JP6620431B2 - High-strength steel sheet with excellent workability and method for producing the same - Google Patents

Description

The present invention relates to a high-strength steel sheet having a tensile strength of 400 N / mm ² or more, which is suitable as a pressed steel sheet for automobiles and has high workability and yield stress, and a method for producing the same.

The demand for high-strength steel sheets with excellent workability is increasing year by year. Taking a steel plate used as a material for the shock absorbing member as an example, there is a demand for both workability for creating a part shape and high strength for improving shock absorbing characteristics.
In order to protect the occupant, it is required to increase the absorbed energy at the time of collision while suppressing the deformation amount of the shock absorbing member. In order to meet this requirement, as shown in Non-Patent Document 1, it is effective to increase the yield stress of the steel sheet used as the material of the shock absorbing member.

  Generally, when increasing the yield stress of a steel sheet, the tensile strength is increased. However, increasing the strength of the steel sheet causes deterioration of workability. For this reason, it becomes a material which is difficult to apply to parts having complicated shapes. Further, as a technique for increasing the yield stress of a steel sheet having the same strength, a steel sheet using precipitation strengthening by adding Ti, Nb or the like has been reported. However, a steel plate using precipitation strengthening has low workability among steel plates of the same strength, and is unlikely to be a solution for both yield stress and workability.

  On the other hand, Patent Document 1 reports a technique that achieves both high yield stress and workability by uniformly dispersing non-recrystallized ferrite containing many transitions. Patent Document 1 proposes controlling the ratio of the lengths of recrystallized ferrite and non-recrystallized ferrite in the rolling direction. However, when further workability improvement is required, the technique described in Patent Document 1 is insufficient.

  Patent Document 2 reports a cold-rolled steel sheet having an unrecrystallized structure and excellent chemical conversion property and workability. However, the technique described in Patent Document 2 is originally a technique specialized for an extremely low C material having high workability, and a similar effect cannot be obtained with a general low carbon steel.

  Patent Document 3 reports a technique for improving local ductility and improving Young's modulus of a cold-rolled steel sheet by utilizing non-recrystallized ferrite that is softer than the hard second phase. However, with the technique described in Patent Document 3, although improvement in local ductility can be expected, improvement in elongation cannot be expected because the ductility of crystal grains of unrecrystallized ferrite is poor.

  Patent Document 4 introduces a technique for increasing Young's modulus and Rankford value (r value) by utilizing non-recrystallized ferrite. However, the technique described in Patent Document 4 is not a technique related to improvement in elongation.

JP 2008-156680 A JP-A-62-161938 JP 2008-106352 A JP 2009-114523 A

Automobile Engineering Society Spring Academic Lecture Proceedings, 1973, P60

The present invention has been made in view of such a conventional situation, and has a tensile strength of 400 N / mm ² or more, a high-strength steel plate excellent in workability having high elongation and high yield stress, and its It is an object to provide a manufacturing method.

The present inventors paid attention to non-recrystallized ferrite, and conducted intensive studies on measures for increasing elongation while making full use of the hardness of non-recrystallized ferrite.
As a result, the elongation rapidly increases when the area ratio of the non-recrystallized ferrite phase contained in the metal structure is less than 20%, and an excellent yield stress can be obtained when the area ratio is 2% or more. I found.

In addition, the present inventors have studied a method for producing a high-strength steel sheet having a metal structure that appropriately contains non-recrystallized ferrite. As a result, after pickling the hot-rolled steel sheet, the cold-rolled steel sheet obtained by performing cold rolling at a rolling rate of 15% or more was subjected to component composition, crystal grain size of the hot-rolled steel sheet, and cold rolling. It has been found that it is important to perform annealing at a maximum temperature that is equal to or lower than the recrystallization temperature (recrystallization completion temperature) determined by the rolling rate of (recrystallization temperature −30) ° C.
This invention is made | formed based on such knowledge, The summary is as follows.

(1) In mass%,
C: 0.03% or more, 0.35% or less,
Si: 0.01% or more, 2.00% or less,
Mn: 0.3% or more and 4.0% or less,
P: 0.001% or more, 0.100% or less,
S: 0.0005% or more, 0.050% or less,
N: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, containing 2.00% or less, the balance consists of Fe and inevitable impurities,
The metal structure contains an unrecrystallized ferrite phase at an area ratio of 2% or more and less than 20%,
The ratio of yield stress to tensile strength (yield stress (N / mm ² ) / tensile strength (N / mm ² )) is 0.75 or more,
A high-strength steel sheet excellent in workability, wherein the product of yield stress and elongation (yield stress (N / mm ² ) × elongation (%)) is 12,000 or more.

(2) The area ratio of 70% or more of the non-recrystallized ferrite phase is made of non-recrystallized ferrite grains having a major axis length of 25 μm or less,
The high-strength steel sheet having excellent workability as set forth in (1), wherein a connection rate of the non-recrystallized ferrite grains is 0.30 or less.

(3) Furthermore, in mass%,
Cr: 0.05% or more, 3.0% or less,
Mo: 0.05% or more, 1.0% or less,
Ni: 0.05% or more, 3.0% or less,
Cu: 0.05% or more and 3.0% or less of 1 type or 2 types or more, The high-strength steel plate excellent in workability as described in (1) or (2) characterized by the above-mentioned.
(4) Furthermore, in mass%,
Nb: 0.005% or more, 0.30% or less,
Ti: 0.18% or more , 0.30% or less,
V: 0.01% or more and 0.50% or less of 1 type or 2 types or more, The high-strength steel plate excellent in workability in any one of (1)-(3) characterized by the above-mentioned.

