JP5549307B2 - Cold-rolled steel sheet excellent in aging and bake hardenability and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet suitable for building materials and home appliances and a method for producing the same, and in particular, the bake-hardening ability of a member formed by relatively mild processing such as bending and shallow overhanging. Regarding improvement. The “steel plate” here includes a steel plate and a steel strip. The “cold-rolled steel sheet” also includes a cold-rolled steel sheet and a cold-rolled steel sheet obtained by subjecting the cold-rolled steel sheet to a surface treatment such as electrogalvanizing or hot dip galvanizing. Further, it includes a steel sheet having a chemical conversion coating on the surface of a cold-rolled steel sheet, or on the surface of a cold-rolled steel sheet further subjected to surface treatment such as electrogalvanizing or hot dip galvanizing.

  In recent years, building materials and household electrical appliances have been strongly required to reduce manufacturing costs. In particular, in order to reduce material costs, the use of thin materials has been rapidly progressing, and the strength of steel plates to be used has been demanded. . The strength of steel sheets used for building materials and household appliances is up to 440MPa in tensile strength. Moreover, it is mainly for members that are subjected to relatively mild processing such as bending and shallow overhang.

  The increase in strength of the steel sheet used in accordance with the demand for reducing the thickness of the material is required not only for building materials and members for home appliances but also for members for automobiles. Since the CAFE regulations in 1989, the weight reduction of automobile bodies has been eagerly desired to improve automobile fuel efficiency, and the strength of steel sheets used has been increased. Against this background, high-strength steel sheets with an appropriate amount of P, Si, Mn, etc. added for automobiles have been developed and used one after another. Many steel plates have a TS of 590 MPa or more. It is difficult to apply such high-strength steel sheets for building materials and home appliances. This is because such a high-strength steel sheet has a high strength, so that it is difficult to process beyond the capacity of the processing machine, and even if it can be processed, the accuracy of the product is lowered. In addition, such high-strength steel sheets have many alloying elements to be added and are often expensive, so that the effect of reducing material costs cannot be expected.

  Further, many bake-hardening type soft steel plates have been proposed and used for automobile outer plates. For example, Patent Document 1 discloses that, by weight, C: 0.002 to 0.008%, Si: 0.5% or less, Mn: 0.05 to 1.2%, P: 0.10% or less, Al: 0.01 to 0.08%, N% × 8 or more. , Nb: C% × 3 or more, (C% × 8 or less + 0.02%) or less hot-rolled steel sheet is cold-rolled at a rolling reduction of 60% or more, under conditions of 750 to 900 ° C. for 10 seconds or more. A method for producing a cold-rolled steel sheet having excellent seizure hardenability is described in which after continuous annealing, cooling is performed at an average cooling rate of 10 ° C./s or more up to at least 650 ° C. in the cooling process. Since the steel sheet manufactured by the technique described in Patent Document 1 is an extremely low carbon type, it has a soft and high workability at the time of forming, and dislocations introduced at the time of forming by chemical conversion treatment or paint baking treatment after forming. Solid solution carbon and solid solution nitrogen adhere to and harden to increase the strength. Such a steel plate has a tensile strength of about 340 MPa to 390 MPa, and is often used as a BH (bake hardening) steel plate for automobile outer plates.

  Patent Document 2 describes a method for producing a cold-rolled steel sheet having excellent strength increasing ability by heat treatment after forming. Patent Document 2 includes C: 0.15% or less, Si: 0.005-1.0%, Mn: 0.01-3.0%, Al: 0.005-0.02%, N: 0.006-0.020%, and P: 0.002-0.10% for N ( %) / Al (%) ≧ 0.3 When the steel slab having a composition that satisfies the condition of 0.3 is hot rolled and wound, when [Mn%] × [Si%] is 1.0 or less, the winding temperature ≦ 700 ° C. On the other hand, when [Mn%] × [Si%] is larger than 1.0, the coiling temperature ≦ 300 + 400 / ([Mn%] × [Si%]), and in the recrystallization annealing step, [ When Mn%] × [Si%] is 1.0 or less, the annealing temperature is 650 to 950 ° C., and when [Mn%] × [Si%] is greater than 1.0, 950−300 / ([Mn% ] × [Si%]) ≦ Annealing temperature ≦ 950 A method of manufacturing a cold-rolled steel sheet is described, in which annealing is performed at a temperature satisfying the following condition. Accordingly, it is said that a cold rolled steel sheet having a product of precipitated Mn% and precipitated Si% in steel of 0.00010% or less, containing solute N of 0.0015% or more and having a ferrite phase or a ferrite-based structure is obtained. In the steel sheet manufactured by the technique described in Patent Document 2, the tensile strength is increased by 60 MPa or more due to the heat treatment after forming due to the interaction between the solid solution N and the dislocations introduced during forming.

