JP5761080B2 - High-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue characteristics, and manufacturing method thereof - Google Patents

Description

  [Technical Field] The present invention relates to a high strength hot-rolled steel sheet having a thickness of about 1.0 to 6.0 mm and excellent in elongation, hole expansibility and fatigue characteristics, and a method for producing the same, mainly for automotive steel sheets to be pressed. Is.

  In recent years, there has been a growing need for weight reduction as a measure to improve automobile fuel economy and cost reduction by integral molding of parts, and the development of hot-rolled high-strength steel sheets with excellent press formability has been promoted. Conventionally, as a hot-rolled steel sheet for processing, a dual-phase steel sheet having a ferrite / martensite structure is known. The dual phase steel sheet is composed of a composite structure of soft ferrite phase and hard martensite phase, and voids are generated from the interface between the two phases with significantly different hardness, causing cracks and poor hole expandability. , It is not suitable for applications that require high hole expansibility, such as suspension parts. On the other hand, Patent Document 1 and Patent Document 2 propose a method of manufacturing a hot-rolled steel sheet with excellent hole expansibility by a structure mainly composed of bainite. There were restrictions.

  As a technique for achieving both hole expandability and ductility, Patent Document 3, Patent Document 4, Patent Document 5, and Patent Document 6 propose a steel plate having a mixed structure of ferrite and bainite. However, in recent years, higher hole expansibility has been demanded against the background of further weight reduction of automobiles, complicated parts, and the like, and there has been a demand for high workability and high strength that cannot be handled by the above technology. Further, Patent Document 7 proposes a steel sheet that satisfies a high level of hole expansibility and ductility and secondary work cracking property by utilizing tempered martensite in the second phase, although it is excellent in formability. However, it has not yet been established a technology to enhance the fatigue characteristics that are indispensable as the characteristics of the suspension parts.

  On the other hand, as a method for producing a hot-rolled steel sheet, it is common to perform hot rolling after heating, and to prepare the material by cooling and scraping on a run-out table (hereinafter referred to as ROT). It was. However, in Patent Document 8, Patent Document 9, Patent Document 10, Patent Document 11, Patent Document 12, and Patent Document 13, after hot rolling, the steel sheet is trimmed below the Ms point and reheated at a high temperature above the A1 point. A technique for improving workability by producing retained austenite by slow cooling has been proposed. However, steel sheets that ensure workability by retained austenite cannot be improved in hole expansibility even if ductility can be ensured. Patent Document 14 proposes a technique for improving workability by generating martensite by reheating and slow cooling at a point A1 or higher in a continuous annealing process or plating process after hot rolling. However, since martensite degrades hole expandability, there is a limit to improving hole expandability. In addition, all of them require a high processing temperature of A1 or higher, and it has been difficult to put it into practical use from the viewpoint of production cost and productivity. On the other hand, Patent Documents 15 and 16 propose a technique for tempering martensite at an intermediate temperature of 420 to 650 ° C. However, in addition to the fact that martensite is tempered on average, and there is no definition of crystal grains, it is not desired that fatigue characteristics be improved in coarse crystal grains.

JP-A-4-88125 Japanese Patent Laid-Open No. 3-180426 JP-A-6-293910 JP 2002-180188 A JP 2002-180189 A JP 2002-180190 A JP 2005-146379 A JP 2002-302734 A JP 2002-309334 A JP 2003-171735 A JP 2010-275627 A JP 2009-209450 A JP 2005-206919 A Japanese Patent Laid-Open No. 2003-247045 JP-A-9-263883 JP-A-9-263484

The present invention has been made to solve the above-mentioned conventional problems, and relates to a hot rolled steel sheet of 590 N / mm ² class or higher and a method for producing the same, and is excellent in elongation, hole expansibility and fatigue characteristics. It intends to provide a high strength hot rolled steel sheet.

