JP4266343B2 - High-strength hot-rolled steel sheet with excellent formability - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having excellent formability. Specifically, the tensile strength [TS (MPa)] × total elongation [El (%)] × work hardening index [n value] is 5000. The present invention relates to a high-strength composite steel sheet having an excellent balance of strength, ductility, and work hardening characteristics (n value).

  The demand for high-strength steel sheets is increasing mainly due to the reduction of fuel consumption associated with weight reduction of automobile steel sheets and ensuring safety in the event of a collision. Recently, the demand for high-strength steel sheets has been increasing. Demand is increasing.

  However, even for a high-strength steel sheet, there is a strong demand for formability, and it is desired that the steel sheet can be formed satisfactorily according to the part shape. In particular, in the manufacture of automobile parts, there are many cases where complicated shape pressing is performed, and realization of a high-strength steel sheet excellent in various processing characteristics is eagerly desired.

As a high-strength steel sheet developed in response to such needs, retained austenite (γ _R ) is generated in the structure, and γ _R is induced to undergo transformation (strain-induced transformation: TRIP) during work deformation to improve ductility. Steel plates (so-called TRIP steel plates) are known. For example, Patent Document 1, the ferrite, bainite, is composed of three phases of gamma _R, and, by controlling the ferrite occupancy and the ratio of the ferrite grain size, and gamma occupancy of _R in a predetermined range, the working effect It has been disclosed that it has been possible to efficiently demonstrate.

  The inventors of the present invention have also achieved a local ductility and strength by setting the metal structure of the steel sheet to a retained austenite: 10% or more and less than 20% by volume, tempered martensite and tempered bainite: 30% or more by volume. A TRIP steel sheet having an excellent balance is realized (for example, Patent Document 2). In Patent Document 3, the tempered bainite contains 15% or more space factor and ferrite as the metal structure of the steel sheet, and the retained austenite is 3 to 30% as the second phase structure and the retained austenite. By defining the C concentration therein, a high-strength steel sheet excellent in both stretch flangeability and total elongation properties is realized.

  However, in recent years, a high-strength steel sheet that is excellent in strength-ductility balance and excellent in stretchability as processing characteristics has been desired. This stretch formability generally tends to be better as the work hardening index (n value) of the steel sheet is higher (hereinafter, “the work hardenability index is higher and better overhang formability can be expected”). In order to impart excellent stretch formability, the n value of the steel sheet may be improved.

In the TRIP steel having an excellent balance between strength and ductility, the steel sheet has a work hardening index of 0.13 or more and a work hardening index × yield strength of 70 or more. A steel plate for automobiles with improved safety is proposed in Patent Document 4. However, it is considered that further study is required to obtain a steel sheet having a stable and high work hardening index and reliably exhibiting excellent stretch formability.
Japanese Patent Laid-Open No. 2-97620 JP 2003-138345 A JP 2003-171736 A JP-A-11-61326

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength hot-rolled steel sheet having an excellent balance of strength, ductility and work-hardening characteristics.

  The high-strength hot-rolled steel sheet of the present invention that has solved the above problems is in mass% (the same applies to the following components), C: 0.06 to 0.23%, Si: 0.5 to 2.5%, Mn: satisfying 0.5 to 2.5%, Nb: 0.03 to 0.10% and / or Mo: 0.1 to 1.0%, balance iron and inevitable impurities, metal structure Area ratio (the same applies to the metal structure below), retained austenite 5 to 20%, polygonal ferrite 20% or more (preferably 60% or less), bainitic ferrite 30% or more (preferably 60% or less And the total area ratio of the polygonal ferrite and bainitic ferrite is 60% or more (preferably 90% or less). The high strength composite steel sheet according to the present invention has Nb and / or Mo compounds precipitated in the polygonal ferrite or the polygonal ferrite and bainitic ferrite when observed at a magnification of 10,000 times or more. You can also confirm that you are doing.

  The steel sheet of the present invention has excellent strength-elongation balance and excellent work hardening characteristics. Therefore, when manufacturing automotive parts having high strength, other industrial machine parts, etc., forming processes including overhang forming are performed. It can be done well.

The present inventors have intensively studied to provide a high-strength steel sheet that can achieve high elongation and is excellent in work hardening characteristics. As a result, the metallographic structure of the steel sheet is mainly composed of three phases of polygonal ferrite, bainitic ferrite and retained austenite (hereinafter sometimes referred to as “γ _R ”). In particular, the inventors have found that Nb and / or Mo may be contained and have arrived at the present invention.

