JP6260087B2 - High-strength hot-rolled steel sheet with excellent workability and fatigue characteristics and method for producing the same - Google Patents

Description

  The present invention mainly relates to a steel sheet for automobiles to be pressed, and relates to a high-strength hot-rolled steel sheet having a thickness of about 1.0 to 6.0 mm and excellent workability and fatigue characteristics, and a method for producing the same. .

  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. Hereinafter, Dual Phase steel is also referred to as DP steel. 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 technique for improving the hole expansion property utilizing softening by tempering of martensite, in Patent Document 8, some martensites are obtained by reheating and gradual cooling at a point A1 or higher in a continuous annealing process or plating process after hot rolling. A technique for improving workability by tempering the steel has been proposed. However, since the remaining martensite deteriorates the hole expandability, there is a limit to improving the hole expandability. On the other hand, Patent Document 9 and Patent Document 10 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 Japanese Patent Laid-Open No. 2003-247045 JP-A-9-263883 JP-A-9-263484

The present invention has been made in order to solve the above-mentioned conventional problems, and relates to a hot rolled steel sheet of 500 N / mm ² class or higher and a method for producing the same, and has a high strength heat excellent in workability and fatigue strength. It is intended to provide a rolled steel sheet.

  So far, in DP steel, the optimization of the hardness and size of each phase and the phase fraction has been promoted, and many results have been seen, but the inventors focused on the arrangement of martensite in DP, We have intensively studied the arrangement of martensite phases suitable for processing. As a result, it has been found that when the coverage of ferrite grains by martensite grains increases, the balance of stretch-hole expansibility improves significantly. In addition, the present inventors have found that the fatigue characteristics that are characteristic of DP are also improved. Here, the coverage is the ratio of the ferrite grain boundaries occupied by the martensite grains to the total ferrite grain boundary length as shown in FIG. 1 when 2D observation is performed with an optical microscope. Show. That is, a coverage of 100% means that there are no ferrite / ferrite grain boundaries and the ferrite grains are completely surrounded by martensite grains. The reason for the improvement of the material due to such an arrangement is not clear, but the martensite connectivity increases, resistance to deformation increases, the strength increases, and the ferrite grain boundary takes a random atomic arrangement compared to the inside of the grain. This is because the strain concentration relaxes the strain concentration at the martensite / ferrite interface and the initial void size is reduced, or the void generation is suppressed. There are several methods for controlling this arrangement. For example, recrystallization control and run-out table cooling (hereinafter also referred to as “ROT cooling”) in a subsequent rolling mill of hot rolling, or re-rolling performed after hot rolling. It has been found that when heating is used, it is possible to control the coverage, and the present invention has been made.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.03% or more, 0.35% or less, Si: 0.01% or more, 2.0% or less, Mn: 0.3% or more, 4.0% or less, P : 0.001% or more, 0.10% or less, S: 0.0005% or more, 0.05% or less, N: 0.0005% or more, 0.010% or less, Al: 0.01% or more, 2 0.0% or less, the balance being Fe and inevitable impurities, and the crystal structure is 10% or more martensite phase and 38% or more ferrite phase by area fraction, and the pearlite phase is less than 10%. A high-strength heat excellent in workability and fatigue characteristics, characterized in that the ferrite grain coverage by the martensite phase is more than 30% and the average grain size of the martensite grains is more than 1 μm and less than 4 μm Rolled steel sheet.
Here, the coverage of the ferrite grains by the martensite phase is the percentage ratio of the ferrite grain boundary portion occupied by the martensite grains when the total ferrite grain boundary length is 100. is there.
(2) The high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics as described in (1) above, wherein the area fraction of the martensite phase is less than 60%.
(3) Further, in mass% in steel, Cr: 0.05% or more, 3.0% or less, Mo: 0.05% or more, 1.0% or less, Ni: 0.05% or more, In the workability and fatigue characteristics according to the above (1) or (2), containing 1 type or 2 types of 0% or less, Cu: 0.05% or more, 3.0% or less Excellent high-strength hot-rolled steel sheet.
(4) Further, in steel, by mass%, Nb: 0.005% or more, 0.3% or less, Ti: 0.005% or more, 0.3% or less, V: 0.01% or more, 0. The high-strength hot-rolled steel sheet having excellent workability and fatigue properties according to any one of the above (1) to (3), comprising one or more of 5% or less.
(5) The processing according to any one of (1) to (4) above, wherein the steel further contains B: 0.0001% or more and 0.1% or less by mass%. High-strength hot-rolled steel sheet with excellent durability and fatigue characteristics.
(6) Further, by mass% in the steel, Ca: 0.0005% or more and 0.01% or less, Mg: 0.0005% or more, 0.01% or less, Zr: 0.0005% or more; The processability according to any one of the above (1) to (5), comprising 01% or less, REM: 0.0005% or more and 0.01% or less High strength hot rolled steel sheet with excellent fatigue properties.
(7) Further, among all the martensite grains, the ratio of the grains satisfying the following formula in which the average hardness (Hv) of the grains with respect to the C concentration (CM) in the martensite is more than 60%. The high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to any one of (1) to (6) above.
Hv / (− 982.1 × CM ² + 1676 × CM + 189)> 0.80 (1)
(8) Further, any one of the above (1) to (7), wherein the surface of the steel sheet has a hot-dip galvanized layer containing Fe of less than 13%, the balance being Zn, Al and inevitable impurities. A high-strength hot-rolled steel sheet having excellent workability and fatigue properties as described in item 1.
(9) The cast slab is directly or once cooled and then heated to 1100 ° C. or higher, finish rolling is started at 1000 ° C. or higher, and the rolling ratio of the stand before the final stand that performs rolling by finish rolling is over 20%, Rolling at the final stand is performed at an Ar3 transformation point or higher and a rolling rate of more than 5% and less than 40%, and after less than 1.5 seconds from the end of rolling, forced cooling at 60 ° C. or higher is carried out. After passing through natural cooling at the time of passing, it is forcibly cooled to 800 ° C. or lower at an average cooling rate of 30 ° C./s or higher, and is spontaneously released from 800 ° C. or lower and above 600 ° C. for 3 seconds or longer and less than 8 seconds The high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics according to any one of the above (1) to ( 6 ), wherein cooling is performed and forced cooling is again performed to 300 ° C. or less. Production method.
(10) The cast slab is directly or once cooled and then heated to 1100 ° C. or higher, finish rolling is completed at the Ar3 transformation point or higher, and after cooling, the steel sheet wound at 300 ° C. or lower is further heated to an average heating rate of 10 ° C. / in s greater was heated to 650 ° C. or higher, (including 0 sec) after holding less than 8 seconds, any one of the above (1) to (6), characterized in that forced cooling to 550 ° C. or less A method for producing a high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics as described in 1.
In the present invention, forced cooling refers to “actively cooling with gas or liquid, or a mixture thereof”, and natural cooling refers to “a phenomenon that is not actively cooled and generally used in air cooling”. means.

