JPS5818970B2 - Method for manufacturing high-strength thin steel sheets with excellent cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high tensile strength thin steel sheet with excellent cold workability.

In recent years, demand for high-tensile hot-rolled thin steel sheets has been increasing in various fields including the automobile industry.

In such applications, the thin steel sheet material is usually subjected to a cold-open forming process such as press working, and is required to have excellent cold workability.

One known method for satisfying these requirements is to adjust the metal structure to a structure in which a ferrite phase and a martensitic phase are dispersed and mixed.

Steel with such a mixed structure exhibits unique mechanical properties such as a low yield point and high tensile strength, and has excellent cold workability.

The reason why high-strength thin steel sheets with a ferrite-martensite mixed structure have excellent cold workability is that the soft ferrite phase determines the strength in the low strain region, and the hard martensite phase determines the strength in the high strain region. Since the strength of the region is determined, the yield ratio is low (because it is highly ductile).

Moreover, these materials undergo extremely high work hardening during processing, and their strength increases through subsequent age hardening, making it possible to obtain a final strength as high as that of conventional high-tensile steel.

Conventionally, to manufacture such steel, hot-rolled steel sheets were heated to α + γ.
The process involved reheating to a two-phase region and then quenching.

However, such a method requires precise adjustment of the reheating temperature, which has the drawback of increasing costs.

Furthermore, even if a mixed structure of ferrite and bainite phases is used instead of a mixed structure of ferrite and martensite phases, the yield ratio will be slightly higher than the former, but the objective can be achieved, and the bainite phase and martensite There is no problem in the presence of a mixed structure of one or two types of phases and a ferrite phase, or a small amount of pearlite phase in this mixed structure, and in order to lower the yield ratio and increase the ductility of steel containing these mixed structures. It is known that it is important not only to adjust the strength level and quantitative ratio of the hard phases bainite phase and martensite phase (hereinafter collectively referred to as low-temperature transformed phases), but also to adjust their dispersion state and ferrite particle size.

However, in order to manufacture steel with such a structure, the objective has traditionally been achieved only by subjecting hot-rolled steel sheets to heat treatment under appropriately selected conditions. No method has been proposed to directly obtain steel with the above structure by selecting conditions.

The object of the present invention is to overcome the drawbacks of the prior art, to obtain a high tensile strength steel plate with a low yield ratio as hot-rolled, and to obtain a steel plate with a low yield ratio and high tensile strength that is superior to that of the conventional technology by further applying heat treatment. The object of the present invention is to provide a method for manufacturing a cold-workable dipped high-strength thin steel sheet that can yield a steel sheet.

This object of the present invention is achieved by the following six inventions.

The gist of the first invention is as follows.

C: 0.05-0.25%, Mn: 0.05% by weight.
1 to 2.0%, Si: 1.0% or less, and the balance is Fe.
and a method for producing a thin steel sheet containing unavoidable impurities, the step of applying a reduction of 20 to 80% when the temperature during hot rolling of the steel sheet is below the equilibrium ferrite transformation starting temperature, and the step of rolling the hot rolled steel sheet from the equilibrium ferrite transformation starting temperature to the ferrite state. The temperature range up to the transformation start temperature is 200℃ over 30℃/sec.
At an average cooling rate of ℃/sec or less, and at a temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature for at least 2 seconds or more, 30℃ over 0.5℃/5ecJU
cooling at an average cooling rate of 50° C./sec or more and 200° C./sec or less after the cooling treatment, and then cooling the coil at a temperature not higher than the bainite transformation starting temperature. and a step of winding the
Forms a mixed structure of one or both of bainite phase and martensite phase and ferrite phase, yielding ratio 80%
There is a method for producing a high-strength thin steel sheet with excellent cold workability, which is characterized by the following.

