Method for producing a steel component having a metal coating protecting it against corrosion

Abstract

A method for producing a steel component from a flat steel sheet is provided. The produced steel component includes a substrate and a coating. The method ensures that the steel component has an H.sub.diff content below a certain level. The low H.sub.diff content minimizes the risk of hydrogen-induced cracking of the steel component after hot forming, including during subsequent use of the steel component. The H.sub.diff content in the hot-formed steel component is ensured to be below a certain level by selecting furnace parameters depending on the rolling degree and the sheet thickness of the flat steel sheet.

Claims

1. A method for producing a steel component having a content of diffusible hydrogen H.sub.diff of up to 0.4 ppm, the method comprising the steps of: (A) providing a steel product having a coating including, in weight percent (wt. %), 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0 alkali and/or alkaline earth metals, remainder Al and unavoidable impurities, the steel product including, in wt. %: 0.20 to 0.25 C, 0.50 to 3.0 Mn, 0.10 to 0.50 Si, 0.01 to 1.00 Cr, up to 0.20 Ti, up to 0.10 Al, up to 0.10 P, up to 0.1 Nb, up to 0.01 N, up to 0.05 S and up to 0.1 B, remainder Fe and unavoidable impurities, rolling the steel product, on which the coating already is present, to a lower sheet thickness to form a rolled steel product having rolling degree of from 2.5% to 60% and having a rolling degree to sheet thickness ratio (WGB) which is within a range from 0.8 to 200, wherein the WGB is a dimensionless value being determined according to the following formula: $\mathrm{WGB}=1.5.Math.\frac{1+\mathrm{rolling}\mathrm{degree}.Math.100}{\frac{1}{2}.Math.\left(1+\sqrt{\mathrm{Sheet}\mathrm{thickness}}\right)}$ wherein, in said formula, the sheet thickness is in mm and is identical to the final thickness of the steel product after rolling, (B) determining a hydrogen-related furnace parameter value (WOP value) for the rolled steel product on the basis of the rolling degree to sheet thickness ratio (WGB) within a surface spanned by straight connecting paths between points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) and P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value is plotted on the y axis and the rolling degree to sheet thickness ratio (WGB) is plotted on the x axis, (C) treating the rolled steel product at an average furnace temperature T.sub.furnace (in Kelvin) for a duration t.sub.furnace (in hour), wherein the dew point temperature of the furnace atmosphere of the furnace T.sub.dew point (in Kelvin), the average furnace temperature T.sub.furnace (in Kelvin) and the duration t.sub.furnace (in hour) are set according to the following equation of general formula (1)
WOP=T.sub.furnace.Math.log (t.sub.furnace+1.15)+(T.sub.dew point−243.15).sup.1.6 (1) and (D) forming the heated rolled steel product from step (C) in a mold while being simultaneously cooled to obtain the steel component.

2. The method according to claim 1, wherein the WOP value is determined according to step (B) within a surface spanned by straight connecting lines between the points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200), P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) and P33 (WGB 50, WOP 300) and P12 (WGB 0.8, WOP 300) in a coordinate system in which the WOP value is plotted on the y axis and the rolling degree to sheet thickness ratio (WGB) is plotted on the x axis.

3. The method according to claim 1, wherein t.sub.furnace is 0.05 to 0.5 h.

4. The method according to claim 1, wherein the rolled steel product is a blank having a structure as a consequence of the rolled steel product having been made of a hot rolled strip or a blank having a structure as a consequence of the rolled steel product having been made of a cold rolled strip.

5. The method according to claim 1, wherein step (A) includes coating the rolled steel product with the coating by hot-dip galvanizing, by an electrolytic coating or by means of a piecework-coating process.

6. The method according to claim 1, wherein the coating is a double sided coating with a coating weight of 20 to 240 g/m.sup.2.

7. The method according to claim 1, wherein step (D) takes place at a cooling rate of from 10 to 500 K/s.

8. The method according to claim 1, wherein the content of diffusible hydrogen H.sub.diff is 0.1, 0.2, 0.3 or 0.4 ppm in the material after hot forming.

