Die construction methodology for reducing quench time for press hardenable steels

Abstract

A method of quenching a press hardenable steel (PHS) is provided. The method includes preparing a die having a material with a thermal conductivity of at least 40W/(m.Math.K) and placing a blank within the die and simultaneously hot stamping and quenching the blank at a heat transfer coefficient of at least 2,950W/(m.sup.2.Math.K). In one form, the step of hot stamping the blank is carried out with greater than 20 MPa of contact pressure between the die and the blank. In another form, the step of hot stamping the blank is carried out with 31 MPa of contact pressure between the die and the blank.

Claims

1. A method of quenching a press hardenable steel (PHS) comprising: preparing a die having a material with a thermal conductivity of at least 40 W/(m.Math.K); and placing a blank within the die and simultaneously hot stamping and quenching the blank at a heat transfer coefficient greater than or equal to 2300 W/(m.sup.2.Math.K) and less than or equal to 2950 W/(m.sup.2.Math.K) to a temperature of less than 130° C. and quench time of less than 5 seconds, the hot stamped blank having a yield strength greater than 1,400 MPa, wherein the step of hot stamping the blank is carried out with 31 MPa of contact pressure between the die and the blank.

2. The method according to claim 1, wherein the PHS has a composition comprising: manganese greater than zero and up to 1.4 wt. %; silicon greater than zero and up to 0.7 wt. %; carbon greater than zero and up to 0.37 wt. %; and boron greater than zero and up to 0.005 wt. %.

3. The method according to claim 1, wherein the die material has a hardness of 48 HRc thermal conductivity of at least 34 W/(m.Math.K) at 600° C. and thermal conductivity of at least 44 W/(m.Math.K) at 0° C.

4. The method according to claim 1, wherein the heat transfer coefficient is achieved by hydraulic pressure control.

5. The method according to claim 1, further comprises a steady state temperature of the die is less than 85° C.

6. The method according to claim 5, wherein the steady state temperature of the die is 65° C.

7. The method according to claim 1, wherein the hot stamped blank has a tensile strength greater than 1,900 MPa.

8. A method of quenching a press hardenable steel (PHS) comprising: preparing a die having a material with a thermal conductivity greater than 28 W/(m.Math.K), a die contact surface, and a cooling channel; and placing a blank within the die and hot stamping the blank with 31 MPa of contact pressure between the die and the blank at a heat transfer coefficient greater than or equal to 2300 W/(m.sup.2.Math.K) and less than or equal to 2950 W/(m.sup.2.Math.K) to a temperature of less than 130° C. and quench time of less than 5 seconds, wherein a distance between a center of the cooling channel and the die contact surface is less than 10 mm.

9. The method according to claim 8, wherein the distance is 8 mm.

10. The method according to claim 8, wherein the PHS has a composition comprising: manganese greater than zero and up to 1.4 wt. %; silicon greater than zero and up to 0.7 wt. %; carbon greater than zero and up to 0.37 wt. %; and boron greater than zero and up to 0.005 wt. %.

11. The method according to claim 8, further comprising a steady state temperature of the die is less than 85° C.

12. The method according to claim 8, wherein the hot stamped blank has a yield strength greater than 1,400 MPa and a tensile strength greater than 1,900 MPa.

Description

DRAWINGS

(1) In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

(2) FIG. 1 illustrates the relationship between strength and part extraction temperature for 36MnB5 grade steel according to the discoveries of the present disclosure;

(3) FIG. 2 illustrates the relationship between the cooling rate and the blank thickness for 22MnB5 and 36MnB5 grade steels according to the discoveries of the present disclosure;

(4) FIG. 3 illustrates the relationship between the cooling rate and the die temperature for 22MnB5 and 36MnB5 grade steels according to the discoveries of the present disclosure;

(5) FIG. 4 illustrates the relationship between blank temperature and time for a 1.5 mm 36MnB5 grade steel blank to cool from about 830° C. to about 100° C. according to the teachings of the present disclosure;

(6) FIG. 5 illustrates the relationship between cooling rate and time for a 1.5 mm 36MnB5 grade steel blank to cool from about 830° C. to about 100° C. according to the teachings of the present disclosure;

(7) FIG. 6 is a flowchart for a method of quenching a press hardenable steel according to the teachings of the present disclosure;

(8) FIG. 7 is a flowchart for another method of quenching a press hardenable steel according to the teachings of the present disclosure; and

(9) FIG. 8 is a flowchart for yet another method of quenching a press hardenable steel according to the teachings of the present disclosure.