Japanese Patent Publication No. 60-17004 JP 2002-226937 A

The BH steel sheet for automobiles described in Patent Document 1 requires 2% or more of distortion and heat treatment such as baking coating treatment, and as a result, an increase in strength of about 30 MPa is recognized. Moreover, even steel sheets for automobiles such as those described in Patent Document 2 require strain application of 5% or more and heat treatment in a low temperature range of 120 to 200 ° C., thereby increasing the tensile strength TS by 60 MPa or more. Strengthening is obtained. However, members in building materials, home appliances, and the like often have relatively mild processing of 2% or less. Alternatively, there are some that are hardly processed and used in a flat state. For this reason, when the steel plate for automobiles described in Patent Document 1 and Patent Document 2 is used as a building material or a member for home appliances, the amount of distortion applied is small, and subsequent baking coating processing, etc. However, there is a problem in that the increase in strength is small and the desired increase in strength cannot be achieved. In addition, the BH steel sheet for automobiles described in Patent Document 1 is age-hardened by being left for a long time, and there is a problem that wrinkles called stretcher strains are generated at the time of forming, and the appearance of the product is remarkably impaired. Moreover, in the technique described in patent document 2, in order to ensure the predetermined amount of solute N, the upper limit of Al content is 0.02%. Al is usually added to deoxidize, that is, to reduce oxygen in the steel as Al ₂ O ₃ . If the amount of Al is small, deoxidation becomes insufficient, the amount of oxygen remaining in the steel increases, the cleanliness decreases, cracks occur during cold rolling, and surface defects are likely to occur. .

In view of the problems of the prior art, the present invention can prevent wrinkles due to aging, can ensure a high bake hardening amount with relatively low strain application of about 2% or less, has excellent aging properties, and also has bake hardenability. It aims at proposing the outstanding cold-rolled steel plate and its manufacturing method.
Here, “excellent bake hardenability” means that after applying a pre-strain of less than 2.0% (including 0%) and performing a heat treatment equivalent to a coating baking process of 100 to 200 ° C. × 5 to 60 min. The difference ΔYS (= YS _HT −YS _PS ) between the yield strength YS _HT and the stress YS _PS at the time of prestraining is 50 MPa or more. “Excellent aging” means that the yield elongation YEl after holding for 3 months in a normal temperature atmosphere of 25 ° C. or less is 2% or less in the rolling direction.

In order to achieve the above-mentioned object, the present inventors diligently studied the influence of alloy elements on bake hardenability. As a result, in order to provide high bake hardenability even with a low strain amount, the inventors have come up with effective use of solid solution N. Moreover, in order not to generate wrinkles due to aging, it has been found that it is important that C exists as a precipitate as much as possible.
In order to reduce solute C as much as possible by further research by the present inventors, the coiling temperature in hot rolling is adjusted appropriately to refine the crystal grains and precipitate carbides in the crystal grains. Further, it has been found that it is important to precipitate the remaining solid solution C as a carbide by appropriately adjusting the heating rate and annealing temperature in annealing after cold rolling. In addition, in order to secure an appropriate amount of solid solution N, it is important to properly adjust the Al content that forms a precipitate by combining with N, and to optimize the coiling temperature and annealing temperature in hot rolling. I found out. Furthermore, it has been found that temper rolling after cold rolling effectively contributes to suppressing wrinkles due to aging.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.020 to 0.070%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.08%, N: 0.005 to Including 0.02%, solid solution N is 0.0010% or more, the composition composed of the balance Fe and inevitable impurities, and the ferrite phase having an average crystal grain size of 7 μm or less in an area ratio of 80% or more. A cold-rolled steel sheet having a structure in which precipitates having an average equivalent circle diameter of 0.05 to 5 μm are precipitated and dispersed in the grains.

(2) The cold-rolled steel sheet according to (1), wherein the precipitate has a precipitation density of 1 to 100 pieces / 0.01 mm ² .
(3) In (1) or (2), in addition to the above-mentioned composition, by mass%, one or two selected from Ti: 0.01% or less, Nb: 0.01% or less, B: 0.005% or less A cold-rolled steel sheet containing at least seeds.

(4) In any one of (1) to (3), in addition to the above composition, in terms of mass%, Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, Cu: 0.01% or less A cold-rolled steel sheet comprising one or more selected from among them.
(5) The cold rolled steel sheet according to any one of (1) to (4), wherein the surface has a galvanized layer.

(6) When a steel material is subjected to a hot rolling step, a cold rolling step, an annealing step, and a temper rolling step in order to obtain a cold rolled steel sheet, the steel material is, in mass%, C: 0.020 to Contains 0.070%, Si: 0.05% or less, Mn: 0.1-0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02-0.08%, N: 0.005-0.02%, the balance Fe and inevitable impurities After the hot rolling step, the steel material is heated to a heating temperature of 1150 ° C. or higher, and then roughly rolled into a sheet bar, and then the finish rolling finish temperature on the sheet bar: Finish rolling to 850 ° C or higher to make a hot-rolled sheet, and then the hot-rolled sheet is expressed by the following formula (1)
[(Al / 28) / (N / 14)] / CT ≤ 5.5 × 10 ^-3 (1)
(Where, Al, N: content of each element (mass%), CT: coiling temperature (° C.))
The cold rolling step is performed by pickling the hot rolled sheet, and then cold rolling to a cold rolling reduction ratio of 60 to 90%. It is a process to make a rolled sheet, and the annealing process is a process in which the cold-rolled sheet is annealed to form a cold-rolled annealed sheet, and the annealing process sets the annealing temperature An to the following formula (2)
1.0 ≦ {[(Al / 28) / (N / 14)] / CT} / {[(Al / 28) / (N / 14)] / An} ≦ 1.5 (2)
(Where, Al, N: content of each element (mass%), CT: coiling temperature (° C), An: annealing temperature (° C))
The heating rate in the temperature range from 300 ° C. to (annealing temperature −20 ° C.) is 1 to 30 ° C./s, and the temperature range from (annealing temperature −20 ° C.) to (annealing temperature) A heating rate is 0.5 to 5 ° C./s, heating to the annealing temperature, and then cooling to 500 ° C. or less at a cooling rate of 5 ° C./s or more, and the temper rolling step is the cold rolling A method for producing a cold-rolled steel sheet, which is a step of subjecting an annealed sheet to temper rolling at an elongation of 0.5 to 5%.

