WO2004104254A1

WO2004104254A1 - High-resistant sheet metal which is cold rolled and aluminized in dual phase steel for an anti-implosion belt for a television and method for the manufacture thereof

Info

Publication number: WO2004104254A1
Application number: PCT/FR2004/001149
Authority: WO
Inventors: Antoine Moulin; Christophe Degand; Dominique Spehner
Original assignee: Usinor
Priority date: 2003-05-19
Filing date: 2004-05-12
Publication date: 2004-12-02
Also published as: EP1627092A1; FR2855184B1; CN1791695A; FR2855184A1

Abstract

The invention relates to a dual phase sheet metal which is cold rolled, can be used for the manufacture of an anti-implosion belt for a television and whose chemical composition comprises the following expressed in percentage by weight:: 0.03 % = CD 0.3 %, 1 % = Mn = 3 %, 0.05 % = Si = 2 %, 0.02 % = Al = 2 %, 0.02 % = Cr = 1 %, Mo = 1 %, S = 0.02 %, P = 0.2 %, N = 0.01 %, and optionally, one or several elements chosen from Ti, V, Zr, Nb, whereby the content of each element ranges from 0.001 and 0.2 %, and the rest of the composition comprises iron and impurities resulting from production. A slab or ingot of steel having the above-mentioned composition is heated at a temperature of 1100 -1300 °C; the slab or ingot is hot-rolled such that the temperature at the end of the rolling is higher than the temperature Ar3 of the steel, the steel thus obtained is cooled at a speed VR of 1 - 500 °C/s; then it is wound at a temperature of 300 -720 °C. After cold-rolling, the sheet metal is annealed at a temperature Tm which is higher than Ac1; it is cooled at a speed of more than 2 °C/s to reach the temperature of the aluminization operation, whereupon it is subsequently hardened in an aluminium-based bath at a temperature of 650 - 720 °C, subsequently cooled at a speed of more than 2 °C/s to reach room temperature. The microstructure of the steel is made up of ferrite and 5 - 30 % martensite and less than 2 % phases with carbides.

Description

Cold rolled and aluminized sheet made of very high strength Dual Phase steel for TV anti-implosion belt, and method of manufacturing this sheet

The present invention relates to the field of anti-implosion belts for cathode ray screens. These elements play an essential role in televisions, because they prevent deformation of the front surface of the tube under the effect of the pressure difference between the inside of the tube (10 ^"7 torr) and atmospheric pressure. The tensioning by hooping of the belt counterbalances the effect of the atmospheric pressure.In the absence of this correction, the colors on the screen are disturbed due to the modification of the distance grid-phosphorescent panel. includes an expansion operation for dimensioning the cathode ray tube, which results in hardening, then heating to around 500 ° C. before the setting up by shrinking. This latter operation leads to a residual deformation of approximately 0, 3 to 0.4% which remains after fixing the belt.

Anti-implosion belts are usually made from calm aluminum steels or non-interstitial aluminized steels, this coating providing protection against corrosion. These traditional solutions do not however allow very high levels to be reached, since the elastic limit Rp0,2 is close to 400 MPa after final positioning of the belt on the tube under these conditions. However, the current trend towards large screens or flat screens leads to particularly high service efforts. We can then increase the section of the anti-implosion belts, but this runs up against the concern of reducing the weight of the televisions. The use of materials with higher mechanical characteristics (an important parameter being the elastic limit after fitting the belt on the tube) is in turn limited by the fact that the elongation of these materials is generally reduced , which leads to formatting problems (cracks) in the folding areas. It should therefore be noted that there has so far been no anti-implosion belt with high mechanical characteristics (elastic limit after fitting greater than 500 MPa, good resistance-ductility compromise)

