JP4747473B2 - Hot-rolled steel sheet and hot-dip galvanized steel sheet excellent in stretch flangeability and methods for producing them - Google Patents

Description

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【発明の効果】
本発明の熱延鋼板および溶融亜鉛めっき鋼板は、大きな伸びフランジ加工性を有する中強度の高成形性鋼板であるから、サイドメンバーなどの耐衝突部品として自動車車体の軽量化に寄与する。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel sheet excellent in workability, particularly stretch flangeability, which is mainly used for automobile bodies, a hot-dip galvanized steel sheet using the hot-rolled steel sheet, and a method for producing them.
[0002]
[Prior art]
Although many thin steel plates are used for automobile bodies, there are parts and applications that require extremely severe flangeability, even though they are relatively light bending and roll forming. For example, it is a reinforcing member for a front side member, a rear side member, a cross member, and components connected thereto. These parts and applications have a high strength level, a tensile strength that ensures shape freezing property is 540 MPa or less, and a hole expansion rate of 80% or more showing excellent flange workability. This is a rough guide.
[0003]
In many cases, cold rolled steel sheets have been used for the above parts and applications.
However, since a cold-rolled steel sheet requires steps of cold rolling and annealing as compared with a hot-rolled steel sheet, it is generally economically advantageous to use a hot-rolled steel sheet. Therefore, at least when the appearance is not required to be beautiful, hot-rolled steel sheet is used, but the hot-rolled steel sheet is inferior in stretch flange workability compared to cold-rolled steel sheets having almost the same composition. There are problems such as large in-plane anisotropy of mechanical properties and poor elongation characteristics.
[0004]
In order to solve these problems, techniques such as reduction of C content and reduction of S content in hot-rolled steel sheets have been proposed, but it is still difficult to improve in-plane anisotropy of mechanical properties. It was.
[0005]
In addition, a technique for improving the stretch flange characteristics of a hot-rolled steel sheet by adding elements such as Nb and Ti to refine the structure has also been proposed, but these added elements reduce the hot deformation resistance of steel. In order to increase greatly, in the case of a thin steel plate having a plate thickness of approximately 2.3 mm or less, there is a problem that hot rolling becomes difficult. At the same time, precipitation strengthening occurs due to these additive elements, and it has been difficult to produce a highly formable steel sheet having a relatively low strength with a tensile strength (TS) of 540 MPa or less. Furthermore, the addition of elements such as Nb and Ti has a problem of increasing the in-plane anisotropy of the mechanical properties of the steel sheet.
[0006]
In this way, it is possible to clear the deep drawability as an automobile part, but there is a technology for industrially inexpensively and stably manufacturing a steel sheet having a very high level of workability from another viewpoint of stretch flangeability. There wasn't.
[0007]
In addition, in a high-strength steel plate with a plate thickness> 3 mm and a TS of approximately 540 MPa, the stretched flange workability is improved by refining the matrix structure and making the structure single-phase, making it an automobile undercarriage part. The technology to be used has been proposed but has not yet reached a sufficient level.
[0008]
[Problems to be solved by the invention]
The present invention solves the above-described problems, and has a medium strength of TS of 540 MPa or less, but has a very excellent stretch flangeability, and a hot-dip galvanized steel sheet using the hot-rolled steel sheet as a raw sheet And a manufacturing method thereof. In addition, the plate | board thickness of the object product is a thin steel plate of about 2.3-0.8 mm in general.
Further, in order to ensure stable moldability, it is desired that the variation (variation) in characteristics such as YS, TS, EL, etc. is small, but this requirement is also met.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventor manufactured steel plates having various components and manufacturing methods and conducted many material evaluation experiments. As a result, in order to realize such a high stretch flangeability, it is important to control the hot rolling conditions and the microstructure through them together with the component composition. It was clarified that the reduction of the amount of S is important, that the control of the microstructure is to make 90% or more a uniform and fine ferrite structure, and the form of the second phase is important.
