JP4158034B2 - Hot rolling method for thin steel sheet - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot rolling method for thin steel sheets containing N as a strengthening element in an amount of 0.005% by mass or more and having excellent strain age hardening characteristics, and in particular, a hot rolling method for thin steel sheets capable of suppressing the occurrence of uneven plating during plating. About.

In recent years, for example, steel plates used in automobiles have a tendency to use steel plates having higher strength and thinner thickness than conventional ones. The reason for this is to reduce the thickness of the car by using a high-strength material, thereby reducing the weight of the car, improving fuel efficiency, and further improving environmental performance associated with reducing CO ₂ emissions. In the background. For example, if it is in contact with human eyes, it is a high-strength plate on a member used on the door surface, fender, bonnet, etc. If it is not in contact with the chassis frame, impact beam, or seat slide bracket, etc. By using many thin ones, the weight of the automobile body can be further reduced. Not only for automobiles, but also for home appliances and beverage cans, the use of thin steel plates with high thickness and high strength has the advantage that the amount of steel sheet purchase can be reduced.

  If it is simply a high-strength one, conventional high-carbon steel or high-strength steel may be used. However, since these steel types are hard, formability typified by press formability etc. It is extremely bad, and there are problems such as springback without bending along the mold and cracking in the middle of molding. For example, the members used on the surface of the door must be able to be pressed without causing cracks or wrinkles, etc., as can be imagined from the body line unique to automobiles. Is strongly required to have high strength and good moldability.

  As one measure for meeting such technical requirements, for example, Patent Document 1 describes a method for producing a high-tensile cold-rolled steel sheet having excellent strain age hardening characteristics. According to the technique described in Patent Document 1, N is used as a strengthening element, and the Al content is controlled to an appropriate range according to the N content, and hot rolling conditions, cold rolling conditions, and cold rolling annealing conditions are set. By optimizing and refining the structure and optimizing the amount of dissolved N, it is said that a high-tensile cold-rolled steel sheet having excellent formability and strain age hardening characteristics is obtained.

  Patent Document 2 describes a method for producing a high-tensile hot-rolled steel sheet having excellent strain age hardening characteristics. According to the technique described in Patent Literature 2, N is used as a strengthening element, the key Al content is controlled within an appropriate range, the hot rolling conditions are optimized, the structure is refined, and the amount of solid solution N Is optimized to provide a high-tensile hot-rolled steel sheet having excellent formability and strain age hardening characteristics.

  Patent Document 3 describes a method for producing a deep-drawn cold-rolled steel sheet having excellent strain age hardening characteristics. According to the technique described in Patent Document 3, N: 0.0050 to 0.02%, Al: 0.005 to 0.020%, and a steel material having a composition of N / (Al + Nb + Ti + V + B): 0.3 or more is subjected to rough rolling and finish rolling. Deep drawing is performed by applying hot rolling under adjusted conditions to obtain a hot rolled sheet, then subjecting the hot rolled sheet to recrystallization annealing and subsequent cold rolling to form a cold rolled sheet, and then subjecting the cold rolled sheet to recrystallization annealing. It is said that it will be a cold-rolled steel sheet with excellent properties and strain age hardening characteristics.

  The techniques described in Patent Literature 1 to Patent Literature 3 adjust hot rolling, cold rolling, cold rolling annealing conditions and the like to control the microstructure of the steel sheet within a certain range, and the Al content according to the N content. By adjusting the amount of solute N within a certain range and controlling the strain age hardening phenomenon due to N after forming, the steel sheet can achieve both excellent formability and high strength after forming. It is said.

  In other words, these steel sheets are high in strength and good by utilizing the property of being hardened by a paint baking process after forming while being relatively soft when performing press forming, which is representative of cold working. It can be said that it meets the technical requirement that it is necessary to combine the contradictory properties of formability.

  By the way, in recent years, in the manufacturing process of hot rolling, for the purpose of reducing the basic unit of energy such as fuel and electric power, molten steel is cast into a steel material such as slab by continuous casting, which is the previous manufacturing process. In the manufacturing process, steel materials such as manufactured slabs are not cooled to room temperature, but are charged in a heating furnace for hot rolling while maintaining the high temperature immediately after manufacturing, and heating and rolling efficiency are not hindered. A method called hot charge rolling (HCR) is widely adopted and applied to actual operations.

  Here, from the viewpoint of thermal efficiency, since it is desirable to be charged into the heating furnace at a higher temperature, the steel material such as slab is charged into the heating furnace while still in the austenite (γ) temperature range, so-called, γ-HCR may also be applied to actual operations.

