JP2004143470A - Steel sheet excellent in paint bake hardenability and retarded natural aging hardenability and its manufacturing process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet of good paint bake hardenability and retarded natural aging hardenability and its manufacturing process. <P>SOLUTION: The steel sheet which contains, by mass%, 0.0005-0.0025% C, ≤2.0% Si, ≤3.0% Mn, ≤0.15% P, ≤0.015% S, 0.2-1.4% Cr, 0.003-0.020% O, ≤0.008% Al, 0.001-0.005% N and the balance Fe and inevitable impurities is offered. It is preferable that solid-soluted C is ≤0.0020% and solid-soluted N is 0.0005-0.004%. A hot rolled and cold rolled steel sheet, and further a cold steel sheet for a galvanized sheet which has good paint bake hardenability and retarded natural aging hardenability is obtained by remaining solid-soluted N and adding Cr and O into an ultralow carbon steel. <P>COPYRIGHT: (C)2004,JPO

Description

【００５９】
【表２】

【００６０】
〔実施例２〕
表１の鋼のうちＢ，Ｇをスラブ加熱温度１１５０℃、仕上げ温度９１０℃、巻取り温度６５０℃で熱間圧延し、４．０ｍｍ厚の鋼帯とした。酸洗後、８０％の圧下率の冷間圧延を施し０．８ｍｍ厚の冷延板とし、ついで連続溶融亜鉛メッキ設備にて加熱速度１４℃／ｓ、最高到達温度８００℃とする焼鈍を行い、６００℃以下４６０℃までを種々の冷却速度で冷却し、４６０℃の亜鉛浴に浸漬させたのち、１５℃／ｓにて５００℃まで再加熱し、１５秒間保持を行った。３００℃から５０℃までの冷却速度も変化させた。その後、さらに０．８％の圧下率の調質圧延をし、ＪＩＳ５号引張試験片を採取しＢＨ、人工時効後の降伏点伸びの測定を行った。
【００６１】
結果を表３に示す。これより明らかなとおり、適正な条件で製造した場合には、高ＢＨ性と常温時遅効性とを両立することができた。
【００６２】
【表３】

【００６３】
【発明の効果】
以上説明したように、本発明は高ＢＨ性と常温時効性とを備え、またＢＨの温度が低温になっても十分なＢＨ量を有する鋼板を提供することが可能となる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a steel sheet having both paint bake hardening performance (BH), normal temperature delayed aging, and formability.
[0002]
The steel sheet according to the present invention is used for automobiles, home appliances, buildings, and the like. Further, it includes a steel plate in a narrow sense without surface treatment and a steel plate in a broad sense subjected to surface treatment such as hot-dip Zn plating, alloyed hot-dip Zn plating, and electric Zn plating for rust prevention. Since the steel sheet according to the present invention is a steel sheet having paint bake hardening performance, it is possible to reduce the thickness of the steel sheet in use, that is, to reduce the weight, in use. Therefore, it is considered that it can contribute to global environmental conservation.
[0003]
[Prior art]
With the recent progress in vacuum degassing of molten steel, it has become easier to produce ultra-low carbon steel. At present, the demand for ultra-low carbon steel sheets having good workability is increasing. Among these, for example, the ultra-low carbon steel sheet to which Ti and Nb are added in combination as disclosed in Patent Literature 1 and the like has extremely good workability, also has paint bake hardening (BH) properties, and has hot-dip galvanizing properties. Because it is excellent, it is becoming an important position. However, the BH amount does not exceed the level of a normal BH steel sheet, and there is a disadvantage that non-aging at room temperature cannot be ensured if a further BH amount is to be added.
[0004]
Patent Literature 2 discloses, for example, a technique relating to a steel sheet having both high BH property and ordinary-temperature delayed aging property. This is because a very large amount of Nb, B, and Ti are added to a very low carbon steel and the structure after annealing is changed to a composite structure of a ferrite phase and a low-temperature transformation generation phase to obtain a high r value, a high BH, a high ductility, and a non-normal temperature. This is to obtain a cold-rolled steel sheet having aging properties. However, it has become apparent that this technology has the following practical operational problems. 1) In steels containing such a large amount of Nb, B and Ti, the α → γ transformation point does not necessarily decrease, and in order to obtain a composite structure, annealing at an extremely high temperature is indispensable. 2) Since the temperature range of α + γ is extremely narrow, the structure changes in the width direction of the sheet, and as a result, the material varies greatly and the composite is changed due to a change in the annealing temperature of several degrees Celsius. It may or may not be an organization, making production extremely unstable.