[0010]
In addition to the conventionally used strengthening elements Si, Mn and P, it has been clarified that positive utilization of N is advantageous from the viewpoint of improving stretch flangeability. In addition, it is possible to stably produce a steel sheet having excellent stretch flangeability by finishing rolling at a higher rolling temperature, cooling it, and winding it at a lower winding temperature compared to the conventional case. Clarified what can be done.
[0011]
Therefore, the present invention is, in mass%, C: 0.01 to 0.08%, Si: 0.2% or less, Mn: 0.05 to 0.80 %, P: 0.04% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.003 to 0.0250%, the balance consisting of Fe and inevitable impurities, The structure fraction of the ferrite phase as the main phase is 90% or more in area ratio, the structure fraction of the second phase is 10% or less in area ratio, and the shape ratio of the second phase in the rolling direction and the plate thickness direction Having a structure in which the average particle diameter of the ferrite is 20 μm or less and the average particle diameter of the second phase is 10 μm or less, and the tensile strength is 540 MPa or less and the heat is excellent in stretch flangeability. It is a rolled steel sheet.
[0012]
It is preferable that the hot-rolled steel sheet further contains 1.0% by mass or less of one or more selected from the group consisting of Cu, Ni, Cr and Mo.
[0013]
Moreover, it is preferable that a hot-rolled steel plate contains 0.0010-0.01 mass% in total of 1 type, or 2 or more types chosen from the group which consists of Ca and REM (rare element) further.
[0014]
Moreover, this invention is a hot dip galvanized steel sheet characterized by providing a hot dip galvanized layer on the surface of the hot rolled steel sheet.
[0015]
Moreover, this invention is mass%, C: 0.01-0.08%, Si: 0.2% or less, Mn: 0.05- 0.80 %, P: 0.04% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.003 to 0.0250% And the balance consists of Fe and inevitable impurities The steel slab is hot-rolled at a slab heating temperature of 1000 ° C. or higher and a finish rolling exit temperature of 860 ° C. or higher, and then cooled to an average cooling rate up to 650 ° C. of 40 ° C./s or more, and a coiling temperature of 550 ° C. The following is a method for producing a hot-rolled steel sheet having excellent stretch flange workability, which is wound around a coil.
[0016]
Moreover, this invention is mass%, C: 0.01-0.08%, Si: 0.2% or less, Mn: 0.05- 0.80 %, P: 0.04% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.003 to 0.0250% And the balance consists of Fe and inevitable impurities The steel slab is hot-rolled at a slab heating temperature of 1000 ° C. or higher and a finish rolling exit temperature of 860 ° C. or higher, and then cooled to an average cooling rate up to 650 ° C. of 40 ° C./s or more, and a coiling temperature of 550 ° C. The following is a method for producing a hot-dip galvanized steel sheet having excellent stretch flange workability, which is wound around a coil and heated to a temperature of 900 ° C. or lower to perform hot-dip galvanizing.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The component composition of the steel plate of the original plate of the present invention is as follows in mass%.
C: 0.01 to 0.08% or less;
If C exceeds 0.08%, the stretch flangeability of the steel sheet is significantly deteriorated due to an increase in the fraction of carbides in the steel, and spot weldability, arc weldability, etc. are also significantly reduced. . Therefore, the C content is 0.08% or less, but 0.05% or less is preferable from the viewpoint of improving moldability. When particularly good ductility is required, 0.03% or less is more preferable. On the other hand, when C is less than 0.01%, the microstructure is significantly coarsened and stretch flangeability is deteriorated. In particular, the undesirable effect of increasing the dependence of the shear end face on the finishing condition appears.