  According to the present invention, a high-strength hot-rolled steel sheet excellent in workability (elongation, hole expansibility) and fatigue characteristics can be provided. is there. 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, improve fuel efficiency and reduce manufacturing costs. Therefore, the industrial value is great.

Image diagram explaining coverage 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, by controlling the arrangement of martensite in DP steel, high fatigue characteristics, which are features of DP, and an improvement in the balance between elongation and hole expansion can be obtained. As a structure, the martensite phase is positively arranged in the ferrite grain boundary, thereby increasing the ferrite grain coverage. As a manufacturing method, there are a method of controlling the recrystallization of austenite by determining the rolling rate at the latter stage of hot rolling, and a method of controlling DP steel once by hot rolling and then by reheating for a short time. 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. Hereinafter, unless otherwise specified,% means mass%.

  C is an element necessary for strengthening martensite, and 0.03% or more is necessary in terms of securing strength. On the other hand, it is an element that affects the workability of steel. As the content increases, the workability deteriorates. In particular, when it exceeds 0.35%, the strengthening ability of martensite is saturated, and carbides (pearlite, cementite) harmful to hole expansibility 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.

  Si is a ferrite forming element, and is an element indispensable for promoting the generation of ferrite during ROT cooling. It also has the effect of suppressing the formation of harmful carbides and improving processability. Although these actions can be replaced by Al, Si is an element that is also effective for securing material strength by solid solution strengthening. From the above, it is desirable to add 0.01% or more, and particularly when 0.1% or more of Al is not added, addition of 0.3% or more is desirable. However, if the amount added is increased, the chemical conversion processability is lowered and the spot weldability is also deteriorated, so 2.0% is made the upper limit.