[0005]
Further, Patent Document 3 discloses that, by controlling the cooling rate after annealing in an ultra-low carbon cold rolled steel sheet to which Nb is added, the carbon concentration in the grain boundaries is increased, thereby achieving both high BH and normal temperature delayed aging. It has been shown that this is possible. However, even by this, the balance between high BH and normal temperature delayed aging cannot be said to be sufficient.
[0006]
Further, in the conventional BH steel sheet, a predetermined BH amount can be obtained if the BH heat treatment condition is 170 ° C. for 20 minutes, but the BH decreases when the condition is 160 ° C. for 10 minutes or 150 ° C. for 10 minutes. There is a problem of doing.
[0007]
[Patent Document 1]
JP-A-59-31827
[Patent Document 2]
Japanese Patent Publication No. 3-2224
[Patent Document 3]
JP-A-7-300623
[0008]
[Problems to be solved by the invention]
As described above, the conventional BH steel sheet has a drawback that stable production is difficult and that the BH content is increased and at the same time the normal-temperature delayed aging is lost. Further, there is a problem that a sufficient BH amount cannot be obtained when the temperature of the paint baking is lower than the current 170 ° C., such as 160 ° C. to 150 ° C.
[0009]
An object of the present invention is to provide a steel sheet having both high BH properties and normal-temperature delayed aging properties, and having a sufficient BH amount even when the BH temperature is low, and a method for producing the same.
[0010]
[Means for Solving the Problems]
The present inventors have earnestly conducted research in order to achieve the above-mentioned goal, and have obtained unprecedented knowledge as described below.
[0011]
That is, by adding Cr to steel in which solid solution N remains, and further adding O (oxygen), it has better BH and room-temperature delayed aging than ever before, and the coating baking condition is reduced to a short time at low temperature. However, it has been found that high BH can be ensured.
[0012]
The present invention is an unprecedented and completely new steel plate constructed based on such ideas and new findings, and the gist thereof is as follows.
(1) In mass%, C: 0.0005 to 0.0025%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.015% or less, Cr: 0.2 to 1.4%, O: 0.003 to 0.020%, Al: 0.008% or less, N: 0.001 to 0.005%, balance Fe and inevitable impurities BH170 is evaluated by applying a heat treatment at 170 ° C. for 20 minutes after 2% tensile deformation, and is evaluated by applying a heat treatment at 160 ° C. for 10 minutes after 2% tensile deformation. BH160 and BH150 evaluated by heat treatment at 150 ° C. for 10 minutes after 2% tensile deformation are each 40 MPa or more.
(2) The steel sheet having excellent paint bake hardening performance and normal temperature delayed aging according to (1), further containing Mo: 0.001 to 1.0% by mass%.
(3) It is characterized in that it further contains one or more of V, Zr, Ce, Ti, Nb, and Mg in a mass% of 0.001 to 0.02% in total (1) or (2) A steel sheet having excellent paint bake hardening performance and normal temperature delayed aging as described in (2).
(4) In any one of (1) to (3), further containing 0.0020% or less of solute C and 0.0005% to 0.004% of solute N in mass%. 2. A steel sheet having excellent baking hardening performance and normal temperature delayed aging according to item 1.
(5) The coating bake hardening performance and the ordinary temperature delayed aging according to any one of (1) to (4), further containing 0.0005 to 0.01% by mass of Ca. Excellent steel plate.
(6) The coating baking hardening performance and the normal temperature delayed aging according to any one of (1) to (5), further containing B: 0.0001 to 0.0015% by mass%. Excellent steel plate.
(7) It is characterized in that it further contains one or more of Sn, Cu, Ni, Co, Zn and W in a total amount of 0.001 to 1.0% by mass. (6) The steel sheet according to any one of (1) and (2), which is excellent in baking hardenability and delayed aging at room temperature.
(8) It is composed of a ferrite single phase structure and has a crystal grain size of 8 μm or more, and is excellent in paint baking hardening performance and ordinary temperature delayed aging according to any one of (1) to (7). steel sheet.
(9) A slab having the chemical component according to any one of (1) to (7) is prepared by (Ar ₃ Hot rolling at a temperature of -100 ° C. or higher, cooling from an end temperature of the hot rolling to 600 ° C. or lower at an average cooling rate of 10 ° C./s or higher, and then winding at 750 ° C. or lower. A method for producing hot-rolled steel sheets with excellent performance and normal-temperature delayed aging.
