PROCESS FOR MANUFACTURING A FLAT STEEL PRODUCT HAVING AN ALUMINUM-BASED CORROSION-RESISTANT COATING, AND FLAT STEEL PRODUCT HAVING AN ALUMINUM-BASED CORROSION-RESISTANT COATING

Abstract

A process for manufacturing a flat steel product having a prealloyed corrosion-resistant coating, comprises providing a coated flat steel product comprising a steel substrate having, at least on one side of the steel substrate, and an aluminum-based corrosion-resistant coating. The coated flat steel product is heat-treated, comprising the following substeps: Heating the coated flat steel product in a furnace at a furnace temperature T of between 950° C. and 1150° C. with a furnace dwell time tv of between 40 seconds and 150 seconds, the furnace temperature being chosen such that the heating rate of the coated flat steel product in the temperature range from 500° C. to 700° C. is more than 10 K/s. The coated flat steel product is held at a temperature above Ac3 for a hold time of between 20 seconds and 60 seconds.

Claims

1. A process for manufacturing a flat steel product having a prealloyed corrosion-resistant coating whose Fe content is between 30 and 45 wt%, comprising at least the following steps: Providing a coated flat steel product comprising a steel substrate having, disposed on both sides of the steel substrate, an aluminum-based corrosion-resistant coating which has a both-sides add-on weight r of between $50\frac{g}{m^2}$ and $200\frac{g}{m^2}$ and contains 0.5 -15 wt% of silicon, up to 5 wt% of iron, up to 5 wt% of alkali metals or alkaline earth metals and up to 15 wt% of Zn optional further constituents, the amounts of which are restricted in total to at most 2.0 wt%, and aluminum as the balance. Heat-treating the coated flat steel product where the heat treatment is divided into a first substep, in which the flat steel product is heated, and a second substep, in which the flat steel product is held at a temperature above Ac3, the two substeps being configured as follows: i. Heating the coated flat steel product in a furnace at a furnace temperature T of between 950° C. and 1150° C. with a furnace dwell time tv of between 40 s and 150 s, the furnace temperature being chosen such that the heating rate of the coated flat steel product in the temperature range from 500° C. to 700° C. is more than 10 K/s; and ii. Holding the coated flat steel product at a temperature above Ac3 for a hold time of between 20 s and 60 s.

2. The process as claimed in claim 1, wherein the dwell time tv is subject to the following criterion: $\begin{array}{l}\left(48+5.5\cdot \left(\frac{1050°\text{C}-T}{50\text{K}}\right)\right)\frac{\text{s}}{\text{mm}}\cdot d\cdot \left(0.3\cdot \ln \left(r\cdot \frac{{\text{m}}^2}{\text{g}}\right)-0.5\right)<t_V\\ <\left(85+8.5\cdot \left(\frac{1050°\text{C}-T}{50\text{K}}\right)\right)\frac{\text{s}}{\text{mm}}\cdot d\cdot \left(0.3\cdot \ln \left(r\cdot \frac{{\text{m}}^2}{\text{g}}\right)-0.5\right)\end{array}$ where T = furnace temperature in °C d = thickness of the flat steel product in mm tv = furnace dwell time in s r = both-sides add-on weight in $\frac{\text{g}}{{\text{m}}^2}$ .

3. The process as claimed in claim 2 wherein the aluminum-based corrosion-resistant coating contains 0.5 - 15 wt% of silicon, up to 5 wt% of iron, up to 5 wt% of magnesium, balance aluminum.

4. A flat steel product having a prealloyed corrosion-resistant coating, comprising a steel substrate having, at least on one side of the steel substrate, a prealloyed, aluminum-based corrosion-resistant coating, more particularly manufactured as claimed in claim 2, wherein the average Fe content of the corrosion-resistant coating is 30 – 45 wt% and where the corrosion-resistant coating comprises: a low-silicon phase which contains, in addition to unavoidable impurities, 1 - 10 wt% of Si, 10 - 50 wt% of Fe, up to 1 wt% of Mn and 40 -80 wt% of aluminum, a silicon-rich phase which contains, in addition to unavoidable impurities, 10 - 15 wt% of Si, 25 - 50 wt% of Fe, up to 1 wt% of Mn and 40 80 wt% of aluminum, with the silicon-rich phase being distributed insularly in the low-silicon phase.

