JP4884687B2 - Hot-rolled steel with excellent corrosion resistance and aesthetics after painting - Google Patents

Description

  The present invention relates to a hot-rolled steel material excellent in corrosion resistance and aesthetic appearance after chemical conversion treatment and coating. In particular, it is a high-tensile hot-rolled steel material that contains Si, Mn, Al, Cr, etc., which is more easily oxidized than iron as a strengthening element, and it has corrosion resistance after chemical conversion and coating by appropriate oxide control in the hot rolling and pickling processes. Is drastically improved.

  In recent years, due to global environmental problems, there is an increasing need for weight reduction for improving the fuel efficiency of automobiles, and high-tensile steel sheets are becoming mainstream automobile steel sheets as suitable materials. However, high-strength steel sheets contain elements that are more easily oxidized than iron such as Si, Mn, Al, Cr, etc. as strengthening elements in the order of a few percent, and these are added during the heat treatment process such as hot rolling and annealing. These elements are concentrated on the surface of the steel sheet to form oxides, which are not removed during chemical conversion treatment or electrodeposition coating in the automobile manufacturing process, and there are cases where the quality after coating is adversely affected.

  Various technical proposals have been made so far to solve such problems. Patent Documents 1 and 2 disclose a method of pickling after performing brush grinding for the purpose of removing an oxide film formed on the surface of a steel sheet. Patent Document 3 contains one or more of Co ions, Ni ions, and Sn ions in the pickling solution after hot rolling, and by substituting and depositing these metals on the surface of the steel sheet, chemical conversion treatment is performed. A technique of imparting crystal nuclei and improving the coverage with a chemical conversion coating is disclosed. In Patent Document 4, pickling in a pickling solution containing a sulfur compound after hot rolling, removing the scale of the surface layer and attaching the sulfur content to the steel sheet surface, the phosphophyllite ratio in the automobile manufacturing process A technique of forming a high-quality and uniform chemical conversion crystal having a high (P ratio) is disclosed.

  Further, in Patent Document 5, heat treatment is performed in an atmosphere in which reduction does not occur substantially with the black scale being adhered to the hot-rolled sheet, and an internal oxide layer is formed on the surface layer portion of the steel sheet, and then In addition, it is disclosed that by removing the black skin scale by pickling, it is possible to form a phosphate film without any scale. Patent Document 6 discloses a technique for producing a multilayer steel sheet by casting using an inner layer as a high strength component and a surface layer as a chemical conversion treatment component.

  On the other hand, in the case of a hot-rolled steel sheet containing Si in particular, examination of hot-rolling conditions has been performed from the viewpoint of eliminating surface property deterioration and post-coating unevenness due to residual Si scale. For example, in Patent Document 7, by maintaining the slab surface temperature at 1170 to 1250 ° C. for 100 to 150 minutes, the Si scale is uniformly generated on the slab surface and removed together with the scale generated in the subsequent hot rolling process. Thus, a technique for improving the surface properties is disclosed. Patent Document 8 discloses a technique for obtaining a hot-rolled steel sheet having excellent surface properties by performing descaling at a slab surface temperature of 1175 ° C. or higher and leaving an ultrathin Si scale uniformly after rolling. Yes. On the other hand, Patent Document 9 discloses a technology that makes it possible to manufacture a hot-rolled steel sheet having improved surface removal properties by improving the scale removability by setting the temperature before final descaling to 1180 ° C. or lower.

JP 2003-226920 A JP-A-5-317949 JP 56-130475 A JP 2003-277959 A JP 2000-248334 A JP-A-6-336647 JP-A-5-279734 JP 2003-55736 A JP 2002-194442 A

  However, all of these conventional techniques have problems. When the techniques of Patent Documents 1 and 2 are applied, in order to sufficiently remove easily oxidizable elements, it is necessary to remove a considerable thickness of the steel sheet surface layer by grinding and pickling, and productivity is poor. Moreover, when it uses for the steel kind with a high content rate of an easily oxidizable element, a chemical conversion processability may deteriorate on the contrary. The techniques of Patent Documents 3 and 4 are both techniques for forming a uniform chemical conversion treatment film on a steel sheet, and this alone does not improve the corrosion resistance after painting. Because, as will be described later, in the steel types targeted by the present invention, there is an example in which the corrosion resistance after coating is extremely poor even if a high-quality and uniform chemical conversion film having a high phosphophyllite ratio (P ratio) is formed. Because it exists.