(7) A method for producing a cold-rolled steel sheet according to (6), wherein the soaking is performed for 150 s or less at the annealing temperature, followed by cooling.
(8) In (6) or (7), in addition to the above composition, in addition to mass, one or two selected from Ti: 0.01% or less, Nb: 0.01% or less, B: 0.005% or less The manufacturing method of the cold-rolled steel plate characterized by including more than seed | species.

(9) In any one of (6) to (8), in addition to the above composition, in addition to mass, Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, Cu: 0.01% or less A method for producing a cold-rolled steel sheet, comprising one or more selected from among them.
(10) In any one of (6) to (9), a galvanizing treatment step for forming a galvanized layer on a steel sheet surface is performed after the annealing step and before the temper rolling step. A method for producing rolled steel sheets.

  (11) In (10), after the cold-rolled sheet is cooled to 500 ° C. or less at the cooling rate of 5 ° C./s or more in the annealing step, it is subsequently immersed in a hot dip galvanizing bath as the galvanizing treatment step. And a hot dip galvanizing process for forming a hot dip galvanized layer on the surface of the steel sheet, or an alloying process for alloying the hot dip galvanized layer. Production method.

  According to the present invention, it is possible to easily and inexpensively provide a cold-rolled steel sheet excellent in aging and bake hardenability, which suppresses aging and exhibits high bake hardenability even with low strain addition, and has a remarkable industrial effect. Play. Further, according to the present invention, office members such as office desk tops, vending machines, refrigerator panels, etc., which are formed by relatively light processing such as bending and shallow overhang, It is useful as a member for home appliances such as an air conditioner outdoor unit, and further as a steel material for building materials, and has an industrially effective effect that it can contribute to weight reduction and cost reduction of the product.

First, the reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.020-0.070%
C is an element that dissolves to increase the strength of the steel, but a large amount has an adverse effect of reducing formability (workability). In particular, when the amount of dissolved C is increased, aging is promoted and adverse effects such as generation of wrinkles at the time of molding are exerted. Therefore, in the present invention, it is desirable to reduce the amount of dissolved C as much as possible. In the present invention, at the time of winding or annealing, C is precipitated as a precipitate such as cementite to reduce the solid solution C as much as possible. In this method, since it is necessary to contain an appropriate amount of C in advance, C is set to 0.020% or more. If C is less than 0.020%, the degree of supersaturation for carbide precipitation is small, and C does not sufficiently precipitate as carbide. On the other hand, if it exceeds 0.070%, the workability is remarkably lowered. For this reason, C was limited to the range of 0.020 to 0.070%.

Si: 0.05% or less
Si is an element that dissolves to increase the strength of steel, but if it is contained in a large amount, it hardens and the workability decreases. In addition, when a large amount of Si is contained, an adverse effect such as generation of Si oxide during annealing and inhibition of plating properties is caused. In addition, Si is an element that has a strong tendency to stabilize ferrite. For example, during hot rolling, the transformation temperature from austenite (γ) to ferrite (α) increases, making it difficult to complete rolling in the austenite region. There is a case. For these reasons, Si was limited to 0.05% or less.

Mn: 0.1-0.5%
Mn has the effect of increasing the strength of steel by solid solution and forming MnS to detoxify harmful S that has harmful effects such as inducing hot cracking and remarkably deteriorating surface properties. It is. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, when the content exceeds 0.5%, adverse effects such as hardening and workability deterioration, and suppression of ferrite recrystallization during annealing become remarkable. For this reason, Mn was limited to the range of 0.1 to 0.5%. In addition, Preferably it is 0.3% or less.

P: 0.05% or less P is an element that contributes to an increase in strength, but segregates at the grain boundaries and has an adverse effect on reducing ductility and toughness. For this reason, it is desirable to reduce as much as possible, especially when it is not necessary to use the increase in strength due to P. However, if it is about 0.05% or less, the above-described adverse effects are acceptable. For this reason, P was limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

S: 0.02% or less S induces hot cracking and significantly deteriorates the surface properties. Furthermore, S is mostly present as inclusions in steel and contributes little to the strength, but also forms coarse MnS and lowers the ductility. For this reason, it is desirable to reduce S as much as possible as an impurity. However, if it is 0.02% or less, the above-described adverse effects are acceptable. For this reason, S was limited to 0.02% or less.

Al: 0.02 to 0.08%
Al is an important element in the present invention. Al is an element that acts as a deoxidizer, and in order to obtain this effect sufficiently, it needs to be contained in an amount of 0.02% or more. More preferably, it is over 0.02%. Moreover, Al has the effect | action which couple | bonds with N and fixes N as AlN. In the present invention, Al is adjusted to an appropriate range in order to stably secure solid solution N contributing to an increase in bake hardenability within a desired range.

