JP4438600B2 - Hot-rolled steel strip and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel strip and a method for producing the same. In particular, the present invention relates to a hot-rolled steel strip suitable for applications that are subjected to severe molding of wheel disks and the like and require a beautiful surface appearance, and a method for producing the same.

  A so-called hot-rolled steel sheet manufactured by hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including automobiles. In recent years, application to automobile undercarriage parts that are subjected to severe processing such as stretch flange processing and produced in large quantities has been increasing in recent years.

  In order to improve stretch flange workability and overhang workability, it is known that Si addition is effective as disclosed in Patent Document 1, for example. However, since the high Si content steel has reduced scale peelability in hot rolling, it has a scale indentation wrinkle called an island-shaped scale wrinkle having an average surface roughness Ra of 2.0 μm or more after pickling, There was a problem with the surface appearance. In order to ensure a beautiful surface, the entire surface after pickling is preferably 1.5 μm or less, more preferably 1.0 μm or less in terms of average surface roughness Ra.

For example, Patent Document 2 discloses a technique for increasing the descaling pressure to, for example, a high pressure of 5.0 to 30.0 kgf / cm ² as a technique for preventing island-like scale defects. However, high-pressure descaling is a very expensive equipment, its maintenance is difficult, and it is disadvantageous in cost. In addition, it is difficult to obtain a higher impact pressure in a wide range, and it is difficult to obtain a uniform and beautiful surface in a wide range such as a short side of 200 mm or more and a long side of 100 m or more that can be provided in a coil shape. Hot-rolled steel sheets are usually supplied in the form of a coil and cut into a cut plate just before forming. If there is a surface defect even in a part of the coil, it is necessary to extract it at the time of cutting or after forming the cut plate. It was a hassle.

As a method that does not use high-pressure descaling, for example, the technique of Patent Document 3 is disclosed, but its effect is insufficient for suppressing island-like scale defects of steel containing Si.
Japanese Unexamined Patent Publication No. 6-17924 JP 7-70649 A JP 2002-28713 A

  An object of the present invention is to solve the problems of the prior art as described above, and to provide a hot-rolled steel strip having high workability and a good surface property over a wide range, and a method for producing the same. In particular, the addition of Si from the viewpoint of workability, and the steel satisfying (5xP + Al) /Si≦0.728 is extremely difficult to suppress island-like scale flaws. It is intended for steel having a component satisfying /Si≦0.728.

  As a result of diligent experiments, the present inventors have been able to obtain a Si-containing hot-rolled steel sheet having a beautiful surface in a wide range, which could not be produced in the past, and completed the invention. The hot-rolled steel strip can be provided in a large amount in the form of a coil, and it is not necessary to extract a defective surface or the like at the time of cutting a plate or in a pressing process.

Here, the present invention is as follows.
(1) By mass%, C: 0.02 to 0.25%, Si: 0.04 to 2.5%, Mn: 0.3 to 2.3%, P: 0.050% or less, S: 0.02% or less, Al: 0.005 to 0.50%, and N: 0.02 %, The balance is iron and impurities, and the P, Al and Si contents in the steel satisfy the following formula (I), and have tensile strength (MPa) and hole expansion A hot-rolled steel strip with a product of rate (%) of 40000 (MPa ·%) or more, no island-like scale ridges on the front and back surfaces, a short side of 200 mm or more and a long side of 100 m or more.

(5xP + Al) /Si≦0.728 (I)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet.
(2) The chemical composition further includes one or more selected from the group consisting of Ti: 0.3% or less, Nb: 0.3% or less, and V: 0.5% or less in terms of mass%. The hot-rolled steel strip described in (1).

  (3) The chemical composition is 1% selected from the group consisting of Mo: 1.0% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less. The hot rolled steel strip according to (1) or (2), wherein the hot rolled steel strip contains seeds or two or more kinds.

