JP2011195956A - High strength thin steel sheet having excellent elongation and hole expansion and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength thin steel sheet having excellent elongation and hole expansibility while securing its high strength, in the retained austenite steel, and to provided a method for producing the same.SOLUTION: The high strength thin steel sheet having excellent elongation and hole expansibility is characterized in that a steel sheet has a specific steel composition and has a metallic structure mainly comprising ferrite or bainite or tempered martensite, and containing 3 to 20% of a retained austenite phase, and the steel sheet has average C concentration in the austenite phase of 0.6 to 1.2% in a phase boundary in contact with the ferrite phase, the bainite phase and a martensite phase, and includes ≥50% austenite grains in which the central C concentration Cgc of the austenite phase and the concentration Cgb of the austenite grain at the grain boundary are in the range satisfying equation (1): Cgb/Cgc>1.3.

Description

  TECHNICAL FIELD The present invention relates to a high-strength thin steel sheet excellent in elongation and hole expansibility suitable for structural materials such as automobiles that are mainly pressed and used, and a method for producing the same.

  In order to achieve both weight reduction and safety of automobile bodies, parts, etc., the strength of steel plates as materials is being increased. In general, when the strength of a steel plate is increased, ductility, hole expansibility and the like are lowered, and formability is impaired. On the other hand, in high-strength steel sheets, retained austenitic steels with retained austenite in the steel structure are known to have a very high elongation despite the high strength using the TRIP effect. In this retained austenitic steel, in order to further increase the elongation, for example, in Patent Document 1, uniform elongation is achieved by controlling two types of ferrites (bainitic ferrite and polygonal ferrite) while ensuring a high fraction of retained austenite. A technique for ensuring the above is disclosed. On the other hand, Patent Document 2 discloses a technique for defining the shape of an austenite phase by an aspect ratio for the purpose of securing elongation and shape freezing property. Patent Document 3 states that higher elongation can be secured by optimizing the distribution of the austenite phase.

  On the other hand, the steel sheet using the TRIP effect transforms the retained austenite phase into martensite when punching before hole expansion, and as a result, stress concentration at the phase interface is promoted during hole expansion. As a result, the hole expandability deteriorates. Patent Document 4 discloses a technique for improving hole expandability by tempering a small amount of martensite phase in retained austenitic steel. However, even with this technology, it is not possible to suppress the hole expansion deterioration due to martensite generated by processing-induced transformation at the time of punching, and the elongation and hole expansion that can meet the high workability currently required for high-strength steel sheets. It cannot be said that sex is compatible.

JP 2006-274418 A JP 2007-154283 A JP 2008-56993 A JP 2006-104532 A

  The present invention has been made to solve the conventional problems, and is a technique found as a result of intensive studies to improve hole expandability in retained austenitic steel. It is an object to provide an excellent high-strength thin steel sheet and a method for producing the same.

  Residual austenitic steel controls the ferrite transformation and bainite transformation during annealing, and increases the C concentration in austenite, leaving austenite in the steel structure of the product, and high strength with high elongation due to the TRIP effect of this retained austenite It is a steel plate. However, martensite was generated at the time of punching, and due to a large hardness difference between martensite and ferrite, stress concentration was likely to occur at the phase boundary, resulting in deterioration of hole expansibility. In addition, in order for the TRIP effect to work effectively, stable retained austenite having a high C concentration is necessary. However, since such retained austenite has high hardness after transformation, the hole expandability is deteriorated. There was a problem. On the other hand, as a result of intensive studies, the present inventors have controlled the concentration gradient in the austenite phase in order to increase the stability of the retained austenite phase while keeping the martensite after transformation low. A technique for producing a stable austenite phase without increasing the martensite hardness after transformation has been found, and the present invention has been completed.