  The precipitation of AlN is affected by temperature. For this reason, in order to ensure a desired solid solution N amount stably, the Al content is expressed by the equation (1) in relation to the coiling temperature, and further in the relationship between the coiling temperature and the annealing temperature (2). Adjust to satisfy the equation. In order to satisfy the expressions (1) and (2), at least 0.02% or more of Al is required. On the other hand, since a large amount of Al increases the transformation point of the γ → α transformation during hot rolling, it is difficult to complete the rolling in the austenite region. For these reasons, Al is limited to 0.02 to 0.10%. In addition, Preferably it is 0.060% or less.

N: 0.005-0.02%
N is an element that increases the strength of steel by solid solution, and in the present invention, solid solution N is utilized for improving bake hardenability. In order to ensure the desired excellent bake hardenability, it is necessary to ensure 0.0010% or more as the solid solution N, and N needs to contain at least 0.005%. On the other hand, if the content exceeds 0.02%, the tendency of slab cracking to increase and surface flaws may occur. For this reason, N was limited to the range of 0.005 to 0.02%. In addition, Preferably it is 0.007 to 0.015%.

Solid solution N: 0.0010% or more Solid solution N adheres to dislocations introduced by applying strain during coating baking, and increases the strength of the steel sheet. In order to secure a desired ΔYS (amount of increase in yield strength after applying a strain and baking treatment), the solid solution N amount is set to 0.0010% or more in the present invention. The solute N amount is adjusted by optimizing the coiling temperature and the annealing temperature after containing the Al content within the range as described above. The amount of solute N is preferably 0.0020% or more. More preferably, it is 0.0040% or more.

The above-mentioned components are basic components, and in addition to the basic composition, depending on the desired strength, further selected as follows: Ti: 0.01% or less, Nb: 0.01% or less, B: 0.005% or less One or more selected from the group consisting of Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, Cu: 0.01% or less, Can be contained.
One or more selected from Ti: 0.01% or less, Nb: 0.01% or less, B: 0.005% or less
Ti, Nb, and B are all elements that have the effect of refining the ferrite of the cold-rolled steel sheet, and can be selected as necessary and contain one or more. In order to obtain such effects, it is desirable that Ti: 0.001% or more, Nb: 0.001% or more, and B: 0.0005% or more, respectively. However, excessive inclusion reduces the amount of dissolved N and causes baking. Curability decreases. For this reason, when it contains, it limits to Ti: 0.01% or less, Nb: 0.01% or less, and B: 0.005% or less, respectively.

One or more selected from Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, Cu: 0.01% or less
Mo, Ni, Cr, and Cu are all elements that have the effect of increasing the strength of the steel sheet by solid solution strengthening, and can be selected as necessary to contain one or more. In order to obtain such effects, it is desirable that Mo: 0.001% or more, Ni: 0.001% or more, Cr: 0.001% or more, and Cu: 0.001% or more, respectively. On the other hand, excessive inclusion leads to a decrease in ductility, so it is limited to Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, and Cu: 0.01% or less, respectively.

The balance other than the components described above consists of Fe and inevitable impurities.
Next, the reason for limiting the structure of the cold-rolled steel sheet of the present invention will be described.
The cold-rolled steel sheet of the present invention has a structure containing a ferrite phase with an area ratio of 80% or more from the viewpoint of ensuring ductility and improving workability. Examples of the second phase other than the ferrite phase include pearlite, martensite, bainite, and retained austenite. When the ferrite phase is less than 80% in area ratio, the structure fraction of the second phase increases and the workability decreases. From the viewpoint of formability, the ferrite phase is preferably 85 to 95% by area ratio.

  The average crystal grain size of the ferrite phase is 7 μm or less. If the average crystal grain size exceeds 7 μm, the distribution of strain introduced by temper rolling becomes non-uniform, and the introduced strain cannot be effectively applied to the entire steel sheet. In addition, Preferably it is 4-7 micrometers. The average crystal grain size of ferrite is observed with 20 or more fields of view with an optical microscope (magnification: 200 to 1000 times), and a value calculated by a cutting method or image analysis based on the JIS method is used.

  Further, precipitates mainly composed of carbide are precipitated and dispersed in the ferrite phase grains. Thereby, the solid solution C can be reduced as much as possible, and the appearance of yield elongation due to aging can be suppressed. The size of the precipitate is 0.05 to 5 μm in terms of an average equivalent circle diameter. Since the bake hardening amount (ΔYS) is greatly influenced by the particle size of the precipitates, the bake hardening amount of ΔYS: 50 MPa or more can be stably secured by the presence of an appropriate amount of precipitates of this size. When the average size of the precipitate is less than 0.05 μm, the bake hardening amount is small, and ΔYS: 50 MPa or more cannot be secured stably. On the other hand, if the average is larger than 5 μm, the amount of bake-hardening is small even if strain is applied, and desired characteristics cannot be obtained.

In addition, it is preferable to make the precipitate of the above-mentioned size exist at a precipitation density of 1 to 100 pieces / 0.01 mm ² . When the precipitation density is less than 1 piece / 0.01 mm ² , the bake hardening amount is small, and ΔYS: 50 MPa or more cannot be secured stably. On the other hand, if it exceeds 100 pieces / 0.01 mm ² , the workability deteriorates.
Next, a preferred method for producing the cold-rolled steel sheet of the present invention will be described.
In the manufacturing method of the cold-rolled steel sheet of the present invention, a hot-rolling process, a cold-rolling process, an annealing process, and a temper rolling process are sequentially performed on the steel material to obtain a cold-rolled steel sheet.