(10) The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

(11) The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

(12) Generally, to address the issues related to press hardening a press hardenable steel while using manufacturing equipment designed for 22MnB5 grade press hardenable steel processing, the present disclosure reduces quench time of a higher grade press hardenable steel to about the quench time for the 22MnB5 grade steel.

(13) The inventors discovered that between production conditions for 22MnB5 grade steel and 36MnB5 grade steel, die quench time for 36MnB5 grade steel would be significantly higher than 22MnB5 grade steel. The inventors also discovered that the yield strength (YS) and the ultimate tensile strength (UTS) of 36MnB5 grade steel would be lower than the specification using existing production equipment/processing for 22MnB5 grade steel. This is reflected below in Table 1:

(14) TABLE-US-00001 TABLE 1 Part YS UTS Extraction Specimen (MPa) (MPa) % EL Temperature 22MnB5 PHS #112 1013 1456 18 ~210° C. 22MnB5 PHS #114 1050 1468 17 ~210° C. Ave 1031.5 1462 17.5 36MnB5 PHS #109 1247 1824 12 36MnB5 PHS #110 1235 1821 15 ~210° C. Ave 1241 1822.5 13.5 ~210° C. Specification ≥1400 ≥1800 ≥4

(15) As shown, 22MnB5 grade steel properties for Yield Strength (YS) and Ultimate Tensile Strength (UTS) are within the specification for typical production part extraction temperature of about 200° C. However, the yield strength for 36MnB5 grade steel processed with 22MnB5 grade steel typical production part extraction temperatures were below the 36MnB5 specification yield strength of greater than or equal to 1,400 MPa.

(16) Referring to FIG. 1, the relationship between strength and part extraction temperature for 36MnB5 grade steel processed with 22MnB5 grade steel hot-stamping tooling and procedures is shown as discovered by the inventors. As illustrated, the tensile strength (TS) of the 36MnB5 grade steel is relatively constant (≤75 MPa) with respect to the part extraction temperature over the range of about 75-200° C. However, the yield strength of the 36MnB5 grade steel is varies by about 300 MPa and is therefore dependent upon the part extraction temperature over the range of about 75-200° C. The desired yield strength for 36MnB5 grade steel is greater than 1,400 MPa, which shows that 36MnB5 grade steel is enabled for production when the part extraction temperature is below about 130° C.

(17) Referring to FIGS. 2-3, the differences to reach the target temperatures for 22MnB5 grade steel and 36MnB5 grade steel are plotted with respect to blank thickness (FIG. 2) and die steady state temperature (FIG. 3) as discovered by the inventors. The inventors discovered that the difference in time to reach the target extraction temperatures for 36MnB5 grade steel versus 22MnB5 grade steel varies by 1.5-5 seconds. These results showed that the mechanical properties of 36MnB5 grade steel are more sensitive to variations in the cooling systems (quenching technology and processes) than 22MnB5 grade steel.

(18) According to the present disclosure, one method to reduce 36MnB5 grade steel quench time is to reduce the Time-Temperature-Transformation (TTT) relationship and therefore the time to quench the 36MnB5 grade steel. Numerous analyses and testing resulted in the processing parameters of TABLE 2 below and the relationships shown in FIG. 4 and FIG. 5.

(19) TABLE-US-00002 TABLE 2 22MnB5 36MnB5 grade steel grade steel Plate thickness (mm) 1.5 1.5 Die Contact Pressure (MPa) 19.1 31 Die contact heat transfer coefficient 2302 2943 (W/K*m{circumflex over ( )}2) Die thermal conductivity (W/K*m) 28 45 Die surface absorptivity 0.6 0.6 Die steady state average temperature (° C.) 83 65 Part quench temperature (° C.) ~200 ± 10 <130 Time to quench (seconds) ~4.7 ~4.8 Distance to cooling channel (mm) 10 8

(20) The die contact pressure is the pressure between the die and the steel blank, and the distance to cooling channel is the distance between the center of the cooling channel to the die contact surface. Further, as die thermal conductivity increases, the abrasive resistance of the die reduces, therefore an abrasive resistant coating and/or surface hardening of the dies may be desired.

