What are some properties of isotropic steel



description


The invention relates to a method for producing a steel strip according to the preamble of claim 1, such steels and their composition belong to the prior art.


Cold-rolled steel strip is widely used for the production of cold-formed products. Depending on the type of forming process, different properties (parameters) are required.


[0003] The increasing requirements with regard to the application and usage properties increasingly require even better mechanical properties, in particular forming properties. Good formability is characterized by the highest possible r-values, which characterize deep-drawability, high n-values, which characterize the stretchability, and high elongation values, which characterize the plane strain properties.


It has proven to be advantageous if the deformation properties in the different directions, in particular in the longitudinal, transverse and diagonal directions, are as equal as possible, d. H. are largely isotropic. If this condition applies to the r-value, this also means that the so-called .DELTA.r-value is very small and after the pressing of rotationally symmetrical parts, extensive freedom from lobes is achieved. The advantages of isotropic properties are essentially expressed in the uniformity of the material flow and a reduction in sheet metal scrap.


The likewise increasing efforts in lightweight construction require the use of thinner metal sheets in order to achieve weight reductions. To compensate for the loss of strength caused by the reduction in sheet thickness, the strength of the sheet must be increased


Because of the natural decrease in formability as a result of an increase in strength, a predominant goal in material development is to keep the loss of formability as low as possible when realizing higher strengths.


According to the state of the art, numerous high-grade steels with suitability for cold formability are known. The status achieved is mainly shown in the steel-iron material sheets 093 and 094 for micro-alloyed and P-alloyed with and without bake-hardening (BH). BH properties can be determined particularly well by one of the new continuous annealing processes, e.g. T. achieve coupled with a hot dip refinement. The belt cleanliness and the uniformity of the properties can be adjusted very well in this continuous belt process.


There have also been successful efforts to achieve isotropic properties for a long time. An isotropic material does not show any lobes when rotationally symmetrical parts are pressed. An example of this is the "B-Factor" display from Brockhaus, "Der Spiegel", No. 19/1966, page 125. However, this example does not expressly include the production of high-strength steels and requires either very high degrees of cold rolling or even normalizing annealing for setting the freedom from corners.


Recently, the example of a higher-strength sheet steel with a Ti alloy to achieve freedom from tips from DE-PS 38 03 064 has been known. However, this development is limited to the hood annealing process and must therefore forego the advantages mentioned when using continuous annealing and surface refinement using a hot-dip process. Furthermore, there remains the possibility of increasing the strength properties, e.g. B. the yield point, to around 220 to 280 N / mm2 limited. Another disadvantage is the exclusively low r-values ​​around 1.0, which affects the production of deep-drawn parts. In addition, with this concept, the higher strength is essentially achieved through the strengthening mechanism of grain refinement. Fine grain requires a comparatively high level of effort when dressing. With only normal skin passages, there would be a risk of flow figure formation and thus failure of the outer skin parts. In the present case, however, the necessarily high skin-pass degrees reduce the deformation properties compared to normal skin-pass passages.


The restriction to the almost exclusive effect of grain refinement via titanium also make a precise coordination of hot, cold rolling and annealing conditions on the given chem. Composition required, which means that there are high demands on accuracy under the manufacturing conditions mentioned. Another disadvantage is the limitation of the final rolling temperature to values ​​above A.r3 can be seen, whereby the rolling of strips with a small final thickness is made more difficult due to the associated higher temperature loss.


This is where the present invention begins with the features specified in claim 1. The method according to the invention is for setting yield points in the range between 200 and 420 N / mm2 suitable. The mechanical properties are isotropic. In addition, the process in its individual variants allows the setting of higher r-values ​​and offers the possibility of achieving bake-hardening. Furthermore, the advantages of continuous annealing or hot-dip refinement can be included. The advantages according to the invention can be achieved with Ti, Nb, V or Zr.


