JP2013119653A - Hot-rolled steel plate as material for cold-rolling, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-rolled steel plate as a material for cold-rolling and a manufacturing method therefor, with which a thin steel sheet after cold-rolling and the steel sheet obtained by applying the surface treatment to the thin steel plate can be manufactured at low cost, even with a cold-rolling mill having small rolling capacity.SOLUTION: The hot-rolled steel plate as the material for cold-rolling has a chemical composition composed, by mass, of 0.016-0.07% C, ≤0.1% Si, 0.05-0.5% Mn, ≤0.03% P, ≤0.03% S, 0.02-0.1% sol.Al, ≤0.005% N, 0.005-0.03% Ti and the balance Fe with inevitable impurities, and has an average crystal grain diameter of ≤13 μm and the aging index AI of ≤10 MPa.

Description

  The present invention is a thin cold-rolled steel sheet or surface-treated cold-rolled steel sheet used in the field of building materials such as roofing materials, in particular, as a material for cold rolling in the manufacture of cold-rolled steel sheets or steel sheets subjected to surface treatment. The present invention relates to a hot-rolled steel sheet for cold rolling material and a method for producing the same.

  In recent years, the demand for building materials has increased with the global population growth and economic development. In particular, the outer walls and roofs of buildings are made of thin cold-rolled steel sheets with a thickness of 0.3 mm or less, or surface-treated cold-rolled steel sheets that have been plated or painted on a substrate such as plywood. There is a case. In such applications, cold-rolled steel sheets are hardly processed, so workability is not questioned, and cold-rolled steel sheets or steel sheets subjected to surface treatment are used.

  In order to reduce the cost of building materials for outer walls and roofs, it is desirable to make these cold-rolled steel sheets and steel sheets with surface treatments thinner, but demand for building materials for outer walls and roofs is in demand. In many cases, cold rolling is performed by a cold rolling mill having a small rolling capacity adjacent to the ground, and there is a problem that thinning cannot be achieved due to insufficient load during cold rolling.

Therefore, the demand for a soft hot-rolled steel sheet having a low cold rolling load as a raw material has increased.For example, Patent Document 1 discloses a technique related to a hot-rolled steel sheet in which the amount of C in the steel is extremely low to 0.010 mass% or less. Patent Document 2 proposes a technique related to a hot-rolled steel sheet in which the N content in steel is reduced to 0.0020% by mass or less. Further, in Patent Document 3, the amount of C in steel is 0.01 to 0.10% by mass, the amount of N is 0.010% by mass or less, and hot rolling at a finishing temperature of 700 ° C. or more and an Ar ₃ transformation point or less [so-called α (ferrite) rolling To obtain a hot-rolled steel sheet having coarse crystal grains has been proposed.

JP-A-3-79726 Japanese Patent Publication No.63-30969 JP 2010-77482 A

However, in the technology relating to the hot rolled steel sheet having a low gas component amount such as C and N described in Patent Documents 1 and 2, it is necessary to perform a vacuum degassing process at the time of steel making, and an increase in manufacturing cost is inevitable. In order to produce a cold-rolled steel sheet with a thickness of 0.3 mm or less or a surface-treated steel sheet using a cold rolling mill with a small rolling capacity, the thickness of the hot-rolled steel sheet is used. it is necessary to 3mm or less, when low C and N content is difficult to secure the temperature finishing above Ar ₃ transformation point, easily inhomogeneous microstructure is formed in the thickness direction, inhibits cold rollability To do.

  In the α rolling technique described in Patent Document 3, the strength of the hot-rolled steel sheet is easily affected by the finishing temperature and the coiling temperature, and the thickness cannot be reduced stably.

  The present invention has been made in order to solve such a problem, and it is possible to inexpensively manufacture a thin steel plate that is still cold-rolled in a cold rolling mill or a surface-treated steel plate even in a cold rolling mill having a small rolling capacity. An object of the present invention is to provide a hot-rolled steel sheet of a material that can be produced and a method for producing the same.