Next, a preferred method for producing the cold-rolled steel sheet of the present invention will be described.
In the manufacturing method of the steel material, C: 0.020 to 0.070%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.08%, N: 0.005 to 0.02% is included as long as a steel material composed of the balance Fe and unavoidable impurities can be obtained. Although there is no particular limitation, the molten steel having the above composition can be converted into a converter method, an electric furnace method, etc. It is preferable to use a conventional melting method to produce a steel material such as a slab by a conventional casting method such as a continuous casting method. The steel material casting method is desirably an intermittent casting method in order to prevent macro segregation of components, but there is no problem with the ingot casting method or the thin slab casting method.

The obtained steel material is then subjected to a hot rolling process, but the heating for hot rolling is performed by cooling to room temperature and then reheating, without cooling to room temperature. Energy-saving processes such as direct feed rolling and direct rolling, in which the material is charged into the heating furnace as it is or after a slight heat retention is performed, can be applied without any problem.
The hot rolling process is a process in which a steel material is heated at a predetermined temperature, subjected to hot rolling including rough rolling and finish rolling to form a hot rolled sheet, and then wound.

The heating temperature is preferably 1150 ° C. or higher.
In heating in hot rolling, it is necessary to once dissolve carbonitride such as AlN and Fe ₃ C during heating and to precipitate only carbide after winding. For this reason, it is preferable to limit the heating temperature of hot rolling to 1150 ° C. or higher. If the heating temperature is less than 1150 ° C, the carbonitride is not sufficiently dissolved, and it is impossible to obtain a precipitate having an appropriate size after winding. The upper limit of the heating temperature is not particularly limited, but is preferably 1300 ° C. or lower from the viewpoint of crystal grain coarsening, scale loss due to oxidation, and the like.

The heated steel material is roughly rolled into a sheet bar having a predetermined size and shape. However, the rough rolling condition is not particularly limited as long as the predetermined size and shape can be secured. Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet.
The finish rolling finishing temperature of finish rolling is preferably 850 ° C. or higher.
In the present invention, finish rolling is preferably performed in the austenite (γ) region. In finish rolling, if the temperature of the steel sheet changes from the γ region to the ferrite (α) region, the rolling load decreases rapidly, making it difficult to control the rolling mill load, and the risk of troubles during sheet passing such as breakage. There is sex. On the other hand, such a risk can be avoided by passing the steel sheet temperature from the inlet side as a temperature in the α range, but the rolling temperature is lowered and the structure of the hot rolled sheet becomes non-recrystallized ferrite. There arises a problem that the rolling load increases during the subsequent cold rolling. For this reason, the finish rolling is finished in the γ region, and the temperature is preferably 850 ° C. or higher within the steel composition range of the present invention. On the other hand, the upper limit of the finish rolling finish temperature is not particularly limited, but if it becomes too high, there is a problem that the crystal grains become coarse and the workability of the cold-rolled sheet is lowered. It is preferable.

The obtained hot rolled sheet is then wound into a coil. The cooling rate until winding is not particularly limited, and a cooling rate higher than air cooling is sufficient. If necessary, forced cooling, for example, rapid cooling at 100 ° C./s or higher may be performed.
In the present invention, the coiling temperature CT is expressed by the following equation (1) in relation to the Al and N contents.
[(Al / 28) / (N / 14)] / CT ≤ 5.5 × 10 ^-3 (1)
(Where, Al, N: content of each element (mass%), CT: coiling temperature (° C.))
Adjust to satisfy.

  In the present invention, in order to suppress precipitation of AlN and secure a desired solid solution N amount in the hot-rolled sheet, the coiling temperature CT is adjusted to satisfy the formula (1) related to the Al amount and the N amount. To do. When the coiling temperature CT does not satisfy the above formula (1), the desired amount of dissolved N in the hot-rolled sheet cannot be secured, and the desired excellent bake hardenability cannot be secured in the cold-rolled steel sheet. In addition, by winding a hot-rolled sheet on the above-mentioned conditions, the solid solution C precipitates as a carbide | carbonized_material, and the crystal grain diameter of a ferrite phase refines | miniaturizes.

The hot rolled sheet is then subjected to a cold rolling process. In the cold rolling step, the hot-rolled sheet is subjected to a pickling treatment, and then cold-rolled to a cold rolling reduction ratio of 60 to 90% to obtain a cold-rolled sheet.
The cold rolling reduction ratio after pickling the hot rolled sheet is preferably determined by the thickness of the hot rolled sheet and the product sheet. Usually, when the cold rolling reduction ratio is 60% or more, there is no particular problem in workability and sheet thickness accuracy. On the other hand, if the cold rolling reduction ratio exceeds 90%, the load on the cold rolling mill becomes too large and operation becomes difficult. For this reason, it is preferable to limit the cold rolling reduction ratio to the range of 60 to 90%.

