JP4333356B2 - Cold rolled steel sheet manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a cold-rolled steel sheet, and more particularly to a method for producing a cold-rolled steel sheet having excellent workability such as ductility and stretch flangeability.

  Cold-rolled steel sheets are usually manufactured through an annealing process in which a steel slab adjusted to a specific component composition is hot-rolled, then pickled, cold-rolled, and imparted with workability. The annealing step is generally performed by either batch annealing or continuous annealing. Batch annealing is an annealing method that uses a box-type annealing furnace to stack several layers of tightly wound tight coils or loosely wound open coils and heat-treat them in a reducing atmosphere. Is about 100 ° C./hr in the case of a direct heating type heating system.

  On the other hand, continuous annealing is an annealing method in which a continuous annealing line is used to heat in a strip state while rewinding a coil, to anneal in a reducing atmosphere, and to cool. The heating in this continuous annealing is generally performed by direct flame reduction heating, non-oxidation heating, radiant tube heating, etc., and the heating rate is about 10 ° C / sec, which is very fast compared to batch annealing. It is a feature. As for the rapid heating annealing method, Non-Patent Document 1 discloses that a strip having a thickness of 0.9 mm and a width of 600 to 750 mm is rapidly heated at a rate of about 200 ° C./sec using an electromagnetic induction heating device. A technique that can suppress the temperature distribution in the width direction within ± 2% has also been proposed.

On the other hand, new materials using the rapid heating annealing as described above have also been developed. For example, in Non-Patent Document 2, a hot-rolled steel sheet containing 0.003 mass% and 0.05 mass% of C is cold-rolled at a reduction rate of 70%, and then the heating rate is changed from 50 ° C / sec to 1000 ° C / sec. A technique is disclosed in which the crystal grain size can be reduced from 20 μm to 12 μm and from 6 μm to 5 μm, respectively, by performing a heat treatment that is increased to 5 μm. Moreover, in patent document 1, in the manufacturing method of the cold-rolled steel plate using low carbon steel, a hot-rolled coil is wound up at high temperature of 650-800 degreeC, a carbide | carbonized_material is coarsened, and it anneals after the subsequent cold rolling. By heating at a rate of 100 ° C / sec or more in the process and suppressing the re-dissolution of the precipitated coarse carbide, the fine precipitation in the cementite grains in the subsequent cooling and overaging treatment is suppressed, and the grains are soft Therefore, a method of manufacturing a cold-rolled steel sheet having improved aging and excellent workability with improved uniform elongation has been proposed. Furthermore, in Patent Document 2, in a method for producing a cold-rolled steel sheet using low-carbon steel, after hot rolling, the steel sheet is rapidly cooled, wound at a temperature of 270 ° C. or less, and a large amount of solute C is contained in the hot-rolled sheet. A method for producing a cold-rolled steel sheet excellent in aging and formability by allowing it to remain and performing heating in the annealing process after cold rolling at a rate of 1 to 100 ° C./sec and finely depositing cementite. Proposed.
Metallurgical Plant and Technology International 6/2001, p.70 D. Muljono, M. Ferry, DPDunne, `` Influence of heating rate on anisothermal recrystallization in low and ultra-low carbon steels '', Materials Science and Engineering A303 (2001), 90-99 Japanese Patent Application Laid-Open No. 07-216459 JP 60-258429 A

  However, since the cooling rate after winding the hot-rolled coil described in Non-Patent Document 2 is described as 20 ° C./hr, in this technique, the coil after winding is naturally cooled. Conceivable. In such a hot-rolled sheet that is slowly cooled, cementite precipitates during cooling and grows greatly. And when such a hot-rolled sheet is cold-rolled and rapidly heated, it is known that the time for coarse cementite to dissolve cannot be secured and remains. The remaining coarse cementite becomes the starting point of cracking when the steel sheet is subsequently processed, and greatly deteriorates the local elongation. In particular, in processing that requires excellent local elongation such as hole expansion processing, there is a problem that workability is significantly impaired. In addition, in order to redissolve coarse cementite, if it is held for a long time at a high temperature after rapid heating, the fine particles obtained by rapid heating become coarse, and the effect of rapid heating cannot be obtained. There's a problem. Furthermore, since the recrystallization is suppressed in the periphery of cementite because the C concentration is high, when coarse cementite is present, recrystallization proceeds non-uniformly and the structure after recrystallization is also non-uniform. Thus, there is a problem that the workability deteriorates.