(5) Furthermore, in mass%,
B: A high-strength steel sheet excellent in workability according to any one of (1) to (4), characterized by containing 0.0001% or more and 0.100% or less.
(6) Furthermore, in the steel by mass%,
Ca: 0.0005% or more, 0.010% or less,
Mg: 0.0005% or more, 0.010% or less,
Zr: 0.0005% or more, 0.010% or less,
REM: A high-strength steel sheet excellent in workability according to any one of (1) to (5), characterized by containing one or more of 0.0005% or more and 0.010% or less.

(7) The high-strength steel sheet having excellent workability according to any one of (1) to (6), further comprising a hot-dip galvanized layer on the surface.

(8) A method for producing a high-strength steel sheet having excellent workability according to any one of (1) to (7), wherein: (1) From the component composition according to any one of (3) to (6) The cast slab is heated to 1100 ° C. or higher, hot rolled, pickled, then cold rolled at a rolling rate of 15% or higher, and the resulting cold rolled steel sheet is below the recrystallization temperature, (Recrystallization temperature-30) After performing an annealing process that anneals at the highest temperature above 30 ° C., a cooling process is performed to cool to 480 ° C. or less at an average cooling rate of 1 ° C./second or more and 200 ° C./second or less. A method for producing a high-strength steel sheet excellent in workability characterized by

(9) The method for producing a high-strength steel sheet having excellent workability according to (8), wherein the annealing step is performed at the maximum temperature for 30 seconds to 300 seconds.
(10) The method for producing a high-strength steel sheet having excellent workability according to (8) or (9), wherein hot-dip galvanization is performed after the cooling step and alloying treatment is performed.

  According to the present invention, it is possible to provide a high-strength steel sheet that has both high workability, particularly high elongation and high yield stress. The high-strength steel sheet of the present invention is suitable, for example, as a material for an impact absorbing member that has a shape that is difficult to process and that requires impact absorption characteristics. In addition, the high-strength steel sheet of the present invention can be used as a material for automobile members such as an impact absorbing member, for example, to reduce the weight of the vehicle body, to integrally form parts, to rationalize the processing process, and to improve fuel efficiency. Manufacturing costs can be reduced. Therefore, the present invention has great industrial value.

Hereinafter, the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described in detail.
Up to now, a technique for achieving both workability and strength by dispersing a transformation hard phase (martensite, bainite, etc.) of DP (Dual Phase) steel in a highly workable ferrite phase has been proposed. . This is achieved by effectively strengthening the steel due to the presence of the hard phase while utilizing the characteristics of the soft phase excellent in ductility.

  In the present invention, an unrecrystallized ferrite phase is used as the hard phase, not a transformed hard phase. The non-recrystallized ferrite phase and the transformed hard phase have greatly different characteristics such as hardness, shape, and arrangement of the grains forming the phase. For this reason, the non-recrystallized ferrite phase and the transformed hard phase differ in suitable shape, area ratio, and the like, and the conditions for obtaining a suitable shape and area ratio differ greatly.

Conventionally, there are few examples of examining the area ratio of the non-recrystallized ferrite phase, and there is no example of examining the possibility of improving the quality of the steel sheet by controlling the connection rate and size of the non-recrystallized ferrite grains.
Therefore, from the knowledge of using the transformed hard phase as the hard phase, the conditions for using the non-recrystallized ferrite phase cannot be easily estimated.

Conventionally, non-recrystallized ferrite has hardly been used to control the properties of a steel sheet that is desired to be workable. This is because when the non-recrystallized ferrite is contained in the metal structure of the steel sheet, the ductility of the steel sheet is significantly deteriorated.
On the other hand, the value of non-recrystallized ferrite as a hard ferrite has long been recognized. For this reason, non-recrystallized ferrite has been used as a means for increasing the strength of a steel sheet at low cost.

  The present invention has been made by clarifying the optimum conditions when non-recrystallized ferrite is dispersed as a hard phase. The inventors of the present invention have made extensive studies on measures for increasing elongation while maximally utilizing the hardness of non-recrystallized ferrite. As a result, it was found that the yield stress had a first-order correlation with the area ratio of unrecrystallized ferrite contained in the metal structure. That is, the higher the area ratio of the non-recrystallized ferrite phase, the higher the yield stress. And it discovered that the outstanding yield stress was obtained by making the area ratio of a non-recrystallized ferrite phase into 2% or more. On the other hand, it has been found that the elongation has a critical value with respect to the area ratio of the non-recrystallized ferrite and rapidly increases by controlling the area ratio to less than 20%.

  Furthermore, as a result of repeated studies by the present inventors, it has been found that the elongation of the steel sheet is strongly influenced by the connection ratio of non-recrystallized ferrite grains and the length of the long axis. When the area ratio of 70% or more of the non-recrystallized ferrite phase is composed of non-recrystallized ferrite grains having a major axis length of 25 μm or less, and the connection ratio of the non-recrystallized ferrite grains is 0.30 or less. In addition, it was confirmed that the elongation was further improved while maintaining a high yield stress. The reason why such an effect can be obtained is not clear, but there is an optimum value that can utilize the hardness of non-recrystallized ferrite while avoiding stress concentration on non-recrystallized ferrite with poor ductility. It is estimated to be.

  The high-strength steel sheet of the present invention improves the balance between elongation and yield stress of the steel sheet by controlling the area ratio of the non-recrystallized ferrite phase and the length and connection ratio of the major axis of the non-recrystallized ferrite grains. It is a thing. In the method for producing a high-strength steel sheet according to the present invention, the rolling rate in cold rolling is controlled, and the maximum temperature in the annealing process is determined by the component composition, the crystal grain size of the hot-rolled steel sheet, and the rolling rate in cold rolling. Strictly control according to the crystal temperature.