The inventors of the present invention have made a ferrite and bainite structure by the ROT cooling after hot rolling and the steel composition, and then the structure of the second phase is once controlled to have an appropriately controlled grain size by low-temperature cutting. The martensite structure is tempered by low-temperature reheating, and the mixed structure of ferrite, bainite, and tempered martensite improves the hole expandability dramatically while ensuring excellent elongation, and is particularly important for suspension parts. Excellent fatigue characteristics can be imparted. In addition, at this time, it has been found that the above balance is drastically improved by strictly controlling the tempering amount of martensite and by changing the tempering amount in the vicinity of the martensite grain boundary and in the central part. . Therefore, as a result of diligent investigation focusing on the tempering conditions of martensite, the structure should specify the amount and size of tempered martensite, the amount of retained austenite, and the amount of martensite. It has been found that excellent fatigue characteristics can be obtained while ensuring excellent elongation and hole expansibility by performing tempering under conditions that satisfy the temperature and time parameters that control the heating rate and amount of tempering achieved. It was. Furthermore, by setting an upper limit on the processing temperature, it was found that residual austenite, which forms between the tempered martensite laths and deteriorates the secondary work cracking property, and martensite that deteriorates the hole expandability can be suppressed. Invented the invention.
(1) By mass% C: 0.01% or more, 0.35% or less,
Si: 2.0% or less,
Mn: 0.1% or more, 4.0% or less,
Al: 0.001% or more, 2.0% or less,
P: 0.2% or less,
S: 0.0005% or more, 0.02% or less,
N: 0.02% or less,
O: 0.0003% or more, 0.01% or less,
Having a component composition consisting of the balance Fe and inevitable impurities, and
Each phase is an area fraction,
More than 5% tempered martensite
Less than 2% residual austenite (including 0),
Martensite is less than 1% (including 0),
Perlite is less than 5% (including 0),
The balance has a steel structure consisting of ferrite and bainite,
A high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue characteristics, wherein the average particle size of the tempered martensite phase is in the range of 0.5 µm or more and 5 µm or less.
(2) In mass%,
Ti: 0.01% or more, 0.20% or less,
Nb: 0.01% or more, 0.10% or less,
A high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue properties according to (1), characterized by containing one or more of the following.
(3) The composition according to (1) or (2), further comprising 0.0005% or more and 0.02% or less of one or more of Ca, Mg, Zr, and REM in mass%. High-strength hot-rolled steel sheet with excellent elongation, hole expansibility and fatigue characteristics.
(4) In mass%,
Cu: 0.04% or more, 1.4% or less,
Ni: 0.02% or more, 0.8% or less,
Mo: 0.02% or more, 0.5% or less,
V: 0.02% or more, 0.1% or less,
Cr: 0.02% or more, 0.3% or less ,
B: 0.0003% or more, 0.0010% or less,
The high-strength hot-rolled steel sheet excellent in elongation, hole expansibility, and fatigue properties according to any one of (1) to (3), characterized by containing one or more of the following.
(5) Yield ratio (YP / TS) exceeds 0.7. High strength excellent in elongation, hole expansibility and fatigue characteristics according to any one of (1) to (4) Hot rolled steel sheet.
(6) The average volume center hardness (Hvc) of tempered martensite grains satisfies the formula (1) using the average mass C concentration (Xc) in tempered martensite (1) To (5), a high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue properties.
0.7 ≦ Hvc / (− 982.1 × Xc ² + 1676 × Xc + 189) ≦ 0.95 (1)
(7) The average ratio of the hardness (Hvc) at the volume center of the tempered martensite grains and the hardness (Hve) at 1/5 position from the grain boundary in a straight line (line segment) connecting the center and the grain boundary is expressed by the formula ( 2) The high-strength hot-rolled steel sheet excellent in elongation, hole expansibility, and fatigue properties according to any one of (1) to (6).
0.2 ≦ Hve / Hvc ≦ 0.8 (2)
(8) After cooling the cast slab having the component composition according to any one of (1) to (4) to a temperature range of 1050 ° C. or higher and 1300 ° C. or lower, or performing reheating, 800 ° C. or higher, 1100 ° C. Exit finish rolling at a temperature below, followed by cooling to 200 ° C. or less at an average cooling rate of 10 ° C. / s or higher, the wound taken steel at temperatures below 100 ° C., again reheating the row physician A method for producing a high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue characteristics, characterized in that a reheating treatment temperature T is 150 ° C. or more and 625 ° C. or less .
(9) In cooling after finishing rolling, an air cooling region of 2 seconds or more and 10 seconds or less is provided in a temperature range of 600 ° C. or higher and 750 ° C. or lower, and the cooling zone excluding this air cooling region is 15 ° C./s. The method for producing a high-strength hot-rolled steel sheet having excellent elongation, hole expansibility, and fatigue characteristics as described in (8), wherein cooling is performed to 200 ° C. or less at the above average cooling rate.
(10) The reheating treatment temperature T and the treatment time t are performed under the conditions shown in the formula (3), and the elongation, hole expansibility and fatigue characteristics described in (8) or (9) are excellent. Manufacturing method of high-strength hot-rolled steel sheet.
4500 ≦ (T + 273) × (log (t / 60) +10) ≦ 7000 (3)
T: Processing temperature (° C)
t: Processing time (min )
(11 ) The reheating treatment time (t) is 10 min or less in the case of continuous treatment , or 10 hours or more in the case of annealing in a coil as in BAF (10 The method for producing a high-strength hot-rolled steel sheet having excellent elongation, hole expansibility and fatigue properties as described in 1 ) .

