EP0870848B1 - Niobium containing steel and process for making flat products from this steel - Google Patents

Abstract

Extra mild niobium steel is free of titanium and comprises no more than following by weight: 0.100% carbon, 1.000% manganese, 0.100% phosphorous, 0.020% sulphur, 0.080% aluminium, 0.012% nitrogen and 0.500% silicon, plus an amount of niobium stoichiometrically less than the N content and enough boron or zirconium to fix all the N which is not fixed by the Nb. The rest is iron plus any impurities. Also claimed is casting the steel into a slab at least 1000 degrees C, then hot rolling to form a strip with a final rolling temperature above Ar3, winding into a reel between 500 and 750 degrees C, cold rolling to the desired thickness, annealing to recrystallise and finally shaping the finished article.

Description

• de 0,010% à au maximum 0,100 % de C,
• au maximum 1,000 % de Mn,
• au maximum 0,100 % de P,
• au maximum 0,020 % de S,
• au maximum 0,080 % de Al,
• au maximum 0,500 % de Si,
• N ayant une teneur supérieure à 0 et au maximum de 0.012%,
• le reste étant du fer et des impuretés résiduelles.

The present invention relates to a niobium steel, extra mild, calmed with aluminum and free from titanium, for cold-rolled and annealed flat products, having a chemical composition in% by weight comprising:

• from 0.010% to a maximum of 0.100% of C,
• maximum 1,000% of Mn,
• maximum 0.100% of P,
• maximum 0.020% of S,
• maximum 0.080% Al,
• maximum 0.500% of Si,
• N having a content greater than 0 and a maximum of 0.012%,
• the remainder being iron and residual impurities.

Niobium steels of this kind have already been known for a long time (see for example EP-0101740, DE-19547181 and EP-A-0421087).

The titanium-free steel, indicated in EP-A-0421087, is a steel with an ultra-low carbon content, namely less than 0.007% by weight, in which the Nb content is very much greater than the nitrogen content, around 20 times. Nitrogen is therefore in this steel entirely fixed by nitrobutene niobium and, if boron is used, it remains free and not nitrided. Boron is intended to protect the joints of ferritic grains in order to avoid brittleness when cold deformed. This steel allows the production to obtain a sheet equivalent or close to IF steels (interstitial free) which have very high drawing coefficients r, but also a very high Δr (high plane anisotropy).

In EP-0101740, it is proposed to manufacture flat products whose Nb content is less than or equivalent to the N content. Following hot rolling at a final temperature below Ar ₃ , rolling when cold and annealed, products are obtained having low mechanical strength properties, sometimes even lower than the usual minimum requirements.

In DE-19547181, a niobium steel is manufactured, in which the Nb content must be at least 6 times that of nitrogen. The manufacturing process here also includes hot rolling at a final temperature below Ar ₃ , cold rolling and annealing, as well as baking after application of varnish. The final products obtained have a significantly higher niobium content, for properties of mechanical resistance that are not much improved.

EP-B-0400031 finally proposes, by way of comparative example, a niobium steel without titanium, having a content comprising more than 12 times the content of N. Following hot rolling at a temperature final higher than Ar ₃ , cold rolling and annealing, a product is obtained which, according to the patent itself, is not suitable for deep drawing, whatever the degrees of reduction used during cold rolling.

Document EP-A-0 816 524 discloses a low carbon steel having a chemical composition by weight: C 0.0010-0.01%; If 0-0.2%; Mn 0.1-1.5%; P 0-0.05%; S 0-0.02%; ground. Al 0.03-0.10%; N 0-0.0040%; at least one element chosen from the group consisting of Nb 0.005-0.08% and Ti 0.01-0.07%; the remainder Fe and inevitable impurities, the contents of Nb, Ti and C having to satisfy additional conditions.

Document EP-A-0 822 266 discloses a low carbon steel having a chemical composition by weight: C 0-0.06%; If 0-0.03%; Mn 0.1-0.3%; P 0-0.02%; S 0.005-0.015%; ground. Al 0.01-0.10%; N 0-0.004%; B 0.0005-0.0015; O 0-0.0025%; the remainder Fe and inevitable impurities, the contents of O and S as well as B and N having to satisfy additional conditions.

The present invention aims to provide a niobium steel having, in terms of mechanical properties on cold-rolled and annealed strips, a favorable compromise between the strength properties, such as for example the elastic limit and the load of rupture, and the properties of ductility, such as uniform elongation, work hardening coefficient and total elongation.

