JP6658708B2 - Method for producing steel sheet having low yield ratio - Google Patents

Description

  The present invention relates to a method for producing a steel sheet having a low yield ratio. The present invention is particularly useful for producing cold-rolled steel sheets having a tensile strength (TS) of 270 MPa or more and less than 500 MPa, a low yield strength, and less wrinkling during press molding, which is useful for the use of automobile skin panels and the like. It is about the method.

2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, improvement of fuel efficiency of automobiles has been demanded in order to regulate CO ₂ emissions. In addition, in order to ensure the safety of the occupant in the event of a collision, there is also a demand for an improvement in safety with a focus on the collision characteristics of the vehicle body. As described above, the weight reduction of the vehicle body and the strengthening of the vehicle body have been actively promoted.

  In order to satisfy both the weight reduction and strengthening of an automobile body at the same time, it is said that it is effective to increase the strength of component materials and reduce the thickness of the parts as long as there is no problem with rigidity. Steel plates are being actively used in automotive parts.

  Since the weight reduction effect increases as the steel plate used increases in strength, the automotive industry tends to use a steel plate having a tensile strength (TS) of 390 MPa or more, for example, as a panel material for an inner plate and an outer plate.

  On the other hand, since most automotive parts made of steel sheets are formed by press working, steel sheets for automobiles need to have excellent press formability. However, since high-strength steel sheets are deteriorated in formability as compared with ordinary mild steel sheets, there are many problems in reducing the weight of automobiles. For the panel material for the outer panel, the surface quality after pressing is strictly controlled, and for example, it is important to eliminate local unevenness called surface distortion around the handle of the door.

  For such a surface strain, it is necessary to lower the yield strength, and an attempt to lower the yield ratio YR [= (YS / TS) × 100], which is the ratio of the yield strength (YS) to the tensile strength (TS), has been made. It has been done.

  For example, Patent Literature 1 discloses a steel sheet having excellent stretch flangeability, low YS characteristics, and excellent press formability, and a method for producing the same. In Patent Document 1, C: 0.0040 to 0.015 wt%, Nb: 0.04 to 0.25 wt%, Nb / (7.75 × C) = 1.5 to 2.5, and carbon is Nb. By sufficiently fixing and heating to a temperature equal to or higher than the recrystallization temperature in the annealing step, and controlling the tension to 0.3 × Nb / (7.75 × C) -0.25 ≦ tensile T ≦ 2.0, A low-density NbC region is formed near the grain boundary to achieve a low YS.

  Further, the steel sheet disclosed in Patent Document 2 has a structure in which a second phase mainly composed of martensite is dispersed in a ferrite structure in an appropriate amount, and YP is reduced as compared with a conventional solid solution strengthened steel such as IF steel. You. Further, the steel sheet has a high BH and has excellent dent resistance. However, when this steel sheet is press-formed in a part such as a door, the amount of surface strain generated becomes larger than that of a conventional 340 MPa class bake hardening steel sheet, so that a further reduction in YP is required.

Patent Document 3 discloses that a hot-rolled sheet having a predetermined component is cold-rolled, annealed in a temperature range of _{1 to 3} points of Ac, and 550 to 750 ° C. at a cooling rate of 3 to 20 ° C./s. A method for producing a steel sheet in which primary cooling is performed to a temperature range of 200 ° C. and then further cooled to 200 ° C. or less at a cooling rate of 100 ° C./s or more is disclosed. However, since the method described in Patent Document 3 requires rapid cooling after annealing, it can be applied to a continuous annealing line that is not subjected to plating treatment, but a zinc plating bath maintained at 450 to 500 ° C. during cooling after annealing. It is difficult in principle to apply to a continuous hot-dip galvanizing line in which a plating process is performed by immersion in a galvanizing process.

JP 2001-131681 A JP-B-62-40405 JP 2006-233294 A

An object of the present invention is to provide a method for producing a steel sheet having a low yield ratio.
The steel sheet having a low yield ratio in the present invention means a steel sheet having a yield ratio (YR) of 50% or less.

  Conventionally, to improve the deep drawability, the atomic ratio of Nb and C (12/93) × (Nb / C) [the content of each element (mass%) of Nb and C is 1.0] However, when there are many precipitates due to NbC, the yield strength (YS) can be lowered by controlling the dispersion, and the tensile strength (TS) can be increased by precipitation strengthening. It was difficult to make (YR) 50% or less. Therefore, further investigation was made. In order to obtain a steel sheet having a sufficiently low yield ratio, the atomic ratio of Nb and C was set to a limited range of 0.8 or more and 1.1 or less, and the cold rolling process was performed. It has been found that it is important to control the cold rolling rate and the heating rate in the annealing step.