5. The flat steel product with prealloyed corrosion-resistant coating as claimed in claim 4, comprising: a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of between 1 .Math.m and 6 .Math.m which is disposed adjacent to the steel substrate.

6. The flat steel product with prealloyed corrosion-resistant coating as claimed in claim 5, comprising: a layer of the silicon-rich phase with a thickness of between 1 .Math.m and 3 .Math.m, which is disposed adjacent to the diffusion layer.

7. A process for manufacturing a steel component having a fully alloyed corrosion-resistant coating, comprising at least the following steps: providing a flat steel product having a prealloyed corrosion coating as claimed in claim 6, more particularly manufacturing a flat steel product having a prealloyed corrosion-resistant coating as claimed in claim 3, heating the flat steel product with prealloyed corrosion-resistant coating to a forming temperature Tu of between Ac3 and 950° C. for a heating time t.sub.E of between 2 s and 600 s,, where the heating is accomplished in particular by means of at least one of rapid conductive heating, rapid inductive heating and rapid contact heating.

8. The process for manufacturing a steel component having a fully alloyed corrosion-resistant coating as claimed in claim 7, wherein the heating time t.sub.E is subject to the following criterion: $\begin{array}{l}\left(55\frac{\text{s}}{\text{mm}}\cdot d+32\text{s}-\left(0.7\frac{\text{s}}{\text{K}}\cdot \left(T_U-890°\text{C}\right)\right)\right)<t_E\\ <\left(85\frac{\text{s}}{\text{mm}}\cdot d+260\text{s}-\left(3\frac{\text{s}}{\text{K}}\cdot \left(T_U-890°\text{C}\right)\right)\right)\end{array}$ where Tu = forming temperature d = thickness of the flat steel product in mm t.sub.E = heating time in s.

9. The process for manufacturing a steel component having a fully alloyed corrosion-resistant coating as claimed in claim 8, comprising: forming the flat steel product in a forming tool to give a steel component.

10. A steel component having a fully alloyed corrosion-resistant coating, comprising a steel substrate having, at least on one side of the steel substrate, a fully alloyed, aluminum-based corrosion-resistant coating, manufactured as claimed in claim 9, wherein the fully alloyed corrosion-resistant coating comprises: a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of between 1 .Math.m and 20 .Math.m which is disposed adjacent to the steel substrate, a low-silicon phase which contains, in addition to unavoidable impurities, 1 - 10 wt% of Si, 40 - 60 wt% of Fe, up to 1 wt% of Mn and 30 – 60 wt% of aluminum, a silicon-rich phase which contains, in addition to unavoidable impurities, 10 - 15 wt% of Si, 40 - 70 wt% of Fe, up to 1 wt% of Mn and 40 80 wt% of aluminum, with the silicon-rich phase being distributed insularly in the low-silicon phase and where the insular distribution of the silicon-rich phase in the low-silicon phase is such that the corrosion-resistant coating has between 10 vol% and 25 vol% of the silicon-rich phase.

11. The steel component as claimed in claim 10, wherein the fraction of the silicon-rich phase in the fully alloyed corrosion-resistant coating is less than 20 vol%.

12. (canceled)

13. The steel component as claimed in claim 11, that wherein the steel component comprises: a layer of the silicon-rich phase with a thickness of between 1 .Math.m and 3 .Math.m, which is disposed adjacent to the diffusion layer.

14. The process of claim 7 wherein the heating time is between 20 s and 400 s.

15. The process of claim 7 wherein the heating time is between 60 s and 240 s.

Description

[0113] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0114] The invention is elucidated in more detail with the following working examples in conjunction with the figures, in which:

[0115] FIG. 1a shows a polished cross section of a flat steel product having a prealloyed corrosion-resistant coating in a first variant embodiment;

[0116] FIG. 1b shows a polished cross section of a steel component having a fully alloyed corrosion-resistant coating in a first variant embodiment;

[0117] FIG. 2a shows a polished cross section of a flat steel product having a prealloyed corrosion-resistant coating in a second variant embodiment;

[0118] FIG. 2b shows a polished cross section of a steel component having a fully alloyed corrosion-resistant coating in a second variant embodiment;

[0119] FIG. 3a shows a polished cross section of a flat steel product having a prealloyed corrosion-resistant coating in a third variant embodiment;

[0120] FIG. 3b shows a polished cross section of a steel component having a fully alloyed corrosion-resistant coating in a third variant embodiment;

[0121] FIG. 4a shows a polished cross section of a flat steel product having a prealloyed corrosion-resistant coating in a fourth variant embodiment;

[0122] FIG. 4b shows a polished cross section of a steel component having a fully alloyed corrosion-resistant coating in a fourth variant embodiment.