The reason why all the strength, ductility and work-hardening properties can be improved in a well-balanced manner by using such a configuration is not yet clear, but is estimated as follows. In the case of a steel plate made of conventional polygonal ferrite, bainitic ferrite and retained austenite, the strength of each structure is higher in the order of polygonal ferrite, bainitic ferrite and γ _R , and stress is applied to such a steel plate. Then, microscopically the softest ferrite starts to deform, bainitic ferrite harder than polygonal ferrite, and γ _R surrounded by bainitic ferrite also deform polygonal ferrite in the early stage of transformation. As a result, transformation (TRIP) is likely to occur.

The present invention steel to which the intensity of each organization, bainitic ferrite appears to be higher in the order of polygonal ferrite, gamma _R. The reason why polygonal ferrite is higher in strength than bainitic ferrite is as follows. That is, as can be confirmed in FIG. 1 (micrograph) described later, many compounds such as NbC are precipitated in the polygonal ferrite, but almost no such compound is precipitated in the bainitic ferrite. In the cooling process after hot rolling, a large amount of a compound such as NbC precipitates in the initially formed polygonal ferrite, and the bainitic ferrite deposited later does not substantially precipitate the compound, resulting in precipitation strengthening. This is probably because there are few.

When stress is applied to a steel sheet having such a metal structure, plastic deformation progresses in stages, so that uniform elongation increases and n value increases. That is, when stress is applied to the steel sheet of the present invention, it begins with the yield of bainitic ferrite, followed by the yield of polygonal ferrite, and γ _R surrounded by soft bainitic ferrite in the second half of the uniform deformation The induced transformation is considered to increase the uniform elongation and increase the n value.

  The present inventors also examined the amount of each structure of bainitic ferrite, polygonal ferrite, and retained austenite in order to effectively express such effects. Hereinafter, the reason why the area ratio of the metal structure is defined will be described in detail.

γ _R (residual austenite): 5 to 20%
γ _R is useful for improving the total elongation, and in order to exert such an effect effectively, it is necessary that the area ratio is 5% or more (preferably 8% or more) with respect to the whole structure. On the other hand, since the local deformability and stretch flangeability deteriorate when a large amount exists, the upper limit was set to 20%. More preferably, it is 17% or less.

Furthermore C concentration in the γ _R (Cγ _R) is recommended to be 0.8% or more. This C gamma _R is largely affects the characteristics of the TRIP (strain-induced transformation process), controlling over 0.8%, in particular, is effective in improving the elongation and the like. Preferably it is 1% or more, More preferably, it is 1.2% or more. The content of the C gamma _R is preferably as large, but the actual operation, the adjustable upper limit is generally considered to 1.6%.

Polygonal ferrite + bainitic ferrite: 60% or more in total In the present invention, polygonal ferrite and bainitic ferrite are made 60% or more in total in order to sufficiently exhibit the above-described effects. Preferably but less than 70%, the area ratio is within the range of 90% or less as capable of exhibiting desired characteristics, gamma due to a balance between the _R (gamma _R 85% in the case of 15% or more Appropriate control is recommended.

Polygonal ferrite: 20% or more In the present invention, a ferrite having higher strength than bainitic ferrite can be formed by precipitating a compound of Nb and / or Mo in polygonal ferrite as described above. It is thought that it contributes to the said effect. The above compound can be precipitated both in the crystal grains and in the grain boundaries, but it is the precipitation in the crystal grains that contributes to the strength improvement. Moreover, as said compound, the carbide | carbonized_material of Nb and / or Mo should just precipitate. FIG. 1 is a TEM observation photograph showing that the above compound is precipitated in polygonal ferrite. In (a), Nb carbide is precipitated, and in (b), Nb and Mo carbides are precipitated.

  In order to sufficiently exhibit such an effect, it is necessary that 20% or more of polygonal ferrite is present. Preferably it is 30% or more. However, if too much polygonal ferrite occupies the entire structure, bainitic ferrite is relatively reduced, and it becomes difficult to obtain high work-hardening properties, so it is recommended that the upper limit be 60%. More preferably, it is 50% or less.

Bainitic ferrite: 30% or more In the present invention, γ _R has a structure included in bainitic ferrite that precipitates after polygonal ferrite, thereby causing induced transformation in the second half of uniform deformation. It is considered that the n value can be improved. In order to sufficiently exhibit such an effect, bainitic ferrite should be present at 30% or more, preferably 35% or more with respect to the entire structure. However, even if the area ratio of bainitic ferrite is excessive, polygonal ferrite is relatively reduced and it becomes difficult to obtain high work-hardening characteristics, so it is suppressed to 60% or less (more preferably 55% or less). Is preferred.