(4) The hot-rolled steel strip according to any one of (1) to (3), wherein the chemical composition further includes, by mass%, Ca: 0.01% or less.
(5) A method for producing a hot-rolled steel strip comprising the following steps (A) to (C):
(A) A steel ingot or steel slab having the chemical composition according to any one of (1) to (4) is held in a heating furnace at 1200 ° C. or more for less than 75 minutes, then extracted from the heating furnace, and then 120 seconds The process of performing rough hot rolling to make the surface temperature at the completion of rolling higher than the eutectic point of FeO and Fe ₂ SiO ₄ given by the following formula (II) to make a rough bar ;
(B) After the descaling of the rough bar, the finish hot rolling is started and the finish hot rolling at a surface temperature of 800 ° C. or higher is completed within 420 seconds after the step (A). And (C) a step of cooling and winding up to 700 ° C. or less at an average cooling rate of 10 ° C./s or more after the finish hot rolling.
( Eutectic point of FeO and Fe ₂ SiO ₄ ) = 168.15x ((5xP + Al) / Si) ² -245.12x (5xP + Al) / Si + 1170
(II)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet, and the unit of eutectic point of FeO and Fe ₂ SiO ₄ is ° C.

(6) A method for producing a hot-rolled steel strip comprising the following steps (a) to (d) :
(A) A steel ingot or steel slab having the chemical composition according to any one of (1) to (4) is held in a heating furnace at 1200 ° C. or more for less than 75 minutes, then extracted from the heating furnace, and then 120 seconds The process of applying descaling within and then subjecting it to rough hot rolling to form a rough bar;
(B) a step of heating the surface temperature of the coarse bar to the eutectic point or more of FeO and Fe ₂ SiO ₄ given by the following formula (II) after the completion of the rough hot rolling;
(C) After the step (b), after the descaling is performed on the rough bar, the finish hot rolling is started, and the finish hot rolling is performed at a surface temperature of 800 ° C. or higher when the rolling is completed (b) And (d) a step of cooling to 700 ° C. or lower at an average cooling rate of 10 ° C./s or higher after the finish hot rolling.
( Eutectic point of FeO and Fe ₂ SiO ₄ ) = 168.15x ((5xP + Al) / Si) ² -245.12x (5xP + Al) / Si + 1170
(II)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet, and the unit of eutectic point of FeO and Fe ₂ SiO ₄ is ° C.

  The hot-rolled steel sheet according to the present invention is optimal for applications that require strict burring forming such as wheels and disks and requires a beautiful surface, and the significance of the present invention having such effects is extremely remarkable.

Next, the invention specific matter and the operation of the present invention will be described in detail. In this specification, “%” indicating the chemical composition of steel is “% by mass” unless otherwise specified.
(A) Chemical composition of steel ingot or steel slab C: C is an element necessary for high strength. If the content is less than 0.02%, the effect is small. If the content exceeds 0.25%, the weldability deteriorates, so the content was determined to be 0.02 to 0.25%. A preferred range is 0.02 to 0.18%.

  Si: Si is an element necessary for achieving both high strength and stretch flangeability. If the content is less than 0.04%, the effect is small, and if the content exceeds 2.5%, the toughness decreases. Accordingly, the content is determined to be 0.04 to 2.5%. A preferred range is 0.15 to 1.7%, and most preferred is 0.4 to 1.5%.

  Mn: Mn is effective for increasing the strength and also has a function of fixing S as MnS and suppressing hot brittleness. If the content is less than 0.3%, the effect is small, and if the content exceeds 2.3%, the weldability deteriorates, so the content was determined to be 0.3-2.3%. Preferably it is 0.8 to 2.3%.

  P: P is an undesirable element that deteriorates toughness. Therefore, the content of P is set to 0.050% or less. However, since P has a function of improving the peelability of the scale, the content is preferably 0.005 to 0.03%, and more preferably 0.01 to 0.03%.

  S: S is an undesirable element that increases the amount of MnS and lowers stretch flangeability. Therefore, the content is determined to be 0.02% or less. Preferably it is 0.008% or less, More preferably, it is 0.004% or less.