[0018]
Si: 0.2% or less;
Since Si has the effect of solid solution strengthening the steel sheet without adversely affecting the stretch flange workability or improving it, the addition amount may be adjusted as appropriate according to the desired strength. However, since a large amount of addition significantly reduces the hot rolling property, it is necessary to make it 0.2% or less. Further, in the case of hot dip galvanized steel sheet, excessive addition may cause non-plating (a situation where a part of the steel sheet is repelled and partially cannot be plated). 2% or less is preferable and 0.05 to 0.15% is more preferable.
[0019]
Mn: 0.05 to 1.5%:
Mn is an effective element for preventing hot cracking due to S, and needs to be 0.05% or more. Mn also acts to refine crystal grains. From the viewpoint of stably fixing S, 0.2% or more is desirable. By increasing the amount of Mn added, there is a great advantage that the sensitivity of the mechanical properties of the steel sheet to the change in hot rolling conditions is remarkably improved, so 0.5% or more is more desirable. However, when Mn is added excessively, the detailed mechanism is unknown, but it tends to increase the hot deformation resistance of the steel sheet, which is not preferable. Furthermore, the weldability and the formability of the welded portion tend to deteriorate. Further, since the formation of ferrite is remarkably suppressed, the ductility is remarkably lowered. From the above, the upper limit is 1.5%. When the content is 0.80% or less, better corrosion resistance and moldability can be obtained.
[0020]
P: 0.04% or less;
P is effective as a solid solution strengthening element for steel, and can be appropriately added depending on the desired steel plate strength. In order to acquire the said effect, 0.001% or more of containing is preferable. However, if added excessively, the steel tends to become brittle and further segregate, so that the stretch flangeability of the steel plate is deteriorated and the welded portion becomes brittle. From the above, the upper limit is 0.04%. In addition, when these characteristics deterioration is regarded as important especially, it is preferable to set it as 0.02% or less.
[0021]
S: 0.005% or less:
S is an element that exists as an inclusion, reduces the ductility of the steel sheet, and further deteriorates the corrosion resistance. Therefore, it is preferable to reduce it as much as possible, but it is acceptable up to 0.005%. In particular, when good workability is required, the content is desirably 0.003% or less.
[0022]
Al: 0.10% or less;
Al is added as a deoxidizing element for steel and is a useful element for improving the cleanliness of steel. Moreover, addition is also desirable for refining the structure of steel, and a content of 0.003% or more is desirable. However, even if it is added in a large amount exceeding 0.10%, the effect of refining the structure is saturated, and the cost is increased by the amount of the alloy to be added. Further, from the viewpoint of material stability, 0.02 to 0.06% is more desirable. On the other hand, there is a concern that the reduction of the Al addition amount leads to the coarsening of the crystal grains, but in the present invention, other alloy elements such as Mn are limited to the optimum amount and the hot rolling conditions are made the optimum range. This can be prevented.
[0023]
N: 0.003-0.0250%;
N is an important additive element in the present invention. In other words, by adding N to control the production conditions of the steel sheet, a structure excellent in stretch flange characteristics mainly composed of uniform and fine ferrite can be obtained, and this has no adverse effect on weldability. Excellent stretch flangeability can be obtained. N also has the effect of lowering the transformation point of steel, and its addition is particularly effective in situations where it is not desired to perform rolling with a thin material that greatly cuts the transformation point. As one of the features, even when N is added, the increase in hot deformation resistance is extremely small, and when trying to manufacture a hot rolled sheet steel as in the present invention, it is particularly advantageous in workability of the hot rolling process. It is.
[0024]
In order to obtain the effect balanced with the strength, N needs to be added in an amount of 0.003% or more. However, if added over 0.0250%, the rate of occurrence of internal defects in the steel sheet such as blow holes increases, and the occurrence of slab cracking during continuous casting becomes significant. Note that the range of 0.0050 to 0.0170% is more preferable from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. Even if N is added, there is no problem of aging deterioration of the mechanical properties at room temperature within the scope of the present invention.
[0025]
The amount of elements to be selectively added to steel is described below.