  Moreover, the technique described in patent document 1 winds up the coil after hot rolling at high temperature, and coarsens cementite. Therefore, similarly to the technique of Non-Patent Document 2, there is a problem that cementite remains undissolved by rapid heating in annealing after cold rolling, the local elongation is greatly reduced, and the workability is remarkably impaired. Furthermore, since the technique of Patent Document 2 lowers the coiling temperature after hot rolling and performs cold rolling with a large amount of solid solution C remaining in the hot rolled sheet, deformation during cold rolling is performed. There is a problem that resistance increases. Furthermore, when a large amount of solute C is present, the shear strain introduced into the steel sheet during cold rolling becomes non-uniform, resulting in non-uniform recrystallized structure after annealing, leading to a decrease in workability. There is also.

  The object of the present invention is to solve the above-mentioned problems of the prior art and to obtain a cold-rolled steel sheet that is excellent in workability, particularly ductility and stretch flangeability, even if the steel sheet after cold rolling is rapidly heated and annealed. It is to propose an advantageous manufacturing method.

  The inventors diligently studied to solve the problems in the rapid heating annealing. As a result, when hot-rolled steel sheet containing a predetermined amount of C is cold-rolled and subjected to rapid heating annealing, the coiling temperature after hot rolling is set to an appropriate range, and after winding on a coil, By forcibly cooling within a predetermined time, the cementite of the hot-rolled steel sheet can be refined and an appropriate amount of solid solution C can be left. Therefore, the structure is uniformly refined by subsequent rapid heating annealing, and cooling with good workability is achieved. The present inventors have found that a rolled steel sheet can be obtained and have come to the present invention.

That is, the present invention, C: 0.02~0.2mass%, Si: 0.1~ 2.0mass%, Mn: 3.0mass% or less, P: 0.2 mass% or less, S: 0.01 mass% Hereinafter, hot rolled steel slab containing Al: 3.0 mass% or less, N: 0.02 mass% or less , the balance consisting of Fe and unavoidable impurities , wound at a temperature of 300-650 ° C., then In the manufacturing method of the cold-rolled steel sheet, which is forcibly cooled within 10 to 60 minutes, then pickled and cold-rolled, and heated to 600 ° C or higher and a soaking temperature at 30 ° C / sec or higher and annealed. is there.

  In addition to the above component composition, the steel slab of the present invention contains at least one selected from Ti: 1.0 mass% or less, Nb: 1.0 mass% or less, and V: 1.0 mass% or less, and / Or in addition to the above component composition, Cr: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 1.0 mass% or less and B: 0.01 mass% or less preferable.

  In addition, the cold-rolled steel sheet of the present invention is not only a cold-rolled steel sheet manufactured by rapid heating annealing in a continuous annealing line after cold rolling, but also a surface on which surface treatment such as electroplating or coating treatment is performed thereafter. Includes treated steel sheet. Furthermore, the cold-rolled steel sheet of the present invention includes a hot-dip galvanized steel sheet that is rapidly heated and annealed in a continuous hot dip galvanizing line and then subjected to a hot dip galvanizing process, and then an alloyed hot dip galvanized steel sheet that is subjected to alloying treatment. Is included.

  According to the present invention, a steel slab containing a predetermined amount of C is used as a raw material, and rapid heating annealing is performed after cold rolling by controlling the winding temperature and the cooling condition after winding in the hot rolling process. In addition, a uniform and fine structure can be obtained, and a cold-rolled steel sheet having excellent workability can be produced.

