MXPA99008354A - Method for producing a highly resistant, very ductile steel strip - Google Patents

Abstract

The invention relates to a method for producing a highly resistant (at least 900MPa), very ductile steel strip. The steel, containing (in mass per cent);0.10 to 0.20%C;0. 30 to 0.60%Si;1.50 to 2.00%Mn;max. 0.08%P;0.30 to 0.80%Cr;up to 0.40%Mo;up to 0.20%Ti and/or Zr;up to 0.08%Nb;the remainder being Fe and unavoidable impurities, is melted, cast in slabs and then rolled out into a hot rolled strip. The roll end temperature is above 800oC, the cooling speed on the delivery roller table is at least 30oC/s and the reel temperature is 300 to 600oC.

Description

METHOD TO PRODUCE A STEEL PLATE IN BANDS WITH HIGH RESISTANCE AND GOOD CONFORMING CAPABILITIES The invention relates to a method for producing a sheet steel in bands with high strength of at least 900 MPa and good forming capabilities. The demand for reducing the fuel consumption of vehicles requires the application of lightweight concepts. Light weight constructions can be achieved by reducing the thickness of the steel sheet. To compensate for any of these losses caused in this manner in the strength of the component it is necessary to increase the strength of the material. Any increase in resistance usually causes a reduction in deficiency. The thin steel plates used in the construction of vehicles must be put into the final form required for design or functional purposes by shaping. If the increase in strength and the resulting decrease in conformation capacities become too high, failures will occur during shaping by local constructions and the REF: 031322 tearing. For this reason, an increase in resistance is limited. The development of steels is always aimed at improving the relationship between deformability and resistance. Substantial success has already been achieved in the resistance range below 500 MPa relative to a reduction in the thickness of the thin sheet when using alloyed or phosphorus or micro-alloyed steels. Even better results were achieved with hardened steels with an oven.

In the range of resistance between 500 and 800 MPa, the developments of the double phase steels and TRIP steels (plasticity induced by transformation, for its acronym in English) produced relatively good values of conformation capacity. The relevant characteristic values for the conformation can be gained with high representativeness for the practical operation of the stress test. In particular, the elongation at break and the value n (quantity per resistance capacity) represent the important dimensional figures. The value n is characteristic for deformation under stress of stretching. This is the predominant deformation mechanism in most parts of steel plate of a vehicle. The value n corresponds relatively well to the ratio of the remaining deformation load to the tensile strength, which is also a representative value in practical operation for the resistance capacity of a material. In order to exploit the advantage of increasing the strength to reduce the thickness of the thin sheet to the highest possible degree, the highest possible values are pursued concerning the elongation at break (A) and the strength value (value n). Steels with very high strengths above 800 MPa can be used very efficiently to reduce the weight of components relevant to shocks such as door impact sleeper, bumper sleepers. However, for this reason it is necessary to reduce the steel thickness from about 2.0 mm to a thickness below 2.0 mm such as 1.5 mm for example. Such super-high strength steel products could only be provided in the past as cold-rolled sheet. Particularly in the area of the higher strengths of over 800 MPa, the deformation properties are insufficient for the shaping of the thin sheets in the useful portions when conventional material concepts are used to produce cold rolled or hot rolled strips. The high resistance is achieved when mounting martensitic structures. However, the apparent remaining deformation loads are also very high in such steels. The resultant values for the ratio of the remaining deformation load to the tensile strength and strength are respectively low. In addition to the low manageability, this leads to high bastard values, so that die-cut parts can only be produced with difficulty or do not really conform. It is the object of the present invention to develop steels in bands which are provided with a high strength property in conjunction with good forming capabilities and high component strength. To achieve this objective, a method according to the invention is proposed in which a steel consisting of (in mass percentage). 0. 10 to 0.20% C 0.30 to 0.60% Si 1.50 to 2.00% of Mn max. 0.08% of P 0.30 to 0.80% of Cr up to 0.40% of Mo up to 0.20% of Ti and / or Zr up to 0.08% of Nb the rest of Fe and unavoidable impurities, It is melted, molded into rectangular slabs and then rolled into a hot strip, with the final rolling temperature being above 800 ° C, the cooling speed at the output roll table being at least 30 ° C / s and the winding temperature which is 300 to 600 ° C. The assembly with a certain purpose of very fine microstructures consisting of soft and hard phases next to each other in combination with an ultrafine precipitation distribution opens up the possibility of previously unknown properties, attractive for processing and use. A hardness of the structure by multiple phases in conjunction with the hardness by a fine grain and fine particles leads to a process of multiple strengthening. The economic, particular relevance of the method according to the invention is the production capacity as a hot strip in thickness below 2.0 mm, for example 1.5 mm. In this way, the production process does not compulsorily require the complex production process of cold rolled strip production with the additional steps of cold rolling and subsequent annealing. The present concept of material also includes the possibility of refining the surface applied by the factory. In this way, for example, an electrolytically deposited zinc layer can be applied. The considerable improvement of the protection against corrosion by a zinc layer can be assumed as a known fact. It is also known that super high strength steels tend towards embrittlement due to the absorption of hydrogen during the electrolytic galvanization process. It could be proved that the strip steel sheet according to the invention is free of these dreaded galvanization problems. The relevance of the alloying elements and the production parameters are described below.