(10) A slab having the chemical component according to any one of (1) to (7) is prepared by (Ar ₃ Hot rolling at -100 ° C or higher, cold rolling at a rolling reduction of 95% or lower, annealing at a maximum temperature of 600 to 1100 ° C, from 600 to 450 ° C Cooling excellent in paint baking hardening performance and normal temperature delayed aging characterized by cooling at an average cooling rate of 10 ° C / s or more and further cooling at a temperature of 300 to 100 ° C at an average cooling rate of 15 ° C / s or less. Manufacturing method of rolled steel sheet.
(11) A slab having the chemical component according to any one of (1) to (7) is prepared by using (Ar) ₃ Hot rolling at -100 ° C or higher, cold rolling at a rolling reduction of 95% or lower, annealing at a maximum temperature of 600 to 1100 ° C, from 600 to 450 ° C A method for producing a cold-rolled steel sheet having excellent baking hardenability and delayed aging at room temperature, wherein the steel sheet is cooled at an average cooling rate of 10 ° C./s or more and then overaged at 150 to 450 ° C. for 120 seconds or more. .
(12) A slab having the chemical component according to any one of (1) to (7) is prepared by (Ar ₃ Hot rolling at -100 ° C or higher, cold rolling at a rolling reduction of 95% or lower, annealing in a continuous hot-dip galvanizing line so that the maximum temperature is 600 ° C or higher and 1100 ° C or lower, After baking at an average cooling rate of 5 ° C./s or more to a galvanizing bath temperature of 600 ° C. or less, and then cooling from 300 ° C. to 100 ° C. or less at an average cooling rate of 15 ° C./s or less, paint baking hardening performance and normal temperature A method for producing galvanized cold-rolled steel sheets with excellent late aging.
(13) After cooling to a lead plating bath temperature of 600 ° C. or less at an average cooling rate of 5 ° C./s or more, heat treatment at 460 to 550 ° C. for 1 second or more, and an average cooling rate of 15 ° C. from 300 ° C. to 100 ° C. or less. The method for producing a galvanized cold-rolled steel sheet according to (12), wherein the steel sheet is cooled at a rate not higher than / s.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Here, the reason why the steel composition and the production conditions are limited as described above in the present invention will be further described.
[0014]
Since C is an element that increases strength at low cost, the amount of addition varies depending on the intended strength level, but making C less than 0.0005% is difficult in steelmaking technology and increases the cost. Further, in order to impart sufficient BH properties, it is preferable that a small amount of C is present, so this is set as the lower limit. On the other hand, if the C content exceeds 0.0025%, not only is the moldability deteriorated, but also it becomes difficult to achieve both high BH property and non-aging property at room temperature, which are important in the present invention. 0.0007% or more and less than 0.0020% is a more preferable range of C.
[0015]
In addition to the function of increasing the strength as a solid solution strengthening element, Si is also effective in obtaining a structure containing martensite, bainite, and residual γ, etc., and the amount of addition depends on the intended strength level. However, if the addition amount exceeds 2.0%, press formability deteriorates or chemical conversion property deteriorates, so this is made the upper limit. When hot-dip galvanizing or alloyed hot-dip galvanizing is performed, problems such as a reduction in plating adhesion and a reduction in productivity due to a delay in the alloying reaction occur. For applications in which the surface quality is particularly important, such as the outer panel of an automobile door or hood, the upper limit is 0.05%. There is no particular lower limit, but the lower limit of 0.001% is a substantial lower limit because the production cost is high. Further, when it is difficult to perform Al deoxidation from the viewpoint of controlling the amount of Al, deoxidation may be performed with Si. In this case, 0.04% or more of Si is contained.
[0016]
Mn is useful as a solid solution strengthening element, forms MnS, suppresses edge cracking due to S during hot rolling, makes a hot rolled sheet structure fine, obtains a structure containing martensite, bainite, and residual γ. Therefore, it is added as necessary. Further, Mn has an effect of suppressing normal temperature aging caused by solid solution N, so that it is preferable to add 0.3% or more. However, when deep drawability is required, it is preferably 0.15% or less, more preferably less than 0.10%. On the other hand, if it exceeds 3.0%, the strength becomes too high and the ductility is reduced, or the adhesion of galvanizing is hindered.
[0017]
P, like Si, is known as an element that increases the strength at low cost, and is added more actively when the strength needs to be increased. P also has the effect of making the hot-rolled structure finer and improving workability. However, when the addition amount exceeds 0.15%, the fatigue strength after spot welding becomes inferior, and the yield strength increases too much, causing poor surface shape at the time of pressing. Further, the alloying reaction during continuous hot-dip galvanizing becomes extremely slow, and the productivity is reduced. Further, the secondary workability also deteriorates. Therefore, the upper limit is set to 0.15%.