The thickness of the high-strength steel plate obtained using the high-strength steel plate of the present invention and the manufacturing method thereof is not particularly limited, but is preferably a thin steel plate of 0.1 to 11.0 mm. A high-strength thin steel sheet having a thickness in the above range can be preferably used as a material for automobile members produced by press working such as an impact absorbing member. In addition, a high-strength thin steel sheet having a thickness in the above range can be easily produced using a thin sheet production line.
The high-strength steel plate of the present invention preferably has a tensile strength of 400 to 780 N / mm ² . A high-strength steel plate having a tensile strength in the above range is particularly suitable as a material for automobile members such as impact absorbing members.

Hereinafter, each component of the present invention will be described in detail.
First, the reasons for limiting the steel plate material will be described.
The high-strength steel sheet of the present invention has a YR which is a ratio of yield stress to tensile strength (yield stress (N / mm ² ) / tensile strength (N / mm ² )) of 0.75 or more. YP * El which is a product of (yield stress (N / mm ² ) × elongation (%)) is 12000 or more.

  The higher the YR, the higher the yield stress (YP) at the same tensile strength. Increasing the tensile strength causes a decrease in elongation, so increasing the yield point at the same tensile strength is a technology that effectively increases the impact absorption characteristics of the member when steel plate is used as the material for the impact absorbing member. sell. If YR is less than 0.75, the yield stress is low with respect to the tensile strength, so that it is difficult to achieve both elongation and yield stress at a high level. In addition, a low difference between the tensile strength and the yield stress (high YR) means that the stress in the low strain region is high. A high stress in the low strain region greatly contributes to an increase in the impact absorption characteristics of the member when a steel plate is used as the material for the impact absorbing member. Therefore, YR is preferably 0.75 or more. YR is preferably 0.99 or less.

  The high-strength steel sheet of the present invention needs to have YR of 0.75 or more and YP * El of 12000 or more. A steel sheet having a low YP (yield stress) can be formed into a member (part) that satisfies the shock absorption characteristics by processing into a shape having excellent shock absorption characteristics. However, if the member has a shape inferior in impact absorption characteristics or if El cannot be processed into a shape excellent in impact absorption characteristics due to lack of El (elongation), the YP of the steel sheet (material) needs to be increased.

That is, the use of a material having a high YP * El results in effectively improving the shock absorption characteristics as a component. Such a characteristic as a part is not limited to the shock absorbing characteristic, and the same applies to the fatigue characteristic and the strength characteristic. If YP * El is less than 12000, either the yield stress or the elongation of the material is insufficient, so that the characteristics of the part using the steel plate cannot be effectively improved. In particular, when high component characteristics are required, it is desirable that YP * El is 12,500 or more.
The high strength steel sheet of the present invention preferably has a yield stress of 330 to 1020 N / mm ² and an elongation of 13.0 to 39.0%.

Next, the reasons for limiting the component composition of the present invention will be described. Hereinafter, unless otherwise specified,% means mass%.
C contributes to the strengthening of steel. The C content is required to be 0.03% or more in order to ensure strength, and is preferably 0.04% or more. On the other hand, it is an element that affects the workability of steel, and when the C content increases, the workability deteriorates. When the C content exceeds 0.35%, a large amount of bainite, pearlite, and martensite are generated as the second phase, and these influence each other with the non-recrystallized ferrite, thereby reducing the elongation. On the other hand, if the C content exceeds 0.35%, the hole-expanding property is remarkably lowered due to generation of harmful carbides (cementite). For this reason, C content shall be 0.35% or less. However, when particularly high hole expansibility is required, the C content is desirably 0.10% or less.

  Si increases the strength of the steel by solid solution strengthening, and has a small decrease in ductility. Therefore, Si is an effective element for increasing the strength of steel. Si also suppresses the generation of harmful carbides and is effective in improving workability. The action of suppressing the formation of carbides can be replaced by Al. From the above, it is desirable to add 0.01% or more of Si. In particular, when not adding 0.10% or more of Al, it is desirable to add 0.30% or more of Si. However, when there is too much Si content, chemical conversion property will fall and spot weldability will also deteriorate. For this reason, Si content sets 2.00% as an upper limit.

  Al is an element effective for suppressing the formation of harmful carbides and improving the elongation, like Si described above. Conventionally, Al is an element necessary for deoxidation and has been added in an amount of about 0.01 to 0.07%. As a result of intensive studies, the present inventors have found that chemical conversion property can be improved without degrading ductility by adding a large amount of Al in a low Si system. However, when there is too much Al content, the effect of ductility improvement will be saturated, and chemical conversion property and spot weldability will also deteriorate. For this reason, it is desirable that the upper limit of the Al content is 2.00%, and the upper limit is 1.00% particularly under severe conditions of chemical conversion treatment. For sufficient deoxidation, 0.01% or more of Al needs to be added.

  Mn is an element necessary for ensuring strength. Mn also contributes to the formation of unrecrystallized ferrite by delaying recrystallization. In order to acquire this effect, it is necessary to make Mn content 0.3% or more. However, when Mn is added in a large amount, microsegregation and macrosegregation are likely to occur and the elongation is deteriorated. Therefore, the upper limit of the Mn content is 4.0%.

