EP1534869B1 - Very high mechanical strength steel and method for making a sheet thereof coated with zinc or zinc alloy - Google Patents

Abstract

The invention concerns a very high mechanical strength steel, whereof the chemical composition comprises in wt. %: 0.060%=C=0.250%; 0.400%=Mn=0.950%; Si=0.300%; Cr=0.300%; 0.100%=Mo=0.500%; 0.020%=AI=0.100%; P=0.100%; B=0.010%; Ti=0.050%, the rest being iron and impurities resulting from preparation. The invention also concerns a method for making a sheet of said steel coated with zinc or zinc alloy.

Description

The present invention relates to a steel with very high mechanical strength, and a method of manufacturing a sheet of this steel coated with zinc or zinc alloy.

There are several families of steels with very high mechanical strength that differ in their compositions and their microstructures. Thus, so-called dual phase steels have a microstructure composed of ferrite and martensite, which enables them to reach tensile strengths ranging from 400 MPa to more than 1200 MPa.

In order to obtain the microstructures which will make it possible to reach high mechanical characteristics, these grades are quite heavily loaded in elements such as chromium, silicon, manganese, aluminum or phosphorus. These grades, however, pose a problem when it is desired to coat them with a protective coating against corrosion, for example by hot dip galvanizing.

Indeed, it is observed that the surface of the sheets has a very poor wettability vis-à-vis zinc or zinc alloys. The sheets then comprise uncoated portions, which constitute privileged areas for the initiation of corrosion.

To overcome this problem, different approaches have been proposed. Thus, there are known methods of pre-coating a metal to provide a better bonding base for zinc. It has been proposed for this purpose to deposit iron, aluminum, copper and other elements, usually by electroplating. These methods have the disadvantage of adding an additional step before the actual galvanizing.

It has also been proposed to pass the sheets in annealing furnaces having, in particular, particular atmospheres, allowing selectively oxidize the iron to form a layer of iron oxide on which zinc deposits well. Such a process is however of a very delicate setting and requires a very strict control of the oxidation conditions.

The present invention therefore aims to provide a steel composition does not have the disadvantages of the compositions of the prior art, and having in particular a good coating ability with zinc or zinc alloys, while maintaining high mechanical characteristics.

For this purpose, a first object of the invention is constituted by a steel with very high mechanical strength, the chemical composition of which comprises, in% by weight: 0.080% ≤ C ≤ 0.120% 0.800% ≤ Mn ≤ 0.950% If ≤ 0.300% Cr ≤ 0.300% 0.150% ≤ Mo ≤ 0.350% 0.020% ≤ Al ≤ 0.100% P ≤ 0.100% B ≤ 0.010% Ti ≤ 0.050% the remainder being iron and impurities resulting from the preparation, its microstructure consisting of ferrite and martensite.

This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 500 MPa.

In another preferred embodiment, the steel comprises: 0.100% ≤ C ≤ 0.140% 0.800% ≤ Mn ≤ 0.950% If ≤ 0.300% Cr ≤ 0.300% 0.200% ≤ Mo ≤ 0.400% 0.020% ≤ Al ≤ 0.100% P≤ 0.100% B ≤ 0.010% Ti ≤ 0.050% the rest being iron and impurities resulting from the elaboration.

This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 600 MPa.

A second object of the invention is constituted by a sheet of steel with very high mechanical strength according to the invention, and coated with zinc or zinc alloy.

• élaborer une brame dont la composition est conforme à l'invention, laminer à chaud, puis à froid ladite brame pour obtenir une feuille,
• chauffer ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température de maintien comprise entre 700 et 900°C,
• refroidir ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température proche de celle d'un bain contenant du zinc ou un alliage de zinc fondu, puis
• revêtir ladite feuille de zinc ou d'un alliage de zinc par immersion dans ledit bain et la refroidir jusqu'à température ambiante à une vitesse de refroidissement comprise entre 2 et 100°C/s.

A third object of the invention is constituted by a method of manufacturing a steel sheet according to the invention coated with zinc or zinc alloy, and which comprises the steps of:

• developing a slab whose composition is in accordance with the invention, rolling hot, then cold said slab to obtain a sheet,
• heating said sheet at a speed of between 2 and 100 ° C / s until a holding temperature of between 700 and 900 ° C is reached,
• cooling said sheet at a rate of between 2 and 100 ° C / s until a temperature close to that of a bath containing zinc or a molten zinc alloy is reached, then
• coating said sheet with zinc or a zinc alloy by immersing in said bath and cooling it to ambient temperature at a cooling rate of between 2 and 100 ° C / s.

In another preferred embodiment, the sheet is maintained at the holding temperature for 10 to 1000 seconds.

In another preferred embodiment, the zinc-containing bath or molten zinc alloy is maintained at a temperature of between 450 and 480 ° C, and the immersion time of the sheet is between 2 and 400 seconds.

In another preferred embodiment, the bath contains mainly zinc.