However, when this γ-HCR is performed, even in the slab surface layer where the temperature is the lowest, austenite (γ) is converted to ferrite (α) during cooling from the end of casting to charging the heating furnace for hot rolling. May not transform. In this case, the structure due to the γ → α transformation and the α → γ reverse transformation during heating cannot be refined, resulting in coarse γ grains, and surface defects due to the coarse grains tend to occur. In particular, in the case of a steel material having a composition containing an alloy element that lowers the Ar ₃ transformation point such as Nb, Ti, or V, the occurrence of such surface defects becomes remarkable.

  With respect to such a problem, although it is not about a thin steel plate but about a thick steel plate, Patent Document 4 describes a hot charge rolling method for a continuous cast slab. In the technique described in Patent Document 4, a slab having an unsolidified phase inside is cooled in a continuous casting machine, the surface temperature is lowered to 500 ° C. or lower and held at 500 ° C. or lower for 15 seconds or more, The slab surface is solidified to the center of the slab while reheating, and then the hot slab cut to a predetermined length is charged into a heating furnace and heated, heated to a predetermined temperature, and then hot rolled. This is a method of making a thick steel plate.

Patent Document 5 describes a cracking prevention method during hot rolling. In the technique described in Patent Document 5, in direct feed rolling or hot charge rolling performed under conditions where the surface temperature of the slab is 600 to 900 ° C. and the total reduction ratio is 1.3 or more, rolling up to a total reduction ratio of less than 1.3 is an Ar ₃ transformation point. Rolling is performed on the above high temperature side, and the slab surface temperature is cooled to a range of Ar ₃ to (Ar ₃ −80 ° C.) for the total reduction ratio exceeding 1.3, and (Ar ₃ + 50 ° C.) to (Ar ₃ −100) In the range of [° C.].

Patent Document 6 describes a method for producing a thick steel plate having excellent low temperature toughness in a hot charge process. In the technique described in Patent Document 6, the surface temperature of the slab is (Ar ₃ −100 ° C.) or less, and is heated to a temperature of 950 ° C. or more after being charged into a heating furnace while being 300 ° C. or more. In this method, rolling with a total reduction ratio of 3 or more is performed, and hot rolling is finished at a surface temperature of the steel sheet of (Ar ₃ −100 ° C.) or more and 950 ° C. or less.
JP 2002-053935 A JP 2002-047536 A JP 2001-335887 A JP 2001-137901 A Japanese Unexamined Patent Publication No. 62-34602 JP 07-331329 A

  According to the study by the present inventors, Patent Documents 4 to 5 are disclosed for the purpose of improving thermal efficiency in the production of high-tensile steel sheets having excellent strain age hardening characteristics as described in Patent Documents 1 to 3. When hot charge rolling (HCR) as described in 6 is applied, the surface at the hot-rolled steel plate stage (for example, 2-3 mm in thickness) or cold-rolled steel plate stage (for example, 0.3-0.5 mm in thickness) Although wrinkles do not occur, there is a case where, in the plating process that is a subsequent manufacturing process, as shown schematically in FIG. 6, there may be a quality defect that uneven plating occurs in the vicinity of the width end portion of the steel sheet 1. Newly found.

  In view of the situation as described above, the present invention contains 0.005% by mass or more of N as a strengthening element, and does not hinder the efficiency of heating and rolling during hot rolling of a thin steel sheet having excellent strain age hardening characteristics. Proposal of a hot rolling method for thin steel sheets that can reduce the energy intensity of fuel, electric power, etc., and can suppress the occurrence of uneven plating during plating after hot rolling or cold rolling The purpose is to do.

  The “thin steel sheet” in the present invention refers to a hot rolled sheet having a thickness of 1.0 mm or more and 6 mm or less, preferably 1.0 mm or more and 4.5 mm or less, and includes all steel sheets that pass through hot rolling as an intermediate step. These hot-rolled sheets are further subjected to a plating treatment to be a hot-rolled plated steel sheet, or further subjected to a cold rolling-plating process, and preferably a cold-rolled plated steel sheet having a thickness of 0.1 mm to 3.2 mm. .

  In order to achieve the above-described problems, the present inventors diligently studied various factors that contribute to the occurrence of uneven plating during the plating treatment of a steel sheet containing 0.005% by mass or more of N as a strengthening element. As a result, it has been found that uneven plating occurs due to a change in structure based on the difference in the position of the temperature of the steel material (slab) before being charged into the heating furnace.