To solve these problems, there is provided according to the invention as defined in claim 1 a niobium steel as described at the start, characterized in that this steel contains a niobium content greater than 0 and at most equal to four times the N content and a boron content greater than 0 and at most equal to 0.012% by weight or a zirconium content greater than 0 and at most equal to 0.080% by weight, this boron or zirconium content being sufficient to fix nitrogen not fixed by niobium.

This steel has the advantage of being able to have a low niobium content, and therefore of not altering the ductility properties of the steel, while obtaining an assured and preferably early fixing of the nitrogen by the simultaneous presence of boron or zirconium and niobium. Advantageously, the niobium content is at most equal to three times this.

According to one embodiment of the invention, the steel contains an Nb content of less than 0.040% by weight, and preferably between 0.005 and 0.030% by weight. Advantageously, it contains a boron content of between 0.0005 and 0.012% by weight, preferably between 0.0015 and 0.012% by weight, or also a zirconium content between 0.020 and 0.080% by weight.

The carbon content is equal to or greater than 0.010% by weight. The amount of Nb can thus be relatively low compared to the carbon content, which makes it possible to obtain a steel with favorable mechanical properties.

Other particular embodiments of the steel according to the invention will emerge from claims 2 to 9 given below.

• une coulée de cet acier en brames,
• un réchauffage des brames à une température supérieure ou égale à 1000°C,
• un laminage à chaud des brames pour former des bandes, avec une température finale de laminage supérieure à Ar₃,
• un bobinage des bandes à une température de bobinage comprise entre 500 et 750°C,
• un laminage à froid des bandes avec un taux de réduction prédéterminé,
• un recuit de recristallisation, et
• un passage d'écrouissage final (de peau).

The invention also relates to a method for manufacturing cold-rolled and annealed flat products, based on a niobium steel having a chemical composition as indicated above. This process defined in claim 10 comprises

• a casting of this steel in slabs,
• reheating the slabs to a temperature greater than or equal to 1000 ° C.,
• hot rolling of the slabs to form strips, with a final rolling temperature greater than Ar ₃ ,
• a winding of the strips at a winding temperature between 500 and 750 ° C,
• cold rolling of the strips with a predetermined reduction rate,
• recrystallization annealing, and
• a final work hardening passage (skin).

This process offers the advantage of a secure fixation of nitrogen in the form of boron nitride or zirconium as well as in the form of niobium carbonitride, and this at a very early stage in the process. The simultaneous presence of boron or zirconium and niobium also promotes a reduced size of the austenitic grain during hot rolling. At the reheating temperature used, the niobium present is advantageously redissolved.

According to one embodiment of the invention, the final temperature of hot rolling is preferably equal to or less than 900 ° C. It is precisely at this temperature, that is to say between the transformation temperature γ → α (Ar ₃ ) and 900 ° C., that the boron nitrides and the carbon nitrides of Nb precipitate in the process according to the invention. , which fixes nitrogen. The maximum temperature mentioned above is not, however, critical and should only be considered as a preferred temperature.

According to a preferred embodiment of the invention, during cold rolling, the reduction rate is of the order of 40 to 85%, preferably 55-80%. This reduction rate is calculated according to the formula: $$\begin{gathered} \text{Reduction rate =} \\ \frac{\mathit{\text{Thickness of hot rolling - thickness of rolling}}}{\mathit{\text{Thickness of hot rolling end}}} \end{gathered}$$

Other particular embodiments of the method according to the invention will emerge from claims 11 to 17 given below.

Other details and particularities of the invention will emerge from the description given below without implied limitation.

The niobium steel according to the invention is usually a conventional production or electrical production steel which is cast continuously. This steel must be extra soft, that is to say have an extremely low carbon content, less than 0.100% by weight, being able to reach minimum contents up to 0.020% or more. Advantageously, however, the carbon content will not exceed a value of less than 0.010% by weight.

This steel must also be quenched with aluminum with a content of less than 0.080% by weight.

It will of course include niobium and will be free from any addition of titanium.