  Al: Al is an important element that is used for deoxidation and improves the peelability of the scale. The effect is insufficient if it is less than 0.005%, and even if it exceeds 0.50%, the effect is saturated, which is disadvantageous in terms of cost. Therefore, it was determined as 0.005 to 0.50%. Preferably it is 0.03-0.50%, More preferably, it is 0.06-0.50%, Most preferably, it is 0.10-0.50%.

N: N is an impurity element. If the content is too high, the high temperature ductility is lowered and the problem of surface flaws occurs. An upper limit of 0.02% is allowed.
Ti, Nb, V: These are optional added components, which can be contained alone or in combination of two or more, and can contribute to increasing the strength of the steel sheet by precipitation strengthening. In order to acquire this effect, it is preferable to contain 0.003% or more of all. Since the effect is saturated even if the addition amount is too large, the content of each element is determined to be Ti: 0.3% or less, Nb: 0.3% or less, and V: 0.5% or less.

  Mo, Cr, Cu, Ni, B: These are optional added components, which can be used alone or in combination of two or more, and contribute to high strength through solid solution strengthening and transformation strengthening. In order to ensure this effect, it is preferable to contain 0.005% or more of each. However, excessive addition causes deterioration of chemical conversion property, deterioration of toughness and workability, so the content is Mo: 1.0% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less , B: determined to be 0.01% or less.

  Ca: Ca is also an optional additive, has a function of spheroidizing sulfide inclusions, and a function of improving workability required for local ductility such as hole expandability. In order to reliably obtain this effect, the content is preferably 0.0001% or more. However, even if the content exceeds 0.01%, the effect is saturated, so the content was determined to be 0.01% or less.

  In the present invention, the chemical composition further satisfies the following formula (I), but this is the object of the present invention is a high Si content steel that secures workability by adding Si. This is because one of the problems is to improve the scale peelability of such a high Si content steel.

(5xP + Al) /Si≦0.728 (I)
When the above formula (I) is not satisfied, the problem of scale peelability does not become serious, and the workability is not sufficient.

The balance is Fe and impurities. Examples of impurities include 0.01% or less of Sn.
According to the present invention, a hot-rolled steel sheet that does not have island-shaped scales on the front and back surfaces can be obtained. In this case, the “island-shaped scales” are residual scales that have not been peeled off by the descaling process as described above. This is an indentation flaw derived from the above, which can be easily discriminated visually, and indicates a surface roughness Ra after pickling of 2.0 μm or more. Therefore, the hot-rolled steel sheet according to the present invention does not have such a portion, and the average surface roughness Ra after pickling is 1.5 μm or less, more preferably 1.0 μm or less.

  Next, although the manufacturing method of the hot-rolled steel sheet concerning this invention is demonstrated, it shows a manufacturing example preferable from a practical use, and the hot-rolled steel sheet concerning this invention is restrict | limited only to what was manufactured by it. That's not to say.

  In the present invention, the steel having such a chemical composition is produced by, for example, a converter, an electric furnace or the like. The steel type may also be any of rimmed steel, capped steel, semi-killed steel or killed steel. Furthermore, as a steel ingot or steel slab as a raw material, either a steel slab obtained by ingot-splitting rolling or a slab obtained by continuous casting may be used.

(B) Hot rolling conditions In order to suppress island-like scale flaws in Si-containing steel, descaling after rough hot rolling and before finish hot rolling is important. The manufacturing method is basically based on performing descaling at a high temperature at which the scale is easily peeled after rough hot rolling with Si-containing steel, and then finishing hot rolling in a short time. Descaling at this time is possible with a low collision pressure, and therefore a good surface can be stably obtained over a wide range. The collision pressure is preferably 9.8 kPa (0.1 kgf / cm ² ) to 490 kPa (5 kgf / cm ² ), and more preferably 9.8 kPa (0.1 kgf / cm ² ) to 196 kPa (2 kgf / cm ² ).