Total of one or more selected from Cu, Ni, Cr and Mo: 1.0% or less;
These may be added singly, and composite addition does not offset the desirable effect as a solid solution strengthening element. By adding these elements, uniform and refinement of the ferrite crystal grain size can be advantageously achieved, and the strength of the steel sheet as a product can be increased. The lower limit of the addition amount of each element preferable for obtaining this effect is Cu: 0.01%, Ni: 0.01%, Cr: 0.01% and Mo: 0.01%. However, by adding these, as a side effect, the increase in hot deformation resistance when hot rolling the steel sheet becomes significant, the deterioration of the chemical conversion property and the surface treatment characteristics in a broader sense becomes remarkable, and further, welding Decrease in weld formability resulting from the hardening of the part is also significant. Since these exhibit the same behavior whether they are added alone or in combination, the upper limit is set as the total content of Cu, Ni, Cr and Mo, and is 1.0% or less.
[0026]
Total of one or more of Ca and REM: 0.0010 to 0.010%;
Ca and REM are effective in controlling the form of inclusions and improving stretch flangeability, and in order to obtain this effect, the total of one or more of Ca and REM is 0.0010% or more. It is preferable to contain. However, if the total amount of Ca and REM added exceeds 0.010%, surface defects are likely to occur, so the total added amount is set to 0.010% or less.
Note that REM preferably uses one or more of La, Ce, Pr, Nd, and Y from the standpoint of a remarkable effect of making inclusions into fine spheres and uniformly dispersing them.
[0027]
Next, the structure, thickness and mechanical properties of the steel sheet will be described below.
Area ratio of ferrite phase: 90% or more, average crystal grain size of ferrite phase: 20 μm or less;
The present invention is intended for automotive steel sheets that require a high degree of stretch flange workability. When ferrite is less than 90% in area ratio, it becomes difficult to ensure the necessary stretch flange workability. 90% or more. Furthermore, when higher stretch flangeability is required, a ferrite fraction of 95% or more is desirable. Further, in order to ensure good stretch flangeability, which is a feature of the present invention, the ferrite needs to have a fine structure with an average crystal grain size of 20 μm or less, preferably 15 μm or less.
In addition to so-called polygonal ferrite, which does not include distortion due to processing, ferrite also does not include carbide precipitation, bainitic ferrite and acicular ferrite also have moderate strength increases, but these are also stretch flangeability Therefore, it is a desirable structure, so it falls within the category of ferrite.
[0028]
The other phase (second phase) other than ferrite, which is the main phase, can be pearlite, bainite, martensite, retained austenite, cementite, etc. Just decide. When the fraction of the second phase exceeds 10% in terms of area ratio, the area ratio of the ferrite phase cannot be secured, so the area ratio is set to 10% or less. In the present invention, from the viewpoint of the balance between strength and ductility, it is preferable that the second phase has a structure with an area ratio of 2 to 5%.
[0029]
The shape ratio between the rolling direction and the plate thickness direction of the second phase is 3 or less, the average particle size of the second phase is 10 μm or less; the shape of the second phase is also an important factor. In the case of a hot-rolled steel sheet, unlike the cold-rolled steel sheet, the number of processes in which recrystallization can occur is less by one step, and therefore, the distribution form of the second phase tends to be layered (or strip-shaped). In this case, it has been found that, along the layered structure, the occurrence of initial cracks or the propagation of cracks during processing occurs during stretch flange molding.
[0030]
The composition and hot rolling conditions were varied widely to vary the structure, particularly the shape ratio and particle size (equivalent circle diameter) of the second phase structure, and the stretch flangeability was investigated. As a result, it has been clarified that when the shape ratio in the rolling direction of the second phase (also referred to as the L direction) and the thickness direction exceeds 3, the stretch flangeability is significantly reduced. Here, the shape ratio of the second phase in the rolling direction and the sheet thickness direction refers to [second phase crystal grains (maximum diameter in the rolling direction) / (maximum diameter in the sheet thickness direction)]. In addition, it is desirable that not only the extension in the rolling direction but also the extension in the direction orthogonal to the rolling direction (also referred to as the c direction) is small, but the definition of the shape ratio between the rolling direction and the plate thickness direction is 3 or less. It is clear that sufficient stretch flangeability can be obtained by making the range preferably from 1 to 2, so that it is only necessary to consider the shape ratio of the rolling direction of the second phase and the thickness direction as described above. became.