  That is, the slab (steel material) immediately after being cast by continuous casting or the like has the highest cooling rate at each side or in the vicinity thereof, and is cooled to a low temperature before being charged into the heating furnace, thereby miniaturizing the structure. However, in other parts, the cooling rate is slow, and it is charged into the heating furnace at a relatively high temperature, and the structure becomes a coarse structure without being refined. The portion corresponding to the boundary between these tissues exists near the end in the slab width direction and extends in the slab longitudinal direction. The present inventors, after the subsequent hot rolling and cold rolling, because the portion corresponding to this boundary is cracking internally at the slab stage or the grain boundary of the same portion is weaker than other normal grain boundaries It appears that it appears as a fine crack in the surface layer of the steel plate, which becomes apparent as a subtle difference in the manner of adhesion of the plating during the plating process, and results in uneven brightness and uneven plating.

  Fig. 1 schematically estimates the change in crystal grains until they are charged into the heating furnace prior to hot rolling, for each part of the slab that is locally cooled to a low temperature. Show.

Slabs that have just been cast by the continuous casting method contain many coarse γ grains (a in Fig. 1), but when cooled and the temperature drops below the Ar ₃ transformation point, α grains precipitate from the γ grain boundary (Fig. In 1), as the particles are further cooled, α grains grow or α grains are precipitated from within the γ grains, and the γ grains are divided. When the temperature is further lowered to a temperature below the Ar ₁ transformation point, the γ → α transformation is completed, and the structure becomes an α grain structure and is refined (c in FIG. 1). After that, the slab is charged into a heating furnace, heated to a temperature higher than the Ac ₁ transformation point, α → γ reverse transformation starts and many fine γ grains precipitate (d in FIG. 1). When the temperature is raised above the Ac ₃ transformation point, the α → γ reverse transformation is completed, and a uniform structure consisting of fine γ grains is obtained (e in FIG. 1). When the temperature is further increased, γ grains grow (f in FIG. 1).

  On the other hand, the change of the crystal grains in the portion charged into the heating furnace is schematically estimated and shown in FIG.

The slab that has just been cast by the continuous casting method contains many coarse γ grains (a in Fig. 2), but when cooled and the temperature drops below the Ar ₃ transformation point, α grains precipitate from the γ grain boundary (Fig. B), α grains precipitate and grow as they are further cooled, but they are not cooled to below the Ar ₁ transformation point and are charged into the heating furnace, so that Ar is no longer hot-rolled. _The temperature does not drop below the ₁ transformation point. Therefore, the region in the vicinity of the old γ grain boundary that has undergone γ → α transformation during cooling becomes α → γ reverse transformation and refines by heating (c in FIG. 2), but the region that has not transformed into α during cooling is coarse. Since it remains as γ, it becomes a non-uniform structure (d, e in FIG. 2) including fine γ grains at the grain boundary of coarse γ grains after being kept at the rolling heating temperature.

  In addition, since the slab (steel material) used in the present invention contains N and Al, solid solution / precipitation of AlN occurs during heating and cooling of the slab.

In the γ region, the precipitation of AlN depends on the solubility, but when the temperature reaches the temperature below the Ar ₃ transformation point in the α region during the cooling process, the precipitation of AlN does not proceed. Thereafter, it is heated in a heating furnace, and AlN begins to precipitate again from above about 1000 ° C. in the temperature raising process. However, as shown in FIG. 2, the portion that is cooled only to a temperature exceeding the Ar ₁ transformation point and is charged into the heating furnace at a relatively high temperature remains a structure containing a large amount of coarse γ grains. As a result of heating, AlN is concentrated in the grain boundaries where precipitation is likely to occur (c in FIG. 2), and AlN exhibits a non-uniform distribution state.

On the other hand, the portion of each side of the slab or its vicinity, which is locally cooled to the low temperature before being charged into the heating furnace, is refined in γ grains. In addition to the precipitation of coarse γ grains at the grain boundaries, they must also precipitate within the γ grains, exhibiting a uniform distribution state.
When FIG. 1 and FIG. 2 are compared, the structure when finally extracted from the heating furnace will be significantly different between the two. In the case of FIG. 1, a uniform microstructure is obtained as a whole, whereas in the case of FIG. 2, a heterogeneous structure in which fine AlN and fine γ grains are present at the grain boundaries of coarse γ grains. .

  The difference in structure depending on the position in the slab remains after hot rolling, and the boundary between the two appears as fine cracks in the surface layer of the steel sheet after hot rolling and cold rolling. It is thought that it becomes a subtle difference in the manner of adhesion of the metal, and it becomes apparent that the luster becomes uneven and the plating becomes uneven. At the boundary between the two, at the slab stage, internal cracking has occurred, or the grain boundary of the same part is weaker than other normal grain boundaries, so the surface layer of the steel sheet after subsequent hot rolling and cold rolling It is assumed that it appears as a fine crack.