The object of the present invention is to provide an anti-implosion belt with high mechanical characteristics, in particular with an elastic limit greater than 500 MPa after the band has been placed on the tube, a process and a sheet of steel for making this belt economically. - To this end, the invention relates to a process for manufacturing a steel sheet Dual Phase (that is to say steel whose structure consists of a hard phase, essentially martensitic, dispersed within a more deformable ferritic matrix) capable of being used for the manufacture of a television anti-implosion belt, characterized in that a steel is produced, the chemical composition of which comprises, the contents being expressed in weight: 0.03% <C <0.3%, 1% ≤ Mn <3%, 0.05% <If <2%, 0.02% <Al <2%, 0.02% ≤Cr <1 %, Mo <1%, S <0.02%, P <0.2%, N <0.01% and, optionally, one or more elements chosen from Ti, V, Zr, Nb, the content of each element being between 0.001 and 0.2%, the rest of the composition consisting of iron and impurities resulting from the production. A steel slab or ingot of said composition is brought to a temperature between 1100 and 1300 ° C., the slab or the ingot is hot rolled, the temperature at the end of hot rolling being higher than the temperature Ar3 of l steel, the sheet thus obtained is cooled at a speed V _R of between 1 and 500 ° C / s, the sheet is _wound at a temperature T _bob such that 300 <T _b0b ^< 720 ° C, the sheet is cold rolled , the cold-rolled sheet is subjected to continuous annealing at a temperature T _m such that T _m > Ad, the sheet is cooled at a speed greater than 2 ° C / s to the aluminizing temperature, said aluminized sheet by soaking in an aluminum-based bath at a temperature between 650 and 720 ° C, and said sheet is cooled to room temperature at a speed greater than 2 ° C / s. According to a preferred characteristic, a skin-pass treatment is applied to said aluminized sheet with a reduction rate of less than 5%. The subject of the invention is also a steel sheet manufactured according to the process described above, characterized in that the structure of the steel consists of a ferritic matrix containing a proportion of martensite of between 5 and 30%, and less than 2% of phases with carbides. The invention also relates to an anti-implosion television belt, characterized in that it is produced from a strip of said sheet steel. Other characteristics and advantages of the invention will appear during the description below, given by way of example and made with reference to the following appended figures: - Figure 1 shows thermal cycles corresponding to continuous annealing with galvanizing or aluminizing cycles of cold-rolled sheets.

- Figure 2 shows, in the form of a continuous cooling transformation diagram, the metallurgical structures formed under annealing conditions with continuous aluminizing or galvanizing cycles.

- Figures 3 and 4 illustrate the microstructures corresponding respectively to the thermal galvanizing and aluminizing cycles. After numerous tests, the inventors have shown that the various requirements reported above can be satisfied by observing the following conditions:

- Regarding the chemical composition of steel, carbon plays a very important role in the formation of the microstructure. Below 0.03%, the hardenability is however insufficient to obtain the desired yield strength and strength characteristics. Above 0.30%, the drawing and weldability properties are very limited.

- In addition to a hardening effect by solid solution, manganese is an element which stabilizes the austenite and provides satisfactory hardenability. A minimum content of 1% is necessary to obtain the desired mechanical properties. However, beyond 3%, its gammagenic nature leads to the formation of an overly marked band structure and also degrades the weldability. - Silicon is an element participating in the deoxidation of liquid steel and hardening in solid solution. In addition, it prevents the precipitation of carbides by promoting the formation of martensitic phase. It plays an effective role from 0.05%. However, beyond an Si content of

2%, the formation of adherent oxides on the surface of the products becomes excessive and the weldability is reduced.

- Aluminum is an effective element for the deoxidation of liquid steel from a content of 0.02%. Above 2% the weldability is degraded, and its addition is no longer effective

- Chromium acts on hardening in solid solution and on hardenability. In this latter respect, it therefore makes it possible to obtain a dual-phase structure with lower cooling rates than for compositions containing no chromium. It is effective from a content of 0.02%. Above 1%, there is an increase in the risk of dusting during stamping, as well as a deterioration in the compromise between strength and ductility.

- Molybdenum acts on hardening in solid solution and on hardenability. In this latter respect, it therefore makes it possible to obtain a dual-phase structure with lower cooling rates than for compositions not containing molybdenum. Above 1%, it significantly degrades the weldability of the steel.

- Above a sulfur content of 0.02%, the ductility is reduced due to the presence of sulphides which reduce the ability to deform, in particular during the hole expansion test.

- Phosphorus is an element that reduces the ability to spot weld and hot ductility, particularly because of its tendency to segregate or co-segregate with manganese. For these reasons, its content must be limited to 0.2%. - When they are present, microalloy elements (Ti, Nb, V, Zr) in contents between 0.001% and 0.2% harden the steel by precipitating in the form of carbides or nitrides. The implementation of the manufacturing process according to the invention is as follows: - Slabs or steel ingots of the above composition are first brought to a temperature between 1100 and 1300 ° C. This is intended to reach at all points the temperature ranges favorable to the strong deformations that the steel will undergo during rolling, as well as to re-dissolve the carbides formed after solidification. However, if the temperature is too high, the austenitic grains grow undesirably. The initial temperature should therefore be limited to 1300 ° C in order to maintain a fine austenitic grain at this stage.

- The rolling is carried out in the austenitic domain and must be finished at a temperature higher than Ar3, depending on the composition of the steel.

- The rolling is followed by cooling at a speed VR between 1 and 500 ° C / s, then by a winding at a temperature between 300 and 720 ° C. These conditions make it possible to avoid the development of pearlitic strip structures on the hot sheet.

- Cold rolling is carried out under conditions identical to those of conventional steels, for example with a reduction rate of between 30 and 80%.