  As described above, Al is an element effective for suppressing the formation of harmful carbides and increasing the ferrite fraction and improving the elongation, similar to Si. 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, in a low Si system, chemical conversion can be improved without degrading ductility by adding a large amount of Al. 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 0.01% or more is necessary for sufficient deoxidation.

  Mn is an element necessary for ensuring the strength, and it is necessary to add at least 0.3%. 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 is 0.10% or less. Especially when corrosion resistance is not a problem, 0.03% or less is desirable with emphasis on workability. Since it takes cost to reduce P, the lower limit is made 0.001% from the viewpoint of removal P cost.

  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 0.05% 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. Due to the above restrictions, the content is set to 0.010% or less. From the viewpoint of cost reduction, the lower limit is made 0.0005% or more.

  Furthermore, you may contain the following elements as needed.

  Ti, Nb, and V 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 one or more. At this time, at least 0.005% of Ti and Nb needs to be added, and 0.01% of V needs to be added. However, if these additions become excessive, the ductility deteriorates due to precipitation strengthening. Therefore, the upper limit is set to 0.3% or less for Ti and Nb, and V is set to 0.5%. 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 1 type (s) or 2 or more types. At this time, 0.0005% or more of each element needs to be added. 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 of each addition amount is set to 0.01%.

  Cu is an element that improves the corrosion resistance when combined with P. To obtain this effect, it is desirable to add 0.05% or more. However, the addition of a large amount increases the hardenability and lowers the ductility, so the upper limit is made 3.0%.

  Ni is an essential element for suppressing hot cracking when Cu is added. In order to obtain this effect, it is desirable to add 0.05% or more. However, a large amount of addition, like Cu, increases the hardenability and decreases the ductility, so the upper limit is made 3.0%.

  Mo is an element effective for suppressing the formation of cementite and improving the hole expansibility. To obtain this effect, 0.05% or more must be added. However, since Mo is also an element that enhances hardenability, the ductility is lowered when excessively added, so the upper limit is made 1.0%.

  Cr, like V, forms carbides and contributes to securing strength. In order to obtain this effect, addition of 0.05% 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 3.0%.

  B, like Mn, is an element that contributes to strength. To obtain this effect, 0.0001% or more must be added. 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 0.1%.

  Next, the crystal structure of the steel sheet of the present invention will be described.

  In general, when a martensite phase is introduced into a structure and a composite structure such as dual phase steel is used, both strength and ductility can be achieved at a high level. However, although there have been many studies related to the phase fraction and size of martensite, there have been few examples of examining the possibility of material improvement by actively utilizing the dispersion state (that is, arrangement) of martensite. Therefore, as a result of intensive research by researchers focusing on this point, a method of controlling recrystallization by rolling after hot rolling, or reheating after forming a DP structure by hot rolling Therefore, it is possible to create a martensitic phase arrangement different from the conventional one, and this arrangement can dramatically improve the elongation, fatigue characteristics, and hole expandability of the steel sheet compared to the conventional DP steel. I found out that it would be a steel plate.

  Martensite forms a composite structure with ferrite and can improve elongation and fatigue properties. This technique is intended to further improve the elongation, hole expansibility, and fatigue characteristics by controlling the existence state of this martensite, and it is necessary to contain a martensite phase with an area fraction exceeding 5%. In particular, when high strength-ductility is desired, it is desirable to contain more than 10%. However, since an increase in martensite phase causes a significant decrease in elongation, the upper limit of the martensite area fraction is preferably less than 60%. In particular, when the required elongation value is high, less than 50% is desirable. The remainder of the structure contains a ferrite phase from the viewpoint of securing elongation. The ferrite phase should be high, and the area fraction needs to exceed 20%. In particular, it is desirable to contain more than 40% when the demand for elongation is severe. In addition, pearlite is preferable to be less because the decrease in elongation and the decrease in hole expansibility are remarkable. In order to minimize this deterioration, the volume fraction of the pearlite phase is less than 10%. In the present invention, the effect is not changed even if a bainite or residual austenite phase is contained in addition to ferrite, martensite and pearlite.

  In the present invention, one of the most important features is the martensite arrangement. In the present invention, martensite is arranged so as to surround ferrite grains. At this time, the ratio of the portion of the ferrite grain boundary occupied by the martensite grains to the total ferrite grain boundary length is defined as the coverage (see FIG. 1). If this coverage exceeds 30%, the martensite connectivity increases, and the elongation, hole expansibility and fatigue strength are improved. However, it is desirable that the coverage is 50% or more for severe stretch flange processing, hole expansion processing, and bending processing.

