JP4715496B2 - Method for producing cold-rolled steel sheets with excellent strain aging resistance and small in-plane anisotropy - Google Patents

Description

  The present invention relates to a method for producing a cold-rolled steel sheet having excellent strain aging resistance and small in-plane anisotropy, particularly suitable for dry battery cans.

  As a method for processing a cold-rolled steel sheet into a dry battery can, a method in which deep drawing and ironing are appropriately combined is used. For example, DI processing for ironing after drawing cups, stretch drawing and bending back after processing for drawing cups, stretch drawing for ironing as needed, and several stages of drawing Later, methods such as multi-stage drawing with ironing can be mentioned.

  In the processing of dry battery cans, it is required that the can heights in the can circumferential direction after processing do not become uneven, that is, the generation of ears is suppressed. It is generally known that the height of the ear has a good correlation with the in-plane anisotropy Δr of the r value (Rankford value) of the steel plate for dry cell cans, and when Δr approaches 0, the height of the ear decreases. It has been. Therefore, in order to suppress the occurrence of ears, it is desirable to set Δr of the steel plate for dry battery cans to 0, but in general, the occurrence of ears is not observed when −0.10 ≦ Δr ≦ 0.10.

  In addition, steel plates for dry cell cans have excellent strain aging resistance, that is, strain aging index, in order to prevent deterioration of can shape caused by wrinkles called stretcher strain during deep drawing. AI is required to be 4.9MPa or less. Further, the steel plate for dry battery cans is required to have a fine crystal grain size in order to suppress rough skin during processing. Further, in the production method, if the rolling reduction during cold rolling is too high, the load on the equipment is large and the production efficiency is lowered, so that it is desired to be less than 90%. On the other hand, if the rolling reduction is too low, the productivity decreases, so a rolling reduction exceeding 60% is desired.

Conventionally, IF steel (Interstitial free steel) suitable for deep drawing has been put to practical use as such a steel plate for battery cans. For example, in Patent Document 1, in mass%, C: 0.001 to 0.003%, Si: 0.03% or less, Mn: 0.1 to 0.6%, P: 0.02% or less, S: 0.04% or less, Cr: 0.03 to 0.10% , Al: 0.012-0.12%, N: 0.0035% or less, B: 5ppm ≦ B- (11/14) N ≦ 40ppm, Nb: 0.6 ≦ Nb / C (atomic equivalent ratio) ≦ 1.2, balance Fe and inevitable impurities An Ni-plated steel sheet for dry cell cans of Nb-added IF steel excellent in anisotropy having an average surface roughness Ra of 0.02 to 0.2 μm is disclosed.
JP 2002-212673 A

  However, the steel sheet for dry battery cans described in Patent Document 1 requires B addition to improve the in-plane anisotropy, which not only increases the alloy cost but also increases the recrystallization temperature of the steel sheet. A high annealing temperature is required, resulting in an increase in energy costs. In addition, in the steel sheet for dry battery cans described in Patent Document 1, since the Nb addition amount is small compared to the atomic equivalent of C, solid solution C tends to remain, and the strain plate has poor strain aging resistance, that is, AI ≦ 4.9MPa cannot be stably obtained, and deterioration of the can shape may appear.

  The present invention uses Nb-added IF steel, does not require an additive element such as B, improves the in-plane anisotropy of the r value, and is cold-rolled suitable as a steel plate for a dry battery can with excellent strain aging resistance. It aims at providing the manufacturing method of a steel plate.

  As a result of various investigations on steel sheets having excellent strain aging resistance and small Δr using the Nb-added IF steel, the present inventors do not need additive elements such as B. As a result, the atomic equivalent ratio Nb / C is appropriate. Nb is added so as to be within the range, and solid solution C and solid solution N remain in the hot-rolled sheet after hot rolling. Specifically, the cooling conditions after hot-rolling are controlled to control the hot-rolled sheet Strain aging index AI is set to 9.8 MPa or more, and after hot rolling this hot-rolled sheet at a specified reduction rate, continuous annealing at a specified temperature results in AI of 4.9 MPa or less and excellent strain aging resistance. It has been found that a steel sheet having a small in-plane anisotropy of ≦ Δr ≦ 0.10 can be produced.