WORKING EXAMPLE 1

[0123] Shaped blanks were cut from a strip of steel type D from table 2, 1.8 mm thick and with a 25 .Math.m thick, aluminum-based corrosion-resistant coating on both sides. The cutting method employed comprised a punching tool and a laser. The initial composition of the protective coating was 8 wt% Si, 3 wt% Fe, 0.3 wt% Mg, balance Al. The add-on weight was

[00007]$70\frac{\text{g}}{{\text{m}}^2}$

on each side. These shaped blanks, in a two-zone continuous furnace, were first heated to more than 900° C. in the first zone at a furnace temperature of 1100° C., in 105 seconds. The heating rate between 500° C. and 700° C. was 12 K/s. The blank was then held at 920° C. temperature for 35 seconds, this being above the Ac3 temperature, which for the type of steel selected is about 860° C. Within this time, prealloying of the corrosion-resistant coating occurred. The Fe content of the corrosion-resistant coating subsequently was between 35 wt% and 40 wt%. Moreover, the Fe content of the corrosion-resistant coating at each point was more than 10 wt% Fe. The mean roughness R.sub.a for this variant embodiment is 0.9 .Math.m.

[0124] The polished cross section of the resultant flat steel product 9 is represented in FIG. 1a. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The areas with silicon-rich phase 15 are distributed over a region which has a thickness, perpendicular to the surface, of about 80% of the thickness of the corrosion-resistant coating 13. What is formed, therefore, is not a single narrow strip with areas of silicon-rich phases 15; instead, the silicon-rich areas are distributed insularly over a strip which occupies about 80% of the corrosion-resistant coating. This ensures that in the hot-pressed steel component, the fraction of the silicon-rich phase 15 in the corrosion-resistant coating is between 10 vol% and 25 vol%.

[0125] With this variant of the flat steel product with prealloyed corrosion-resistant coating, the flat steel product comprises a diffusion layer 19 comprising Fe3Al and Fe2Al5 with a thickness of 3 .Math.m, disposed adjacent to the steel substrate 11. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The corrosion-resistant coating 13 further comprises a layer 21 of the silicon-rich phase 15 with a thickness of 1 .Math.m, disposed adjacent to the diffusion layer 19. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0126] In the case of this working example, moreover, the flat steel product comprises an oxide layer at the surface of the corrosion-resistant coating, having a thickness of 60 nm, which because of the resolution in FIG. 1a is not visible.

[0127] The shaped blanks treated in this way were processed further elsewhere on a hot forming line. The further processing took place in a short hot forming furnace at a forming temperature of 920° C. Complete austenitization and full alloying of the coating required a heating time of 150 seconds, in which the treated shaped blanks were brought to the forming temperature and held there. The furnace rollers of the hot forming furnace showed no Al soiling even after throughput of multiple 500 blanks.

[0128] After the hot press hardening, the corrosion-resistant coating contained an average of 51 wt% Fe and had silicon-rich areas with Si contents of between 10% and 14%. The corrosion-resistant coating does not show any clear layering of the silicon-rich and low-silicon areas. Instead, the silicon-rich phase is distributed insularly in the low-silicon phase. As a result of this, there are both low-silicon and silicon-rich phases present in the near-surface region. Not only the joining properties but also the coating-material adhesion properties were therefore better than for standard commercial aluminum-based corrosion-resistant coatings, in which a layering is formed with a continuous low-silicon layer in the near-surface region.

[0129] The polished cross section of the resultant steel component 23 is represented in FIG. 1b. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The near-surface region has around 18 vol% of the silicon-rich phase 15. Moreover, the corrosion-resistant coating 13 and the diffusion layer 19 together have a pore fraction of 1.1%. The mean roughness of the surface is Ra = 1.5 .Math.m. The fraction of the silicon-rich phase in the corrosion-resistant coating is 14 vol%. About 85% of the silicon-rich phase is present in the form of small “islands” with a surface area of less than 100 .Math.m.sup.2.

