JPWO2020003986A1 - Cold rolled steel sheet, hot dip galvanized steel sheet, and method for manufacturing alloyed hot dip galvanized steel sheet - Google Patents

Abstract

An object of the present invention is to provide a method for manufacturing a cold rolled steel sheet which is excellent in formability and bake hardenability. The present invention is a method for producing a cold rolled steel sheet, comprising a hot rolling step of obtaining a hot rolled steel sheet from a steel material, a cold rolling step of obtaining a cold rolled steel sheet, and an annealing step of obtaining a cold rolled steel sheet that has been annealed. The steel material is, by mass%, C: 0.0010% or more and 0.0030% or less, Nb: 0.010% or more and 0.025% or less, Al: 0.01% or more and 0.10% or less. , Si: 0.05% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.010% or less, B: 0.0030% or less, N: 0.010% or less , The balance Fe and unavoidable impurities, the dew point of the furnace atmosphere during the annealing is −35° C. or lower, and the annealing is held at a temperature of 850° C. or higher and lower than 910° C., and then 5 It is a method for producing a cold-rolled steel sheet, which is a process of cooling to a temperature of 750° C. or lower at an average cooling rate of C/s or higher.

Description

The present invention relates to a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a method for manufacturing an alloyed hot-dip galvanized steel sheet.

In recent years, exhaust gas regulations have been implemented in order to reduce pollutants emitted from the viewpoint of conservation of the global environment, and automobiles are strongly required to improve fuel consumption by reducing the weight of the vehicle body. The strength of thin steel sheets used as one of the promising methods for reducing the weight of vehicle bodies is increasing, and the amount of high-strength steel sheets used is increasing year by year.
As a method for increasing the strength of a thin steel sheet, a method of using solid solution strengthening by adding a solid solution strengthening element, a precipitation strengthening method of using precipitation by adding a precipitation strengthening element, and martensite which is a hard phase are used. The strengthened method is common. However, each of the methods has a problem in that as the strength is increased, the formability is decreased due to the decrease in ductility and r-value, the spot weldability is deteriorated, and the secondary work brittleness resistance is decreased. Therefore, when manufacturing a high-strength steel sheet having excellent formability, the method of adding the above-mentioned strengthening element or the method of utilizing martensite which is a hard phase is not necessarily a preferable method.
Further, as another means for reducing the weight of the vehicle body, there is a method of improving the formability of the thin steel sheet. By improving the formability of thin steel sheets, it is possible to change from the conventional welding assembly method in which many parts are welded and assembled to the integrated molding method, and the overlap margin for welding is reduced, and the weight of the automobile body is reduced. Is possible. In addition, the integral molding has an advantage that the manufacturing cost can be reduced in addition to the reduction in weight.
As a punch that achieves both high moldability and high strength, it achieves high strength in the final part (product) after assembly and painting due to the softness during molding, the processing strain introduced during molding and the subsequent paint baking treatment. There is a way to do it. A steel plate using this method is called a bake hardening type steel plate. By using this method, soft and good formability at the time of forming, on the other hand, the strain aging phenomenon of steel caused by the work strain introduced into the steel sheet and the paint baking treatment after forming, so-called bake hardening is utilized. The deformation strength can be increased, and the final component (product) can have higher strength. Although this method limits the amount of increase in strength, it has the advantage that it is soft when it is molded into the shape of the final part (product) and that the final part (product) can be made stronger after baking coating. There is. Moreover, this method has an advantage that the spot weldability is good because no strengthening element is added, and the presence of solid solution C that increases the grain boundary strength makes it difficult to reduce the secondary work embrittlement resistance. Further, since no additional element is used, there is an advantage that the manufacturing cost can be suppressed.
Several proposals have been made so far regarding a method for producing a bake hardenable steel sheet having such advantages, for example, Patent Document 1 and Patent Document 2.