The reason why the above conditions are optimal in the Si-containing steel according to the present invention is considered as follows.
A characteristic of Si-containing steel is that the generation rate of scale is slow. If the scale is too thin, it becomes difficult for the scale to exfoliate due to descaling.For efficient exfoliation, increase the end temperature of rough hot rolling or reheat to high temperature after completion of rough hot rolling to perform descaling. There is a need to do.

Here, according to the findings of the present inventors, this is the temperature at which Fe ₂ SiO ₄ to generate on the surface of the coarse bar is melted, in other words, the FeO and Fe ₂ SiO ₄ to produce a surface roughness bar It can be seen that if the temperature of the coarse bar is raised above the eutectic point, the strong oxidation inhibition effect of Fe ₂ SiO ₄ can be lost, so that even a coarse bar made of steel with high Si content can sufficiently generate oxide scale. It was.

  Furthermore, since the scale at the steel interface of the coarse bar is close to melting, peeling of the scale is easy. Since the descaling can be facilitated by these, the descaling can be surely performed without performing the descaling of the high collision pressure after that, and scale flaws are not generated.

In order to sufficiently generate the oxide scale, it is preferable to perform descaling after one second or more from the state of the high-temperature coarse bar.
As a result of experiments based on the new technical idea of the present inventors, a preferable temperature for improving the descaling is 168.15x ((5xP + Al) / Si) ² -245.12x (5xP + Al) / Si + It was 1170 (° C) or higher.

  If a thicker scale is generated after descaling, surface defects due to scale indentation will occur, but with Si-containing steel, even if it takes as much time as necessary to produce a wide area of 100 m on the long side, it will be a problem. Thick scale does not generate. However, since the scale becomes thick enough to cause a problem if it takes too long time, the rough bar is in a state above the above temperature, that is, after the end of the rough hot rolling in such a state, for example, or with a rough bar heater. After heating to the desired state, descaling and finish hot rolling must be completed within 420 seconds, that is, finished. Preferably it is within 300 seconds, more preferably within 180 seconds.

The descaling of the coarse bar is preferably performed at or above the eutectic point from the viewpoint of scale peeling.
In addition, the finish hot rolling is usually hot-rolled by a finishing mill with 5 or more stands and a hot rolling mill within 10 seconds between each pass. In order to bring the scale of the steel interface of the coarse bar into the above molten state, a method for increasing the temperature of the coarse hot rolling or heating with a coarse bar heater, a method of winding the coarse bar into a coil and putting it in a furnace, A method of holding the coarse bar at a high temperature with a heat insulating cover can be considered.

Furthermore, according to the preferred embodiment of the present invention, it is effective to join the rough bars before and after and perform continuous finish hot rolling before finish hot rolling in order to pass the plate stably.
In addition to the above, the scale generated in the heating furnace is a condition that is easy to remove after heating at a temperature of 1050 ° C or higher, within 120 seconds after extraction of the heating furnace, and at least once before the start of rough rolling. is there. For the above-mentioned reasons, by increasing the scale thickness by high-temperature heating and further performing descaling while maintaining the high temperature within 120 seconds, it is possible to effectively scale the scale generated in the heating furnace. . The heating temperature is preferably 1150 ° C or higher, more preferably 1200 ° C or higher, and most preferably 1230 ° C or higher. The time until descaling is preferably within 90 seconds, and more preferably within 60 seconds.

The heating time is not particularly specified, but the in-furnace time of 1200 ° C. or more is preferably less than 75 minutes from the viewpoint of saving labor and preventing the removal of a thick scale.
As other conditions, from the viewpoint of workability, it is preferable that the finishing temperature is 800 ° C. or higher, the winding temperature is 700 ° C. or lower, and the average cooling rate until winding is 10 ° C./s or higher. If the finishing temperature is less than 800 ° C, the structure becomes band-like and stretch flangeability decreases, and if the cooling rate is less than 10 ° C / s, or if the coiling temperature exceeds 700 ° C, carbides and the structure become coarse. Occurs, the local ductility is lowered, and the stretch flangeability may be lowered.

