JP6613738B2 - Straightening method for high-tensile steel plate shape - Google Patents

Description

The present invention, in producing a high-strength steel by quenching, time and until the Shi tempering after quenching, over time from tempered to straightening operation, by utilizing the phenomenon that the yield strength of the steel sheet is increased or decreased, By performing correction with a leveler or a hydraulic press leveler during the time zone when the yield strength of the steel plate after heat treatment is low, the steel plate has a greater degree of processing than the correction in other time zones, and good post-correction flatness Is the technology to get

High-strength steel is defined in various ways depending on the manufacturer and application. In general, however, high-strength steel generally refers to steel having a tensile strength of approximately 490 MPa or higher (50 kg / mm ² class in the old display). The thing of the intensity | strength exceeding 1000 MPa has also appeared. High-tensile steel is generally produced by quenching immediately after hot rolling or immediately after heating in a heat treatment furnace to cause martensitic transformation. Depending on the application, tempering heat treatment may be performed after quenching to adjust hardness and toughness.

  In this manufacturing process, quenching is performed by spraying water on the front and back surfaces of the steel sheet or immersing the steel sheet in a water tank. In the first half of cooling, the steel sheet has a non-uniform temperature distribution, and the hot part is compressed and the cold part is tensile plastic. Deformation is likely to occur, and the high temperature part may buckle in this process, and the part that was low temperature in this process has a long wire length, so the cooling ends and buckles when the steel plate temperature becomes uniform. There is a case.

  That is, in the quenching process, waves due to buckling tend to occur in each part of the steel sheet according to the steel sheet dimensions and cooling conditions. In order to reduce such deformation, for example, with respect to quenching by a roller quench device (Roller Quench, hereinafter referred to as RQ), conventionally, a technique of uniform cooling in the width direction as in Patent Document 1, or As in Patent Document 2, a technique for reducing the temperature gradient in the longitudinal direction by increasing the speed of the passing plate and a technique for suppressing out-of-plane deformation by pressing the restraining roll have been proposed.

Japanese Patent No. 3449295 JP 2008-231476 A

  However, there are equipment limitations such as cooling nozzle uniformity, sheet feeding speed, pressing force of high-speed rolls, etc. Especially for thin high-tensile steel with a thickness of less than 10 mm, general RQ has sufficient shape control. Is difficult to do. On the other hand, in the case of high-strength steel with a large plate thickness, the difference in cooling characteristics of the steel plate itself, particularly the difference in cooling rate between the four circumferences and the inside of the steel plate, and the upper and lower surfaces is large. It is difficult to completely suppress deformation.

  In addition, a steel plate with buckling deformation generally needs to be corrected by bending with the roller leveler CL shown in FIG. 1, but high-tensile steel has high yield strength. It is difficult to correct flatly. The correction by the hydraulic press leveler OL is also a bending process, and it is the same as the roller leveler in that correction of high-tensile steel is difficult compared to mild steel.

Here, the degree of processing is obtained by dividing the curvature κ given to the material to be corrected during correction by the elastic limit curvature κ ₀ of the material to be corrected, and is given by Expression (1).

  In leveler correction, as shown in FIG. 1 and FIG. 2, positive and negative bending is repeatedly applied to the steel sheet by passing the steel sheet between work rolls arranged in a staggered manner, as shown in FIG. 2, Levels # 4, # 6, # 8 and # 10 have a mechanism for tilting, and the amount of indentation, that is, the height difference between the steel plate surface on the straightening machine entry side and the lower surface of the top work roll # 2 on the upper side It is generally performed to set δi to a positive value and to set the exit side push amount, that is, the height difference δo between the straightening machine exit side steel plate surface and the upper surface of the final work roll # 10 to zero. .

  By setting the work roll position of such a leveler, it is possible to give a large degree of processing to the steel plate at the initial stage of correction, then repeatedly bend it while gradually reducing the degree of processing, and finally finish to zero curvature, that is, flat. As the maximum value of the degree of work imparted to the steel sheet during this correction process increases, the plastic deformation region in the thickness direction of the material to be corrected extends from the surface layer to the vicinity of the center of the thickness, and the correction effect increases.