Nb: 0.03-0.10% and / or Mo: 0.1-1.0%
As described above, in order to precipitate a Nb and / or Mo compound in the ferrite to realize a hard ferrite, it is necessary to make these elements exist in appropriate amounts in the present invention. Specifically, Nb is contained in an amount of 0.03% or more, preferably 0.04% or more. However, if the Nb content is excessive, the hardness increases more than necessary, leading to deterioration of ductility and the like, so it is necessary to keep it to 0.10% or less, and preferably 0.08% or less. In addition, when Mo is added, the content may be 0.1% or more, preferably 0.15% or more. However, when the Mo content is excessive, the hardness increases and the intended level of ductility can be secured. Therefore, when Mo is contained, the content should be suppressed to 1.0% or less, and preferably 0.8% or less.

  The reason why other components are specified will be described below.

C: 0.06-0.23%
C is an element essential for securing the strength and γ _R of the steel sheet. If it is less than 0.06%, after winding the hot-rolled steel sheet, γ _R present in each steel sheet becomes extremely small, and the desired TRIP effect by γ _R cannot be sufficiently obtained. Preferably it is 0.08% or more, More preferably, it is 0.10% or more. However, if the C content is excessive, characteristics such as weldability deteriorate, so the content is suppressed to 0.23% or less, preferably 0.20% or less, more preferably 0.15% or less.

Si: 0.5 to 2.5%
Si is an element that contributes to the generation of γ _R. If it is less than 0.5%, the predetermined γ _R cannot be obtained, and the TRIP effect by γ _R cannot be sufficiently obtained. Preferably it is 1.0% or more, More preferably, it is 1.2% or more. However, if added over 2.5%, cracks may occur and workability deteriorates. Preferably it is 2% or less, More preferably, it is 1.8% or less.

Mn: 0.5 to 2.5%
Mn, like Si, is an element that contributes to the generation of γ _R. In order to exhibit such an action effectively, it is preferable to contain 0.5% or more. Preferably it is 1.0% or more, More preferably, it is 1.5% or more. However, if Mn is contained excessively, the load during hot rolling becomes high and the productivity is remarkably deteriorated. Preferably it is 2.2% or less.

The contained elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities, but P (phosphorus) is 0.0 as an unavoidable impurity brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, and the like. Of course, it is allowed to be contained within a range of 02% or less, S (sulfur) 0.01% or less, Al 0.05% or less, N (nitrogen) 0.008% or less, As long as the effects of the present invention are not adversely affected, other elements can be actively incorporated as follows.

Ni: 2% or less (not including 0%) and / or Cu: 2% or less (not including 0%)
These elements are both austenite stabilizing elements and contribute to the generation of gamma _R. In order to effectively exert such actions, Ni: 0.1% or more (more preferably 0.3% or more), Cu: 0.1% or more (more preferably 0.3% or more) should be added. Is preferred. However, since there is a risk of cracking if added excessively, it is recommended that the upper limit be Ni: 2% (more preferably 1%), Cu: 2% (more preferably 1%). The above elements may be added alone or in combination.

Cr: 1.0% or less (excluding 0%)
Cr is an element that contributes to strength improvement. In order to effectively exhibit such an action, it is preferable to add Cr by 0.1% or more (more preferably by 0.2% or more). However, if Cr is added excessively, carbides are generated and the production of γ _R decreases, so the Cr content is preferably 1.0% or less (more preferably 0.5% or less).

Ti and / or V: 0.1% or less in total Ti and V are elements that exhibit precipitation strengthening action. In order to effectively exhibit such an action, it is recommended to add at least 0.01% or more (more preferably 0.05% or more) of at least one of the above elements in total. However, if the total amount of the above elements exceeds 0.1%, carbides are generated and the desired γ _R amount cannot be obtained. More preferably, it is 0.08% or less in total.

  The present invention is not limited to the above-described method for producing a steel sheet, and is cast after melting as usual, and as exemplified in the examples described later, at a temperature of 1050 ° C. to 1250 ° C. After heating for a minute, it may be hot-rolled, but to achieve the above metal structure, to efficiently obtain a steel sheet excellent in the balance of strength, ductility (total elongation) and work-hardening characteristics intended in the present invention. In particular, cooling and winding after hot rolling may be performed under the following conditions.

  That is, after finish rolling at 800 ° C. or higher during hot rolling, it is rapidly cooled to a temperature range of 700 to 750 ° C. at 50 ° C./s or more (for example, water cooling), and then for 5 seconds or more in this temperature range (preferably Leave it for 10 seconds or more. By leaving it in the temperature range, a specified amount of ferrite can be secured, and precipitation of Nb and / or Mo compounds into the ferrite can be promoted.

