DE3007560A1 - METHOD FOR PRODUCING HOT-ROLLED SHEET WITH LOW STRETCH STRESS, HIGH TENSILE STRENGTH AND EXCELLENT SHAPING CAPACITY - Google Patents

Description

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The invention relates to a method for producing a hot rolled Steel sheet with low yield stress and high tensile strength as well as an excellent Deformability. The invention relates in particular to a method for producing a hot-rolled steel sheet with low yield stress, high Tensile strength and excellent ductility, which is a two-phase structure which consists essentially of ferrite and martensite and has a yield strength ratio of a maximum of 65 *.

More recently, there has been a demand for increased strengths increased by steel sheets. Until now, there has only been brisk demand for a high tensile strength steel sheet for use in the vehicle body, for example of a motor vehicle to increase the safety of the vehicle occupant and to reduce the weight of the vehicle.

The reason for the requirement for high tensile strength is that the aim is to compensate for the weight increases resulting from the arrangement an exhaust gas purification device and the like result by changing the thickness dimensions of the steel used.

For the steel sheet used for the above purpose, it is necessary to high tensile strength together with excellent ductility to have.

In general, the greater the tensile strength a steel sheet, the ductility of the same is lowered, so that it is for the purpose Manufacture of a steel sheet with both high tensile strength and excellent ductility is also required, an appropriate selection is required of the alloying elements and to strictly monitor the manufacturing process. As methods for improving the tensile properties of a steel, there are methods have been used, in which the proportion of elements hardening in solid solution, how carbon, silicon and manganese are used as the main components, and in addition special elements such as niobium, titanium and vanadium are used, through which a precipitation hardening effect as a result of the resulting carbides and nitride is induced. In addition, the above-mentioned elements to grain refinement effects. In general, these are improved in their tensile strength Steels, however, are characterized by the disadvantage that their yield stress is high and their deformability is low.

In recent years it has been found and attention Excited to the public that the balance between the tensile strength and the ductility has been increased to a considerable extent by the fact that the structure of the steel material two-phase made of ferrite and at low temperatures transforming products. One with a two-phase structure provided steel has excellent tensile strengths, and not just elongation is particularly favorable, but no elongation appears, which is the result of a Variety of free dislocations is occurring during the low temperature transition have been generated. Furthermore, the yield stress is considerable with these materials low, so that the yield strength ratio, that is, the ratio of the yield strength to tensile strength can be limited to a maximum of 70 *.

If the yield stress is as low as mentioned above, so result such advantages as that the steel sheet has a satisfactory dimensional stability (shape fixability) after compression molding. Also owns such a Material during deformation a certain softness and can be the tensile strength increase of the steel sheet by aging after the compression molding.

In order to produce such two-phase steel sheets, the so-called continuous annealing has been used in which a cold-rolled steel sheet or a hot rolled steel sheet heated above its transformation point Al and then rapid cooling to the Ms temperature or a Lower temperature took place, so that a double structure of ferrite and martensite could be obtained. However, this method requires a continuous annealing device, wherein the coiling of the annealed sheets also necessarily needs to be below the Ms temperature has to take place, which leads to the disadvantage that a particularly large amount of mechanical energy is required.

In view of the above-mentioned disadvantages of the prior art Technique, the invention aims to provide a method for producing a hot rolled Steel sheet with low yield stress, high tensile strength and excellent Deformability, the tensile strength being strong after deformation through aging can be increased to create.

The above-mentioned object of the invention is achieved by a method for producing a hot-rolled steel sheet with low yield strength, high Tensile strength, excellent ductility and one substantial the end Ferrite and martensite are two-phase structures with a yield strength ratio of a maximum of 65 *, in which a steel plate with 0.02 to 0.15 wt .- * carbon, 0.9 to 2.0% by weight manganese, 0.2 to 3.0% by weight silicon, maximum 0.10% by weight aluminum, a maximum of 0.02% by weight of sulfur, 0.2 to 2.0% by weight of chromium, the remainder being essentially iron and production-related impurities of a final rolling with a degree of rolling from 5 to 40% within a temperature range in which ferrite and austenite can exist side by side, is final rolled, this so a final rolling subjected to hot-rolled steel sheet with a cooling rate of at least Cooled 100 per second and at a temperature of 250 to less than 54,000 is reeled, whereupon the reeled sheet material is cooled.