  For example, when straightening mild steel with a plate thickness of 8 mm and a yield strength of 300 MPa, the work roll position setting for giving a predetermined degree of processing to the steel sheet is calculated assuming that the work roll is a rigid body. Become. The actual leveler may be set to a value slightly different from Table 1 in consideration of the deformation of the roll assembly and the frame, the influence of work roll bending, and the like.

In the right side of the equation (1), the curvature κ is determined by the leveler pressing amount, and E is a substantially constant value of about 2.1 × 10 ⁵ N / mm ^{2 in the case} of carbon steel. Therefore, when the steel material having the same plate thickness is corrected with the same pressing amount, the degree of work decreases as the yield strength σ _{y of the} material to be corrected increases.

  While the yield strength of mild steel is around 300 MPa, the yield strength of high-strength steel produced by quenching is around 900 MPa, which is about three times that of mild steel. Therefore, under the same straightening conditions, the maximum workability imparted to the high-strength steel is approximately 1/3 of the maximum workability imparted to the mild steel.

  In leveler correction, the relationship between the steepness before and after correction and the maximum degree of processing is expressed as in equation (2).

  As shown in FIG. 3, the steepness of the poorly shaped material after quenching is distributed around 0.7%, and as understood from the curve showing the cumulative frequency, about 85% of the whole is 1.0%. Is less than. The steepness that can be shipped as a product is often 0.5% or less. In order to reduce the steepness of the steel sheet from 1.0%, which covers 85% of the steepness after quenching, to 0.5%, which is a general flatness that can be shipped, by leveler correction, the formula (2) From the relationship, the maximum processing degree needs to be 4 or more. FIG. 4 is a graph of this, and shows that the post-correction steepness decreases to a value obtained by dividing the entry-side steepness by the half power of the processing degree.

  On the other hand, a normal leveler is often designed to give a maximum workability of 5 to 6 to mild steel. When high-strength steel is corrected with these levelers, the maximum workability is calculated according to equation (1). Is 1/3, and the material steepness on the exit side of the leveler is increased by √3 times that of mild steel according to the equation (2).

  In order to give a higher maximum workability to high-strength steel, it is necessary to increase the roll push-in amount, but the load and driving force exceed the equipment limit, increasing the risk of jamming and equipment damage. For this reason, for example, the increment of the push amount from the setting of the roll push amount that gives the maximum workability 6 to the mild steel to the equipment limit is often 10 to 20% at most. Here, when the roll pressing amount is δ and the roll interval is p, the curvature κ given to the steel plate by the roll is expressed by the following expression (3).

  That is, the curvature is proportional to the push amount, and therefore the degree of processing is also proportional to the push amount from the equation (1). When the indentation amount is increased by 20% from the setting for giving the maximum workability 6 to the mild steel, the maximum workability 7.2 is given to the mild steel. However, when straightening a high-strength steel that has three times the yield strength of mild steel with the same setting, the maximum workability decreases to 7.2 / 3 = 2.4, and the entry side steepness is 1.0%. The exit-side steepness is 1.0 ÷ √ (2.4) = 0.65%, and the acceptance criterion cannot be reached.

  As described above, high-strength steel is likely to buckle and deform during quenching, and shape correction after quenching is extremely difficult compared to mild steel because of its high yield strength. There was a drawback that flattening was difficult.

In order to solve the above problems, the yield strength of the present inventors have conducted intensive studies and as a result, steel sheets were tempered Shi treatment after quenching, the elapsed time t1 to tempering Shi processed from quenching, and tempering The knowledge that it increased / decreased according to progress of time t3 from a process to a correction process was acquired.

Figure 5 is a diagram showing an example of a time schedule for performing quenching and tempering on the steel plate.
The steel plate was heated and held at 930 ° C. for 1200 seconds, and then roller quenching (RQ; sandwiched between cooling rolls and rapidly cooled) at a cooling rate of 70 ° C./s. After holding for 2 seconds, it is cooled to about 300 ° C. at 1 ° C./s, and then cooled to a cooling end temperature of 50 ° C. at 0.2 ° C./s to 1 ° C./s. Here, the timing for starting the count of t3 is defined as the time after reaching 50 ° C.

6, to fix the time t3 from tempered until end correction operation start point by leveler 15 minutes, when changing the time t1 to the start temperature increase for the skip back baked from quenching, straightening operation start It is the graph which showed the change of the yield strength of the high strength steel plate at the time.