The cold-rolled sheet is then subjected to an annealing process.
An annealing process is a process of giving an annealing process to a cold-rolled sheet, and setting it as a cold-rolled annealed sheet. In annealing treatment, the annealing temperature An is expressed by the following equation (2)
1.0 ≦ {[(Al / 28) / (N / 14)] / CT} / {[(Al / 28) / (N / 14)] / An} ≦ 1.5 (2)
(Where, Al, N: content of each element (mass%), CT: coiling temperature (° C), An: annealing temperature (° C))
Is satisfied. In the annealing treatment of the present invention, it is preferable to first adjust the annealing temperature An to a temperature satisfying the expression (2) in order to secure a desired amount of dissolved N and obtain a desired bake hardenability. The amount of solid solution N depends on the amount of Al, the amount of N, the coiling temperature, and the annealing temperature. Therefore, in order to ensure the desired amount of solid solution N, the annealing temperature is the amount of Al, the amount of N, and the coiling. It is important to adjust so as to satisfy the equation (2) which is a relational expression between the temperature and the annealing temperature. When the annealing temperature does not satisfy the formula (2), it becomes difficult to secure a desired amount of dissolved N.

The heating up to the annealing temperature An described above is preferably heating in which the heating rate is changed in two stages. By changing the heating rate up to the annealing temperature in two stages, the size of the precipitates (carbides) in the grains and the amount of precipitation can be set to a desired distribution state.
The first stage heating is heating in a temperature range from 300 ° C. to (annealing temperature −20 ° C.), and the heating rate is preferably 1 to 30 ° C./s. When the heating rate in this temperature range is less than 1 ° C./s, the carbide generated in the hot-rolled sheet is dissolved and the amount of solute C increases. On the other hand, when it exceeds 30 ° C./s, precipitation of carbides and the like in the ferrite phase grains becomes insufficient, the amount of solid solution C increases, and the yield elongation cannot be reduced after the strain imparting-paint baking process. For this reason, the heating rate in this temperature range is preferably limited to the range of 1 to 30 ° C./s.

  The second-stage heating is heating in a temperature range from (annealing temperature −20 ° C.) to (annealing temperature), and the heating rate is preferably 0.5 to 5 ° C./s. When the heating rate in this temperature range is less than 0.5 ° C./s, the carbide is dissolved, the amount of solute C increases, and the yield elongation cannot be reduced after the strain imparting-paint baking process. On the other hand, if it exceeds 5 ° C./s, precipitation of carbide and the like in the ferrite phase grains becomes insufficient, the amount of solid solution C increases, and the yield elongation cannot be reduced after the strain imparting-paint baking process. For this reason, the heating rate in this temperature range is preferably limited to the range of 0.5 to 5 ° C./s.

After heating to the annealing temperature An by the two-stage heating described above, it is preferable to perform soaking at the annealing temperature An of 150 s or less. When the soaking time is longer than 150 seconds, the grains grow and become coarse grains, which causes rough skin during processing and the surface properties are lowered. For this reason, it is preferable to limit the soaking time at the annealing temperature An within a range of 150 s or less. In the present invention, the soaking time at the annealing temperature An is 0 s, that is, includes the case where the cooling is started immediately after reaching the annealing temperature An. If it is less than 15 s, recrystallization will not be completed or even if it is completed, grain growth may be suppressed and ductility (elongation) may be reduced, and more preferably 15 s or more.
Cooling after soaking is performed at a cooling rate of 5 ° C./s or higher to 500 ° C. or lower.

  If the cooling rate after soaking is less than 5 ° C / s, the coarsening of the precipitates in the ferrite phase grains becomes remarkable, it becomes difficult to secure the desired precipitate size, and the desired bake hardening amount cannot be ensured. . When the cooling stop temperature exceeds 500 ° C., coarsening of the carbide proceeds by subsequent cooling. For this reason, after soaking, it is preferable to cool to 500 ° C. or less at a cooling rate of 5 ° C./s or more. The cooling rate in the region lower than 500 ° C. does not need to be particularly limited. Further, a heat history such as holding during cooling may be taken.

  In addition, if necessary, a galvanizing treatment process for forming a galvanized layer on the steel sheet surface, such as hot dip galvanizing and electrogalvanizing, to improve corrosion resistance after the annealing process is performed before the temper rolling process. You may go. For example, after the annealing step, a hot dip galvanizing treatment step in which hot dip galvanizing treatment is performed near 480 ° C. may be performed. In the hot dip galvanizing process, preferably, after the annealing process, the cold-rolled sheet is cooled to a predetermined temperature of 500 ° C. or lower, preferably about 450 ° C., at a cooling rate of 5 ° C./s or higher. Subsequently, a hot dip galvanizing treatment for forming a hot dip galvanized layer on the surface of the steel sheet by immersing in a hot dip galvanizing bath maintained at a temperature close to 480 ° C., or a further formed hot dip galvanized layer, preferably 450 ° C. It is preferable to apply an alloying treatment for alloying the plating layer by heating to 550 ° C. or lower to form an iron / zinc alloy layer.