A fourth object of the invention is the use of a very high-strength steel sheet coated with zinc or zinc alloy, for the manufacture of auto parts.

The present invention is based on the new observation that by limiting the manganese, silicon and chromium contents to the maximum values claimed, excellent heatability of the shades thus produced can be obtained. Depending on the level of mechanical characteristics sought, the contents of quenching elements such as carbon and molybdenum, which have been found to be not detrimental to this coating, will be adjusted.

où Cγ représente la teneur en carbone de l'austénite avant le refroidissement.For this purpose, it is possible for example to use the conventional formula giving the decimal logarithm of the critical quenching velocity V (in ° C / s): $$\log \left(\mathrm{V}\right)\mathrm{=}\mathrm{4}\mathrm{,}\mathrm{5}\mathrm{-}\mathrm{2}\mathrm{,}\mathrm{7}\mathrm{\%}{\mathrm{VS}}_{\mathrm{\gamma }}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{95}\mathrm{\%}\mathrm{mn}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{18}\mathrm{\%}\mathrm{Yes}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{38}\mathrm{\%}\mathrm{Cr}\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{17}\mathrm{\%}\mathrm{MB}\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{29}\left(\mathrm{\%}\mathrm{VS}\mathrm{\times }\mathrm{\%}\mathrm{Cr}\right)\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{33}\left(\mathrm{\%}\mathrm{Cr}\mathrm{\times }\mathrm{\%}\mathrm{MB}\right)$$

where Cγ represents the carbon content of the austenite before cooling.

The steel composition according to the invention contains between 0.080 and 0.120%, by weight of carbon, since it has been observed that for a carbon content of less than 0.060%, the grade was no longer hardenable, and no longer allowed obtain the desired high mechanical characteristics. Beyond 0.250% by weight, the carbon greatly deteriorates the weldability of the grade.

The composition also contains between 0.800 and 0.950% by weight of manganese. As for carbon, the lower limit is required to obtain a hardenable steel grade, while the upper limit must be respected in order to ensure a good coating of the grade.

The composition also contains up to 0.300% by weight of silicon. The upper limit must be respected in order to ensure a good coating of the grade.

The composition further contains up to 0.300% by weight of chromium. The upper limit must be respected in order to ensure a good coating of the grade.

Finally, the composition according to the invention must contain between 0.150 and 0.0350% by weight of molybdenum since it has been observed that for a content of less than 0.100%, the grade no longer makes it possible to obtain the desired high mechanical characteristics. Beyond 0.500% by weight, molybdenum greatly deteriorates the weldability of the grade.

The composition may also optionally contain up to 0.010% by weight of boron which will then be protected if necessary by a content of 0.050% by weight maximum of titanium. This latter element has a greater affinity for nitrogen than boron, the trap by formation of titanium nitrides.

The steel composition may also contain various unavoidable residual elements, among which may be mentioned N, Nb, Cu, Ni, W, V.

It is particularly preferred to limit the nitrogen content which can make the steel susceptible to aging.

Thanks to its improved galvanizability, the steel according to the invention finds particular applications in the field of the manufacture of parts for the automobile, and more particularly for the manufacture of visible parts such as bodywork elements, which will present a good appearance after painting, unlike those manufactured so far with the steels of the prior art.

The present invention will now be illustrated on the basis of the following observations and examples, given by way of nonlimiting examples, Table 1 giving the chemical composition of the steels tested, in 10 ^-3 % by weight. <U> Table 1 </ u> VS mn Yes Cr MB al B Ti NOT P S Cu Or V AT 59 1195 121 491 - 38 - - 5.4 11 2 6 23 - B 83 1546 361 204 - 24 - - 5.1 15 2 8 22 - VS* 95 906 12 15 102 33 - - 2.3 25 4 9 20 - D ** 93 909 10 15 205 33 - - 2.3 25 4 9 23 3 E ** 85 900 11 14 305 35 - - 2.6 25 4 9 25 3 F ** 90 900 11 15 306 33 1 27 2.5 25 4 9 25 4 ** according to the invention

These different compositions were produced in the form of 15 kg ingots. The ingots were then reheated at 1250 ° C for 45 minutes, then hot rolled in 7 passes, the end of rolling temperature being 900 ° C.

The sheets thus obtained were cooled by quenching with water with a retarder at a cooling rate of the order of 25 ° C./s, and then wound at 550 ° C. before being cooled.

• chauffage à une vitesse de l'ordre de 30°C/s jusqu'à atteindre une température de maintien variant entre 770 et 810°C pendant un temps variant entre 50 et 80 secondes, pour simuler des vitesses de ligne allant de 80 à 150 m/min,
• refroidissement de la feuille à une vitesse de l'ordre de 10°C/s jusqu'à atteindre 470°C.