  In order to achieve this arrangement, the martensite size should be small. This is because coarse martensite has a relatively high martensite phase fraction in order to increase the coverage, thereby causing a decrease in elongation. For this reason, the average particle size of martensite is preferably less than 4 μm. In particular, when there are many hole expanding processes or when strict molding is forced, it is desirable to make it less than 3 μm. On the other hand, if the particle size is too small, the martensite / martensite grain boundary is easily cut against deformation, and the hole expandability and local ductility are reduced. In order to suppress this, the average particle size of martensite is set to more than 1 μm.

Furthermore, from the viewpoint of fatigue strength, the hardness of the martensite phase that suppresses the propagation of cracks is an important factor. Since the hardness of martensite varies depending on the C concentration, when the average C concentration (CM) in the martensite and the average hardness (Hv) of the martensite particles are used, martensite particles satisfying the formula (1) are obtained. When it is 60% or less of all martensite grains, the fatigue limit ratio decreases and YR increases, which is not preferable.
Hv / (− 982.1 × CM ² + 1676 × CM + 189)> 0.80 (1)

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.
Any method can be used as long as the tissue fraction can be accurately measured. For example, the steel plate is subjected to repeller etching or nital etching, and the structure at a position of 1/4 t in the cross section in the hot rolling direction is subjected to an optical microscope or SEM. The phase fraction is determined and the area fraction of each phase is measured using an image analyzer or the like.

  The hardness of martensite grains is basically measured by Vickers measurement. However, when the particle size is small and the Vickers hardness cannot be obtained, 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.

  The measurement of the martensite size and coverage must be quantified by selecting a visual field at a position of a thickness of 1/4 t at random and measuring at least 10 visual fields and 500 martensite grains.

  Next, the plating layer will be described.

  When corrosion resistance is desired, a galvanized layer composed of Fe, Zn, Al, and inevitable impurities can be provided on the surface of the steel sheet. At this time, the amount of Fe is limited from the viewpoint of plating adhesion, and the upper limit is made less than 13%. Exceeding this will impair the adhesion of the plating layer itself, causing the plating layer to break or drop off during processing and adhere to the mold, causing defects during molding.

  When spot weldability or paintability is desired, these characteristics can be enhanced by alloying treatment. Specifically, after immersion in a hot dip galvanizing bath, an alloying treatment is performed, whereby Fe is taken into the plating layer, and a high-strength hot dip galvanized steel sheet excellent in paintability and spot weldability can be obtained. . If the Fe content after alloying is less than 7%, spot weldability is insufficient.

  Further, when the alloying treatment is not performed, even if the amount of Fe in the plating layer is less than 7%, the corrosion resistance, the formability and the hole expandability, which are the effects excluding spot welding obtained by alloying, are good. At this time, the amount of Fe includes 0%.

The plating adhesion amount is not particularly limited, but is preferably 5 g / m ² or more in terms of single-sided adhesion from the viewpoint of corrosion resistance. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.

  Next, a manufacturing method will be described.

  The high-strength hot-rolled steel sheet of the present invention is firstly heated to 1100 ° C. or higher after the cast slab having the components of the present invention is directly or once cooled, and finish rolling is started at 1000 ° C. or higher. The rolling rate of the stand before the final stand to be implemented is over 20%, the rolling at the final stand is performed at the Ar3 transformation point or more, the rolling rate is over 5% and less than 40%, and after the end of rolling is less than 2 seconds. Then, forced cooling of at least 50 ° C. or more is carried out, followed by forced cooling to 800 ° C. or less at an average cooling rate of more than 25 ° C./s, from 800 ° C. or less, over 600 ° C. to over 2 seconds, less than 10 seconds In addition, the crystal structure of the hot-rolled steel sheet contains a martensite phase of more than 5% and a ferrite phase of more than 20% in terms of area fraction. Perlite phase is less than 10% , The ferrite grains coverage by martensite phase can be 30 percent (hereinafter referred to as "first method".).

  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 reheating is less than 1100 ° C., both homogenization and dissolution become insufficient, resulting in a reduction in strength and workability. On the other hand, when the temperature exceeds 1300 ° C., the production cost and productivity are lowered, and the initial austenite particle size is increased, so that it tends to be finally mixed. Therefore, it is necessary to set the temperature to 1100 ° C. or higher, and preferably less than 1300 ° C.

