ES2569190T3 - Hot stamping molded article, method for producing it, and stainless steel sheet for hot stamping molding - Google Patents

Abstract

A hot stamped product, comprising a steel sheet formed by a hot stamping method, and having a metallic structure containing 30% to 80% ferrite by area, bainite ferrite in less than 30% by area not including 0% per area, martensite 30% per area or less, not including 0% per area, and retained austenite 3% to 20% per area.

Description

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[During formation, an average cooling rate of 20 ° C / s or more is maintained in the press tool, and the formation is terminated at a temperature not less than (the bainitic transformation start temperature Bs -100 ° C)]

5 To change the austenite, which was formed in the previous heating stage, in a prescribed fraction of retained austenite, while cementite formation is avoided, the average cooling rate during formation and the formation termination temperature must be properly control From this point of view, the average cooling rate during formation must be controlled to be 20 ° C / s or more, and the formation termination temperature must be controlled to be a temperature not exceeding (the bainitic transformation start temperature Bs -100 ° C, sometimes abbreviated as "B -100 ° C"). The average cooling rate during formation may preferably be 30 ° C / s or more (more preferably 40 ° C / s or more). With respect to the formation termination temperature, the formation can be terminated, while cooling to room temperature at an average cooling temperature as described above. Alternatively, cooling stops after cooling at a temperature not exceeding B -100 ° C, and

15 after the training can be finished. The control of the average cooling rate during formation can be achieved by means of, for example, (a) the control of the temperature of a press tool (using a cooling medium shown in Figure 1 above) or (b) the thermal conductivity control of a press tool (the same applies to cooling in the method described below).

As another method of producing the stamped product of the present invention, when a steel sheet is stamped with a press tool, the stainless steel sheet can be heated at a temperature not lower than the Acs transformation point and not higher than 1000 ° C , and then the stainless steel sheet is cooled to a temperature not exceeding 700 ° C and not less than 500 ° C to an average cooling temperature of 10 ° C / s or lower, and then the formation of the stainless steel sheet can be start, during whose training a speed of

The average cooling of 20 ° C / s or more can be maintained in the press tool, and formation that can be completed at a temperature not exceeding (the bainitic transformation start temperature Bs -100 ° C). The reasons for defining the respective requirements in this process are the following (the same as described above applies to the cooling termination temperature):

[Heat a sheet of stainless steel at a temperature not lower than the Ac3 transformation point and not exceeding 1000 ° C]

To properly adjust the structure of a hot stamped product, the heating temperature must be controlled in a prescribed range. Proper control of the heating temperature makes it possible

35 causing the transformation into a structure composed mainly of ferrite while ensuring a prescribed fraction of austenite retained in the subsequent cooling stage to provide the final hot stamped product with a desired structure. When the heating temperature of the stainless steel sheet is below the Ac3 transformation point, a sufficient fraction of austenite cannot be obtained during heating, and therefore, a prescribed fraction of retained austenite cannot be secured in the structure final (the structure of a formed product). When the heating temperature of the stainless steel sheet is greater than 1000 ° C, the austenite grain size increases during heating, and therefore, the ferrite cannot be formed in subsequent cooling.

[Cool at a temperature not exceeding 700 ° C and not below 500 ° C at an average cooling rate of 45 ° C / s or lower, and then start formation]

This cooling stage is an important stage for the formation of ferrite during cooling. When the average cooling rate at this cooling stage exceeds 10 ° C / s, a prescribed fraction of ferrite cannot be ensured. The average cooling rate may preferably be 7 ° C / s or lower, more preferably 5 ° C / s or less. The cooling stop temperature of this cooling stage (this temperature can sometimes be referred to as the "cooling rate change temperature") must be controlled not to exceed 700 ° C and not be less than 500 ° C. When the cooling stop temperature exceeds 700 ° C, a sufficient fraction of ferrite cannot be ensured. When the cooling stop temperature becomes below 500 ° C, the ferrite fraction becomes too high, and therefore, the

55 prescribed resistance cannot be assured. The cooling stop temperature may preferably be not more than 680 ° C as the preferred upper limit (more preferably not more than 660 ° C) and not less than 520 ° C as the preferred lower limit (more preferably not less than 550 ° C).