• 0,010 < C < 0,100
• 0,100 < Mn < 1,000
• P < 0,100
• S < 0,020
• Al < 0,080
• N < 0,012
• Si < 0,500

avec des additions volontaires de niobium combinées à une addition de bore ou de zirconium, de par exemple :

• Nb ≤ 0,040 % en poids, et de
• 0,0015 ≤ B ≤ 0,0120 % en poids ou de
• 0,020 ≤ Zr ≤ 0,080 % en poids,

le reste étant du fer et des impuretés résiduelles de Cu, Ni, Cr, Sn par exemple.The chemical composition of this steel could therefore be as follows, in% by weight:

• 0.010 <C <0.100
• 0.100 <Mn <1.000
• P <0.100
• S <0.020
• Al <0.080
• N <0.012
• If <0.500

with voluntary additions of niobium combined with an addition of boron or zirconium, for example:

• Nb ≤ 0.040% by weight, and
• 0.0015 ≤ B ≤ 0.0120% by weight or
• 0.020 ≤ Zr ≤ 0.080% by weight,

the remainder being iron and residual impurities of Cu, Ni, Cr, Sn for example.

In fact the adjusted values of Nb, B and Zr are calculated mainly as a function of the nitrogen present in the steel being treated.

The amount of Nb added is therefore in reality much lower stoichiometrically than nitrogen. Nitrogen not fixed by niobium is fixed by B or Zr, which allows an addition of Nb lower than what is usually necessary, to obtain sufficient mechanical resistance properties from a niobium steel , without titanium. This minimal addition of Nb makes it possible to simultaneously maintain good ductility properties. It also offers appreciable economic advantages given the significant cost of niobium.

The steel described above is poured into slabs, which are reheated in a conventional oven, for example an oven with movable beams or a pushing oven, so that they reach a core temperature greater than or equal to 1000 ° C. sufficient to re-dissolve the precipitated niobium.

• un dégrossissage pour réaliser une ébauche de 35 mm ± 10 mm d'épaisseur, à une température moyenne de 1050°C, et
• une finition pour réaliser une bande à chaud d'une épaisseur de 1 à 10 mm, en respectant une température minimale de laminage à chaud qui soit supérieure à la température de transformation de la phase γ à la phase α (Ar₃).

Hot rolling is then carried out on a conventional rolling train, generally in two stages:

• roughing to produce a blank 35 mm ± 10 mm thick, at an average temperature of 1050 ° C, and
• a finish to produce a hot strip with a thickness of 1 to 10 mm, while respecting a minimum hot rolling temperature which is higher than the transformation temperature from the γ phase to the α phase (Ar ₃ ).

It is between 900 ° C. and this transformation temperature that the boron nitrides and the niobium carbonitrides precipitate, with consequently a very early nitrogen fixation.

The strip is then cooled in a controlled manner and finally wound up at a temperature of the order of 625 ° C ± 125 ° C.

After continuous pickling in conventional lines (HCl or H ₂ SO ₄ ), the strip is cold re-rolled, with a thickness reduction rate of between 40 and 85%.

The cold rolled strip is then subjected to recrystallization annealing to give it the necessary mechanical properties. This annealing can be carried out in the form of a static annealing, for example in a tight or expanded coil, at a temperature of the order of 620-680 ° C, or in the form of a continuous annealing at a temperature of 680- 850 ° C. This latter annealing may or may not be combined with any covering by dipping or other methods.

A final rolling step is also carried out, in the form of a final work hardening, in order to eliminate the phenomena of "Lüders bands" and to ensure a good surface roughness as well as a flatness of the product.

The invention will now be explained in more detail, using examples given without limitation.

Comparison example 1

Extremely low carbon steel, without niobium, but with the addition of boron. Chemical composition (10 ^-3 %). VS mn Yes P S al N ₂ B Nb 35 250 6 11 8 44 4.2 3.6 0 Hot rolled strip with a thickness of 3 mm. Final hot rolling temperature: 870 ° C Winding temperature 620 ° C HCl pickling Reduction rate 66% Cold rolled strip to a thickness of 1 mm.
Continuous recrystallization annealing at 700 ° C for 40 sec. followed by quenching in hot water at 50 ° C / sec. up to 400 ° C, application of aging at 400 ° C for 120 sec. and cooling by nozzles to a temperature of 120 ° C., formic pickling, rinsing, and drying, then application of a final work hardening rate of 0.8%. Mechanical properties Elasticity limit Rp 0.2 = 235 MPa Breaking load Rm = 340 Mpa Elongation at break A% = 38% Work hardening coefficient n = 0.190 / 0.200 Anisotropy coefficient r through = 1.35 Coefficient of plane anisotropy Δr = 0,350 Coefficient of normal anisotropy r avg. 1.1

Comparison example 2

Same steel as that used in comparison example 1.