The gist of the second invention is that, in addition to the basic composition of the steel of the first invention, Cr s of 0.5 or less, M □ s L of 1.0 or less, and Ni s 1-0 of 0% or less. %
Cu less than or equal to 0.10%, Nb less than 0.20%, vlo less than 0.20%, Ti less than 50%, Zr less than 0.50%
sO, 0.10% or less (7) B, 0.10% or less (7) A
I, containing one or more types of W with a coefficient of 1.0 or less in a total amount of 1.0% or less, the remainder consisting of Fe and unavoidable impurities, and other processes are This is the same as the first invention.

The gist of the third invention is that a hot rolled steel sheet having the same composition as the first invention is manufactured by taking the same step as the first invention and wound into a coil, and then the hot rolled steel sheet is further wound into an Ac1 This method includes a step of reheating to a temperature higher than the transformation point and then rapidly cooling, forming a mixed structure of a ferrite phase and a martenside phase, and making it possible to have a yield ratio of 70 coefficients or less.

The gist of the fourth invention is that after the second invention and the second invention, the hot rolled steel sheet is wound into a coil using the same manufacturing process, and is then further wound. A rapid cooling process is added, and a mixed structure of ferrite phase and martensitic phase is formed similarly to the third invention, resulting in a yield ratio of 70.
This is a method that can reduce the amount of time required.

The gist of the fifth invention is that the steel sheet is further rapidly cooled in the same manufacturing process as the third invention, in which the hot-rolled steel sheet is reheated to a temperature equal to or higher than the Act transformation point and then quenched. In this method, an excellent high-tensile steel sheet with a yield ratio of 70 coefficients or less can be obtained.

The gist of the sixth invention is that the same manufacturing process as that of the fourth invention is used to reheat the hot-rolled steel sheet to a temperature higher than the ACI transformation point and then quench it. This method, which includes a tempering step at the following temperature, is also a manufacturing method that can produce a high-tensile steel sheet with a yield ratio of 70 or less and particularly excellent cold workability.

As described above, as a result of numerous studies, the inventors of the present invention have found that basically, as shown in the first invention and the second invention, cold workability exhibiting properties of a low yield ratio in the hot-rolled state is obtained. By finding manufacturing conditions for producing a high-strength thin steel sheet with excellent properties and further heat-treating the same, a high-tensile thin steel sheet with particularly excellent cold workability, which has never been seen before, was obtained.

The details of the limiting conditions of the present invention will be explained below.

First, the reasons for limiting the hot finish rolling and cooling conditions, which are most important in the present invention, will be described.

According to the present invention, when producing hot-rolled thin steel sheets, the equilibrium ferrite transformation start temperature (described later) during finish rolling (
(Hereinafter abbreviated as Ts) Perform rolling at a temperature range of at least 20 hairs or more and a maximum reduction rate of 80% or less.

This condition is essential for adjusting the properties and dispersion state of the low-temperature transformed phase that refines the austenite grains, introduces strain into the austenite grains, and ultimately contributes to an increase in strength, and is less than 20%. It is difficult to expect this effect when the rolling reduction is 80%, and it is extremely difficult to prevent strain-induced transformation when the rolling reduction exceeds 80%.
A ferrite phase is generated during rolling, and rolling strain is applied to this ferrite phase, reducing the ductility of the manufactured steel sheet.
The rolling reduction was limited to 80%.

Next, as the first cooling condition, the temperature range from Ts to the ferrite transformation start temperature (hereinafter abbreviated as TF), which will be explained later, is an average cooling rate of 30°C/sec or more and 200°C.
/sec or less.

This lower limit of cooling rate not only has a negative meaning of prolonging the slow cooling cycle in the next step within the thermal cycle which is limited in time, but also means that finish rolling, which is performed below the equilibrium transformation temperature, is terminated. The cooling rate is less than 30℃/sec for two purposes: to prevent ferrite transformation from starting until the next step, and to allow work strain to remain in the austenite phase until the next slow cooling process. This is because this purpose is not achieved.

Furthermore, although the upper limit is not determined by metallurgical significance, it is often difficult to achieve a cooling rate of more than 200°C/application from the viewpoint of rolling equipment, manufacturing costs, and productivity. This is because it is not desirable.