  According to the present invention, a high-strength hot-rolled steel sheet excellent in elongation, hole expansibility, and fatigue characteristics can be provided, so that it is suitable as a high-strength hot-rolled steel sheet having high workability. The high-strength hot-rolled steel sheet according to the present invention can reduce the weight of the vehicle body, integrate the parts, and streamline the machining process. It can improve fuel efficiency and reduce manufacturing costs, and is excellent as a part. It has a great industrial value as having fatigue characteristics.

The graph which shows the effect of this invention steel on the elongation with respect to tensile strength. The graph which shows the effect of this invention steel on the hole expansibility with respect to tensile strength.

  In the present invention, a base structure (ferrite, bainite structure) is formed by steel components and hot rolling ROT cooling, and the second phase is once converted into a martensite structure having an appropriate size and then tempered martensite by reheating. A steel sheet that can be made by incorporating the material of the site. As a structure, the retained austenite and the martensite fraction that has not been tempered are reduced as much as possible, while using the tempered martensite phase to ensure excellent elongation, It is a steel sheet that dramatically improves the expansibility and has excellent fatigue characteristics. In addition, the reheating conditions are optimized to strictly control the tempering state, and the average tempering amount and intra-grain tempering amount distribution (hardness gradient) are controlled to greatly increase elongation, hole expansibility, and fatigue characteristics. It can be a steel plate secured at a high level. The individual constituent requirements of the present invention will be described in detail below.

  First, the reasons for limiting the components of the present invention will be described. Unless otherwise specified,% means mass%.

  C is an element that affects the workability of steel, and the workability deteriorates as the content increases. In particular, if it exceeds 0.35%, carbides (pearlite, cementite) harmful to the hole expandability are generated, so the content is made 0.35% or less. However, when particularly high hole expansibility is required, it is desirable to set it to 0.10% or less. Moreover, 0.01% or more is necessary in terms of securing strength.

  Si is an effective element to suppress the formation of harmful carbides, increase the ferrite fraction and improve elongation, and is also an element effective for securing material strength by solid solution strengthening, so it is desirable to add it However, if the addition amount increases, the chemical conversion processability decreases and spot weldability also deteriorates, so 2.0% is made the upper limit. Si may not be contained.

  Al, like Si, is an element effective for suppressing the formation of harmful carbides and increasing the ferrite fraction and improving the elongation. In particular, it is an element necessary to achieve both ductility and chemical conversion treatment. Al is an element necessary for deoxidation, and has been usually added in an amount of about 0.01 to 0.07%. As a result of intensive studies, the present inventors have found that, by adding Al, the ductility is improved and the chemical conversion property is excellent. However, if the amount added is increased, the effect of improving ductility is saturated, and the chemical conversion treatment performance is deteriorated and the spot weldability is also deteriorated, so the upper limit is set to 2.0%. It is desirable that the upper limit is 1.0%. Addition of 0.001% or more is necessary for sufficient deoxidation.

  Mn is an element necessary for ensuring strength, and it is necessary to add at least 0.1%. However, if added in a large amount, microsegregation and macrosegregation are likely to occur, and these deteriorate the hole expandability. Therefore, the upper limit is 4.0%.

  P is an element that increases the strength of the steel sheet, and is an element that improves the corrosion resistance when added simultaneously with Cu. However, if the addition amount is high, it is an element that causes deterioration of weldability, workability, and toughness. From this, it should be 0.2% or less. Especially when corrosion resistance is not a problem, 0.03% or less is desirable with emphasis on workability.

  S is an element that forms sulfides such as MnS, becomes a starting point of cracking, and reduces hole expansibility. Therefore, it is necessary to make it 0.02% or less. However, the desulfurization cost increases to adjust to less than 0.0005%, so this is made 0.0005% or more.

