JP4528137B2 - Manufacturing method of high strength and high ductility steel sheet with excellent hole expandability - Google Patents

Description

The present invention, building materials, household appliances, excellent hole expandability and ductility suitable for automotive, tensile strength method of manufacturing a more high-strength thin steel plate 850 MPa.

  In recent years, there has been an increasing demand for high-strength steel sheets with good workability for the purpose of improving fuel efficiency and durability particularly in automobile bodies. In addition, it is desired to increase the strength of the steel sheets for the members because of the need for collision safety and expansion of cabin space. In fact, regulations regarding automobile collision safety are becoming stricter year by year, as represented by N-CAP. Against this background, there has been an active movement to use high strength steel sheets of 780 MPa class.

  However, in order to withstand further stringent regulations, a steel plate of 980 MPa class with a high tensile strength is considered necessary particularly for members such as a sheet member and a part of reinforcement for side collision. When a member is assembled using such a high-strength material, ductility, bendability, hole expansibility, energy absorption at the time of collision, and the like become serious problems, and measures for these are required.

  Although hole expansibility and ductility are contradictory characteristics, the following measures have been taken to improve individual characteristics. For example, regarding hole expansibility, CAMP-ISIJ vol. 13 (2000) p. As disclosed in 391 (Non-Patent Document 1), it is disclosed that the hole expandability can be improved with a tensile strength of 980 MPa or more by increasing the volume ratio of martensite as the main phase.

  However, since martensite is the main phase, the ductility is low, and Si is added in order to achieve both hole expansion and ductility. Here, since the addition amount of Si exceeds 1%, the chemical conversion and electrodeposition coating properties necessary for use in cold-rolled steel sheets are clearly deteriorated. Furthermore, since water quenching is required as the heat treatment, it is inevitable that the plate shape becomes worse than a heat treatment pattern without water quenching.

  In addition, CAMP-ISIJ vol. 13 (2000) p. As shown in 395 (Non-patent Document 2), the main phase is bainite and the hole expandability is improved. Furthermore, with regard to the formability, the austenite is formed in the second phase to form the retained austenite steel. It is disclosed to exhibit the overhanging property. Furthermore, it is also shown that when retained austenite having an area ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, there is no mention of ductility as a steel sheet, and furthermore, since these are additions of Si exceeding 1%, the problem of chemical conversion and electrodeposition coating property deterioration remains.

CAMP-ISIJ vol. 13 (2000) p. 391 CAMP-ISIJ vol. 13 (2000) p. 395

The present invention, above problems to resolve, primarily in tensile strength than 850MPa is the object to provide a method for producing a high strength thin steel plate having improved simultaneously hole expandability and ductility of high-strength steel sheet 980MPa grade To do.

As a result of various studies, the present inventors have specified the steel plate components and the microstructure structure as a method for simultaneously improving the hole expansibility and ductility in a region where the tensile strength is 850 MPa or higher and mainly 980 MPa or higher. Thus, it was found that hole expandability and ductility can be secured while maintaining a high strength of 850 MPa or more.
The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) By mass%, C: 0.001 to 0.3%, Si: 0.001 to 0.60%, Mn: 0.01 to 3%, Al: 0.001 to 4.0%, Mo : 0.001 to 0.30%, P: 0.0001 to 0.3%, S: 0.0001 to 0.1%, B: 0.0001 to 0.0050%, N: 0.0001 to 0 .0070%, O: 0.0021% to 0.0037%, satisfying B / 11 + (Al / 27−O / 16) −N / 14> 0, and the balance being Fe and inevitable impurities The cast slab is cast as it is or once cooled and then heated again to 1150 to 1250 ° C., and after hot rolling is finished at 800 to 950 ° C., it is cooled to 550 ° C. or lower at an average cooling rate of 5 to 200 ° C./s. Te, the hot-rolled steel sheet was wound at 550 ° C. or less rolled pickling after cold, then, Ac ₁ (° C.) or higher Ac ₃ After annealing at a temperature range of 50 (° C) or lower for 10 seconds to 30 minutes, it is cooled to a temperature range of 750 to 600 ° C at an average cooling rate of 0.1 to 20 ° C / second, and subsequently 20 to 200 ° C / second. A high-strength, high-ductility sheet steel excellent in hole expansibility, characterized by cooling to a temperature of 450 to 250 ° C. at an average cooling rate and holding in that temperature range for more than 100 seconds and less than 3000 seconds, and then cooling to room temperature . Production method.