As the second cooling condition, the temperature range from TF to a temperature equal to or higher than the pearlite transformation start temperature (hereinafter abbreviated as TP), which will be explained later, is set at 0.5°C/sec or more and 30'C/sec.
The average cooling rate below is at least 2 seconds, preferably 5 seconds or more.

The purpose of this is to advance ferrite transformation while adjusting the ferrite grain size by reducing the degree of supercooling during ferrite transformation, and to diffuse and concentrate C in the remaining austenite so that it can be used at a low temperature in the next quenching process. The two points are to facilitate the formation of the transformed phase and to make the formed low-temperature transformed phase harder.

The reason why the time limit under this requirement is effective even if it is shorter than in the case of normal heat treatment is due to the effect of the pot remaining in the austenite phase due to the low-temperature rolling and rapid cooling treatment in the previous process, as described above. This point is also one of the technical features of the present invention.

If the time required to pass through this temperature range is 2 seconds or less, the above effect will be weak (5 seconds or more is desirable).

Note that the upper limit of this transit time is determined by the time dependence of the pearlite transformation initiation conditions.

Further, from a metallurgical point of view, the cooling rate in the temperature range between TF and TP is slow, but preferable results can be obtained.

Therefore, when carrying out the present invention using ordinary hot rolling equipment, it is preferable to select air cooling, which provides the slowest cooling rate.

However, even greater effects can be obtained if a slower cooling rate can be achieved, for example by installing a heating device or the like in the run-out table.

The lower limit of the cooling conditions that can be practically implemented by such a method is set at 0.5° C./boiled.

On the other hand, the reason why the upper limit of the cooling rate is set to 30°C/sec is as follows.
If this exceeds this, it becomes difficult to accurately regulate the transit time between the TP and TF, and the progress of ferrite transformation and the diffusion and concentration of C into the retained austenite become insufficient, and the effects of the present invention become less effective. This is because it is not desirable because it reduces the number of deaths.

As the third cooling condition, following the slow cooling step, the temperature range is 50°C from a temperature higher than TP to a temperature lower than the bainitic transformation start temperature (hereinafter abbreviated as TB), which will be explained later.
A mixed structure of a ferrite phase and a low-temperature transformation phase is obtained by rapidly cooling at a cooling rate of not less than /see and not more than 200° C./sec.

The reason why the lower limit of this cooling rate is set to 50° C./sec is that it is necessary to suppress pearlite transformation and generate a required amount of low-temperature transformed phase.

Also, the reason why the upper limit is set at 200°C/sec is that in order to obtain a cooling rate exceeding this, the burden on the cooling equipment will increase (it will only increase), and there will be no big difference in the effect obtained ( It's for a reason.

The hot-rolled steel sheet cooled under such cooling conditions is wound into a coil at a temperature below TB in the final step.

In this case, the wound pot coil undergoes some degree of tempering due to the self-annealing effect after the completion of transformation, so the steel plate of the present invention can be used as a final product as is, but the yield ratio can be improved by making further fine adjustments to the strength. In order to improve this, by reheating the hot-rolled steel sheet to a temperature above the Act transformation point and then rapidly cooling it, a mixed structure of ferrite and martensitic phases is formed, which can be used as a final product. .

In this case, the reheating temperature needs to be higher than the □transformation point because the effect is hardly recognized at a temperature lower than the Act transformation point.

Furthermore, the mechanical properties of the hot-rolled steel sheet subjected to the above-mentioned quenching treatment are further improved by changing the structure to a mixed structure of ferrite phase, martensite phase, and bainite phase at a temperature below the Act transformation point. good.

Regarding the tempering temperature in this case, in the temperature range exceeding the Act transformation point, both strength and yield ratio deteriorate, and when tempering in the temperature range below the Act transformation point, the strength is improved.
Since it has the effect of lowering the yield ratio to 70% or less, the temperature should be limited to the ACI transformation point or lower.