The present invention has been made based on such knowledge, and has the following configuration.
[1] In mass%,
C: 0.0040 to 0.0120%,
Si: 0.70% or less,
Mn: 0.50 to 1.80%,
P: 0.005 to 0.05%,
S: 0.01% or less,
Al: 0.005 to 0.3%,
N: 0.005% or less,
Nb: 0.025 to 0.110%
Containing, and
In a steel slab having an atomic ratio of Nb to C represented by the following formula (1) of 0.80 or more and 1.10 or less and a balance of Fe and unavoidable impurities,
A hot rolling step of performing finish rolling at a finish-rolling exit temperature of 880 ° C. or higher, winding at 550 ° C. or higher and 670 ° C. or lower, and forming a hot-rolled sheet;
A cold rolling step of subjecting the hot rolled sheet to cold rolling at a cold rolling rate of 65% or more and 92% or less to form a cold rolled sheet;
An annealing step of heating the cold-rolled sheet and performing annealing at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less,
A steel sheet having a low yield ratio, in which the relationship between the true thickness strain εt in the cold rolling step and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing step satisfies the following expression (2). Manufacturing method.
(12/93) × (Nb / C) (1)
However, Nb and C in the above formula (1) represent the contents (% by mass) of each element.
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = -ln (1-CR / 100) (3)
Here, CR in the above equation (3) is a cold rolling rate (%) in the cold rolling step.
[2] In addition to the above composition, the steel slab further comprises,
B: The method for producing a steel sheet having a low yield ratio according to [1], containing 0.0010% or less.
[3] The method for producing a steel sheet having a low yield ratio according to [1] or [2], further comprising, after the annealing step, plating the steel sheet surface.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the steel plate which has a low yield ratio can be provided.
The steel sheet manufactured by the manufacturing method of the present invention has an unprecedented low yield ratio. Further, since the addition of alloying elements such as Mo and Cr is limited, the plating property and the surface appearance are good. Therefore, it can be suitably used as a steel plate for automobile outer panels.

  In the present invention, when the C content is in the range of 0.0040 to 0.0120%, the atomic ratio of Nb to C (12/93) × (Nb / C) is 0.80 ≦ (12/93). ) × (Nb / C) ≦ 1.10. By fixing carbon at a lower level than the conventional IF steel, and controlling the cold rolling rate in the cold rolling step and the heating rate in the annealing step. It has been found that ferrite single-phase steel can achieve a lower yield ratio than ever before. The reason for this is not necessarily clear, but is considered as follows.

  Until now, from the viewpoint of deep drawability, Ti and Nb exceeding the equivalent amount of carbon have been added to reduce the amount of solid solution carbon to the limit. However, the carbides are inhibited and the yield strength of the ferrite structure is sufficiently reduced. I couldn't. Thus, by adding Nb in an amount equal to or slightly lower than that of carbon, the carbide was able to be brought into a state in which steel easily yielded. In this case, since the precipitation state of carbides in hot rolling is unstable, a part of the carbides (NbC) is melted by cold rolling, and the carbides are reprecipitated in the heating step (annealing step). It has become clear from such evaluations. Dissolution of carbides depends on the strain energy of cold rolling, that is, the cold rolling rate, and the dispersion of carbides when reprecipitated affects the heating rate in the annealing step, so optimize the two. Thus, it is considered that a lower yield ratio than ever before can be achieved even with a ferrite single phase.

  Hereinafter, the present invention will be described in detail. Hereinafter, unless otherwise specified, the contents of the elements are shown by mass%.

  First, the reason for limiting the component composition of the steel slab of the present invention will be described.

C: 0.0040 to 0.0120%
C is an important element in the present invention together with Nb described later. The upper limit is set to 0.0120% in consideration that excessive addition is not preferable in order to obtain good deep drawability of an automobile outer panel. C is preferably as low as possible from the viewpoint of deep drawability, but 0.0040% or more is required from the viewpoint of utilizing the control of NbC precipitation. C content is preferably 0.0050% or more. Further, the C content is preferably 0.0110% or less.