( 2 ) The steel further contains 0.001 to 0.50% of one or more of Nb, Ti, V, Zr, Hf, and Ta in a total mass of 0.001 to 0.50%. The manufacturing method of the high intensity | strength high ductility thin steel plate excellent in the hole expansibility described in ) .

( 3 ) The steel according to (1) or (2 ), wherein the steel further contains 0.001 to 5% in total of one or more of Cr, Ni, Cu, Co, and W in mass%. The method for producing a high-strength and high-ductility thin steel sheet excellent in hole expansibility described in the item ) .
( 4 ) The above-mentioned (1) to ( 3 ), wherein the steel further contains 0.0001 to 0.5% in total of one or more of Y, Rem, Ca and Mg in mass%. The method for producing a high-strength, high-ductility thin steel sheet excellent in hole expansibility described in any one of items 1) .

  The high-strength thin steel sheet of the present invention has both excellent hole expansion and ductility, and is extremely effective for applications such as automobile frames and its reinforcing members, as well as building materials and home appliances.

Hereinafter, the present invention will be described in detail.
The inventors have melted the steel ingot added with each alloy element, heated again as cast or once cooled, hot-rolled hot-rolled steel sheet after hot rolling, cold-rolled after pickling, and then annealed, A cold-rolled annealed plate was created. The steel sheet was subjected to microstructural observation, hole enlargement test specified by the Federation of Iron and Steel, and tensile test based on JIS, and each characteristic was compared and evaluated. As a result, it was found that a tensile strength of 850 MPa or more was obtained by the microstructure control finally obtained, and a high-strength steel sheet excellent in hole expansibility and ductility could be produced.

A preferable microstructure of the steel sheet will be described.
In general, it is effective to make the main structure a bainite phase (also referred to as bainite), bainitic ferrite, or a martensite single phase in order to ensure sufficient hole expandability. However, when such a hard phase is increased, the ductility deteriorates. On the other hand, when the number of relatively soft ferrite phases (also called ferrite) is increased, the ductility tends to be improved, but it is not suitable for securing the strength and hole expandability. Therefore, strength-hole expansibility can be achieved by balancing each of these microstructures without degrading chemical formation and electrodeposition, and in addition, without any treatment that may cause plate shape deterioration such as water quenching in the continuous annealing process. -It is important to ensure a sufficient level of ductility.

  That is, the space factor of each microstructure is 20% or more of the soft ferrite phase, 10% or more of the bainite phase, 5% or more of the martensite phase (also called martensite), and the residual austenite phase (residual). By setting the austenite) to 5% or less, a good material can be obtained. Strength-ductility is ensured by the balance between ferrite and martensite, and strength-hole expansibility is ensured by the balance between bainite and martensite. Here, retained austenite is effective in securing ductility, but is less effective in improving hole expansion, and rather tends to deteriorate, so it is desirable that it be less than 3%. Most preferably, it is 0%.

For example, in order to obtain a good hole expansion-ductility balance while ensuring a tensile strength of 980 to 1180 MPa, ferrite is 20 to 70%, bainite is 20 to 60%, and martensite is 5 to 20%. The retained austenite is preferably less than 3%.
Furthermore, the relatively high volume fraction of the soft ferrite, which is the main phase, is effective for improving ductility, and the fact that it is fine is effective for improving the balance between strength and hole expansibility. For this reason, the upper limit of the average particle diameter of the ferrite was set to 3 μm. In addition to the above, the present invention includes a case in which one or two or more of carbides, nitrides, sulfides, oxides, and the like are contained as a remaining structure of the microstructure in a volume fraction of 1% or less.

  In addition, identification of each phase of the above microstructure, ferrite, bainite, austenite, martensite, interfacial oxidation phase and remaining structure, observation of the existing position, and measurement of the space factor are performed using the Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. Can be quantified by observing the steel sheet in the rolling direction or in the direction perpendicular to the rolling direction with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. . Observation of 20 or more fields of view can be performed, and the space factor of each tissue and the average particle size of the main phase can be obtained by a point counting method or image analysis. Moreover, the space factor of each microstructure can be obtained from the investigation of each phase transformation behavior from the expansion / contraction curve by Formaster. The total of each phase of the microstructure is 100%, but phases that cannot be observed or identified by an optical microscope or an expansion curve such as carbides, oxides, and sulfides are included in the area ratio of the main phase.

