To reduce the weight and improve the crash safety of automobiles, hot stamping of quenchable steel sheets is useful. By heating the steel sheets, the forming load is remarkably reduced, springback is prevented, and formability is improved [1
]. The stamped parts are hardened by quenching with dies, and thus ultra-high strength steel parts with a tensile strength of approximately 1.5 GPa are obtained under a low forming load. In hot stamping, the quenchable steel sheets are heated to approximately 900 °C to transform the sheets to the austenite phase during heating [2
]. The sheets are generally heated by a furnace in the conventional warm and hot stamping. So, the temperature decreases due to the takeout procedure of the specimen from the furnace. In addition, the oxidation of the heated sheets occurs due to the forming setup and procedure. Therefore, rapid heating of the sheet is vital to reduce the oxidation scale. By changing the layout of the electrodes, the resistance heating is applicable to heat the sheets. Utilising resistance heating is a rapid way to synchronise with a press, and has higher energy efficiency than the other heating methods such as induction heating.
Mori et al. [3
] developed a hot stamping process using rapid resistance heating to improve the productivity and investment cost. The sheets are heated for only 2 s to 900 °C required for quenching. The oxide scale of the products is hardly generated by rapid heating [4
]. The efficiency of resistance heating is higher than that of induction heating employed by Kolleck et al. [5
] due to the direct passage of current through the sheets. Maki et al. [6
] studied the feasibility of hot stamping and press quenching of ultra-high strength steel sheet of SPFC980Y using resistance heating. Ozturk et al. [7
] used resistance heating in the hot stamping process of titanium alloy sheets.
Aluminium-coated sheets are mostly utilised in hot stamping process to prevent the oxide scale; however, utilising coated steel sheets are limited in hot stamping using resistance heating due to the evolution of intermetallic layer in the furnace. Lee et al. [8
] examined the heating of aluminium-coated sheets using direct resistance heating and pointed out the related difficulty. Maeno et al. [9
] investigated ultrasonic cleaning with a diluted hydrochloric acid solution to remove the thin oxide scale in hot stamping of non-coated 22MnB5 sheet using resistance heating. The results indicated that the ultrasonic cleaning with diluted hydrochloric acid could successfully remove the thin oxide scale and the cleaned surface had sufficient quality for welding and painting. However, the heating of the coated steel sheet using resistance heating is significantly longer than that of uncoated steel sheet and the productivity of hot stamping of coated steel sheets are low.
Roller hearth furnace is generally employed in hot stamping to heat steel sheets continuously. The heating speed, in this case, is comparatively slow, and thus the heated sheets are fully austenitised to undergo a martensitic transformation by rapid cooling. In addition, the gradual heating speed is suitable to generate an intermetallic compound having high oxidation prevention for aluminium- and zinc-coated sheets [10
]. On the other hand, rapid resistance heating is too fast to austenitise the steel sheets fully. Quan et al. [11
] investigated the effect of austenite’s holding time on phase transformation by finite element analysis. They showed that the final martensite volume fraction changed with an increase of the holding time in the austenite temperature range of 800–900 °C. Zhang et al. [12
] studied the phase transformation in the hot stamping process of USIBOR 1500 high-strength steel based on the Kirkaldy-Venugopalan model during the pressure holding quenching process. Pedraza et al. [13
] studied the effect of rapid heating and fast cooling on the transformation behaviour and mechanical properties of 22MnB5 steel. Mori et al. [14
] developed smart hot stamping of ultra-high strength steel products using rapid resistance heating and mechanical servo press. Löbbe et al. [15
] examined the influence of various austenitisation parameters such as rapid heating and cooling on the mechanical properties in hot stamping of ultra-high strength steel sheets.