Si: 0.70% or less Si has the effect of solid solution strengthening without significantly lowering the deep drawability. In order to obtain the above effects, it is preferable that Si is contained at 0.20% or more, more preferably 0.35% or more. On the other hand, if the content of Si exceeds 0.70%, red scale is generated at the time of hot rolling, so that the surface appearance of the steel sheet is deteriorated. Further, when hot-dip galvanizing is performed, the wettability of the plating is deteriorated, which causes uneven plating and deteriorates the plating quality. Therefore, the Si content is set to 0.70% or less. The Si content is preferably 0.50% or less.

Mn: 0.50 to 1.80%
Mn is an element that is effective in increasing the strength and is also effective in preventing hot cracking due to S. From such a viewpoint, Mn needs to be contained at 0.50% or more. Mn is preferably contained at least 0.55%, more preferably at least 1.00%. On the other hand, excessive addition deteriorates deep drawability and weldability, so that the upper limit of the Mn content is 1.80%. The Mn content is preferably 1.40% or less.

P: 0.005 to 0.05%
P has the effect of solid solution strengthening. However, if the content is less than 0.005%, not only the effect is not exhibited, but also the dephosphorization cost increases in the steel making process. Therefore, P should be contained at 0.005% or more. On the other hand, an excessive addition exceeding 0.05% causes P to segregate at the grain boundary, thereby deteriorating secondary work brittleness resistance and weldability. Therefore, the upper limit of the P content is set to 0.05%. The P content is preferably 0.010% or more. Further, the P content is preferably 0.03% or less.

S: 0.01% or less S is an impurity and causes hot cracking. In addition, S is present as an inclusion in steel and deteriorates various properties of the steel sheet. % Is acceptable, so the S content is set to 0.01% or less. The S content is preferably 0.005% or less.

Al: 0.005 to 0.3%
Al is useful as a deoxidizing element of steel, and has an effect of fixing solid solution N to improve aging resistance at room temperature. Therefore, Al is contained in an amount of 0.005% or more. On the other hand, the addition exceeding 0.3% causes an increase in cost due to high alloying and further induces surface defects. Therefore, the Al content is set to 0.3% or less. The Al content is preferably 0.015% or more. Further, the Al content is preferably 0.15% or less.

N: 0.005% or less If N is too much, the aging resistance at normal temperature is deteriorated and a large amount of Al or Ti needs to be added. Therefore, it is preferable to reduce as much as possible, and the upper limit is made 0.005%. The N content is preferably 0.0030% or less.

Nb: 0.025 to 0.110%
Nb is an element that has a function of miniaturizing the structure of the hot-rolled sheet and precipitating and fixing C as NbC in the hot-rolled sheet, and contributing to deep drawability. From such a viewpoint, Nb is contained at 0.025% or more. On the other hand, excessive addition of Nb increases the cost and increases the hot rolling load. Further, since the yield strength increases as the NbC content increases, the Nb content is set to 0.110% or less.

Atomic ratio of Nb and C is 0.80 or more and 1.10 or less In the present invention, it is important to control the atomic ratio of Nb and C. When the atomic ratio of Nb to C represented by the following formula (1) is less than 0.80, a large amount of solute carbon is present at the hot rolling stage, and the deep drawability after cold rolling annealing is reduced. Further, the presence of solute carbon in the cold-rolled annealed material is not preferable because it increases the yield strength. Therefore, the atomic ratio between Nb and C needs to be 0.80 or more and 1.10 or less. The atomic ratio between Nb and C is preferably 0.85 or more. The atomic ratio between Nb and C is preferably 1.05 or less.
(12/93) × (Nb / C) (1)
However, Nb and C in the above formula (1) represent the contents (% by mass) of each element.

  The above are the basic components of the steel slab of the present invention. Further, in the present invention, the balance other than the above-mentioned components has a composition of iron and inevitable impurities.

  The composition of the steel slab of the present invention may further contain B: 0.0010% or less in addition to the above components.

B: 0.0010% or less B is an element having an effect of improving the brittleness of secondary working of steel, and can be contained as necessary. However, if the content exceeds 0.0010%, the effect is saturated, so the B content is preferably 0.0010% or less. The B content is more preferably 0.0008% or less.

  As the unavoidable impurities, V, Mo, and Cu can be added in a total amount of 0.1% or less as long as the desired characteristics are not affected.