Next, the reason for limiting the preferable range of the steel plate component in the present invention will be described.
C is an important additive element in order to ensure a good material balance. It is the most important additive element for controlling the fraction of ferrite, bainite and martensite. Addition of 0.001% or more to ensure strength, and when aiming for 980 MPa or more, addition of 0.1% or more is desirable. On the other hand, if the amount added is increased, the hole expandability is deteriorated, so the content was made 0.3% or less. In order to obtain a good balance between hole expandability and ductility at a strength level of 980 to 1180 MPa class and to avoid deterioration of weldability as much as possible, 0.15% or less is desirable. Preferably, C: 0.05 to 0.15% can provide a good balance between ductility and hole expansibility.

  Si is desirable to be low because it deteriorates chemical conversion and electrodeposition properties. On the other hand, since it is a strengthening factor, it was set to 0.01 to 0.6%. However, considering the refining ability and the raw material composition, there is a concern that making it less than 0.03% leads to a significant cost increase. Moreover, when considering especially with chemical conversion and electrodeposition property, it is desirable that it is 0.1% or less. In addition, in the case of complex addition with a relatively large amount of Al, it is desirable that the content be 0.001 to 0.1% from the viewpoint of securing the welding quality.

  Mn is added for the purpose of increasing the strength. Further, it is added for the purpose of suppressing carbide precipitation and pearlite formation, which are one cause of strength reduction and material deterioration. From these things, it was set as 0.01 mass% or more. On the other hand, 3 mass% was made the upper limit because it delayed the bainite transformation that contributes to improving hole expansibility. Preferably, a good balance between strength and hole expandability can be obtained by setting Mn to 1.5 to 3.0%.

  In addition to the deoxidizing element, Al is added for the purpose of controlling ferrite and bainite. It is effective in improving ductility by promoting ferrite and improving hole expandability by promoting bainite. For this reason, it was set as 0.001 mass% or more addition. On the other hand, excessive addition causes a decrease in the bainite fraction and excessive martensite accompanying the excessive promotion of ferrite, which not only deteriorates hole expandability but also deteriorates weldability, so the upper limit was made 4%. In addition, in order to obtain a good balance between ductility and hole expansibility, particularly at a strength level of 980 to 1180 MPa, addition of 0.20% to 1.5% is desirable. Moreover, in order to obtain especially high ductility, addition of 0.75% or more is desirable.

Mo is an element added for the purpose of suppressing the formation of carbide and pearlite which deteriorates the strength and ductility balance, and is made 0.001% or more. Furthermore, excessive addition, since it leads to ductility degradation by delaying the ferrite and bainite, 1. 0%. In particular, in order to suppress pearlite and carbide precipitation in the continuous annealing process as much as possible, addition of 0.05% or more is desirable, especially to obtain a good balance of ductility and hole expansibility at a strength level of 980 to 1180 MPa class. Is preferably added in an amount of 0.30% or less. On the other hand, when the cooling rate from 700 ° C. to 400 ° C. is 50 ° C./s or more, a lower addition (0.15% or less) may be used. Further, in order to obtain a particularly good hole expansion-ductility balance, a range of 0.01% or more, preferably 0.1% or more and 0.3% or less is desirable.

P is a strengthening element, and extremely low reduction is economically disadvantageous, so 0.001% by mass, preferably 0.0001% by mass was set as the lower limit. Moreover, since addition in a large amount adversely affects weldability, manufacturability at the time of casting or hot rolling, 0.3%, preferably 0.1% was made the upper limit. More preferably, it is 0.03% or less.
Since S is extremely disadvantageous because it is economically disadvantageous, 0.001% by mass, preferably 0.0001% by mass is set as the lower limit, and 0.1% by mass, preferably 0.05% is set as the upper limit. This is because addition of an amount exceeding this adversely affects weldability and manufacturability during casting and hot rolling.