  Next, finish rolling start temperature shall be 1000 degreeC or more. For refining hot rolling, recrystallization needs to occur repeatedly before the finish rolling. Below this temperature, recrystallization is insufficient and final martensite refinement becomes difficult.

  The finish rolling finish temperature is Ar3 or higher. Below this temperature, the processed ferrite remains and the elongation is significantly reduced.

  The coverage can be increased by increasing the recrystallization rate after rolling on the stand before the final stand and before the final stand. In order to achieve this, the rolling rate in this stand (the stand before the final stand) is set to more than 20%.

  The rolling rate of the final stand is more than 5% and less than 40%. A higher rolling reduction is more effective for refinement of martensite. If it is 5% or less, it becomes a mixed grain structure by partial recrystallization, so that the coverage ratio is lowered and elongation and fatigue properties are also lowered. On the other hand, if it exceeds 40%, the martensite becomes too small or more likely to be extended, and the hole expandability and ductility deteriorate. In addition, the definition of the last stand in this invention means the stand located last among the stands which are rolling 2% or more. For example, even if it is located at the end, if the stand is only rolled under light pressure (less than 2%) to the extent of draining, the stand before that is set as the final stand.

  Forced cooling is performed as soon as possible after rolling. During the period from processing to forced cooling, grain growth occurs and both ferrite and martensite grains generated by transformation tend to be coarse. Moreover, since the loss of transformation driving force can be suppressed by rapid cooling, the ferrite structure becomes fine and the coverage tends to be high. In order to obtain this effect, forced cooling is started within 2 seconds after rolling, and the temperature is lowered by at least 50 ° C. or more. After this, when passing through the measuring instrument zone of the hot rolling facility, it is naturally cooled, but after passing the measuring instrument zone, it is necessary to forcibly cool to 800 ° C. or less at an average cooling rate of more than 25 ° C./s. is there. The shorter the start of cooling, the better. However, in practice, it is set to 0.05 seconds or more due to the limitation of the arrangement of the cooling equipment. When the cooling rate is 25 ° C./s or less or the stop temperature exceeds 800 ° C., a loss of transformation driving force occurs.

  After forced cooling to 800 ° C. or lower, natural cooling is started in the region of 800 ° C. or lower and higher than 600 ° C. for more than 2 seconds and less than 10 seconds. Ferrite formation occurs during this time, and C concentration to austenite occurs due to C diffusion. The ductility is improved by the formation of this ferrite, and C concentrated to austenite is important because it changes to martensite by subsequent forced cooling. If the natural cooling start temperature exceeds 800 ° C., the ferrite rate cannot be sufficiently obtained, the grains become too large, and the final martensite grains exceed the specified size. When the natural cooling start temperature is 600 ° C. or less or the natural cooling time is 2 seconds or less, a predetermined ferrite fraction cannot be obtained, and the martensite ratio also increases. On the other hand, if the natural cooling time exceeds 10 seconds, the formation of pearlite lowers the elongation and fatigue characteristics, and the martensite grains become smaller. From the viewpoint of suppressing the formation of pearlite, it is desirable to set it to 7 seconds or less.

  In order to cause martensitic transformation of C-enriched austenite, it is naturally cooled and then forcedly cooled to 300 ° C. or lower and wound. The cooling rate at this time is desirably 10 ° C./s or more on average. When the winding temperature exceeds 300 ° C., self-tempering of martensite occurs, and elongation and fatigue characteristics are deteriorated.

In the first method of the present invention, when the natural cooling time is 3 seconds or more, the area fraction of the martensite phase can be less than 60%. In the first method of the present invention, the natural cooling time is 3 seconds or more, the time from immediately after rolling to the start of forced cooling is less than 1.5 seconds, and the forced cooling amount immediately after rolling is 60 ° C. or more to 800 ° C. When the cooling rate is 30 ° C./s or more , all martensite grains are less than 4 μm, and the coverage is further improved. On the other hand, when the natural cooling time is less than 8 seconds, the martensite grains exceed 1 μm. Further, in the first method of the present invention, when the coiling temperature is less than 200 ° C., the ratio of martensite satisfying the formula (1) is more than 60%.