  As described above, in the prior art, in order to reduce the cold rolling load, studies are made with emphasis on softening a hot-rolled steel sheet as a raw material. However, the present inventors may increase the cold rolling load even when using a soft hot-rolled steel sheet, and the cause thereof is a small amount of solute C or C which does not greatly affect the strength of the hot-rolled steel sheet. Found N. In addition, if Ti, which is an inexpensive element, is added, even if the coiling temperature varies or the coil coiled to increase productivity is water-cooled, there is almost no solid solution C or N, and it is stable. It has been found that the cold rolling load can be reduced, that is, the strength increase due to cold rolling can be suppressed and the cold rolling load can be reduced.

  Next, the experimental results on which the present invention is based will be described.

  Using steel containing, by mass%, C: 0.002-0.05%, Si: 0.015%, Mn: 0.15%, P: 0.012%, S: 0.008%, Sol.Al: 0.035%, N: 0.003%, Heating temperature: after heating at 1250 ° C, hot rolling at finish temperature: 920 ° C, winding at winding temperature: 650 ° C, allowing to cool, pickling, temper rolling with elongation rate of 1% And a hot-rolled steel sheet was obtained. And this hot-rolled steel sheet was cold-rolled at a reduction ratio of 78% to obtain a cold-rolled steel sheet. From the obtained hot-rolled steel sheet and cold-rolled steel sheet, a JIS No. 5 tensile test piece with the rolling direction as the longitudinal direction was sampled and subjected to a tensile test according to JIS Z 2241 to obtain the tensile strength TS. . For hot-rolled steel sheet, in order to evaluate the amount of solute C and N, using JIS No. 5 tensile test piece with the rolling direction as the longitudinal direction, after applying 7.5% pre-strain by tensile processing, A 30 minute heat treatment was performed, and an aging index AI defined by the strength difference before and after the heat treatment was obtained. Further, the microstructure of the hot-rolled steel sheet was observed, and the average crystal grain size was determined by the cutting method described in JIS G 0552 (1998).

  FIG. 1 shows the relationship between the average grain size of the hot-rolled steel sheet and the TS of the hot-rolled steel sheet and the TS of the cold-rolled steel sheet.

  FIG. 2 shows the relationship between AI of hot-rolled steel sheet and TS of cold-rolled steel sheet.

  As apparent from FIG. 1, TS of the hot-rolled steel sheet depends on the average crystal grain size, and decreases when the C content is less than 0.016% and the average crystal grain size is large. However, the TS of the steel sheet after cold rolling of these hot-rolled steel sheets (the steel sheet as cold-rolled) is contrary to the TS of the hot-rolled steel sheet, and the average crystal grain size of the hot-rolled steel sheet is 13 μm or less. As a result, a low value is obtained.

  In addition, as is clear from FIG. 2, when C <0.016%, where the degree of supersaturation is low, and C <0.016%, the hot-rolled steel sheet has a higher AI compared to C ≧ 0.016%, which is still cold-rolled. Steel sheet TS is high.

  The reason for this is not clear, but the TS of the hot-rolled steel sheet is a TS with a strain of at most about 0.3 obtained in a tensile test, whereas the TS of a cold-rolled steel sheet is a TS with a high strain of 1.8. Therefore, it is presumed that solute C and N exhibit larger work hardening ability in a high strain region and increase TS of the steel sheet as cold-rolled. That is, in order to reduce the TS of a cold-rolled steel sheet, it is advantageous to reduce the amount of dissolved C and N and to reduce the AI of the hot-rolled steel sheet to 10 MPa or less.

  The present invention has been made based on the above findings, and in mass%, C: 0.016-0.07%, Si: 0.1% or less, Mn: 0.05-0.5%, P: 0.03% or less, S: 0.03% Hereinafter, Sol.Al: 0.02-0.1%, N: 0.005% or less, Ti: 0.005-0.03%, the balance has a chemical composition consisting of Fe and inevitable impurities, the average crystal grain size is 13μm or less, Also provided is a hot-rolled steel sheet for cold rolling material characterized by an aging index AI of 10 MPa or less.