Moreover, even when they existed at the same fraction, it has been found that when the average particle size of the second phase is 10 μm or less, preferably 8 μm or less, a remarkable improvement in stretch flangeability is achieved.
[0031]
In the present invention, when applied to a hot-rolled steel sheet having a medium strength of TS of 540 MPa or less, the excellent characteristics are remarkably exhibited. In steel plates with higher strengths above 540 MPa, it is relatively easy to refine the structure, so it is relatively easy to improve the hole expandability, but in addition to the reduction in ductility, the shape freezeability is significantly reduced. .
[0032]
Further, in the present invention, the stretch flange characteristic of the steel sheet is evaluated by hole expansion workability (hole expandability), which corresponds extremely well to the occurrence of defects in stretch flange cracks occurring in actual parts as described below. Confirmed to be an evaluation method.
That is, it is performed in accordance with the Japan Iron and Steel Federation standard (JFS T1001), the initial hole diameter is 10 mm, and the hole diameter D is measured when the crack penetrates the plate thickness, and (D-10) / 10 × 100 (% If the hole expansion workability calculated in step) is 80% or more, it can be confirmed that press forming is possible without causing problems such as stretch flange cracking for the parts targeted by the present invention. It was. Desirable is 100% or more.
One of the features of the present invention is that a stable and high hole expansion workability can be obtained even if the clearance during punching changes within a wide range.
[0033]
The thickness of the steel plate is not particularly limited, and the effect of the present invention is exhibited regardless of whether the steel plate is thick or thin. However, when applied to a thin hot-rolled steel sheet having a sheet thickness of 2.3 mm or less, the improvement effect becomes remarkable.
[0034]
In addition, even when a hot-dip galvanized layer is applied to the surface of a hot-rolled steel sheet having the composition and structure as described above, the effect of the plated layer on the properties of the present invention is small, and the strength level equivalent to that of the hot-rolled steel sheet And good stretch flangeability. Here, the hot dip galvanized layer includes a so-called alloyed hot dip galvanized layer obtained by alloying after hot dip galvanizing.
Also, electroplating such as electrogalvanization has little influence on the mechanical properties of the plating itself, and there is no problem as an electroplated steel plate using the hot-rolled steel plate of the present invention as a plating base plate.
[0035]
The hot-rolled steel sheet of the present invention is manufactured as follows.
The slab is desirably produced by a continuous casting method to prevent macro segregation of components, but can also be produced by an ingot casting method or a thin slab casting method. Also, after manufacturing the slab, in addition to the conventional method of once cooling to room temperature and then heating again, without cooling, by inserting into a heating furnace as a warm piece, or after performing a slight heat retention, A hot-rolled steel sheet can be produced without problems by an energy-saving process such as direct feed rolling and direct rolling that are immediately rolled. In particular, direct rolling is one of useful techniques for effectively securing N in a solid solution state.
[0036]
Hot rolling conditions are regulated as follows.
Slab heating temperature: 1000 ° C. or higher;
The lower limit of the slab heating temperature is defined as 1000 ° C. from the viewpoint of securing solid solution N as an initial state before hot rolling and obtaining the effect of N as described above. The upper limit is not particularly defined, but is preferably 1280 ° C. or lower because of an increase in loss accompanying an increase in oxidized weight. More preferred is 1050 to 1250 ° C.