  P is an element that increases the strength of the steel sheet, and is an element that improves corrosion resistance when added simultaneously with Cu. However, when the P content is high, deterioration of weldability, workability, and toughness is caused. Accordingly, the P content is 0.100% or less. In particular, when the corrosion resistance is not a problem, it is desirable that the P content is 0.030% or less with emphasis on workability. However, since it takes a cost to reduce the P content, the lower limit is set to 0.001% from the viewpoint of de-P cost.

  S forms a sulfide such as MnS, becomes a starting point of cracking, and is an element that reduces hole expansibility among workability, and significantly reduces the total elongation. Therefore, the S content needs to be 0.050% or less. However, desulfurization cost becomes high in order to make S content less than 0.0005%. For this reason, S content shall be 0.0005% or more.

  N deteriorates workability. In addition, when Ti and / or Nb is added together with N, the effective amount of Ti and Nb is reduced due to the generation of TiN and NbN, and the generated nitride lowers the elongation and hole expandability. For this reason, it is better that the N content is small. From the above constraints, the N content is set to 0.010% or less. From the viewpoint of removing N cost, the lower limit of the N content is set to 0.0005%.

The high-strength steel sheet of the present invention may further contain the following elements as necessary.
Ti, Nb, and V all form carbides and are effective in increasing the strength. In order to effectively exhibit this effect, it is necessary to add one or more of Ti, Nb, and V. Ti, Nb, and V delay recrystallization and contribute to the formation of non-recrystallized ferrite. In order to obtain these effects, it is necessary to add 0.005% or more of Ti, 0.005% or more of Nb, and 0.01% or more of V. However, when Ti, Nb, and V are excessively added, elongation deteriorates due to precipitation strengthening. Therefore, the Ti content is 0.30% or less, the Nb content is 0.30% or less, and the V content is 0.50% or less.

  Ca, Mg, Zr, and REM control the shape of sulfide inclusions and are effective in improving workability. In order to effectively exhibit this effect, it is necessary to add one or more of Ca, Mg, Zr, and REM. At this time, 0.0005% or more of each element needs to be added. On the other hand, a large amount of addition deteriorates the cleanliness of the steel, leading to a decrease in elongation. From this, when it contains 1 type, or 2 or more types of Ca, Mg, Zr, and REM, the upper limit of each addition amount shall be 0.010%.

Cu is an element that improves the corrosion resistance when combined with P. In order to obtain this effect, it is desirable to add 0.05% or more of Cu. However, the addition of a large amount of Cu increases the hardenability and decreases the ductility. For this reason, the upper limit of Cu content is set to 3.0%.
Ni is an element that suppresses hot cracking when Cu is added. In order to obtain this effect, it is desirable to add 0.05% or more of Ni. However, the addition of a large amount of Ni increases the hardenability and lowers the ductility like Cu. For this reason, the upper limit of Ni content is set to 3.0%.

  Mo is an element effective for suppressing the formation of cementite, contributing to strength, and improving hole expansibility. In order to acquire this effect, it is necessary to add 0.05% or more of Mo. However, since Mo is also an element that enhances hardenability, if Mo is added excessively, ductility decreases. For this reason, the upper limit of the Mo content is set to 1.0%.

  Cr, like V, forms carbides and is an element that contributes to securing strength. In order to obtain this effect, it is necessary to add 0.05% or more of Cr. However, since Cr is an element that enhances the hardenability, elongation is reduced when added in a large amount. Therefore, the upper limit of Cr content is set to 3.0%.

  B is an element that contributes to strength in the same manner as Mn. In order to obtain this effect, the B content needs to be 0.0001% or more. However, since B is also an element that enhances the hardenability, the ductility decreases when added in a large amount. For this reason, the upper limit of the B content is set to 0.100%.

Next, the metal structure of the high strength steel plate of the present invention will be described.
In the present invention, one of the most important features is the area ratio of the non-recrystallized ferrite phase. The less unrecrystallized ferrite phase is better in terms of ensuring the elongation of the steel sheet. However, the elongation of the steel sheet does not become a linear function with the area ratio of the non-recrystallized ferrite phase, and increases rapidly when the area ratio of the non-recrystallized ferrite phase is less than 20%. On the other hand, the yield stress of the steel sheet decreases with a substantially linear function as the area ratio of the non-recrystallized ferrite phase decreases.

  Therefore, in order to increase the balance between the yield stress and the elongation of the steel sheet, the area ratio of the non-recrystallized ferrite phase contained in the metal structure is 2% or more and less than 20%. In particular, when a steel sheet having a high elongation is desired, the area ratio of the non-recrystallized ferrite phase is preferably 15% or less. Further, in the case of a steel plate used for a member application that strongly requires yield stress, it is desirable that the area ratio of the non-recrystallized ferrite phase is 5% or more.

"Calculation method of area ratio of non-recrystallized ferrite phase"
The area ratio of the non-recrystallized ferrite phase contained in the high-strength steel sheet of the present invention is determined by the following method.
A sample is taken with a plate thickness cross section parallel to the rolling direction as an observation surface, and the observation surface is polished, nital etched, and if necessary, repeller etched, then observed with an optical microscope and photographed. By analyzing the image of the obtained micrograph, the ferrite phase and other than the ferrite phase are distinguished, and the area ratio of the ferrite phase is calculated. In addition, the area ratio of the ferrite phase is calculated by calculating the area ratio of the ferrite phase by analyzing one image of 10 or more micrographs having different fields of view, with one field of view of the micrograph being 200 μm in length and 200 μm in width. Calculate by averaging.