  Although the technique of Patent Document 5 has an effect of eliminating the scale of chemical conversion treatment as in Patent Documents 3 and 4, it has no effect of improving the corrosion resistance after coating. Moreover, it is not economical because the number of processes increases. The technique of Patent Document 6 is difficult to control in the casting process and has poor productivity. The techniques of Patent Documents 7 to 9 are originally intended to improve the surface properties of hot-rolled steel sheets, and by simply applying these as they are, it is not possible to improve chemical conversion properties and post-coating corrosion resistance.

  The present invention solves these problems and drastically improves the corrosion resistance after chemical conversion and coating while containing a total of 0.5 mass% or more elements that are more easily oxidized than iron, and has excellent aesthetics after coating. It provides steel materials.

  In order to solve the above-mentioned problems, the present inventors have studied in detail the cause of the poor chemical resistance and post-coating corrosion resistance of hot-rolled steel materials containing a total of 0.5 mass% or more of elements that are more easily oxidized than iron. In particular, we focused on the factors on the steel side, focusing on the case where the corrosion resistance after coating is extremely poor even when a high-quality and uniform chemical conversion film having a high phosphophyllite ratio (P ratio) is formed.

  In general, the chemical resistance of steel materials after coating and corrosion resistance is that the steel material after chemical conversion and painting is exposed to a corrosive environment after scratches that reach the ground iron, and blisters (film blisters) are generated around the scratched part. It is customary to evaluate by the coating film peeling width when the periphery is tape-peeled. The corrosion in this case is so-called under-coating corrosion, and the corrosion proceeds with the scratch being the anode and the blister around the scratch portion being the cathode. However, in the case of hot-rolled steel materials containing 0.5 mass% or more of easily oxidizable elements, many blisters are generated at a location several mm or more away from the scratch, and when the tape is peeled off, these blisters are connected and peeled off. Therefore, the present inventors have noticed that the coating film peeling width is increased and the post-coating corrosion resistance is judged to be poor.

  Therefore, when the blister away from the scratch was analyzed in more detail, it was found that the chemical conversion crystals were sequentially dissolved from the upper side inside the blister, and this blister was also the cathode. However, since the corrosion resistance test is conducted with scratches reaching the base iron, it is considered that the anode reaction rate of the scratch itself, that is, the dissolution rate of iron, does not vary greatly depending on the steel type. Nevertheless, considering hot-rolled steel materials containing 0.5 mass% or more of easily oxidizable elements, a large number of cathode blisters are generated, and these are generated at positions away from the scratches. It was thought that there was a possibility that an anode was generated in addition to scratches in the steel material.

  Therefore, as a result of further detailed analysis of the surroundings of the blister away from the scratch, a portion where the base iron is dissolved under the chemical conversion crystal is seen just outside the blister, which corresponds to the blister away from the scratch. It became clear that it was an anode. In other words, on the surface of hot-rolled steel material containing 0.5 mass% or more of an easily oxidizable element, there are many parts where iron easily dissolves from the surroundings and easily becomes a local anode, and as a result, in a place away from scratches. However, the mechanism that a large number of cathode blisters were generated and the coating film peeling width became large was clarified.

  Furthermore, the present inventors have obtained the result that, as described above, in order to reduce the portion where iron is easily dissolved from the surroundings and easily becomes a local anode, different hot rolling and pickling conditions are required. It has been found that a hot-rolled steel material excellent in corrosion resistance after painting has a characteristic surface structure, and has completed the present invention.

The gist of the present invention is the following (1) to ( 4 ).

(1) A hot rolled steel material containing 0.5 mass% or more of one or more of C, Si, Mn, P, S, Al, N, and Cr, which are more easily oxidized than iron, and oxidized from iron Among the elements that are most likely to be oxidized, the concentration at the surface of the steel material that is most easily oxidized does not exceed the concentration in the steel, and there is an oxide layer that covers the crystal grains of the steel on the surface of the steel material. Cationic electrodeposition coating after forming a phosphate film having a P ratio of 0.9 or more on the surface of the hot-rolled steel material by immersion zinc phosphate treatment on the surface of the hot-rolled steel material. After immersing this in a 7% NaCl aqueous solution at 55 ° C for 253 hours, the number of paint peeling parts with a major axis of 0.5 mm or more generated by tape peeling of the coated surface should be less than 1 per 1 cm ² Hot rolled steel with excellent corrosion resistance and aesthetics after painting.