The aim of the invention can also be achieved by a method for producing a hot-rolled steel sheet with low yield stress, high Tensile strength, excellent ductility and essentially one Ferrite and martensite existing two-phase structure with a yield strength ratio of 65% or less, in which a steel plate with 0.02 to 0.15 wt .-% carbon, 0.9 to 2.0 wt. * Manganese, 0.2 to 3.0 wt. O / o silicon, 0.1 or less wt.% Aluminum, 0.02 or less weight percent sulfur, 0.2 to 2.0 weight percent chromium, and 1.0 or less wt% molybdenum, 1.0 or less wt% copper, 1.0 or less wt% Nickel, 0.01 to 0.1% by weight niobium, 0.01 to 0.2% by weight titanium, 0.01 to 0.2% by weight Vanadium, 0.2 or less weight percent phosphorus, 0.0005 to 0.01 weight percent boron and at least one of the rare earth metals zirconium, calcium and magnesium, their content in each case is less than 0.1% by weight, the remainder being essentially iron and production-related Impurities in a final roll with a degree of rolling of 5 to 40% within a temperature range is subjected to what ferrite and Austenite can be present side by side, such that the final rolled hot rolled Steel sheet is cooled at a cooling rate of 100 / second or more and is reeled in a temperature range of 250 cm to less than 54000, whereupon the reeled sheet material is cooled.

A preferred idea of the invention lies in creating a method for producing a hot-rolled steel sheet with low yield strength, high Tensile strength and excellent formability with a yield strength ratio of 65% or less, in which a steel in which the contents of carbon, Manganese, silicon, aluminum, sulfur and chromium are chosen so that a low Yield stress and high tensile strength can be achieved and whichever if necessary defined proportions of molybdenum, copper, nickel, niobium, titanium, vanadium, phosphorus, Boron and at least one of the elements zirconium, calcium and magnesium and rare earth metals contains, a final roll with a degree of rolling of 5 to 40 * within a temperature range is subjected, in which ferrite and austenite can coexist, this hot-rolled steel sheet slowly at a cooling rate of 1 ° C / second or more is cooled so that the decomposition of the residual austenite does not complete the sheet material is then within a temperature range of 2500C to less than 54,000 is reeled, so that one at room temperature is essentially Two-phase structure consisting of ferrite and martensite is achieved, whereupon the Material is cooled to room temperature.

Further features, advantages and details of the invention result from the following description based on exemplary embodiments and with reference on the drawing.

In this shows: 1 is a graphical diagram, which the influence of different contents of manganese, silicon and chromium on the yield point ratio and FIG. 2 is a graphical diagram showing the influence of changed proportions on manganese, silicon and chromium shows the yield elongation.

It goes without saying that the invention is in no way represented by the drawing is limited, since this only serves to explain the concept of the invention.

The following are the reasons for the invention Determination of the salary ranges for the individual steel components.

Carbon: At least 0.02 wt .- * carbon is required, to increase tensile strength. However, if the carbon content rises to more than 0.15% by weight, the weldability, in particular the weldability, is improved Spot welding deteriorated significantly. Because of this, the carbon content is set to the range of 0.02 to 0.15 wt. *. In terms of satisfactory Ductility and weldability properties is a carbon content of 0.02 up to 0.09% by weight preferred.

Manganese, Silicon and Chromium: Figures 1 and 2 illustrate the Results of investigations carried out by the inventors and show the of tongs, silicon and chromium on the yield point ratio and the yield point elongation influences exerted on the steel according to the invention in the tensile test. In the drawing A curve A denotes the situation with changed deficiency proportions and 0.06% by weight Carbon, 1.0% by weight silicon and 1.0% by weight * chromium. A curve B denotes the ratios with changed chromium content and 0.06 wt .- * Carbon, 1.7 wt .- * manganese and 1.0 wt .- * silicon.

