A method for quickly evaluating the aging degree of bake-hardening steel

Abstract

The present invention discloses a method for rapidly evaluating the aging degree of bake-hardened steel, the method comprising the following steps: (1) establishing an initial temperature-time curve when the bake-hardened steel initially produces discontinuous yield; (2) measuring the aging time t corresponding to the discontinuous yield elongation Ae=x to be evaluated at 100°C of the bake-hardened steel; x ; (3) Temperature Tn and aging time t x The initial temperature-time curve is shifted along the time axis until the point corresponding to Tn on the initial temperature-time curve coincides with the reference point. This yields the temperature-time curve for the bake-hardened steel that produces a discontinuous yield elongation Ae = x. This method combines accelerated aging with natural aging to construct a temperature-time curve and functional relationship for the same yield elongation. This allows for rapid estimation of the time required to achieve the same degree of natural aging at room temperature. This method is simple, convenient, highly operational, accurate, and efficient, significantly shortening experimental cycles.

Description

Technical Field

[0001] The invention relates to an alloy performance evaluation method, in particular to a method for quickly evaluating the aging degree of bake hardening steel. Background Art

[0002] Bake-hardening steel is widely used in exterior panel parts for automobiles, home appliances, and other applications due to its excellent formability and dent resistance. However, to maintain its bake-hardening properties, a portion of interstitial C and N atoms not fixed by alloying elements such as Nb and Ti remain in the bake-hardening steel. These atoms still have a certain diffusion capacity at room temperature. After a period of time, the interstitial atoms diffuse to dislocations, pinning them and forming Coriolis gas clusters. This causes discontinuous yielding of the bake-hardened steel during stretching, a phenomenon known as natural aging. When stamping sheet metal that has aged to a certain degree, aging defects such as tensile strain marks and orange peel will appear, affecting the appearance quality of the part. This is unacceptable for exterior panel parts such as automobiles and home appliances, which require high quality and must be scrapped, resulting in serious losses. However, in the actual production process, due to changes in sales, production rhythm, long-distance transportation, etc. of automobile or home appliance manufacturers, it is impossible to always coordinate well with the sheet material production plan of the material supplier. As a result, many sheets cannot be stamped into parts within several months after production, and the natural storage process will cause aging. In addition, uncontrollable factors such as seasonal changes and temperature differences between the north and the south will also accelerate the aging of materials and increase the risk of quality problems.

[0003] However, due to the different deformation degrees and strain paths of parts during the stamping process, the corresponding aging degrees when tensile strain marks are generated during the stamping process of the parts are also different, resulting in tensile strain marks when some materials are stamped, while other materials do not produce tensile strain marks; or the same material will produce tensile strain marks when stamping this part, but will not produce tensile strain marks when stamping other parts with different strain paths. Therefore, quickly and effectively evaluating the aging degree of materials can effectively reduce unnecessary losses such as scrap and rework, improve customer production efficiency, and increase material utilization and supplier satisfaction.

[0004] Currently, there is no accurate method for evaluating the aging resistance of bake-hardened steel. Chinese Patent Publication No. CN109517951A discloses a method for evaluating the aging resistance of ultra-low carbon bake-hardened steel. This method heat treats samples at different temperatures and aging times, tests the critical aging time corresponding to the onset of yield in the samples, and then tests the yield elongation (Ae) of the samples. If Ae ≤ 0.2%, the ultra-low carbon bake-hardened steel corresponding to this aging scheme has good aging resistance. However, based on actual stamping production processes, it was found that when the yield elongation (Ae) of the material is ≤ 0.2%, tensile strain marks will not appear during the stamping process. However, most sheet metal with a yield elongation of 0.2% < Ae ≤ 0.5% will not show tensile strain marks during the stamping process. Some materials will not show tensile strain marks during the stamping process even if the yield elongation (Ae) exceeds 0.5%. Therefore, the yield elongation (Ae) ≤ 0.2% used in this method as a criterion for good aging resistance is too strict. Furthermore, this method specifies that after heat treatment, the yield elongation (Ae) at the critical time of yielding is measured, and then the aging performance is evaluated based on the magnitude of Ae. If the heat treatment equipment can ensure temperature uniformity and time control is sufficiently precise, samples with a yield elongation (Ae) ≤ 0.2% can be obtained at each heat treatment temperature. This method cannot effectively determine the aging performance of the material. Furthermore, this method only provides a rough assessment of aging performance and does not involve an assessment of the degree of aging. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an accurate and efficient method for quickly evaluating the aging degree of bake hardening steel.

