STEEL STRIP, SHEET OR BLANK FOR PRODUCING A HOT FORMED PART, PART, AND METHOD FOR HOT FORMING A BLANK INTO A PART

Abstract

A steel strip, sheet or blank for producing hot formed parts containing at least the following composition in weight %: C: 0.03-0.17, Mn: 0.65-2.50, Cr: 0.2-2.0, Ti: 0.01-0.10, Nb: 0.01-0.10, B: 0.0005-0.005, N: ≤0.01, wherein Ti/N≥3.42.

A hot formed part produced from such a steel strip, sheet or blank, to the use of such a hot formed part, and to a method for forming such a steel blank or a preformed part made from such a blank, into a part.

Claims

1. The steel strip, sheet or blank for producing hot formed parts having the following composition in weight %: C: 0.03-0.17, Mn: 0.65-2.50, Cr: 0.2-2.0, Ti: 0.01-0.10, Nb: 0.01-0.10, B: 0.0005-0.005, N: ≤0.01, wherein Ti/N≥3.42, and optionally one or more of the elements selected from: Si: ≤0.1, Mo: ≤0.1, Al: ≤0.1, Cu: ≤0.1, P: ≤0.03, S: ≤0.025, O: ≤0.01, V: ≤0.15, Ni: ≤0.15 Ca: ≤0.15 the remainder being iron and unavoidable impurities.

2. The steel strip, sheet or blank according to claim 1, wherein: C: 0.05-0.17, and/or Mn: 1.00-2.10, and/or Cr: 0.5-1.7, and/or Ti: 0.015-0.07, and/or Nb: 0.02-0.08, and/or B: 0.0005-0.004, and/or N: 0.001-0.008, Ca: ≤0.01.

3. The steel strip, sheet or blank according to claim 1, wherein the sum of the amount of Mn and Cr is less than 2.7.

4. The steel strip, sheet or blank according to claim 1, wherein Mn, Cr and B are used in such amounts that (B×1000)/(Mn+Cr) is in the range of from 0.185-2.5.

5. The steel strip, sheet or blank according to claim 1, provided with a zinc based coating or an aluminium based coating or an organic based coating.

6. The steel strip, sheet or blank according to claim 5, wherein the zinc based coating is a coating containing 0.2-5.0 wt % Al, 0.2-5.0 wt % Mg, optionally at most 0.3 wt % of one or more additional elements, the balance being zinc and unavoidable impurities.

7. A hot formed part produced from a steel strip, sheet or blank according to claim 1, the hot formed part having a tensile strength of at least 750 MPa.

8. The hot formed part according to claim 7, having a total elongation (TE) of at least 5% and/or a bending angle (BA) at 1.0 mm thickness of at least 100°.

9. The hot formed part according to claim 7, the hot formed part having a microstructure comprising at most 60% bainite, the remainder being martensite.

10. A method of use of a hot formed part according to claim 7, comprising forming the hot formed part into a structural part in a body-in-white of a vehicle.

11. A method for hot-forming a steel blank or a pre-formed part into a part comprising the steps of: a. heating the blank, or a pre-formed part produced from the blank, wherein the blank or the blank from which the pre-formed part is produced is according to claim 1, to a temperature T1 and holding the heated blank at T1 during a time period t1, wherein T1 is higher than the Ac3 temperature of the steel, and wherein t1 is at most 10 minutes; b. transferring the heated blank or pre-formed part to a hot-forming tool during a transport time t2 during which the temperature of the heated blank or preformed part decreases from temperature T1 to a temperature T2, wherein the transport time t2 is at most 20 seconds; c. hot forming the heated blank or preformed part into a part; and d. cooling the part in the hot-forming tool to a temperature below the Mf temperature of the steel with a cooling rate of at least 30° C./s.

12. The method according to claim 11, wherein the temperature T1 in step (a) is 50-100° C. higher than the Ac3 and/or the temperature T2 is above Ar3.

13. The method according to claim 11, wherein the time period t1 in step (a) is at least 1 minute and at most 7 minutes and/or the time period t2 in step (b) is at most 12 seconds.

14. The method according to claim 11, wherein the part is cooled in step (d) with a cooling rate in the range of 30-150° C./s.

15. A vehicle comprising at least one hot formed part according to claim 7.

16. A vehicle comprising at least one hot formed part produced according to claim 11.

17. The steel strip, sheet or blank according to claim 1, wherein: C: 0.07-0.15, Mn: 1.20-1.80, Cr: 0.8-1.5, Ti: 0.025-0.05, Nb: 0.03-0.07, B: 0.001-0.003, N: 0.002-0.005, and Ca: ≤0.01.

18. The steel strip, sheet or blank according to claim 1, wherein the sum of the amount of Mn and Cr is between 0.5 and 2.5.