Patent Document 1 contains C: 0.0005 to 0.0035 mass% and Nb: 5C to 9C mass% (where C is a C content (mass %)) so as to satisfy a specific relationship. After hot-rolling and cold-rolling a steel material containing Si, Mn, B, Al, and N in specific amounts, continuous annealing is performed from 730° C. to a specific temperature T determined by the amounts of Nb and C. Disclosed is a method for producing a thin steel sheet having high bake hardenability and surface roughening resistance, characterized in that the warm time is 30 seconds or longer and the residence time at the specific temperature T or higher is 40 seconds or longer. In Patent Document 1, by the above method, the progress of recrystallization and the timing of dissolution of Nb carbide can be appropriately adjusted, a high BH amount (bake hardening amount) can be secured, and the rough skin resistance and deep drawing can be improved. It is supposed that the moldability can be improved.

Further, in Patent Document 2, C: 0.0008 to 0.0025 mass% and Nb: 0.008 to 0.020 mass% are defined as the amount of excess C = C-(12/93)Nb>-0.0005%. Hot-dip galvanized zinc line after hot-rolling, pickling and cold-rolling a steel material containing a specific amount of Si, Mn, P, S, Al and B The annealing temperature at 800° C. or more and less than 850° C. when the Excess C amount exceeds 0%, and 850° C. or more and less than the Ac3 transformation point when the Excess C amount is −0.0005% or more and 0% or less, Further disclosed is a method for producing a hot-dip galvanized steel sheet having excellent bake hardenability, characterized by cooling to a temperature of 750° C. or lower at an average cooling rate of 5° C./s or more after annealing at the annealing temperature. .. In Patent Document 2, the melting temperature of Nb carbide can be appropriately adjusted by the above method, and a high BH amount (bake hardening amount) can be secured.

Japanese Patent Laid-Open No. 8-100221 JP, 2007-270167, A

However, when the present inventors examined the techniques described in Patent Document 1 and Patent Document 2, for example, the BH amount (bake hardening amount) greatly fluctuates due to variations in the C amount and Nb amount during tapping. Therefore, although the ductility is excellent, the BH amount is lower than the target value and the characteristics required by the user cannot be satisfied, or conversely, the BH amount is high, but the ductility is lowered, which may cause problems such as press cracking. It became clear. It was also clarified that due to variations in the C content and Nb content during tapping, stretcher strain is likely to occur during press working, and the surface appearance may be significantly impaired.
In addition, in the technique described in Patent Document 1, as a specific relationship between C and Nb,
730≧−9100/{log(Nb·C)−3.7}−273
In the steelmaking stage, the C content and Nb content are adjusted so as to satisfy
T=-9100/{log(Nb·C)-3.7}-273
In the actual operation, it is not easy to operate while satisfying such a complicated condition.
Moreover, also in patent document 2, in addition to C: 0.0008-0.0025 mass% and Nb: 0.008-0.020 mass %, the amount of Excess C=C-(12/93)Nb>-0.0005. %, and the C amount and the Nb amount are severely limited, and it is difficult to operate under such a limitation.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for manufacturing a cold-rolled steel sheet having excellent formability and bake hardenability.

In order to achieve the above-mentioned object, the present inventors have studied in detail the influence of the composition of steel and the manufacturing conditions on the BH content. As a result, the C content is controlled to be 0.0010% or more and 0.0030% or less, and the Nb content is controlled to be in the range of 0.010% or more and 0.025% or less, and the atmosphere in the furnace for the annealing treatment after cold rolling is controlled. By adjusting the dew point and the annealing temperature and adjusting the cooling rate in the specific temperature range to an appropriate range, the solid solution C amount was kept constant without depositing C as a precipitate, and thus stable at 30 MPa or more. The inventors have found that the amount of BH can be secured, and completed the present invention.
That is, the inventors have found that the above problems can be solved by the following configurations.