9. The method according to claim 1, wherein the rolled steel product includes, in wt. %: 0.20 to 0.225 C, 0.50 to 3.0 Mn, 0.10 to 0.50 Si, 0.01 to 1.00 Cr, up to 0.20 Ti, up to 0.10 Al, up to 0.10 P, up to 0.1 Nb, up to 0.01 N, up to 0.05 S and up to 0.1 B, remainder Fe and unavoidable impurities.

Description

DRAWINGS

(1) FIG. 1 shows a graph in which the WOP value is plotted against the rolling degree to sheet thickness ratio.

(2) On said graph, the numbering means the following:

(3) 1 WOP value (hydrogen-related furnace parameter value)

(4) 2 WGB (rolling degree to sheet thickness ratio)

(5) 3 Partial surface “3”

(6) 4 Partial surface “4”

(7) Partial surface “5”

(8) FIG. 2 shows, by way of example, how the WOP value is determined with a known rolling degree to sheet thickness ratio according to the invention, and in this case the numbering means the following:

(9) E1 rolling degree 0.5%, starting sheet thickness 3.0 mm, rolling degree to sheet thickness ratio 1.6, resulting in WOP value of from 300 to 790.

(10) E2 rolling degree 2.5%, starting sheet thickness 3.0 mm, rolling degree to sheet thickness ratio 3.8, resulting in WOP value of from 300 to 780.

(11) E3 rolling degree 30%, starting sheet thickness 1.5 mm, rolling degree to sheet thickness ratio 41.8, resulting in WOP value of from 300 to 630.

(12) E4 rolling degree 50%, starting sheet thickness 1.98 mm, rolling degree to sheet thickness ratio 63.6, or rolling degree 47%, starting sheet thickness 1.5 mm, rolling degree to sheet thickness ratio 64.7, resulting in WOP value of from 200 to 600 in each case.

EXAMPLES

Example 1

(13) The following embodiments serve to explain the invention in greater detail.

(14) Blanks are used which have been obtained from melts having the alloy components according to Table 1.

(15) TABLE-US-00001 TABLE 1 Melt composition of the flat steel products used Alloy Alloy component in wt. % elements Melt A Melt B Melt C Melt D C 0.224 0.212 0.219 0.212 to 0.225 Si 0.23 0.22 0.26 0.21 to 0.27 Mn 1.20 1.11 1.14 1.11 to 1.20 P 0.014 0.009 0.013 0.009 to 0.016 S 0.0029 0.0013 0.0023 0.0006 to 0.0029 Al total 0.035 0.027 0.032 0.026 to 0.038 Cr 0.190 0.187 0.183 0.180 to 0.190 Nb 0.001 0.001 0.001 0.001 to 0.001 Mo 0.0055 0.0018 0.0040 0.0016 to 0.0055 Ti 0.028 0.029 0.025 0.020 to 0.033 B 0.0022 0.0024 0.0026 0.0021 to 0.0028 All data in wt. %, remainder Fe and unavoidable impurities

(16) The flat steel products used have a coating containing 9 to 10 wt. % Si, 2 to 3.5 wt. % iron, remainder aluminum and the amount of Mg set out in Table 2. The coating weight, the sheet thickness and the rolling degree of the flat steel products used are likewise set out in Table 2. The corresponding WOP value is then determined in the graph according to FIG. 1 by means of the rolling degree to sheet thickness ratio (formula 3), and T.sub.furnace, t.sub.furnace, and T.sub.dew point of the furnace atmosphere are subsequently determined and set by means of formula (1). The thus heated flat steel product is then removed from the furnace and inserted into a mold after a transport time of 6 seconds. After insertion into the mold, this mold immediately closes and remains in the closed state for approx. 20 seconds, in order to thereby cool the component to <80° C. by contact with the cooled molds. Samples are taken from the manufactured steel components, which are analyzed with regard to the amount of diffusible hydrogen contained (H.sub.diff) by means of desorption mass spectrometry using heated samples (thermal desorptlon mass spectrometry (TDMS)).