The method for producing a cold-rolled steel sheet according to the present invention comprises hot rolling a steel slab containing 0.02 to 0.2 mass% of C as a basic component, forcibly cooling the steel slab within a predetermined time after winding at a predetermined temperature, and pickling. Then, after cold rolling, rapid heating annealing is performed.
The reason for limiting the component composition of the cold-rolled steel sheet according to the present invention will be described.
C: 0.02-0.2 mass%
C promotes the structural change by precipitation of fine cementite or pearlite or the formation of low-temperature transformation phase such as bainite or martensite, and is therefore an effective element for increasing the strength. However, if the amount of C is less than 0.02 mass%, it becomes difficult to precipitate cementite during cooling after winding the hot-rolled coil, and a large amount of solute C remains in the steel. This large amount of solute C promotes the introduction of non-uniform shear strain in the subsequent cold rolling, makes the recrystallized structure after annealing non-uniform, and causes a decrease in workability. On the other hand, if the C content exceeds 0.2 mass%, it causes an increase in the amount of pearlite and coarsening of cementite, which deteriorates the workability (ductility, stretch flangeability) of the steel sheet after annealing, and also adversely affects weldability. . Therefore, the addition range of the C amount is 0.02 to 0.2 mass%. Preferably, it is 0.04 to 0.1 mass%.

Cold-rolled steel sheet according to the present invention, components other than C, the Si, Mn, P, S, it is preferable that Al and N contained in the following range.
Si: 0.1 to 2.0 mass %
Si solidifies and strengthens ferrite without degrading workability, and improves the balance between strength and workability, so it can be added according to the required strength level. However, since addition exceeding 2.0 mass % deteriorates toughness and weldability, the upper limit of the Si addition amount is preferably 2.0 mass %. In addition, when slab heating is performed before hot rolling, Si promotes the generation of firelite on the surface and promotes the generation of a surface pattern called a so-called red scale. May cause unplating. Therefore, when manufacturing cold-rolled steel sheets and hot-dip galvanized steel sheets that require good surface properties, the upper limit of the amount of Si added is preferably 0.5 mass%, and desirably 0.2 mass% or less. is there. Incidentally, when adding Si to securing strength purposes, if the content is too small, since the strength and workability of the balance of the steel sheet is low, it added 0.1mass %% more.

Mn: 3.0mass% or less
Since Mn is an element effective for increasing the strength through solid solution strengthening and structure strengthening, it can be added according to the required strength level. However, since addition of a large amount of Mn causes deterioration of weldability, the upper limit is preferably set to 3.0 mass%. More preferably, it is 2.5 mass% or less. In addition, since Mn is effective in fixing S which degrades hot workability as MnS and avoiding hot cracking, it is preferable to add 0.2 mass% or more.

P: 0.2 mass% or less P is an element that can be solid-solution strengthened without harming workability, and is therefore effective for increasing the strength. In addition, in the case of Si-added steel, there is an effect of suppressing the generation of red scale, so that it can be added as necessary. However, the addition of a large amount of P promotes segregation to grain boundaries, lowers ductility and toughness, and degrades weldability. Therefore, the upper limit of P is preferably 0.2 mass%. In addition, P causes a delay in the alloying speed of the hot dip galvanized steel sheet, so when manufacturing an galvannealed steel sheet, it is desirable that the upper limit is 0.1 mass%. On the other hand, P is an element inevitably contained in the steel, and an extreme reduction in the P content only causes an increase in production cost. Therefore, the lower limit is preferably set to 0.005 mass%.

S: 0.01 mass% or less S is an element that remarkably reduces hot ductility, induces cracking during hot rolling, and significantly deteriorates surface properties. S does not contribute to the strength, but also forms coarse MnS as an impurity element. Further, in the case of Ti-added steel, a large amount of coarse Ti-based sulfides are produced, and corrosion resistance, ductility, Since stretch flangeability is deteriorated, S is preferably reduced to 0.01 mass% or less. In order to further improve the ductility and stretch flangeability, S is more preferably 0.005 mass% or less.