That is, the high-strength thin steel sheet excellent in elongation and hole expansibility of the present invention is
(1) In mass%,
C: 0.05 or more and 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
N: 0.0010% or more, 0.010% or less,
Al: 0.01% or more, 2.0% or less,
A steel composition comprising the balance iron and unavoidable impurities and having a metal structure mainly composed of ferrite, bainite or tempered martensite, and containing 3% or more and 30% or less of the retained austenite phase. In the phase interface in contact with the ferrite phase, bainite phase and martensite phase, the average C concentration in the austenite phase is 0.6% or more and 1.2% or less, and the central concentration Cgc of the austenite phase and the austenite grains A high-strength thin steel sheet excellent in elongation and hole expansibility, characterized in that the austenite grains having a grain boundary concentration Cgb in a range satisfying (Equation 1) are 50% or more.
Cgb / Cgc> 1.3 (Formula 1)
(2) The high-strength thin steel sheet excellent in elongation and hole expansibility as described in (1) above, wherein the austenite phase has an average particle size of 5 μm or less.
(3) The total structure of the ferrite, the bainite, and the tempered martensite is 50% or more in terms of volume fraction with respect to the entire structure, as described in (1) or (2) above A high-strength thin steel sheet with excellent elongation and hole expandability.
(4) Furthermore, in mass%,
Mo: 0.02% or more, 0.5% or less,
A high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of (1) to (3) above.
(5) Furthermore, in mass%,
Nb: 0.01% or more, 0.10% or less,
Ti: 0.01% or more, 0.20% or less,
V: 0.005% or more, 0.10% or less,
Cr: 0.1% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
A high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of (1) to (4) above, comprising one or more of the following.
(6) Furthermore, in mass%,
Elongation and hole according to any one of (1) to (5) above, containing one or more of Ca, Mg, Zr, and REM in an amount of 0.0005% to 0.05% A high-strength thin steel sheet with excellent spreadability.
(7) Furthermore, in mass%,
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less A high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of (1) to (6) above.
(8) For the cast slab, after casting, or once cooled to 1100 ° C. or lower and then reheated to 1100 ° C. or higher to perform hot rolling, the finishing temperature is 850 ° C. or higher and 970 ° C. Finished below, and then cooled to a temperature range of 650 ° C. or less at an average of 10 ° C./second or more and 200 ° C./second or less, wound up in a temperature range of 650 ° C. or less, pickled, and then cooled to 40% or more. After annealing and annealing at a maximum temperature of 700 ° C. or higher and 900 ° C. or lower after annealing, the average temperature is 350 ° C. or higher and 480 ° C. or lower at a cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower. After cooling to a temperature range and holding for 20 seconds or more and 800 seconds or less in the same temperature range, the temperature range from 350 ° C to 220 ° C is primary at a cooling rate of 5 ° C / second or more and 25 ° C / second or less. Cool, and further from 120 ° C to near room temperature And a final cooling step in which the temperature range is secondary cooled at an average cooling rate of 100 ° C./second or more or 5 ° C./second or less, and the elongation according to any one of (1) to (7) above A method for producing high-strength thin steel sheets with excellent hole expandability.
(9) With respect to the cast slab, as it is after casting or once cooled to 1100 ° C. or lower, when re-heating to 1100 ° C. or higher and performing hot rolling, the finishing temperature is 850 ° C. or higher and 970 ° C. Finished below, and then cooled to a temperature range of 650 ° C. or lower on average at 10 ° C./second or higher and 200 ° C./second or lower, wound up in a temperature range of 650 ° C. or lower, pickled, and cooled to 40% or higher. After annealing and annealing at a maximum temperature of 700 ° C. or higher and 900 ° C. or lower after annealing, the average temperature is 350 ° C. or higher and 480 ° C. or lower at a cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower. After cooling to the temperature range and holding for 20 seconds or more and 800 seconds or less in the same temperature range, it is immersed in the hot dip galvanized layer and the temperature range from 350 ° C to 220 ° C is 5 ° C / second to 25 ° C. Primary cooling at a cooling rate of less than Any one of the above (1) to (7), comprising a final cooling step of secondary cooling the temperature range from 0 ° C. to near room temperature at an average cooling rate of 100 ° C./second or more or 5 ° C./second or less. A method for producing a high-strength thin steel sheet excellent in elongation and hole expandability according to crab.
(10) When the cast slab is cast as it is or once cooled to 1100 ° C. or lower and then reheated to 1100 ° C. or higher to perform hot rolling, the finishing temperature is 850 ° C. or higher and 970 ° C. Finished below, and then cooled to a temperature range of 650 ° C. or lower on average at 10 ° C./second or higher and 200 ° C./second or lower, wound up in a temperature range of 650 ° C. or lower, pickled, and cooled to 40% or higher. After annealing and annealing at a maximum temperature of 700 ° C. or higher and 900 ° C. or lower after annealing, the average temperature is 350 ° C. or higher and 480 ° C. or lower at a cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower. After cooling to a temperature range and subsequently holding in the same temperature range for 20 seconds or more and 800 seconds or less, it is immersed in a hot dip galvanized layer and alloyed in the range of 440 ° C. or more and 580 ° C. or less. Temperature range from ℃ to 100 ℃ is 8 ℃ / The method for producing a high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of the above (1) to (7), characterized by cooling at a temperature of at least 2 seconds.