After the annealing step, or after the annealing step, the hot dip galvanizing step or the galvanizing step, a temper rolling step is performed. The temper rolling step is preferably a step of subjecting the cold-rolled annealed sheet to temper rolling with an elongation of 0.5 to 5%.
The cold-rolled annealed sheet can be subjected to temper rolling to correct the shape, and an appropriate strain can be applied to the steel sheet (surface) to suppress the occurrence of wrinkles due to aging in the cold-rolled steel sheet. Especially in the case of light processing with a small amount of processing added to the steel plate during processing to the member, in order to ensure the desired bake hardening amount after the processing-paint baking process, the amount of strain applied in the temper rolling is important. Become. In this invention, it is desirable to limit the elongation rate in temper rolling to the range of 0.5 to 5%. If the elongation in temper rolling is less than 0.5%, it is difficult to ensure a desired bake hardening amount, especially in the case of light processing with a small amount of processing on the member. On the other hand, if the elongation exceeds 5%, the strength of the steel sheet increases due to work hardening, and the formability decreases, so that a shape defect often occurs after processing. The elongation is preferably 3% or less.

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

A steel material (slab) having the composition shown in Table 1 is used as a starting material, and subjected to a hot rolling process, a cold rolling process, an annealing process, and a temper rolling process under the conditions shown in Table 2, and the cold thickness of the sheet shown in Table 2 A rolled steel sheet was used.
About the obtained cold-rolled steel sheet, the amount of solid solution N, structure | tissue, tensile characteristics, bake hardenability, and aging were investigated. The test method was as follows.
(1) Measurement of solid solution N amount A test piece for electrolytic extraction was collected from the obtained cold-rolled steel sheet, and the N amount in the electrolytic extract extracted by the potentiostatic electrolysis method in the acetylacetone-based electrolytic solution was analyzed. The amount. The obtained amount of precipitated N was subtracted from the total amount of N in the steel to obtain a solid solution N amount.

(2) Microstructure observation test Samples for microstructural observation were collected from the obtained cold-rolled steel sheet, the cross section in the rolling direction was polished and corroded (Nital solution), and about 1/4 to 3/4 of the plate thickness Using an optical microscope (magnification: 200 to 1000 times) or a scanning electron microscope (magnification: 500 to 2000 times), observing the number of fields of view: 20 fields or more, imaging and identifying tissues, and using an image analyzer The average crystal grain size of the ferrite phase and the structure fraction of each phase were determined. Further, the size of precipitates precipitated in ferrite grains and the precipitates per unit area using a scanning electron microscope (magnification: 2000 to 5000 times) or a transmission electron microscope (magnification: 2000 to 5000 times). The number (precipitation density) was measured. Here, the average crystal grain size of the ferrite phase was calculated by a cutting method in accordance with the provisions of JIS G 0552-1998. Further, the size of the precipitate is obtained by calculating the area of each precipitate, calculating the equivalent circle diameter of each precipitate from the area, obtaining the arithmetic average of the obtained values, and displaying the average equivalent circle diameter. did.

(3) Tensile test From the obtained cold-rolled steel sheet, a JIS No. 5 tensile test piece was taken so that the rolling direction was the tensile direction, and a tensile test was performed at a tensile rate of 10 mm / min to obtain tensile properties (yield strength YS , Tensile strength TS, elongation El).
(4) Bake hardenability test From the obtained cold-rolled steel sheet, a JIS No. 5 tensile test piece was taken so that the rolling direction would be the tensile direction, and the pre-strain shown in Table 3 was added in the tensile test. YS _PS was obtained (pre-strain processing). Then, after the pre-straining process, a heat treatment equivalent to the coating baking process under the conditions shown in Table 3 (paint baking equivalent heat treatment) was performed. After pre-straining-heat treatment, a tensile test was performed to determine yield strength YS _HT and yield elongation (El _Y ) _HT . As bake hardenability, yield strength increases amount ΔYS before and after the heat treatment _- was calculated (= yield strength YS _HT predistortion stress YS _PS during _processing).

(5) Aging After storing the obtained cold-rolled steel sheet at 25 ° C for 3 months, a JIS No. 5 tensile test piece was taken so that the rolling direction was the tensile direction, a tensile test was conducted, and the yield elongation YEl was determined. And aging was evaluated. When YEl was 2% or less, it was evaluated that the aging property was excellent.
The obtained results are shown in Table 3.

  In each of the inventive examples, a structure in which a ferrite phase having an average particle diameter of 7 μm or less was 80% or more in area ratio and precipitates having an equivalent circle diameter (average) of 0.005 to 5 μm dispersed in the ferrite grains was observed. . In addition, pearlite, bainite, etc. were recognized as 2nd phases other than a ferrite. Each of the inventive examples has a tensile strength TS: strength of 340 MPa or more, excellent workability, excellent aging property, and a strain of less than 2.0%. The amount of increase in yield strength is 50 MPa or more, and it is a cold-rolled steel sheet with excellent bake hardenability that can obtain a large bake hardening amount even when lightly processed. On the other hand, a comparative example out of the scope of the present invention provides a cold-rolled steel sheet having desired characteristics when elongation is low and workability is reduced, YEl is aged greatly, or the amount of bake hardening is small. Not.

  A part of the steel material (slab) having the composition shown in Table 1 is subjected to a hot rolling process, a cold rolling process, an annealing process, a hot dip galvanizing process, and a temper rolling process under the conditions shown in Table 4. Table 4 It was set as the cold rolled steel plate (plated steel plate) which has the plating layer of the board thickness shown to. In addition, in the hot dip galvanizing process, after cooling to 440 ° C. during the cooling of the annealing process, the hot dip galvanizing process to be immersed in a hot dip galvanizing bath (bath temperature: 480 to 520 ° C.) is subsequently performed. A hot-dip galvanized layer having the adhesion amount shown was formed. In some of the hot dip galvanizing treatment steps, in addition to the hot dip galvanizing treatment, heating was further performed at 480 to 530 ° C., and the alloying treatment was performed by alloying the plating layer into an alloyed hot dip galvanizing layer.