The hot rolling conditions and cooling conditions for the manufacturing method of the low yield ratio, high tensile strength thin steel sheet according to the present invention are as described above (this is a precise working heat treatment technique and is applicable to steels having an extremely wide range of chemical compositions). However, in order to achieve economical efficiency and to make the material custom-made as required for its use, and to maximize the heat treatment effect mentioned above, the chemical composition has been regulated.

The reason for limiting each of these components is as follows.

C: At least a modulus of 0.05 or higher is required in order to achieve reinforcement through low-temperature transformation phases, but if it exceeds 0.25%, weldability and workability deteriorate, so it should be in the range of 0.05 to 0.25%. And so.

Mn: At least 0.1 in order to make the processing heat treatment effect effective
0% or more is required, but if it exceeds 2.0%, weldability
Since it has an adverse effect on workability, it is set in the range of 0.10 to 2.0%.

Si: If the amount is within an appropriate range, it will have an advantageous effect on the formation of a low-temperature transformation phase, but if the content exceeds 1.0%, the heat treatment effect will be hindered, which is undesirable, so it was limited to 1.0% or less. .

A high-strength thin steel sheet with a low yield ratio and excellent cold workability can be produced according to the present invention using the above three main elements as basic components, and in addition, one kind of each of the following components is added within a limited amount. Alternatively, the effects of the present invention can be further exhibited by containing two or more kinds.

In this case, the total amount of each of these components is limited to 1.0% or less.

Cr, Mo, W: All are elements that improve the processing heat treatment effect, and it is preferable to use them in appropriate amounts. %, weldability deteriorates, so CrO, 5% or less, Mo 1.0% or less,
Wl was limited to 0% or less.

Ni-Cu: 1. Both are elements that provide strength and weather resistance.
Even if it is used in the range of 0% or less, the effect of the present invention is not inhibited and the original effect of the element is imparted, which is preferable. However, if it exceeds 1.0%, it will have a negative effect on the heat treatment effect in the present invention, so it may not be used. was also limited to 1.0 fO or less.

Nb, Vt Tip Zr, and B2 are all preferably used in appropriate amounts in order to expand the non-recrystallized temperature range of austenite grains and enhance the processing heat treatment effect, but Nb is 0.1% and ■ is 0.2%. %.

Even if Tip Zr is used in excess of 0.5% and B in excess of 0.01%, the effect will be saturated, so these contents were set as the upper limits.

A1: An element useful as a deoxidizing element for the purpose of improving the internal properties of steel, but if it exceeds 0.10%, nonmetallic inclusions will increase, which is undesirable, so it is limited to 0.10% or less.

Next, the above Ts, which is a standard for setting manufacturing conditions according to the present invention,
TF, TP, and TB will be explained.

These transformation temperatures generally vary depending on the chemical composition, austenite grain size, residual processing strain in the austenite phase, and cooling rate.

Therefore, in the past, the only suitable way to quantify this was to conduct experiments that simulated the processing and heat treatment histories of the manufacturing sites for materials with different components, and to analyze the results through trial and error. It goes without saying that applying such a method to the present invention is complicated and unrealistic.

The present inventor developed a mathematical model for quantitatively calculating the metamorphosis phenomenon as a function of the above factors, and previously filed a patent application for
It was disclosed as No. 98.

The outline is as follows.

In other words, the incubation period of transformation is calculated using an approximate cooling curve obtained by dividing the isothermal transformation curve calculated based on the chemical composition, austenite grain size, and working degree into steps, and the given cooling curve. i/Z i (T i ) = 1...
...(1)-0 where Zi (Ti): Incubation period at each temperature (Ti) of isothermal transformation quantified as a function of temperature △ti: Approximate by assuming that the temperature is kept isothermally in stepwise manner After the transformation starts, the transformation that progresses during the holding time Δtj at that temperature is calculated as isothermal transformation, and the state is determined in the next step T. is constructed based on the logic that calculation is continued as being equivalent to the ``+1'' state of tj+1 where the untransformed rate is equal.