(1) a hot rolling step of hot rolling a steel material to obtain a hot rolled steel sheet;
A cold rolling step of performing cold rolling on the hot rolled steel sheet to obtain a cold rolled steel sheet, and
An annealing process of obtaining a cold rolled steel sheet that has been subjected to an annealing treatment by subjecting the cold rolled steel sheet to an annealing treatment, and
A method for manufacturing a cold rolled steel sheet, comprising:
The above steel material is mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.01% or more and 0.10% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.010% or less, a steel material having a composition of balance Fe and inevitable impurities,
The dew point of the furnace atmosphere during the annealing treatment is −35° C. or lower,
A method for manufacturing a cold-rolled steel sheet, wherein the annealing treatment is a treatment of holding at a temperature of 850° C. or higher and lower than 910° C. and then cooling it to a temperature of 750° C. or lower at an average cooling rate of 5° C./s or higher.
(2) A method for producing a hot-dip galvanized steel sheet, wherein the cold-rolled steel sheet after the annealing treatment obtained by the production method according to (1) above is subjected to hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet.
(3) A method for producing an alloyed hot-dip galvanized steel sheet, which comprises subjecting the hot-dip galvanized steel sheet obtained by the production method according to (2) above to an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.

As shown below, according to the present invention, it is possible to provide a method for manufacturing a cold-rolled steel sheet having excellent formability and bake hardenability.

Below, the manufacturing method of the cold-rolled steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet of this invention is demonstrated.
In addition, in this specification, the numerical range represented using "-" means the range which includes the numerical value described before and after "-" as a lower limit and an upper limit.

[Cold Rolled Steel Sheet Manufacturing Method]
The method for producing a cold-rolled steel sheet of the present invention (hereinafter, also referred to as “method of the present invention”) is
A hot rolling step of subjecting a steel material to hot rolling to obtain a hot rolled steel sheet;
A cold rolling step of performing cold rolling on the hot rolled steel sheet to obtain a cold rolled steel sheet, and
An annealing process of obtaining a cold rolled steel sheet that has been subjected to an annealing treatment by subjecting the cold rolled steel sheet to an annealing treatment, and
A method for manufacturing a cold rolled steel sheet, comprising:
The above steel material is mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.01% or more and 0.10% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.010% or less, a steel material having a composition of balance Fe and inevitable impurities,
The dew point of the furnace atmosphere during the annealing treatment is −35° C. or lower,
A method for manufacturing a cold-rolled steel sheet, wherein the annealing treatment is a treatment of holding at a temperature of 850°C or higher and lower than 910°C, and then cooling it to a temperature of 750°C or lower at an average cooling rate of 5°C/s or higher.

Hereinafter, each step will be described.

[Hot rolling process]
The hot rolling step is a step of performing hot rolling on a steel material having a specific composition to obtain a hot rolled steel sheet.

<Steel material>
The composition of the steel material used in the hot rolling process will be described below. In addition, "%" in the composition means "mass %" unless otherwise specified.

(C: 0.0010% or more and 0.0030% or less)
C is an element that increases the strength and develops the bake hardenability, and it is desirable to add a large amount, but if the addition amount is large, the ductility and the formability are deteriorated. In the present invention, the content of 0.0010% or more is required in order to secure a solid solution C amount of a predetermined amount or more and obtain a bake hardening amount of 30 MPa or more. On the other hand, if the amount of addition is large, the ductility is greatly reduced. Therefore, C is limited to 0.0030% or less. From the reason that the effect of the present invention is more excellent, C is preferably 0.0015% or more. C is preferably 0.0025% or less, more preferably 0.0020% or less.

(Nb: 0.010% or more and 0.025% or less)
Nb is an element that improves the formability of the steel sheet, and is an element that fixes C as a precipitate (carbide) before annealing. In order to obtain such effects, the content of 0.010% or more is required. On the other hand, if it is contained excessively, the formability of the steel sheet is deteriorated, C is excessively fixed as carbide, and the amount of solid solution C cannot be secured without melting of carbide in the annealing process, resulting in bake hardening. The amount decreases. Therefore, Nb is limited to 0.025% or less. Nb is preferably 0.020% or less, more preferably 0.015% or less.