B is also an additional element necessary for maintaining a good balance between hole expansibility and ductility. When B is added in an amount of 0.0001% by mass or more, the grain boundary is strengthened, the strength of the steel material is increased, and the balance between hole expansibility and ductility is effectively improved. However, when the addition amount exceeds 0.0050 mass%, not only the effect is saturated but also the workability is lowered, so this was made the upper limit.
N is not a preferable element for exhibiting the effect of addition of B. Therefore, 0.0070% or less is added, and extremely low is economically disadvantageous, so 0.0001% by mass was made the lower limit.

  Ti forms fine carbides, nitrides or carbonitrides and is effective for strengthening the steel sheet. Further, since it is particularly effective for forming hard ferrite by composite addition with B, the addition was made 0.001% by mass or more. Further, in the case of composite addition with B, addition of 0.01% or more is desirable for forming hard ferrite. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 0.5 mass%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.005% to 0.020% is desirable.

  Nb forms fine carbides, nitrides or carbonitrides or is in a solid solution state, and is effective for strengthening the steel sheet. Further, it is an important additive element for forming hard ferrite, and 0.001% by mass or more is added, and 0.01% or more is desirable for forming hard ferrite. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 0.5 mass%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.005% to 0.020% is desirable.

In addition to the above, B is an element that promotes the fine homogenization of the hot-rolled sheet structure and, as a result, is effective in improving the hole expansion after annealing. For this reason, it was made into 0.0001% or more of addition. This effect is particularly effective when combined with Ti. On the other hand, excessive addition causes ductility deterioration, so the upper limit was made 0.0050%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.0003% to 0.0020% is desirable.
Cr is an additive element effective for improving the hole expansion property in order to promote the formation of hard ferrite and the refinement of carbides, so it was added in an amount of 0.01% or more. Moreover, since excessive addition causes ductility fall, the upper limit was made 5%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.1% to 0.8% is desirable.

  Relationship between B, Al, O and N: As described above, addition of B is an important item of the present invention, and in order to sufficiently exhibit this effect, it is effective not to combine B with N as much as possible. Therefore, it is important for exhibiting the effect of B and improving the material balance that the sum of the amount of Al not bonded to O as a nitride-forming element stronger than B and the amount of B added is larger than the amount of N. Therefore, it is desirable that B / 11 + (Al / 27−O / 16) −N / 14> 0. Further, when the formula is a value of 0.02 or more, a better hole expansion-ductility balance is obtained. can get.

  Furthermore, the steel targeted by the present invention can contain one or two of Nb, Ti, V, Zr, Hf, and Ta, which are strong carbide forming elements, for the purpose of further improving the strength and refining the structure. These elements form fine carbides, nitrides or carbonitrides and are extremely effective for strengthening the steel sheet. Therefore, if necessary, one or more of these elements may be added in a total amount of 0.001% by mass or more. It was set as addition. On the other hand, since it inhibits ductility deterioration and concentration of C in retained austenite, the upper limit of the total amount of one or more types is set to 0.50% by mass. Of these, Nb, Ti, Zr, Hf, and Ta, which are stronger nitride-forming elements than B, are also effective in utilizing the B addition effect, and it is desirable to add them in consideration of economy.

Furthermore, the steel targeted by the present invention can contain one or more of Cr, Ni, Cu, Co, and W for the purpose of further improving the strength.
Cr is an element added for the purpose of strengthening and suppressing the formation of carbides and the purpose of forming bainite and bainitic ferrite. The addition of 0.001% or more exhibits an effect, and Cr, Ni, Cu, Co, W 1 Addition of more than 5% in total of seeds or two or more kinds adversely affects processability, so this was made the upper limit.

Ni exhibits an effect by addition of 0.001% by mass or more for the purpose of strengthening by improving hardenability, and exceeds 5% by mass in total of one or more of Cr, Ni, Cu, Co, and W. Addition of the amount has an adverse effect by contributing to the increase in workability, in particular the hardness of martensite, so this was made the upper limit.
Cu is effective when added in an amount of 0.01% by mass or more for the purpose of strengthening, and when added in an amount exceeding 5% by mass in total of one or more of Cr, Ni, Cu, Co, and W, it is processed. Adversely affects performance and manufacturability.

Co exhibits an effect when added in an amount of 0.001% by mass or more in order to improve the strength ductility balance by controlling the bainite transformation. On the other hand, since it is an expensive element, the addition of a large amount impairs the economy, so it is desirable that the total of one or more of Cr, Ni, Cu, Co, and W be 5% by mass or less.
When W is added in an amount of 0.001% by mass or more, a strengthening effect appears. Addition of more than 5% by mass of one or more of Cr, Ni, Cu, Co, and W adversely affects workability. Effect.