  N causes stretcher strain formation during steel plate processing and degrades workability. When Ti and Nb are added, N contributes to the formation of (Ti, Nb) N, and extends and expands holes. Less is better to reduce. 0.02% or less due to the above restrictions.

  O forms an oxide in the steel. This oxide has an effect of suppressing coarsening of austenite grains during hot rolling, and as a result, contributes to refinement of martensite grains. In order to obtain this effect, at least 0.0003% is added. On the other hand, if it is contained in a large amount, miniaturization proceeds too much, and the oxide becomes the starting point of cracking, so that the hole expandability and elongation are significantly reduced. For this reason, 0.01% is made the upper limit.

  Furthermore, it is good also as containing the element shown below.

  Ti and Nb form carbides and are effective in increasing the strength, and contribute to uniform hardness and improve hole expansibility. In order to exhibit these results effectively, it is necessary to add at least 0.01% of both Nb and Ti. However, if these additions become excessive, the ductility deteriorates due to precipitation strengthening, so the upper limit is set to 0.20% or less for Ti and 0.10% or less for Nb. These elements are effective even when added alone, and are effective even when added in combination.

  Ca, Mg, Zr, and REM control the shape of sulfide inclusions and are effective in improving the hole expansibility. In order to exhibit this effectively, it is necessary to add at least one or two or more of 0.0005% or more. On the other hand, if a large amount is added, the cleanliness of the steel is worsened and the hole expandability and ductility are impaired. Accordingly, the upper limit is set to 0.02%.

  Cu is an element that improves the corrosion resistance when combined with P. To obtain this effect, it is desirable to add 0.04% or more. However, the addition of a large amount increases the hardenability and decreases the ductility, so the upper limit is 1.4%.

  Ni is an essential element for suppressing hot cracking when Cu is added. In order to obtain this effect, it is desirable to add 0.02% or more. However, the addition of a large amount, like Cu, increases the hardenability and decreases the ductility, so the upper limit is made 0.8%.

  Mo is an element effective for suppressing the formation of cementite and improving the hole expansibility. In order to obtain this effect, it is necessary to add 0.02% or more. However, since Mo is also an element improving the hardenability, the ductility is lowered when it is excessively added, so the upper limit is made 0.5%.

  V forms carbides and contributes to securing the strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, since addition of a large amount reduces elongation and costs are high, the upper limit is made 0.1%.

  Cr, like V, forms carbides and contributes to securing strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, since Cr is an element improving the hardenability, elongation is reduced by adding a large amount. Therefore, the upper limit is set to 1.0%.

  B, like Mn, is an element that contributes to strength. In order to obtain this effect, addition of 0.0003% or more is necessary. However, since B is also an element improving the hardenability, the ductility is lowered by adding a large amount, so the upper limit is made 0.0010%.

  In general, when a martensite phase is introduced into a structure to form a composite structure such as dual phase steel (hereinafter referred to as DP steel), high strength and ductility can be secured. However, when a hard phase such as martensite phase is introduced into the structure, the hole expandability is significantly deteriorated. As a result of intensive research by researchers, by using a tempered martensite phase with an appropriate size, hole expansion can be achieved while maintaining excellent elongation like DP steel without significantly reducing strength and elongation. It has been found that the steel sheet can be improved dramatically and has excellent fatigue characteristics. In particular, by optimizing the reheating conditions and controlling the average tempering amount and tempering distribution of martensite during tempering, it is superior to DP steel without significantly reducing the strength and ductility obtained by the martensite phase. It has been found that the hole expandability can be dramatically improved while maintaining elongation. In addition, by increasing the tempering amount in the vicinity of the grain boundary of martensite and decreasing the tempering amount in the central part, the above effect is further increased, and further, deterioration of fatigue characteristics due to tempering is minimized, and DP steel It has been found that it has the same fatigue characteristics.

  Hereinafter, the area ratio of a specific phase, for example, the area ratio of a tempered martensite phase is referred to as a tempered martensite fraction.

  In order to obtain the effect of the tempered martensite as described above, it is necessary to contain 5% or more in terms of the tempered martensite fraction. Below this, ductility deteriorates and the effect of improving hole expansibility is small. At this time, in order to achieve both elongation and hole expansibility, the balance needs to have a ferrite and bainite structure. In particular, the residual austenite generated during tempering generates residual stress due to deformation of the work and reduces the hole expandability. For this reason, the retained austenite fraction needs to be less than 2%. When martensite that has not been tempered remains in the structure, it becomes a starting point and cracks are generated during hole expansion processing, so the martensite fraction must be less than 1%. In addition, if pearlite is present in the structure, cracking occurs at the pearlite ferrite / cementite boundary during hole expansion processing, so the pearlite fraction is preferably less than 5%.