Al: 3.0mass% or less
Al is a solid solution strengthening element and is an element effective for increasing the strength. However, since Al is a ferrite stabilizing element, the addition of a large amount leads to an increase in the transformation point from austenite to ferrite in hot rolling, coarsening the grain size of the hot rolled steel sheet, and processing the steel sheet after annealing. Reduce sex. Moreover, the addition of Al as a strengthening element causes an increase in cost. Therefore, Al is preferably added as necessary within a range of 3.0 mass% or less. Note that Al is an effective element as a deoxidizing element and has an effect of reducing inclusions in the steel. However, when a large amount is added, alumina inclusions increase, leading to deterioration of ductility as well as deterioration of surface properties and internal properties. Therefore, when these characteristics are regarded as important, Al is more preferably 0.1 mass% or less. Moreover, since Al improves the aging resistance of a steel plate by fixing solid solution N as AlN, it is preferable to contain 0.02 mass% or more.

N: 0.02 mass% or less N is an element that combines with Al to form fine AlN and contributes to an increase in the strength of the steel sheet. Moreover, bake hardenability can be acquired by making solid solution N remain in steel as needed. However, if it is added in a large amount exceeding 0.02 mass%, slab cracking may occur during hot rolling, and surface defects may occur. Therefore, it is preferable that the N content has an upper limit of 0.02 mass%. Note that N is preferably reduced to 0.005 mass% or less when aging resistance is required.

In addition to the above components, the cold-rolled steel sheet of the present invention is further one or more selected from Ti, Nb and V, and / or one or more selected from Cr, Ni, Mo and B. Can be contained in the following range.
One or more selected from Ti: 1.0 mass% or less, Nb: 1.0 mass% or less, and V: 1.0 mass% or less
Ti, Nb and V are all carbonitride-forming elements, and while carbonitrides are finely precipitated, the recrystallization and grain growth are suppressed and the steel grains are refined to increase the strength. It is a contributing element and can be added as necessary. In order to acquire the said effect, it is preferable to add Ti: 0.02 mass% or more, Nb: 0.005 mass% or more, and V: 0.05 mass% or more, respectively. However, when a large amount of Ti, Nb, V is added, the carbonitride cannot completely dissolve at the time of reheating in the normal hot rolling process, and coarse carbonitride remains, so that the strength Does not contribute much to the rise. In addition, when steel slabs obtained by continuous casting are hot-rolled as they are without cooling and reheating, the addition of Ti, Nb and V increases the strength even if added over 1.0 mass%. The contribution to is small and only increases the alloy costs. Furthermore, addition of a large amount of Ti, Nb, V increases the strength of the hot-rolled steel sheet, so the rolling load of cold rolling increases, leading to an increase in manufacturing cost. Therefore, Ti: 1.0 mass% or less, It is preferable to add in the range of Nb: 1.0 mass% or less and V: 1.0 mass% or less.

One or more selected from Cr: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 1.0 mass% or less, and B: 0.01 mass% or less
Cr, Ni, Mo, and B are all elements that contribute to strength increase by improving the hardenability and strengthening the structure, and can be added as necessary. In order to acquire the said effect, it is preferable to add Cr: 0.1 mass% or more, Ni: 0.1 mass% or more, Mo: 0.1 mass% or more, and B: 0.0005 mass% or more, respectively. However, addition of a large amount of Cr, Ni, Mo, and B leads to an increase in alloy cost. Therefore, Cr is added in a range of 1.0 mass% or less, Ni: 1.0 mass% or less, and Mo: 1.0 mass% or less, respectively. It is preferable. Addition of a large amount of B not only saturates the effect of improving hardenability, but also significantly increases the load load during hot rolling due to the recrystallization suppressing action, so B is added in the range of 0.01 mass% or less. It is preferable.

  In the present invention, the balance other than the above components contained in the material is preferably Fe and inevitable impurities, but may contain other elements as long as the effects and effects of the present invention are not impaired. I do not care.