  The steel sheet of the present invention also includes a so-called plated steel sheet which has been subjected to a surface treatment such as electroplating or hot-dip plating. Plating is, in addition to pure zinc, an element that forms an alloy, zinc-based alloy plating in which zinc is the main component and an alloy element is added, or Al-based alloy plating in which Al or Al is the main component and an alloy element is added, Mg, It also includes a plating layer conventionally applied to the surface of a steel sheet, such as Ni or an alloy containing these elements.

  The composition of the steel sheet in the present invention is the same as the composition of the steel slab.

  Next, a method for manufacturing a steel sheet according to the present invention will be described. In the following description, unless otherwise specified, the temperature is the surface temperature of a steel slab, a hot rolled sheet, or the like.

  In the present invention, in the hot rolling step, the steel slab having the above composition is subjected to finish rolling in which the finish-rolling exit temperature is 880 ° C. or higher.

  The steel slab used in the manufacturing method of the present invention is desirably manufactured by a continuous casting method in order to prevent macro segregation of components, but may be manufactured by an ingot making method or a thin slab casting method. In addition to the conventional method in which steel slabs are manufactured and then cooled to room temperature and then heated again, direct-rolling in which hot slabs are placed in a heating furnace without cooling and hot-rolled, or a slight heat retention The energy saving processes, such as direct rolling and direct rolling, in which hot rolling is performed immediately after the rolling, can be applied without any problem.

  The steel slab heating temperature is desirably lower because the precipitates are coarsened to develop {111} recrystallization texture and improve deep drawability. However, if the heating temperature is lower than 1000 ° C., the rolling load increases, and the risk of occurrence of trouble during hot rolling increases, so the steel slab heating temperature is preferably set to 1000 ° C. or higher. Note that the upper limit of the heating temperature of the steel slab is preferably set to 1300 ° C. because of an increase in scale loss due to an increase in oxidation weight.

  The steel slab heated under the above conditions is subjected to hot rolling for performing rough rolling and finish rolling. Here, the steel slab is turned into a sheet bar by rough rolling. It should be noted that the conditions for the rough rolling need not be particularly specified, and may be performed according to a conventional method. It is needless to say that utilizing a so-called sheet bar heater for heating the sheet bar is an effective method from the viewpoint of lowering the heating temperature of the steel slab and preventing troubles during hot rolling.

  Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet. In the present invention, the finish rolling exit side temperature (FT) is 880 ° C. or higher. This is for obtaining a fine hot-rolled sheet structure capable of obtaining excellent deep drawability after cold rolling and recrystallization annealing. If the FT is less than 880 ° C., not only does the structure have a worked structure and the {111} texture does not develop after cold rolling annealing, but also the rolling load during hot rolling increases. Therefore, FT is set to 880 ° C. or higher. On the other hand, if the FT exceeds 980 ° C., the structure becomes coarse, which may hinder the formation and development of the {111} recrystallized texture after cold rolling annealing, and may not ensure the deep drawability. Therefore, FT is preferably set to 980 ° C. or lower.

  In order to reduce the rolling load at the time of hot rolling, lubricating rolling may be performed between some or all of the passes of finish rolling. Performing lubricating rolling is also effective from the viewpoint of uniformizing the shape of the steel sheet and homogenizing the material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Further, it is also preferable to adopt a continuous rolling process in which successive sheet bars are joined and finish rolling is continuously performed. Applying the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

Winding temperature (CT): 550 ° C or more and 670 ° C or less The coil winding temperature is 550 ° C or more and 670 ° C or less. This temperature range is a suitable temperature range for precipitating NbC in the hot-rolled sheet, and particularly when the CT exceeds the upper limit, the crystal grains become coarse and the strength is reduced, and the deep drawability after cold rolling annealing is increased. Will hinder. It is necessary to control the NbC generated in the hot-rolling stage so that a part of the NbC is melted by cold rolling and the yield strength after annealing is reduced. Therefore, the upper limit of the winding temperature is set to 670 ° C.

  After the above-mentioned hot rolling step, the hot-rolled sheet is subjected to cold rolling to form a cold-rolled sheet. The hot rolled sheet may be pickled after the hot rolling step and before the cold rolling step. The pickling may be performed under ordinary conditions.