  The hot-rolled steel sheet of the present invention can further contain B: 0.0003 to 0.0030% by mass.

The hot-rolled steel sheet of the present invention is a slab of steel having the above chemical composition, finishing temperature: Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C.), reduction rate in the final pass: 20% or more heat It can be manufactured by a method characterized by cooling to 700 ° C. or lower at a cooling rate of 10 ° C./second or higher within 0.4 seconds and winding in a coil shape at a winding temperature of 450 to 650 ° C. . At this time, it is preferable to water-cool the coiled steel plate after winding.

  According to the present invention, even a cold rolling mill with a small rolling capacity can be manufactured at low cost, as a thin steel plate as it is cold-rolled or a surface-treated steel plate. A thin cold-rolled steel sheet produced by using the hot-rolled steel sheet of the present invention and a steel sheet that has been subjected to surface treatment can greatly contribute to cost reduction of building materials such as wall materials and roofing materials.

It is a figure which shows the relationship between the average crystal grain diameter of a hot-rolled steel plate, TS of a hot-rolled steel plate, and TS of the steel plate as cold-rolled. It is a figure which shows the relationship between AI of a hot-rolled steel plate, and TS of the steel plate as cold-rolled.

  Details of the present invention will be described below. The “%” below represents “% by mass” unless otherwise specified.

1) Chemical composition
C: 0.016-0.07%
When the C content exceeds 0.07%, a large amount of cementite is generated, and the cold rolling load is increased. On the other hand, if the amount of C is less than 0.016%, the degree of supersaturation is low, so that cementite hardly precipitates, and C remains in a solid solution state to increase the cold rolling load. Therefore, the C content is 0.016 to 0.07%.

Si: 0.1% or less
When the Si content exceeds 0.1%, the strength increases and the cold rolling load increases. Therefore, the Si content is 0.1% or less. The lower limit of the Si amount does not need to be specified in particular, but excessive reduction causes an increase in cost, so 0.001% is desirable.

Mn: 0.05-0.5%
Mn fixes S as MnS and has the function of improving hot ductility, so the amount needs to be 0.05% or more. However, if the Mn content exceeds 0.5%, the steel is hardened and the cold rolling load is increased. Therefore, the Mn content is 0.05 to 0.5%.

P: 0.03% or less
P is a solid solution strengthening element. If the amount exceeds 0.03%, the steel is hardened and the cold rolling load is increased. Therefore, the P content is 0.03% or less. The lower limit need not be specified, but excessive reduction causes an increase in cost, so 0.001% is desirable.

S: 0.03% or less
S is an element that inhibits hot ductility. If the amount exceeds 0.03%, ear cracks occur at the coil edge. Therefore, the S amount is 0.03% or less. The lower limit is not particularly required, but excessive reduction causes an increase in cost, so 0.001% is desirable.

Sol.Al: 0.02-0.1%
Al has the effect of reducing the cold rolling load by reducing solid solution N by fixing N that is not fixed by Ti as AlN. In order to obtain such an effect, the amount of Sol.Al needs to be 0.02% or more. However, if the amount exceeds 0.1%, the manufacturing cost increases. Therefore, the amount of Sol.Al is 0.02 to 0.1%.

N: 0.005% or less
N is likely to remain in the hot-rolled steel sheet in a solid solution state, and increases the cold rolling load, so the amount needs to be 0.005% or less. The lower limit need not be specified, but excessive reduction leads to an increase in cost, so 0.001% is desirable and 0.002% is more desirable.

Ti: 0.005-0.03%
Since Ti is a strong nitriding element, the cold rolling load is reduced by fixing N as TiN and reducing the solid solution N. In order to obtain such an effect, the Ti amount needs to be 0.005% or more. However, if the amount exceeds 0.03%, fine TiC precipitates, increasing the cold rolling load. Therefore, the Ti amount is 0.005 to 0.03%.