Using this model, T s p T under any cooling condition in the composition range of the low alloy steel targeted by the present invention
p t Tp 9TB sFurthermore, it is possible to calculate the martensitic transformation start temperature TM, and therefore, the manufacturing conditions of the present invention are within the constraints determined by the rolling equipment, technical constraints, and cooling device capacity. Optimum conditions that satisfy the limiting conditions of the invention can be easily determined.

The accompanying drawings are schematic diagrams showing the hot rolling history according to the present invention described above.

The symbols in the figure are as follows.

Ts: Equilibrium ferrite transformation start temperature TF: Ferrite transformation start temperature TP: Pearlite transformation start temperature TB: Paynite transformation start temperature CT: Coiling temperature R: Total rolling reduction ratio between T5 and Tp C1: Cooling between Ts □Tp Speed C2: Cooling rate to TF-TP or more t: Time to cool at the cooling rate of C2 C3: Cooling rate from TP to CT L in the drawing
Hot rolling was carried out while changing C□, c2*-t and C3, and all were wound into coils at 480°C.

A test piece was taken from each coil and the yield strength (YS), tensile strength (TS), elongation (El), and yield ratio (YS/TS) of the inventive steel and comparative steel were measured. The results are shown in Table 2. That's right.

As is clear from Table 2, the steels of the present invention that were hot-rolled while satisfying all the requirements of the present invention all had a yield ratio of 80 degrees or less, whereas those that met even one of the requirements of the present invention The comparative steels that did not satisfy the requirements all had a yield ratio of 80% or more, and when comparing the inventive steel with the same strength and the comparative steel, the elongation of the inventive steel is much lower.

Note that the underlined values for the comparative steels in Table 2 do not meet the requirements of the present invention.

Example 2 Using the test materials A2, 3, 4, 5, and 6 shown in Table 1, hot rolling was carried out under conditions according to the present invention indicated by symbol C in Table 2 and conditions that did not satisfy the requirements of the present invention indicated by symbol C. The results of comparing their mechanical properties are shown in Table 3.

As is clear from Table 3, the comparative steel hot-rolled under conditions different from the requirements of the present invention has an increased strength but also a large increase in yield ratio, whereas the steel of the present invention does not yield even though the strength increases. It was found that the increase in ratio was extremely small (all showed a yield ratio of less than 80%, and the total elongation was also much better).

Example 3 Coils hot-rolled using sample material A1 in Table 1 according to the requirements of the present invention indicated by symbol C in Table 2 were annealed in a continuous annealing furnace at different heating temperatures, and then quenched.

In addition to investigating the mechanical properties of these coils, we performed tempering treatment on the coils that had been rapidly cooled after annealing and the coils that were as hot-rolled.

The influence of heating temperature was investigated. The results are shown in Table 4.

As is clear from Table 4, regarding the reheating temperature conditions, it was found that the yield ratio was improved at temperatures above the Act transformation point, while almost no effect was observed at temperatures below the Act transformation point. Regarding the tempering conditions, it was found that when tempered at a temperature above the ACI transformation point, both strength and yield ratio deteriorated, and when tempered at a temperature below the Ac1 transformation point, the yield ratio decreased to below 70 coefficients.

As is clear from the many embodiments described above, the present invention achieves a low yield ratio while remaining hot-rolled by controlling various conditions during the finish rolling and subsequent cooling process during hot rolling using precise processing and heat treatment techniques. (Therefore, we were able to provide a method for manufacturing high-strength hot-rolled thin steel sheets with cold force measurement properties.

Furthermore, when this material was heat-treated, a high-strength thin steel sheet with a lower yield ratio that was better than when a hot-rolled sheet under conventional manufacturing conditions was used as the material could be obtained.

[Brief explanation of drawings]

The accompanying drawings are schematic diagrams showing the hot rolling history of the method for producing high-strength thin steel sheets with excellent cold workability according to the present invention. Ts: Equilibrium ferrite transformation start temperature, TF: Ferrite transformation start temperature, TP: Barite transformation start temperature, TB:
Bainite transformation start temperature, CT: coiling temperature, R: T
s ”' = total rolling reduction rate between TF, C1: T 5
- Cooling rate between Tp, C2: Cooling rate from TF to TP or higher, t: Time to cool at the cooling rate of 02, C3: TP
Cooling rate from above to CT.