Coal is required for the hardening of the structure and the formation of ultra-fine rainfall. For reasons of weldability the content should be restricted to 0.1 to 0.2%.

Silicon increases the hardness of the solid solution, for which at least 0.3% is required. For reasons of weldability and to avoid unfavorable formation of floor scales the content should be restricted to 0.6%.

Manganese in a content of at least 1.5% retards the transformation and leads to the formation of hard transformation products. In order to avoid strong microegregation, intolerable content must be recorded at 2.0%.

Phosphorus can be used for the additional increase in the hardness of the solid solution, but it must not exceed a content of 0.08% for reasons of weldability.

Chromium promotes the formation of a high end structure in bainite by at least 0.3%. In order to avoid the delay of the transformation too strong, the content must be restricted to a maximum of 0.80%.

Titanium or zirconium can be used for the formation of ultra-fine precipitations with a hardening effect. However, the effect decreases considerably in contents over 0.2%. That's because the maximum value has been set at 0.2%.

Niobium can also be used for hardening precipitation. It must be added preferably by the alloy with at least 0.04%. The content is restricted to a maximum of 0.08% for reasons of effectiveness.

Boro improves the hardenability in contents in the range of 0.0005 to 0.005%. According to current knowledge it is used for the martensitic transformation of steels. It has been surprisingly observed that boron also causes in the present case a significant increase in the strength in the basic structure, bainitica with only a low reduction of the capacities of conformation. The final rolling temperature should be in the range of homogeneous austenite and thus should not be below 800 ° C in order to ensure a resistance to sufficiently low dimensional change and keep other precipitations induced by deformation low. The cooling conditions should be selected such that a transformation in the pearlite is avoided and the transformation occurs to the highest possible degree in the bainitic stage. The martensite portions can contribute to further strengthening. further, strengthening must be achieved by precipitation of ultrafine particles. For this purpose, a cooling of the final rolling temperature is required with a cooling rate of at least 30 ° C / s. This cooling process must be completed at a temperature below 600 ° C when winding the belt in a winder and then letting it cool in the coil. The invention is now described by reference to the following examples. Table 1 shows the chemical compositions of steel plates in strips 1 and 2 produced according to the invention and steel 3, a martensitic steel of reference. Table 2 shows the characteristic mechanical properties of the steels in strips 1 and 2 produced according to the invention and of the reference steel 3 which is aged or artificially stabilized by a heat treatment subsequent to the values given in table 2. A comparison of the properties clearly shows the great advantages of the strip steel sheet produced according to the invention. This shows a higher rupture elongation and a better residual strain load ratio to the tensile strength as a ratio for strengthening. Table 3 shows the influence of a low winding temperature and a subsequent heat treatment on the properties of a strip steel sheet produced according to the invention having the composition of steel 1 in table 1. As a result of the With a low winding temperature of preferably 330 ° C, it is possible to achieve considerable increases in strength properties (see example 4 in table 3). A further object of the invention is the achievement of an advantageous effect of a subsequent heat treatment. It has been surprisingly observed that by thermal treatment of the strip steel sheet produced according to the invention at a temperature range between 500 ° C and 850 ° C, the properties of forming capacity can be further improved. Examples 4, 5 and 6 in table 3 show the effect of such heat treatment on steel 1 with the composition according to table 1. A state of the material is achieved which offers advantages to the components which require the general high resistances and in particular apparent high remaining deformation loads in the good conformation capacities. This property chart can be used for the production of cold-rolled sections with a high energy absorption capacity (example 5a). By selecting higher annealing temperatures it is possible to achieve high resistances with exceptionally low ratios between the remaining deformation load to tensile strength and high strength, similar in favorable extensibility values (examples 5b, 6a and 6c). Many hot-rolled products have the disadvantage that these lose their advantageous properties once they are subsequently cold-rolled and annealed with recrystallization. It was found that the strip steel sheet according to the invention, however, has advantageous properties also after cold rolling, subsequent and annealing. In this way, example 7 in table 3 shows that the strip steel sheet 1 produced according to the invention also achieves high strengths in even more improved ratios between the remaining deformation load to the tensile strength after a cold rolled with a degree of deformation of 50% as compared to the steels in bands 1 and 2 that were only hot rolled.