  When the tempered martensite has an average particle size of 0.5 μm or more, both hole expansibility and extensibility can be achieved, and fatigue characteristics can be improved. On the other hand, tempered martensite grains having an average grain size of more than 5 μm increase the stress concentration at the grain boundaries more than necessary, thereby causing a decrease in hole expansibility and elongation. Accordingly, the tempered martensite average particle size is 0.5 μm or more and 5 μm or less.

  When martensite is tempered, YP increases and the yield ratio (YP / TS) increases. For the same steel type, it is necessary to increase the yield ratio in order to increase the fatigue strength. Therefore, YP / TS is more than 0.7.

The tempering amount of martensite is one of the most important features in the preferred embodiment of the present invention. When the tempering amount is large, the strength is remarkably lowered, and not only the desired strength cannot be obtained, but also the strength-elongation balance is remarkably lowered. On the other hand, if the tempering amount is too small, the boundary between the adjacent ferrite and bainite phases becomes a point of void formation, and the hole expandability is remarkably lowered. Therefore, by controlling the ratio of the volume center hardness (Hvc) of the tempered martensite grains to the average hardness of the martensite before tempering, the tempering amount can obtain an excellent elongation-hole expansion-strength balance. it can. However, in general, it is difficult to measure the martensite hardness before tempering. Therefore, in the present invention, the tempering amount is evaluated by the formula (1) using the average of the volume center hardness (Hvc) of the tempered martensite grains and the average of the mass C concentration (Xc) in the tempered martensite. In order to obtain an excellent elongation-hole expansibility-strength balance, the tempered martensite grains must satisfy the formula (1).
0.7 ≦ Hvc / (− 982.1 × Xc ² + 1676 × Xc + 189) ≦ 0.95 (1)

The tempering distribution of martensite is a preferred embodiment of the present invention and one of the most important features. As a result of intensive research by researchers, it has been found that the above-mentioned effect obtained by tempering martensite is further increased by increasing the tempering amount in the vicinity of the grain boundary of martensite and decreasing the tempering amount in the center. . Although the details are not clear, by making a hardness gradient for the elongation, the martensite hardness in the center can be kept relatively high, so there is little strength reduction and the strength-elongation balance by tempering This seems to be because degradation can be suppressed to the limit. Further, regarding hole expansibility, the hardness gradient is considered to have an effect of delaying the formation of voids because the hardness of the phase boundary where voids are easily formed becomes low. In addition, regarding fatigue properties, the effect of suppressing the propagation of martensite cracks can be as good as DP, and the stress concentration at the phase boundary where stress concentration tends to occur is mitigated by a decrease in the hardness ratio at the phase boundary. . At this time, the tempered martensite particle size contributes to effectively forming a hardness gradient. At this time, if the hardness gradient is small, the above effect cannot be obtained, so that either or both of the elongation and the hole expansibility are remarkably lowered. On the other hand, if the gradient is too large, the boundary hardness becomes too small and the strength is greatly reduced. In order to obtain a steel sheet having excellent elongation, hole expansibility and fatigue strength, the hardness (Hvc) of the volume center of the tempered martensite phase and the grain boundary in the straight line (line segment) connecting this center and the grain boundary are used. It is necessary that the average ratio of the hardness (Hve) at the 1/5 position satisfies the formula (2).
0.2 ≦ Hve / Hvc ≦ 0.8 (2)

In the present invention, the method of measuring the tissue fraction is not limited as long as it is a measurement method with excellent accuracy. For example, the determination of each phase and the measurement of the fraction were performed as follows.
[1] Martensite, ferrite, bainite, pearlite, tempered martensite Any method can be used as long as the structural fraction can be accurately measured. For example, the steel sheet is subjected to reperal etching or nital etching to perform hot rolling. Observe the structure at 1 / 4t of the directional section with an optical microscope or SEM, determine each phase, and measure the fraction of each phase using an image analyzer.
[2] Residual austenite The surface of a steel sheet is ground to 1/4 t, and then chemically polished and then subjected to X-ray diffraction using a Mo tube to determine the diffraction intensity Iα (200) of ferrite and (211 of ferrite) ) Diffraction intensity Iα (211) and austenite (220) diffraction intensity Iγ (220) and (311) diffraction intensity Iγ (311).
Vγ (% by volume) = 0.25
× {Iγ (220) / (1.35 × Iα (200) + Iγ (220))
+ Iγ (220) / (0.69 × Iα (211) + Iγ (220))
+ Iγ (311) / (1.5 × Iα (200) + Iγ (311))
+ Iγ (311) / (0.69 × Iα (211) + Iγ (311))}