  Y, an abbreviation for Rem (Rare Earth Metal), a lanthanoid element starting from La. Industrially, it is added in the form of misch metal. In this case, the main component is La and Ce.) Ca, Mg are added in appropriate amounts to control the form of inclusions, especially finely dispersed. From the viewpoint, the total of one or more of Y, Rem, Ca and Mg is 0.0001% or more, while excessive addition reduces the manufacturability such as castability and hot workability and the ductility of the steel sheet product. Therefore, the total of one or more of Y, Rem, Ca, and Mg is set to 0.5 mass%. Of these, La and Ce, which are stronger nitride forming elements than B, are also effective in utilizing the B addition effect, and it is desirable to add them while considering manufacturability. Inevitable impurities include, for example, Sn, but even if these elements are contained in the range of 0.02% by mass or less, the effect of the present invention is not impaired.

A method for producing a high-strength and high-ductility thin steel sheet having such a structure and excellent hole expansibility will be described below.
When the steel sheet of the present invention is manufactured by cold rolling and annealing after hot rolling, the slab adjusted to a predetermined component is cast as it is or once cooled and then reheated for hot rolling. In this case, the reheating temperature is desirably 1150 ° C. or higher and 1250 ° C. or lower. When the reheating temperature is high, coarse grains and thick oxide scales are formed. On the other hand, low temperature heating increases the rolling resistance. Moreover, after hot rolling, if the surface scale is removed by using a high-pressure descaling device or pickling, the surface of the product is better cleaned, which tends to be advantageous for plating properties.

In addition, it is important to control the structure from hot rolling in order to achieve both ductility and hole expansibility. Making the hot rolled sheet structure uniform and fine contributes greatly to improving the ductility and hole expansibility of continuous hot-dip galvanized steel sheets. Therefore, the finishing temperature for hot rolling is set to 800 to 950 ° C., and in order to obtain a better material, it is set to 850 to 970 ° C. When the finishing temperature exceeds 970 ° C., and preferably exceeds 950 ° C., there are concerns that the structure becomes coarse and the transformation controllability during cooling becomes difficult.
On the other hand, if it is less than 800 ° C., preferably less than 850 ° C., there is a concern that two-phase region rolling occurs, and it may be difficult to ensure the target gauge thickness accuracy.

  Subsequent cooling is performed at an average cooling rate of 5 ° C./s or more for the purpose of suppressing pearlite transformation. On the other hand, the fast cooling side was set to 200 ° C./s or less as a possible range without a special cooling device. The cooling stop was 550 ° C. or less up to the bainite generation temperature range. After that, it was decided to wind up to 550 ° C. or less for the purpose of homogenizing the hot rolled structure by bainite or martensite formation. The homogenization of the hot-rolled sheet is particularly important for ensuring the same hole expandability and ductility as well as the effect of adding B.

  On the other hand, it is desirable to wind as high a temperature as possible from the problem of an increase in reaction force during cold rolling, and winding at 500 to 400 ° C. is desirable. In order to obtain good ductility and hole expansibility, the cooling rate after finishing is set to 10 to 100 ° C./s, cooled to a temperature range of 550 ° C. or less, and wound at 550 ° C. or less, thereby forming a layered partite structure. It is desirable to suppress formation. After that, for cold rolling, if the total reduction ratio set from the relationship between the final sheet thickness and the cold rolling load is 40% or more, it is sufficient from the viewpoint of recrystallization and transformation control, and the final steel sheet characteristics are not deteriorated. .

As for the conditions of the continuous annealing process, the temperatures Ac ₁ and Ac ₃ are determined depending on the chemical components of the steel (for example, “Steel Material Science” by W.C. Leslie, translated by Koyasu Naruyasu, Maruzen P273) When it is less than the expressed Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, and bainite and martensite cannot be left in the final steel sheet. For this reason, this was made into the minimum of annealing temperature. In addition, when the annealing temperature exceeds Ac ₃ +50 (° C.), the structure becomes coarse and the manufacturing cost increases, so the upper limit of the annealing temperature is set to Ac ₃ +50 (° C.). The annealing time at this temperature requires 10 seconds or more in order to ensure uniform temperature of the steel sheet and to ensure recrystallization and austenite. However, if it exceeds 30 minutes, not only will the effect be saturated, but also an increase in cost and coarsening of the structure will be caused, so this was made the upper limit.