  The balance is Fe and inevitable impurities.

  In addition to the above component composition, the hot-rolled steel sheet of the present invention can also contain B: 0.0003 to 0.0030% for the following reasons.

B: 0.0003-0.0030%
B, like Ti, is a strong nitride-forming element. Therefore, the cold rolling load is reduced by fixing N as BN and reducing solute N. In order to obtain such an effect, the B content needs to be 0.0003% or more. However, when the amount exceeds 0.0030%, iron boride is formed and the cold rolling load is increased. Therefore, the B content is 0.0003 to 0.0030%.

2) Average crystal grain size: 13 μm or less As shown in FIG. 1, it is effective to reduce the average crystal grain size of the hot-rolled steel sheet to 13 μm or less in order to reduce the TS of the cold-rolled steel sheet. . This is presumably because by reducing the average crystal grain size, the crystal grain boundaries, which are the precipitation sites of cementite, increase and the solid solution C decreases. In particular, it is effective to make the average crystal grain size 13 μm or less in order to precipitate cementite even during the water cooling of the coiled coiled steel sheet. More preferably, it is less than 12 μm.

3) AI: 10MPa or less
AI is an indicator of solute C content and solute N content.As shown in Fig. 2, AI of hot-rolled steel sheet should be 10MPa or less to reduce TS of cold-rolled steel sheet. Although effective, this is considered to be because the amount of dissolved C and the amount of dissolved N are reduced by making AI 10 MPa or less.

4) Manufacturing method The hot-rolled steel sheet of the present invention can be manufactured by hot rolling a steel slab having the above chemical composition under the following conditions.

Finishing temperature: Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C) temperature range, final pass reduction: 20% or more Finishing temperature, which is the exit temperature of the final pass of hot rolling, is less than Ar ₃ transformation point In this case, a heterogeneous microstructure is likely to occur in the sheet thickness direction, and variations in sheet thickness are likely to occur after cold rolling. Here, the final pass means rolling at the final rolling stand in hot rolling. On the other hand, when the finishing temperature exceeds (Ar ₃ transformation point + 49 ° C) or the rolling reduction (also called rolling rate) of the final pass is less than 20%, it is coarsened by recrystallization in the γ (austenite) region after the final pass. After the transformation, a hot-rolled steel sheet having a structure with an average crystal grain size exceeding 13 μm is obtained, and the grain boundaries, which are cementite precipitation sites, are reduced and the cold rolling load is increased. Therefore, it is necessary to set the finishing temperature within the temperature range of Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C.), and the rolling reduction in the final pass should be 20% or more.

Cooling after hot rolling: cooling to 700 ° C or less at a cooling rate of 10 ° C / second or more within 0.4 seconds After hot rolling, that is, immediately after the final pass in the hot rolling described above, the cooling was not started immediately but was allowed to cool. In this case, the crystal grain grows during cooling and becomes a hot-rolled steel sheet having a structure with an average crystal grain size exceeding 13 μm, and the grain boundary, which is a cementite precipitation site, decreases, and the cold rolling load increases. Similarly, when the cooling rate is as low as less than 10 ° C./second, the cold rolling load is similarly increased. Therefore, it is necessary to cool immediately after hot rolling, that is, at a rate of 10 ° C./second or more within 0.4 seconds. Cooling needs to be performed to 700 ° C or less where the crystal grain growth rate is fast.

Winding temperature: 450-650 ° C
When the coiling temperature exceeds 650 ° C., scale defects are likely to occur. In addition, when the coiling temperature is less than 450 ° C., the coil shape tends to be disturbed. Therefore, the coiling temperature is set to 450 to 650 ° C. In addition, as in the present invention, in the steel sheet in which Ti is added and N is fixed, and the average crystal grain size is 13 μm or less and the number of cementite precipitation sites is increased, even if the wound coil is cooled with water, Since C precipitates sufficiently and the amount of dissolved C can be reduced, the coil after winding is preferably water-cooled from the viewpoint of increasing productivity.