[0006] In order to solve the above technical problems, the method steps taken by the present invention are:

[0007] (1) Establish the initial temperature-time curve when bake hardening steel initially produces discontinuous yield;

[0008] (2) Measure the aging time t corresponding to the discontinuous yield elongation Ae = x at 100 °C for the bake hardened steel to be evaluated. x ;

[0009] (3) Temperature Tn and aging time t x is the reference point, Tn is selected from 80°C to 100°C; the initial temperature-time curve is translated along the time direction until the point corresponding to Tn in the initial temperature-time curve coincides with the reference point; the temperature-time curve when the bake hardening steel produces a discontinuous yield elongation Ae=x can be obtained.

[0010] Furthermore, the step (1) is: heating the tensile specimen of the bake-hardened steel at different aging temperatures, obtaining the initial aging time when the specimen initially produces discontinuous yielding and the yield elongation Ae is less than 0.1% at different aging temperatures; and drawing an initial temperature-time curve according to the corresponding aging temperature and initial aging time.

[0011] Furthermore, the different aging temperatures include temperature Tn.

[0012] Furthermore, the step (2) is as follows: taking a number of tensile specimens of bake-hardened steel and conducting an aging test at a temperature Tn, and after the aging time reaches the initial aging time corresponding to Tn in the initial temperature-time curve; taking out the tensile specimens at intervals and conducting a tensile test to obtain the aging time t corresponding to the discontinuous yield elongation Ae=x to be evaluated. x .

[0013] The beneficial effect of the above technical solution is that the present invention combines room temperature natural aging with high temperature artificial aging to construct a temperature-time curve when the material initially undergoes discontinuous yielding. Then, through experiments, the time required for artificial aging to a certain degree at 80-100°C is obtained. This point is used as a benchmark to construct a temperature-time curve that produces the same degree of aging. The curve can then be used to calculate the time required for the same degree of aging at room temperature.

[0014] The present invention is not limited by composition, strength, etc. Any bake-hardened steel that is time-sensitive and used for cold stamping can be evaluated using this method. The present invention combines artificial accelerated aging with natural aging to construct a temperature-time curve and functional relationship under the same yield elongation, which can quickly evaluate the time required for natural aging at room temperature to reach the same level. The method is simple and convenient, highly operational, accurate, and efficient, greatly shortening the experimental cycle and providing targeted guidance for customer production applications. This application defines the temperature at 80-100°C, and can be tested by water bath heating. Sampling is convenient, and the mid-sampling process will not produce temperature fluctuations. The test repeatability, accuracy, and precision are all optimal. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] Figure 1 This is the temperature-time curve diagram described in Example 1 of the present invention;

[0017] Figure 2 This is the temperature-time curve diagram of Example 2 of the present invention. DETAILED DESCRIPTION

[0018] The method for rapidly evaluating the aging degree of bake-hardened steel is to sample the ultra-low carbon bake-hardened steel and perform aging heat treatment in the temperature range of 25±5℃ to 170±5℃, find out the time when the tensile specimen begins to age at different temperatures, take the yield elongation of the specimen when tensile 0<Ae<0.1% as the aging start criterion, and establish a temperature-time curve for the onset of aging; then carry out accelerated aging for a longer time in a water bath at temperature Tn, with Tn selected from 80℃ to 100℃, and obtain the aging time t corresponding to the yield elongation Ae reaching x. x , the yield elongation x is between 0.2% and 2.0%, which is determined according to the actual part characteristics; and then the point (t x , Tn°C) as the reference, the temperature-time curve for the onset of aging is right-shifted to obtain the temperature-time curve for the bake-hardened steel when the yield elongation reaches x after aging. The function expression is then best fitted to obtain its expression, and the time required for the yield elongation Ae of the bake-hardened steel to reach x after natural aging at room temperature can be calculated. The time required for the yield elongation Ae of the bake-hardened steel to reach x after natural aging calculated using this method is consistent with the time required for the same degree of aging to occur naturally at room temperature. The specific process steps are as follows:

[0019] (1) Take the bake-hardened steel to be subjected to aging evaluation and process it into a tensile specimen. It is necessary to ensure that there is no work hardening on the edge of the gauge section of the tensile specimen. Wire cutting or CNC milling machine can be used for processing;

[0020] (2) Artificially age the specimens at different temperatures in a constant temperature drying oven to obtain the time required for initial discontinuous yielding; the aging temperature is 25±5°C to 170±5°C, and the temperature gradient is set to 10°C to 20°C, with equal or unequal intervals. Tn°C is the required aging temperature;