[0130] The steel component 23 further comprises a diffusion layer 19 with Fe3Al and Fe2Al5, which is disposed adjacent to the steel substrate 11 beneath the corrosion-resistant coating 13. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The diffusion layer has a thickness of 7 .Math.m. The steel component 23 further comprises a layer 21 of the silicon-rich phase 15, having a thickness of 2 .Math.m, which is disposed adjacent to the diffusion layer 19 beneath the corrosion-resistant coating 13. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0131] In this working example, the steel component further comprises an oxide layer at the surface of the corrosion-resistant coating, with a thickness of 100 nm, which because of the resolution in FIG. 1b is not visible.

WORKING EXAMPLE 2

[0132] Shaped blanks were cut from a strip of steel type E from table 2, 1.5 mm thick and with a 20 .Math.m thick, aluminum-based corrosion-resistant coating on both sides. The cutting method employed was a laser. The initial composition of the protective coating was 10 wt% Si, 3 wt% Fe, balance Al. The add-on weight was

[00008]$60\frac{\text{g}}{{\text{m}}^2}$

on each side.

[0133] These shaped blanks, in a two-zone continuous furnace, were first heated to more than 900° C. in the first zone at a furnace temperature of 1050° C., in 90 seconds. The heating rate between 500° C. and 700° C. was 11 K/s. This part of the furnace had an O2-containing atmosphere with a dew point DP of -10°. The blank was then held at 920° C. temperature for 60 seconds, this being above the Ac3 temperature, which for the type of steel selected is about 845° C. This part of the furnace had no dew point regulation.

[0134] Within this time, full alloying of the AS coating occurred. The Fe content of the corrosion-resistant coating subsequently was between 35 wt% and 40 wt%. Moreover, the Fe content of the corrosion-resistant coating at each point was more than 10 wt% Fe.

[0135] The polished cross section of the resultant flat steel product 9 is represented in FIG. 2a. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The areas with silicon-rich phase 15 are distributed over a region which has a thickness, perpendicular to the surface, of about 90% of the thickness of the corrosion-resistant coating 13. What is formed, therefore, is not a single narrow strip with areas of silicon-rich phases 15; instead, the silicon-rich areas are distributed insularly over a strip which occupies about 90% of the corrosion-resistant coating. This ensures that in the hot-pressed steel component, the near-surface region of the corrosion-resistant coating has between 10 vol% and 25 vol% of the silicon-rich phase.

[0136] With this variant of the flat steel product with prealloyed corrosion-resistant coating, the flat steel product comprises a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 2 .Math.m, disposed adjacent to the steel substrate 11. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The corrosion-resistant coating 13 further comprises a layer 21 of the silicon-rich phase 15 with a thickness of 1 .Math.m, disposed adjacent to the diffusion layer 19. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0137] In the case of this working example, moreover, the flat steel product comprises an oxide layer at the surface of the corrosion-resistant coating, having a thickness of 60 nm, which because of the resolution in FIG. 2a is not visible.

[0138] The shaped blanks thus treated were processed further in a separate operating step on an inductive rapid heating line. The heating time to 900° C. was 15 seconds. The blanks were subsequently held at this temperature by infrared radiation for a further 30 seconds at this temperature. This was accompanied by complete austenitization of the base material and full alloying of the coating. The heating time was therefore 45 seconds.

[0139] After the hot press hardening, the corrosion-resistant coating contained an average of 56 wt% Fe and had silicon-rich areas with Si contents of between 10 wt% and 13.5 wt%. Above the diffusion layer, the corrosion-resistant coating does not show any clear layering of the silicon-rich and low-silicon areas. Instead, the silicon-rich phase is distributed insularly in the low-silicon phase. As a result of this, there are both low-silicon and silicon-rich phases present in the near-surface region. In this working example, the near-surface region of the corrosion-resistant coating had 18 vol% of the silicon-rich phase. Moreover, the corrosion-resistant coating 13 and the diffusion layer 19 together have a pore fraction of 2.3%. Not only the joining properties but also the coating-material adhesion properties were therefore better than for standard commercial aluminum-based corrosion-resistant coatings, in which a layering is formed with a continuous low-silicon layer in the near-surface region.