As a result of further investigation based on the above-mentioned idea, the present inventors adjusted the surface temperature of the steel material to 400 ° C. or higher and Ar ₁ transformation point or less after casting, and then charged it into the heating furnace and It has been found that the occurrence of uneven plating during the plating process can be suppressed by rolling.

  The present invention has been completed based on the above findings and further studies.

That is, the present invention provides a steel material having a composition containing, by mass%, Al: 0.02% or less, N: 0.005-0.025%, and N / Al: 0.3 or more, A steel sheet characterized by adjusting the surface temperature to 400 ° C. or more and below the Ar ₁ transformation point, charging it into a heating furnace and heating it, and then subjecting the steel material to hot rolling to form a steel sheet This is a hot rolling method.

  According to the present invention, it is possible to reduce the energy intensity of fuel, electric power, etc. without impairing the heating and rolling efficiency of hot rolling, and to prevent the occurrence of uneven plating during the plating process, thereby plating thin steel plates Quality defects can be suppressed, and there are significant industrial effects.

  First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, the mass% in the composition is simply referred to as%.

  The steel material used in the present invention contains 0.005% or more of N as a strengthening element and 0.02% or less of Al so that N / Al: 0.3 or more in order to make a thin steel plate with excellent strain age hardening characteristics. To do.

Al: 0.02% or less
Al acts as a deoxidizer, is an element effective for improving the cleanliness of steel, and is also an element having an action of refining the structure of a thin steel sheet. Such an effect becomes remarkable when the content is 0.001% or more. On the other hand, an excessive content exceeding 0.02% deteriorates the surface properties of the steel sheet, further reduces the amount of N in a solid solution state that contributes to strain age hardening, and variation in strain age hardening characteristics when manufacturing conditions vary. It tends to occur. For this reason, in the present invention, Al is limited to 0.02% or less. From the viewpoint of material stability, Al is preferably 0.015% or less.

N: 0.005-0.025%
N is an element that improves the strength of the steel sheet by solid solution strengthening and strain age hardening. In the present invention, N is contained by 0.005% or more. N also has a function of lowering the transformation point of steel, and the content of N is also useful for stabilizing the operation in a situation where rolling that is a thin material and greatly interrupts the transformation point is avoided. If the N content is less than 0.005%, the above-described strength improvement effect is not likely to appear stably. On the other hand, if N exceeds 0.025%, the rate of occurrence of internal defects in the steel sheet increases, and slab cracking during continuous casting occurs frequently. For this reason, N was limited to the range of 0.005 to 0.025%. Note that N is preferably in the range of 0.0070 to 0.0170% from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. If the N amount is within the range of the present invention, there is no adverse effect on weldability such as spot welding and arc welding.

N / Al (ratio of N content to Al content): 0.3 or more In the product state, preferably in a solid solution state of preferably 0.0010% or more, N in a solid solution state is stably left, and a desired strain age hardening amount (for example, strain age hardening) In order to ensure an increase in tensile strength before and after the treatment ΔTS40 MPa or more), it is necessary to limit the amount of Al, which is an element that strongly fixes N. As a result of examining the combination of the N content and the Al content within the composition range of the steel material used in the present invention, in order to make the solid solution N content in cold-rolled products and plated products 0.0010% or more, the Al content is If limited to 0.02% or less, N / Al must be 0.3 or more. For this reason, N / Al was limited to 0.3 or more.

In the steel material used in the present invention, chemical components other than the above-described Al and N can be appropriately selected according to required characteristics. In the steel material used in the present invention, Al: 0.02% or less, N: 0.005 to 0.025% is added to the composition containing N / Al so as to satisfy 0.3 or more, and C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, or the following a group to d group a group: one or more of Cu, Ni, Cr, and Mo are totaled 1.0% or less b group: Nb, Ti, or V in total, 0.1% or less c group: B: 0.0030% or less d group: Ca, REM 1 or 2 types in total 0.0010-0.010%
It is good also as a composition which contains 1 group or 2 groups or more selected from these, and the remainder consists of Fe and an unavoidable impurity. The reasons for limiting the content of each component are as follows.