Moreover, in order to obtain a favorable structure fraction, annealing in the temperature range of 0.1 × (Ac ₁ −Ac ₃ ) + Ac _{1 to} 0.8 × (Ac ₁ −Ac ₃ ) + Ac ₁ is desirable. Moreover, in order to make the balance of hole expansion property and ductility better, 0.2 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) to 0.5 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) It is desirable to anneal in the range of 60-200S in the temperature range.

  Subsequent primary cooling is important to control the transformation from austenite to ferrite, bainite and martensite to obtain the optimum structure fraction. Setting the average primary cooling rate following annealing to less than 0.1 ° C / second causes manufacturing disadvantages such as lengthening the required production line length and extremely slowing the production rate, and excessively during cooling. The lower limit of the cooling rate was set to 0.1 ° C./second in order to generate ferrite or promote pearlite formation. On the other hand, when the primary cooling rate exceeds 20 ° C./second, ferrite transformation does not occur sufficiently and the hard phase becomes excessive, so this was made the upper limit.

  About the primary cooling stop temperature, if it is less than 600 ° C., ferrite is excessively generated or pearlite promotes the generation, so that it is 600 ° C. or more, and if it exceeds 750 ° C., ferrite cannot be sufficiently controlled and secured. The appropriate primary cooling stop temperature range was set to 750 to 600 ° C. From the viewpoint of ferrite control and production line length, the primary cooling is desirably stopped at 700 to 650 ° C. at 3 to 10 ° C./s. Subsequent secondary cooling, but if the average is less than 20 ° C / second, there is a concern about the formation of pearlite, and if it exceeds 200 ° C / second, the pearlite suppression effect is saturated and the plate shape It was set to 20 to 200 ° C./s because deterioration was difficult and operation was difficult.

  Furthermore, in order to generate bainite or bainitic ferrite or martensite while suppressing the transformation from the austenite phase to the ferrite phase to some extent, it is possible to reduce the cooling rate to less than 0.1 ° C./second. In addition, there is a concern that the generation of pearlite is promoted and the strength is lowered, so that the lower limit of the cooling rate is preferably 0.1 ° C./second. On the other hand, when the cooling rate is higher than 100 ° C./second, the hard steel phase such as the martensite phase in the final steel sheet becomes large and it is difficult to operate. . Further, at the time of cooling, for the purpose of controlling a small amount of ferrite, a two-stage cooling pattern may be taken in which, after annealing, it is once cooled to a temperature range of 600 to 750 ° C. and then rapidly cooled to avoid pearlite and carbide precipitation.

  When the secondary cooling stop temperature is less than 250 ° C., there is a concern that the amount of martensite is excessive or the plate shape is deteriorated. Moreover, when it exceeds 450 degreeC, there exists a concern of controllability of a bainite transformation and carbide precipitation, and 450 degreeC was made into the upper limit. From the bainite transformation and the hardness adjustment by tempering the already generated martensite, the secondary cooling is stopped and the holding is performed in the temperature range of 250 to 450 ° C. for 100 to 3000 seconds. If it is less than 100 seconds, bainite transformation does not occur sufficiently, and if it exceeds 3000 seconds, there is a concern that hole expansion and ductility decrease due to carbide precipitation.

  The treatment including the secondary cooling and the subsequent holding is mainly for the purpose of controlling the bainite transformation and suppressing a large amount of carbide formation (including pearlite structure), with a secondary average cooling rate of 30 to 150 ° C./s, It is desirable that the stop temperature is 270 to 400 ° C. and the holding time is 200 to 600 seconds. In addition, when cooling, if it is moderately cooled in a temperature range exceeding 400 ° C. or retained for a relatively long time, there is a concern that although it tends to be effective for hole expansibility, it may lead to a decrease in strength and ductility. It is preferable to cool and stop to a temperature range of 400 ° C. or lower while securing the average cooling speed.