  The high-strength thin steel sheet of the present invention can obtain high hole expansibility while ensuring extremely high elongation due to the TRIP effect of retained austenite, despite its high strength. In addition, the C concentration gradient in the austenite phase, which is the basis of the present invention, is an extremely efficient and effective method for improving stability without increasing the average C concentration in retained austenite. It is possible to achieve both a high level of elongation and hole expandability.

  This effect can be continued if it is possible to create a C concentration gradient in the retained austenite phase after generating the retained austenite. That is, the same effect can be obtained not only in cold-rolled steel sheets and hot-dip galvanized steel sheets but also in hot-rolled steel sheets. Moreover, since the effect can be continued without breaking this concentration gradient, it can also be applied to electroplated steel sheets.

  The present invention is not affected by casting conditions. For example, a special casting-hot rolling method such as a thin slab may be used without being affected by a casting method (continuous casting or ingot casting) or a difference in slab thickness.

It is a figure which shows that the steel plate of this invention has the outstanding elongation compared with comparative steel. It is a figure which shows that the steel plate of this invention has the outstanding hole expansibility compared with comparative steel.

  The high-strength thin steel sheet of the present invention focuses on improving the stability of the retained austenitic steel without increasing the average C concentration of the retained austenite phase. As a result of extensive studies, the concentration gradient of the retained austenite phase It has been found that this can be realized by controlling, and that strength, elongation and hole expandability can be achieved at a high level.

  The microstructure is mainly composed of ferrite phase, bainite phase and tempered martensite phase, and it is necessary to contain 3% or more of retained austenite phase. When higher strength is desired, martensite may be contained, but when the ferrite phase and bainite phase are not mainly used, the elongation is remarkably reduced. Further, even if pearlite is contained in an amount of 5% or less, the material is not significantly deteriorated.

  The average C concentration of retained austenite greatly contributes to the stability of retained austenite and the martensite hardness after processing-induced transformation. If the average C concentration is less than 0.6%, the stability of retained austenite cannot be sufficiently increased even if the C concentration gradient is used, so that the TRIP effect cannot be obtained effectively and the elongation deteriorates. On the other hand, if it exceeds 1.2%, the martensite hardness after transformation becomes too strong, causing deterioration of hole expansibility. For this reason, it is 0.6% or more and 1.2% or less.

The C concentration distribution in the retained austenite phase is one of the most important in the present invention. Each retained austenite grain has high stability of retained austenite at the phase boundary by keeping the C concentration higher than the concentration at the center at the phase interface in contact with the ferrite phase, bainite phase, and martensite phase. Increased stability. In order to obtain an excellent TRIP effect under the condition that the average C concentration in the retained austenite phase is low, the concentration gradient must be kept large, that is, from the central concentration Cgc of the retained austenite grains and the concentration Cgb of the grain boundaries of the retained austenite grains. It is necessary to enlarge the left side of (Formula 1). When the value on the right side of (Formula 1) is 1.3 or more, the effect is remarkably exhibited and high elongation is obtained. Therefore, the range of (Formula 1) is 1.3 or more. In order to secure this effect in the entire structure, it is necessary that the remaining austenite grains satisfying (Equation 1) among the entire retained austenite grains be 50% or more.
Cgb / Cgc> 1.3 (Formula 1)
Here, Cgc and Cgb may be any measurement method as long as accuracy is guaranteed under the condition that the decomposition concentration can be accurately obtained. For example, using EPMA attached to FE-SEM, It can be obtained by carefully measuring the C concentration with a pitch of 0.5 μm or less. However, it is currently impossible to measure the local C concentration at the interface. Therefore, the ratio shown in (Equation 1) is 1.3, and as a result of repeated investigations by the present inventors, it has been determined that a sufficient effect can be seen when this value is met at the minimum in normal measurement. Invented.