[0018]
If S exceeds 0.015%, it causes hot cracking and deteriorates workability. Therefore, the upper limit is set.
[0019]
Cr is important in the present invention. Only by adding 0.2% or more of Cr, it becomes possible to achieve both the high BH property and the aging resistance at normal temperature. It is known that since N has a higher diffusion rate than C, it is difficult to ensure normal-temperature aging resistance. Therefore, a BH steel sheet utilizing N is not applied to a member whose appearance is important such as an outer panel of an automobile. However, it has been newly found that by positively adding Cr, it is possible to obtain normal-temperature delayed aging without impairing the BH property. The mechanism by which these elements improve the aging resistance at room temperature is not necessarily clear, but is presumed as follows. In the vicinity of room temperature, these elements and N form pairs or clusters, and the room temperature aging resistance is secured to suppress the diffusion of N. On the other hand, in the baking treatment at 150 to 170 ° C., N Escapes from these pairs and clusters and fixes the dislocations, thereby exhibiting high BH properties. The upper limit of Cr is determined from the viewpoint of ensuring workability, plating adhesion and cost, and is set to 1.4%. 0.4 to 1.0% is a more preferable range.
[0020]
O (oxygen) is a particularly important element in the present invention. It has been found that by controlling O to a predetermined amount, the above-described contribution of BH of Cr and ordinary-temperature delayed aging becomes large. The reason for this is not necessarily clear, but it is presumed that Cr and N segregate preferentially around the oxide, and as described above, Cr promotes the effect of suppressing the diffusion of N at room temperature. . Since the effect becomes clear when the O content is 0.003% or more, the lower limit is set. On the other hand, if O exceeds 0.020%, not only does such an effect tend to be saturated, but also the workability such as the r value and ductility deteriorates, so 0.020% is made the upper limit. 0.005 to 0.015% is a more preferable range of O. O is usually present as an oxide of Fe, but may be present as an oxide of Al, Ce, Zr, Mg, Si or the like or a composite oxide thereof. However, Al-based oxides make only a small contribution to the compatibility between high BH and delayed aging at room temperature, and also degrade the surface properties. The form, size, and distribution of the oxide are not particularly limited, but are preferably spherical from the viewpoint of increasing the surface area, the average diameter is 1.0 μm or less, and the proportion of the oxide present at the grain boundaries of the product plate is the volume ratio. Is preferably 20% or less. These requirements are all from the viewpoint of increasing the number of effective sites for segregation of Cr and N as much as possible. From the same viewpoint, it is effective to finely disperse not only oxides but also MnS, CaS, CuS and the like.
[0021]
Al may be used as a deoxidizing agent. However, since Al combines with N to form AlN, the BH property is reduced. Therefore, it is desirable that the addition of Al be kept to the minimum necessary within a range that is reasonable in production technology. From this viewpoint, in the case of a cold-rolled steel sheet, the upper limit is made less than 0.008%. If the Al content exceeds 0.008%, a large amount of the total N must be added in order to secure solid solution N, which is disadvantageous in terms of manufacturing cost and moldability. Less than 0.005% is more preferred, and less than 0.003% is an even more preferred upper limit.
[0022]
N is important in the present invention. That is, in the present invention, a high BH property is achieved mainly by N. Therefore, addition of 0.001% or more is essential. On the other hand, if the N content is too large, excessive Cr must be added to secure the normal temperature delayed aging property, or the workability is deteriorated. Therefore, the upper limit is made 0.005%. More preferably, it is 0.001 to 0.004%.
[0023]
Further, since N is easily combined with Al to form AlN, it is preferable to satisfy N−0.52Al> 0 in order to secure N that contributes to BH. More preferably, N-0.52Al> 0.0005%. This formula is determined because a necessary condition is that the N amount is stoichiometrically larger than the Al amount.
[0024]
Mo may be mainly contained as a solid solution strengthening element in an amount of 0.001% or more. Further, when a large amount is added, strengthening by carbonitride formation can be expected, but ductility deterioration is remarkable, so the upper limit is set to 1.0%.
[0025]
If V is added in the presence of Cr, it will effectively act to ensure normal-temperature delayed aging, so it is preferably added at 0.001% or more. On the other hand, in combination with the following Zr, Ce, Ti, Nb, and Mg, the addition of one or more of them in a total amount of more than 0.02% promotes the formation of nitrides, so the upper limit is set.