The steel sheets tested were produced as follows: Each steel sheet was heated to a temperature of 1200 ° C, then including the final rolling (finish rolling) with a reduction degree of 25 * at a temperature of 7500C, in which a double phase from + γ can be achieved, hot-rolled, and then cooled at a cooling rate of 20 ° C / second, reeled at 5000C and cooled as a bunch.

Fig. 1 shows that when the contents of manganese, silicon and chromium below the respectively prescribed lower salary limit, i.e. below 0.9% by weight, 0.2% by weight and 0.2% by weight, respectively, are essentially the desired two-phase structure consisting of ferrite and martensite cannot be achieved and hence the yield point elongation is large and the yield point ratio is high. The two-phase structure that the invention is aimed at achieving can, however can be obtained even if the contents of manganese, silicon and chromium are the exceed the upper content limits, i.e. 2.0 wt. * or 3.0 wt. * and 2.0 wt.%. Alloy contents that exceed the specified upper content limits are However, unnecessary from a practical point of view and even lead to the disadvantage that the weldability of the material is reduced.

Accordingly, according to the invention, the manganese content is 0.9 to 2.0% by weight, the silicon content 0.2 to 3.0% by weight and the chromium content 0.2 to 2.0% by weight. Preferably the manganese content is 1.2 to 2.0% by weight, the silicon content 0.7 to 2.0% by weight * and the chromium content 0.5 to 1.5 wt .- * to keep the material quality stable.

Aluminum: Aluminum is preferably used as a deoxidation element.

However, it is not necessary to use aluminum in amounts above of 0.10% by weight, since such high aluminum proportions lead to increased alumina inclusions can lead. Accordingly, aluminum is limited to 0.10 wt% or less.

Sulfur: Preferably, the sulfur content should be as high as possible can be reduced in terms of deformability. However, as can be seen from the relation between the desulfurization and the steel sheet manufacturing cost, a above the sulfur content limit of 0.02 wt .- * can be tolerated, as these are comparatively easily achieved and able to meet the conditions which cannot be met the presence of sulfur can be influenced.

According to the invention, the aforementioned chemical composition corresponds the basic composition of the steel to be processed according to the invention. After a preferred embodiment can be the steel in addition to the components of Basic composition still molybdenum, copper, nickel, niobium, titanium, vanadium, phosphorus, Boron, zirconium, calcium, magnesium and at least one rare earth metal are added will. The reasons for adding the above elements are as follows discussed.

Molybdenum, copper and nickel: these elements have a hardening effect in solid solution hardening effect, are not only effective in achieving a high stability, but also support the achievement of an essentially two-phase structure consisting of ferrite and martensite.

However, these elements are expensive and consequently are economical From the point of view, their upper content limits are set at 1.0% by weight.

Niobium, titanium and vanadium: To increase the strength of the steel it is favorable to add 0.01% by weight or more of the respective elements.

However, if alloy additions are made to these elements, the are above the upper limits1, their influence on the increase in firmness saturated, with the effect that such excessively large additions make no sense.

For this reason, the niobium content is 0.01 to 0.1% by weight, the Titanium content 0.01 to 0.2% by weight * - and the vanadium content 0.01 to 0.2% by weight phosphorus: This element increases the strength of the steel. An addition of more than 0.1 wt .- * however, it makes the steel brittle, which is why the upper limit of the content is set at 0.2 wt .- * has been.

Boron: The addition of a very small amount of boron, greater than 0.0005% by weight leads to an effective retardation of the ferrite-pearlite transformation of the low-temperature transformation product, to which the present invention is based. Boron contents of more than 0.01 wt .- * However, do not lead to an improved effect, since contents of more than 0.01 wt .-% Boron do not bring about any further increase in the influence mentioned. So is the boron content is limited to the range from 0.0005 to Q01% by weight.