  The average particle size of the retained austenite needs to be 5 μm or less. If it exceeds 5 μm, the dispersion of the retained austenite phase is coarse, and the TRIP effect cannot be fully exhibited, so the elongation decreases. On the other hand, if it is less than 0.5 μm, it is difficult to obtain a concentration gradient at the phase interface, and the elongation deteriorates. Therefore, 0.5 μm or more is desirable.

The total of ferrite, bainite, and tempered martensite structure, which are the parent phase, needs to be 50% or more in terms of volume fraction with respect to the entire structure. If it is less than 50%, the C concentration in the austenite phase cannot be increased, so that it becomes difficult to ensure stability even if a concentration gradient is used, and the elongation deteriorates. On the other hand, if it exceeds 95%, it becomes difficult to secure the necessary fraction of the retained austenite phase, and this causes deterioration of elongation.
The reason for limiting the chemical components of the high-strength thin steel sheet of the present invention will be described below.
C is an essential element from the viewpoint of securing strength and as a basic element for stabilizing austenite. If C is less than 0.05%, the strength is not satisfactory, and no retained austenite is formed. On the other hand, if it exceeds 0.35%, the strength is excessively increased, the ductility is insufficient, and it cannot be used as an industrial material. In addition, spot weldability is significantly degraded. When high elongation is required, 0.2% or more is desirable. On the other hand, when weldability is required, it is desirable to make it 0.25% or less.
Si is an element that delays the formation of cementite and is an element that is effective in generating retained austenite, and is usually an element that is added to ensure ductility. However, even if added over 2.0%, the effect is saturated and it is easy to cause embrittlement. When hot dip galvanizing and ease of chemical conversion are required, 1.5% or less is desirable. On the other hand, when it is added less than 0.05%, the cementite suppressing effect cannot be obtained. Therefore, 0.05% is the lower limit. When the amount of Al added to obtain the same effect as Si is 0.1% or less, addition of 1% or more is desirable.
Mn is an element that delays the formation of carbides and is effective in the formation of retained austenite, in addition to the need to be added from the viewpoint of securing strength. If Mn is less than 0.8%, the strength is not satisfied, and the formation of retained austenite becomes insufficient, resulting in deterioration of ductility. In addition, when the Mn addition amount exceeds 3.0%, the hardenability is improved, so that martensite is generated instead of retained austenite, which easily causes an increase in strength. It cannot be used as an industrial material due to lack of ductility. Therefore, the range of Mn in the present invention is 0.8% or more and 3.0% or less. In terms of material, 1.0 to 2.4% is preferable.

  P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if the addition amount is large, segregation to the grain boundary causes deterioration of local ductility. In addition, the weldability is deteriorated. Therefore, the P upper limit is set to 0.1%. On the other hand, if it is less than 0.0010%, the deterioration effect of P can be ignored, and if it is less than this, the cost increases.

  S is an element that degrades local ductility and weldability by generating MnS, and is preferably an element that does not exist in steel. Therefore, the upper limit is made 0.05%. On the other hand, if it is less than 0.0005%, the cost increases, so the lower limit is made 0.0005%.

  Al, like Si, has an effect of promoting ferrite formation and is one of important elements that can also suppress cementite. That is, it has the effect of stabilizing the retained austenite. This effect cannot be expected when Al is added in an amount of less than 0.01%. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so 2.0% was made the upper limit. In consideration of hot dip galvanizing properties, Al deteriorates this, so the upper limit is desirably 1.8%.

  N is an element that is inevitably included, but if it is contained in a large amount, not only the aging property is deteriorated, but also the AlN precipitation amount is increased to reduce the effect of Al addition, so 0.010% The following contents are preferred. Further, unnecessarily reducing N increases the cost in the steelmaking process, so it is usually preferable to control the N to 0.0010% or more.