On the other hand, the lower limit of the cooling stop temperature is preferably stopped at 250 ° C. or higher because, in addition to the problem of plate shape, the strength increases and the ductility is lowered as a result. In addition, when emphasizing ductility in particular, it is desirable that the cooling stop and stop be in a temperature range of 350 ° C. or lower.
The retention time is preferably not longer than 1000 seconds because a long time is not preferable in terms of productivity and carbides are generated. Moreover, in order to make the balance between hole expandability and ductility better, it is desirable to stop for 200 to 450 seconds.

Example 1
Hereinafter, the present invention will be described in more detail with reference to examples.
A steel plate having a composition as shown in Table 1 was heated to 1180 to 1250 ° C., completed in hot rolling at 800 to 950 ° C., wound up after cooling, pickled and then cold rolled to a thickness of 1.2 mm. .
Thereafter, it was determined by calculating the Ac ₁ and Ac ₃ transformation temperature according to the following equation from a component of the steel (mass%).
_{Ac 1 = 723-10.7 × Mn% -16.9} × Ni% + 29.1 × Si% + 16.9 × Cr%,
Ac ₃ = 910-203 × (C%) × 1 / 2-15.2 × Ni% + 44.7 × Si% + 104 × V% + 31.5 × Mo% −30 × Mn% −11 × Cr% − 20 × Cu% + 700 × P% + 400 × Al% + 400 × Ti%,

After raising and maintaining the annealing temperature calculated from these Ac ₁ and Ac ₃ transformation temperatures in a 5% H ₂ —N ₂ atmosphere, 250 to 450 ° C. in the cooling rate range of 0.1 to 100 ° C./second. The holding treatment was performed in the temperature range. JIS No. 5 tensile test specimens were collected from these steel plates and measured for mechanical properties. In addition, a hole expansion test was performed in accordance with the Steel Federation standard, and the hole expansion rate was obtained. From Tables 2 to 3, the steels of the present invention all have a tensile strength of 850 MPa or more, and are excellent in hole expansion / elongation balance even with a low Si content. On the other hand, all the comparative examples not satisfying the scope of the present invention are inferior in strength, hole expansion, and elongation balance. Moreover, the steel sheet manufactured in the scope of the claims of the present invention has the microstructure described above, and is excellent in strength, hole expansion, and elongation balance.

Claims (4)

% By mass
C: 0.001 to 0.3%,
Si: 0.001 to 0.60%,
Mn: 0.01 to 3%
Al: 0.001 to 4.0%,
Mo: 0.001 to 0.30%,
P: 0.0001 to 0.3%,
S: 0.0001 to 0.1%,
B: 0.0001 to 0.0050%,
N: 0.0001 to 0.0070%,
O: 0.0021 to 0.0037%,
1150-1250 after casting or once cooling a cast slab of steel that contains B and 11+ (Al / 27-O / 16) -N / 14> 0, and the balance is Fe and inevitable impurities. After heating at 800 to 950 ° C, the hot rolled steel sheet was cooled to 550 ° C or lower at an average cooling rate of 5 to 200 ° C / s and wound up at 550 ° C or lower. It is cold-rolled after washing, and then annealed in a temperature range of Ac ₁ (° C.) or higher and Ac ₃ +50 (° C.) or lower for 10 seconds to 30 minutes, and then 750 to 600 at an average cooling rate of 0.1 to 20 ° C./second. Cooling to a temperature range of ℃, subsequently cooling to a temperature of 450 to 250 ℃ at an average cooling rate of 20 to 200 ℃ / second, holding in that temperature range for more than 100 seconds and less than 3000 seconds, and then cooling to room temperature High strength with excellent hole expansibility Method of manufacturing a sex thin steel plate. The steel according to claim 1, further comprising 0.001 to 0.50% of Nb, Ti, V, Zr, Hf, and Ta in a total of 0.001 to 0.50% by mass. A method for producing a high-strength, high-ductility sheet steel with excellent hole expansibility . The hole according to claim 1 or 2 , wherein the steel further contains 0.001 to 5% in total of one or more of Cr, Ni, Cu, Co, and W in mass%. A method for producing a high-strength, high-ductility thin steel sheet with excellent spreadability . Steel, with further mass%, Y, Rem, Ca, any one of claims 1 to 3, characterized in that it contains from 0.0001 to 0.5 percent by total one or two or more of Mg A method for producing a high-strength and high-ductility thin steel sheet excellent in hole expansibility described in 1.