We apply the same process unlike the recrystallization annealing which this time is static at 640 ° C cold point (with maximum temperature of 700 ° C) for 2 hours. Then the treatment is completed in the manner described above. Mechanical properties Rp 0.2 = 175 MPa Rm = 310 MPa A% = 40% n = 0,230 r through = 1.25 Δr = 0,050 r avg. 1.01

Comparison example 3

Niobium steel with extremely low carbon content, without boron. Chemical composition (10 ^-3 %) VS mn Yes P S al N ₂ B Nb 50 350 8 12 6 40 6.0 0 50

The process applied is the same as that of comparison example 1, with these few differences: Winding temperature 600 ° C Reduction rate 50%

Static recrystallization annealing at 660 ° C cold point (with maximum temperature of 680 ° C) for 2 hours, or continuous annealing at around 790 ° C for 1 minute and aging at 400 ° C for 180 seconds, then application of a rate 1.4% final work hardening. Mechanical properties (in length) Rp 0.2 = 350 MPa Rm = 440 Mpa A% = 26% n = 0.155 r through = 1.2 r long = 0.7 Δr = -0.250 r avg. 1.1

Example 4

Niobium steel according to the invention, with addition of boron. Chemical composition (10 ^-3 %) VS mn Yes P S al N ₂ B Nb 55 300 7 14 3 50 5.6 4.5 7

The process applied is the same as that described in comparison example 1, with these few differences: Winding temperature 500 ° C Reduction rate 80%

Static recrystallization annealing at 660 ° C cold point (with a maximum temperature of 710 ° C) for 2 hours, then application of a final work hardening rate of 1.5%. Mechanical properties Rp 0.2 = 290 MPa Rm = 390 Mpa A% = 36.5% n = 0.195 r through = 1.1 Δr = -0.005 r avg. 1.0

Example 5

Niobium steel according to the invention, with addition of boron. Chemical composition (10 ^-3 %) VS mn Yes P S al N ₂ B Nb 45 270 19 12 6 43 6.0 4.0 12

The process applied is the same as that described in comparison example 1, with these few differences: Final hot rolling temperature 875 ° C Winding temperature 640 ° C Reduction rate 55%

Continuous galvanizing annealing at 850 ° C (zinc pot temperature: 480 ° C) with aging at 480 ° C, then application of a final work hardening rate of 1.2%. Mechanical properties Rp 0.2 = 300 MPa Rm = 400 Mpa A% = 33% n = 0,175 r through = 1.1 Δr = 0.005 r avg. 1.0

Comparison example 6

Extremely low carbon steel, without niobium, but with the addition of zirconium. Chemical composition (10 ^-3 %) VS mn Yes P S al N ₂ B Nb Zr 36 216 50 7 6 55 3.2 0 0 48

The process applied is the same as that of comparison example 1, with these few differences: Final hot rolling temperature: 885 ° C. Winding temperature: 650 ° C. Static recrystallization annealing (base annealing) at 610 ° C. Final work hardening rate: 0.9%. Mechanical properties Rp 0.2 = 224 MPa Rm = 351 Mpa A% = 37.6% n = 0.206 Δr = 0.308 r avg. 0.96

Example 7

Niobium steel with an extremely low carbon content, with the addition of zirconium. Chemical composition (10 ^-3 %) VS mn Yes P S al N ₂ B Nb Zr 35 200 5 9 4 47 4.9 0 10 30

The process applied is the same as that of comparison example 1, with these few differences: Winding temperature 640 ° C. Reduction rate 58.3%. Static recrystallization annealing (base annealing) at 700 ° C. Final hardening rate 0.8% Mechanical properties Rp 0.2 = 255 MPa Rm = 361 Mpa A% = 36.4% n = 0,190 Δr = 0,040 r avg. 1.01

As can be seen from these examples, extra mild steels with boron or zirconium, without niobium, if they are well ductile, have low to poor mechanical strength values, relatively close to the minimum values required by users (R _p O, 2 greater than or equal to 220 MPa and Rm greater than or equal to 320 MPa).

The extra mild niobium steel, without boron and without zirconium, of Comparative Example 3 has good mechanical strength values, but its ductility properties are perfectly unsatisfactory, whereas it is generally required to be elongated. rupture greater than or equal to 32% and a work hardening coefficient greater than or equal to 0.170.

The niobium steels according to the invention offer both mechanical strength properties much greater than the usual lower limits and good ductility properties, thus providing a compromise which is entirely favorable for subsequent treatments.