Next, the manufacturing conditions of the cold rolled steel sheet according to the present invention will be described.
As a method for melting steel having the above component composition, generally known methods such as a converter method and an electric furnace method can be appropriately applied. The melted steel is made into a steel slab by a continuous casting method or an ingot-bundling rolling method, and is subjected to hot rolling as it is or after being cooled and reheated. The rolling conditions such as the rolling temperature and the rolling reduction in the hot rolling need not be particularly defined, and may be performed according to ordinary methods.
The hot-rolled steel sheet after hot rolling is wound at the following winding temperature, forcibly cooled within the following predetermined time, and then subjected to normal pickling and cold rolling, and then an annealing process with rapid heating. To manufacture.

Winding temperature: 300-650 ° C
The coiling temperature after hot rolling needs to be in the range of 300 to 650 ° C. When the coiling temperature exceeds 650 ° C., cementite precipitated in the steel sheet becomes coarse during cooling after winding. Therefore, in rapid heating annealing performed after cold rolling, cementite cannot be completely dissolved and remains, resulting in significant deterioration of workability. In addition, the remaining cementite can be dissolved by holding it at a soaking temperature for a long time after rapid heating, but an increase in the soaking time leads to a coarsening of the crystal grain size, resulting in fine grains by rapid heating. The effect is lost. Furthermore, since the C concentration is high around the cementite and recrystallization is suppressed, when coarse cementite is present, the recrystallization proceeds non-uniformly and the steel sheet structure after annealing becomes non-uniform and processed. Deteriorates. On the other hand, when the coiling temperature is below 300 ° C., cementite cannot be precipitated during cooling, and a large amount of solute C remains. Therefore, shear strain is introduced non-uniformly during cold rolling, the recrystallized structure after annealing becomes non-uniform, and the workability is reduced. Therefore, the coiling temperature after hot rolling needs to be in the range of 300 to 650 ° C.

Cooling start time: within 10 to 60 minutes after winding The cooling condition after winding the hot-rolled coil is a part of the present invention together with the winding temperature described above, and between 10 and 60 minutes from the end of winding. In addition, forced cooling is required. As a result, cementite is appropriately precipitated and solid solution C is moderately secured. Specifically, fine carbide having a diameter of approximately 1 μm or less is precipitated, and about 0.001 to 0.008 mass% of C is dissolved. This is because excellent workability can be obtained after cold rolling and annealing. When the time from the end of winding to the start of cooling is less than 10 minutes, the precipitation of cementite becomes insufficient, and a large amount of solid solution C exists in the steel. As a result, introduction of non-uniform shear strain is promoted in cold rolling, resulting in deterioration of workability in the steel sheet after recrystallization annealing. Conversely, when the time from the end of winding to the start of cooling exceeds 60 minutes, the precipitated cementite becomes coarse, and this coarse cementite cannot be completely dissolved in the rapid heating process of annealing, so it remains in the steel. This is the starting point of processing cracks. Moreover, when precipitation progresses so that cementite coarsens, most of the solid solution C in steel becomes cementite and hardly remains. Further, C, which is decomposed and re-dissolved during rapid heating, is present in the vicinity of the old cementite, so the C concentration in this part temporarily increases, and recrystallization of that part is suppressed, and the steel sheet after annealing The structure becomes non-uniform and the workability is reduced. In this respect, when the coil is forcibly cooled for 10 to 60 minutes after coil winding is completed, a certain amount of solid solution C (about 0.001 to 0.008 mass%) remains in the steel. Even if the amount of solute C in the vicinity of cementite increases, the difference from the amount of solute C in the matrix is small. Therefore, recrystallization proceeds uniformly, the structure after annealing becomes uniform, and workability can be improved.

  In addition, it is necessary to wind the hot-rolled coil at a predetermined temperature and hold it for a certain period of time, followed by forced cooling. The cooling at this time is forced to a cooling rate of 200 ° C or lower and 100 ° C / hr or higher Cooling is preferred. This is because when the wound hot-rolled coil is allowed to cool naturally, it has a very slow cooling rate of about 15 ° C./hr. Therefore, when the coil after hot rolling is allowed to cool naturally, the result is the same as extending the holding time after winding, resulting in precipitation of coarse cementite in the steel and deterioration of workability. . In order to suppress such coarse cementite precipitation, the coil after being held for a certain period of time after winding needs to be forcibly cooled to a temperature of 200 ° C. or less that does not affect the precipitation of cementate. The cooling rate is preferably 100 ° C./hr or more. From the standpoint that the solid solution C is uniformly dispersed and left, it is preferably 300 ° C./hr or more. As a method for forcibly cooling the coil, any method such as a method of immersing the coil after winding in cooling water as it is, a method of spraying cooling water, or a cooling method by blowing air may be used. Moreover, the method of using the said cooling method together may be sufficient, rewinding a coil.