Table 1 (in percentage by mass [mass-%]) reference artensitic steel Table 2 reference steel Re resistance limit Rm ultimate tensile strength Ag uniform elongation A5 elongation at break A8o elongation at break WET final rolling temperature HT winding temperature Table 3 cold rolling with 50% It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, the content of the following claims is claimed as property.

Claims (15)

1. A method for producing a sheet steel in bands with high strength of at least 900 MPa and good forming capabilities, characterized in that it comprises (in mass percentage) 0. 10 to 0.20% C 0.30 to 0.60% Si 1.50 to 2.00% of Mn max. 0.08% P 0.30 to 0.80% Cr up to 0.40% Mo up to 0.20% Ti and / or Zr up to 0.08% Nb the rest of Fe and unavoidable impurities, which is melted, molded into rectangular slabs and then rolled into hot strips, with the final rolling temperature being above 800 ° C, the cooling speed at the exit laminating table being at least 30 ° C / s and the winding temperature which is 300 to 600 ° C.

2. A method according to claim 1, characterized in that the hot band is rolled at a temperature of not more than 550 ° C.

3. A method according to claim 1, characterized in that the hot band is rolled at a temperature of not more than 350 ° C.

4. A method according to claims 1 to 3, characterized in that the hot band is not wound below a temperature of 330 ° C.

5. A method according to one or more of claims 1 to 4, characterized in that the hot strip is laminated to a final thickness of not more than 2.0 mm.

6. A method according to one or more of claims 1 to 5, characterized in that the hot strip is hardened by rolling.

7. A method according to one or more of claims 1 to 6, characterized in that the strip is treated with acid and coated metallicly.

8. A method according to claim 7, characterized in that the metal coating is applied electrolytically.

9. A method according to claim 1, characterized in that the metal coating is applied by hot bath electroplating.

10. A method according to one or more of claims 1 to 6, characterized in that the hot strip is annealed in the range of 500 to 850 ° C.

11. A method according to one or more of claims 1 to 6, characterized in that after hot rolling a cold rolling of at least 30% and continuous annealing is carried out at temperatures between 700 and 900 ° C.

12. A method according to one or more of claims 1 to 11, characterized in that the steel is added by the alloy of not more than 0.15% Mo.

13. A method according to one or more of claims 1 to 12, characterized in that the steel is added by alloying at least 0.04% Ti and / or Zr.

14. A method according to one or more of claims 1 to 13, characterized in that the steel is added by the alloy of 0.0005% to 0.005% of B.

15. A method according to one or more of claims 1 to 14, characterized in that the steel is added by alloying at least 0.04% Nb.