They were then cold rolled with a 70% reduction rate before undergoing the following thermal cycle:

• heating at a rate of the order of 30 ° C / s until a holding temperature of between 770 and 810 ° C is reached for a time varying between 50 and 80 seconds, to simulate line speeds ranging from 80 to 150 m / min,
• cooling the sheet at a rate of the order of 10 ° C / s to reach 470 ° C.

The sheets are then subjected to galvanization by dipping in a zinc bath, with a residence time in the bath depending on the selected line speed (between 80 and 150 m / min), then cooled at a speed of 5 ° C. / s to room temperature.

• Rm : résistance à la traction en MPa
• Rel : limite d'élasticité en MPa,
• A : allongement à la rupture en %
• Ag : allongement réparti en %.
• P : palier en %,

ainsi que la proportion de martensite des feuilles (%M).For each sheet, the following mechanical characteristics are then measured:

• Rm: tensile strength in MPa
• Rel: yield strength in MPa,
• A: elongation at break in%
• Ag: elongation distributed in%.
• P: level in%,

as well as the proportion of leaf martensite (% M).

Test 1: Influence of the molybdenum content and the presence of boron

This influence has been studied for grades A to F, for a holding temperature of 790 ° C and a line speed of 120 m / min. rm rel AT Ag P % M AT 480 375 28.2 18.8 2.3 1 B 540 360 28.3 17.6 - 3 VS* 466 380 28.8 19.9 4.6 1 D ** 526 324 29.0 18.8 0.6 4 E ** 563 282 26.6 17.9 0 7 F ** 673 393 15.2 11.8 0 6 ** according to the invention

For the shades according to the invention, it is found that by increasing the molybdenum content, the martensite content is increased, which makes it possible to increase the tensile strength and to lower the yield strength.

On the other hand, the addition of boron does not lead to an increase in the percentage of martensite, but rather leads to a refinement of the martensite and the carburized phases.

Test 2: Influence of heat treatment

This influence has been studied for grade D for three line speeds and for three holding temperatures (in m / min): T maintaining V line rm AT % M Nuance D 770 80 502 29.4 1 120 528 27.6 4 150 534 27.3 6 790 80 500 26.2 2 120 526 29.0 4 150 530 28.6 6 810 80 505 29.9 3 120 521 25.8 4 150 530 26.4 6

It is found that the holding temperature and the line speed have a small influence on the mechanical characteristics obtained. This is of great interest for an industrial application that should not be sensitive to this type of variations.

This influence was then studied for the grade F: T maintaining V line rm AT % M Nuance F 770 80 692 18.6 6 120 687 15.3 6 150 715 13.7 6 790 80 664 17.3 6 120 673 15.2 6 150 688 16.6 6 810 80 634 15.9 6 120 654 16.0 6 150 666 17.7 6

It can be seen that the addition of boron to the grade according to the invention remarkably stabilizes the proportion of martensite formed which does not vary at all, whatever the parameters of the heat treatment.

Test 3: Galvanizability

The sheets of shades A, B, C and F are hot dip galvanized and the dew point is adjusted to -40 ° C. The sheets made in shades A and B have gaps in their coatings, unlike shades C and F which have continuous coatings.

Claims (7)

1. Very high-strength steel, characterized in that its chemical composition comprises, in percent by weight: 0. 080% ≤ C ≤ 0.120% 0.800% ≤ Mn ≤ 0.950% Si ≤ 0.300% Cr ≤ 0.300% 0.150% ≤ Mo ≤ 0.350% 0.020% ≤ Al ≤ 0.100% P ≤ 0.100% B ≤ 0.010% Ti ≤ 0.050% the balance being iron and impurities resulting from the smelting, its microstructure consisting of ferrite and martensite.
2. Very high-strength sheet of steel according to Claim 1, characterized in that it is coated with zinc or a zinc alloy.
3. Process for manufacturing a steel sheet according to Claim 2, characterized in that it comprises the steps consisting in:

   - producing a slab, the composition of which is in accordance with Claim 1;

   - hot rolling, and then cold rolling said slab in order to obtain a sheet;

   - heating said sheet at a rate of between 2 and 100°C/s until reaching a soak temperature of between 700 and 900°C;

   - cooling said sheet at a rate of between 2 and 100°C/s until reaching a temperature close to that of a bath containing molten zinc or a molten zinc alloy; and then

   - coating said sheet with zinc or a zinc alloy by immersion in said bath and cooling it down to room temperature, at a cooling rate of between 2 and 100°C/s.
4. Process according to Claim 3, characterized in that the sheet is held at said soak temperature for 10 to 1000 seconds.
5. Process according to either of Claims 3 and 4, characterized in that said bath containing molten zinc or a molten zinc alloy is maintained at a temperature of between 450 and 480°C and in that the immersion time of said sheet is between 2 and 400 seconds.
6. Process according to any one of Claims 3 to 5, characterized in that said bath contains mainly zinc.
7. Use of a very high-strength steel sheet coated with zinc or a zinc alloy, according to Claim 2, for the manufacture of motor vehicle parts.