  After the hot-rolled coil is forcibly cooled, it is pickled and cold-rolled according to a conventional method. In this cold rolling, it is preferable to set the rolling reduction to 50% or more from the viewpoint of giving processing strain necessary for recrystallization and refining the crystal grains after recrystallization. On the other hand, an excessive reduction ratio is preferably 85% or less because it increases the rolling load.

Annealing heating rate: 600 ° C. or higher up to a soaking temperature of 30 ° C./sec or higher Continuous cold rolling is followed by continuous annealing to recrystallize the steel sheet and impart workability. The heating rate of this annealing is a portion that forms the basis of the present invention together with the above-described coiling temperature after hot rolling, and it is necessary to rapidly heat from 600 ° C. to a soaking temperature at 30 ° C./sec or more. When the heating rate is as low as less than 30 ° C./sec, strain recovery is preceded during heating, and the frequency of recrystallization nucleation decreases, so that fine recrystallized grains cannot be obtained. Furthermore, the decrease in the frequency of nucleation promotes non-uniform growth of crystals and makes the recrystallized grains non-uniform in size, leading to a decrease in workability. On the other hand, when the heating rate is set to 30 ° C./sec or more, recovery of strain during heating is suppressed, and recrystallization nuclei are randomly generated at once, so that uniform and fine recrystallized grains are obtained. be able to.

  The reason why the temperature range of the heating rate is limited to 600 ° C. or higher and the soaking temperature is that in continuous annealing, the temperature that substantially affects strain recovery and nucleation is in the high temperature range of 600 ° C. or higher. This is because the influence of the heating rate in the temperature range below 600 ° C. is small. Therefore, the heating rate up to 600 ° C. is not particularly limited. The soaking temperature in the present invention means the highest temperature reached during annealing that is 600 ° C. or higher and the recrystallization temperature or higher, and the chemical composition of the steel sheet, the characteristics required for the steel sheet after annealing (mechanical characteristics) ) And the like. A suitable soaking temperature is in the range of 700-900 ° C, more preferably 800-850 ° C. The holding time at the soaking temperature, so-called annealing time, is too long, leading to the coarsening of the crystal grains and causing the workability to deteriorate, so it is preferably set to 10 seconds or less, immediately after reaching the soaking temperature. It may be so-called soaking for 0 seconds at which cooling is started, and can be appropriately determined according to the chemical composition of the material, the characteristics (mechanical characteristics) required for the steel sheet after annealing, and the like. The cooling after the soaking is not particularly required, and may be slow cooling or rapid cooling, and may be further subjected to an overaging treatment after cooling. In particular, when it is produced as a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet, it may be processed by a heat treatment cycle in the process.

  As the furnace used for annealing, a continuous hot dip galvanized steel sheet, a continuous hot dip galvanized steel line for alloyed hot dip galvanized steel sheets and the like can be used in addition to a continuous annealing line for ordinary cold rolled steel sheets. In addition, the method of rapid heating is not particularly limited, but the method of heating from room temperature to soaking temperature by induction heating or direct current heating method, or the method of heating directly from room temperature to 600 ° C, direct heating method or radiant tube heating method It is possible to use a method in which heating is performed slowly, etc., and rapid heating is performed at 600 ° C. or higher by induction heating or direct current heating.