  The hardness measurement is basically performed by Vickers measurement. However, in the case where the Vickers hardness cannot be measured with a small particle size, the measurement may be performed using nanoindentation. In that case, the value converted into Vickers hardness is used. For this conversion, it is necessary to obtain a converted value with high accuracy, such as using a standard sample having similar hardness.

  The C concentration of the martensite grains may be any measurement method as long as the accuracy is guaranteed under the condition that the decomposition concentration can be obtained accurately. For example, using the EPMA attached to the FE-SEM, It can be obtained by carefully measuring the C concentration with a pitch of 0.5 μm or less.

  Next, a manufacturing method will be described.

  Cast slabs need to be homogenized and carbonitride dissolved before hot rolling. When this is done, the continuously cast slab may be kept hot or reheated. If the temperature for maintaining at a high temperature or the temperature for reheating is less than 1050 ° C., both homogenization and dissolution become insufficient, resulting in a decrease in strength and workability. On the other hand, when the temperature exceeds 1300 ° C., the production cost and productivity are reduced, and the initial austenite particle size is increased, so that it tends to be mixed eventually, and the martensite particle size is increased. And workability deteriorates after tempering.

  The finish rolling finish temperature needs to be 800 ° C. or higher in order to prevent the formation of ferrite and improve the hole expandability. On the other hand, if the temperature is too high, the strength is reduced due to the coarsening of the structure, and the ductility is lowered. Moreover, when high elongation is required, it is desirable to set it as 950 degrees C or less.

  In the present invention, it is important that the second phase is once martensite. In order to generate martensite, an average cooling rate of 10 ° C./s or more is required in ROT. If it is less than this, pearlite will be generated, resulting in a decrease in strength and deterioration in hole expansibility.

  In the present invention, in order to obtain a particularly excellent balance between elongation and hole expansibility, it is desirable to produce a ferrite phase during ROT cooling. In order to achieve this, it is necessary to air-cool in a temperature range of 600 ° C. or higher and 750 ° C. or lower. If the temperature is lower than 600 ° C. or higher than 750 ° C., ferrite transformation does not occur sufficiently, so that the elongation decreases. The air cooling time needs to be 2 seconds or more and 10 seconds or less. If it is less than 2 seconds, ferrite cannot be obtained sufficiently, and the elongation decreases. On the other hand, if it exceeds 10 seconds, pearlite is generated, resulting in a decrease in strength and deterioration in hole expansibility. In the cooling pattern in which air cooling is provided, a cooling rate of 15 ° C./s or more is required in the cooling zone excluding the air cooling region. If it is less than this, pearlite is produced, resulting in a decrease in strength and a deterioration in hole expansibility.

  The aforementioned cooling stop temperature needs to be 200 ° C. or lower. If the temperature exceeds 200 ° C., the desired tempering effect cannot be obtained, so that a good balance between stretchability and hole expansibility cannot be obtained. In addition, the coiling temperature needs to be less than 100 ° C. When it winds up at 100 degreeC or more, since self-annealing will occur, it becomes difficult to obtain a desired hardness gradient by subsequent reheating process.

  The present invention is characterized in that the steel cut after hot rolling is reheated again. By reheating, the martensite once generated can be tempered martensite.

  The high strength hot-rolled steel sheet defined in claim 1 of the present invention can be manufactured by satisfying the above manufacturing conditions after containing the components specified in the present invention, and the yield ratio (YP / TS). Can be greater than 0.7.