19. The steel strip, sheet or blank according to claim 1, wherein Mn, Cr and B are used in such amounts that (B×1000)/(Mn+Cr) is in the range of from 0.5-1.5.

20. The hot formed part produced from a steel strip, sheet or blank according to claim 1, the part having a tensile strength of at most 1400 MPa.

21. The hot formed part according to claim 7 having a total elongation (TE) of at least 7% and/or a bending angle (BA) at 1.0 mm thickness of at least 140°.

22. The hot formed part according to claim 7, the part having a microstructure comprising at most 40% bainite, the remainder being martensite.

23. The method according to claim 11, wherein the time period t1 in step (a) is at least 1 minute and at most 7 minutes and/or the time period t2 is between 2 and 10 seconds.

24. The method according to claim 11, wherein the part is cooled in step (d) with a cooling rate of 30-100° C./s.

Description

[0101] One or more steps of the method according to the present invention may be conducted in a controlled inert atmosphere of hydrogen, nitrogen, argon or any other inert gas in order to prevent oxidation and/or decarburisation of said steel.

[0102] FIG. 1 shows a schematic representation of an embodiment of the method according to the invention.

[0103] FIG. 2 shows a cross-section through a drop tower for axial crash tests.

[0104] In FIG. 1, the horizontal axis represents the time t, and the vertical axis represents the temperature T. The time t and temperature T are indicated diagrammatically in FIG. 1. No values can be derived from FIG. 1.

[0105] In FIG. 1, a steel blank or preformed part is (re)heated up to the austenitizing temperature above Ac1 at a particular (re)heating rate. Once the Ac1 has been exceeded the (re)heating rate is lowered until the blank or preformed part has reached a temperature higher than the Ac3. Then the strip, sheet or blank is held at this particular temperature for a period of time. Subsequently, the heated blank is transferred from the furnace to the hot forming tool, during which cooling of the blank by air occurs to some extent. The blank or preformed part is then hot-formed into a part and cooled down (or quenched) at a cooling rate of at least 30° C./s. After reaching a temperature below the Mf temperature of the steel, the hot-forming tool is opened and the formed article is cooled down to room temperature.

[0106] The different temperatures as used throughout the patent application are explained below. [0107] Ac1: Temperature at which, during heating, austenite starts to form. [0108] Ac3: Temperature at which, during heating, transformation of the ferrite into austenite ends. [0109] Ar3: The temperature at which transformation of austenite to ferrite starts during cooling. [0110] Ms: Temperature at which, during cooling, transformation of the austenite into martensite starts. [0111] Mf: Temperature at which, during cooling, transformation of the austenite into martensite ends.

[0112] The invention will be elucidated by means of the following, non-limiting Examples.

EXAMPLES

Steel Composition a (According to the Invention)

[0113] Steel blanks with dimensions of 220 mm×110 mm×1.5 mm were prepared from a cold-rolled steel sheet having the composition as shown in Table 1. These steel blanks were subjected to hot forming thermal cycles in a hot dip annealing simulator (HDAS) and an SMG press. The HDAS was used for slower cooling rates (30-80° C./s) whereas the SMG press was used for fastest cooling rate (200° C./s). The steel blanks were reheated to a T1 of respectively 900° C. (36° C. above Ac3) and 940° C. (76° C. above Ac3), soaked for 5 min. in nitrogen atmosphere to minimize surface degradation. The blanks were then subjected to transfer cooling for a drop in temperature of 120° C. in 10s, so at a cooling rate V2 of about 12° C./s and then subjected to cooling to 160° C. at the following cooling rates V3: 30, 40, 50, 60, 80, 200° C./s. From the heat treated samples, longitudinal tensile specimens with 50 mm gauge length and 12.5 mm width (A50 specimen geometry) were prepared and tested with quasistatic strain rate. Microstructures were characterized from the RD-ND planes. Bending specimens (40 mm×30 mm×1.5 mm) from parallel and transverse to rolling directions were prepared from each of the conditions and tested till fracture by three-point bending test as described in the VDA 238-100 standard. The samples with bending axis parallel to the rolling direction were identified as longitudinal (L) bending specimens whereas those with bending axis perpendicular to the rolling direction were denoted as perpendicular (T) bending specimens. The measured bending angles at 1.5 mm thickness were also converted to the angles for 1 mm thickness (=original bending angle×square root of original thickness). For each type of test, three measurements were done and the average values from three tests are presented for each condition.