  Secondly, the high-strength hot-rolled steel sheet of the present invention can be achieved by reheating. That is, the cast slab having the components of the present invention is directly or once cooled and then heated to 1100 ° C. or higher, finish rolling is completed at the Ar3 transformation point or higher, and after cooling, the steel sheet wound at 300 ° C. or lower is further added. Crystal structure of hot-rolled steel sheet by heating to 650 ° C or higher at an average heating rate of 5 ° C / s or higher, holding for less than 10 seconds (including 0 seconds), and forcibly cooling to 600 ° C or lower (reheating) However, the area fraction contains a martensite phase exceeding 5% and a ferrite phase exceeding 20%, the pearlite phase is less than 10%, and the coverage of ferrite grains by the martensite phase can be more than 30%. (Hereinafter referred to as “second method”).

  When creating a structure by reheating, the minimum requirement is that the hot rolling heating condition and the finish rolling finish temperature satisfy the above-mentioned ranges. However, since the formation of martensite is performed during the reheating process, the winding temperature is set to 300 ° C. or lower. When it exceeds 300 degreeC, the martensite production amount at the time of a reheating process will fall.

  As reheating conditions, it is necessary to heat at an average heating rate of 5 ° C./s or more. Below this, the martensite size tends to increase.

  In reheating, the heating temperature is set to 650 ° C. or higher. If the temperature is lower than 650 ° C., the reverse transformation of martensite formed by hot rolling into austenite does not occur, and martensite having a predetermined hardness cannot be obtained. The holding time after reaching the above temperature is less than 10 seconds (including 0 seconds). As the holding time becomes longer, the martensite becomes coarser and agglomerated, and the coverage decreases. In forced cooling, it is desirable to set it as 10 degreeC s / s or more, and it is necessary to perform forced cooling to 600 degrees C or less. If this temperature is exceeded, the tissue cannot be frozen, and similarly, the covering rate decreases due to coarsening and agglomeration. In order to further improve the stretch-hole expansibility balance, it is desirable to forcibly cool to 500 ° C. or lower at a cooling rate of 30 ° C./s or higher.

  Since the two hot-rolling recrystallization controls and the array control by reheating have an additive effect, a higher coverage can be obtained by executing them simultaneously.

  After cooling, the effect of the present technology is lost even if the steel sheet is immersed in a plating bath for the purpose of providing plating to the surface layer, and then alloyed in a temperature range of 600 ° C. or lower. Absent.

In the second method of the present invention, when the reheating retention time is less than 8 seconds and the reheating rate is more than 10 ° C./s, the martensite grains are less than 4 μm, and the coverage is improved. On the other hand, when the forced cooling stop temperature after reheating is 550 ° C. or less, the martensite grains exceed 1 μm. Further in the second method of the present invention, when the reheating temperature is 680 ° C. greater than the proportion of grains satisfying the formula (1) is 60 percent.

  Next, this invention is demonstrated based on an Example.

  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 the components according to the present invention, steels a and d are addition amounts of C, steels b are addition amounts of Mn and P, steels c are addition amounts of Nb, steels e are S The amount of steel added is N, the amount of Ti added, and the amount of Ca is outside the scope of the present invention. For example, “A1” in the steel codes in Table 2 and later means that the steel code in Table 1 is the first example. In Tables 1 to 5, numerical values and symbols outside the scope of the present invention are underlined.

  These steels were hot-rolled and ROT-cooled under the conditions shown in Table 2. 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. Some slabs were hot rolled and reheated under the conditions shown in Tables 3 and 4.

  Tables 2, 3 and 5 show the structural fraction of the obtained steel sheet, the C concentration in the martensite, the martensite particle size, the martensite ratio satisfying the formula (1), and the coverage. The mechanical characteristics and fatigue characteristics are shown in Table 2, Table 3, Table 5, and FIGS. The fatigue characteristics are expressed as a fatigue limit ratio (= fatigue strength / tensile strength), and 0.5 or less was inferior. All the plated materials satisfy the plating characteristics of claim 9.

  Groups satisfying the production methods of claims 10 and 11 of the present invention (invention steels 1 and 2) exhibit superior properties compared to comparative steels. Furthermore, the invention steel 3 satisfying the production method of claim 12 has a higher coverage than the invention steels 1 and 2, and exhibits high elongation and hole expansibility. On the other hand, comparative steels outside the scope of the present invention are inferior to the inventive steels in terms of elongation, hole expansibility and fatigue characteristics. That is, it can be seen that only those satisfying the present invention provisions can have both excellent workability (elongation and hole expandability) and fatigue characteristics.