  Steel A having a component composition of C: 0.08 mass%, Si: 0.2 mass%, Mn: 1.5 mass%, P: 0.01 mass%, S: 0.003 mass%, Al: 0.03 mass% and N: 0.002 mass% is vacuumed Melted in a melting furnace, cast into a 50kg steel ingot, this steel ingot was reheated to 1250 ° C, hot rolled to a final finishing temperature of 870 ° C with an experimental rolling mill, and a plate thickness of 3.0mm Then, after cooling to a temperature corresponding to the coiling temperature (coiling temperature) and performing a heat treatment for holding at that temperature for a certain period of time, under the conditions shown in Table 1, It was cooled to room temperature by performing furnace cooling corresponding to natural cooling or air cooling corresponding to forced cooling. As shown in Table 1, the winding treatment temperature was changed between 280 and 670 ° C., and the holding time at that temperature was changed between 5 and 80 minutes. The cooling rate after the winding process was 15 ° C./hr for furnace cooling (corresponding to natural cooling after winding) and 600 ° C./hr for air cooling (corresponding to forced cooling after winding). . About the said hot-rolled steel plate, the cementite diameter and the amount of solute C were measured. As for the cementite diameter, after grinding a cross section parallel to the rolling direction and corroding with a picral solution, three fields of view were imaged at 3000x using a scanning electron microscope, and image analysis of the equivalent circle diameter (diameter) of all cementite was performed. The average value was defined as the cementite diameter. Moreover, the amount of solid solution C was calculated | required by measuring the internal friction of a hot-rolled sheet and converting from snake peak height.

  Further, the hot-rolled steel sheet is pickled to remove the surface scale, and cold-rolled at a reduction rate of 60% to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. Heated at a heating rate of 100 ° C / sec, heated from 600 ° C to a soaking temperature (850 ° C) at a heating rate of 20 to 300 ° C / sec, held at the soaking temperature for 1 second, and then cooled. It was cooled to 300 ° C. at a rate of 50 ° C./sec, subjected to an overaging treatment that was maintained at that temperature for 300 seconds, and a heat treatment corresponding to continuous annealing was performed to air-cool to room temperature.

About the cold-rolled steel plate after the said annealing, the hole expansion rate for evaluating a tensile characteristic and stretch flangeability was measured. For tensile properties, JIS No. 5 test piece specified in JIS Z 2201 was sampled in the direction perpendicular to the rolling direction, subjected to a tensile test according to JIS Z 2241, and measured for tensile strength (TS) and elongation at break (El). . In addition, the hole expansion rate is determined by punching a 10mmφ hole into a cold-rolled steel sheet, pressing a 60 ° conical punch into the hole, and expanding the hole diameter. And measure the following formula:
Hole expansion ratio λ (%) = (d−d ₀ ) / d ₀ × 100
Here, d: hole diameter when the crack penetrates the plate thickness (mm), d ₀ : initial hole diameter (= 10 mm)
Was used to determine the hole expansion ratio λ.

  The production conditions, the tensile test, and the measurement results of the hole expansion rate are shown together in Table 1. The results are shown in FIGS. FIG. 1 shows the influence of the coiling temperature, that is, the coiling temperature after hot rolling, on the elongation at break (El) and the hole expansion ratio (λ), and the coiling temperature is 350 to 650 ° C. It can be seen that good characteristics can be obtained within this range. FIG. 2 shows the effect on the El and λ of the holding time at the winding processing temperature, that is, the time from coil winding to the start of cooling, between 10 and 60 minutes after winding. It can be seen that good characteristics can be obtained when forced cooling is performed. FIG. 3 shows the effect of the cooling rate after winding on El and λ. Air cooling corresponding to forced cooling provides better characteristics than furnace cooling corresponding to cooling. I understand. FIG. 4 shows the influence of the heating rate during annealing on El and λ. It can be seen that good characteristics can be obtained at a heating rate of 30 ° C./sec or more. FIG. 5 is a comparison of the TS × El balance and TS × λ balance of the inventive example that meets the manufacturing conditions of the present invention described in Table 1 and a comparative example that deviates from the manufacturing conditions of the present invention. From this, according to the production method of the present invention, even if materials having the same component composition are used, a cooling property having excellent characteristics such as a TS × El balance of 20000 MPa ·% or more and a TS × λ balance of 65000 MPa ·% or more. It turns out that a rolled steel plate can be obtained.