[0114] For selected conditions (SMG press samples with reheating at 940° C.), J-integral fracture toughness and drop tower axial crash tests were conducted. Compact tension specimens according to NFMT76J standard were prepared from both longitudinal and transverse directions for fracture toughness tests. For the transverse specimen, the crack runs along the rolling direction and the loading is transverse to the rolling direction, whereas the opposite applies for the longitudinal specimens. The specimens were tested according to ASTM E1820-09 standard at room temperature. The pre-cracks were introduced by fatigue loading. The final tests were done with tensile loading with anti-buckle plates to keep the stress in plane for sheet material. Three tests for each conditions were done and following the guidelines in BS7910 standard the minimum values of three equivalents (MOTE values) for different fracture toughness parameters are presented. A brief description of the fracture toughness parameters is given below. CTOD is the Crack Tip Opening Displacement and is a measure of how much the crack opens at either failure (if brittle) or maximum load. J is the J-integral and is a measure of toughness that takes account of the energy, so it is calculated from the area under the curve up to failure or maximum load. KJ is the stress intensity factor determined from the J integral using an established expression, given as KJ=[J(E/(1−v.sup.2))].sup.0.5 where E is the Young's modulus (=207 GPa) and v is the Poisson's ratio (=0.03). K.sub.q is the value of stress intensity factor measured at load P.sub.q, where P.sub.q is determined by taking the elastic slope of the loading line, then taking a line with 5% less slope and defining P.sub.q as the load where this straight line intersects the loading line.

[0115] Drop tower axial crash tests were done in SMG-pressed condition with a load of 200 kg and a loading speed of 50 km/hour for the load to hit the crash boxes having a closed top hat geometry (FIG. 2) with 500 mm height (transverse to the rolling direction). The dimensions of the cross-section of the drop tower are given in FIG. 2 in millimetres (t=1.5 mm, R.sub.0=3 mm). The back plates of 100 mm width were spot-welded to the profiles to prepare the crash boxes.

[0116] For some selected conditions, a paint bake thermal cycle was also given to the samples, and the tests were done as will be reflected from the results directly.

Steel Compositions B and C (not According to the Invention)

[0117] For comparison reasons a commercially available cold-formed CR590Y980T-DP (steel composition B and commonly known as DP1000 steel) was also tested since it has a similar strength level as the steel blank in accordance with the invention. In addition, and also for comparative reasons, a standard hot-formed 22MnB5 steel product (steel composition C) was tested.

[0118] In Table 1, the chemical compositions in wt % of steel compositions A-C are specified.

[0119] In Table 2, the transformation temperatures of steel composition A are shown.

[0120] The results of the various tests are presented in Tables 3 to 8.

[0121] In Table 3, the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE), and total elongation (TE) are shown for steel composition A after a variety of cooling rates V3. In addition, Table 3 shows the microstructure in terms of martensite (M) and bainite (B). It will be clear from Table 3 that an ultimate tensile strength of greater than 800 MPa was achieved at the different cooling rates V3.

[0122] In Table 4, bending angles (BA) at 1.0 mm thickness are shown for steel composition A as obtained after different cooling rates V3. It is clear from Table 4 that high bending angles of greater than at least 130° were achieved for both the longitudinal (L) and transverse (T) orientations.

[0123] In Table 5, the various mechanical properties have been shown for steel composition A after said composition has been subjected to a horforming and baking treatment simulating the paint baking treatment used during automobile manufacturing. Steel composition A was heated to 900° C., soaked for 5 min. and then cooled at a V3 of 200° C./s, following the transfer cooling. The baking treatment was carried out at 180° C. for 20 minutes. From Table 5, it will be clear that approximately the same minimum levels of yield strength YS), ultimate tensile strength (UTS), ultimate elongation (UE), total elongation (TE) and bending angels (BA) are also achieved after steel composition A has been subjected to a baking treatment. This means that in automotive manufacturing after paint baking, the properties claimed will be ensured in service condition.

[0124] In Table 6, the various mechanical properties of steel compositions B (DP1000) and C (22MnB5) are shown. These steel compositions B and C were tested under the same test conditions as steel composition A. When the contents of Tables 4 and 6 are compared it will become immediately evident that the steel part in accordance with the present invention (steel composition A) constitutes a major improvement in terms of bendability when compared with conventional cold-formed steel products DP1000 (steel composition B) and conventional hot-formed steel product 22MnB5 (steel composition C).

[0125] From Table 7, it is also clear that the fracture toughness parameters of the steel part in accordance with the present invention (steel composition A) is also higher than that of blanks made of DP1000 (steel composition B).

[0126] In Table 8, the crash behavior of the steel compositions A and B is shown. From Table 8 it is clear that the crash behavior of steel composition A is better than that of DP1000 (steel composition B) in both hot pressed as well as hot pressed and baked conditions. The baking conditions are the same as described here above. The crash boxes of steel composition A did not show any indication of cracking after the tests, whereas the crash boxes of DP1000 (steel composition B) showed severe cracking in the folds. Moreover, steel composition A shows a higher energy absorption capability.

