FI120203B - Process for treating steel and steel treated according to the procedure - Google Patents

Description

Method for treating steel and steel treated according to the method

Background of the Invention

The present invention relates to a process for the preparation of steels having a composition of 0.08% to 0.16% C, 0.5% to 1.5% Cr and / or 0.1% to 0.5% Mo, 0.6% by weight 1.1% Mn, 0.1 -0.3% Si, 10 up to 0.015% S, up to 0.03% P, 0.01-0.08% Al, residual Fe and unavoidable impurities, comprising the steps of hot rolling the 15 steel to a thickness of 2 to 12 mm so that the final rolling temperature is in the range of 860 to 960 ° C, then quenching the steel no later than 15 s after the last rolling to a rewinding temperature of 20 -470 ° C; at least 30 ° C / s.

Such thermomechanical treatment is known from EP 1375694 20 for steel of the above composition. The microstructure of the steel after said thermomechanical treatment is almost completely (at least 95% by volume) martensitic-bainitic. The steel has a high strength: tensile strength (Rm) is about 1000 MPa and yield strength (Rp0.2) is about 5-10% lower than the breaking strength. The elongation (A5) is about 9-10%. Steel can be made into structural steel and is well suited for applications where wear resistance is required. The hardness of the steel is in the order of 300 - 400 HB.

When workpieces intended to be edged have been made from thermomechanically treated steel in the aforementioned manner, problems have occurred in the edging of the workpieces. At the edging, the deformation has become more inclined, whereby the shape of the resulting edge no longer corresponds to that of the edging tool. One or more lines of bending radius smaller than the radius of the crimping tool (ram) have been detected in the formed edge. At its worst, the steel is bent so much locally that the structure is cracked. In less severe cases, only a shape defect is observed which is mut-35-sharp or discontinuous. Surface cracks have also been observed on the outer surface of the beam. Thus, the deformation of the steel at the edge 2 has been very inhomogeneous and the workpiece to be manufactured has not met its requirements.

Brief Description of the Invention

It is an object of the invention to provide a treatment for steel having the above composition, which eliminates the problems that occur particularly in steel edging.

To achieve said object, the method according to the invention is characterized in that the edging of the steel is improved by reducing the tendency of the steel to inhomogeneate at the edge 10 by annealing the steel at a temperature of 160 to 450 ° C after quenching.

Thanks to the above-mentioned treatment, the edging of the steel pieces can be carried out without problems. It has been very surprising that, thanks to this treatment, the breaking and yield strength values of steel have also typically increased. The rise of 15 has been up over 10%.

Preferably the annealing is carried out at a temperature in the range of 170 to 250 ° C. The annealing time is preferably 9 to 20 hours.

Cold forming of the steel after quenching and before annealing has proved highly recommended. Tine molding results in a steel with high strength values and very good edging.

Preferred embodiments of the invention are set forth in the appended claims 2-17.

The major advantages of the process according to the invention are that it can provide a steel with a good edge flanging in a simple manner even though its strength values such as yield strength and tensile strength are very high and even higher than without the treatment included in the process. Stretch values are also good, especially when considering the high strength of steel.

The invention also relates to steel of the above composition, wherein the steel of the invention is characterized by being treated by the process of the invention and having a yield strength of greater than 950 MPa and an elongation of A 5 of at least 10%. Preferably, the yield strength of the steel is greater than 1000 MPa and its elongation at A5 is at least 12%. The result is a steel with good edging, despite its high strength values. The ductility properties of the steel (elongation and impact strength values) are also good.

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BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of example with reference to the accompanying drawing, in which Fig. 1 illustrates the method of the invention, 5 Fig. 2 shows the edges properties of the steel treated by the method, Figures 3-5 show the strength values achieved by the method. you reach -10 inches of elongation.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a block diagram illustrating the steps of the method of the invention.

The starting point is steel having a composition by weight of 0.08 to 0.16% C, 0.5 to 1.5% Cr and / or 0.1 to 0.5% Mo, 0.6 to 1.1% Mn , 0.1-0.3% Si, up to 0.015% S, up to 0.03% P, 0.01-0.08% Al, and the remaining Fe and unavoidable impurities. The copper content should preferably not exceed 0.3% if excellent surface quality is required. If the copper content exceeds 0.3%, it is advisable to add nickel at least 0.25 times the amount of copper-20, but the nickel content of the steel should not exceed 1.5%.