[0140] The polished cross section of the resultant steel component 23 is represented in FIG. 2b. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The near-surface region has around 23 vol% of the silicon-rich phase 15. The mean roughness is Ra = 1.5 .Math.m. The fraction of the silicon-rich phase in the corrosion-resistant coating is 11 vol%. More than 90% of the silicon-rich phase is present in the form of small “islands” with a surface area of less than 100 .Math.m.sup.2.

[0141] The steel component 23 further comprises a diffusion layer 19 with Fe3Al and Fe2Al5, which is disposed adjacent to the steel substrate 11 beneath the corrosion-resistant coating 13. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The diffusion layer has a thickness of 6 .Math.m. The steel component 23 further comprises a layer 21 of the silicon-rich phase 15, having a thickness of 1 .Math.m, which is disposed adjacent to the diffusion layer 19 beneath the corrosion-resistant coating 13. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0142] In this working example, the steel component further comprises an oxide layer at the surface of the corrosion-resistant coating, with a thickness of 110 nm, which because of the resolution in FIG. 2b is not visible.

WORKING EXAMPLE 3

[0143] Shaped blanks were cut from a strip of steel type D from table 2, 1.2 mm thick and with a 20 .Math.m thick, aluminum-based corrosion-resistant coating on both sides. The cutting method employed was a laser. The initial composition of the protective coating was 6 wt% Si, 3 wt% Fe, 0.3 wt% Mg, balance Al. The add-on weight was

[00009]$60\frac{\text{g}}{{\text{m}}^2}$

on each side.

[0144] These shaped blanks, in a two-zone continuous furnace, were first heated to more than 900° C. in the first zone at a furnace temperature of 1000° C., in 90 seconds. The heating rate between 500° C. and 700° C. was 13 K/s. This part of the furnace had an O2-containing atmosphere with a dew point of -15° C. The blank was then held at 920° C. temperature for 30 seconds, this being above the Ac3 temperature, which for the type of steel selected is about 860° C. Within this time, prealloying of the corrosion-resistant coating occurred. The Fe content of the corrosion-resistant coating subsequently was between 35 wt% and 40 wt%. Moreover, the Fe content of the corrosion-resistant coating at each point was more than 10 wt% Fe. The mean roughness R.sub.a for this variant embodiment is 1.2 .Math.m.

[0145] The polished cross section of the resultant flat steel product 9 is represented in FIG. 3a. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The areas with silicon-rich phase 15 are distributed over a region which has a thickness, perpendicular to the surface, of about 80% of the thickness of the corrosion-resistant coating 13. What is formed, therefore, is not a single narrow strip with areas of silicon-rich phases 15; instead, the silicon-rich areas are distributed insularly over a strip which occupies about 80% of the corrosion-resistant coating. This ensures that in the hot-pressed steel component the corrosion-resistant coating, in particular the near-surface region of the corrosion-resistant coating has between 10 vol% and 25 vol% of the silicon-rich phase.

[0146] With this variant of the flat steel product with prealloyed corrosion-resistant coating, the flat steel product comprises a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 2.5 .Math.m, disposed adjacent to the steel substrate 11. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The corrosion-resistant coating 13 further comprises a layer 21 of the silicon-rich phase 15 with a thickness of 0.5 .Math.m, disposed adjacent to the diffusion layer 19. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0147] In the case of this working example, moreover, the flat steel product comprises an oxide layer at the surface of the corrosion-resistant coating, having a thickness of 70 nm, which because of the resolution in FIG. 3a is not visible.

[0148] The shaped blanks thus treated were processed further elsewhere on a contact heating line. The heating time to the forming temperature of 900° C. was 20 seconds. The blank was subsequently held at this temperature via the infrared radiation of the contact plates for a further 75 seconds at this temperature, with the plates occupying a distance of around 5 mm from the blank surface. This was accompanied by complete austenitization of the steel substrate and by full alloying of the corrosion-resistant coating. The heating time was therefore 95 seconds. Because of the prealloyed corrosion-resistant coating, there were no instances of sticking to the contact plates. The layer thickness of the corrosion-resistant coating also remained constant over the plate area.