  As is apparent from FIG. 6, when t3 is fixed to 15 minutes and the elapsed time t1 from the quenching to the start of temperature rise for tempering is changed, the yield strength of the steel sheet at the start of the straightening operation increases as t1 increases. It can be seen that there is a tendency to decrease.

On the other hand, FIG. 7 shows that the time t1 from the end of tempering to the start of the straightening operation by the leveler is kept constant at 1440 minutes, and the time t3 from the end of quenching to the start of temperature increase for tempering is changed. It is the figure which plotted the change of the yield strength of the steel plate at the time of a straightening operation start at the time.
According to the figure, the steel sheet yield strength after the completion Shi tempering it is found that the shorter the t3, tends to be smaller.
That is, when the temperature started to be raised for tempering the 1440 minutes after quenching completion, as the elapsed time after the completion Shi tempering is short, since the yield strength of the steel sheet becomes small, the straightening operation, after the tempering ends, Starting as soon as possible is advantageous because the yield strength decreases.

  FIG. 8 is a graph in which changes in yield strength due to changes in t1 and t3 shown in FIGS. 6 and 7 are superimposed, and changes in t1 and t3 indicate changes in yield strength of the high-tensile steel sheet at the end of tempering. It can be understood that this has a strong correlation.

Based on the results shown in FIGS. 6-8, the hardening and tempering after performed on the steel plate, in carrying out the correction operation for shape control, after quenching, at a certain level or more times Therefore, if the temperature rise for tempering is started and straightening operation for shape control is started as soon as possible after tempering, the high strength steel sheet can be straightened with low yield strength. It can be seen that the maximum effect can be obtained.

In comparison with mild steel, as shown in Table 2, when a leveler set to give a maximum workability of 5.0 to 6.0 is applied to mild steel, it is the highest in high-strength steel that has reached its original yield strength. The degree of work remains at 1.7 to 2.1, and the exit side steepness of a steel sheet with an entrance side steepness of 1.0% is 0.8% to 0.7%.
On the other hand, when the high-strength steel is corrected when the yield strength is 360 MPa, the maximum working degree is 4.2 to 5, and the exit steepness can be reduced to about 0.45 to 0.48%. It is.

The present invention was made based on the above findings, the yield strength is low value at which time zone, i.e., after sufficiently securing the time at the tempering it from the after quenching completion, heating for tempering At the point of time when the yield strength is much lower than the inherent yield strength (final ultimate yield strength) of the high-tensile steel sheet by starting straightening operation with a leveler as soon as possible after tempering is completed. By performing correction, a high degree of workability is imparted to the high-tensile steel sheet, and good flatness is obtained, and the gist thereof is as follows.

(1) In mass%, C: 0.50% or less, Si: 0.70% or less, P: 0.05% or less, S: 0.05% or less, O: 0.007% or less, N: 0.00. 01% or less, including at least one of Ni, Cr and Mo in a range of Ni: 9.5% or less, Cr: 2.5% or less, Mo: 1.2% or less the steel sheet comprising the balance inevitable impurities and Fe, after the steel sheet structure and austenite is heated in a heat treatment furnace, a quenching step of performing quenching to Ru is transformed to steel sheet structure with water cooling to martensite the steel sheet, tempering been has a tempering process to high-tensile steel plate, a straightening step for straightening of the high tensile steel plate subjected to the tempering, the, the said shape in the correction process, the time until hardening after-tempering (t1 ) was greater than or equal to 24 hours, the time from the tempering until the end correction As within 60 minutes between (t3), straightening method of high-tensile steel plate shape and performing correction.
(2) pre-Symbol steel plate, further, the Cu 0.5% or less including, straightening method of high-tensile steel plate shape according to (1).
(3) yield strength after quenching and tempering, the final 60% or less times of arrival intensity, cormorants row corrective, wherein (1) or method of correcting high-tensile steel plate shape according to (2).
(4) the yield strength after quenching and tempering, is predicted on the basis of t1, t3 and previously determined .sigma.y _0.2%, in the time zone is .sigma.y _0.2% is low, you implement a corrective, the (1) The correction method of the high-tensile steel plate shape according to any one of to ( 3 ).