(Al: 0.01% or more and 0.10% or less)
Al acts as a deoxidizer in the steelmaking process, has a strong affinity with N, has a strong tendency to form AlN, and contributes to improving the yield of elements having a strong tendency to form nitrides. To obtain such an effect, the content of 0.01% or more is required. The amount of Al is preferably 0.02% or more because the formability and the bake hardenability of the obtained cold rolled steel sheet are more excellent (hereinafter, also referred to as “the effect of the present invention is more excellent”). On the other hand, even if Al is contained in an amount of 0.10% or more, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. In addition, AlN becomes coarse and becomes a factor that reduces the ductility of the steel sheet, so the content is limited to 0.10% or less. From the reason that the effect of the present invention is more excellent, Al is more preferably 0.03% or more. Al is preferably 0.08% or less, more preferably 0.06% or less.

(Si: 0.05% or less)
Si is an effective element that acts as a deoxidizer in the steelmaking process and strengthens the steel. In order to obtain such an effect, it is desirable to contain 0.005% or more, but if it exceeds 0.05%, the formability and the plating property are deteriorated. Therefore, Si is limited to 0.05% or less. Si is preferably 0.02% or less, more preferably 0.01% or less.

(Mn: 1.0% or less)
Mn is an effective element that acts as a deoxidizer in the steelmaking process and strengthens the steel. In order to obtain such effects, it is desirable to contain 0.10% or more. Further, for the reason that the effect of the present invention is more excellent, Mn is preferably more than 0.5%, more preferably more than 0.6%. On the other hand, the excessive Mn content exceeding 1.0% increases the strength of the steel sheet more than necessary and makes it brittle. Therefore, Mn is limited to 1.0% or less, and Mn is preferably 0.9% or less, more preferably 0.8% or less.

(P: 0.10% or less)
P has the effect of strengthening the steel, and it is desirable to contain at least 0.02% or more in order to increase the strength, but if it exceeds 0.10%, the secondary work embrittlement resistance decreases. Therefore, P is limited to 0.10% or less. From the reason that the effect of the present invention is more excellent, it is preferably 0.05% or less.

(S: 0.010% or less)
S exists as an inclusion in steel and reduces formability. Therefore, it is desirable to reduce S as much as possible, but as S is reduced, the manufacturing cost of the steel material increases. Therefore, the S content is set to 0.010% or less. In addition, S is preferably 0.009% or less, and more preferably 0.008%.

(B: 0.0030% or less)
B is an element effective for fixing N because it combines with N to form BN, and is also an element effective for improving the secondary work embrittlement resistance. In order to obtain such effects, the content of 0.0003% or more is desirable. On the other hand, if the content exceeds 0.0030%, the formability is lowered. Therefore, B is limited to the range of 0.0030% or less. Note that B is preferably 0.0003% or more, more preferably 0.0005% or more, because the effect of the present invention is more excellent. B is preferably 0.0020% or less, more preferably 0.0015% or less.

(N: 0.010% or less)
If N is contained in the steel sheet, carbonitrides are formed and the bake hardening amount is reduced, so it is desirable to reduce N as much as possible. Therefore, the N content is set to 0.010% or less. When the content of N is 0.010% or less, N is combined with Al and B to form a nitride, so that the bake hardening amount can be secured. From the reason that the effect of the present invention is more excellent, it is preferably 0.0030% or less.

(The rest)
The balance other than the above components is Fe and inevitable impurities.
In addition, O as an unavoidable impurity is preferably 0.0050% or less because the effect of the present invention is more excellent.

<Preferable embodiment>
A preferred embodiment of the hot rolling step is, for example, a step of heating the above-mentioned steel material, then hot rolling it and winding it.