[0037]
Finish rolling temperature during hot rolling: 850 ° C. or higher;
By setting the finish rolling temperature to 850 ° C. or higher, it is possible to obtain a uniform and fine hot-rolled structure with small in-plane anisotropy, which is an important characteristic of the present invention. Can be molded. In other words, the shape ratio of the second phase can be 3 or less. However, when the finish rolling temperature is lower than 850 ° C., the second phase structure is non-uniform in a layered (band) shape, and the shape ratio of the second phase increases. Even if it is used as it is or when it is made a hot-dip galvanized steel sheet that is reheated, such a non-uniformity of the structure remains, so various problems such as cracks occur during severe stretch flange processing. Can be a cause.
[0038]
Further, in the case of a lower rolling temperature than this, even if a high winding temperature is adopted in the subsequent winding process in order to avoid the remaining of the layered structure, the same problem with the generation of coarse grains occurs. Two-phase expansion cannot be eliminated. In order to further improve the mechanical properties, it is desirable that the temperature be 860 ° C. or higher. In particular, the upper limit temperature is not restricted, but when rolled at an excessively high temperature, it causes scale wrinkles and the like, but is generally applicable up to 950 ° C.
[0039]
Average cooling rate up to 650 ° C. after the end of hot rolling: 40 ° C./s or more;
After rolling, cooling is performed by water cooling or the like, and the average cooling rate up to 650 ° C. during the cooling is 40 ° C./s or more to refine the structure, especially to reduce the diameter of the second phase (equivalent circle diameter). It is essential to do. Further, it is advantageous for stably leaving the solid solution N. In addition, the upper limit of the cooling rate is not particularly limited, but if it exceeds approximately 300 ° C./s, it is disadvantageous in terms of achieving both material uniformity and shape uniformity. Preference is given to 50 to 150 ° C./s.
[0040]
Hot rolling coiling temperature: 650 ° C. or lower;
When the hot rolling coiling temperature is lowered, the strength tends to increase and improve. In order to obtain a sufficiently high tensile strength that contributes to the weight reduction of the vehicle body, a coiling temperature of 650 ° C. or less is used in order to refine the ferrite crystal grains and the average crystal grains of the second phase and obtain excellent characteristics. is necessary. In order to obtain higher tensile strength, it is desirable that the temperature be 600 ° C. or lower. The lower limit is not strictly limited in terms of material, but if the temperature is lower than 200 ° C., the shape of the steel sheet is significantly disturbed, which increases the possibility of causing problems in actual use. Moreover, when the uniformity of a material is requested | required, it is desirable to set it as 300 degreeC or more.
[0041]
Furthermore, when using it as a hot dip galvanized steel plate, what is necessary is just to hot-dip galvanize, after pickling the hot-rolled steel plate manufactured as mentioned above according to a conventional method. Usually, hot dip galvanizing of steel strip is performed in a continuous hot dip galvanizing line. At that time, a treatment for reducing the surface by heating is usually performed before hot dip galvanization, but when the heating temperature before hot dip galvanization exceeds 900 ° C., the ferrite phase and the second phase of the parent phase are coarse. Since the stretch flange workability is remarkably reduced, the heating temperature before hot dip galvanization is set to 900 ° C. or lower. The heating temperature before hot dip galvanization is preferably 700 ° C. or higher in order to sufficiently reduce the surface of the steel plate before hot dip galvanization and prevent unplating.
[0042]
After the above heat treatment, hot dip galvanizing or further alloying hot dip galvanizing can be performed according to a conventional method, and temper rolling or leveler processing can be performed in a normal range for hot dip galvanized steel sheets or hot rolled steel sheets. .
[0043]
Note that the steel sheet to which the present invention is directed is a hot-rolled thin steel sheet mainly used for an automobile body, and naturally, it is required to improve the shape and dimensional accuracy of the steel sheet and to make the material in the longitudinal and width directions uniform. . For this purpose, the continuous rolling technology that has been put into practical use in the hot rolling process is an extremely effective technology, and at the same time, the rolling temperature is made uniform in the longitudinal and width directions of the coil in order to make the material uniform. It is extremely effective to add a seat bar edge heater or a seat bar heater to be used. In the cooling after rolling, the technology of masking the cooling water in the width direction to prevent overcooling of the edge part is one of the important technologies from the viewpoint of material uniformity, and these technologies are uniform in shape and material. It is very effective in securing the property, and can be adopted as necessary.