[0127] The high and improved crash behavior of hot formed steel composition A in accordance with the present invention when compared to the conventional steel products of similar strength is due to the higher bending angle and higher fracture toughness properties. In this respect it is observed that during a crash, the steel component need to fold which is determined by its bendability, whereas on the other hand the energy absorption capability before failure is determined by its fracture toughness parameters.

[0128] In view of the above, it will be clear to the skilled person that the steel products in accordance with the present invention constitute a considerable improvement over conventionally known cold-formed and hot-formed steel products.

TABLE-US-00001 TABLE 1 chemistry (wt %) Steel C Mn Si Nb B Cr Ti N Remainder A 0.075 1.48 — 0.05 0.0025 1.01 0.03 0.0045 Fe + impurities B 0.15 2.3 0.1 0.01 — — 0.015 0.0035 Fe + impurities C 0.23 1.25 0.2 — 0.003 — — 0.004 Fe + impurities

TABLE-US-00002 TABLE 2 Transformation temperatures steel composition A A.sub.c1 (° C.) A.sub.c3 (° C.) M.sub.s (° C.) M.sub.f (° C.) 770 864 486 287

TABLE-US-00003 TABLE 3 Mechanical properties and microstructures for steel composition A T1 V3 YS UTS UE TE Microstructure (° C.) (° C./s) (MPa) (MPa) (%) (%) (vol. %) 900 30 696 893 2.8 5.6 55M + 45B 900 40 699 911 2.8 5.6 73M + 27B 900 50 741 955 3.2 6.2 79M + 21B 900 60 772 998 3.2 5.8 91M + 9B  900 80 784 1003 3.7 6.4 94M + 6B  900 200 879 1090 3.2 6.1 100M 940 30 757 962 3.6 6.9 60M + 40B 940 40 763 975 3.7 6.8 70M + 30B 940 50 741 985 4.4 8.1 82M + 18B 940 60 782 1006 4.4 8.3 93M + 7B  940 80 777 1021 4.4 8.1 960M + 4B  940 200 892 1089 3.2 6.3 100M

TABLE-US-00004 TABLE 4 Bending angles for steel composition A BA BA BA BA (1.5 mm) (1.5 mm) (1 mm) (1 mm) T1 V3 L sample T sample L sample T sample (° C.) (° C./s) (°) (°) (°) (°) 900 30 126.8 123 155.3 150.7 900 40 123.5 123.5 151.2 151.2 900 50 126.2 126.4 154.5 154.8 900 60 123 124.1 150.7 152 900 80 119.2 115.3 146 141.3 900 200 111.7 113 136.8 138.5 940 30 120.7 122.4 147.8 149.9 940 40 127.8 121 156.5 148.1 940 50 121.2 125.9 148.5 154.2 940 60 122.6 120.5 150.2 147.6 940 80 118.6 132.5 145.3 162.3 940 200 122.1 117.9 149.5 144.4

TABLE-US-00005 TABLE 5 Mechanical properties Steel composition A after baking BA BA BA BA (1.5 mm) (1.5 mm) (1 mm) (1 mm) YS UTS UE TE L sample T sample L sample T sample (MPa) (MPa) (%) (%) (°) (°) (°) (°) 937 1072 2.5 5.7 113.6 116 139.1 142.1

TABLE-US-00006 TABLE 6 Mechanical properties Steel compositions B (DP1000), and C (22MnB5) BA BA BA BA (1.5 mm) (1.5 mm) (1 mm) (1 mm) YS UTS UE TE L sample T sample L sample T sample Steel (MPa) (MPa) (%) (%) (°) (°) (°) (°) DP1000 747 1022 7.3 14.4 65.4 74.4 71.1 81.5 22MnB5 912 1374 4.1 6.6 83.5 69.2 102.3 84.7

TABLE-US-00007 TABLE 7 Fracture toughness parameters for steel compositions A and B (DP1000) Orien- CTOD J KJ K.sub.Q Steel tation (mm) (J/mm.sup.2) (MPa .Math. m.sup.0.5) (MPa .Math. m.sup.0.5) Compo- L 0.361 0.638 381 90 sition A Compo- T 0.245 0.434 314 104.7 sition A DP1000 L 0.139 0.231 229 86 DP1000 T 0.146 0.243 235 79.2

TABLE-US-00008 TABLE 8 Crash test results for steel compositions A and B (DP1000) Mean force at Visual Steel Condition 1.5 mm (NM) observation Steel heated and 107 good folding; compo- pressed no cracking sition A Steel heated, 98 good folding; compo- pressed no cracking sition A and baked DP1000 as annealed 82 severe cracking in folds