  As described above, according to the present invention, when a predetermined indentation amount is applied to the high-tensile steel plate with a leveler, the yield strength at the time of correction is low, so the same strength as that of the high-tensile steel plate that has reached the final yield strength is obtained. Compared with the case where an indentation amount is provided, a greater degree of processing and a straightening effect can be obtained, and the steel plate shape can be improved.

It is a figure which shows the basic structure of a roller leveler, and the bending correction of a steel plate. It is a figure which shows the relative relationship of a steel plate and a roller leveler, the entrance side pushing amount, and the exit side pushing amount. In leveler correction, it is a figure which shows a mode that the steepness of a steel plate reduces according to the maximum work degree provided with a leveler from 1.0% before correction as a starting point. It is a graph which shows the influence which the increase in a processing degree has on the fall of steepness. It is a figure which shows an example of the time schedule of the quenching and tempering process using RQ. It is a figure which shows the yield strength change at the time of fixing t3 to 15 minutes and changing t1. It is a figure which shows the yield strength change at the time of fixing t1 to 1440 minutes (1 day) and changing t3. It is a figure which shows the yield strength change at the time of changing t1 and t3 by a multiple regression equation.

  Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

  In FIG. 5, the steel sheet is heated to 930 ° C. in a heat treatment furnace, held for 20 minutes (1200 seconds) and subjected to austenitizing treatment, and then extracted from the heat treatment furnace and quickly cooled with water to cause martensitic transformation. Thus, a high-tensile steel plate is obtained. Here, the heat treatment is performed in a continuous heat treatment furnace or a batch heat treatment furnace, and the water cooling is performed in an RQ or a water tank.

Heating for tempering is started after t1 seconds, heated to 500 ° C at a heating rate of 1.4 ° C / s, held for 20 minutes (1200 seconds), then to 1300 ° C at 1 ° C / s, then Cool to room temperature at 0.2 ° C./s, and after 3 seconds, send to a cold roller leveler (CL) and / or a hydraulic press leveler (Oil Leveler, OL) downstream of the line to correct the shape.

In the present invention, if the time (t1) from quenching to tempering is 24 hours or more and the time (t3) from the end of tempering to straightening is within 60 minutes, the straightening device for ordinary steel (yield stress is about 400 MPa) Even if the following steel can be straightened), the yield stress can be expected to be softened to such an extent that a predetermined shape straightening effect can be obtained with respect to high-strength steel. Preferably, t1 is 36 hours (2160 min) or more, If t3 is within 40 minutes, more preferably t1 is 48 hours or longer and t3 is within 20 minutes, a further decrease in yield strength can be expected. At that time, if the correction is performed in a time zone in which the yield strength is 60% or less of the final strength, the effect of the present invention can be fully enjoyed. However, since increasing t1 leads to a decrease in productivity and an increase in storage locations such as a steel yard, it is necessary to set the balance in consideration of these factors.

The present invention can be applied to various high-strength steels.
<C: 0.50% or less>
Needless to say, C is a basic element that secures hardenability and creates the strength of steel. However, when the concentration is high, the hardness is improved, but the toughness is reduced, and cracks are easily generated. Therefore, the upper limit is 0.5%.

<Si: 0.70% or less>
Si is contained in the steel in the deoxidation operation and has a solid solution strengthening effect. However, if excessively contained, the weldability and HAZ toughness are deteriorated, so the upper limit was made 0.70%.

<P: 0.05% or less>
P also has a side as a solid solution strengthening element, but an increase in the amount thereof is not preferable because the workability and weldability of the steel sheet are significantly reduced. In particular, if it exceeds 0.05%, these adverse effects become large, so the upper limit was made 0.05%.

<S: 0.05% or less>
Since S is an element that causes cracking during hot rolling and cracking during processing, the upper limit of the content is defined as 0.05%.

<Mn>
Mn is one of the five basic elements of the steel component, but in the present invention, no action beyond capturing S contained in the component as MnS is planned. Therefore, it is only necessary to include an amount commensurate with the content of S contained as an impurity, and the upper limit of the content is about 1.5% as in the case of structural steel.

Ni, together with an element that improves the hardenability les of steel, a component can be secured low temperature toughness, actually 9% Ni-containing high strength steel, although is where widely used as LNG tanks material, The upper limit was set at 9.5% due to high cost and saturation of effects.