[0149] After the hot press hardening, the corrosion-resistant coating contained an average of 60 wt% Fe and had silicon-rich areas with Si contents of between 11% and 15%. Above the diffusion layer, the corrosion-resistant coating does not show any clear layering of the silicon-rich and low-silicon areas. Instead, the silicon-rich phase is distributed insularly in the low-silicon phase. As a result of this, there are both low-silicon and silicon-rich phases present in the near-surface region. Not only the joining properties but also the coating-material adhesion properties were therefore better than for standard commercial aluminum-based corrosion-resistant coatings, in which a layering is formed with a continuous low-silicon layer in the near-surface region.

[0150] The polished cross section of the resultant steel component 23 is represented in FIG. 3b. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The near-surface region has around 15 vol% of the silicon-rich phase 15. Moreover, the corrosion-resistant coating 13 and the diffusion layer 19 together have a pore fraction of 1.3%. The mean roughness is Ra = 1.5 .Math.m. The fraction of the silicon-rich phase in the corrosion-resistant coating is around 13 vol%. More than 90% of the silicon-rich phase is present in the form of small “islands” with a surface area of less than 100 .Math.m.sup.2.

[0151] The steel component 23 further comprises a diffusion layer 19 with Fe3Al and Fe2Al5, which is disposed adjacent to the steel substrate 11 beneath the corrosion-resistant coating 13. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The diffusion layer has a thickness of 9 .Math.m. The steel component 23 further comprises a layer 21 of the silicon-rich phase 15, having a thickness of 2 .Math.m, which is disposed adjacent to the diffusion layer 19 beneath the corrosion-resistant coating 13. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0152] In this working example, the steel component further comprises an oxide layer at the surface of the corrosion-resistant coating, with a thickness of 220 nm, which because of the resolution in FIG. 3b is not visible.

WORKING EXAMPLE 4

[0153] Shaped blanks were cut from a strip of steel type D from table 2, 1.6 mm thick and with a 25 .Math.m thick, aluminum-based corrosion-resistant coating on both sides. The cutting method employed was a laser. The initial composition of the protective coating was 8 wt% Si, 3 wt% Fe, 0.3 wt% Mg, balance Al. The add-on weight was

[00010]$75\frac{\text{g}}{{\text{m}}^2}$

on each side. The chemical composition was identical to working example 3.

[0154] These shaped blanks, in a two-zone continuous furnace, were first heated to more than 900° C. in the first zone by means of an infrared radiation array, in 60 seconds. The furnace temperature was 1050° C. The heating rate between 500° C. and 700° C. was 20 K/s. This part of the furnace had an O2-containing atmosphere with a dew point of -15° C. The blank was then held at 920° C. temperature for 45 seconds in a roller hearth furnace, this being above the Ac3 temperature, which for the type of steel selected is about 860° C. This part of the furnace had an O2-containing atmosphere with a dew point of +20° C. Within this time, prealloying of the corrosion-resistant coating occurred. The Fe content of the corrosion-resistant coating subsequently was between 40 wt% and 45 wt%. Moreover, the Fe content of the corrosion-resistant coating at each point was more than 10 wt% Fe. The mean roughness R.sub.a for this variant embodiment is 1.3 .Math.m.

[0155] The polished cross section of the resultant flat steel product 9 is represented in FIG. 4a. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The areas with silicon-rich phase 15 are distributed over a region which has a thickness, perpendicular to the surface, of about 80% of the thickness of the corrosion-resistant coating 13. What is formed, therefore, is not a single narrow strip with areas of silicon-rich phases 15; instead, the silicon-rich areas are distributed insularly over a strip which occupies about 80% of the corrosion-resistant coating. This ensures that in the hot-pressed steel component the near-surface region of the corrosion-resistant coating has between 10 vol% and 25 vol% of the silicon-rich phase.

[0156] With this variant of the flat steel product with prealloyed corrosion-resistant coating, the flat steel product comprises a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 3 .Math.m, disposed adjacent to the steel substrate 11. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The corrosion-resistant coating 13 further comprises a layer 21 of the silicon-rich phase 15 with a thickness of 0.5 .Math.m, disposed adjacent to the diffusion layer 19. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0157] In the case of this working example, moreover, the flat steel product comprises an oxide layer at the surface of the corrosion-resistant coating, having a thickness of 110 nm, which because of the resolution in FIG. 4a is not visible.