(Heating temperature)
The heating temperature of the steel material is not particularly limited, but is preferably 1100 to 1250° C. because the effect of the present invention is more excellent.

(Rolling end temperature)
The rolling end temperature (FDT) of hot rolling is preferably 900° C. or higher from the viewpoint of formability. The rolling end temperature is preferably 900 to 980° C. because the crystal grain coarsening of the hot rolled steel sheet is suppressed and, as a result, the press workability (deep drawability) of the obtained cold rolled steel sheet is more excellent. More preferably, the rolling end temperature is 950°C or lower.

(Winding temperature)
The winding temperature is preferably 700° C. or lower. When the winding temperature exceeds 700° C. and becomes high, the effect of improving the material is saturated and the pickling property is deteriorated. The winding temperature is preferably 500 to 700° C. from the viewpoint of promoting the precipitation of AlN and NbC and improving the formability by coarsening the precipitate. More preferably, the winding temperature is 600°C to 650°C.

[Cold rolling process]
The cold rolling step is a step of performing cold rolling on the hot-rolled steel sheet obtained in the hot-rolling step or the hot-rolled steel sheet after the pickling treatment described below to obtain a cold-rolled steel sheet.

<Reduction rate>
The reduction ratio of cold rolling is not particularly limited, but is preferably 50% or more because the effect of the present invention is more excellent.

[Annealing process]
The annealing step is a step of performing annealing treatment on the cold rolled steel sheet obtained in the cold rolling step to obtain a cold rolled steel sheet subjected to the annealing treatment.
Here, the annealing treatment is a treatment of holding at a temperature of 850° C. or higher and lower than 910° C. and then cooling it to a temperature of 750° C. or lower at an average cooling rate of 5° C./s (seconds) or higher.
Further, the dew point of the atmosphere in the furnace during the annealing treatment is −35° C. or lower.
The annealing treatment is performed, for example, in a continuous annealing line. When the method of the present invention further includes a plating step described below, the annealing treatment is performed in, for example, a continuous hot dip galvanizing line.

<annealing temperature>
As described above, the holding temperature (annealing temperature) of the annealing treatment is 850° C. or higher and lower than 910° C.
In the annealing treatment, the precipitated NbC is dissolved into solid solution C, and annealing is performed at 850° C. or higher in order to secure a bake hardening amount of 30 MPa or higher. On the other hand, when the annealing temperature is high, the deep drawability (r value) and the elongation (El) are reduced, and the crystal grains are coarsened to cause rough surface defects during press forming, which deteriorates the surface quality. Further, in the case of the hot-dip galvanized steel sheet described below, the surface properties after the hot-dip galvanizing treatment deteriorate. Therefore, annealing is performed at a temperature lower than 910° C. at the highest. The annealing temperature is preferably 870° C. or higher. The annealing temperature is more preferably 890°C or lower.
In the annealing treatment, it is preferable to maintain the annealing temperature for about 50 to 300 s for the reason that the effect of the present invention is more excellent.

<Average cooling rate and cooling stop temperature>
As described above, the temperature is maintained at 850° C. or higher and lower than 910° C., and then the steel sheet is cooled to a temperature of 750° C. or lower at an average cooling rate of 5° C./s or higher. In addition, the average cooling rate here means the average from the annealing temperature to the cooling stop temperature. When the average cooling rate is less than 5° C./s, C dissolved in the annealing treatment is re-precipitated as NbC and the bake hardenability is deteriorated. Therefore, a cooling rate of 5° C./s or more is required. If the cooling rate is too high, the ductility of the steel sheet is reduced, so a cooling rate of 10° C./s or less is preferable. Further, when the cooling stop temperature exceeds 750° C. and reaches a high temperature, the solid solution C is re-precipitated as NbC by the subsequent slow cooling, and the bake hardenability is deteriorated. Therefore, it is necessary to cool to 750° C. or less. The cooling stop temperature is preferably 700° C. or lower.