  The present invention has been made based on such knowledge, in mass%, C: 0.0030% or less, Si: 0.02% or less, Mn: 0.15-0.25%, P: 0.02% or less, S: 0.015% or less, N: 0.004% or less, Al: 0.020 to 0.070%, Nb: 1.0% Nb / C (atomic equivalent ratio) ≦ 5.0 steel slab having a composition composed of Fe and inevitable impurities with the balance (%) , Hot rolled into a hot rolled sheet having a strain aging index AI of 9.8 MPa or more, then cold rolled at a rolling reduction of 65 to 88% to obtain a cold rolled sheet, and then the cold rolled sheet Provided is a method for producing a cold-rolled steel sheet having excellent strain aging resistance and low in-plane anisotropy, characterized by continuously annealing a sheet at an annealing temperature of 700 to 820 ° C.

For example, in mass%, C: 0.0030% or less, Si: 0.02% or less, Mn: 0.15 to 0.25%, P: 0.02% or less, S: 0.015% or less, N: 0.004% or less, Al: 0.020 to 0.070%, Nb: 1.0 ≤ Nb / C (atomic equivalent ratio) ≤ 5.0 in the amount of steel (%) that satisfies the composition consisting of Fe and unavoidable impurities balance, the rolling finish temperature (finishing) of Ar ₃ transformation point or higher Strain aging index AI by hot rolling at the temperature at the rolling side), starting cooling within 2 seconds after the hot rolling, and cooling over a temperature range of 250 ° C or higher at an average cooling rate of 100 ° C / second or higher. Is a hot-rolled sheet of 9.8 MPa or more, then cold-rolled the cold-rolled sheet at a rolling reduction of 65 to 88%, and then the cold-rolled sheet is continuously used at an annealing temperature of 700 to 820 ° C. It is the manufacturing method which anneals.

  According to the present invention, Nb-added IF steel is used, no additional elements such as B are required, AI is 4.9 MPa or less, excellent strain aging resistance, and -0.10 ≦ Δr ≦ 0.10 and small in-plane anisotropy Steel plates for dry battery cans can be manufactured.

  Below, the detail of the manufacturing method of the steel plate for dry battery cans which is excellent in the strain aging resistance which is this invention, and has small in-plane anisotropy is demonstrated.

1) Component (“%” below represents “% by mass”)
The smaller the C: C amount, the softer and the better the elongation, and the better the press workability. When solid solution C is precipitated as carbide, it does not cause strain age hardening and improves deep drawability. If the amount of C exceeds 0.0030%, it becomes difficult to precipitate the entire amount as carbide by adding Nb, and hardening due to solute C and deterioration of elongation appear. Therefore, the C content is 0.0030% or less.

  Si: If the amount of Si exceeds 0.02%, it will lead to hardening and deterioration of plating properties. Therefore, the Si content is 0.02% or less.

  Since Mn: Mn is an element effective for preventing red hot brittleness during hot rolling by S, the amount of Mn needs to be 0.15% or more. On the other hand, if the Mn content exceeds 0.25%, it hardens and deteriorates workability, so the upper limit of the Mn content is 0.25%.

  P: If the P content exceeds 0.02%, the workability is lowered, so the upper limit of the P content is 0.02%.

  When the S content exceeds 0.015%, red hot brittleness is caused during hot rolling. Therefore, the upper limit of the S content is 0.015%, but the smaller the better.

  If the N: N content exceeds 0.004%, AlN precipitates during continuous casting of steel slabs and induces steel slab cracking due to hot brittleness, so the upper limit of N content is 0.004%.

  Al: Al is an element necessary for deoxidation of steel, so the Al content needs to be 0.020% or more. On the other hand, if the Al content exceeds 0.070%, inclusions increase and surface defects are likely to occur, so the upper limit of Al content is 0.070%.

  Nb: Nb precipitates solute C in the steel as NbC and suppresses deep drawability deterioration due to solute C. From such a viewpoint, it is necessary to add Nb in such an amount that the atomic equivalent ratio Nb / C is 1.0 or more. In addition, if unprecipitated solid solution C remains, it is difficult to stably obtain AI ≦ 4.9 MPa of the annealed plate.Therefore, in order to stabilize the precipitation of NbC, Nb is set so that Nb / C is 1.2 or more. It is desirable to add, and it is more desirable to add Nb so that Nb / C is 1.4 or more. However, when Nb is added excessively, an increase in the recrystallization temperature appears. Therefore, it is necessary to add Nb in such an amount that the atomic equivalent ratio Nb / C is 5.0 or less.

  The balance is Fe and inevitable impurities.