[0044]
Hereinafter, manufacturing conditions that are preferably applied in an auxiliary manner will be described.
It is desirable to join the sheet bars together on the entry side of the finish rolling mill and continuously roll them. By rolling continuously, so-called rolling unsteady portions at the front and rear ends of the coil are eliminated, and stable hot rolling conditions can be achieved over the entire length and width of the coil. This is extremely effective in improving not only the hot-rolled steel sheet but also the cross-sectional shape and dimensions of the cold-rolled steel sheet. There is no particular restriction on the joining method at this time, and it can be applied in the same way with either the pressure welding method, laser welding method, electron beam welding method, or other joining methods, so that the shape of the steel sheet can be kept good. is there.
From this point of view, it is desirable to apply continuous hot rolling.
[0045]
In addition, by performing continuous rolling, the coil tip can be stably threaded, and therefore, lubrication rolling that cannot be applied from the viewpoints of sheeting property and penetration property under normal single-shot batch rolling conditions can be applied. . Thereby, since the rolling load can be reduced and the surface pressure of the roll can also be reduced, the life of the roll can be extended. From the above, it is very effective to perform continuous hot rolling on a thin hot-rolled steel sheet.
[0046]
The edge part is heated by the sheet bar edge heater and the sheet bar heater on the entrance side of the finish rolling mill. First, it is desirable to compensate for the difference in rolling temperature in the width direction with an edge heater. The heating amount at this time is recommended to be such that the temperature difference in the final finish rolling is approximately 20 ° C. or less, but it varies depending on the steel composition and the like. Next, it is possible to make the temperature drop part at the leading end of the coil more uniform by the sheet bar heater. In this case, the heating amount in the range of approximately 20 ° C. with respect to the central portion is recommended from the viewpoint of uniforming the material.
In addition, performing lubrication rolling to reduce the load during hot working is also effective from the viewpoint of uniform shape and uniform material. The friction coefficient at this time is preferably in the range of 0.25 to 0.10, and it is further desirable to apply the above-described continuous rolling process from the viewpoint of operational stability of hot rolling.
[0047]
【Example】
(Invention Examples 1-8, Comparative Examples 1-6)
A steel containing the components shown in Table 1 and the balance substantially consisting of Fe is melted in a converter, and the conditions shown in Table 2 (slab heating temperature, finish rolling exit temperature, hot rolling thickness, Hot rolling and pickling at an average cooling rate of up to 650 ° C., coiling temperature, cooling rate), and for some of the conditions shown in Table 2 (heating temperature before plating) in a continuous hot dip galvanizing line The galvanized steel sheets A to N were manufactured. Note that a La-Ce alloy was used as the REM. The hot-rolled steel sheet or hot-dip galvanized steel sheet was subjected to temper rolling with the elongation shown in Table 2. About these, mechanical properties, such as tensile strength, hole expansibility, etc. were evaluated.
[0048]
The hole expandability was measured according to the Federation of Iron and Steel standards as described above.
The structure was determined by observing a cross section parallel to the rolling direction of the steel sheet.
The area ratios of the ferrite phase and the second phase were investigated by image analysis of an enlarged image of a cross-sectional structure parallel to the rolling direction.
The crystal grain size of the ferrite phase and the second phase was set to the larger one of the nominal grain sizes obtained by the quadrature method or the cutting method defined in ASTM Designation E112-82.
The shape ratio of the second phase is the ratio of the maximum lengths in the plate thickness direction and the rolling direction of the second phase structure, and is an average value obtained by measuring at least 20 pieces.