  Mo is an element that suppresses the formation of pearlite in steel, and is an especially important element when the cooling rate during annealing is slow, or when reheating is performed by alloying treatment of plating or the like. In order to obtain this effect, the minimum addition amount of Mo was set to 0.02%. Below this, the formation of pearlite is not suppressed, and the retained austenite ratio is reduced. On the other hand, excessive addition of Mo may deteriorate ductility and chemical conversion properties, so the upper limit was made 0.5%. In order to obtain a higher strength-ductility balance, the content is preferably 0.3% or less.

  Nb, Ti, V, Cr, and W are elements that generate fine carbides, nitrides, or carbonitrides, and are effective in securing strength. Therefore, one or more elements may be added as necessary. Is possible. In order to achieve this, it is necessary to add 0.01% for Nb, Ti, and W, 0.005% for V, and 0.1% for Cr. On the other hand, excessive addition increases the strength too much and lowers the ductility, so Nb is 0.10% or less, Ti is 0.20% or less, V is 0.10% or less, and Cr is 5.0%. Hereinafter, W needs to be 5.0% or less.

  The steel can further contain one or more of Ca, Mg, Zr, and REM (rare earth elements) alone or in total from 0.0005% to 0.05%. Ca, Mg, Zr, and REM improve the local ductility and hole expansibility by controlling the shapes of sulfides and oxides. For this purpose, it is necessary to add one or more of these elements alone or in total of 0.0005% or more. However, excessive addition deteriorates workability, so the upper limit was made 0.05%.

  Further, the steel is Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less. More than seeds can be contained. These elements can delay the transformation and increase the strength of the steel. However, when Cu is less than 0.04%, Ni is less than 0.02%, and B is less than 0.0003%, the hardenability is weak and ferrite is formed at a high temperature. The necessary strength cannot be obtained. On the other hand, if the addition exceeds this range, the hardenability becomes too strong, and the ferrite and bainite transformation is slowed down, so that the C concentration to the retained austenite phase is delayed.

In addition to the above elements, steel contains elements inevitably mixed such as Sn and As, and is made of the remaining iron.
Below, the manufacturing method of the high intensity | strength thin steel plate based on this invention is demonstrated.

As a result of intensive studies, the inventors of the present invention have produced a high-strength steel sheet of the present invention by controlling the cooling conditions after promoting C concentration to the austenite phase by bainite transformation or tempering martensite. It was found that concentration gradient control in the phase was possible. In addition, it is possible to increase the stability of the retained austenite phase by combining it with the previous enrichment in the austenite phase. In order to realize this effect, one of the most important things in the present invention is the cooling condition after bainite transformation and martensite tempering. In the cooling after over-aging (OA) treatment, the primary cooling is performed at an average cooling rate from 350 ° C. to 220 ° C. at 5 ° C./second or more and 25 ° C./second or less, and further, the temperature range from 120 ° C. or less to near room temperature. Secondary cooling is performed at an average cooling rate of 100 ° C./second or more or 5 ° C./second or less.
The slight transformation that occurs during cooling after OA plays an important role in increasing the C concentration near the grain boundary in austenite. Therefore, in the primary cooling, the cooling rate in the temperature range of 350 ° C. to 220 ° C. is 25 ° C. / If it exceeds 2 seconds, transformation does not proceed during this time, and C concentration does not occur in austenite. On the other hand, when the cooling rate in the temperature range from 350 ° C. to 220 ° C. is less than 5 ° C./second, C diffusion in austenite proceeds and the C concentration gradient becomes small.
In addition, C diffusion is further limited in a low temperature range of 120 ° C. or lower, and transformation hardly occurs. For this reason, in the secondary cooling, the steel sheet is cooled at an average cooling rate of 100 ° C./second or more from 120 ° C. to near room temperature, so that the C concentration gradient in the austenite is still achieved in the 350 ° C. to 220 ° C. temperature range. can do. Alternatively, in the secondary cooling, the C concentration gradient in the austenite phase can be made more remarkable by cooling at an average cooling rate from 120 ° C. to around room temperature at 5 ° C./second or less. In the secondary cooling, if it exceeds 5 ° C./second and less than 100 ° C./second, not only transformation does not occur, but also the C concentration at the grain boundary decreases.