  If the standing time is too long, polygonal ferrite becomes excessive, so it is 45 seconds or less (preferably 30 seconds or less).

  Then, it is good to wind up, after cooling again to 50 degree-C / s or more to a winding temperature range.

  The winding may be performed at 300 to 550 ° C., and the winding temperature may be adjusted based on the examination results shown in FIGS. 2 to 6 below in order to enhance the tensile strength, ductility and work hardening characteristics.

  FIG. 2 shows the relationship between the coiling temperature and TS (tensile strength) for each steel type. From FIG. 2, in order to secure a strength of 780 MPa or more, a specified amount of Nb and / or Mo is used. It turns out that the steel materials to be included may be wound at 550 ° C. or lower.

  FIG. 3 shows the relationship between the coiling temperature and El (total elongation) for each steel type. It can be seen that in order to ensure an excellent elongation (total elongation) of 20% or more, it is preferable to wind a steel material containing a specified amount of Nb and / or Mo at 350 ° C. or higher. FIG. 4 is a graph showing the relationship between the coiling temperature and the uniform elongation (U-El). From the viewpoint of ensuring excellent uniform elongation, it is preferable that the coiling is performed at 350 ° C. or higher and 450 ° C. or lower. Recognize.

  FIG. 5 is a graph showing the relationship between the coiling temperature and [TS (tensile strength) × El (total elongation)]. From FIG. 5, TS × El is 16000 or more, and the strength-ductility balance is excellent. It can be seen that a steel material containing a specified amount of Nb and / or Mo is preferably wound in a range of 300 ° C. or higher and 550 ° C. or lower in order to obtain a steel material. In order to obtain a steel material having a TS × El value of 20000 or more and a more excellent balance between strength and ductility, it is preferable to wind up a steel material containing a specified amount of Nb at 350 ° C. or more and 450 ° C. or less, and most preferably about 400 ° C. You can wind up with.

  FIG. 6 is a graph showing the relationship between the coiling temperature and the n value. From this graph, in order to achieve an n value of 0.15 or more, a steel material containing a specified amount of Nb is 300 ° C. or more and 550 ° C. or less. It can be seen that it is sufficient to wind the steel material in a range of about 400 to 500 ° C. or a steel material containing a specified amount of Mo. In particular, it is understood that a steel material containing a specified amount of Nb may be wound at about 400 to 450 ° C. in order to obtain a steel sheet having an n value of 0.2 or more and excellent work hardening characteristics. The winding may be performed for 10 minutes to 1 hour depending on the component composition, and the cooling method after winding is not particularly limited, but usually slow cooling such as air cooling is performed.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the purpose described above and below. Any of these may be included in the technical scope of the present invention.

Example A test steel having the composition shown in Table 1 (the unit in the table is mass%) was vacuum-melted to obtain a slab for experiment, followed by hot rolling, followed by cooling and winding. .

  Specifically, as shown in FIG. 7, each slab was heated at 1200 ° C. for 30 minutes and then hot-rolled under a finishing temperature (FDT) at hot rolling: 850 ° C. After hot rolling, after water cooling to about 750 ° C., water cooling is stopped and allowed to stand for 10 seconds (air cooling), then water cooled again to 400 ° C. and wound up at about 400 ° C. for 10 minutes, Air cooling was performed to obtain a steel plate having a thickness of about 3.0 mm. Then, the front and back surfaces of the 3.0 mm steel plate were each ground by 0.5 mm, and a 2.0 mm steel plate was used as a test plate.

  Each steel plate thus obtained was measured for tensile strength (TS), elongation [total elongation (EI)], work hardening index (n), and observation of the metal structure in the following manner.

  First, a tensile test was performed using a JIS No. 5 test piece, and tensile strength (TS), elongation (EI), and work hardening index (n) were measured. In addition, the strain rate of the tensile test was 10 mm / sec, and the n value when the strain amount was 5 to 10% was obtained as the work hardening index. In the present invention, a steel sheet having a strength [TS (MPa)] × elongation [El (%)] × work hardening characteristic [n value] of 5000 or more was evaluated as “Example of the present invention”.

  Regarding the metal structure of the steel sheet, the steel sheet was corroded with nital, and the area ratio was determined by distinguishing polygonal ferrite, bainitic ferrite and retained austenite as follows by SEM (magnification: 1000 times or 2000 times) observation.

  Polygonal ferrite (PF): Black in SEM photograph, polygonal shape, and does not contain second phase (residual γ or martensite) inside.