Tensile properties are tensile strength TS, yield stress YS and elongation EL measured according to JIS Z2241 using JIS No. 5 test specimens taken from the steel sheet in the direction perpendicular to the rolling direction.
The evaluation results are shown in Table 3.
[0049]
(Invention Examples 9-12, Comparative Examples 7-8)
Contains C: 0.035%, Si: 0.005%, Mn: 0.55%, P: 0.009%, S: 0.001%, Al: 0.055% and N: 0.0035% And the steel slab whose remainder is Fe was used as a raw material, and the hot dip galvanized steel sheet was manufactured under the manufacturing conditions shown in Table 4. The obtained steel sheet was examined for characteristics such as mechanical properties, and the results are shown in Table 5.
Further, a peel test of the welded part in spot welding was conducted separately, but it was found that the nugget breaks in Comparative Examples 4 and 6 having Mn content and N content exceeding the range of the present invention, which is not preferable. did.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
[Table 3]

[0053]
[Table 4]

[0054]
[Table 5]

[0055]
【The invention's effect】
Since the hot-rolled steel sheet and hot-dip galvanized steel sheet according to the present invention are medium-strength, high-formability steel sheets having large stretch flangeability, they contribute to reducing the weight of automobile bodies as collision-resistant parts such as side members.

Claims (6)

In mass%, C: 0.01 to 0.08%, Si: 0.2% or less, Mn: 0.05 to 0.80 %, P: 0.04% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.003 to 0.0250%, the balance is composed of Fe and inevitable impurities, and the structure fraction of the ferrite phase as the main phase is 90% or more in area ratio, The structure fraction of the second phase is 10% or less in area ratio, the shape ratio in the rolling direction and the plate thickness direction of the second phase is 3 or less, the average grain size of the ferrite is 20 μm or less, the second A hot-rolled steel sheet excellent in stretch-flange formability having a tensile strength of 540 MPa or less, characterized by having a structure in which the average grain size of the phase is 10 μm or less.   The stretch-flange workability according to claim 1, wherein the hot-rolled steel sheet further contains one or more selected from the group consisting of Cu, Ni, Cr and Mo in total of 1.0% by mass or less. Excellent hot-rolled steel sheet.   The hot-rolled steel sheet further contains 0.0010 to 0.01 mass% of one or more selected from the group consisting of Ca and REM, and stretch flange processing according to claim 1 or 2 Hot rolled steel sheet with excellent properties. A hot-dip galvanized steel sheet comprising a hot-dip galvanized layer on the surface of the hot-rolled steel sheet according to any one of claims 1 to 3. In mass%, C: 0.01 to 0.08%, Si: 0.2% or less, Mn: 0.05 to 0.80 %, P: 0.04% or less, S: 0.005% or less, A steel slab containing Al: 0.10% or less, N: 0.003 to 0.0250% , the balance being Fe and inevitable impurities , a slab heating temperature of 1000 ° C. or higher, and a finish rolling exit temperature of 860 ° C. or higher. After the hot rolling at 650 ° C., the average cooling rate up to 650 ° C. is cooled to 40 ° C./s or higher, and the winding temperature is 550 ° C. or lower. A method for producing rolled steel sheets. In mass%, C: 0.01 to 0.08%, Si: 0.2% or less, Mn: 0.05 to 0.80 %, P: 0.04% or less, S: 0.005% or less, A steel slab containing Al: 0.10% or less, N: 0.003 to 0.0250% , the balance being Fe and inevitable impurities , a slab heating temperature of 1000 ° C. or higher, and a finish rolling exit temperature of 860 ° C. or higher. After hot rolling at 650 ° C., the average cooling rate up to 650 ° C. is cooled to 40 ° C./s or higher, the coil is wound at a coiling temperature of 550 ° C. A method for producing a hot-dip galvanized steel sheet excellent in stretch flangeability, characterized by galvanizing.