[0026]
Zr, Ce, Ti, Nb, and Mg are effective as deoxidizing elements and are difficult to float in molten steel, so that they tend to remain as oxides in the steel, and thus effectively work as Cr and N segregation sites. Since it is widely known that Nb and Ti have an effect of improving workability, it is preferable to add 0.001% or more of each in the case of single addition. However, if the addition amount is too large, nitrides are formed, and it becomes difficult to secure solid solution N. Therefore, when one or more of them including V is added, the total amount is 0.02% or less. I do.
[0027]
The amount of solid solution C is preferably set to 0.0020% or less. In the present invention, the high BH property and the normal temperature aging property are secured mainly by N. Therefore, if the amount of solid solution C is too large, it is difficult to secure the normal temperature aging property. More preferably, the solid solution C is less than 0.0015%. The adjustment of the amount of solid solution C may be performed by making the total amount of C equal to or less than the upper limit described above, or may be reduced to a predetermined level depending on the winding temperature and the overaging condition.
[0028]
It is preferable that the total amount of solute N is 0.0005 to 0.004%. Here, the solute N includes not only N existing alone in Fe but also N forming a pair or a cluster with a substitutional solute element such as Cr, Mo, V, Mn, Si, or P. The amount of solute N can be determined from a value obtained by subtracting the amount of N present as a compound such as AlN, NbN, VN, TiN, BN, or ZrN (determined from the chemical analysis of the extraction residue) from the total N amount. Further, it may be obtained by an internal friction method or FIM (Field Ion Microscopy). If the solute N is less than 0.0005%, sufficient BH cannot be obtained. Further, even if the content exceeds 0.004%, the BH property is improved, but it is difficult to obtain normal temperature delayed aging. More preferably, it is 0.0008 to 0.0022%.
[0029]
Ca is useful as a deoxidizing element and also has an effect on controlling the form of sulfide. Therefore, Ca may be added in the range of 0.0005 to 0.01%. If the content is less than 0.0005%, the effect is not sufficient, and if the content exceeds 0.01%, the workability is deteriorated.
[0030]
B is effective in preventing the embrittlement of secondary working, and is added as necessary. If the addition amount is less than 0.0001%, there is almost no effect. If the addition amount exceeds 0.0015%, not only the effect is saturated, but also BN is easily formed, and it is difficult to secure solid solution N. Become. 0.0001 to 0.0004% is a more desirable range.
[0031]
In order to increase the mechanical strength and the fatigue properties of steel containing these as main components, one or more of Sn, Cu, Ni, Co, Zn and W are contained in a total amount of 0.001 to 1%. It does not matter.
[0032]
The structure of the hot-rolled steel sheet obtained by the present invention is preferably composed of 100% ferrite (single-layer ferrite structure) excluding a phase having a maximum area ratio of ferrite and excluding a precipitation phase such as carbide, nitride, or sulfide. In the present invention, when the content of ferrite is 100%, it is possible to obtain the best ductility and the balance between BH and normal temperature aging. However, bainite or acicular ferrite may be mixed in order to satisfy required characteristics such as strength of steel. The average crystal grain size of the ferrite is desirably 8 μm or more. This is because good formability is ensured. More preferably, it is more than 10 μm. There is no particular upper limit, but if the crystal grain size is too large, the surface is likely to be rough after pressing, so that it is preferably less than 50 μm.
[0033]
Next, reasons for limiting the manufacturing conditions will be described.
[0034]
The slab to be subjected to hot rolling is not particularly limited. That is, it may be any one manufactured with a continuous cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
[0035]
When using a hot-rolled steel sheet as a final product, it is necessary to limit the manufacturing conditions as follows. That is, the finishing temperature of hot rolling is (Ar ₃ -100) C or higher. (Ar ₃ If the temperature is lower than (−100) ° C., it is difficult to secure workability or a problem of thickness accuracy occurs. Ar ₃ A point or more is a more preferable range. The effect of the present invention can be obtained without any particular upper limit of the finishing temperature, but it is preferably 1000 ° C. or lower in order to secure the r value.
[0036]
The heating temperature of hot rolling is not particularly limited, but is preferably set to 1150 ° C. or higher when it is necessary to dissolve in order to secure solid solution N.
[0037]
After hot rolling, cooling must be performed so that the average cooling rate is 10 ° C./s or more from the hot rolling end temperature to at least 600 ° C. Thereby, the normal temperature delayed aging is improved. If the cooling rate is 30 ° C./s or more, it is more preferable for the BH property and the aging resistance at room temperature. The upper limit of the average cooling rate is not particularly defined, but if it is too high, the structure is likely to be non-uniform.