C: 0.15% or less C is an element that improves the strength of the steel sheet, and is preferably contained in an amount of 0.005% or more from the viewpoint of securing a desired strength. More preferably, it is 0.03% or more. On the other hand, if the content exceeds 0.15%, the carbide fraction in the steel sheet becomes excessive, the ductility is remarkably lowered and the formability is lowered, and the spot weldability and arc weldability are further lowered. For this reason, it is preferable to limit C to 0.15% or less. In addition, it is more preferably 0.10% or less, and more preferably 0.08% or less for uses requiring good ductility.

Si: 2.0% or less
Si is a useful element that can improve the strength of the steel sheet without significantly reducing the ductility of the steel, and is preferably contained in an amount of 0.1% or more. On the other hand, a Si content exceeding 2.0% greatly increases the transformation point during hot rolling, making it difficult to ensure quality and shape, or lowering the surface properties and chemical conversion properties, resulting in the beauty of the steel sheet surface. Adversely affected. For this reason, it is preferable to limit Si to 2.0% or less. If Si is 2.0% or less, a remarkable increase in the transformation point can be suppressed by adjusting the amount of Mn added in combination, and good surface properties can be secured.

Mn: 3.0% or less
Mn is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. From the viewpoint of stably fixing S, Mn is preferably contained in an amount of 0.2% or more. Mn is an element that improves the strength of the steel sheet, and can be appropriately contained according to the strength requirement. From the viewpoint of stably securing the strength, it is more preferably 1.5% or more. When the Mn content is increased to this level, variations in the mechanical properties and strain age hardening characteristics of the steel sheet with respect to fluctuations in production conditions including hot rolling conditions are reduced, which is effective in stabilizing the quality. On the other hand, if Mn is contained in a large amount exceeding 3.0%, the hot deformation resistance of the steel sheet tends to increase, the spot weldability and the formability of the welded portion tend to decrease, and further, the formation of ferrite occurs. Since it is suppressed, the ductility tends to decrease remarkably. For this reason, it is preferable to limit Mn to 3.0% or less. In applications where better corrosion resistance and formability are required, Mn is preferably 2.5% or less.

  Further, Mn has a function of lowering the transformation point during hot rolling, and inclusion with Si can offset an increase in transformation point due to the inclusion of Si. Especially for products with thin plate thickness, quality and shape change sensitively due to changes in transformation point, so it is important to balance Mn and Si contents precisely. For these reasons, it is more preferable to set Mn / Si to 3.0 or more.

P: 0.08% or less P is an element useful as a solid solution strengthening element of steel. From this viewpoint, P is preferably contained in an amount of 0.001% or more. However, if excessively contained, steel makes the steel brittle and further improves the formability of the steel sheet. Reduce. Moreover, since P has a strong tendency to segregate in steel, it causes embrittlement of the weld due to it. For this reason, it is preferable to limit P to 0.08% or less. In view of elongation and workability, it is more preferably 0.04% or less, and further preferably 0.02% or less from the viewpoint of weld zone toughness.

S: 0.02% or less S is an element which exists as an inclusion in a steel sheet and causes a decrease in ductility and further corrosion resistance of the steel sheet, and is preferably limited to 0.02% or less. In applications where particularly good workability is required, it is 0.015% or less, more preferably 0.008% or less.

  In addition to the above-described components, one group or two or more groups selected from group a to group d can be contained according to required characteristics.

Group a: 1.0 or less in total of one or more selected from Cu, Ni, Cr, and Mo Group a: Cu, Ni, Cr, and Mo all contribute to improving the strength of the steel sheet It is an element to be selected and can be selected alone or in combination as required. In order to obtain such effects, it is preferable to contain Cu: 0.01% or more, Ni: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. On the other hand, if the total content exceeds 1.0%, the hot deformation resistance increases, or the chemical conversion property and the surface treatment characteristics in a broad sense are deteriorated, and the weld is hardened and the weld formability is deteriorated. For this reason, it is preferable that the elements in group a be 1.0% or less in total.

Group b: Total of one or more selected from Nb, Ti, and V, 0.1% or less Group b: Nb, Ti, and V all contribute to refinement and uniformity of crystal grains It is an element and can be selected alone or in combination as required. Such an effect becomes noticeable by containing Nb: 0.002% or more, Ti: 0.002% or more, and V: 0.002% or more. On the other hand, when the total amount of elements in the group b exceeds 0.1%, the content is too large, the hot deformation resistance increases, and the chemical conversion property and the surface treatment characteristics in a broad sense are deteriorated. For this reason, it is preferable that the total amount of elements in group b is 0.1% or less.

c group: B: 0.0030% or less c group: B is an element that improves the hardenability of the steel and increases the fraction of the low-temperature transformation phase other than the ferrite phase to improve the strength of the steel sheet. Can be contained. This effect is observed when the content of B is 0.0002% or more. However, when the content is too large, the hot deformability is lowered, and the solute N is reduced by generating BN. For this reason, B is preferably 0.0030% or less.

d group: One or two of Ca and REM in total, 0.0010 to 0.010%
d group: Ca and REM are elements useful for controlling the form of inclusions, and are preferably contained alone or in combination as required. When the total amount of elements in the d group is less than 0.0010%, the effect of controlling the shape of inclusions is insufficient. On the other hand, when the content exceeds 0.010%, surface defects are conspicuous. For this reason, it is preferable to limit the elements of the d group to a total range of 0.0010 to 0.010%.