<Cr: 2.5% or less>
Cr improves both the strength and toughness of the steel material. However, if the addition amount is too large, the toughness deteriorates and the weldability also deteriorates. Therefore, the upper limit is specified to 2.5%.

<Mo: 1.2% or less>
Mo is an element that improves hardenability, but at the same time has a high solid solution strengthening ability, and precipitates as MoC ₂ , causing a strong secondary hardening phenomenon. If added over 1.2%, the curing becomes too strong and the material becomes brittle, so 1.2% was made the upper limit.

<Cu: 0.5% or less>
Although it has a precipitation strengthening action as Cu particles, it has a low melting point, and therefore melts and penetrates into grain boundaries at high temperatures, causing red-hot embrittlement. For this reason, 0.5% was made the upper limit.
In the presence of Ni, there is an effect of suppressing cracking during hot rolling.
Note that O is 0.007%, and N is about 0.05% as a guide for the upper limit of the content. Other unavoidable impurities that are generally contained in steel materials as well as high-tensile steels are the same as those in ordinary steel materials, and do not set a specific content range.

(Example)
Table 3 shows the components of the high-strength steel (steel A) used in the test.
This steel was heat-treated under the same conditions. After quenching, 15 minutes, 60 minutes (1 hour), 1440 minutes (1 day), 2160 minutes (1.5 days), 2880 minutes (2 days) and 5760 minutes Table 4 shows the results of performing the tempering operation after 4 days and measuring the yield strength after 15, 20, 40 and 60 minutes, 1, 2 and 4 days.

As understood from the yield strength values in Table 4, the time t1 from quenching to the start of tempering is 1 day (1440 minutes) or longer, and the elapsed time t3 from the end of tempering is within 60 minutes, yield strength. In any case, the yield strength is less than 400 MPa, and it is understood that the plate shape can be effectively corrected even with straightening equipment for ordinary steel. As a combination of t1 and t3, t1 is preferably 1.5 days or more, t3 is 40 minutes or less, more preferably t1 is 2 days or more, and t3 can be defined as 20 minutes or less. In terms of productivity and securing a waiting yard, it is difficult to unnecessarily take a long time.
On the other hand, as shown in the comparative example, when tempering is performed in less than one day after quenching and / or after 60 minutes or more have elapsed after tempering, the yield strength of the high-strength steel is 400 MPa. As a result of the above, the plate shape straightening operation using the straight steel straightening device cannot meet the shipping standards.

  According to the steel sheet straightening method according to the present invention, it is possible to give an appropriate degree of processing to the high-tensile steel plate after quenching treatment, and to obtain good post-correction flatness, so industrial applicability is There is a big one.

RQ Roller quench equipment CL Cold roller leveler OL Hydraulic press leveler

Claims (4)

C: 0.50% or less, Si: 0.70% or less, P: 0.05% or less, S: 0.05% or less, O: 0.007% or less, N: 0.01% or less in mass% And containing at least one of Ni, Cr and Mo in a range of Ni: 9.5% or less, Cr: 2.5% or less, Mo: 1.2% or less, the remainder being inevitable the steel sheet consisting of impurities and Fe, after the steel sheet structure and austenite is heated in a heat treatment furnace, a quenching step of performing quenching Ru the steel sheet is transformed to steel sheet structure with water cooling to martensite,
A tempering step of the high-tensile steel plate subjected to tempering,
A shape correction step of correcting the shape of the high-tensile steel sheet that has been tempered;
Have
In the straightening process, the time until hardening after-tempering (t1) of not less than 24 hours, the time from the tempering ends to correct (t3) as within 60 minutes, high-tensile steel plate shape and performing corrective Correction method. Before Symbol steel plate, further, 0.5% or less including the Cu, straightening method of high-tensile steel plate shape according to 請 Motomeko 1. Yield strength after quenching and tempering, the final to 60% or less times of arrival intensity, cormorants row corrective straightening method of high-tensile steel plate shape according to 請 Motomeko 1 or 2. The yield strength after quenching and tempering, is predicted on the basis of t1, t3 and previously determined .sigma.y _0.2%, in the time zone .sigma.y _0.2% is low, we implement corrective, of 請 Motomeko 1-3 The correction method of the high-tensile steel plate shape according to any one of the above items.

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