[0038]
The winding temperature is 750 ° C. or less. Although there is no particular lower limit, the temperature is preferably set to 200 ° C. or higher in order to obtain good normal-temperature delayed aging.
[0039]
After the hot rolling, pickling may be performed if necessary, and then a skin pass with a rolling reduction of 10% or less or cold rolling up to a rolling reduction of about 40% may be performed in-line or off-line.
[0040]
Next, the manufacturing conditions when the cold rolled sheet is used as the final product will be described.
[0041]
The finishing temperature of hot rolling is determined from the viewpoint of ensuring the workability of the product sheet (Ar ₃ (−100) ° C. or higher. The upper limit of the finishing temperature is preferably set to 1000 ° C. or lower, similarly to the hot-rolled sheet.
[0042]
The rolling reduction of the cold rolling is set to 95% or less. If it exceeds 95%, not only does the load on the equipment become excessive, but also the anisotropy of the mechanical properties of the product increases. Preferably, it is 86% or less. Although the lower limit of the rolling reduction is not particularly defined, it is preferably at least 30% in order to secure workability.
[0043]
Annealing is performed at a maximum temperature of 600C to 1100C. If the annealing temperature is lower than 600 ° C., recrystallization is not completed, and the workability becomes poor. On the other hand, when the annealing temperature is higher than 1100 ° C., the structure becomes coarse and the workability is lowered. 650-900 degreeC is a more preferable range.
[0044]
Cooling after annealing is important in the present invention. In other words, the cooling rate from 600 ° C. to 450 ° C. or less after annealing is set to 10 ° C./s or more, and the average cooling rate from 300 ° C. to 100 ° C. is set to 15 ° C./s or less, whereby high BH property and room temperature delay are achieved. It is possible to manufacture a steel plate having both aging properties. The cooling rate to 450 ° C. is more preferably 30 ° C./s or more, more preferably 50 ° C./s or more, and the average cooling rate from 300 ° C. to 100 ° C. is more preferably 10 ° C./s or less. However, this is not always the case when performing overage processing. Although the upper limit of the cooling rate up to 450 ° C. is not particularly defined, it is preferably 200 ° C./s or less in order to make the plate shape uniform, and the lower limit of the average cooling rate from 300 ° C. to 100 ° C. is not particularly defined. However, in order to secure productivity, the temperature is preferably set to 3 ° C./s or more.
[0045]
The overaging treatment after cooling is effective for improving the normal temperature delayed aging property. For this purpose, the overaging temperature is preferably set to 150 to 450 ° C., and the time is set to 120 seconds or more. The upper limit of the overaging treatment time is not particularly defined, but if it is too long, the productivity is reduced.
[0046]
On the other hand, when hot-dip galvanizing is performed, the average cooling rate to a galvanizing bath temperature of 600 ° C. or less after annealing is 5 ° C./s or more. Also in this case, in order to further improve the high BH property and the normal temperature delayed aging property, the temperature is preferably 10 ° C./s or more, more preferably 20 ° C./s or more.
[0047]
The upper limit of the average cooling rate is not particularly defined, but is preferably 200 ° C./s or less in order to maintain a good plate shape.
[0048]
Thereafter, when a Zn-Fe alloying treatment is required, reheating is performed in a range of 460 ° C to 550 ° C for 1 second or more. The upper limit of the heating time is not particularly limited, but is preferably 40 seconds or less from the viewpoint of securing the productivity.
[0049]
Thereafter, the average cooling rate from 300 ° C. to 100 ° C. or less is set to 15 ° C./s or less. The lower limit of the average cooling rate is not particularly limited, but is preferably 3 ° C./s or more from the viewpoint of securing productivity.
[0050]
Although the reason why these conditions are suitable for improving the normal-temperature delayed aging property is not necessarily clear, it is presumed that Cr and N promote the segregation around the oxide.
[0051]
The temper rolling is preferably performed in a range of a rolling reduction of 3% or less for further improving the normal temperature delayed aging property and forcing the shape. If it exceeds 3%, the yield strength increases and the load on the equipment increases, so the upper limit is set.