Note that air cooling for 1 second or more during the cooling from the completion of finish rolling to winding is also effective in improving elongation because ferrite formation is promoted.
The manufactured hot-rolled steel sheet is usually subjected to shape correction by skin pass rolling or scale removal by pickling, and rust preventive oil is applied to the surface.

  Thus, the steel plate manufactured by the present invention has good workability and good surface properties. In particular, with regard to the surface properties, a hot-rolled steel strip having no island-like scale ridges on the front and back surfaces and having excellent workability and surface properties with a short side of 200 mm or more and a long side of 100 m or more can be produced. When a customer uses such a hot-rolled steel strip, the preferable range of the surface is 400 mm or more on the short side, more preferably 700 mm or more, and the long side is 400 m or more, more preferably 600 m or more. In other words, according to the present invention, it is possible to provide a hot-rolled steel strip having an excellent surface property in which no island-like scale defects are observed in such a range.

  Next, the effects of the present invention will be described more specifically with reference to examples.

  Steel having the chemical composition shown in Table 1 was melted in a converter and then made into a slab by continuous casting. The hot slab is subjected to rough hot rolling to form a rough bar with a thickness of 36 mm, then finish hot rolling to a thickness of 3.6 mm, and a hot rolled steel strip having a thickness of 3.6 mm, a width of 1230 mm, and a length of 600 m. Manufactured.

  Table 2 shows the hot rolling conditions. Note that descaling was performed 30 seconds after the slab was extracted from the heating furnace. Moreover, when heating a rough bar, it performed with the rough bar heater which is high frequency induction heating.

  The rough bar after completion of the rough hot rolling is descaled after passing through the rough bar heater equipment, and is subjected to finish hot rolling. Note that descaling is not performed between the end of rough hot rolling and the passage of the rough bar heater.

  The time from the end of rough rolling shown in Table 2 to the completion of rough bar heater passing plate and the time from the completion of rough bar heater passing plate to the end of finish rolling are both determined by the tip of the rolled steel plate, that is, the top position. There was a difference of 50 seconds between the rolling top and the bottom from the completion of rough hot rolling to the completion of rough bar heater installation. The temperature is measured at the center position of the sheet width, and Table 2 shows the temperature at the rolling bottom position. When heating was not performed with the coarse bar heater, the bottom portion was 60 ° C. lower than the top portion at the temperature at which the passage through the coarse bar heater facility was completed.

  The obtained hot-rolled steel sheet was subjected to flattening with a skin pass and scale removal by pickling, and then the front and back surfaces were inspected while rewinding the coil. In addition, a JIS No. 5 tensile test piece was taken in the direction perpendicular to the rolling, and a tensile test was conducted, and further a hole expansion test was conducted to evaluate stretch flangeability. These results are shown in Table 3.

  In addition, the front and back inspection was performed after performing the pickling process on the conditions immersed in 5-20% hydrochloric acid aqueous solution at 60-90 degreeC. The length of the center is 1000mm and the length is 600m, and there is no island scale 疵. The range of the width center is 1000mm and there is one or more island scale 疵, but the length is 400m or more and there is no island scale 島. The one that was able to manufacture the coil was ◯, and there were one or more island-shaped scale ridges, but the one that was able to produce a coil without an island-shaped scale moth with a width of 200 mm and a length of 100 m or more was △, Even if the width was cut with a slit, a coil having a width of 200 mm and a length of 100 m or more without an island-like scale was not indicated by x, and the results of front and back inspection are shown in Table 3. In addition, more than (triangle | delta) ((triangle | delta), (circle), (double-circle)) was set as the pass. Further, when the average roughness Ra of the island-shaped scale portion was measured, it was 2.0 μm or more, and when the Ra of the portion having no island-shaped scale defects was measured, it was 1.0 μm or less.

  Test Nos. 1 to 4 and 15 to 32, which are steels of the present invention, showed good hole expansion properties of 50% or more, and also exhibited good surface properties satisfying the scope of the present invention. Test Nos. 5, 6, 9, 10, 11 in which both the rough rolling end temperature and the coarse bar heater heating temperature were low, and test Nos. 7 and 8 in which the time from the end of the rough rolling or the heating of the coarse bar heater to the completion of the finish rolling was long The surface was bad. Test No. 12 where the finishing temperature was low and Test Nos. 13 and 14 where the cooling rate after hot rolling was low or the coiling temperature was high were inferior in hole expansibility.