(21) Referring to FIG. 6, in one form of the present disclosure, a method 20 of quenching a press hardenable steel (PHS) is provided. At step 22, the method 20 comprises preparing a die having a material with a thermal conductivity of at least 40 W/(m.Math.K). At step 24, the method 20 comprises placing a blank within the die and simultaneously hot stamping and quenching the blank at a heat transfer coefficient of at least 2,950 W/(m.sup.2.Math.K).

(22) In another method of the present disclosure, the step of hot stamping the blank is carried out with greater than 20 MPa of contact pressure between the die and the blank. In at least one method of the present disclosure, the step of hot stamping the blank is carried out with 31 MPa of contact pressure between the die and the blank.

(23) In variations of the method of the present disclosure, the PHS has a composition comprising: manganese greater than zero and up to 1.4 wt. %; silicon greater than zero and up to 0.7 wt. %; carbon greater than zero and up to 0.37 wt. %; and boron greater than zero and up to 0.005 wt. %, as shown below in TABLE 3:

(24) TABLE-US-00003 TABLE 3 Minimum Maximum Element wt. % wt. % Boron >0 0.005 Carbon >0 0.37 Manganese >0 1.4 Silicon >0 0.7 Iron Balance Balance

(25) In yet another method of the present disclosure, the die material has a hardness of 48 HRc, thermal conductivity of at least 34 W/(m.Math.K) at 600° C., and thermal conductivity of at least 44 W/(m.Math.K) at 0° C. This die material has a composition, as shown in TABLE 4:

(26) TABLE-US-00004 TABLE 4 Minimum Maximum Die Material Die Material Element wt. % wt. % 600 wt. % 620 wt. % Carbon 0.32 0.5 0.32 0.32 Chromium 0 5 0 0 Manganese 0.2 0.3 0.25 0.25 Molybdenum 3.0 3.3 3.3 3.3 Nickel 0 2 2 0 Silicon 0.1 0.2 0.1 0.12 Tungsten 0 2 1.8 1.8 Vanadium 0 0.6 0 0 Iron Balance Balance Balance Balance

(27) In a variation of the present disclosure, the heat transfer coefficient is achieved by hydraulic pressure control.

(28) In a method of the present disclosure, the method further comprises a steady state temperature of the die is less than 85° C. In some of these methods of the present disclosure, the steady state temperature of the die is 65° C.

(29) In other methods of the present disclosure, the hot stamped blank has a yield strength greater than 1,400 MPa and a tensile strength greater than 1,900 MPa.

(30) Now referring to FIG. 7, in another form of the present disclosure, a method 40 of quenching a press hardenable steel (PHS) is provided. At step 42, the method 40 comprises preparing a die having a material with a thermal conductivity greater than 28 W/(m.Math.K). At step 44, the method 40 comprises placing a blank within the die and hot stamping the blank at a heat transfer coefficient of at least 2,300 W/(m.sup.2.Math.K). At step 46, the method 40 comprises transferring the hot stamped blank to a cooling channel at a distance of less than 10 mm. In variations of the methods of the present disclosure, the distance is 8 mm.

(31) Referring to FIG. 8, in yet another form of the present disclosure, a method 60 of quenching a press hardenable steel (PHS) is provided. At step 62, the method 60 comprises preparing a die having a material with a thermal conductivity of at least 28 W/(m.Math.K). At step 64, the method 60 comprises placing a blank within the die and hot stamping the blank at a heat transfer coefficient of at least 2,300 W/(m.sup.2.Math.K), wherein a steady state temperature of the die is less than 85° C. At 66, the method 60 comprises cooling the hot stamped blank. In this method, the PHS has a composition comprising: manganese greater than zero and up to 1.4 wt. %; silicon greater than zero and up to 0.7 wt. %; carbon greater than zero and up to 0.37 wt. %; and boron greater than zero and up to 0.005 wt. %.

(32) In other methods of the present disclosure, cooling the hot stamped blank comprises simultaneously quenching the blank with the hot stamping, and the thermal conductivity of the die is at least 40 W/(m.Math.K).

(33) Furthermore, at least one part is manufactured according to the methods of the present disclosure.

(34) The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.