- An annealing is then carried out in the two-phase domain (annealing temperature between Ad and Ac3) or in the austenitic domain

(temperature higher than Ac3) in order to allow the transformation of the austenite formed during this annealing into constituents of high hardness during cooling.

- In order to keep sufficient proportions of this austenite during cooling and before the aluminizing operation, the cooling rate after maintenance must be greater than 2 ° C / s.

- The holding temperature after annealing, which is the aluminizing temperature in the case of the manufacture of aluminized products, is an important element of the invention: in fact, FIG. 1 presents an example (1 1) of associated annealing to an aluminizing cycle. For comparison, a typical thermal cycle corresponding to annealing with subsequent galvanization has also been shown (12) After maintaining in the two-phase range, it will be noted that the coating step is carried out towards 680 ° C (aluminizing) or 450 ° C (galvanizing) The superposition of the preceding cycles on a transformation diagram in continuous cooling ("TRC") of a grade (C = 0.12%, Mn = 1.4%, If = 0.35%) shows that these two types of cycles lead to very different microstructures: in fact, when the coating is carried out at typical temperatures of galvanization, there is the appearance of phases comprising carbides, in particularly of the bainitic phases. On the other hand, in the case where the coating step is carried out at a sufficiently high temperature (greater than 650 ° C. for the compositions mentioned here), only the ferrite is likely to form partially at high temperature. This partial ferritic transformation cannot however be very significant, since the holding temperatures associated with the aluminizing cycle are close to the temperature Ar1 of the dual-phase steel compositions. After maintenance, continuous cooling to room temperature with a sufficient speed (greater than 2 ° C / s) allows an appreciable proportion of martensite to be obtained. In this way, it is possible to manufacture dual-phase steels with an almost totally ferrito-martensitic character, to the exclusion practically of all other phases containing carbides, such as bainite or perlite.

It will indeed be observed in FIG. 2 that the bainitic and especially pearlitic zones of the diagram are crossed rapidly during cooling, which implies that these phases will possibly be present only in very small quantities. In other words, thanks to the annealing with aluminizing cycle, it is possible to advantageously manufacture steels with essentially ferritomartensitic structure (Dual Phase) whose interesting properties are:

- A lower ratio (elastic limit / resistance)

- An increase in the parameter (resistance x Elongation) - A systematic absence of bearing in the raw annealing state, which practically eliminates the need for a skin-pass.

A proportion of martensite greater than 5% guarantees a minimum resistance of 450 MPa after 2% of cold deformation. Through against the ductility properties are lowered when the proportion of martensite is greater than 30% and when the proportion of phases containing carbides is more than 2%.

- After aluminizing, it may nevertheless be advantageous to carry out a skin-pass treatment: This operation, which can be carried out with a rate ranging from 0 to 5%, allows the production of sheets with different levels of elastic limit in depending on the level of mechanical characteristics desired. This deformation comes naturally to accumulate with the possible subsequent work hardening during the operations of manufacturing parts from these sheets, which contributes to obtaining very high elastic limits on part in the final state. By way of example, the following results will show that the invention described makes it possible to obtain mechanical properties clearly superior to those of steel sheets of the same composition, but with a different annealing cycle, or to those of materials used. conventionally in the manufacture of television belts with identical cycles (annealing with aluminizing). It will of course be noted that the advantages conferred by the invention can also be exploited in other industrial applications combining the need for a high yield strength, good deformability, and protection by aluminizing .

Example 1:

The example is based on steel sheets whose composition is shown in Table 1 (analyzes in% by weight)

Table 1

Steels A1 to A3 were heated to a temperature of 1250 ° C, then subjected to hot rolling with an end of rolling temperature of 900 ° C followed by cooling at a speed v _R of 25 ° C / s and a winding at 570 ° C.

The cold-rolled sheets to a thickness of 1 mm were then subjected to continuous annealing at a temperature of 800 ° C for 60 s, to an aluminizing cycle (reference "I" in Table 2, corresponding conditions to the invention) at 680 ° C, then cooled to 20 ° C / s to ambient. For comparison, the properties were also indicated after annealing and then galvanizing cycle at 450 ° C. (reference “R” in table 2) The mechanical properties measured on 12.5 × 50 mm ² test pieces and the microstructures were reported in the table 2, with:

Re: elastic limit

Rm: Resistance

P: length of the bearing

A: total elongation

M: Proportion of martensite

(P + B): Proportion of perlite and bainite

These results clearly show that:

- A manufacturing process according to the invention leads to structures composed almost entirely of ferrite and martensite, practically without phases containing carbides. This point is illustrated in Figures 3 and 4 where we can compare the structures of A3 steel in the case of a cycle galvanizing and aluminizing respectively. - These microstructures obtained after the aluminizing cycle are associated with mechanical characteristics superior to those resulting from a reference treatment: low Re / Rm ratio, absence of plateau, high values of the parameter associating resistance and elongation. It will be noted for example that the application of the invention makes it possible to increase the resistance from 40 to 80 MPa compared to a treatment with annealing with galvanization. It will also be noted in the three examples above that the steels make it possible to obtain a guaranteed minimum resistance of 450, 500 and 600 MPa respectively. Example 2:

Table 3 presents the composition (analysis in% by weight) of a steel making it possible to obtain a resistance of 750 MPa when the latter is subjected to a galvanizing cycle (cf. conditions of the "R" cycle above) A 1 mm cold rolled steel sheet, produced under conditions identical to those specified above, was subjected to continuous annealing at 800 ° C for 50 s or 100 s then to an aluminizing cycle at 680 ° C for 10 or 20s, then cooled respectively to 40 ° C / s or 20 ° C / s to ambient. These conditions, respectively designated by 11 and 12, therefore correspond to those of the invention.

Table 3

The mechanical properties and the microstructures have been reported in Table 4, with the same conventions as in Table 2.

Table 4 These results again show the advantages conferred by the invention:

- The microstructure is practically free of phases containing carbides

- With comparable elongation, the resistance obtained under the conditions of the invention is much higher than that of the reference treatment, since the latter goes from 750 MPa to more than 850 MPa. With given mechanical characteristics, it is therefore understood that the implementation of the invention makes it possible to lower the content of alloying elements necessary to obtain these properties, which is advantageous in terms of costs and subsequent ease of implementation. of the product (welding, shaping)

Example 3:

Table 5 presents two examples of compositions (analyzes in% by weight) of reference steels: These reference steels R1 (calmed aluminum) or without interstitials (reference R2) are usually used for the manufacture of television belts. The table also presents two compositions of dual phase steels corresponding to the invention (references 13 and 14) Steel sheets of approximately 1 mm thick were produced on the bases indicated in Example 1. These sheets were then continuously annealed in a range between 780 and 820 ° C, subjected to an aluminizing treatment at 680 ° C, then to a skin-pass treatment with a deformation of between 1 to 3%.

Table 5 The sheets obtained then underwent a treatment corresponding to a cycle for manufacturing television belts: ε = 3% (expansion operation for fitting the belt) and heating to 375-550 ° C for shrinking. The elastic limit measured under these conditions is indicated in Table 6.

Table 6 The microstructure of the reference steel R1 consists of ferrite and precipitates of titanium carbonitrides, that of the steel R2 of ferrite and cementite.

These results clearly show that the steels according to the characteristics of the invention make it possible to obtain a much higher elastic limit than conventional solutions, greater than 500 MPa. With given mechanical characteristics, it is therefore possible to achieve a significant weight gain in the production of anti-implosion belts for cathode ray tubes.

Claims

1 - Method for manufacturing a Dual Phase steel sheet capable of being used for the manufacture of a television anti-implosion belt, characterized in that:

- A steel is produced, the chemical composition of which comprises, the contents being expressed by weight:

0.03% <C <0.3% 1% <Mn <3%

0.05% <If <2%

0.02% <Al <2%

0.02% <Cr <1%

Mo ≤ 1% S <0.02%

P <0.2%

N ≤ 0.01% and, optionally, one or more elements chosen from Ti, V, Zr, Nb, the content of each element being between 0.001 and 0.2%, the rest of the composition consisting of iron and impurities resulting from processing,

- A slab or a steel ingot of said composition is brought to a temperature between 1100 and 1300 ° C.

- Said slab or ingot is hot rolled, the temperature at the end of hot rolling being higher than the temperature Ar3 of the steel

- The sheet thus obtained is cooled to a speed VR of between 1 and 500 ° C / s

- Said sheet is wound at a temperature T _bob such that 300 <T _bob <720 ° C

- The said sheet is cold rolled - The said cold-rolled sheet is subjected to continuous annealing at a temperature T _m such that T _m > Ad - Said sheet is cooled at a speed greater than 2 ° C / s to the aluminizing temperature

- Said sheet is aluminized by soaking in an aluminum-based bath at a temperature between 650 and 720 ° C. - Said sheet is cooled to ambient temperature at a speed greater than 2 ° C / s

2 - Manufacturing process according to claim 1, characterized in that a skin-pass treatment is applied to said aluminized sheet with a reduction rate of less than 5%

3 - Steel sheet manufactured according to claim 1 or 2, characterized in that the structure of said steel consists of a ferritic matrix containing a proportion of martensite between 5 and 30%, and less than 2% of phases with carbides.

4- television anti-implosion belt characterized in that it is produced from a strip of sheet steel manufactured according to claim 3.