  The tensile test conformed to JIS Z2241, and the hole expansion test conformed to JIS Z2256. Moreover, about the fatigue test, it carried out by the plane bending test and was based on JISZ2275.

Claims (10)

% By mass
C: 0.03% or more, 0.35% or less,
Si: 0.01% or more, 2.0% or less,
Mn: 0.3% or more and 4.0% or less,
P: 0.001% or more, 0.10% or less,
S: 0.0005% or more, 0.05% or less,
N: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, 2.0% or less,
And the balance is composed of Fe and inevitable impurities, and the crystal structure contains 10% or more of martensite phase, 38% or more of ferrite phase in area fraction, and less than 10% of pearlite phase, martensite A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics, characterized in that the coverage of ferrite grains by phase is more than 30%, and the average grain size of martensite grains is more than 1 μm and less than 4 μm.
Here, the coverage of the ferrite grains by the martensite phase is the percentage ratio of the ferrite grain boundary portion occupied by the martensite grains when the total ferrite grain boundary length is 100. is there.   Furthermore, the area fraction of a martensite phase is less than 60%, The high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to claim 1. Furthermore, Cr: 0.05% or more and 3.0% or less in mass% in steel,
Mo: 0.05% or more, 1.0% or less Ni: 0.05% or more, 3.0% or less,
Cu: 0.05% or more, 3.0% or less,
The high-strength hot-rolled steel sheet excellent in workability and fatigue properties according to claim 1 or 2, characterized by containing at least one of the following. Furthermore, in steel,
Nb: 0.005% or more, 0.3% or less,
Ti: 0.005% or more, 0.3% or less,
V: 0.01% or more, 0.5% or less,
The high-strength hot-rolled steel sheet excellent in workability and fatigue properties according to any one of claims 1 to 3, wherein the high-strength steel sheet is excellent in workability and fatigue characteristics. Furthermore, in steel,
B: The high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to any one of claims 1 to 4, characterized by containing 0.0001% or more and 0.1% or less. Furthermore, in steel,
Ca: 0.0005% or more, 0.01% or less,
Mg: 0.0005% or more, 0.01% or less,
Zr: 0.0005% or more, 0.01% or less,
REM: 0.0005% or more, 0.01% or less,
The high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to any one of claims 1 to 5, characterized by containing one or more of the following. Furthermore, among all the martensite grains, the ratio of grains satisfying the following formula with respect to the C concentration (CM) in the martensite is more than 60%. A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to any one of claims 1 to 6.
Hv / (− 982.1 × CM ² + 1676 × CM + 189)> 0.80 (1)   Furthermore, it has less than 13% of Fe on the surface of a steel plate, and the remainder has a hot dip galvanized layer which consists of Zn, Al, and an unavoidable impurity, The any one of Claims 1-7 characterized by the above-mentioned. High-strength hot-rolled steel sheet with excellent workability and fatigue characteristics. The cast slab is directly or once cooled and then heated to 1100 ° C or higher, finish rolling is started at 1000 ° C or higher, and the rolling ratio of the stand before the final stand where the rolling is performed by finish rolling is over 20%. The rolling is performed at an Ar3 transformation point or higher and the rolling rate is higher than 5% and lower than 40%. After the rolling is finished for less than 1.5 seconds, forced cooling is performed at 60 ° C. or higher. After natural cooling, forced cooling to 800 ° C or lower at an average cooling rate of 30 ° C / s or higher is performed, and natural cooling is provided for temperatures of 800 ° C or lower and over 600 ° C for 3 seconds or longer and less than 8 seconds. Furthermore, forced cooling to 300 degrees C or less is performed again, The manufacturing method of the high strength hot-rolled steel plate excellent in workability and fatigue characteristics of any one of Claims 1-6 characterized by the above-mentioned. The cast slab was heated directly or temporarily 1100 ° C. or higher after cooling, to complete the finish rolling at Ar3 transformation point or higher, after cooling, to the wound steel sheet at 300 ° C. or less, further, the average heating rate 10 ° C. / s greater at heated to 650 ° C. or higher, (including 0 sec) after holding less than 8 seconds, and workability according to any one of claims 1 to 6, characterized in that forced cooling to 550 ° C. or less A method for producing high-strength hot-rolled steel sheets with excellent fatigue properties.