[0158] The shaped blanks thus treated were processed further in a separate operating step in a contact heating operation by thermal conduction. The heating time to the forming temperature of 900° C. was 12 seconds. The blank was subsequently held at this temperature in a roller hearth furnace for a further 110 seconds at this temperature. This was accompanied by complete austenitization of the steel substrate and by full alloying of the corrosion-resistant coating. The heating time was therefore 122 seconds. Because of the prealloyed corrosion-resistant coating, there were no instances of sticking to the contact plates or furnace rollers. The layer thickness of the corrosion-resistant coating also remained constant over the plate area.

[0159] After the hot press hardening, the corrosion-resistant coating contained an average of 58 wt% Fe and had silicon-rich areas with Si contents of between 11% and 16%. Above the diffusion layer, the corrosion-resistant coating does not show any clear layering of the silicon-rich and low-silicon areas. Instead, the silicon-rich phase is distributed insularly in the low-silicon phase. As a result of this, there are both low-silicon and silicon-rich phases present in the near-surface region. Not only the joining properties but also the coating-material adhesion properties were therefore better than for standard commercial aluminum-based corrosion-resistant coatings, in which a layering is formed with a continuous low-silicon layer in the near-surface region.

[0160] The polished cross section of the resultant steel component 23 is represented in FIG. 4b. On a steel substrate 11, a corrosion-resistant coating 13 is disposed. The corrosion-resistant coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. Clearly apparent is the insular distribution of the silicon-rich phase 15 in the low-silicon phase 17. The near-surface region has around 17 vol% of the silicon-rich phase 15. Moreover, the corrosion-resistant coating 13 and the diffusion layer 19 together have a pore fraction of 1.2%. The mean roughness is Ra = 1.5 .Math.m. The fraction of the silicon-rich phase in the corrosion-resistant coating is 10.5 vol%. More than 90% of the silicon-rich phase is present in the form of small “islands” with a surface area of less than 100 .Math.m.sup.2.

[0161] The steel component 23 further comprises a diffusion layer 19 with Fe3Al and Fe2Al5, which is disposed adjacent to the steel substrate 11 beneath the corrosion-resistant coating 13. The diffusion layer 19 therefore borders the steel substrate 11 directly and contacts the steel substrate 11. The diffusion layer has a thickness of 5 .Math.m. The steel component 23 further comprises a layer 21 of the silicon-rich phase 15, having a thickness of 1 .Math.m, which is disposed adjacent to the diffusion layer 19 beneath the corrosion-resistant coating 13. The layer 21 of the silicon-rich phase 15 therefore borders the diffusion layer 19 directly and contacts the diffusion layer 19.

[0162] In this working example, the steel component further comprises an oxide layer at the surface of the corrosion-resistant coating, with a thickness of 170 nm, which because of the resolution in FIG. 4b is not visible.

FURTHER WORKING EXAMPLES

[0163] The table below reports the process parameters for shaped blanks in a variety of thicknesses. The type of steel in the steel substrate is likewise reported in the table. All of the shaped blanks include an aluminum-based corrosion-resistant coating 25 .Math.m thick on both sides. The initial composition of the resistant coating was 9 wt% of Si, 3 wt% of Fe, balance Al. The add-on weight was

[00011]$70\frac{\text{g}}{{\text{m}}^2}$

on each side.

[0164] These shaped blanks were heated to above 900° C. in a two-zone continuous furnace first in the first zone at 1050° C. furnace temperature. The heating rate between 500° C. and 700° C. was in all cases more than 10 K/s. The blanks were subsequently held in the second zone at a temperature of 950° C. for the hold time reported in table 1.

[0165] The shaped blanks thus treated were processed further elsewhere on a hot forming line. The further processing took place in a short hot forming furnace at a forming temperature of 900° C. or 920° C. After this, the thickness of the diffusion layer, the weldability, the coating-material adhesion and the tool abrasion were ascertained.

[0166] The weldability was ascertained in the form of suitability for resistance spot welding according to SEP1220-2.

[0167] For ascertaining the coating-material adhesion, firstly a stone chip test was carried out according to EN ISO 20567. The coated sample is subjected to defined bombardment with a defined amount of “corundum” and subjected to a VDA corrosion test. Secondly, the coating-material adhesion was also ascertained in accordance with EN ISO 2409 (cross-cut testing). In this case the coated sample is scored in a defined way, using a cross pattern, down to the metallic protective layer, and is likewise subjected to the VDA corrosion test.