Cold rolling ratio (CR): 65% or more and 92% or less The cold rolling step is an important step for controlling the distribution state of NbC for obtaining a low yield ratio. In the cold rolling process, NbC precipitated in the hot rolling stage needs to be cut by cold working to dissolve fine and unstable NbC, and the CR is at least 65% or more. Also, from the viewpoint of deep drawability, a high cold rolling reduction is generally effective. If the reduction (cold rolling reduction) is less than 65%, {111} recrystallization texture does not develop and excellent deep drawability is obtained. I can't get it. On the other hand, if the CR exceeds 92%, not only the effect is saturated, but also the load on the roll during cold rolling increases, the upper limit is set to 92%.

Annealing temperature: Annealing temperature T in the temperature range from 760 ° C to 950 ° C
Next, the cold-rolled sheet that has been subjected to the cold rolling step is heated and subjected to an annealing step of annealing at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less. In this annealing step, it is necessary to perform at least recrystallization. For this reason, annealing in a temperature range of 760 ° C. or more is required at a minimum. On the other hand, at a high temperature exceeding 950 ° C., the recrystallized grains become extremely coarse, and the characteristics are significantly deteriorated. Therefore, the annealing in the annealing step is performed at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less. The annealing temperature T is preferably 800 ° C. or higher. Further, the annealing temperature T is preferably 900 ° C. or less.

The relationship between the true thickness strain εt in the cold rolling step and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing step satisfies the following equation (2).
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = -ln (1-CR / 100) (3)
Here, CR in the above equation (3) is a cold rolling rate (%) in the cold rolling step.

  In order to obtain a low yield ratio, it is necessary to control the distribution of NbC in the final annealing step. In this case, it is necessary to control the generation and growth state of NbC at the stage of heating in the annealing step after being divided and disappeared by the energy of the cold working, and the cold rolling in the cold rolling step is required. It is necessary to control both the rate and the heating rate in the annealing step. Regarding the heating rate in the annealing step, considering the diffusion of carbon and Nb, controlling the heating rate at less than 650 ° C. has almost no effect, and the heating rate from 650 ° C. to the annealing temperature (annealing temperature T) is important. .

  As a result of the study made by the present inventors, the relationship between the true thickness strain εt in the cold rolling process and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing process is described in the above (2). By satisfying the expression, it has been found that the yield ratio of the steel sheet after cold rolling annealing can be made extremely low. When the heating rate in the annealing step is low, the effect of the energy of the cold working is insensitive, but when the heating rate in the annealing step is high, the working ratio in the cold rolling step needs to be further increased.

  The heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing step is not particularly limited as long as the above-mentioned formula (2) is satisfied, but from the viewpoint of production efficiency and the like, 1.0 ° C. / S or more is preferable. In addition, from the viewpoint of threading stability and the like, the temperature is preferably 30 ° C / s or less.

  Although the cooling rate after the above-mentioned annealing step is not particularly specified, it is preferable to cool from the annealing temperature T to 300 ° C. at an average cooling rate of 5 ° C./s or more. It is preferable that the average cooling rate from the temperature T to the overaging temperature is 5 ° C./s or more.

  After the annealing step, a plating process such as an electroplating process or a hot-dip plating process may be performed to form a plating layer on the surface of the steel sheet.

  For example, as a plating process, when performing a hot-dip galvanizing process often used for automotive steel sheets, perform the above-described annealing process in a continuous hot-dip galvanizing line, and immerse it in a hot-dip galvanizing bath following cooling after the annealing process. Then, a hot-dip galvanized steel sheet may be manufactured by forming a hot-dip galvanized layer on the surface or by further performing an alloying treatment. In that case, it is preferable that the cooling after leaving the hot-dip plating pot or after further alloying treatment is performed so that the average cooling rate up to 300 ° C. is 5 ° C./s or more.

  Further, the process up to the cooling after the annealing step may be performed in an annealing line, and once cooled to room temperature, hot-dip plating may be performed in a hot-dip plating line, or alloying may be further performed.

  Here, the plating layer is not limited to pure zinc and zinc-based alloy plating, and it is of course possible to use various plating layers conventionally applied to the steel sheet surface, such as Al and Al-based alloy plating.

  Further, the steel sheet (a cold-rolled annealed sheet and a plated steel sheet) may be subjected to temper rolling or leveler processing for the purpose of shape correction, adjustment of surface roughness and the like. It is preferable that the elongation percentage of the temper rolling or leveling is within a range of less than 0.5% in total. If the elongation is 0.5% or more, the yield strength increases, and surface distortion during pressing is likely to occur.