  The balance other than the above components is Fe and inevitable impurities. As an inevitable impurity, O: 0.0050% or less is allowed.

The steel material used in the present invention is preferably prepared by melting a molten steel having the above composition by a generally known melting method, and preferably casting it by a generally known continuous casting method to obtain a slab having a predetermined size. The steel material that has been cast is then cooled, adjusted so that the surface temperature of the steel material is not lower than 400 ° C. and not higher than the Ar ₁ transformation point, and charged into the heating furnace. The cooling from the completion of casting to charging the heating furnace is preferably natural cooling in the atmosphere or forced cooling by a blower, mist, spray or the like. In the case of forced cooling by a blower, mist, spray, etc., as shown in FIG. 3, it is preferable to install a forced cooling device 80 in the middle of the conveyance route, in the continuous casting machine slab yard 50 or in the hot rolling slab yard 60. .

If the surface temperature of the steel material before charging the furnace is less than 400 ° C, the steel material temperature will be too low, the heating time for hot rolling will be long, and it will not be effective for reducing fuel and power energy. Efficiency decreases and productivity decreases. On the other hand, when the surface temperature of the steel material before charging the furnace is high beyond the Ar ₁ transformation point, the grain refinement effect due to the γ → α transformation and further the α → γ reverse transformation cannot be used. At this time, coarse γ grains are formed, and surface defects are likely to occur. For this reason, the surface temperature of the steel material before charging the heating furnace was limited to 400 ° C. or higher and Ar ₁ transformation point or lower. The surface temperature of the steel material described above is an average value measured at the plate width center and the plate length end (100 to 300 mm) of the steel material.

  It is preferable to directly measure the surface temperature of the steel material with a continuous casting machine outlet side thermometer 20 installed on the outgoing side of the continuous casting machine 5 as shown in FIG. 3, for example. Further, the temperature of a medium (for example, a roll in the continuous casting machine, a machine part that is in contact with the roll cooling water or a steel material when handling the steel material, etc.) disposed in the continuous casting machine 5 is measured by another thermometer (not shown) and measured. An indirect method of predicting and estimating the surface temperature of the steel material in the computer 40 based on the temperature may be taken. Further, as shown in FIG. 3, the slab is installed in the continuous casting machine slab yard 50 or in the hot rolling slab yard 60 in the middle of the conveyance route T from the exit side of the continuous casting machine 5 to the heating furnaces 8 and 8. The slab surface temperature may be measured using the slab yard thermometer 70 or the heating furnace entrance-side thermometer 30. Further, it may be predicted and calculated based on the actual time elapsed since the torch cut at the outlet side of the continuous casting machine 5. In addition, you may take the method of using together two or more of the above-mentioned methods.

  Based on information on the surface temperature of the steel material measured or indirectly measured by the continuous casting machine outlet side thermometer 20 and the heating furnace inlet side thermometer 30, the steel material before charging into the furnaces 8 and 8 It is preferable that the surface temperature is adjusted by dissipating heat by leaving in the atmosphere for a predetermined time so that the surface temperature is within the predetermined temperature range described above, and then charged into the heat retaining pit 65 and suspended until charging in the heating furnace. By holding in the heat retaining pit 65, the steel material temperature can be held so as not to decrease as much as possible. In place of leaving in the atmosphere, there is no problem even if forced cooling such as cooling by air blowing, mist cooling with a medium such as water or oil, spray cooling or the like is performed in order to shorten the standing time.

  Further, the required standing time in the atmosphere can be calculated and calculated by the computer 40 based on the measured value of the surface temperature of the steel material and the attribute data of each steel material such as the thickness of the cross section of the steel material.

  In addition, the thermometers 20 and 30 arranged on the continuous casting machine outlet side or the heating furnace inlet side, and the computer 40 based on the data of various tracking sensors (not shown), through the information transmission route X, the continuous casting machine outlet side, the slab yard Among them, instructions are sent to various facilities on the heating furnace entrance side. The information transmission route Y is configured to send information regarding the actual state of these facilities to the computer 40.