[0021] (3) After determining the gradient aging temperature, adjust the furnace temperature to one of the temperatures T1. After the furnace temperature stabilizes, quickly put the sample into the furnace. After the furnace temperature stabilizes to the set temperature T1 again, start timing to obtain the time h when the sample initially produces discontinuous yield and the yield elongation Ae is less than 0.1%. T1 ;

[0022] (4) Then adjust the furnace temperature to the next temperature T2 and repeat step (3) to obtain the aging time h at temperature T2. T2 , until all the experiments at all aging temperatures are completed; the aging time h corresponding to each temperature T is obtained T ;

[0023] (5) According to the various temperatures T and corresponding aging times h obtained in steps (3) and (4) T, draw the initial temperature-time curve when the bake hardening steel initially produces discontinuous yield;

[0024] (6) Take several tensile specimens and conduct aging test in a water bath at Tn℃, where Tn is selected from 80℃ to 100℃, preferably Tn = 100℃; wait until the aging time reaches h Tn After the temperature reaches Tn℃, a group of samples are taken out every few minutes for tensile test to obtain the aging time and the time t corresponding to different yield elongation Ae;

[0025] (7) To evaluate the aging performance of bake-hardened steel when the discontinuous yield elongation Ae is x, x is 0.2% to 2.0%; the aging time corresponding to the yield elongation Ae reaching x in the Tn℃ water bath of step (6) is t x , the initial temperature-time curve is 100℃ and the aging time t x The corresponding coordinate point is the reference point, and the initial temperature-time curve is translated along the time direction until the point corresponding to Tn°C in the initial temperature-time curve coincides with the reference point; the temperature-time curve when the bake hardening steel produces a discontinuous yield elongation Ae=x can be obtained; Figure 1 As shown, this is a temperature-time curve of the bake-hardened steel of Example 1, wherein the solid line is the initial temperature-time curve when the bake-hardened steel initially produces discontinuous yielding, the triangular point is the time point when the yield elongation reaches x after artificial aging at Tn=100°C, and the dotted line is the temperature-time curve when the aging degree is x, obtained by translating the solid line with the triangular point as the reference;

[0026] (8) Performing optimal fitting on the temperature-time curve obtained in step (7) when Ae=x, obtaining the temperature-time function relationship when the discontinuous yield elongation Ae=x, as shown in the following formula (I),

[0027] T=a*ln(-b*ln(t)) (Ⅰ)

[0028] In formula (I), T is temperature, °C; t is aging time, min; a and b are constants obtained by fitting the temperature-time curve when Ae=x.

[0029] (9) According to the temperature-time function relationship obtained in step (8), the aging time required for the bake-hardened steel to reach the yield elongation Ae x at other temperatures can be calculated. That is, according to the local temperature, the temperature T can be assigned a value to obtain the time required for the bake-hardened steel to reach the yield elongation x by natural aging when placed at room temperature.

[0030] Example 1: This method for rapidly evaluating the aging degree of bake-hardened steel adopts the following specific process.

[0031] When the yield elongation of the 180MPa-grade bake-hardened steel used in the door outer panels of a certain vehicle model reaches 0.5%, there is a high probability of tensile strain marks appearing at the keyhole during stamping production. The aging cycle of the raw material is now evaluated:

[0032] The specimens were processed into tensile specimens by wire cutting and artificially aged at 25°C, 40°C, 60°C, 80°C, 100°C, 110°C, 120°C, 140°C, 150°C and 170°C in a constant temperature drying oven. The time when discontinuous yielding (Ae < 0.1%) initially occurred at each temperature was found, and the initial temperature-time curve of the bake-hardened steel initially producing discontinuous yield was plotted. At 100°C, it took 30 minutes for discontinuous yielding to occur.

[0033] Place the sample in 100℃ water and heat it. After the heating time exceeds 30 minutes, take a group of samples every 5 minutes for testing. Find out that it takes 45 minutes for the yield elongation to reach 0.5% during the tensile process. Then, use (45 minutes, 100℃) as the reference point to translate the initial temperature-time curve of the initial discontinuous yield, and obtain the temperature-time curve when the discontinuous yield length of the product is 0.5%, as shown in the figure. Figure 1 As shown, the best fit expression is: T = -64*ln(-0.055*ln(t)).

[0034] The product's storage temperature is approximately 25°C, and t is calculated to be 153 days at this point. Therefore, if the product is stamped and used within 5 months, no tensile strain marks will be produced. Subsequent experiments, in which the product was stored at 25±5°C and samples were taken for stamping on days 50, 100, 150, 153, 160, and 170, showed no tensile strain marks until day 160, but tensile strain marks appeared on day 170.