In a particularly surprising manner, the niobium steels according to the invention exhibit, on cold-rolled and annealed strips, mechanical properties in the plane of the strip which are substantially independent of the direction relative to the direction of rolling as well as 'a rational contraction in width substantially identical to a rational contraction in thickness. They therefore meet all the conditions for undergoing treatments of the difficult stamping type and the like.

Claims (17)

1. Extremely low-carbon, aluminium-killed steel containing niobium and free of titanium, for cold-rolled, annealed flat products, having a chemical composition based on % by weight as follows:

   from 0.010 % up to a maximum 0.100 % of C,

   a maximum of 1.000 % of Mn,

   a maximum of 0.100 % of P,

   a maximum of 0.020 % of S,

   a maximum of 0.080 % of Al,

   a maximum of 0.500 % of Si,

   N being in a proportion in excess of 0 and at most 0.012 %,

   the rest being iron and residual impurities,

   the steel containing a proportion of niobium higher than 0 and at most equal to four times the proportion of N and containing a proportion of boron higher than 0 and at most equal to 0.012 % by weight or containing a proportion of zirconium higher than 0 and at most equal to 0.080 % by weight, this proportion of boron or zirconium being sufficient to fix the nitrogen that is not fixed by the niobium.
2. Steel containing niobium as claimed in claim 1, characterised in that the niobium content is at most equal to three times the proportion of N.
3. Steel containing niobium as claimed in one of claims 1 and 2, characterised in that it contains a proportion of Nb which is less than 0.040% by weight and is preferably between 0.005 and 0.030 % by weight.
4. Steel containing niobium as claimed in any one of claims 1 to 3, characterised in that it contains a proportion of boron ranging between 0.0005 and 0.012 % by weight, preferably between 0.0015 and 0.012 % by weight.
5. Steel containing niobium as claimed in any one of claims 1 to 3, characterised in that it contains a proportion of zirconium ranging between 0.020 and 0.080 % by weight.
6. Steel containing niobium as claimed in any one of claims 1 to 5, characterised in that when cold-rolled and annealed into strips, it has an elastic limit, the minimum values of which are in excess of 220 MPa, and a breaking load, the minimum values of which are in excess of 320 MPa.
7. Steel containing niobium as claimed in claim 6, characterised in that said elastic limit is in excess of 250 MPa, preferably in excess of 280 MPa, and the breaking load is in excess of 350 MPa, preferably in excess of 380 MPa.
8. Steel containing niobium as claimed in any one of claims 1 to 7, characterised in that when cold-rolled and annealed into strips, it has an elongation at break higher than or equal to 32 % and a work-hardening coefficient higher than or equal to 0.17.
9. Steel containing niobium as claimed in one of claims 1 to 8, characterised in that it has a coefficient of planar anisotropy Δr of between -0.200 and +0.200, preferably between -0.100 and +0.100 and an average coefficient of normal anisotropy r of between 0.9 and 1.1.
10. Process of producing flat cold-rolled annealed products based on a steel containing niobium with a chemical composition as claimed in any one of claims 1 to 9, consisting in

    casting this steel into slabs,

    re-heating the slabs to a temperature in excess of or equal to 1000°C,

    hot-rolling the slabs into strips at a final rolling temperature higher than Ar₃,

    winding the strips at a winding temperature ranging between 500 and 750°C,

    cold-rolling the strips at a predetermined rate of reduction,

    recrystallisation annealing and

    a final work hardening pass.
11. Production process as claimed in claim 10, characterised in that the slabs are preferably re-heated at a temperature in the order of 1250°C.
12. Production process as claimed in one of claims 10 and 11, characterised in that the final hot-rolling temperature is equal to or less than 900°C.
13. Production process as claimed in one of claims 10 to 12, characterised in that the rate of reduction is in the order of 40 to 85 %, preferably 55 - 80 %.
14. Production process as claimed in one of claims 10 to 13, characterised in that recrystallisation annealing is operated as a static annealing process.
15. Production process as claimed in claim 14, characterised in that the static annealing process is carried out on tightly wound or expanded reels at a cold point temperature of 620 to 680 °C.
16. Production process as claimed in one of claims 10 to 13, characterised in that recrystallisation annealing is operated in the form of continuous annealing, with or without a coating.
17. Production process as claimed in claim 16, characterised in that the continuous recrystallisation annealing is operated at a temperature of 680 to 850°C.