[0052]
The structure of the cold-rolled steel sheet obtained by the present invention is preferably made of ferrite 100% (single-layer ferrite structure) excluding a phase having a maximum area ratio of ferrite and excluding a precipitation phase such as carbide, nitride, or sulfide. In the present invention, when the content of ferrite is 100%, it is possible to obtain the best ductility and the balance between BH and normal temperature aging. However, bainite, acicular ferrite, and martensite may be mixed in order to satisfy required characteristics such as strength of steel. The average crystal grain size of the ferrite is preferably 8 μm or more. Thereby, a steel sheet excellent in formability such as ductility and deep drawability can be obtained. More preferably, it is more than 10 μm. There is no particular upper limit, but if the crystal grain size is too large, the surface is likely to be rough after pressing, so that it is preferably less than 50 μm.
[0053]
The steel plate obtained by the present invention has BH170 of 45 MPa or more, and BH160 and BH150 of all 40 MPa or more. It is more preferable that BH170 be 60 MPa or more, and BH160 and BH150 be 50 MPa or more. The upper limit of BH is not particularly limited. However, if BH 170 exceeds 140 MPa and BH 160 and BH 150 exceed 125 MPa, it becomes difficult to secure normal temperature aging resistance. BH170 is subjected to a heat treatment at 170 ° C. for 20 minutes after 2% tensile deformation, BH160 is subjected to a heat treatment at 160 ° C. after 2% tensile deformation, and BH150 is further subjected to a 2% tensile deformation. It represents BH evaluated by heat treatment at 150 ° C. for 10 minutes after deformation.
[0054]
Cold aging is evaluated by the yield point elongation after artificial aging. The steel sheet obtained by the present invention has a yield point elongation of 0.4% or less, more preferably 0.3% or less in a tensile test after heat treatment at 100 ° C. for 1 hour.
[0055]
Next, the present invention will be described with reference to examples.
[0056]
【Example】
[Example 1]
The steel shown in Table 1 was hot-rolled at a slab heating temperature of 1200 ° C., a finishing temperature of 930 ° C., and a winding temperature of 650 ° C. to form a 3.5 mm thick steel strip. After pickling, cold rolling is performed at a rolling reduction of 80% to form a cold-rolled sheet having a thickness of 0.7 mm. Then, annealing is performed in a continuous annealing facility at a heating rate of 10 ° C./s and a maximum ultimate temperature of 800 ° C. The range of 600 ° C. to 450 ° C. was cooled at various cooling rates shown in Table 2, and the overaging temperature was also changed. Note that the overaging treatment time was fixed at 180 seconds. The cooling rate from 300 ° C. to 50 ° C. was also varied. Further, temper rolling was performed at a rolling reduction of 1.0%, JIS No. 5 tensile test pieces were collected, and BH and the yield point elongation after artificial aging were measured.
[0057]
Table 2 shows the results. As is clear from this, when the steel having the chemical composition of the present invention was annealed under appropriate conditions, it was possible to achieve both the high BH property and the normal temperature delayed aging property. On the other hand, when the O content is less than the range of the present invention, the normal temperature aging property is deteriorated, and when it is too large, the elongation and the r value are deteriorated.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
[Example 2]
Of the steels in Table 1, B and G were hot-rolled at a slab heating temperature of 1150 ° C., a finishing temperature of 910 ° C., and a winding temperature of 650 ° C. to form a 4.0 mm thick steel strip. After pickling, cold rolling is performed at a rolling reduction of 80% to form a cold-rolled sheet having a thickness of 0.8 mm. Then, annealing is performed at a continuous hot-dip galvanizing equipment at a heating rate of 14 ° C / s and a maximum temperature of 800 ° C. After cooling at various cooling rates from 600 ° C. to 460 ° C., and immersing in a zinc bath at 460 ° C., it was reheated to 500 ° C. at 15 ° C./s and held for 15 seconds. The cooling rate from 300 ° C. to 50 ° C. was also varied. After that, temper rolling was further performed at a rolling reduction of 0.8%, JIS No. 5 tensile test pieces were collected, and BH and yield point elongation after artificial aging were measured.
[0061]
Table 3 shows the results. As is evident from the above, when manufactured under appropriate conditions, both high BH property and delayed action at room temperature could be achieved.
[0062]
[Table 3]

[0063]
【The invention's effect】
As described above, the present invention can provide a steel sheet having high BH properties and normal-temperature aging, and having a sufficient BH amount even when the temperature of BH becomes low.