  The steel material charged in the heating furnace is heated to a predetermined temperature and then subjected to hot rolling.

In the present invention, the heating temperature of the steel material is not particularly limited as long as it is a temperature at which hot rolling is possible, but is preferably in the range of 1050 ° C to 1250 ° C. When the heating temperature is less than 1050 ° C., the deformation resistance becomes high, so that the rolling load becomes too high and rolling becomes difficult. In addition, it is difficult to set the finish rolling mill outlet temperature to the Ar ₃ transformation point or higher. On the other hand, when the temperature exceeds 1250 ° C., the scale loss due to heating increases and the yield decreases.

  Note that the heating temperature of the steel material (slab) is equal to or higher than a predetermined temperature, for example, as shown in FIG. 4 based on the results of the atmospheric temperature and the time spent in each heating zone in the heating furnace. Although it is preferable to obtain the slab (steel material) with such a mesh by predictive calculation using a model that virtually divides the slab (steel material), the present invention is not necessarily limited to this. In FIG. 4, a mesh is assumed at the two ends of the slab longitudinal direction and the central portion, and the temperatures of the slab longitudinal direction central portion and the end portion are calculated separately, but the method of cutting the mesh is not limited to this, or It is also possible to use a method that does not depend on the difference method.

  As for the heating temperature of the steel material, it is preferable to use one that is convenient in actual operation, whether it is the surface temperature or the average cross-sectional temperature. Moreover, you may use the temperature of the steel raw material longitudinal direction center part, and the temperature of an edge part.

  Next, the steel material heated to a predetermined temperature in the heating furnace is hot-rolled by a hot rolling line shown in a typical example in FIG.

  A hot rolling line 100 shown in FIG. 5 extracts a steel material (hereinafter, a material to be rolled: S), which is a material to be rolled, to several hundred to several hundreds of degrees Celsius, and then extracts the material on the hot rolling line. It is a production line which extends thinly by rotating the roll while pinching the material to be rolled with the roll. This is because, in order from the upstream to the downstream in the conveying direction of the material to be rolled S, the heating furnaces 8, 8, a plurality of roughing mills (Rougher) 12, a cropping machine 14, a descaling device 16, a finishing mill (Finisher) 18, A cooling zone 22, a coiler (winding device) 24, and the like are sequentially arranged.

  In general, in a hot rolling line, the rough rolling mill 12 is often provided with four units, of which a part (often one) is reciprocally rolled, and the remaining rough rolling mill (in many cases 3). There are many types called “3/4 continuous” in which the base) performs unidirectional rolling. However, three of the four groups are not limited to the one-way type, and for example, one of the three groups is a one-way type. In addition, as a type other than 3/4 continuous, after one or two rough rolling mills were reciprocally rolled, a type called semi-continuous finish rolling or one-way rolling was performed inside and outside six rough rolling mills. After that, there is a type called complete continuous that is finish-rolled. The hot rolling line to which the present invention can be applied is not limited to the 3/4 continuous type hot rolling line that reciprocally rolls two out of the three shown in FIG. Needless to say, it can be applied to a hot rolling line. Note that the material to be rolled S during rolling by the roughing mill 12 or after rolling and before rolling by the finishing mill 18 is also referred to as a sheet bar.

  The steel material heated to a predetermined temperature in the heating furnace is then subjected to finish rolling in which the finish rolling mill outlet temperature is in a predetermined temperature range and hot rolling in which the winding temperature is wound in a predetermined temperature range. , Hot-rolled sheet (thin steel sheet). Note that the finish rolling mill delivery side temperature is preferably measured by a finish mill delivery side thermometer 21 installed on the delivery side of the finishing mill 18 (on the downstream side in the material transport direction). In addition, the winding temperature is preferably measured by a winding thermometer 23 installed on the entry side (upstream side in the conveyance direction of the material to be rolled) of the winding device 24. The finish rolling mill outlet temperature and the coiling temperature are appropriately determined based on characteristics such as mechanical properties required for the product.

  When manufacturing high-tensile cold-rolled steel sheets with excellent strain age hardening characteristics, the finish rolling mill outlet temperature should be 800 ° C or higher, and the winding temperature should be 600 ° C or lower. In the case of a high-tensile hot-rolled steel sheet, the finish rolling mill outlet temperature is 800 ° C. or higher, and the winding temperature is 650 ° C. or lower, preferably 550 to 650 ° C.