Zirconium, Calcium, Magnesium, and Rare Earth Elements: These elements affect the sulphide shape and are considered beneficial additives as they contribute to a lead to improved anisotropy of the mechanical material properties.

Additions of these elements in amounts of more than 0.1 wt .- * are, however disadvantageous because of an associated increase in the proportion of non-metallic Inclusions.

For this reason, the content of these elements is 0.1 wt .- * Or less.

The hot-rolled steel sheets according to the invention with low yield stress, high tensile strength and excellent ductility can be passed through Establish and control the hot rolling conditions and the coiling temperature, as hereinafter with reference to steels having compositions as discussed above, will be described. The reasons for setting the hot rolling conditions and the reel temperature are discussed below.

First its the hot rolling conditions explained. That essentially consisting of ferrite and martensite two-phase structure, which for the invention produced steel is characteristic, can even after the completion of hot rolling can be achieved in a single y phase, as in the Japanese patent application 21872/78 described by the inventor, the tensile properties of this steel being a have a low yield strength ratio and a satisfactory ratio between the tensile strength and ductility. It was also found that the Relation between tensile strength and ductility very strong by hot rolling can be increased within a temperature range in which a two-phase structure, consisting of ferrite (K-phase) and austenite (y-phase) is present. A on inventive A wise two-phase structure shows far better properties than a structure consisting only of the y-phase. In particular, by such a Two-phase structure the ductility can be increased without reducing the strength must be accepted.

Thus, in a comparison between one, Table II shows only the t-phase steel with the sample designation A-3 (hot rolling finish temperature 8500C) and a two-phase (d + y) steel with the sample designation according to the invention A-1 (hot rolling sinish temperature 7200C) that both steels matching Have tensile strengths, but the steel according to the invention has a greater ductility than the comparative steel A3.

The cause of the ductility due to hot rolling in the dl / two-phase area is increased has not yet been clarified. However, it becomes the following context assumed: Die-FestigReit of the essentially consisting of ferrite and martensite Two-phase steel material, i.e. the steel according to the invention, is first Line determined by the martensite phase, while the ductility of the steel material depends essentially on the ferrite phase. The lower the proportion of alloying elements is in the ferrite, the more the ductility of the ferrite phase is improved. on the other hand it is known that the concentration of alloying elements from the ferrite in the austenite phase is favored by the fact that the rolling in the + y two-phase region is made, has the consequence that the proportion of the alloy elements present in the ferrite decreases and the ductility of this phase increases.

It is possible to improve the ductility by finishing the rolling (finishing rolling) is carried out within a temperature range in which the ct + y dual phase can exist as described above. In this case The reduction ratio, i.e. the degree of rolling in the two-phase area, plays one of the following role to be explained. If the degree of rolling is less than 5 k, the equilibrium becomes between tensile strength and ductility increased to the same level, as in the case of final rolling in the single-phase area, as shown on the basis of the comparative steel A-6 in UafelII listed. On the other hand, if the degree of rolling exceeds 40 *, where does the ferrite phase thermoformed and thermoset by rolling even at a lower level Temperature get what to the Ductility leads to the achievement of which is sought by the invention. For this reason, the degree of rolling is at the final rolling in the two-phase α + y region according to the invention to a range from 5 to 40 * limited.

After hot rolling under the conditions described above it is necessary to cool the steel sheet with a cooling speed prior to reeling of 1 ° C / second or more, so as not to cause the residual austenite to decompose to complete. In addition, it is not necessary to specifically define the upper limit, but it is quite advantageous in terms of practical cooling capacity that the Cool steel within a range of 10 to 500C.

The reeling conditions are discussed below.

The coiling temperature after hot rolling is within that of the present invention Process of concern and this temperature is in the range of 250 to 54,000 has been set for the following reasons. Especially in hot-rolled sheet steel According to the invention, the decomposition of the retained austenite is close to the coiling temperature controlled to a considerable extent by the added elements manganese, silicon and chromium and the y-phase is further enhanced by the carbon concentration of the y-phase increased during slow cooling after reeling, which leads to that at low temperatures during cooling to room temperature austenite is formed.

