cooling rate, to determine whether a fully martensitic structure can be achieved, distinguishing between indirect and direct press hardening processes, and estimating necessary alloy compositions and tool parameters for achieving desired martensite content and hardness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel is cooled at a rate above the critical hardening rate to achieve full martensitic structure, then the hardness and strength are improved, but undesired diffusion-controlled conversions (ferrite formation) occur at high temperatures during transfer and handling
Solution Approach 1:
The patent applies preliminary action by heating the steel to austenitization temperature and maintaining it long enough to ensure complete austenitic structure formation before the cooling and forming operations begin. This preliminary austenitization step ensures that when cooling starts, the steel is fully austenitic and ready for martensitic transformation, preventing unwanted ferrite formation during transfer and handling operations.
2Temperature
If the hot sheet is transferred from furnace to press with high emissivity, then heat radiation increases, but undesired diffusion-controlled conversions at high temperatures occur
Solution Approach 1:
The patent applies parameter changes by carefully controlling the temperature parameters during transfer and handling. By maintaining the steel at austenitization temperature long enough to ensure complete austenitic structure before cooling begins, and by controlling the cooling rate parameters, the process prevents the temperature range where diffusion-controlled ferrite formation would occur, even during high-radiation transfer operations.
3Productivity
If deep-drawing is performed on hot sheets, then forming is accelerated, but ferrite and bainite formation is promoted before martensite formation
Solution Approach 1:
The patent applies preliminary action by completing the deep-drawing forming operation while the steel is still in the austenitic state, before any diffusion-controlled transformations can occur. The forming is performed during the brief window when the steel is heated to austenitization temperature and held just long enough to ensure complete austenitic structure, but before cooling begins and ferrite or bainite formation starts.
Solution Approach 2:
The patent applies the skipping principle by rapidly transitioning the steel through the temperature range where diffusion-controlled transformations occur. The process heats to austenitization, performs forming quickly while austenitic, then immediately begins rapid cooling to skip through the ferrite and bainite transformation ranges and directly achieve martensitic structure.
4Strength
If alloying elements like boron are used to increase hardenability, then the critical hardening rate is reduced, but the temperature below which full martensitic structure cannot be achieved is lowered
Solution Approach 1:
The patent applies parameter changes by adjusting the heating temperature and holding time parameters to ensure complete austenitic structure formation, which compensates for the lowered martensite formation temperature caused by boron addition. By controlling these thermal parameters appropriately, the process achieves full martensitic structure despite the presence of conversion-retarding alloying elements.
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
This approach simplifies process control, reduces trial and error, and minimizes scrap by accurately predicting the suitability of steel for press hardening processes, ensuring a fully martensitic structure and optimal hardness, thereby enhancing process reliability and efficiency.
Implementation Method 1
a steel blank with an alloy composition matched to the press hardening is raised to a temperature which enables austenitization, preferably complete austenitization
Implementation Method 2
After heating and complete austenitizing, such a steel is cooled at a rate which is above the so-called critical hardening rate. This results in a fully martensitic structure
Implementation Method 3
by transferring the hot sheet from the furnace to the press and in particular due to high emissivities (high heat radiation behavior) of the sheet or the sheet metal plate, undesired diffusion-controlled conversions at high temperatures (ferrite) can take place
Implementation Method 4
cool it there by all-round contact of the cooling tool
Data Source
Figure 1
Figure 2~3
AI summary
Press hardening of steel, comprises cold pre-forming a steel sheet made of a hardenable steel alloy, transforming in a mold, which has the contour of the preformed component, heating to perform a complete austenitization, and cooling at a speed higher than the critical hardening speed, such that a quenching of the preformed element takes place. A press hardness number is determined e.g. for adjusting the suitable steel alloy to known plant geometry, where the press hardness number is equal to the cooling rate in the mold/theoretical press cooling rate. Press hardening of steel, comprises either: cold pre-forming a steel sheet made of a hardenable steel alloy, transforming in a mold, which has the contour of the preformed component, heating to perform a complete austenitization, and cooling at a speed higher than the critical hardening speed, such that a quenching of the preformed element takes place; or heating a sheet of a steel with a composition, which allows a press hardening, to a temperature above the austenitizing temperature, hot molding in a mold, cooling at a speed which is higher than the critical hardening speed, such that hardening takes place, where the austenitic structure is transformed into a martensitic structure optionally with residual austenite. A press hardness number is determined for: adjusting the suitable steel alloy to known plant geometry, and the cooling rate achieved during operation in the mold; or adjusting a required mold to a given grade of steel, where the press hardness number is equal to the cooling rate in the mold/theoretical press cooling rate. The cooling rate in the mold is predetermined for a required thickness. The determination of the theoretical press cooling rate for steel material, which contain greater than 5 ppm boron dissolved in the starting material is represented by: theoretical press cooling rate is equal to 1750/(28.5% carbon + 3.5% silicon + 2.3% manganese - 2% aluminum + 4% chromium + 3% nickel + 25% molybdenum - 20% niobium - 6.3) 2>.7. The determination of the theoretical press cooling rate for steel material, which contain less than 5 ppm boron dissolved in the starting material is represented by: 2750/(28.5% carbon + 3.5% silicon + 2.3% manganese - 2% aluminum + 4% chromium + 3% nickel + 25% molybdenum - 20% niobium - 7) 1>.8, where: when press hardness number is less than 1 the complete hardening by martensite formation does not takes place; when press hardness number is equal to 1 an undeformed or preformed sheet is cured via an indirect process; and when press hardness number is less than 1, in addition to the indirect process, the sheet is thermoformed or increased security against plastic deformation during hardening takes place.