  Test No. 33 in which P was out of the scope of the present invention had a good surface, but was inferior in hole expansibility. Test No. 34 in which Si was out of the scope of the present invention was inferior in hole expansibility. Test number 35 with a high C was inferior in hole expansibility.

Claims (6)

In mass%, C: 0.02 to 0.25%, Si: 0.04 to 2.5%, Mn: 0.3 to 2.3%, P: 0.050% or less, S: 0.02% or less, Al: 0.005 to 0.50%, and N: 0.02% or less It has a chemical composition in which the balance is composed of iron and impurities, and the contents of P, Al, and Si in the steel satisfy the following formula (I): tensile strength (MPa) and hole expansion rate (% ) Product of 40000 (MPa ·%) or more, no island-scale scales on the front and back surfaces, hot-rolled steel strip with a short side of 200 mm or more and a long side of 100 m or more.
(5xP + Al) /Si≦0.728 (I)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet.   The chemical composition further includes one or more selected from the group consisting of Ti: 0.3% or less, Nb: 0.3% or less, and V: 0.5% or less in terms of mass%. The hot rolled steel strip according to 1.   The chemical composition is 1% or 2 selected from the group consisting of Mo: 1.0% or less; Cr: 1.0% or less; Cu: 1.0% or less; Ni: 1.0% or less; and B: 0.01% or less. The hot-rolled steel strip according to claim 1 or 2, comprising a seed or more.   The hot-rolled steel strip according to any one of claims 1 to 3, wherein the chemical composition is mass% and further contains Ca: 0.01% or less. A method for producing a hot-rolled steel strip comprising the following steps (A) to (C):
(A) The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is held in a heating furnace at 1200 ° C. or more for less than 75 minutes, and then extracted from the heating furnace, and then within 120 seconds A step of performing rough hot rolling to make the surface temperature at the time of completion of rolling higher than the eutectic point of FeO and Fe ₂ SiO ₄ given by the following formula (II) to form a rough bar after performing descaling ;
(B) After the descaling of the rough bar, the finish hot rolling is started and the finish hot rolling at a surface temperature of 800 ° C. or higher is completed within 420 seconds after the step (A). And (C) a step of cooling and winding up to 700 ° C. or less at an average cooling rate of 10 ° C./s or more after the finish hot rolling.
( Eutectic point of FeO and Fe ₂ SiO ₄ ) = 168.15x ((5xP + Al) / Si) ² -245.12x (5xP + Al) / Si + 1170
(II)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet, and the unit of eutectic point of FeO and Fe ₂ SiO ₄ is ° C. A method for producing a hot-rolled steel strip comprising the following steps (a) to (d) :
(A) A steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is held at 1200 ° C. or more for less than 75 minutes in a heating furnace, then extracted from the heating furnace, and then within 120 seconds The step of descaling and then rough hot rolling to form a rough bar;
(B) a step of heating the surface temperature of the coarse bar to the eutectic point or more of FeO and Fe ₂ SiO ₄ given by the following formula (II) after the completion of the rough hot rolling;
(C) After the step (b), after the descaling is performed on the rough bar, the finish hot rolling is started, and the finish hot rolling is performed at a surface temperature of 800 ° C. or higher when the rolling is completed (b) And (d) a step of cooling to 700 ° C. or lower at an average cooling rate of 10 ° C./s or higher after the finish hot rolling.
( Eutectic point of FeO and Fe ₂ SiO ₄ ) = 168.15x ((5xP + Al) / Si) ² -245.12x (5xP + Al) / Si + 1170
(II)
Here, P, Al, and Si in the formula are the contents (mass%) of each element in the steel sheet, and the unit of eutectic point of FeO and Fe ₂ SiO ₄ is ° C.