Often found in daily life, such as mild steel plate and other materials after hot or cold working, placed at room temperature for a period of time, its mechanical properties have changed, the performance of such metal materials with the extension of time and the phenomenon known as aging.
The conditions of aging aging tends to make the performance of the material worse, in the production practice should pay attention to prevent, but also can master the change rule, so that it can be used in production. Due to the different chemical composition of steel, pre hot or cold working and use of different temperatures, the aging of steel also has different performance. Steel by solid solution treatment after rapid cooling to the aging temperature, the alloying elements will be in a supersaturated state, at this time, if the alloying elements still have the ability to diffuse, then with the prolongation of time, the alloying elements in the steel will be from the solid solution in the fall off (or precipitation) resulting in changes in the material's properties, which is called aging. Definition of aging process: all the material properties over time are collectively referred to as the aging process. Aging conditions: 1) the alloying elements have a certain solubility; 2) solubility with the reduction of temperature and reduce; 3) high temperature solid solution of alloying elements, after rapid cooling to become supersaturated; 4) in the low temperature state, the alloying elements still have a certain diffusion rate.
Aging phenomenon is a spontaneous phenomenon from the non-equilibrium state to the equilibrium state. If the solution treatment is cooled very slowly to achieve equilibrium without cold deformation, aging will not occur.
Steel aging phenomenon is mainly caused by the steel carbon, nitrogen interstitial atoms. Carbon, nitrogen is a gap atom in steel, gap atoms generally have a certain diffusion capacity at room temperature, their solubility is reduced with the reduction of temperature, so as long as the solution treatment after the fast cooling caused by supersaturation, can produce aging phenomenon. Therefore, aging phenomenon can be divided into quenching aging and strain aging (deformation aging, mechanical aging). Quenching aging is the precipitation hardening of a solid solution caused by rapid cooling to a certain temperature.
At this temperature, the second phase elements become supersaturated. Precipitation occurs at higher temperatures and with multiple applications and results in an increase in yield strength, tensile strength and hardening. Strain ageing is a phenomenon that occurs in certain materials after plastic deformation. For mild steel plates, strain ageing results in the reproduction of discontinuous yielding, an increase in yield strength and hardening, and a decrease in toughness with no significant change in tensile strength.
Changes in Properties As a result of aging the material, its properties will undergo large changes, mainly the following changes.
1) The hardness of the material increases;
(2) the strength of steel (yield strength increased, tensile strength increased or unchanged), plasticity and toughness (elongation, section shrinkage ratio, impact resistance) decreased; 3) some electrical properties and physical properties also changed, such as making the electrical resistance decreased, the magnetic coercivity increased.
The aging process is a spontaneous phenomenon from the non-equilibrium state to the equilibrium state, is carbon, nitrogen and other interstitial atoms due to oversaturation, at low temperatures by the diffusion ability, from the solid solution to fall off (or precipitation), resulting in changes in the properties of the material process.
In order to ensure that we require a variety of properties of the strip, we must use the corresponding production process measures to prevent the occurrence of aging phenomenon of the strip. These production process measures are the so-called ageing.
In the continuous annealing furnace is set up in the over-aging section, for some over-aging requirements of the steel grade (such as DQ-AK, DDQ-AK, DP steel, TRIP steel) aging treatment, that is, in the range of over-aging temperature of the steel grade, so that the strip to maintain enough over-ageing time, so that the carbon, nitrogen and other interstitial atoms to fully precipitate, but their precipitation and the precipitation of the ordinary low carbon steel is very different, mainly Due to the simultaneous addition of aluminium, vanadium, niobium and other alloying elements in this type of steel, this part of the elements will form a stable nitride with nitrogen precipitation at the same time, so that the ferrite matrix strengthened (known as the second phase of the diffuse hardening), and the grain refinement, so that the steel's strength and toughness can be significantly improved, while the lower temperature aging phenomenon is suppressed.
The following are specific analyses of the effects of ageing on stamping and countermeasures: 1. Effects of ageing on stamping
(1) Changes in mechanical properties Yield strength rises: the yield strength (YS) of steel rises after aging, leading to an increase in the forming force required for stamping and increased wear of the die. Decrease in plasticity: Elongation is reduced, which is easy to cause stamping cracks, especially for complex shaped parts (such as deep-drawn parts and flanging parts). Increased anisotropy: aging may aggravate the anisotropy of the material, resulting in uneven forming (such as the lug effect).
(2) Residual stress and dimensional stability - If residual stress exists in the stamped part, long-term storage may cause warping, distortion and other deformation problems due to stress release, affecting assembly accuracy.
(3) Deterioration of formability - After age-hardening, the forming limit diagram (FLD) of the material is shifted to the left, which reduces the material's ultimate drawing ratio (LDR) and increases the risk of deep drawing rupture. - For high-strength steels (e.g. DP steels, TRIP steels) and bake-hardening steels (BH steels), ageing needs to be specifically considered in the process design.
(4) Surface quality and springback - Increased strength leads to increased springback, which affects the dimensional accuracy of the part and makes mould commissioning more difficult. - Surface strain aging may cause Lüders bands (Lüders bands), resulting in uneven strain stripes on the surface of the stamped part.
(5) The effect of storage conditions - humid environment, some steel grades (such as phosphorus-containing high-strength steel) may be due to aging and corrosion together, exacerbating the deterioration of performance.
2. Countermeasures
(1) Material selection and treatment Selection of materials with low ageing sensitivity: e.g. ultra-low carbon steels (IF steels) or steels with stabilising elements (Ti, Nb). Application of bake-hardening steels (BH steels): Use artificial ageing (e.g. paint drying) to improve strength while maintaining good formability before stamping. Stress relief annealing: low-temperature annealing of stamped parts to remove residual stresses.
(2) Process optimisation to control inventory cycle time: Avoid long-term storage of cold rolled steel sheets (<3 months is recommended), and adopt first-in-first-out (FIFO) management. Adjustment of stamping parameters: optimise crimping force, lubrication conditions and die clearance for the strength of the material after ageing. Pre-strain treatment: Accelerate the aging process by pre-stretching or pre-distortion to stabilise the material properties.
(3) Mould design and simulation Compensation of rebound design: based on the rebound trend of the material after aging, correct the mould profile.CAE simulation: use finite element analysis (e.g. AutoForm, Dynaform) to predict the impact of aging on stamping and optimize the process window.
(4) Quality control Regular testing of material properties: Monitor the yield strength, elongation and aging index (e.g. BH value) of stock steel. Accelerated ageing test: simulate long-term natural ageing by heating (e.g. 100°C x 1h) to assess stamping risks in advance.
3. Typical application scenarios
Automotive cladding parts: BH steels are strengthened by ageing through coating and drying, taking into account both stamping formability and part strength. Precision electronic parts: storage conditions need to be strictly controlled to prevent overdimensioning due to aging. High-strength steel parts: DP steel and martensitic steel need to enter the next process quickly after stamping to avoid fluctuation of performance.
The effect of steel ageing on stamping is mainly reflected in material hardening, plasticity decline, increased rebound and reduced dimensional stability. Through reasonable material selection, process optimization and mould design, the negative effects of ageing can be effectively controlled, and at the same time, properties such as baking hardening can be used to improve part performance. In actual production, it is necessary to combine material properties, storage cycles and part requirements to develop a targeted strategy.