(17) TABLE-US-00002 TABLE 2 Mg Coating weight Sheet Rolling Serial content on both sides thickness degree T.sub.furnace t.sub.furnace T.sub.dew point H.sub.diff WOP no. Melt [wt. %] [g/m.sup.2] [mm] [%] WGB [K] [h] [K] [ppm] value Region V1  A 0.3 80 1.50 0 1.3 1193.15 0.100 288.15 0.12 557 — V2  A 0.3 80 1.50 0 1.3 1193.15 0.167 288.15 0.17 584 — V3  A 0.3 140 1.50 0 1.3 1193.15 0.222 288.15 0.13 606 — V4  A 0.3 140 1.50 0 1.3 1193.15 0.100 288.15 0.14 557 — V5  A 0 140 1.50 0 1.3 1193.15 0.222 288.15 0.45 606 —  6 A 0.3 140 1.10 27 41.0 1193.15 0.100 248.15 0.07 129 A  7 A 0.3 140 1.10 27 41.0 1193.15 0.167 248.15 0.05 156 A  8 A 0.3 140 1.10 27 41.0 1193.15 0.222 248.15 0.05 177 A  9 A 0.3 140 1.10 27 41.0 1193.15 0.100 268.15 0.26 288 A 10 A 0.3 140 1.10 27 41.0 1193.15 0.167 268.15 0.14 315 B 11 A 0.3 140 1.10 27 41.0 1193.15 0.222 268.15 0.12 336 B 12 A 0.3 140 0.80 47 76.0 1193.15 0.100 248.15 0.15 129 A 13 A 0.3 140 0.80 47 76.0 1193.15 0.167 248.15 0.13 156 A 14 A 0.3 140 0.80 47 76.0 1193.15 0.222 248.15 0.05 177 A 15 A 0.3 140 0.80 47 76.0 1193.15 0.100 268.15 0.34 288 A 16 A 0.3 140 0.80 47 76.0 1193.15 0.167 268.15 0.29 315 B 17 A 0.3 140 0.80 47 76.0 1193.15 0.222 268.15 0.22 336 B V18 A 0.3 140 0.80 47 76.0 1193.15 0.222 288.15 0.89 606 — V19 C 0 140 1.50 0 1.3 1193.15 0.083 288.15 0.47 550 — V20 C 0 140 1.50 0 1.3 1193.15 0.167 288.15 0.59 584 — V21 C 0 140 1.50 0 1.3 1193.15 0.083 268.15 0.20 281 — V22 C 0 140 1.50 0 1.3 1193.15 0.083 248.15 0.10 122 — 23 B 0.4 140 1.35 30 43.0 1193.15 0.083 268.15 0.20 281 A 24 B 0.4 140 1.35 30 43.0 1193.15 0.167 268.15 0.17 315 B 25 B 0.4 140 1.35 30 43.0 1193.15 0.083 288.15 0.22 550 B 26 B 0.4 140 1.35 30 43.0 1193.15 0.167 288.15 0.31 584 B 27 B 0.4 140 1.00 50 76.5 1193.15 0.083 268.15 0.11 281 A 28 B 0.4 140 1.00 50 76.5 1193.15 0.083 268.15 0.13 281 A 29 B 0.4 140 1.00 50 76.5 1193.15 0.167 268.15 0.11 315 B 30 B 0.4 140 1.00 50 76.5 1193.15 0.167 268.15 0.12 315 B 31 B 0.4 140 1.00 50 76.5 1193.15 0.083 288.15 0.26 550 B 32 B 0.4 140 1.00 50 76.5 1193.15 0.167 288.15 0.28 584 B V33 D 0.3 140 1.50 0 1.3 1253.15 0.083 288.15 0.27 554 — V34 D 0.3 140 1.50 0 1.3 1153.15 0.167 288.15 0.27 579 — V35 D 0 140 1.50 0 1.3 1153.15 0.250 288.15 0.47 610 — 36 D 0 140 1.50 25 35.1 1193.15 0.083 268.15 0.29 281 — V37 D 0 140 1.50 25 35.1 1193.15 0.050 288.15 0.52 536 — 38 D 0 140 1.50 25 35.1 1193.15 0.083 258.15 0.05 185 A 39 D 0 140 1.50 25 35.1 1193.15 0.167 258.15 0.22 219 A 40 D 0.5 140 1.00 0 1.5 1193.15 0.167 288.15 0.27 584 B 41 D 0.5 140 1.97 0 1.2 1193.15 0.083 288.15 0.10 550 B 42 D 0.5 140 1.97 0 1.2 1193.15 0.250 288.15 0.30 616 B V43 D 0 140 1.50 25 35.1 1193.15 0.083 288.15 0.85 550 — V44 D 0 140 1.50 25 35.1 1193.15 0.167 268.15 0.49 315 — 45 D 0.3 140 1.50 0 1.3 1193.15 0.083 298.15 0.24 718 B 46 D 0 140 1.30 30 43.5 1193.15 0.083 268.15 0.27 281 A 47 D 0 140 1.30 30 43.5 1193.15 0.083 268.15 0.27 281 A V48 D 0 140 1.30 30 43.5 1193.15 0.083 288.15 0.57 550 — V49 D 0 140 0.95 50 77.5 1193.15 0.083 268.15 0.58 281 — V50 D 0 140 0.95 50 77.5 1193.15 0.167 268.15 0.47 315 — V Comparative example