  Further, the steel sheet is reduced to a predetermined plate thickness by mechanical polishing or the like, and the distortion is removed by electrolytic polishing or the like, and at the same time, a sample is prepared so that the 1/4 thickness surface becomes the measurement surface. Crystal orientation measurement data in an electron backscattering analysis image (referred to as EBSP) is obtained for the measurement surface of the prepared sample. EBSP measures 5 points or more in each crystal grain of the sample. Crystal orientation measurement data obtained from each EBSP measurement result is output as a pixel.

  Next, the obtained crystal orientation measurement data is analyzed by the Kernel Average Misoration (KAM) method, the unrecrystallized ferrite contained in the ferrite phase is discriminated, and the area ratio of the unrecrystallized ferrite in the ferrite phase is calculated. In the KAM method, the crystal orientation difference between adjacent pixels (measurement points) can be quantitatively shown. In the present invention, grains having an average crystal orientation difference of 1 ° or more from adjacent measurement points are defined as non-recrystallized ferrite.

  Next, the area ratio of unrecrystallized ferrite in the metal structure is calculated using the area ratio of the ferrite phase in the metal structure of the high-strength steel sheet and the area ratio of unrecrystallized ferrite contained in the ferrite phase. .

  The effect of the area ratio of the non-recrystallized ferrite phase on the elongation of the steel sheet has not been clarified. However, under production conditions with a high area ratio, the major axis length of each grain of the non-recrystallized ferrite phase increases. It is easy to increase the connection rate. Moreover, the non-recrystallized ferrite grains having a low ductility extending in the longitudinal direction are likely to be the starting point of void generation, and it is considered that the ductility is significantly reduced. Therefore, in order to suppress the decrease in ductility, in addition to the definition of the area ratio of the non-recrystallized ferrite phase, the area ratio of 70% or more of the non-recrystallized ferrite phase has a long axis length of 25 μm or less. Non-recrystallized ferrite grains are desirable. From the relation of the generation of voids, when the area ratio of non-recrystallized ferrite grains having a major axis length of 25 μm or less among the non-recrystallized ferrite phases is 70% or more, Deterioration of ductility due to inclusion of less than 20% can be substantially suppressed.

  Incidentally, the area ratio of 70% or more of the non-recrystallized ferrite phase is the non-recrystallized ferrite grain having a major axis length of 25 μm or less, and for each of the two or more non-recrystallized ferrite phases, The length of about 100 unrecrystallized ferrite grains is measured by the method shown in FIG. 1, and the ratio of the unrecrystallized ferrite grains whose major axis length is 25 μm or less to the number of measurements {( Number / number of measured non-recrystallized ferrite grains) × 100}, which means that the average value is 70% or more.

"Measuring method of length of major axis of non-recrystallized ferrite grains"
The length of the major axis of the non-recrystallized ferrite grains is measured using the observation result of the non-recrystallized ferrite grains obtained from the crystal orientation analysis in a two-dimensional optical microscope or EBSP. However, when two or more non-recrystallized ferrite grains are present in contact with each other and the following conditions are satisfied, a part or all of the lump of two or more non-recrystallized ferrite grains is combined into one non-recrystallized ferrite. Treat as grains. That is, the length of the interface between the non-recrystallized ferrite grains is 1/5 or more of the length of the interface with the shorter interface length among the non-recrystallized ferrite grains existing in contact with each other. Is the case. In this case, the length of the major axis of the non-recrystallized ferrite grains is defined as the length of the major axis of the mass of the non-recrystallized ferrite grains.

  On the other hand, the low connectivity of each grain of the non-recrystallized ferrite phase also seems to contribute to the elongation of the steel sheet by suppressing the stress concentration on the non-recrystallized ferrite phase with poor ductility. Elongation increases as the connection ratio decreases, but when the connection ratio of non-recrystallized ferrite grains is 0.30 or less, an effect of improving elongation can be obtained.

"Calculation method of the connection ratio of non-recrystallized ferrite grains"
The connection ratio of non-recrystallized ferrite grains is the number B of the total number A of crystal grains adjacent to non-recrystallized ferrite grains, where the crystal grains adjacent to non-recrystallized ferrite grains are other non-recrystallized ferrite grains. It is a ratio (B / A). The connection rate of non-recrystallized ferrite grains is calculated by calculating the connection rate of about 100 non-recrystallized ferrite grains for each of two or more non-recrystallized ferrite phases and averaging them.

  The total number A of the crystal grains adjacent to the non-recrystallized ferrite grains and the number B where the adjacent crystal grains are other non-recrystallized ferrite grains are the non-recrystallized obtained from the crystal orientation analysis in a two-dimensional optical microscope or EBSP. Measured using observation results of ferrite grains. However, when two or more non-recrystallized ferrite grains are present in contact with each other and the following conditions are satisfied, a part or all of the lump of two or more non-recrystallized ferrite grains is combined into one non-recrystallized ferrite. Treat as grains. That is, the length of the interface between the non-recrystallized ferrite grains is 1/5 or more of the length of the interface with the shorter interface length among the non-recrystallized ferrite grains existing in contact with each other. Is the case.

  The present invention is an invention that makes the best use of the non-recrystallized ferrite phase, and in the metal structure of the high-strength steel sheet of the present invention, there are no particular restrictions on the phases other than the non-recrystallized ferrite phase. .

  The high-strength steel sheet of the present invention may have a hot dip galvanized layer on the surface. Corrosion resistance is improved by providing a galvanized layer on the surface. The hot dip galvanized layer preferably contains Zn and Al and has a Fe content limited to less than 13%. When the Fe content in the hot dip galvanized layer is less than 13%, the plating adhesion, formability, and hole expandability are excellent. When the Fe content is 13% or more, the adhesiveness of the hot dip galvanized layer itself is impaired. For this reason, when processing a steel plate, the hot dip galvanized layer breaks and falls off and adheres to the mold, which causes wrinkles.