  The temperature in the heating prior to hot rolling may be a temperature at which a finishing temperature can be secured, generally 1050 ° C. or higher.

  The hot-rolled steel sheet of the present invention does not change its characteristics even if it is pickled or remains black (non-pickled). Moreover, even if temper rolling or leveling for the purpose of improving pickling properties or correcting the shape, the characteristics are not changed.

  Moreover, since the hot-rolled steel sheet of the present invention is a cold-rolled steel sheet or a surface-treated steel sheet, cold rolling is performed. Since the hot-rolled steel sheet of the present invention has good cold rolling properties, the cold rolling reduction ratio of the cold rolling is as large as about 95% to produce a thin cold-rolled steel sheet with a thickness of 0.5 mm or less. Can be rolled without any problems. In general, the reduction ratio of cold rolling of a thin cold-rolled steel sheet is 60% or more. As described above, since the hot-rolled steel sheet of the present invention has good cold-rollability, the surface treatment is performed on the cold-rolled steel sheet as cold-rolled as a thin material or the cold-rolled steel sheet as cold-rolled. It can be preferably used as a hot-rolled steel sheet for a material for cold rolling of a steel sheet.

After melting steel Nos. 1 to 7 having the chemical composition shown in Table 1 and making them into slabs, they were heated to 1200 ° C and hot-rolled under the hot rolling conditions shown in Table 2 to obtain a plate thickness of 1.8 mm. Rolled steel sheets A to G were produced. Here, the Ar ₃ transformation point in Table 1 was determined by the following equation.
Ar ₃ transformation point (℃) = 901-325 × [C] + 33 × [Si] -92 × [Mn] + 287 × [P]
However, [M] represents the content (mass%) of the element M.

  Next, the obtained hot-rolled steel sheet was pickled and then subjected to temper rolling with an elongation of 1%, and the average crystal grain size and AI were measured by the above methods. In addition, the hot-rolled steel sheet after pickling is cold-rolled at a reduction rate of 80% to produce a cold-rolled steel sheet (sheet thickness: 0.36 mm), and the TS of the cold-rolled steel sheet as described above is used. Measurements were made.

  The results are shown in Table 2.

  It can be seen that the hot-rolled steel sheets B and E, which are examples of the present invention, are soft with a TS of 700 MPa or less as cold-rolled and can reduce the cold-rolling load.

  On the other hand, the hot rolled steel sheet A, which is a comparative example, has a large average crystal grain size and does not sufficiently precipitate cementite, so the AI is high, and the hot rolled steel sheets C, D, and F have solid solution N. However, the AI becomes high and the cold-rolled TS is harder than 700 MPa, so the cold-rolling load cannot be reduced. In the hot rolled steel sheet G having a high Ti content of 0.053%, fine TiC precipitates, so the TS as cold-rolled is harder than 700 MPa, and the cold rolling load cannot be reduced.

Claims (4)

  In mass%, C: 0.016-0.07%, Si: 0.1% or less, Mn: 0.05-0.5%, P: 0.03% or less, S: 0.03% or less, Sol.Al: 0.02-0.1%, N: 0.005% or less , Ti: 0.005 to 0.03%, the remainder has a chemical composition consisting of Fe and inevitable impurities, the average grain size is 13 μm or less, and the aging index AI is 10 MPa or less Hot rolled steel sheet for rolling material.   The hot-rolled steel sheet for cold rolling material according to claim 1, further comprising B: 0.0003 to 0.0030% by mass%. In the steel slab having the chemical composition according to claim 1 or 2, the finishing temperature: Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C), the rolling reduction in the final pass: 20% or more hot rolling For cold rolling material, which is cooled to 700 ° C or less at a cooling rate of 10 ° C / second or more within 0.4 seconds and coiled at a winding temperature of 450 to 650 ° C Manufacturing method for hot-rolled steel sheets.   4. The method for producing a hot-rolled steel sheet for cold rolling material according to claim 3, wherein the coiled steel sheet after winding is water-cooled.

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