The steel blank of said composition is hot-rolled to a thickness of 2 to 12 mm. The end temperature of the rolling is 860 - 960 ° C. From said temperature range above A3, the steel is extinguished at high speed, no later than 15 seconds after the te-25 steel leaves the last roll of the mill. The quench rate shall be at least 30 ° C / s. From said direct quenching, the steel is directed to a coil, wherein the coil temperature is in the range of 20 to 470 ° C. Preferably, the winding temperature is up to 460 ° C. After winding, it is highly recommended that the steel be cold-formed. Since it is conceivable that cold forming will not be performed, an arrow from direct quenching to direct annealing has been added to Figure 1. Depending on the manner in which said cold forming is performed, it is difficult to give a numerical value for its size, but it can be said that it is preferably at least about 1%. The cold forming preferably comprises, for example, roll straightening and straight rolling. Alternatively, cold forming is 35 mere roll straightening without straight rolling. In roll alignment, the stress state of the plate-like steel is changed by alternating bending in different directions. In Oi-rolling, sheet steel is preferably cold-rolled with a reduction of 0.5 to 1.5%, whereby the steel is stretched longitudinally (in the rolling direction) and the internal length differences and stresses are equalized. After cold forming, the steel 5 is cut into sheets. The steel is then annealed at a temperature of 170 to 400 ° C, preferably 170 to 250 ° C. In this case, the annealing time is preferably 9-20 hours. After annealing, the steel is allowed to cool slowly, for example in the air or, alternatively, very slowly in the oven at a cooling rate of 1.5-2 ° C / s.

As a result of this treatment, the edging properties of the steel are improved, and its fracture and yield strength has even been found to be increased compared to no treatment (shaping and strain), cf. Figures 2-5, which show measured values for approximately 1% cold-formed, in this case straightening caliper, for steel. Cold shaping can be considerably greater than 1%, for example 5% and more.

From the drawing of Figure 2, it is seen that the radius of curvature of the inside of the edge is obtained by the heat treatment of the invention to approximately correspond to a radius of curvature of the ram of 15 mm for at least 10 hours, then annealing temperature of 200 ° C, 400 ° C, or 550 ° C. After annealing, the steel is air-cooled. Without annealing, the radius of curvature is significantly smaller than that of the press 20.

From the drawing of Fig. 3, it can be seen that the tensile strength (Rm) increases by 3-6% depending on the annealing time, and the yield strength (Rp0.2) by 11-14% when the annealing temperature has been 200 ° C. With an annealing time of 48 hours, the highest strengths are achieved as compared to no annealing.

From the drawing of Figure 4, it can be seen that 48 hours of heat treatment in the temperature range 150-250 ° C increases the transverse yield strength (Rp0.2) for both the initial "900" steel with a yield strength of 960 MPa and the initial "960" steel yield strength is 1000 MPa. The effect of said heat treatment on the tensile strength (R m) is negligible for steel starting from "900" but lowering the breaking strength of steel starting from "960" when the annealing temperature is above 180 ° C.

From the drawing of Figure 5, it can be seen that 48 hours of heat treatment in the temperature range 150-250 ° C significantly increases the fatigue yield strength (Rp0.2) for steel starting at "900". Also for steel starting level "960", said heat treatment increases yield strength when the annealing temperature is in the range of 150-210 ° C. Figure 5 also shows the effect of said heat treatment on the breaking strength (R m).

From the drawing of Fig. 6, it can be seen that, compared to the situation where no heat treatment is performed ("starting level" in the figure), the elongation at break 5 increases significantly with the annealing temperature of 400 ° C and only slightly with the annealing temperature of 200 ° C. The elongation increases from about 12% to open at up to 15% when the annealing temperature is 400 ° C.

Figure 7 shows how a 48-hour heat treatment in the temperature range 150-250 ° C affects the transverse elongation at break of the "900" and "960" steels, see Figs. the "900 vs" curve and the "960 vs" curve, respectively. The figure also shows how said heat treatment affects the Vals dry-elongation at "900" and "960" steels, cf. the "900 boy" curve and the "960 boy" curve, respectively. It can be seen from the figure that heat treatment significantly increases the elongation values at baseline 9.5% and 10.3%.

Figures 3 and 6 show that heat treatment can substantially increase the yield strength (Rp0.2) even though the elongation is not reduced but even increased. By annealing at 200 ° C / 10 h (i.e., annealing at 200 ° C for 10 hours), the yield strength (Rp0.2) of the steel of the invention is obtained at 1040 MPA sa-20 clubs, with an elongation A5 of about 12%. The tensile strength (Rm) is 1070 MPa, respectively. The steel of the invention has a yield strength (Rp0.2) of greater than 950 MPa, preferably greater than 1000 MPa and an elongation at A5 of at least 10% and 12%, respectively.