The effectiveness of the aforementioned additional elements manganese, silicon and Chromium on retarding the austenitic fall is greatly hindered when the temperatures rise above 54,000. The double structure consisting of ferrite and martensite, which a characteristic of the present Invention is, can not can be reached at a reel temperature of more than 540 ° C, as in Table II shown using the comparative steel A-5. Consequently, yield point elongation occurs on, the yield strength ratio is high and it is the balance between the tensile strength and the ductility impaired.

On the other hand, this retarding influence can be applied to the Austenitz fall effective at all temperatures below 54,000, being those of the invention underlying task quite satisfactory at reel temperatures of 230 and 3000C can be solved, as in Table II using the comparative steel A-4 and the inventive Stahls A-2 demonstrated. The mechanical to be applied by the reel device However, performance is increased by lowering the coiling temperature. Especially with a temperature of less than 2500C occurs consisting of ferrite and martensite Two-phase area instead of the two-phase structure consisting of ferrite and retained austenite during the winding process, the strength of the steel sheet being considerable Extent is increased, with the result that the reeling operation is made difficult.

For this reason, the coiling temperature is within the scope of the invention to be selected within a temperature range of 250 to 54000. Higher coiling temperatures are to be aimed for from the point of view of the practical operating conditions and consequently, coiling temperatures in the range of 400 to 450C are preferred.

After reeling, the reeled bundle is cooled down as it is and the two-phase structure of ferrite and martensite which is the objective of the present Invention is can be achieved either by water cooling or by cooling in air reach.

The steel according to the invention has a yield point elongation of 1% or less, a yield point ratio of 65 * or less, one high tensile strength, excellent formability and high ductility (formability).

Example panel 1 Sample name chemical composition (% by weight) A a C Si Mn Cr PS Al 0.06 1.0 1.7 Q, 9 0.008 0.007 0.035 The steel given in Table I with the composition according to the invention is poured, annealed at a temperature of 12000 ° C. and then processed into a hot-rolled sheet under the conditions according to the invention for hot rolling, for cooling after hot rolling and the coiling temperatures. The results of comparative tests with regard to the tensile properties between the steel produced (according to the invention) and the comparative steel not produced according to the invention are summarized in Table II.

Table II shows the yield point, tensile strength, yield point ratio, the elongation at stretch and the total elongation are listed. The comparative steels listed under conditions deviating from the conditions according to the invention have been hot-rolled are underlined in Table II. All steels listed in Table II, that is, both the steels according to the invention and the comparative steels were under the hot rolling conditions given in Table II to give steel sheets with a thickness of 2.5 mm has been hot rolled, then with a cooling rate of about 2000 / second has been cooled at the reel temperatures indicated in each case has been reeled and then after reeling at a cooling rate from 30 ° C / hour been cooled.

Plate II Hot rolling conditions Final Rolling Reel Stretching Tensile Stretching Stretching Total rolling degree temp. limit strengthening limit elongation elongation temp. speed ratio (° C) (%) (° C) (kg / mm²) (kg / mm²) (%) (%) (%) Invention A-1 720 20 510 34 62 55 0 41 A-2 730 15 300 36 65 56 0 39 Comparative A-3 850 20 520 36 62 58 0 36 try A-4 730 20 230 39 66 59 0 37 A-5 740 10 620 37 51 72 23 38 A-6 720 4 520 34 60 57 0 37 As can be seen from Table II, the hot-rolled steel sheet of the present invention has no yield elongation, a low yield elongation, a low yield strength ratio of 50 to 60% and an excellent balance between strength and ductility.

In contrast, those not hot-rolled in the manner according to the invention show Comparative steels have lower tensile strengths than those produced according to the invention Steels.