One of the most important production methods for realizing the preferred embodiment of the present invention (martensite tempering amount and martensite tempering amount distribution) is the tempering condition of the martensite phase. Excessive tempering not only reduces the strength of the material and the desired strength cannot be obtained, but also tempering coarsens cementite in bainite, thus reducing elongation and hole expansion. On the other hand, if tempering is insufficient, the martensite remains hard, so cracks are likely to occur at the phase boundary with the soft phase (ferrite phase, bainite phase), and the hole expandability decreases. As a result of intensive studies, the present inventors have developed a function of temperature and time in Equation (3), and in this function, reheating is performed under conditions of 4500 or more and 7000 or less, thereby extending and expanding the hole. It has been found that excellent characteristics can be obtained. In particular, in order to ensure high hole expansibility, it is desirable to set it to 5000 or more and 6500 or less.
4500 ≦ (T + 273) × (log (t / 60) +10) ≦ 7000 (3)
T: Heat treatment temperature (° C)
t: Processing time (min)

  Temperature is an important factor among the above-mentioned post-treatment conditions. When the heating temperature exceeds 600 ° C, a part of the structure is transformed into austenite, and in the subsequent cooling, martensite that has not been tempered and a residual austenite phase is generated between the tempered martensite laths, and the hole is expanded. Degradation of sex. Therefore, it is necessary to suppress transformation to austenite during reheating, and the heating temperature is preferably 600 ° C. or lower. On the other hand, if the heating temperature is less than 150 ° C., the tempering of martensite is insufficient and the hole expandability deteriorates.

  Moreover, when processing continuously, if processing time becomes long, it will cause productivity fall and a large heating equipment will be needed, and cost will become high too much. In continuous processing, the processing time (t) needs to be 10 min or less. On the other hand, it is also possible to anneal the coil as in BAF. In this case, a treatment time of at least 10 hours or more is required to make the coil temperature uniform.

The average heating rate in the heating process of the reheating process is one of the very important control conditions for obtaining a desired hardness gradient. As a result of intensive studies, the present inventors have found that the hardness gradient can be increased by increasing the average heating rate and CR (° C./s) .

Next, this invention is demonstrated based on an Example. The present invention is intended for high-strength hot-rolled steel sheets of 590 N / mm ² class or higher. Specifically, if the tensile strength of the steel sheet is 490 N / mm ² or higher, the present invention is subject to the high-strength hot-rolled steel sheets of the present invention. .

  Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. Reference signs A to Z are steels of components according to the present invention. On the other hand, steels with symbols a and d are added with C, steel with b is added with Mn and P, steel with c is added with Nb, steel with e is added with S and O, steel with f is N, Ti The amount of steel added, g, is outside the scope of the present invention.

  These steels were hot-rolled and ROT-cooled in the heating furnace under the conditions shown in Tables 2 and 4. Subsequently, reheating was performed under the conditions shown in Tables 2 and 4. Here, Table 2 shows reheating in a high temperature region, and Table 4 shows reheating in a low temperature region. The plate thickness was 2.6 to 3.2 mm, and the plate was then pickled, subjected to 0.5% skin pass rolling, and subjected to material evaluation.

  Tables 3 and 5 show the tempered martensite fraction, retained austenite fraction, martensite fraction, pearlite fraction, ferrite + bainite fraction, C concentration in tempered martensite, and tempered martensite particle size of the obtained steel sheet. Shown in Tables 3 and 5 and FIGS. 1 and 2 show the hardness ratio of the middle side of the formula (1), the hardness ratio of the middle side of the formula (2), YR, mechanical characteristics, and fatigue characteristics. The fatigue characteristics are represented by a fatigue limit ratio (= fatigue strength / tensile strength), and 0.4 or less was inferior. In the table, numerical values outside the scope of the present invention are underlined. In addition, numerical values outside the preferable range of the present invention are bold.

  The group of invention steel 1 satisfies the claims of the present invention, and the group of invention steel 2 satisfies claims 1, 8, and 9 but does not satisfy some of the dependent claims. Invention steel 2 is inferior to invention steel 1 in some characteristics.

  Further, out of the scope of the present invention, the comparative steel 1 is inferior to the inventive steel in terms of elongation, hole expansibility and fatigue characteristics, and the comparative steel 2 has characteristics similar to DP steel, and the elongation and fatigue characteristics are good. The hole expandability is extremely poor. That is, it can be seen that only those satisfying the present specification can have both excellent elongation, hole expansibility, and fatigue characteristics.

  In addition, the hole expansion rate was used as an index of local deformability, the tensile test was based on JIS Z2241, and the hole expansion test was based on JIS Z2256. Moreover, about the fatigue test, it carried out by the plane bending test and was based on JISZ2275.