Claims (1)

[Claims] 1. C: 0.05 to 0.25%, Mn: 0 in weight ratio
．． 1 to 2.0%, Si: 1.0% or less, and the balance is F.
e and unavoidable impurities, the method includes a step of applying a reduction of 20 to 80% at a temperature during hot rolling of the steel sheet below the equilibrium ferrite transformation starting temperature, and a step of rolling the hot rolled steel sheet from the equilibrium ferrite transformation starting temperature. The temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature is at least 30°C/sec and the average cooling rate is at most 200°C/sec, and the temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature is at least 2 seconds or more. Time 0.5℃A or more 30℃/s
A step of cooling at an average cooling rate of ec or less, and further cooling at an average cooling rate of 50° C./sec or more and 200° C./sec after the cooling treatment.
After cooling with the following average cooling, winding it into a coil at a temperature below the bainite transformation starting temperature,
Forms a mixed structure of one or both of bainite phase and martensite phase and ferrite phase, yielding ratio 80%
A method for producing a high-strength thin steel sheet with excellent cold workability, characterized by the following: 2 C: 0.05-0.25%, Mn: 0 in weight ratio
．． 1 to 2.0%, Si: 1.0% or less, and further 0.
5% or less Cr, 1.0% or less MO, 1.0% or less Ni, 1.0% or less Cu, 0.10% or less Nb, 0
．． 20% or less (7) V, 0.50% or less T 1sO0
One or more of the following: Zr of 50% or less, B of 0.010% or less, AI of 0.10% or less, and W of 1.0% or less in a total amount of 1.0% or less. In the method for manufacturing a thin steel sheet containing Fe and the remainder being Fe and unavoidable impurities, the hot-rolled steel sheet is subjected to a reduction of 20 to 80 degrees at a temperature at which the hot-rolled steel sheet is below the equilibrium ferrite transformation start temperature, and the hot-rolled steel sheet is balanced. The temperature range from the ferrite transformation start temperature to the ferrite transformation start temperature is 30 ° C / Both times are 0.5℃ for 2 seconds or more / 30℃ or more
A step of cooling at an average cooling rate of 50°C/sec or more and 200°C or less after the cooling process, and then cooling at a temperature not higher than the bainite transformation starting temperature. and a step of winding it into a coil, forming a mixed structure of one or both of bainite phase and martensite phase and ferrite phase, and having a yield ratio of 80 coefficients or less. A method for manufacturing high-strength thin steel sheets with excellent workability. 3 C: 0.05-0.25%, Mn: 0.3% by weight.
1 to 2.0, Si: contains 1.0 or less, and the balance is Fe
A method for manufacturing a thin steel sheet comprising diagonal and unavoidable impurities, comprising: applying a reduction of 20 to 80% when the temperature during hot rolling of the steel sheet is below the equilibrium ferrite transformation starting temperature, and rolling the hot rolled steel sheet from the equilibrium ferrite transformation starting temperature. The temperature range up to the ferrite transformation start temperature is 30°C/5e=J'), the average cooling rate is 200°C/equation or less, and the temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature is at least 2 seconds. 30℃ for a period of 0.5℃/se or more
A step of cooling at an average cooling rate of /Sω or less, and after the cooling treatment, further cooling at an average cooling rate of 50 ° C A or more and 200 ° C / sec or less, and then winding into a coil at a temperature equal to or lower than the bainite transformation start temperature. and a step of reheating the coiled hot-rolled steel sheet to a temperature similar to Act transformation 7 and then rapidly cooling it to form a mixed structure of a ferrite phase and a martensitic phase and yielding. 1. A method for producing a high tensile strength thin steel sheet with excellent cold workability, characterized in that the steel sheet has a steel sheet of 70% or less. 4 C: 0.05-0.25%, Mn: 0 in weight ratio
．． 1 to 2.0%, Si: 1.0% or less, and further 0.