[0035] Example 2: This method for rapidly evaluating the aging degree of bake-hardened steel adopts the following specific process.

[0036] When the yield elongation of the 220MPa grade bake-hardened steel used in the hood outer panel of a certain vehicle model reaches 0.7%, there is a high probability of tensile strain marks appearing in the die area during stamping production. The aging degree cycle of the raw material is now evaluated:

[0037] The specimens were processed into tensile specimens by wire cutting and artificially aged at 30°C, 50°C, 70°C, 90°C, 100°C, 120°C, 130°C, 150°C and 170°C in a constant temperature drying oven. The time when discontinuous yielding (Ae < 0.1%) initially occurred at each temperature was found, and the initial temperature-time curve of the bake-hardened steel initially producing discontinuous yield was plotted. At 100°C, it took 34 minutes for discontinuous yielding to occur.

[0038] Place the sample in 100℃ water and heat it. After the heating time exceeds 34 minutes, take a group of samples every 5 minutes for testing. Find out that it takes 50 minutes for the yield elongation to reach 0.7% during the tensile process. Then, use (50 minutes, 100℃) as the reference point to translate the initial temperature-time curve of the initial discontinuous yield, and obtain the temperature-time curve when the discontinuous yield length of the product is 0.5%, as shown in the figure. Figure 2 As shown, the best fitting expression is: T = -64.5*ln(-0.054*ln(t)).

[0039] The product's storage environment temperature is relatively high, approximately 30°C. The calculated time t is 80 days, so the product needs to be used within three months to ensure that no tensile strain marks appear during use. Lowering the temperature of the coil storage area to below 25°C can ensure that the material will not produce tensile strain marks within six months of use. Subsequent experiments tested the product at 30±5°C, with samples taken for stamping at the 20th, 40th, 60th, 153rd, 70th, 80th, 90th, and 95th days. No tensile strain marks appeared until the 90th day, but they did appear on the 95th day.

[0040] Subsequent experimental statistics: The applicant randomly selected 90 samples of bake-hardened steel products, placed them in three storage environments at 20°C, 25°C, and 30°C, and used this method to calculate the aging time; then, samples were taken for stamping tests 10 days, 5 days, 2 days, and on the day before the calculated aging time, as well as on the 2nd, 5th, and 10th days after the calculated aging time; according to statistics, none of the 90 samples showed tensile strain marks on or before the calculated aging time, of which 19 samples showed tensile strain marks on the 2nd day, 63 samples on the 5th day, and 8 samples on the 10th day.

[0041] Usage case statistics: The applicant randomly selected 100 bake-hardened steel products and used this method to calculate the aging time; subsequently tracking the customer's usage, 84 of them underwent subsequent stamping within the calculated aging time, and none of them showed tensile strain marks; 16 of them underwent stamping after exceeding the calculated aging time, 9 of which showed no tensile strain marks and 7 showed tensile strain marks.

Claims

1. A method for rapidly evaluating the aging degree of bake-hardened steel, characterized in that: The method steps are: (1) Establish the initial temperature-time curve when bake hardening steel initially produces discontinuous yield; (2) Measure the aging time t corresponding to the discontinuous yield elongation Ae=x to be evaluated when the bake hardened steel reaches 100℃ x ; (3) Temperature Tn and aging time t x is the reference point, Tn is selected from 80°C to 100°C; the initial temperature-time curve is translated along the time direction until the point corresponding to Tn in the initial temperature-time curve coincides with the reference point; the temperature-time curve when the bake hardening steel produces a discontinuous yield elongation Ae=x can be obtained.

2. A method for rapidly evaluating the aging degree of bake hardening steel according to claim 1, characterized in that: The step (1) comprises: heating a tensile specimen of bake-hardened steel at different aging temperatures, obtaining an initial aging time at which the specimen initially produces discontinuous yielding and the yield elongation Ae is less than 0.1% at the different aging temperatures; and drawing an initial temperature-time curve according to the corresponding aging temperature and initial aging time.

3. The method for rapidly evaluating the aging degree of bake-hardened steel according to claim 2, wherein: The different aging temperatures include temperature Tn.

4. A method for rapidly evaluating the aging degree of bake hardening steel according to claim 1, 2 or 3, characterized in that: The step (2) is: taking a number of tensile specimens of bake-hardened steel and conducting an aging test at a temperature Tn, and after the aging time reaches the initial aging time corresponding to Tn in the initial temperature-time curve; taking out the tensile specimens at intervals and conducting a tensile test to obtain the aging time t corresponding to the discontinuous yield elongation Ae=x to be evaluated. x .

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