  The slab before hot rolling is set to 1100 ° C. or higher as it is after continuous casting or by reheating. Below this temperature, the homogenization process is insufficient, causing a decrease in strength and elongation.

  Next, the slab is hot-rolled at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. If the finishing temperature is less than 850 ° C., it becomes (α + γ) two-phase rolling, resulting in a decrease in ductility. If it exceeds 970 ° C., the austenite grain size becomes coarse, the ferrite phase fraction becomes small, and the ductility is reduced. This is because of a decrease.

  Thereafter, it is cooled to a temperature range of 650 ° C. or less at an average of 10 ° C./second or more and 200 ° C./second or less, and then wound in a temperature range of 650 ° C. or less. Below this cooling rate and above the coiling temperature, a pearlite phase that significantly degrades the bendability is formed. When the average cooling rate exceeds 200 ° C./second, the effect of suppressing pearlite is saturated, and the variation in the cooling end point temperature becomes large, making it difficult to secure a stable material. Therefore, it shall be 200 ° C./second or less.

  After pickling, the prototype material can be cold-rolled by 40% or more. Below this, recrystallization and reverse transformation during annealing are suppressed and elongation decreases.

  The maximum temperature during annealing is 700 ° C or higher and 900 ° C or lower. If the temperature is lower than 700 ° C., the recrystallization of the ferrite phase during annealing is delayed, causing a decrease in elongation. On the other hand, at a temperature higher than 900 ° C., the martensite fraction increases and the elongation deteriorates.

In cooling after soaking in the annealing process, a faster cooling rate is better for freezing the structure and causing bainite transformation efficiently. However, the transformation cannot be controlled at less than 0.1 ° C./second. On the other hand, even if it exceeds 200 ° C./second, the effect is saturated, and the temperature controllability of the cooling end point temperature, which is most important for the formation of retained austenite, is significantly deteriorated. For this reason, the cooling rate after annealing is set to 0.1 ° C./second or more and 200 ° C./second or less on average.
The cooling end point temperature and subsequent holding is an important technique for controlling the bainite formation and determining the C concentration of retained austenite. When the cooling end point temperature is less than 350 ° C., a large amount of martensite is generated, the steel strength is excessively increased, and in addition, it becomes difficult to leave austenite, so that the elongation deterioration becomes extremely large. On the other hand, when the temperature exceeds 480 ° C., the bainite transformation is delayed, and in addition, cementite is generated during the holding, and the concentration of C in the retained austenite is lowered. Therefore, the cooling stop temperature and the holding temperature are 350 ° C. or higher and 480 ° C. or lower.

  The longer the holding time, the better in terms of C concentration to retained austenite. If it is less than 20 seconds, the amount of bainite transformation is small and the amount of C discharged is reduced, so that it is difficult to produce a concentration gradient. On the other hand, when the time is long, the concentration of C becomes too large, and the hardness of martensite after the processing-induced transformation is increased to deteriorate the hole expandability. If it exceeds 800 seconds, the bainite transformation is slowed down, the concentration gradient becomes small, and cementite is generated in the austenite phase, which tends to cause a decrease in C concentration. Accordingly, the holding time is 20 seconds or more and 800 seconds or less.

The present technology can also be applied to a hot-dip galvanized steel sheet. In this case, the technique is immersed in a hot-dip galvanized layer after being held at 350 ° C. to 480 ° C. In addition, the present technology can also be alloyed after immersion. At this time, the alloying treatment of the plating is performed in the range of 440 ° C. or higher and 580 ° C. When the temperature is lower than 440 ° C, alloying becomes insufficient, and when it exceeds 580 ° C, it becomes an overalloy and the corrosion resistance is remarkably deteriorated.
Hereinafter, the present invention will be described in detail based on examples.

  Steel having the composition shown in Table 1 was manufactured, reheated to 1200 ° C. after cooling and solidification, finish-rolled at 880 ° C., cooled to 550 ° C., cooled at an average cooling rate of 60 ° C./second, wound I took it. Thereafter, this hot-rolled sheet was cold-rolled by 50%. Thereafter, annealing was performed under the conditions shown in Tables 2 and 3 by continuous annealing. After annealing, 1% skin pass rolling was performed to suppress the yield point elongation.