In any of these methods, the formation termination temperature should be controlled not to exceed (Bs -100 ° C), but preferably it can be controlled in a temperature range not less than the start temperature of martensitic transformation Ms ( a temperature in this range can sometimes be referred to as the "cooling temperature change temperature), in which the retention of the temperature range can preferably be performed for 10 seconds or more. The bainitic transformation can proceed from supercooled austenite to form a structure composed primarily of ferrite by retention in the above temperature range 65 for 10 seconds or more.The retention time may preferably be 50 seconds or more (more preferably 100 seconds or more) When the retention time it becomes very long,

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the austenite begins to decompose, so that the fraction of retained austenite cannot be assured. Therefore, the retention time may preferably be 1000 seconds or shorter (more preferably 800 seconds or shorter).

5 The retention as described above may be any isothermal retention, monotonous cooling, and the reheating stage, provided that it is in the previous temperature range. With respect to a relationship between said retention and formation, retention as described above can be added at the stage in which the formation is terminated. Alternatively, a retention stage can be added within the previous temperature range during the completion of the formation. After the formation is completed in such a way, the steel sheet can be left as it is for cooling or cooled at a suitable cooling rate to room temperature (25 ° C).

The process for producing the hot stamped product of the present invention can be applied, not only in the case where a hot stamped product having a simple shape as shown in Figure 1 above.

15 occurs (ie, direct method), but also in the case where a formed product is produced that has a relatively complicated shape, even if any of the methods described above are adopted. However, in the case of a complicated product form, it can be difficult to provide a product with the final form in a single stamping stage. In such a case, a cold stamping method can be used at a stage before hot stamping (this method has been called an "indirect method"). This method includes previously shaping a portion that is difficult to form in an approximate manner by cold processing and then hot stamping the other portions. When such a method is used to produce, for example, a formed product having three projections (profile peaks) by forming, two projections are formed by cold stamping and the third projection is then formed by hot stamping.

The present invention is intended for a hot stamped product made from a high strength steel sheet, the grade of steel of which is acceptable, if it has a composition of ordinary chemical elements such as a high strength steel sheet, in the However, the contents of C, Si, Mn, P, S, Al and N can preferably be controlled in their respective suitable ranges. From this point of view, the preferred ranges of these chemical elements and the reasons for limiting their ranges are as follows:

[C from 0.1% to 0.3%]

C is an important element to ensure retained austenite. The concentration of austenite during

Heating at a temperature within the biphasic region or at a temperature within the monophasic region, which is not less than that of transformation Ac3, allows the formation of retained austenite after quenching. It also contributes to an increase in the fraction of martensite. When the C content is less than 0.1%, a prescribed fraction of retained austenite cannot be assured, so it is impossible to obtain excellent ductility. When the C content becomes greater than 0.3%, the result is that the resistance is too high. The C content may, more preferably, be not less than 0.15% as the most preferred lower limit (even more preferably not less than 0.20%) and not more than 0.27% as the most preferred upper limit ( even more preferably not more than 0.25%).

[If from 0.5% to 3%]

45 If the austenite is suppressed after heating at a temperature within the biphasic region or at a temperature within the monophasic region, which is not less than the transformation point Ac3, if formed into cementite, and exhibits the increasing action of the fraction of retained austenite. In addition, it exhibits the action of increasing resistance by increasing the solid solution without deteriorating the ductility too much. When the Si content is less than 0.5%, retained austenite cannot be guaranteed in a prescribed fraction, so it is impossible to obtain excellent ductility. When the Si content exceeds 3%, the degree of increase in the solid solution becomes too high, which results in drastic deterioration of ductility. The Si content may more preferably be not less than 1.15% as the most preferred lower limit (even more preferably not less than 1.20%) and not more than 2.7% as the most preferred upper limit (even more preferably not

55 greater than 2.5%).