Not only the hot stamping process but also the warm stamping process is also attractive and useful due to low heating temperature and oxidation. Mori et al. [16
] investigated the warm stamping of ultra-high strength steel sheets using rapid resistance heating around a heating temperature of 300 °C. The increase in hardness in this case is related to the transformation of retained austenite into martensite at comparatively low temperatures using rapid resistance heating and cooling. Sun et al. [17
] investigated the ductility and post-form strength to form a martensitic steel MS1180 into a complex-shaped component using a fast-warm stamping technique at the forming temperature range of 400–450 °C with the heating rate over 50 °C/s. Liu et al. [18
] studied the characterization of thermomechanical boundary conditions of non-alloy martensitic steel for a fast forming process.
Rapid cooling is one of the essential parameters to harden the formed parts in die quenching. On the other hand, the cooling rate of die quenching is lower than that of water quenching due to low heat transfer. Hoffmann et al. [19
] concluded that the cooling speed of die quenching is influenced by the die pressure and a large holding force is required to increase the cooling speed during die quenching. Nürnberger et al. [20
] studied the hot stamping of heat treatable steel 22MnB5 using water-air spray cooling. Ota et al. [21
] improved the formability by partial air cooling of potential cracking regions of blank to harden them before forming. Zhao et al. [22
] showed high hardness and minimum springback of a hot-stamped part by using rapid cooling to a forming temperature between 700 and 750 °C after ejecting from the furnace. Lee et al. [23
] designed a slice die to improve the cooling rate of the blank during hot stamping and the quenching process.
The productivity of hot stamping is lower than that in cold stamping due to the requirement of holding at the bottom dead centre for die-quenching. Behrens et al. [24
] decreased the holding time by spraying water cooling just after removal of the stamped-parts from the tools. Maeno et al. [25
] improved and shortened the holding time for die quenching by the water and die quenching process in the hot deep drawing test. They reduced the holding time for hardening due to the high cooling speed of the water. Nakagawa et al. [26
] reduced the holding time at bottom dead centre in hot stamping by water and die quenching using submerged tools.
In the present study, the hardening behaviour of an uncoated quenchable steel sheet for hot stamping using rapid resistance heating was examined in a hot bending experiment to obtain full hardening of the products. To achieve full hardening, the heating rate was decreased, and the holding at the austenitising temperature was performed in hot stamping using resistance heating.
3. Effect of Heat Microstructure of Blank Sheet on Quenchability in Die Quenching
To investigate the effect of the microstructure of the blank sheet on the die-quenchability for v
= 275 °C/s and t
= 0 s as comparatively short heating time, normalise and anneal heat treatment were performed before hot stamping. Then, the heat-treated sheets were resistance-heated, stamped, and quenched for the experiments. The heating path used for the heat treatment and the microstructures of the as-received, normalised and annealed sheets before hot stamping are shown in Figure 7
and Figure 8
, respectively. At first, the cut section was polished with emery paper to get a smooth mirror-like surface, and then it was lapped with alumina powder. The polished surface was etched with Nital solution. The optical microscope was utilised for observation. As illustrated in Figure 8
, the dark areas show perlite and the light-grey areas show ferrite. All of the sheets in these experiments contain ferrite and perlite structures. The as-received sheet reveals the band structure due to cold rolling; however, the granular perlite with the precipitated ferrite is obtained from normalised sheet. The annealed sheet shows the band structure, because the manganese forms the band structure by rolling and carbon is also segregated according to the segregation of manganese. The grain size of the normalised sheet is considerably small, whereas that for the annealed sheet is large.
An effect of the heat treatment of the blank before hot stamping on Vickers hardness after die- and water-quenching using direct resistance heating for v
= 275 °C/s and t
= 0 s is shown in Figure 9
. The die-quenched hardness of the as-received sheet using furnace heating of 240 s reaches 450 HV, whereas the die-quenched hardness of the as-received and annealed sheets using direct resistance heating reduce. Figure 10
illustrates the effect of heat treatment of blank on the distributions of Vickers hardness in thickness direction after die-quenching using direct resistance heating for v
= 275 °C/s and t
= 0 s. The hardness scatter of the normalised sheet becomes approximately steady in thickness direction, whereas it increases for the annealed sheet and decreases for the as-received one. The result of Figure 9
was arranged based on the grain size of the microstructure of the blank and is shown in Figure 11
All heat-treated sheets have enough hardness in case of water quenching. Therefore, the quenchability of the sheet is a reason for insufficient hardness. The manganese and boron have a large effect on the quenchability. However, since the band structure was formed for as-received and annealed sheets as shown in Figure 8
, it expects the segregation of manganese and boron near perlite and grain boundary. So, it is difficult to diffuse those to ferrite for short heating time, hence the low quenchability portion partially appears.