  Tensile properties were evaluated by C direction tension of JIS No. 5 tensile test pieces. Identification of structure, observation of existing position, and measurement of average particle size (average equivalent circle diameter) and occupancy ratio are 500 times to 1000 times by corroding the steel sheet rolling direction cross section or the cross section perpendicular to the rolling direction with the Nital reagent. Was quantified by observation with an optical microscope.

In the hole expansion test, a sample punched with a hole diameter of 10 mm and clearance of 12% was pushed up with a punch with a vertex angle of 60 °, and the hole expansion ratio (%) from the hole diameter d1 where the crack penetrated the plate thickness = (D1-10) / 10 × 100
I asked for.

  The volume fraction of retained austenite and its average carbon concentration were determined by X-ray diffraction as described in JP-A-11-193435. That is, the volume fraction Vγ of retained austenite can be calculated from the data obtained using the Mo—Kα ray by the following equation.

Vγ = (2/3) {100 / (0.7 × α (111) / γ (200) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1) }
However, α (211), γ (200), α (211), and γ (311) represent surface strength.

  The carbon concentration Cγ of retained austenite is obtained by calculating the lattice constant (unit: angstrom) from the reflection angles of the (200), (220), and (311) surfaces of austenite by X-ray analysis using Cu-Kα rays. Can be calculated.

Cγ = (Lattice constant-3.572) /0.033
Among samples A to g which are invention examples, a does not satisfy the upper limit of the C content and b does not satisfy the lower limit of the C content. c, d, e, and g do not satisfy the upper limits of S, Si, Mn, and Al contents, respectively. f does not satisfy the lower limits of the Si and Al contents. Of the experimental results in Table 2, A3 has a retention time below the lower limit, and the average C concentration in austenite is high. Since B3 has an annealing temperature and an austenite grain size that exceeds the upper limit, the austenite grain size is too large. For D3, since the annealing temperature is below the lower limit, the elongation and the hole expansion rate are reduced. In F3, the holding temperature is below the lower limit, and the ferrite + bainite fraction and the austenite fraction are both out of range. F4 has a holding temperature exceeding the upper limit, and the ferrite + bainite fraction and the average C concentration in austenite are low. H3 has a high average C concentration in austenite, and the elongation and hole expansion rate are reduced. H4 has a final primary cooling rate outside the range of the present invention, and austenite grains satisfying the formula (1) are less than 50% of the whole. a1 has a high average C concentration in austenite, and the elongation and the hole expansion rate are reduced. b1 has an austenite fraction below the range. d1 has a high C concentration in austenite. e1 has a low average C concentration in austenite. f1 cannot secure an austenite fraction. g1 has a high average C concentration in austenite. In G3 and Q3, the final secondary cooling rate is out of the range, and austenite satisfying the formula (1) is out of the range. Although these materials are shown in FIGS. 1 and 2, only the steel plate of the present invention has both elongation and hole expansibility, compared with the comparative steel,
It shows an extremely high value and achieves the object of the present invention.

Claims (10)