B to N steels having the composition shown in Table 2 were melted in a vacuum melting furnace, cast into a 50 kg steel ingot, this steel ingot was reheated to 1250 ° C, and the final finishing temperature in an experimental rolling mill Was hot rolled to 870 ° C. to obtain a hot-rolled steel sheet having a thickness of 3.0 mm. The hot-rolled steel sheet was cooled to 600 ° C., held in a furnace set at 600 ° C. for 30 minutes, and then subjected to air cooling equivalent to forced cooling in which the temperature was reduced to 200 ° C. at 600 ° C./hr. In addition, about said hot-rolled steel plate, it carried out similarly to Example 1, and measured the cementite diameter and the amount of solute C.
Thereafter, the hot-rolled steel sheet is pickled to remove the surface scale, cold-rolled at a reduction rate of 60%, to obtain a cold-rolled steel sheet having a thickness of 1.2 mm, and further to this cold-rolled steel sheet at 600 ° C. Is heated at a heating rate of 100 ° C / sec, heated from 600 ° C to a soaking temperature (850 ° C) at a heating rate of 100 ° C / sec, held at the soaking temperature for 1 second, and then cooled at a cooling rate of 50 ° C / sec. After cooling to 300 ° C., an overaging treatment was performed to maintain that temperature for 300 seconds, and then a heat treatment corresponding to continuous annealing was performed to air-cool to room temperature. The cold rolled steel sheet obtained as described above was subjected to a tensile test and a hole expansion test in the same manner as in Example 1.

  The measurement results for the hot-rolled steel sheet and the cold-rolled steel sheet are shown together in Table 2. Note that No. 1 in Table 2 shows the result (No. 1 in Table 1) of steel A measured in Example 1 under the same conditions as in this example. FIG. 6 shows a comparison of the TS × El balance and the TS × λ balance of the invention examples described in Table 2 and the comparative example. From these results, according to the production method of the present invention, all of Ti, Nb, V, Cr, Ni, Mo, C, except for No. 15 (steel B) whose C content is outside the scope of the present invention. Despite being a steel strengthened by adding B, it is possible to obtain a cold-rolled steel sheet having excellent characteristics with a TS × El balance of 18000 MPa ·% or more and a TS × λ balance of 60000 MPa ·% or more. I understand that.

It is a graph which shows the influence which the coiling process temperature after hot rolling has on breaking elongation (El) and a hole expansion rate ((lambda)). It is a graph which shows the influence which holding time in winding process temperature has on El and (lambda). It is a graph which shows the influence which the cooling method (speed) after winding has on El and (lambda). It is a graph which shows the influence which the heating rate at the time of annealing has on El and (lambda). It is the graph which compared TS * El balance and TS * lambda balance of the example of the present invention and a comparative example. It is the graph which compared TS * El balance and TS * lambda balance of the example of the present invention and a comparative example.

Claims (3)

C: 0.02~0.2mass%, Si: 0.1~ 2.0mass%, Mn: 3.0mass% or less, P: 0.2 mass% or less, S: 0.01 mass% or less, Al: 3. A steel slab containing 0 mass% or less, N: 0.02 mass% or less and the balance being Fe and inevitable impurities is hot-rolled and wound at a temperature of 300 to 650 ° C., and then within 10 to 60 minutes A method for producing a cold-rolled steel sheet, comprising forcibly cooling, pickling and cold rolling, and heating and annealing at 600 ° C. or higher and a soaking temperature at 30 ° C./sec or higher. In addition to the above chemical composition, Ti: 1.0 mass% or less, Nb: 1.0 mass% or less and V: claim 1, characterized in that it contains one or more selected from the following 1.0 mass% A method for producing a cold-rolled steel sheet according to 1. In addition to the above component composition, it further contains one or more selected from Cr: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 1.0 mass% or less, and B: 0.01 mass% or less. The manufacturing method of the cold-rolled steel plate according to claim 1 or 2 .

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