Claims (13)

In mass%, C: 0.0005 to 0.0025%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.15% or less, S: 0.015% or less, Cr: 0 0.2 to 1.4%, O: 0.003 to 0.020%, Al: 0.008% or less, N: 0.001 to 0.005%, the balance being Fe and unavoidable impurities, BH170 evaluated by performing a heat treatment at 170 ° C. for 20 minutes after 2% tensile deformation is 45 MPa or more, and BH160 evaluated by performing a heat treatment at 160 ° C. for 10 minutes after 2% tensile deformation, and BH150 evaluated by heat treatment at 150 ° C. for 10 minutes after 2% tensile deformation is 40 MPa or more. 2. The steel sheet according to claim 1, which further contains Mo: 0.001 to 1.0% by mass%, and further has excellent baking hardenability and delayed aging at room temperature. 3. The method according to claim 1, wherein one or more of V, Zr, Ce, Ti, Nb, and Mg are contained in a total of 0.001 to 0.02% by mass%. 4. Steel sheet with excellent paint bake hardening performance and delayed aging at room temperature. The coating according to any one of claims 1 to 3, further comprising 0.0020% or less of solid solution C and 0.0005% to 0.004% of solid solution N in mass%. A steel sheet with excellent bake hardening performance and normal temperature delayed aging. The steel sheet according to claim 1, further comprising 0.0005 to 0.01% by mass of Ca in terms of mass%, and having excellent baking hardenability and delayed aging at room temperature. The steel sheet according to any one of claims 1 to 5, wherein the steel sheet has excellent paint bake hardening performance and ordinary-temperature late aging property, further containing B: 0.0001 to 0.0015% by mass%. 7. The composition according to claim 1, further comprising 0.001 to 1.0% by mass in total of one or more of Sn, Cu, Ni, Co, Zn and W. 4. A steel sheet having excellent paint bake hardening performance and normal temperature delayed aging according to item 1. The steel sheet according to any one of claims 1 to 7, comprising a ferrite single-phase structure and having a crystal grain size of 8 µm or more, and having excellent baking hardenability and delayed aging at room temperature. The slab having the chemical composition according to any one of claims 1 to 7 is hot-rolled at (Ar ₃ points-100) C or higher, and the average cooling rate is 10C from the hot-rolling end temperature to 600C or lower. A method for producing a hot-rolled steel sheet having excellent baking hardening performance and delayed aging at room temperature, wherein the hot-rolled steel sheet is cooled at 750 ° C./s or higher and then wound at 750 ° C. or lower. The slab having the chemical composition according to any one of claims 1 to 7 is hot-rolled at a temperature of (Ar ₃ points-100) ° C or higher, and cold-rolled at a rolling reduction of 95% or less to achieve a maximum value. Anneal so that the temperature becomes 600 ° C. or more and 1100 ° C. or less, cool from 600 ° C. to 450 ° C. or less at an average cooling rate of 10 ° C./s or more, and further reduce the temperature range of 300 to 100 ° C. to an average cooling rate of 15 ° C./s. A method for producing a cold-rolled steel sheet having excellent baking hardening performance and delayed aging at room temperature, characterized by cooling below. The slab having the chemical composition according to any one of claims 1 to 7 is hot-rolled at a temperature of (Ar ₃ points-100) ° C or higher, and cold-rolled at a rolling reduction of 95% or less to achieve a maximum value. Annealing so that the temperature is 600 ° C or higher and 1100 ° C or lower, cooling from 600 ° C to 450 ° C or lower at an average cooling rate of 10 ° C / s or higher, and then performing an overage treatment at 150 to 450 ° C for 120 seconds or longer A method for producing a cold-rolled steel sheet having excellent paint bake hardening performance and normal-temperature delayed aging characteristics. The slab having the chemical composition according to any one of claims 1 to 7 is hot-rolled at (Ar ₃ points-100) ° C or higher, cold-rolled at a rolling reduction of 95% or lower, and continuously melted. Anneal in a galvanizing line so that the maximum temperature is 600 ° C or higher and 1100 ° C or lower, and cool to a galvanizing bath temperature of 600 ° C or lower at an average cooling rate of 5 ° C / s or higher, and then from 300 ° C to 100 ° C or lower. A method for producing a galvanized cold-rolled steel sheet having excellent baking hardening performance and delayed aging at room temperature, characterized by cooling at an average cooling rate of 15 ° C./s or less. After cooling to a lead plating bath temperature of 600 ° C or less at an average cooling rate of 5 ° C / s or more, heat treatment is performed at 460 to 550 ° C for 1 second or more, and an average cooling rate of 15 ° C / s or less from 300 ° C to 100 ° C or less. The method for producing a galvanized cold-rolled steel sheet according to claim 12, wherein the galvanized cold-rolled steel sheet has excellent baking hardening performance and normal-temperature aging property.