% By mass
C: 0.05 or more and 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
N: 0.0010% or more, 0.010% or less,
Al: 0.01% or more, 2.0% or less,
A steel composition comprising the balance iron and inevitable impurities, the metal structure being mainly composed of ferrite, bainite or tempered martensite, and containing 3% or more and 30% or less of the retained austenite phase. In the phase interface in contact with the ferrite phase, bainite phase and martensite phase, the average C concentration in the austenite phase is 0.6% or more and 1.2% or less, and the central concentration Cgc of the austenite phase and the austenite grains A high-strength steel sheet excellent in elongation and hole expansibility, characterized in that there are 50% or more of austenite grains having a grain boundary concentration Cgb in a range satisfying the formula (1).
Cgb / Cgc> 1.3 (1)   The high-strength thin steel sheet excellent in elongation and hole expansibility according to claim 1, wherein an average particle size of the austenite phase is 5 μm or less.   3. The elongation and hole expansibility according to claim 1, wherein the total of the ferrite, the bainite, and the tempered martensite structure is 50% or more in terms of volume fraction with respect to the entire structure. Excellent high strength thin steel sheet. Furthermore, in mass%,
Mo: 0.02% or more, 0.5% or less,
The high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of claims 1 to 3, characterized in that Furthermore, in mass%,
Nb: 0.01% or more, 0.10% or less,
Ti: 0.01% or more, 0.20% or less,
V: 0.005% or more, 0.10% or less,
Cr: 0.1% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
The high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of claims 1 to 4, characterized by containing one or more of the following. Furthermore, in mass%,
The elongation according to any one of claims 1 to 5, characterized by containing one or more of Ca, Mg, Zr, and REM in an amount of 0.0005% to 0.05%. A high-strength thin steel plate with excellent hole expandability. Furthermore, in mass%,
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less The high-strength thin steel sheet excellent in elongation and hole expansibility according to any one of claims 1 to 6, characterized in that:   For the cast slab, after casting, or once cooled to 1100 ° C. or lower, reheated to 1100 ° C. or higher and perform hot rolling at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. After that, after cooling to a temperature range of 650 ° C. or lower and an average of 10 ° C./second or higher and 200 ° C./second or lower to a temperature range of 650 ° C. or lower, winding in a temperature range of 650 ° C. or lower, pickling, and cold rolling of 40% or higher After annealing at a maximum temperature of 700 ° C or higher and 900 ° C or lower after annealing, the average temperature is 0.1 ° C / second or higher and 200 ° C / second or lower to a temperature range of 350 ° C or higher and 480 ° C or lower. After cooling and subsequently holding in the same temperature range for 20 seconds or more and 800 seconds or less, the temperature range from 350 ° C. to 220 ° C. is primarily cooled at a cooling rate of 5 ° C./second to 25 ° C./second, Furthermore, temperatures from 120 ° C to near room temperature It is equipped with the last cooling process which carries out the secondary cooling with the average cooling rate of 100 degrees C / sec or more or 5 degrees C / sec or less, The elongation and hole expansibility of any one of Claims 1-7 characterized by the above-mentioned. An excellent method for producing high strength thin steel sheets.   For the cast slab, after casting, or once cooled to 1100 ° C. or lower, reheated to 1100 ° C. or higher and perform hot rolling at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. After that, after cooling at an average temperature of 10 ° C./second or more and 200 ° C./second or less to a temperature range of 650 ° C. or less, winding in a temperature range of 650 ° C. or less, pickling, and cold rolling at 40% or more After annealing at a maximum temperature of 700 ° C. or more and 900 ° C. or less after annealing, the temperature is 350 ° C. or more and 480 ° C. or less at an average cooling rate of 0.1 ° C./second or more and 200 ° C./second or less. After cooling and holding at the same temperature range for 20 seconds or more and 800 seconds or less, it is immersed in the hot dip galvanized layer and the temperature range from 350 ° C to 220 ° C is 5 ° C / second or more and 25 ° C / second or less. Primary cooling at a cooling rate of 120 ° C 8. A final cooling step of performing secondary cooling of the temperature range from about 1 to about room temperature to an average cooling rate of 100 ° C./second or more or 5 ° C./second or less is provided. Of high-strength steel sheet with excellent elongation and hole expandability.   For the cast slab, after casting, or once cooled to 1100 ° C. or lower, reheated to 1100 ° C. or higher and perform hot rolling at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. After that, after cooling at an average temperature of 10 ° C./second or more and 200 ° C./second or less to a temperature range of 650 ° C. or less, winding in a temperature range of 650 ° C. or less, pickling, and cold rolling at 40% or more After annealing at a maximum temperature of 700 ° C. or more and 900 ° C. or less after annealing, the temperature is 350 ° C. or more and 480 ° C. or less at an average cooling rate of 0.1 ° C./second or more and 200 ° C./second or less. After cooling and subsequently holding in the same temperature range for 20 seconds or more and 800 seconds or less, it is immersed in a hot dip galvanized layer and alloyed in the range of 440 ° C. or more and 580 ° C. or less, and 350 ° C. to 100 ° C. The temperature range up to ℃ is 8 ℃ / second or more. Method for producing a high strength thin steel sheet excellent in elongation and hole expandability according to any one of claims 1 to 7, characterized in that cooling in degrees.

Images