Further, in the case of a cold-rolled steel sheet for deep drawing having excellent strain age hardening characteristics, the finish rolling mill outlet temperature is preferably 600 ° C. or higher, and more preferably Ar ₃ transformation point or lower. In this case, the winding temperature need not be particularly limited. Moreover, it is preferable that the rough rolling mill delivery side temperature (rough rolling finish temperature) measured with a rough rolling mill delivery thermometer or the like is 1000 ° C. or lower and the Ar ₃ transformation point or higher.

  The hot-rolled sheet obtained by the above hot rolling is rolled or, if necessary, further subjected to hot-rolled sheet annealing, pickling treatment, and cold rolling to form a cold-rolled sheet, and further subjected to recrystallization annealing. After being made into a cold-rolled annealed plate, it is plated and made into a plated steel plate, which is used as a product.

  Hereinafter, based on an Example, this invention is demonstrated in detail.

  Molten steel having the composition shown in Table 1 was melted in a converter, and a slab (steel material) having the thickness shown in Table 1 was obtained by a continuous casting method. The slab was charged into the heating furnace via the conveyance path shown in FIG.

  First, after completion of casting, the slab surface temperature was measured with a continuous caster outlet thermometer 20. The upper limit of the furnace charging temperature of each slab is determined by the computer 40 using the attribute of each slab, and the upper limit of the heating furnace charging temperature is determined by the calculator 40 from the measured slab surface temperature and the attribute of each slab. The air standing time for heat dissipation until charging the furnace was calculated and set. After being left in the air based on the set air leaving time, the heat retaining pit 65 was inserted. In the heat retaining pit, almost no decrease in the slab surface temperature was observed.

  The slab held in the heat retaining pit 65 was then charged into a heating furnace, heated to the heating temperature shown in Table 2, and then hot-rolled under the conditions shown in Table 2 to obtain the plate thickness shown in Table 2. It was a hot-rolled sheet.

  Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing-pickling treatment, and then cold-rolled sheets having the thicknesses shown in Table 2 by cold rolling. Subsequently, the cold-rolled sheet was subjected to finish annealing (700 ° C. × 20 min) and then subjected to electrotin plating to obtain a plated steel sheet.

  After the plating treatment, the surface of the plated steel sheet was visually observed to investigate the occurrence of uneven plating. The case where uneven plating occurred was evaluated as x, and the case where it did not occur was evaluated as ○. In addition, the case where the uneven plating which looks like stripes by visual observation exists was set as the case where the uneven plating occurred.

  The results obtained are shown in Table 2.

  In all of the inventive examples, the occurrence of uneven plating is not observed. On the other hand, in the comparative example outside the scope of the present invention, uneven plating occurs. In the conventional example in which the range of Al, N, and N / Al is outside the range of the present invention, uneven plating does not occur. However, due to the difference in the component system, there has been no problem of uneven plating in the first place. It is.

It is explanatory drawing which shows typically the mode of the change of the crystal structure in the case of satisfy | filling the conditions of this invention. It is explanatory drawing which shows typically the mode of the change of the crystal structure when deviating from the conditions of this invention. It is an outline explanatory view showing typically the whole conveyance route of a steel material, and the whole control system suitable for implementation of the present invention. It is explanatory drawing which shows an example of the model for calculating steel raw material temperature. It is explanatory drawing which shows the outline | summary of the whole of the hot rolling line suitable for implementation of this invention. It is explanatory drawing which shows typically the generation | occurrence | production state of plating unevenness.

Explanation of symbols

1 Steel plate (steel strip)
5 Continuous Casting Machine 8 Heating Furnace 12 Rough Rolling Mill 13 Work Roll 14 Crop Sha 16 Descaling Device 18 Finishing Roller 19 Work Roll 20 Continuous Casting Machine Delivery Side Thermometer 21 Finishing Roller Delivery Side Thermometer 22 Cooling Zone 23 Winding Thermometer 24 Coiler 30 Heating furnace inlet side thermometer 40 Computer 50 Continuous casting machine slab yard 60 Hot-roll slab yard 65 Thermal insulation pit 70 Slab yard thermometer 80 Forced cooling device 90 Sub computer 95 Control device 100 Hot rolling line W, G , X, Y Information transmission route S Rolled material T Transport route

Claims (1)

The surface temperature of the steel material is 400 ° C. or higher after the completion of casting with a steel material having a composition that satisfies Al: 0.02% or less, N: 0.005 to 0.025%, and N / Al: 0.3 or more. A method of hot rolling a thin steel sheet, characterized by adjusting the Ar ₁ transformation point or less, charging it into a heating furnace and heating it, and then subjecting the steel material to hot rolling to form a thin steel sheet.

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