<Dew point>
As described above, the dew point of the furnace atmosphere during the annealing treatment is −35° C. or lower.
In the annealing step, if the dew point of the atmosphere in the furnace is high, the solid solution C is decarburized by the following reaction, which causes a decrease in the bake hardening amount.
C (in steel) + H ₂ O → CO + H ₂
Further, B is oxidized and consumed, and the formation of BN is reduced, so that the bake hardening amount is similarly reduced. The dew point of the atmosphere in the furnace during the annealing treatment is −35° C. or lower in order to prevent a decrease in the amount of solid solution C due to decarburization and a decrease in the amount of BN formed, and a decrease in the amount of bake hardening due to the decrease in the amount of solid solution C. The lower limit of the dew point is not particularly specified, but if it is less than -80°C, it is disadvantageous in terms of cost, and therefore, -80°C or higher is preferable.
The dew point is preferably less than -40°C because the effect of the present invention is more excellent.

[Other processes]
The method of the present invention may include steps other than the steps described above.

<Pickling process>
The method of the present invention preferably further comprises a pickling treatment step, and more preferably further comprises a pickling treatment step after the hot rolling step and before the cold rolling step. When the hot-rolled steel sheet after hot rolling is subjected to pickling treatment, the scale (oxide film) on the surface is removed, scratches are less likely to occur in cold rolling, and the appearance is improved.

[Method for producing hot dip galvanized steel sheet]
The method for producing a hot-dip galvanized steel sheet of the present invention is a hot-dip galvanized steel sheet obtained by subjecting the cold-rolled steel sheet subjected to the annealing treatment obtained by the method of the present invention to a hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. Is a manufacturing method.
The conditions of the hot dip galvanizing treatment are not particularly limited. Any conventional hot dip galvanizing treatment conditions can be applied.

[Method for producing galvannealed steel sheet]
The method for producing an alloyed hot-dip galvanized steel sheet according to the present invention is an alloying treatment for the hot-dip galvanized steel sheet obtained by the method for producing a hot-dip galvanized steel sheet described above to obtain an alloyed hot-dip galvanized steel sheet, alloying It is a manufacturing method of a hot-dip galvanized steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

[Production of cold-rolled steel sheet, hot-dip galvanized steel sheet and galvannealed steel sheet]
Cold-rolled steel sheets, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets were manufactured as described below.

<Hot rolling process>
A slab (steel material) having the composition (% by mass) shown in Table 1 was heated at a heating temperature of 1200° C. and then hot-rolled to obtain a hot-rolled steel sheet having a sheet thickness of 3.2 mm. The rolling end temperature and the coiling temperature of the hot rolling were as shown in Table 2, and after coiling, they were cooled to room temperature.

<Pickling process>
Then, the obtained hot rolled steel sheet was subjected to pickling treatment.

<Cold rolling process>
Then, the hot-rolled steel sheet after the pickling treatment was cold-rolled at a rolling reduction of 75% to obtain a cold-rolled steel sheet having a thickness of 0.8 mm.

<annealing process>
Next, the obtained cold rolled steel sheet was annealed under the conditions shown in Table 2 in a continuous annealing line. Thus, a cold rolled steel sheet which was annealed was obtained. The "dew point" in Table 2 represents the dew point of the atmosphere in the furnace during the annealing treatment.

<Plating process>
Next, with respect to the steel plates described as "Yes" in the column of "Plating treatment" in Table 2, hot dip galvanizing treatment was performed using a continuous hot dip galvanizing line to obtain hot dip galvanizing steel sheets.

<Alloying process>
Further, for the steel sheets described as "Yes" in the "Alloying treatment" column of Table 2, the hot dip galvanized layer was subjected to an alloying treatment to obtain an alloyed hot dip galvanized steel sheet.

[Evaluation]
The following evaluation was performed about the obtained cold rolled steel plate, hot dip galvanized steel plate, and alloyed hot dip galvanized steel plate.