  Bainitic ferrite (BF): obtained by subtracting the second phase from the bainite structure containing the second phase (residual γ and martensite) in the SEM photograph. The SEM photograph shows a dark gray color and often cannot separate and distinguish bainitic ferrite and the second phase. In such a case, the area ratio of bainitic ferrite was obtained by subtracting residual γ by the saturation magnetization method described later from the bainite structure (assuming bainitic ferrite + residual γ).

-Residual γ (γ _R ): The area ratio was measured by saturation magnetization measurement (see R & D Kobe Steel Engineering Reports Vol. 52, No. 3).

  These results are shown in Table 2. In Table 2, the total percentage of the metal structure in the examples does not become 100 because martensite exists (the value obtained by subtracting the residual γ amount by the saturation magnetization method from the second phase is the martensite). Equivalent to site volume).

  From Tables 1 and 2, it can be considered as follows. No. in Table 2 Nos. 2, 5, 6, 8, and 9 (showing Example Nos., The same applies hereinafter) satisfy the requirements of the present invention, and have high strength excellent in balance of strength, ductility, and work hardening characteristics (n value). It turns out that the steel plate is obtained. That is, in all cases, the strength is 780 MPa or more, and the value of (TS × E1 × n) is 5000 or more.

  In contrast, no. 1, 3, 4, 7, and 10 to 13 lack any of the requirements defined in the present invention, and the balance of the above three characteristics is not preferable. No. No. 1 was poor in balance because both bainitic ferrite and retained austenite were insufficient. No. No. 3 does not contain Nb and Mo, so No. 4 contains Nb and Mo, but the content balance is insufficient, so that the property balance is poor. No. In No. 7, since the retained austenite was insufficient, the total elongation was small and the balance of the above three characteristics was not preferable. No. No. 10 had an excessive amount of C, so the amount of residual γ deviated from the upper limit, and the elongation characteristics were not excellent.

  No. Since 11 to 13 were not manufactured under preferable conditions, the intended structure was not obtained, and excellent characteristics were not obtained. That is, no. No. 11 was not allowed to stand (air-cooled) at 700 ° C., so that there was little polygonal ferrite and the balance of the above three characteristics was not preferable. No. In No. 12, since air cooling at 700 ° C. was performed for a long time (60 seconds), polygonal ferrite was excessive and sufficient bainitic ferrite could not be obtained, which resulted in an unfavorable balance of the above three characteristics.

  No. In No. 13, since the coiling temperature was too high, bainitic ferrite and residual γ could not be secured, and the above three characteristics were markedly inferior.

  In addition, said No. SEM photographs of 3-6 steel plates are shown in FIGS. From these micrographs, no. 3 (FIG. 8) and no. In FIG. 4 (FIG. 9), the black portion and the white portion are clearly separated, and it can be seen that dark gray bainitic ferrite does not exist so much. In contrast, no. 5 (FIG. 10) and No. 6 (FIG. 11) has many dark gray portions, and it can be seen that a prescribed amount of bainitic ferrite is secured.

It is a TEM observation photograph which shows that the said compound has precipitated in polygonal ferrite. It is the graph which showed the relationship between coiling temperature and TS (tensile strength) according to steel classification. It is the graph which showed the relationship between coiling temperature and El (total elongation) according to steel classification. It is the graph which showed the relationship between coiling temperature and U-El (uniform elongation) according to steel classification. It is the graph which showed the relationship between coiling temperature and TSxEl for each steel type. It is the graph which showed the relationship between coiling temperature and n value according to steel classification. It is a figure which shows the manufacturing process (part) in an Example. No. in the examples. 3 is an SEM observation photograph of No. 3. No. in the examples. 4 is an SEM observation photograph of No. 4; No. in the examples. 5 is an SEM observation photograph of No. 5. No. in the examples. 6 is an SEM observation photograph of No. 6;

Claims (1)

In mass% (the same applies to the ingredients below)
C: 0.06 to 0.23%,
Si: 0.5 to 2.5%
Mn: 0.5 to 2.5%
While satisfying
Nb: 0.03-0.10% and / or Mo: 0.1-1.0%
The balance is iron and inevitable impurities,
In area ratio in metal structure (the same applies to metal structure below),
5-20% residual austenite,
Polygonal ferrite is 20% to 60% ,
Bainitic ferrite is 60% or less than 30% and I the polygonal ferrite and bainitic total area of 60% or more and 90% or less <br/> der ferrite, the polygonal ferrite in A high-strength hot-rolled steel sheet having excellent formability, wherein a compound of Nb and / or Mo is precipitated in the polygonal ferrite and bainitic ferrite .