When performing the method according to the invention, the first chemical composition of the steel is adjusted and the steel sheet is produced subjected to a finishing rolling imid + y two-phase region, see above that a two-phase structure consisting of ferrite and martensite can be obtained can. The rolling takes place with a rolling degree of 5 to 40% in order to achieve the To increase ductility. In addition, the decay of the retained austenite is controlled by that the steel sheet at a coiling temperature of 250 to 540 ° C after hot rolling is reeled so that the nartensite phase is at low temperatures during of cooling to room temperature. Consequently, a hot-rolled steel sheet can can be produced with excellent ductility, its yield strength ratio 65 * or less, which has no elongation at stretch, has high tensile strength of 60 kg / mm2 or more and an elongation of about 40 *.

Another advantage that can be achieved with the aid of the invention is therein to see that the high tensile strength steel sheet produced in accordance with the invention does not requires the addition of many alloying elements and that the rolling conditions are comparatively simple, thereby lowering the overall production cost can.

L e r s e i t e

Claims (5)

Process for the production of hot-rolled sheet with low yield stress, high tensile strength and excellent ductility claims 1. Method for producing a hot-rolled steel sheet with low yield stress, high tensile strength, excellent ductility and one essentially consisting of ferrite and martensite two-phase structure and one Yield strength ratio of up to 65%, therefore not too much, that a steel plate containing 0.02 to 0.15 wt .-% carbon, 0.9 to 2.0 % By weight manganese, 0.2 to 3.0% by weight silicon, maximum 0.1% by weight aluminum, maximum 0.02% by weight sulfur, 0.2 to 2.0% by weight chromium, the remainder essentially iron and production-related Impurities, a final rolling with a degree of rolling of 5 to 40 °% within a temperature range in which ferrite and austenite coexist can be rolled out, and that this hot-rolled steel at a cooling rate from 10 C is cooled per second or more and at a temperature from 250 to a maximum of 54O0C, whereupon the wound bundle is cooled will. 2. Method of producing a hot rolled steel sheet with lower Yield stress, high tensile strength, excellent ductility and a two-phase structure, which consists essentially of ferrite and martensite, and with a yield point ratio of up to 65%, thus g e k e n n s e i c Note that a steel plate containing 0.02 to 0.15% by weight of carbon, 0.9 up to 2.0% by weight manganese, 0.2 to 3.0% by weight silicon, maximum 0.10% by weight aluminum, maximum 0.02% by weight sulfur, 0.2 to 2.0% by weight chromium, maximum 1.0% by weight molybdenum, maximum 1.0% by weight copper, maximum 1.0 1.0% by weight nickel, 0.01 to 0.1% by weight niobium, 0.01 to 0.2% by weight titanium, 0.01 to 0.2% by weight vanadium, maximum 0.2% by weight phosphorus, 0.0005 to 0.01 wt .-% boron and at least one of the elements zirconium, calcium, magnesium and rare earth elements in a proportion of less than 0.1%, the remainder being essentially Iron and production-related impurities in a final roll with a 5 to 40% rolling degree is subjected at a temperature at which Ferrite and austenite can coexist in this hot-rolled sheet is cooled at a cooling rate of 1 CC ae second or more and reeled at a temperature of 250 to less than 5400C and then cooled will. 3. Method of manufacturing a hot rolled steel sheet with lower Yield stress, high tensile strength and excellent ductility according to claim 1 or 2, characterized in that a steel sheet is hot rolled, which contains 0.02 to 0.09% by weight of carbon, 1.2 to 2.0% by weight of manganese, 0.7 to 2.0 Contains wt .-% silicon and 0.5 to 1.5 wt .-% chromium. 4. Method of manufacturing a hot rolled steel sheet with lower Yield strength, high tensile strength and excellent deformability according to Claim 1 or 2, characterized in that the cooling rate for the hot rolled steel sheet from final rolling to cooling inside a range from 10 to 5000 per second. 5. Method of manufacturing a hot rolled steel sheet with lower Yield stress, high tensile strength and excellent ductility according to Claim 1 or 2, characterized in that the reeling temperature for the hot-rolled steel sheet is in a range of 400 to 50,000.