The hot dip galvanized layer may be alloyed. In the alloyed hot-dip galvanized layer, Fe is taken into the hot-dip galvanized layer by the alloying treatment, so that excellent spot weldability and paintability are obtained. In the alloyed hot-dip galvanized layer, the Fe content is preferably 7% or more. If the Fe content is less than 7%, the effect of improving the spot weldability by performing the alloying treatment may be insufficient.
In the case of an unalloyed hot dip galvanized layer, if the Fe content is less than 13%, even if it is less than 7%, the effect of having the hot dip galvanized layer is not affected, and is 0%. Also good.

The plating adhesion amount is not particularly limited, but is preferably 5 g / m ² or more in terms of one-side adhesion amount from the viewpoint of corrosion resistance.
In the high-strength steel sheet of the present invention, upper plating may be performed on the hot dip galvanized layer for the purpose of improving paintability and weldability. In the high-strength steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment and the like may be performed on the hot dip galvanized layer.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
In order to manufacture the high-strength steel sheet of the present invention, first, a cast slab having any one of the above component compositions is prepared. Next, the cast slab is directly or once cooled, and then heated to 1100 ° C. or higher, and hot rolling is performed.

In this embodiment, before performing hot rolling, heat treatment is performed to heat to 1100 ° C. or higher in order to homogenize the cast slab and dissolve the carbonitride. The heat treatment of the cast slab may be performed directly on the cast slab as it is at a high temperature after casting the cast slab, or may be performed by reheating the cast slab once cooled after casting.
When the heat treatment temperature of the cast slab is less than 1100 ° C., the homogenization of the cast slab and the dissolution of carbonitride are insufficient, resulting in a decrease in strength and workability. On the other hand, when the heat treatment temperature of the cast slab exceeds 1300 ° C., the manufacturing cost increases and the productivity decreases. On the other hand, if the heat treatment temperature exceeds 1300 ° C., the initial austenite grain size becomes large, and eventually it becomes easy to become mixed grains, which may reduce ductility. Therefore, the heat treatment temperature of the cast slab needs to be 1100 ° C. or higher, and is preferably less than 1300 ° C.

As for hot rolling, it is desirable to perform finish rolling at a temperature of ₃ or more points of Ar. If the finishing temperature of the hot rolling is lower than the Ar ₃ point, cracking in the cold rolling is induced, and there is a concern about deterioration of the material.
The Ar ₃ transformation temperature is the content of C, Si, P, Al, Mn, Mo, Cu, Cr, and Ni, expressed in mass%, respectively (% C) (% Si) (% P) (% Al) ( What is necessary is just to calculate by the following formula | equation using% Mn) (% Mo) (% Cu) (% Cr) (% Ni). In addition, Mo, Cu, Cr, and Ni, which are elements that are selectively added, are calculated as 0 when the content is about an impurity.
Ar ₃ = 901-325 × (% C) + 33 × (% Si) + 287 × (% P)
+ 40 × (% Al) −92 × (% Mn +% Mo +% Cu) −46 × (% Cr +% Ni)

The hot-rolled steel sheet obtained after hot rolling is subjected to cold rolling after removing the scale layer by pickling. Cold rolling is performed at a rolling rate of 15% or more. When the rolling rate in cold rolling is less than 15%, recrystallization nuclei are unlikely to form, and grain growth starts due to coarsening of recovered grains. For this reason, recrystallization becomes insufficient, and it becomes difficult to obtain a metal structure in which the area ratio of non-recrystallized ferrite is less than 20%. In order to reduce the area ratio of non-recrystallized ferrite and further improve the elongation of the steel sheet, the rolling ratio in cold rolling is preferably 50% or more.
In addition, the rolling rate in cold rolling is preferably 80% or less in order to prevent deterioration of the steel sheet shape due to a high rolling load.

Next, an annealing process is performed in which the obtained cold-rolled steel sheet is annealed at a maximum temperature higher than (recrystallization temperature−30) ° C. below the recrystallization temperature.
The annealing process is the most important process in forming the metal structure of the high-strength steel sheet of the present invention. The maximum reached temperature (maximum temperature) in the annealing process is controlled with respect to the recrystallization temperature. That is, it is necessary to make the maximum temperature not more than the recrystallization temperature and above (recrystallization temperature−30) ° C. When the maximum temperature reaches the recrystallization temperature, it becomes difficult to leave the non-recrystallized ferrite phase. The maximum ultimate temperature is more preferably −10 ° C. or less of the recrystallization temperature in order to easily secure the area ratio of the non-recrystallized ferrite. On the other hand, if the maximum temperature reached is (recrystallization temperature −30) ° C. or less, the non-recrystallized ferrite phase remains excessively, and remarkable elongation deterioration occurs.

The recrystallization temperature is determined in advance by the following method depending on the component composition, the crystal grain size of the hot-rolled steel sheet, and the rolling rate in cold rolling, which are the main factors that change the recrystallization temperature.
"Calculation method of recrystallization temperature"
A cold-rolled steel sheet (as cold-rolled material) prepared with a predetermined component composition, crystal grain size of hot-rolled steel sheet, and rolling rate in cold rolling is heated with a dilatometer at a rate of temperature increase of 10 ° C./s. When it reaches the temperature reached, it is cooled to obtain a test specimen. In this embodiment, the ultimate temperature is changed at a pitch of 10 ° C. or less to create a plurality of test bodies having different ultimate temperatures. The area ratio of the non-recrystallized ferrite phase of each test specimen obtained is examined using the above-described “calculation method of the area ratio of the non-recrystallized ferrite phase”. And the highest temperature of the ultimate temperature of the test body in which the area ratio of the recrystallized ferrite phase was 98% or more is defined as the recrystallization temperature.