Example 2

(18) Determination by way of example of allowable values for T.sub.furnace, t.sub.furnace and T.sub.dew point to maintain an H.sub.diff value of 0.4 ppm in produced components made of flat steel products.

(19) Example E3 from FIG. 2:

(20) h.sub.0=2.143 mm; h.sub.1=sheet thickness=1.5 mm; Δh=0.643 mm; coating with Mg 0.35 wt. %

(21) $\mathrm{rolling}\mathrm{degree}=\frac{\Delta h}{h_0}=\frac{0.643\mathrm{mm}}{2.143\mathrm{mm}}=0.3=30\%\mathrm{WGB}=1.5.Math.\frac{1+\mathrm{rolling}\mathrm{degree}.Math.100}{\frac{1}{2}\left(1+\sqrt{\mathrm{Sheet}\frac{\mathrm{thickness}}{\mathrm{mm}}}\right)}=1.5.Math.\frac{1+0.3.Math.100}{\frac{1}{2}.Math.\left(1+\sqrt{\frac{1.5\mathrm{mm}}{\mathrm{mm}}}\right)}=41.8$

(22) For the WGB value of 41.8, a WOP value of from 300 to 630 can be read out from FIG. 1 or calculated using the specified points. Now the three parameters T.sub.furnace, t.sub.furnace and T.sub.dew point can be set to result in a WOP value of: 300≤WOP≤630, for example: T.sub.furnace=930° C.=1203.15 K; t.sub.furnace=400 s=0.111 h; and T.sub.dew point=10° C.=283.15 K

(23) $300\le WOP\le 630\iff 300\le \frac{T_{furnace}}{K}.Math.\log \left(\frac{t_{furnace}}{h}+1.15\right)+{\left(\frac{T_{\mathrm{dew}\mathrm{point}}}{K}-243.15\right)}^{1.6}\le 630\iff 300\le \frac{1203.15K}{K}.Math.\log \left(\frac{0.111h}{h}+1.15\right)+{\left(\frac{283.15K}{K}-243.15\right)}^{1.6}\le 630\iff 300\le 487\le 630\iff \mathrm{true}\mathrm{statement}$

(24) Since the calculated WOP value of 487 is between 300 and 630, the selected parameters allow a maximum H.sub.diff value of 0.4 ppm to be maintained in the component.

INDUSTRIAL APPLICABILITY

(25) The steel component produced according to the invention has a low tendency toward hydrogen-induced fractures under load stresses and can therefore advantageously be used in the automotive sector, aircraft construction or rail vehicle construction.