2) Manufacturing conditions As described above, one of the features of the present invention is to hot-roll the steel slab with the components adjusted as described above, and to leave the solid solution C or solid solution N in the hot-rolled sheet. By setting the strain aging index AI to 9.8 MPa or more, preferably 29.4 MPa or more, the in-plane anisotropy Δr after cold rolling and annealing is to be reduced to −0.10 ≦ Δr ≦ 0.10. The reason why Δr after annealing is reduced by setting AI of hot-rolled sheet to 9.8 MPa or more is unknown, but solid solution C and N that did not form Nb precipitates affect the texture of hot-rolled sheet. It is presumed to affect.

In order to produce a hot-rolled sheet with AI of 9.8 MPa or more, the steel having the above composition is melted and cast into a slab by continuous casting, and the Ar ₃ transformation point is used to make the crystal grain size uniform after rolling. Hot rolling at the above rolling end temperature, after completion of hot rolling, cooling is started within 2 seconds in order to prevent precipitation of NbC as much as possible and leave solid solution C, and an average of 100 ° C / second or more It is preferable to cool over a temperature range of 250 ° C. or higher at a cooling rate.

The slab after continuous casting may be directly or slightly heated and then rolled, or may be cooled and reheated and then rolled. The heating temperature for reheating is preferably in the range of 1050 to 1300 ° C. This is because when the heating temperature is lower than 1050 ° C., it is difficult to make the rolling end temperature of the hot rolling higher than the Ar ₃ transformation point, and when the heating temperature exceeds 1300 ° C., the amount of oxide generated on the slab surface is small. This is because the surface defects are likely to occur. In addition, the Ar ₃ transformation point that serves as a reference for the rolling end temperature of hot rolling may be obtained by a conventionally known method. For example, after a test piece is heated by a four master test apparatus, a change in thermal expansion coefficient during cooling is observed. Can be obtained. The time from the end of hot rolling to the start of cooling is preferably 1.5 seconds or less, and more preferably 1.0 seconds or less. If the average cooling rate is 100 ° C./second or more, it is not necessary to define an upper limit, and rapid cooling may be performed by, for example, water cooling. The temperature range for rapid cooling is preferably 400 ° C. or higher, more preferably 500 ° C. or higher. Further, after the rapid cooling, there is no particular need to define, and it may be wound around a coil.

  The hot-rolled sheet having an AI of 9.8 MPa or more produced in this way is cold-rolled at a rolling reduction of 65 to 88% after pickling to remove scale formed on the surface, thereby forming a cold-rolled sheet. At this time, pickling may be performed by a conventional method. In addition, as described above, it is desirable that the rolling reduction is more than 60% and less than 90% from the viewpoint of manufacturing, but as in the present invention, a hot rolled sheet having an AI of 9.8 MPa or more is used. By doing so, a steel sheet having a rolling reduction of 65 to 88% and small in-plane anisotropy can be obtained. When the rolling reduction is less than 65% or exceeds 88%, Δr increases, and a small in-plane anisotropy of −0.10 ≦ Δr ≦ 0.10 cannot be obtained.

  The cold-rolled sheet is continuously annealed at an annealing temperature of 700 to 820 ° C. The lower limit of the annealing temperature is set to 700 ° C. because it cannot be completely recrystallized at a lower temperature. The reason why the upper limit is set to 820 ° C. is that crystal grains become coarse at a temperature higher than this, and rough skin is likely to occur during processing. The reason why annealing is performed by continuous annealing is that high-efficiency production is possible. The soaking time for annealing does not need to be specified in particular, but is about 30 seconds or more to ensure the stability of the material properties, and it is 180 seconds or less because it only increases the cost even if annealing is performed for a long time. It is preferable. As other continuous annealing conditions, it is not particularly necessary to define the annealing temperature as described above, and the annealing may be performed in a normal continuous annealing line. Further, after annealing, it is preferable to perform temper rolling for the purpose of adjusting the shape and surface roughness of the steel sheet, and the elongation rate of temper rolling is preferably in the range of 0.3 to 2.0%, which is the normal range. .

  The annealed steel sheet may be subjected to Ni plating, Sn plating, Cr plating or alloy plating thereof as necessary.