5 or less Cr, 1.0% or less Mo, 1.0% or less Ni, 1.0% or less Cu, 0.10% or less Nb, 0
．． V of 20 or less, T of 0.50% or less 15O050
% Zr, 0.10% or less B, 0.10% or less A1
．． In a method for producing a thin steel plate containing one or more types of W in a total amount of 1.0% or less in a limit of 1.0% or less, the remainder being Fe and unavoidable impurities, A step of applying a reduction of 20 to 80 degrees when the rolling temperature is below the equilibrium ferrite transformation start temperature, and a step of rolling the hot rolled steel sheet from the equilibrium ferrite transformation start temperature to the ferrite transformation start temperature in a temperature range of 30°C/sec or more and 200°C/sec. se
Cool the temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature at an average cooling rate of 0.5°C/sec or more and 30°C/sec or less for at least 2 seconds at an average cooling rate of 0.5°C/sec or more and 30°C/sec or less. After the cooling treatment, the average cooling rate of 50° C./sec to 200° C./sec is further carried out, and then winding into a coil at a temperature below the bainite transformation start temperature, and the step of winding the wound hot rolled steel sheet to the Ac1 transformation point. reheating to the following temperature and then rapidly cooling, forming a mixed structure of a ferrite phase and a martensitic phase and having a yield ratio of 70 coefficients or less. A method for manufacturing excellent high-tensile steel sheets. 5 C: 0.05-0.25%, Mn: 0 in weight ratio
．． 1 to 2.0, Si: 1.0% or less, and the balance is Fe and unavoidable impurities. The step of applying a reduction of 80% and the temperature range from the equilibrium ferrite transformation start temperature to the ferrite transformation start temperature of the hot rolled steel sheet at a rate of 30°C/sec or more 20
At an average cooling rate of 0°C/sec or less, and in a small temperature range from the ferrite transformation start temperature to the pearlite transformation start temperature (2 seconds or more, 0.5°C/SeC or more 30°C
cooling at an average cooling rate of /sec or less, and after the cooling treatment, further cooling at an average cooling rate of 50 ° C / sec to 200 ° C / sec, and then winding into a coil at a temperature equal to or lower than the bainitic transformation start temperature. a step of reheating the coiled hot-rolled steel sheet to a temperature equal to or higher than the Act transformation point and then rapidly cooling it; and a step of tempering the rapidly cooled steel sheet at a temperature lower than or equal to the Ac1 transformation point. The yield ratio is 70
% or less. 6 C: 0.05-0.25%, Mn: 0.6% by weight.
1 to 2.0, Si: 1.0% or less, and further 0.5
Cr below 1.0%, MO below 1.0%, N below 1.0%
1% 1i0% or less Cu, 0.10% or less Nb,
V less than 0.20%, T I %0. less than 0.50%.
One or more of the following: Zr of 50% or less, B of 0.010% or less, At of 0.10% or less, and W of 1.0% or less in a total amount of 1.0% or less. A method for manufacturing a thin steel sheet containing Fe and the remainder comprising Fe and unavoidable impurities, the step of applying a rolling reduction of 20 to 80 degrees at a temperature during hot rolling of the steel sheet equal to or lower than the equilibrium ferrite transformation initiation temperature;
The hot rolled steel sheet is heated in a temperature range from the equilibrium ferrite transformation start temperature to the ferrite transformation start temperature at an average cooling rate of 30°C/sec to 200°C/sec, and from the ferrite transformation start temperature to the pearlite transformation start temperature. Temperature range: 0.5℃/30℃ for at least 2 seconds or more
A step of cooling at an average cooling rate of 50°C/'SeC or more and 200°C/sec or less after the cooling treatment, and then cooling the coil at a temperature less than the bainite transformation starting temperature. a step of reheating the wound hot rolled steel sheet to a temperature higher than the Ac1 transformation point and then rapidly cooling it; and a step of tempering the rapidly cooled steel sheet at a temperature lower than the Ac1 transformation point. The yield ratio is 70
% or less.