The obtained cold-rolled steel sheet (plated steel sheet) was examined for the amount of solute N, structure, tensile properties, bake hardenability, and aging. The test method was the same as in (Example 1). Furthermore, about the obtained cold-rolled steel plate (plated steel plate), the surface was observed visually and the presence or absence of non-plating was investigated.
The results obtained are shown in Table 5.

None of the inventive examples are non-plated, have excellent surface properties, and have a tensile strength of TS: 340 MPa or more after plating treatment, excellent workability, excellent aging, and light processing. It is a cold-rolled steel plate (plated steel plate) excellent in bake hardenability that can obtain a large bake-hardening amount even if it is applied. On the other hand, a comparative example outside the scope of the present invention is not a cold-rolled steel sheet (plated steel sheet) having desired characteristics.

Claims (11)

% By mass
C: 0.020 to 0.070%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.02 to 0.08%,
N: 0.005-0.02%
A solid solution N is 0.0010% or more, the composition comprising the balance Fe and inevitable impurities, and a ferrite phase having an average crystal grain size of 7 μm or less in an area ratio of 80% or more, and within the crystal grains of the ferrite phase And a structure in which precipitates having an average equivalent circle diameter of 0.05 to 5 μm are deposited and dispersed. The cold-rolled steel sheet according to claim 1, wherein a precipitation density of the precipitates is 1 to 100 pieces / 0.01 mm ² .   2. In addition to the above composition, the composition further contains, in mass%, one or more selected from Ti: 0.01% or less, Nb: 0.01% or less, and B: 0.005% or less. Or the cold-rolled steel plate of 2.   In addition to the above composition, the composition further contains one or more selected from Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, and Cu: 0.01% or less. The cold-rolled steel sheet according to any one of claims 1 to 3.   The cold-rolled steel sheet according to any one of claims 1 to 4, further comprising a galvanized layer on the surface. The steel material is subjected to a hot rolling process, a cold rolling process, an annealing process, and a temper rolling process in order to obtain a cold rolled steel sheet.
C: 0.020 to 0.070%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.02 to 0.08%,
N: 0.005-0.02%
A steel material having a composition comprising the balance Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to a heating temperature of 1150 ° C. or higher, and then roughly rolled into a sheet bar, and then the finish rolling to the sheet bar is finished to a finish rolling temperature of 850 ° C. or higher. Applying hot rolled sheet, and then winding the hot rolled sheet at a winding temperature CT satisfying the following formula (1):
The cold rolling step is a step of subjecting the hot-rolled sheet to pickling treatment, and then cold-rolling the cold-rolled reduction ratio: 60 to 90% to obtain a cold-rolled plate,
The annealing step is a step in which the cold-rolled sheet is annealed to form a cold-rolled annealed sheet, and the annealing process sets the annealing temperature An to a temperature satisfying the following expression (2), and is 300 ° C. to (annealing temperature −20 The annealing temperature is 1 to 30 ° C./s in the temperature range up to (° C.) and 0.5 to 5 ° C./s in the temperature range from (annealing temperature −20 ° C.) to (annealing temperature). Is then annealed to cool to 500 ° C. or lower at a cooling rate of 5 ° C./s or higher,
The temper rolling step is a step of subjecting the cold-rolled annealed sheet to temper rolling with an elongation of 0.5 to 5%.
A method for producing a cold-rolled steel sheet.
Record
[(Al / 28) / (N / 14)] / CT ≤ 5.5 × 10 ^-3 (1)
1.0 ≦ {[(Al / 28) / (N / 14)] / CT} / {[(Al / 28) / (N / 14)] / An} ≦ 1.5 (2)
Here, Al, N: Content (mass%) of each element,
CT: Winding temperature (℃),
An: Annealing temperature (° C).   The method for producing a cold-rolled steel sheet according to claim 6, wherein soaking is performed for 150 s or less at the annealing temperature, and then cooling is performed.   7. In addition to the above composition, the composition further contains one or more selected from the group consisting of Ti: 0.01% or less, Nb: 0.01% or less, and B: 0.005% or less. Or the manufacturing method of the cold-rolled steel plate of 7.   In addition to the above composition, the composition further contains one or more selected from Mo: 0.01% or less, Ni: 0.01% or less, Cr: 0.01% or less, and Cu: 0.01% or less. The method for producing a cold-rolled steel sheet according to any one of claims 6 to 8. 10. The manufacturing of a cold-rolled steel sheet according to any one of claims 6 to 9, wherein a galvanizing treatment process for forming a galvanized layer on the steel sheet surface is performed after the annealing process and before the temper rolling process. Method. In the annealing step, the cold-rolled sheet is cooled to a predetermined temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more, and subsequently immersed in a hot dip galvanizing bath as the galvanizing treatment step. The hot-dip galvanizing process which forms the hot-dip galvanized layer in this, or the alloying process which further alloyes this hot-dip galvanized layer is given, The cold rolling of Claim 10 characterized by the above-mentioned A method of manufacturing a steel sheet.