[0168] The tool abrasion was ascertained by removing the pulverulent and adhering residues from the tool after a particular number of components produced, and was determined gravimetrically. Table 1 reports the mass of the abrasion after 100 formed components (cts) relative to one square meter of coated material. Beyond 100 g of abrasion per 1000 components (i.e., 10 g per 100 components), cleaning interventions outside of the plan might be necessary. Values below 10 g/100 components are therefore regarded as acceptable.

TABLE-US-00001 Sheet thickness Steel type Dwell time in 1st furnace zone Hold time in 2nd furnace zone Forming temperature Tu Heating time tE Thickness of diffusion layer RS weldability Cathodic coating-material adhesion Tool abrasion As per invention (mm) Seconds seconds °C Seconds .Math.m > 1 kA = ok ≤1=ok ≤10 g = ok 1.00 B 60 30 940 20 4 1.5 kA 0-1 2 g / 100 cts yes 1.25 D 75 38 880 60 <1 1.2 kA 1 10 g / 100 cts no 1.25 D 75 38 890 100 5 1.5 kA 0 0 g / 100 cts yes 1.25 D 75 38 890 260 14 0.6 kA 2 0 g / 100 cts no 1.50 D 90 45 910 80 2 1.7 kA 0-1 4 g / 100 cts yes 1.50 D 90 45 920 150 5 2.0 kA 0 0 g / 100 cts yes 1.50 D 90 45 920 360 17 0.3 kA 1 0 g / 100 cts yes 2.00 D 120 60 920 105 1 1.3 kA 1 15 g / 100 cts no 2.00 D 120 60 920 180 5 1.8 kA 0 0 g / 100 cts yes 2.00 D 120 60 920 400 15 0.5 kA 2-3 0 g / 100 cts no 1.25 D 75 38 880 75 1-2 1.3 kA 0-1 8 g / 100 cts yes 1.25 D 75 38 890 120 5 1.6 kA 0 0 g / 100 cts yes 1.25 D 75 38 890 360 14 0.5 kA 1 0 g / 100 cts no 1.50 D 90 45 920 90 2 1.3 kA 0-1 5 g / 100 cts yes 1.50 D 90 45 920 180 6 1.8 kA 0 0 g / 100 cts yes 1.50 D 90 45 920 360 16 0.3 kA 1-2 0 g / 100 cts no 2.00 D 120 60 900 90 1 1.4 kA 1 12 g / 100 cts no 2.00 D 120 60 900 205 4 1.6 kA 0-1 0 g / 100 cts yes 2.00 D 120 60 900 480 15 0.4 kA 2 0 g / 100 cts no 1.50 B 90 45 920 150 5 1.7 kA 0 2 g /100 cts yes 1.50 B 90 45 920 150 6 1.8 kA 0 4 g /100 cts yes 1.50 B 90 45 920 150 5 1.4 kA 0-1 0 g /100 cts yes 1.50 C 90 45 920 150 5 1.7 kA 0 5 g /100 cts yes 1.50 C 90 45 920 150 5 1.7 kA 0-1 0 g /100 cts yes 1.50 C 90 45 920 150 7 2.0 kA 0 0 g /100 cts yes 1.50 D 90 45 920 150 6 1.6 kA 0 1 g /100 cts yes 1.50 D 90 45 920 150 5 1.9 kA 0-1 0 g /100 cts yes 1.50 D 90 45 920 150 6 1.8 kA 0-1 2 g /100 cts yes 2.50 E 150 75 900 200 3 1.6 KA 1 3 g / 100 cts yes

TABLE-US-00002 Steel type min / max c Si Mn s Al Nb Ti Cr+Mo B A min 0.05 0.05 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 max 0.10 0.35 1.00 0.030 0.025 0.075 0.100 0.150 0.0050 B min 0.05 0.03 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 max 0.10 0.50 2.00 0.030 0.025 0.075 0.100 0.150 0.0050 C min 0.05 0.05 1.00 0.000 0.000 0.015 0.005 0.000 0.00 0.0010 max 0.16 0.40 1.40 0.025 0.010 0.150 0.050 0.050 0.50 0.0050 D min 0.10 0.05 1.00 0.000 0.000 0.005 0.000 0.00 0.0010 max 0.30 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0050 E min 0.25 0.10 1.00 0.000 0.000 0.015 0.000 0.00 0.0010 max 0.4 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0500