Steel having the component composition shown in Table 1 was melted and made into a steel slab by a continuous casting method. After heating the steel slab to 1250 ° C, it was hot-rolled at a rolling finish temperature of 900 ° C, which is higher than the Ar ₃ transformation point of this steel, and cooled by cooling conditions No. 1 and No. 2 shown in Table 2. A rolled sheet was produced. Here, the cooling condition No. 1 is within the scope of the present invention, and the AI of the hot rolled sheet is also 33.3 MPa within the scope of the present invention. On the other hand, the cooling condition No. 2 is outside the scope of the present invention, and the AI of the hot-rolled sheet is also 4.9 MPa, which is outside the scope of the present invention. The hot-rolled sheet thus prepared was pickled, cold-rolled at the rolling reduction shown in Table 3, and after recrystallization annealing at an annealing temperature of 750 ° C. and a soaking time of 45 seconds in a continuous annealing line, 0.5% The samples of steel plates No. 1 to 6 were made by temper rolling at an elongation rate. And about the obtained sample, (DELTA) r, AI, and the crystal grain size were investigated by the following method.

Δr: JIS13B tensile test specimens were taken from the obtained steel sheet samples in the 0 °, 45 °, and 90 ° directions with respect to the rolling direction, and r in the 0 °, 45 °, and 90 ° directions according to JIS Z 2241. Measure r ₀ , r _{45, and} r ₉₀ , and obtain Δr = (r ₀ + r ₉₀ -2r ₄₅ ) / 2. If Δr is ± 0.10 or less, the in-plane anisotropy is small, The object of the invention is achieved.

AI: JIS No. 13 B tensile test specimen was taken from the obtained steel sheet sample at 0 ° to the rolling direction, and 8.0% tensile strain was added to introduce movable dislocation, then 100 ° C x 1 hour constant temperature After processing, the AI was calculated by the following formula. If AI was 4.9 MPa or less, the strain aging resistance was excellent and the object of the present invention was achieved.
AI = (yield load after isothermal treatment−yield load before isothermal treatment) / (cross-sectional area of the parallel part of the specimen before strain introduction)
Crystal grain size: The average crystal grain size of the ferrite structure of the obtained steel sheet sample was measured according to the cutting method described in JIS G 0552. The crystal grain size is preferably 12.0 μm or less so that rough skin does not occur even when processed into a dry battery can.

  The results are shown in Table 3. Further, FIG. 1 shows the relationship between the rolling reduction during cold rolling and Δr. Steel plates Nos. 1 and 2 according to the present invention both exhibit Δr of ± 0.10 or less and AI of 4.9 MPa or less, and are excellent in strain aging resistance and small in-plane anisotropy. Further, the crystal grain size is 12.0 μm or less, and rough skin does not become a problem even if it is processed into a dry battery can. As shown in FIG. 1, it can be seen that by using a hot-rolled sheet within the range of the present invention, Δr can be made ± 0.10 or less at a rolling reduction of 65 to 88% during cold rolling.

It is a figure which shows the relationship between the rolling reduction at the time of cold rolling, and (DELTA) r.

Claims (2)

  In mass%, C: 0.0030% or less, Si: 0.02% or less, Mn: 0.15 to 0.25%, P: 0.02% or less, S: 0.015% or less, N: 0.004% or less, Al: 0.020 to 0.070%, 1.0 ≦ A steel slab having a composition containing Nb amount (%) satisfying Nb / C (atomic equivalence ratio) ≦ 5.0 and the balance consisting of Fe and inevitable impurities is hot-rolled to have a strain aging index AI of 9.8 MPa or more. The hot-rolled sheet is then subjected to cold rolling at a rolling reduction of 65 to 88% to form a cold-rolled sheet, and then the cold-rolled sheet is subjected to continuous annealing at 700 to 820 ° C. A method for producing cold-rolled steel sheets with excellent strain aging and small in-plane anisotropy. In mass%, C: 0.0030% or less, Si: 0.02% or less, Mn: 0.15 to 0.25%, P: 0.02% or less, S: 0.015% or less, N: 0.004% or less, Al: 0.020 to 0.070%, 1.0 ≦ A steel slab having a composition comprising Nb amount (%) satisfying Nb / C (atomic equivalence ratio) ≦ 5.0 and the balance consisting of Fe and inevitable impurities is hot-rolled at a rolling finish temperature not lower than the Ar ₃ transformation point. The cooling starts within 2 seconds after the hot rolling, and is cooled over a temperature range of 250 ° C. or more at an average cooling rate of 100 ° C./second or more to obtain a hot rolled sheet having a strain aging index AI of 9.8 MPa or more, Next, the hot-rolled sheet is cold-rolled at a rolling reduction of 65 to 88% to obtain a cold-rolled sheet, and then the cold-rolled sheet is continuously annealed at 700 to 820 ° C. The manufacturing method of the cold-rolled steel plate with small in-plane anisotropy.

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