According to the current state of knowledge from DE-PS 38 03 064, the production is carried out while maintaining a rolling end temperature above Ar3 required. Accordingly, it was not previously known under which conditions the advantages of a lowered rolling end temperature could be used.


According to the present invention, a low rolling end temperature is combined with a high coiling temperature. Surprisingly, this resulted in properties and features that were previously unknown for steel with isotropic behavior:
  • Reduced build-up of scale during hot rolling
  • Reduced skin pass requirements on sheet metal


The inventive method allows the production of isotropic steel strips not only by the hood but also by the continuous process and thus allows the achievement of Bakehardening and the refinement in the hot-dip process.


When using vacuum decarburization in the steelworks and continuous annealing of the cold strip, a high r-value can surprisingly also be achieved in addition to bakehardening.


A few examples are intended to illustrate the result of the method according to the invention.


In Table 1, the chemical composition of the steels is listed.
The steels were alloyed with at least the amount of the elements Ti and / or Nb or V required for stoichiometric nitrogen fixation. In addition, steels 4 and 9 were alloyed with phosphorus to increase strength.


Table 2 shows the manufacturing conditions of the steels.
It has the combination of properties according to the invention, low end rolling temperature under A.r3 and high reel temperature (> 650 ° C).


In Table 3, the mechanical quality values, the skin pass and the grain size of the steels of 70% cold-rolled strip are listed.
Due to the hot strip production according to the invention, it was possible to set the skin-pass degree on the cold strips about 1/3 lower. Furthermore, with the vacuum decarburized steels 1 - 4 high rm Values ​​(1.4 - 1.65), at low Δr Values ​​(<± 0.1) achieved.


In Fig. 1, the tip height is plotted graphically above the degree of cold rolling for the continuously annealed steels and in Fig. 2 that for the hood-annealed steels.
The recordings show that both the continuous-annealed and the hood-annealed steels at cold rolling degrees between 50 and 85% were produced with low-pitched cold strips. With the degree of cold rolling of around 70% customary for cold strip production, the tip height of all examples was free of tips.


In addition, it can be seen from Fig. 2 that a different from the invention, low coiling temperature (steel 7.2.1, 600 ° C) causes a high lobing. This underlines the requirement of the combination according to the invention of a high coiling temperature with a low end rolling temperature.








Expectations

1. Process for the production of a cold-rolled, very high strip steel with good formability and isotropic properties from steel with the following composition in percent by weight:

max .: 0.08% C0, 010-0.10% P

max .: 1.0% Si max .: 0.02% S.

max .: 1.8% Mn max .: 0.08% Al
max .: 0.008% N

and one or more of the elements titanium, vanadium, niobium, zirconium, the remainder being iron, by hot rolling, cold rolling, recrystallizing annealing and subsequent tempering, where either the content of Ti or V is at least three times the content of N or the content of Nb or Zr corresponds to at least six times the content of N, and the steel is cast into slabs, heated to a temperature of at least 1000 ° C. before hot rolling, rolled into hot strip, the final rolling temperature being below Ar3and the coiling temperature is above 650 ° C, and the steel after hot rolling is cold rolled with a degree of rolling between 55 and 85%, then annealed for recrystallization and finally passaged, and the yield point of the steel after additional simulated paint baking treatment is at least 200 N / mm2This treatment is carried out for at least 20 minutes at at least 170 ° C.
 

2. The method according to claim 1, characterized in that the steel is annealed to recrystallize in a hood furnace after cold rolling.
 

3. The method according to claim 1, characterized in that the steel is annealed to recrystallize in a continuous furnace after cold rolling.
 

4. The method according to claims 1 and 3, characterized in that the steel is then hot-dip refined after cold rolling and annealing.
 

5. The method according to one or more of claims 1 to 4, characterized in that the phosphorus content is 0.035 to 0.10%.
 

6. The method according to one or more of claims 1 to 5, characterized in that the final rolling temperature is less than or equal to 850 during hot rolling0C is.