When determining the recrystallization temperature, accuracy is confirmed, and a physical model corresponding to the area ratio of the non-recrystallized ferrite phase of the steel sheet is used to select one of the plurality of test bodies. The area ratio of part or all of the unrecrystallized ferrite phase may be calculated, or the recrystallization temperature may be calculated.
Further, based on the area ratio of the non-recrystallized ferrite phase of a plurality of specimens, any one of the conditions (for example, the component composition) of the component composition, the crystal grain size of the hot-rolled steel sheet, and the rolling rate in cold rolling A map showing the relationship with the crystal temperature may be created and used to determine the recrystallization temperature, or an empirical formula between one or more of the above conditions and the recrystallization temperature may be created and used. Thus, the recrystallization temperature may be determined.

  In the present embodiment, in the annealing step, it is preferable to hold for 30 seconds or more and 300 seconds or less in the temperature range of the maximum temperature or lower and the maximum temperature of −30 ° C. or higher. By holding for 30 seconds or more in the temperature range of the maximum temperature or lower and the maximum temperature of −30 ° C. or higher, the length of the major axis of the non-recrystallized ferrite grains is reduced and the connection rate is reduced. On the other hand, even if the holding time in the temperature range of not more than the maximum temperature and not more than the maximum temperature −30 ° C. exceeds 300 seconds, the effect of reducing the length of the major axis of the non-recrystallized ferrite grains and the coupling ratio is saturated. Since productivity falls and manufacturing cost increases, it is not preferable.

In this embodiment, after performing an annealing process, the cooling process which cools to 480 degrees C or less with the average cooling rate of 1 degree C / sec or more and 200 degrees C / sec or less is performed.
In the cooling step, it is necessary to cool at an average rate of 1 ° C./second or more in order to suppress material deterioration due to a change in structure during cooling. Moreover, even if the average cooling rate exceeds 200 ° C./second, the characteristics of the high-strength steel plate do not change greatly, and the accuracy of the cooling stop temperature is lowered and the cooling cost is increased. For this reason, the upper limit of an average cooling rate shall be 200 degrees C / sec.
In the cooling step, the cooling stop temperature is set to 480 ° C. or lower. When the cooling stop temperature exceeds 480 ° C., the elongation of the high-strength steel sheet is significantly deteriorated due to the formation of pearlite.

In the present embodiment, hot dip galvanization may be performed after the cooling step. As a result, a high-strength steel sheet having a hot-dip galvanized layer on the surface can be obtained.
Furthermore, in this embodiment, after performing hot dip galvanization, you may perform an alloying process. When the alloying treatment is performed, it is preferably performed at a temperature of 600 ° C. or lower. When the temperature of the alloying treatment is 600 ° C. or less, it is preferable because the metal structure of the steel sheet after the cooling step can be suppressed from being changed by performing the alloying treatment.

Next, this invention is demonstrated based on an Example.
Steels having the composition shown in Table 1 were melted and cast slabs by a continuous casting method according to a conventional method.
In Table 1, as for the steel of code | symbol AL, a component composition has satisfy | filled this invention. Steel with symbol a contains C and Ca, steel with b contains Mn and P, steel with c contains Nb, steel with d contains C, steel with e contains Si and S The amount of f steel is N and Ti content is outside the scope of the present invention.

In Tables 2 and 3, the letters of steel represent the types of steel shown in Table 1, and the numerals represent the numbers of the examples. For example, “A1” means the first example using the steel A in Table 1.
In Tables 1 to 3, numerical values outside the scope of the present invention are underlined.

The cast slab having the component composition shown in Table 1 was heated, hot-rolled, pickled, cold-rolled, and subjected to an annealing process and a cooling process to obtain a steel sheet having a thickness of 1.4 mm.
Table 2 shows the casting slab heating temperature, Ar ₃ , hot rolling finishing temperature, cold rolling rolling rate, recrystallization temperature in annealing process—maximum temperature reached (maximum temperature), holding time at maximum temperature, cooling process The average cooling rate and cooling stop temperature are shown.

  The obtained steel sheet was subjected to 0.5% skin pass rolling, and some of the steel sheets were subjected to hot dip galvanization according to a conventional method, and some of the hot dip galvanized parts were immersed in hot dip galvanization. An alloying treatment was performed at a temperature of 600 ° C. or lower to obtain a specimen. Table 2 shows the presence or absence of a hot-dip galvanized layer and the presence or absence of an alloying treatment.

Each specimen was evaluated for mechanical properties (YP (yield stress), tensile strength, elongation, YR (yield stress / tensile strength), YP * El (yield stress × elongation)) in accordance with JIS Z2241. The results are shown in Table 3.
Further, for the specimen, by using the above-described method, the area ratio of the non-recrystallized ferrite phase, the area ratio of the non-recrystallized ferrite grains whose major axis length in the non-recrystallized ferrite phase is 25 μm or less, The connection rate of the crystal ferrite grains was examined. The results are shown in Table 3.

As shown in Table 3, specimens satisfying claim 1 of the present invention (invention steels 1 Contact and invention steel 2 in remarks in Table 3) is not satisfied according to a first aspect of the present invention specimens (Table 3 Remarks YP * El was superior to that of Comparative Steel). From this, it was found that excellent workability (elongation) and high yield stress can be obtained by satisfying claim 1 of the present invention.
Furthermore, the specimens satisfying Claims 1 and 2 of the present invention (Invention Steel 1 in the remarks of Table 3 ) exhibited even better YP * El.