<Tensile properties>
From the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet and hot-dip galvanized steel sheet, JIS No. 5 tensile test pieces with the major axis perpendicular to the rolling direction were sampled and stretched in accordance with JIS Z2241. A test was conducted to evaluate the tensile properties (yield strength, tensile strength, elongation).
The results are shown in Table 3. Practically, the elongation is preferably 40% or more.

<Moldability>
JIS No. 5 tensile test pieces were taken from the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, and the r value was measured. Specifically, after applying a 15% pre-strain to the test piece by a tensile test, the r value was measured by a three-point method, and the rolling direction, the direction of 45° with respect to the rolling direction, and the angle of 90° with respect to the rolling direction. The r values in the direction of were taken as rL, rD, and rC, respectively. Then, the average value (=(rL+2rD+rC)/4) was obtained.
The results are shown in Table 3. The larger the r value, the better the moldability. Practically, it is preferably 1.40 or more.

<Bake hardenability>
From the obtained cold rolled steel sheet, hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet, JIS No. 5 tensile test pieces were sampled in the same manner as in the evaluation of tensile properties, and 2% tensile strength was applied in accordance with JIS G3135. A pre-deformation process that gives a pre-strain (plastic strain) is performed, and then a heat treatment at 170° C. for 20 minutes is performed to increase the amount of deformation stress before and after the heat treatment (BH amount) (bake hardening amount) (= ( The yield point (MPa))-(pre-deformation stress before heat treatment (MPa)) was determined.
The results are shown in Table 3. If the BH amount is 30 MPa or more, it can be said that the bake hardenability is excellent.

In Tables 1 to 3, the underlined portions indicate those outside the scope of the present invention.

As can be seen from Tables 1 to 3, the steel sheets A to F and M obtained by subjecting a steel material having a specific composition to hot rolling and cold rolling, and further performing an annealing treatment under specific conditions, It showed excellent moldability and bake hardenability. Among them, the steel plates A to F in which the B content of the steel material was 0.0003% or more showed more excellent bake hardenability. Among them, the steel plates of B to F, in which the dew point of the atmosphere in the furnace during the annealing treatment was −40° C. or lower, showed further excellent formability. Among them, the steel sheets B to C and E to F in which the dew point of the atmosphere in the furnace during the annealing treatment was less than −40° C. showed further excellent bake hardenability. Among them, the steel sheets of C and F in which the dew point of the atmosphere in the furnace during the annealing treatment was less than -46°C showed particularly excellent bake hardenability.

On the other hand, steel sheets of H, K and N in which the steel material does not have a specific composition, and steel sheets of G, I to J and L in which the steel material having a specific composition is used but the annealing condition is not a specific condition are However, at least one of moldability and bake hardenability was insufficient.

Claims (3)

A hot rolling step of subjecting a steel material to hot rolling to obtain a hot rolled steel sheet;
A cold rolling step of performing cold rolling on the hot rolled steel sheet to obtain a cold rolled steel sheet,
An annealing process, in which the cold rolled steel sheet is subjected to an annealing treatment to obtain an annealed cold rolled steel sheet,
A method for manufacturing a cold rolled steel sheet, comprising:
The steel material, in mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.01% or more and 0.10% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.010% or less, a steel material having a composition of balance Fe and inevitable impurities,
The dew point of the furnace atmosphere during the annealing treatment is −35° C. or lower,
The method for producing a cold rolled steel sheet, wherein the annealing treatment is a treatment of holding at a temperature of 850° C. or higher and lower than 910° C. and then cooling it to a temperature of 750° C. or lower at an average cooling rate of 5° C./s or higher. A method for producing a hot-dip galvanized steel sheet, wherein the cold-rolled steel sheet after the annealing treatment obtained by the production method according to claim 1 is subjected to hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises subjecting the hot-dip galvanized steel sheet obtained by the method according to claim 2 to an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.