Claims (11)

In mass% C: 0.01% or more, 0.35% or less,
Si: 2.0% or less,
Mn: 0.1% or more, 4.0% or less,
Al: 0.001% or more, 2.0% or less,
P: 0.2% or less,
S: 0.0005% or more, 0.02% or less,
N: 0.02% or less,
O: 0.0003% or more, 0.01% or less,
Having a component composition consisting of the balance Fe and inevitable impurities, and
Each phase is an area fraction,
More than 5% tempered martensite
Less than 2% residual austenite (including 0),
Martensite is less than 1% (including 0),
Perlite is less than 5% (including 0),
The balance has a steel structure consisting of ferrite and bainite,
A high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue characteristics, wherein the average particle size of the tempered martensite phase is in the range of 0.5 µm or more and 5 µm or less. % By mass,
Ti: 0.01% or more, 0.20% or less,
Nb: 0.01% or more, 0.10% or less,
The high-strength hot-rolled steel sheet having excellent elongation, hole expansibility, and fatigue properties according to claim 1, comprising one or more of the following.   The composition according to claim 1 or 2, further comprising 0.0005% or more and 0.02% or less of one or more of Ca, Mg, Zr, and REM, respectively, by mass%. High-strength hot-rolled steel sheet with excellent elongation, hole expansibility and fatigue characteristics. % By mass,
Cu: 0.04% or more, 1.4% or less,
Ni: 0.02% or more, 0.8% or less,
Mo: 0.02% or more, 0.5% or less,
V: 0.02% or more, 0.1% or less,
Cr: 0.02% or more, 0.3% or less ,
B: 0.0003% or more, 0.0010% or less,
The high-strength hot-rolled steel sheet excellent in elongation, hole expansibility, and fatigue characteristics according to any one of claims 1 to 3, characterized by containing at least one of the following.   The high-strength hot-rolled steel sheet having excellent elongation, hole expansibility, and fatigue characteristics according to any one of claims 1 to 4, wherein a yield ratio (YP / TS) exceeds 0.7. . The average of the volume center hardness (Hvc) of the tempered martensite grains satisfies the formula (1) using the average of the mass C concentration (Xc) in the tempered martensite. 5. A high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue properties according to any one of 5 above.
0.7 ≦ Hvc / (− 982.1 × Xc ² + 1676 × Xc + 189) ≦ 0.95 (1) The average ratio of the hardness (Hvc) of the volume center of tempered martensite grains and the hardness (Hve) at 1/5 position from the grain boundary in a straight line (line segment) connecting the center and the grain boundary is expressed by Equation (2). The high-strength hot-rolled steel sheet excellent in elongation, hole expansibility, and fatigue characteristics according to any one of claims 1 to 6, characterized in that:
0.2 ≦ Hve / Hvc ≦ 0.8 (2) The cast slab having the component composition according to any one of claims 1 to 4 is cooled to a temperature range of 1050 ° C or higher and 1300 ° C or lower, or reheated, and 800 ° C or higher and 1100 ° C or lower. the ends finish rolling temperature, followed by cooling to 200 ° C. or less at an average cooling rate of 10 ° C. / s or higher, the wound taken steel at temperatures below 100 ° C., again, have rows reheating, the reheating A process for producing a high-strength hot-rolled steel sheet excellent in elongation, hole expansibility and fatigue characteristics, characterized in that the treatment temperature T is 150 ° C. or more and 625 ° C. or less .   In the cooling after finish rolling, an air cooling region of 2 seconds or more and 10 seconds or less is provided in a temperature range of 600 ° C. or higher and 750 ° C. or lower, and an average of 15 ° C./s or higher is provided in the cooling zone excluding this air cooling region. The method for producing a high-strength hot-rolled steel sheet having excellent elongation, hole expansibility, and fatigue characteristics according to claim 8, wherein the steel sheet is cooled to 200 ° C or lower at a cooling rate. The high-strength heat excellent in elongation, hole expansibility and fatigue characteristics according to claim 8 or 9, wherein the reheating treatment temperature T and the treatment time t are performed under the conditions shown in the formula (3). A method for producing rolled steel sheets.
4500 ≦ (T + 273) × (log (t / 60) +10) ≦ 7000 (3)
T: Processing temperature (° C)
t: Processing time (min) 11. The elongation according to claim 10, wherein the reheating treatment time t is 10 min or less when the treatment is continuously performed , or 10 hours or more when the coil is annealed as in BAF. And high-strength hot-rolled steel sheet manufacturing method with excellent hole expansibility and fatigue characteristics.

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