On the other hand, in the comparative steel, YP * El is inferior to that of Invention Steel 1 , Reference Example 1 and Invention Steel 2, and YR is also inferior in Steels B1, a1, b1, and d1, and Steel d1 has strength. Was also lacking.
Steel A1 had a low rolling ratio of cold rolling, so that the area ratio of the non-recrystallized ferrite phase was large and the elongation was insufficient, so that YP * El was small.
Steel B1 had a low average cooling rate in the cooling step, and therefore the metal structure changed during cooling, so that the area ratio of the non-recrystallized ferrite phase was small. As a result, the yield stress was insufficient, and YR and YP * El were reduced.

In steels E4 and E5, the maximum temperature was too low, so that the area ratio of the non-recrystallized ferrite phase was increased, the elongation was insufficient, and YP * El was decreased.
Steel G1 had a cooling stop temperature that was too high, so that pearlite was generated and YP * El was reduced.
In steel L2, since the casting slab heating temperature was too low, the area ratio of the non-recrystallized ferrite phase was increased, the elongation was insufficient, and YP * El was decreased. When steel L1 and steel L2 are compared, steel L1, which is invention steel 1, has the same component composition, and the area ratio of unrecrystallized ferrite does not satisfy claim 1 of the present invention. * El was twice as high.

Steel a1 had an excessive content of C and Ca, and therefore, the elongation was insufficient, and YR and YP * El were both small.
Steel b1 had an excessive content of Mn and P, so that the elongation was insufficient, and YR and YP * El were both small.
In steel c1, since the Nb content is excessive, the area ratio of the non-recrystallized ferrite phase is increased, the elongation is insufficient, and YP * El is decreased.

Since the steel d1 has an insufficient C content, the yield stress and the tensile strength are insufficient, and both YR and YP * El are reduced.
In steel e1, since the contents of Si and S are excessive, YP * El was small.
In steel f1, since the contents of N and Ti were excessive, the area ratio of the non-recrystallized ferrite phase was large and YP * El was small.

Claims (10)

% By mass
C: 0.03% or more, 0.35% or less,
Si: 0.01% or more, 2.00% or less,
Mn: 0.3% or more and 4.0% or less,
P: 0.001% or more, 0.100% or less,
S: 0.0005% or more, 0.050% or less,
N: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, containing 2.00% or less, the balance consists of Fe and inevitable impurities,
The metal structure contains an unrecrystallized ferrite phase at an area ratio of 2% or more and less than 20%,
The ratio of yield stress to tensile strength (yield stress (N / mm ² ) / tensile strength (N / mm ² )) is 0.75 or more,
A high-strength steel sheet excellent in workability, wherein the product of yield stress and elongation (yield stress (N / mm ² ) × elongation (%)) is 12,000 or more. The area ratio of 70% or more of the non-recrystallized ferrite phase is composed of non-recrystallized ferrite grains having a major axis length of 25 μm or less,
The high-strength steel sheet with excellent workability according to claim 1, wherein a connection ratio of the non-recrystallized ferrite grains is 0.30 or less. Furthermore, in mass%,
Cr: 0.05% or more, 3.0% or less,
Mo: 0.05% or more, 1.0% or less,
Ni: 0.05% or more, 3.0% or less,
Cu: 0.05% or more and 3.0% or less of 1 type or 2 types or more, The high-strength steel plate excellent in workability of Claim 1 or Claim 2 characterized by the above-mentioned. Furthermore, in mass%,
Nb: 0.005% or more, 0.30% or less,
Ti: 0.18% or more , 0.30% or less,
V: High strength excellent in workability according to any one of claims 1 to 3, characterized by containing one or more of 0.01% or more and 0.50% or less. steel sheet. Furthermore, in mass%,
B: 0.0001% or more and 0.100% or less is contained, The high-strength steel plate excellent in workability of any one of Claims 1-4 characterized by the above-mentioned. Furthermore, in steel,
Ca: 0.0005% or more, 0.010% or less,
Mg: 0.0005% or more, 0.010% or less,
Zr: 0.0005% or more, 0.010% or less,
REM: 0.0005% or more, 0.010% or less of 1 type or 2 types or more is contained, The high intensity | strength excellent in workability of any one of Claims 1-5 characterized by the above-mentioned steel sheet.   Furthermore, it has a hot-dip galvanized layer on the surface, The high-strength steel plate excellent in workability of any one of Claims 1-6 characterized by the above-mentioned. It is a manufacturing method of the high strength steel plate excellent in workability of any one of Claims 1-7, Comprising: The component of any one of Claim 1, Claims 3-6 The cast slab made of the composition is heated to 1100 ° C. or higher, hot-rolled, pickled, cold-rolled at a rolling rate of 15% or higher, and the resulting cold-rolled steel sheet is subjected to a recrystallization temperature. Hereinafter, after performing an annealing step of annealing at a maximum temperature exceeding (recrystallization temperature −30) ° C., a cooling step of cooling to 480 ° C. or less at an average cooling rate of 1 ° C./second or more and 200 ° C./second or less. A method for producing a high-strength steel sheet excellent in workability characterized by being performed. The method for producing a high-strength steel sheet with excellent workability according to claim 8, wherein the annealing step is performed at the maximum temperature for 30 seconds or more and 300 seconds or less.   The method for producing a high-strength steel sheet with excellent workability according to claim 8 or 9, wherein after the cooling step, hot dip galvanization is performed and alloying treatment is performed.