Particularly preferred is a treatment in which the steel is wound at a temperature of 20-100 ° C, followed by said annealing.

The invention has been described above by way of example only and it is therefore noted that the details of the invention may vary in many ways within the scope of the appended claims. Thus, unlike the examples, the annealing temperature may be as low as 150-160 ° C, but in this case the result is not the best possible and the desired result is not achieved by working and subsequent annealing. Generally speaking, the lower the annealing temperature, the longer the annealing time is recommended. It is difficult to give an exact recommendation as to how the annealing time should be selected in relation to the annealing temperature. The thickness of the steel also influences this. In Kek-35 sintering, the upper limit of the annealing temperature is selected to be about 450 ° C. If the annealing temperature is significantly above 400 ° C, it has been found that the breaking and tensile strength values of the steel are reduced. It should be noted that steel has good properties even when the annealing temperature is as high as about 400 ° C and the annealing time is several tens of hours. However, it is not economically meaningful to burn for long periods of time. It is expected that annealing times greater than 5 to about 20 hours will not provide any significant benefits. As a result, it makes no sense to burn for more than 70 hours, and especially for more than 100 hours. The annealing time can be much shorter than 9 hours, for example 10 minutes to 3 hours. This is economically advisable, for example, when heat-treating the tape disc individually. Even if the annealing time is very short, only a few seconds, or even just 1 second, gives the steel improved properties (edging, strength, elongation) compared to not annealing. The cooling rate after annealing may also vary. As mentioned above, it is conceivable that cold working is not performed. This simplifies the handling of steel, but does not provide the same strength properties.

Claims (19)

A process for treating a site whose composition, expressed in weight percent, is 0.08-0.16% C, 0.5-1.5% Cr and / or 0.1-0.5% Mo, 0.6-1.1% Mn, 0.1-0.3% Si, maximum 0.015% S, maximum 0.03% P, 0.01-0.08% AI, residual Fe and inevitable pollutants , the method comprising the stages in which the noise is hot-rolled to a thickness of 2-12 mm, thus, that the final temperature of the rolling is 860-960 ° C, after which the noise is slackened to a rinsing temperature of 20-470 ° C at the latest after 15 seconds after the last of the rolling thus, the extinguishing rate is at least 30 ° C / s, characterized in that the bending properties of the site are improved by reducing the propensity for non-homogeneous deformation at the site of the bending by annealing the noise at a temperature of 160-450. ° C after quenching to the flushing temperature. Process according to claim 1, characterized in that the annealing is carried out in the temperature range 170-450 ° C. 15 Method according to claim 2, characterized in that the annealing is carried out in the temperature range 170-250 ° C. 4. A method according to claim 1, characterized in that the annealing process is carried out in the temperature range 200-450 ° C. Process according to claim 4, characterized in that the annealing process is carried out in the temperature range 200-400 ° C. Method according to claim 1, characterized in that the temperature of said annealing is higher than the lowest temperature of the site during the flushing. Process according to claim 6, characterized in that the flushing temperature is 20-100 ° C. Method according to claim 1, characterized in that the noise is annealed for 3-100 hours. 9. A method according to claim 8, characterized in that the noise is annealed for 3-70 hours. 30 Method according to claim 8, characterized in that the noise is annealed for 5 to 100 hours. Method according to claim 9, characterized in that the noise is annealed for 9-70 hours. Method according to claim 11, characterized in that the noise is annealed for 9-20 hours. 13. A method according to claim 1, characterized in that the noise is annealed for 3 minutes - 3 hours. Process according to some previous claims, characterized in that the steel is air-cooled after annealing. 15. A method according to some previous claims, characterized in that the steel is cooled after annealing at a cooling rate of not more than 2 ° C / s. Method according to some previous claim, characterized in that after quenching to the flushing temperature, the steel is cold worked at least about 1% before said annealing. 10 17. A method according to some previous claims, characterized in that after quenching to the flushing temperature, the steel is cold-worked at least 5% before said annealing. 18. A frame, characterized in that it is treated according to the preceding claims 1-17 and that its tensile strength is over 950 MPa and that its extension A5 is at least 10%. The rack according to claim 18, characterized in that its tensile strength is over 1000 MPa and that its extension A5 is at least 12%.