Next, examples of the present invention will be described.
A steel slab having the composition shown in Table 1 was heated to 1250 ° C. and roughly rolled to form a sheet bar, and then a hot-rolled sheet was subjected to a finish rolling under the conditions shown in Table 2 by hot rolling. After pickling these hot rolled sheets, a cold rolling step was performed at a cold rolling rate shown in Table 2 to obtain a cold rolled sheet. Subsequently, continuous annealing was performed on these cold-rolled sheets under the conditions shown in Table 2 in a continuous annealing line. Further, the obtained steel sheet (cold rolled annealed sheet) was subjected to temper rolling at an elongation of 0.2%.

  With respect to the obtained cold-rolled annealed sheet, the tensile properties and the r value were measured. The measuring method is as follows.

(1) Tensile properties A JIS No. 5 tensile test piece was sampled from each of the obtained cold-rolled annealed sheets in a direction (C direction) at 90 ° to the rolling direction, and a crosshead speed of 10 mm / Then, a tensile test was carried out in a minimum time, and a yield stress (YS) and a tensile strength (TS) were determined. Further, the yield ratio (YR) was obtained from (YS / TS) × 100 from the YS and TS.

(2) r value measurement JIS No. 5 tensile test of each obtained cold rolled annealed sheet from the rolling direction (L direction), 45 ° direction (D direction) with respect to the rolling direction, and 90 ° direction (C direction) with respect to the rolling direction Pieces were collected. When 10% uniaxial tensile strain is applied to these test pieces, the width strain and the thickness strain of each test piece are obtained, and the average r value (average plastic strain ratio) is obtained in accordance with JIS Z 2254. This was taken as the r value. In addition, it can be evaluated that the larger the r value is, the more excellent the deep drawing property is.

  As is clear from Table 2, in each of the examples of the present invention, a steel sheet having a low yield ratio with a YR of 50% or less was obtained. In each of the examples of the present invention, a steel sheet having an average r value of 1.3 or more was obtained. On the other hand, in Comparative Examples manufactured under conditions outside the range of the present invention, a steel sheet having a sufficiently low YR could not be obtained. In addition, when the structure | tissue observation was performed about the steel plate of this invention example, all the steel plates of this invention had ferrite single phase structure.

  According to the present invention, a cold rolled steel sheet having a high r value of YR of 50% or less and an average r value of 1.3 or more can be manufactured inexpensively and stably, which has a remarkable industrial effect. For example, when the cold-rolled steel sheet of the present invention is applied to a part for an automobile outer panel, it is possible to secure the surface quality after press molding even in a part where the surface strain has been strictly controlled and difficult to pass. Since it is possible to make the strength higher than that of the steel plate for an outer panel panel, there is an effect that it can sufficiently contribute to collision safety and weight reduction of an automobile body. In addition, the present invention can be applied not only to automobile parts but also to home electric parts.

Claims (3)

In mass%,
C: 0.0040 to 0.0120%,
Si: 0.70% or less,
Mn: 0.50 to 1.80%,
P: 0.005 to 0.05%,
S: 0.01% or less,
Al: 0.005 to 0.3%,
N: 0.005% or less,
Nb: 0.025 to 0.110%
Containing, and
In a steel slab having an atomic ratio of Nb to C represented by the following formula (1) of 0.80 or more and 1.10 or less and a balance of Fe and unavoidable impurities,
A hot rolling step of performing finish rolling at a finish-rolling exit temperature of 880 ° C. or higher, winding at 550 ° C. or higher and 670 ° C. or lower, and forming a hot-rolled sheet;
A cold rolling step of subjecting the hot-rolled sheet to cold rolling at a cold rolling rate of 65% or more and 92% or less to form a cold-rolled sheet;
An annealing step of heating the cold-rolled sheet and performing annealing at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less,
A steel sheet having a low yield ratio, in which the relationship between the true thickness strain εt in the cold rolling step and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing step satisfies the following expression (2). Manufacturing method.
(12/93) × (Nb / C) (1)
However, Nb and C in the above formula (1) represent the contents (% by mass) of each element.
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = -ln (1-CR / 100) (3)
Here, CR in the above equation (3) is a cold rolling rate (%) in the cold rolling step. In addition to the composition, the steel slab further comprises,
The method for producing a steel sheet having a low yield ratio according to claim 1, wherein the steel sheet contains B: 0.0010% or less.   The method for producing a steel sheet having a low yield ratio according to claim 1 or 2, wherein a plating treatment is performed on the surface of the steel sheet after the annealing step.