The microstructures of the die- and water-quenched sheets from as-received, normalised and annealed sheets for v
= 275 °C/s and t
= 0 s are shown in Figure 12
. On the results of die quenching, ferrite is observed for as-received and annealed sheets. In particular, ferrite forms a thick layer in the result of the annealed sheet, corresponding to the microstructure shown in Figure 8
c. As a result of water quenching, a small ferrite layer was observed for both the as-received and the annealed sheets. A full martensitic transformation occurred for the result from the normalised sheet even for the comparatively short heating time of direct resistance heating.
4. Full Hardening of Products by a Decrease in Heating Rate and by Temperature Holding in Hot Stamping Using Direct Resistance Heating
The effect of the heating rate on the Vickers hardness and the standard deviation of the as-received sheet without pre-heating treatment for t
= 0 s is shown in Figure 13
. As the heating rate decreases, the standard deviation of the hardness improves and the hardness slightly increases. The heating rate is required below a certain value for full hardening.
The heating temperature was held at the target temperature for several seconds to obtain full hardening even for a short heating time in direct resistance heating. The variations in temperature in resistance heating with and without holding at 900 °C above the austenitising temperature for v
= 275 °C/s are shown in Figure 14
. After reaching 900 °C, the temperature was kept almost constant by resistant-heating under low current.
The hot hat-shaped bent parts for v
= 275 °C/s and t
= 0, 5, and 10 s are shown in Figure 15
. The appearances about oxidation of the formed products are similar to the parts without temperature holding due to the short temperature holding time.
The effect of temperature holding during austenitising on the Vickers hardness and standard deviation of that for v
= 275 °C/s is shown in Figure 16
. As the holding time during austenitising increases, the hardness slightly increases up to t
= 5 s. The standard deviation of the hardness is remarkably reduced by temperature holding above 1 s and is similar to the result of furnace heating.
The microstructures of hot hat-shaped bent parts with and without temperature holding during austenitising for v
= 275 °C/s are shown in Figure 17
. By temperature holding, formation of the ferrite is prevented, and the fully martensitic structures are obtained for t
= 5 and 10 s.
The decrease in heating rate and temperature holding during austenitising are effective for full hardening of products in hot stamping using direct resistance heating. The total heating times with and without temperature holding during austenitising were compared. The relationships between the Vickers hardness and total heating time with and without temperature holding during austenitising are shown in Figure 18
. For resistance heating with temperature holding, the sufficient hardness was obtained above 4 s, whereas 8 s is required without temperature holding. The minimum heating time for full hardening is decided to 4 s based on the hardness in this quenchable steel sheet in hot stamping using direct resistance heating. The rapid heating rate with temperature holding during austenitising is effective to minimise the heating time for full hardening.
The effect of the total heating time on the thickness of the oxide layer of hat-shaped bent parts in hot stamping using direct resistance heating is shown in Figure 19
. The oxide layer was removed by the soaking into the 3% hydrochloric acid with ultrasonic vibration. Then, the thickness was measured after removing the oxide layer. The thickness of the oxide layer is half of the difference in thickness of between that before and after oxide removing. The thicknesses of the oxide layer of both heating paths increase with the increase of the total heating time. For the result using temperature holding, the thicknesses of the oxide layer are slightly thicker than that without temperature holding owing to the fact that the time at the high temperature was slightly long.
From the aforementioned results, it was found that the increase in the heating time due to holding at the austenitising temperature and decrease in heating rate is small, and this time is included in the time for transferring the heated sheet to dies. For example, it is conceivable to maintain the temperature by installing an electrode on the transfer manipulator as shown in Figure 20