[MN from 0.5% to 2%]

Mn is an element to stabilize austenite and contributes to an increase in retained austenite. To make such an effect exhibited, Mn may preferably be contained 0.5% or more. However, when the content of Mn becomes excessive, the formation of ferrite is avoided, which makes it impossible to ensure a prescribed fraction of ferrite, and therefore, the content of Mn may preferably be 2% or less. In addition, a considerable improvement in austenite resistance increases a hot rolling load, which makes it difficult to produce steel sheets, and therefore, even from a productivity point of view, it is not preferable that the

65 Mn have a content of more than 2%. The Mn content may more preferably be not less than 0.7% as the most preferred lower limit (even more preferably not less than 0.9%) and not more than 1.8% as the

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[Table 3]

Test No.

Steel grade Structure of the product formed (% by area) tensile strength TS (MPa) EL Elongation (%)

Ferrite

Bainitic Ferrite martensite retained austenite Others*

one

TO 41 25 26 8 994 17

2

B 43 29 22 6 1020 16

3

C 48 2. 3 twenty-one 8 994 17

4

re 49 2. 3 twenty-one 7 1002 17

5

Y 49 24 twenty 7 1023 17

6

F 48 24 twenty-one 7 1031 17

7

G 44 25 2. 3 8 1028 17

8

H 46 25 22 7 1011 17

9

I Four. Five 24 2. 3 8 1019 17

10

J 48 24 22 6 1018 17

eleven

K 49 26 24 one 1022 12

12

L 51 25 22 2 1302 14

13

M 36 28 28 8 1095 16

14

N Four. Five 27 28 0 989 13

fifteen

C 48 24 twenty 8 1023 17

16

C 25 Four. Five 25 5 1082 13

17

C 81 - fifteen - : 4 745 14

18

C - - 95 5 1523 10

19

C 65 8 twenty 7 984 17

twenty

C 62 9 22 7 999 17

twenty-one

C 43 26 22 9 1032 18

22

C 12 61 twenty 7 1233 12

2. 3

C 83 17 - - 921 14

24

C 56 twenty - - P: 24 893 14

25

C 47 2. 3 2. 3 8 994 17

*  and P indicate cementite and perlite, respectively.

From these results, the descriptions can be made as follows: Tests No. 1 to 10, 13, 15, 19 to 21, and 25 are examples that meet the requirements defined in the present invention, thus indicating that those 5 parts that have satisfactory balance between resistance and ductility were obtained.

In contrast, Tests 11 to 12, 14, 16 to 18, and 22 to 24 are comparative examples that do not meet any of the requirements defined in the present invention, thereby deteriorating any of the features. More specifically, Test No. 11 was the case in which steel with insufficient C content was used (degree of

10 K steel shown in Table 1), so that retained austenite was not secured, thereby obtaining only low elongation (EL). Test No. 12 was the case in which the steel had insufficient Si content (grade of steel L shown in Table 1), so that the retained austenite was not secured, thus obtaining only low elongation (EL) .

15 Test No. 14 was intended for conventional 2MnB5 equivalent steel (grade of N steel shown in Table 1), so that retained austenite was not secured, thus obtaining only low elongation (EL), although It obtained a high resistance. Test No. 16 was the case in which the cold rolled steel sheet was used with a low reduction, so that the product formed had a structure containing 25% ferrite per area, which reduced elongation (EL ). Test No. 17 was the case where the temperature of

20 heating was lower than the Ac1 transformation point, so that the product formed had a structure containing 81% ferrite per area (the rest was martensite and cementite) and the retained austenite was not insured, therefore the elongation was reduced (EL) and tensile strength. Test No. 18 was the case where the heating temperature was higher than a value, so that the ferrite and bainitic ferrite were not ensured by excessive martensite formation, which reduced elongation (EL).

25 Test No. 22 was the case where the average cooling rate in cooling 1 was high, so that the ferrite was not ensured by the formation of bainitic ferrite, which reduced elongation (EL). Test No. 23 was the case where the average cooling rate in cooling 1 was low and the temperature of changing the cooling rate was low, so that the product formed had a structure containing ferrite at 83

30% per area (the rest was of bainitic ferrite) and retained austenite was not insured, which reduced elongation (EL). Test No. 24 was the case where the formation termination temperature was high, so that perlite was formed in the structure of the product formed and the retained austenite was not assured, therefore the elongation (EL) was reduced.

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Claims (1)

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