(2) A hot-rolled steel material containing 0.5 mass% or more of one or more of elements C, Si, Mn, P, S, Al, N, Cr, which are more easily oxidized than iron, and oxidized from iron Among the elements that are most likely to be oxidized, there is a region where the concentration on the surface of the steel material that is most likely to be oxidized is higher than the concentration in steel, and there is an oxide layer that covers the crystal grains of the steel on the surface of the steel material. After forming a phosphate film having a P ratio of 0.9 or more on the surface of the hot-rolled steel material by dipping-type zinc phosphate treatment on the surface of the hot-rolled steel material. After applying cationic electrodeposition coating, immersing it in a 7% NaCl aqueous solution at 55 ° C for 253 hours and then peeling the coated surface with tape, the number of coating peeling parts with a major axis of 0.5 mm or more is less than 1 per 1 cm ² A hot-rolled steel material excellent in corrosion resistance and aesthetics after painting.

(3) as likely element to be oxidized than the iron, corrosion resistance and appearance after coating according to Si, Mn, Al, said that, characterized in that the one or more of Cr (1) or (2) Excellent hot rolled steel material.

(4) heat the total content of the iron from being oxidized easily element 1.5 mass% or more, excellent in corrosion resistance and appearance after coating according to any one of the at most 4.0 mass% (1) ~ (3) Rolled steel.

  According to the present invention, a high-strength hot-rolled steel material having excellent post-coating corrosion resistance can be provided. In particular, when applied as an automobile undercarriage part such as a wheel, the generation of blisters due to corrosion under the coating film is small in the flat part as well as the scratch part, so that it can maintain aesthetics for a long time even after running as a real car. it can.

  The present invention is described in detail below.

  First, (1) defines the upper limit of the local anode density on the surface of the steel material, which is necessary to ensure the corrosion resistance after coating. As described above, if there is a portion on the steel surface where iron is more likely to melt than the surroundings, a local anode is formed, and blisters are generated at positions away from the cut, resulting in an increase in tape peeling width. Therefore, in order to reduce the tape peeling width, the local anode density on the steel material surface may be reduced. The local anode density is correlated with the density of the paint peeling part produced by exposing the steel material to zinc corrosion treatment and electrodeposition coating, then exposing it to a corrosive environment without cutting, and then peeling the painted surface with tape. Yes, this can be used as a guide. If the surface of the steel material is uniform and no local anode is generated, even if the coated surface is tape-peeled after being exposed to a corrosive environment without being cut, a paint-peeled portion is not generated.

However, in order to estimate the local anode density by this method, the zinc phosphate treatment and electrodeposition coating must be sound and uniform. This is because if these are defective, they themselves act as local anodes. As the zinc phosphate treatment, an immersion type trication (Zn-Ni-Mg) type is preferable. As the surface conditioning solution, a conventional type containing a Ti colloid may be used, but more preferably, a zinc-based dispersant having a high crystal refining effect is contained regardless of the type of the base steel. Surface conditioning liquid is recommended. Adhesion amount of the zinc phosphate treatment good 2.0~2.5g / m ^2. The phosphophyllite ratio (P ratio) should be 0.9 or more. P ratio is measured as (X-ray diffraction intensity of phosphophyllite) / (X-ray diffraction intensity of hopite + X-ray diffraction intensity of phosphophyllite) . Electrodeposition coating uses a cation type that is currently widely used for automobile bodies, and has a film thickness in the range of 15 to 20 μm. The paint baking conditions are in accordance with the specifications of each paint, but it should be noted that the hot-rolled steel material has a large plate thickness width, and therefore the temperature rise time to a predetermined baking temperature depends on the plate thickness. That is, it should be managed not by the total baking time but by the soaking time. The soaking time should be 70-80% of the total baking time specified in the specification. If it is shorter than this, the coating film is insufficiently cured and a sound electrodeposition coating film cannot be obtained. On the other hand, if it is longer than this, the coating film becomes excessively hardened, the water permeability and the barrier property become higher than those of the actual vehicle coating film, and the corrosion resistance of the steel material may be underestimated.

The dimensions of the hot-rolled steel used for the test are steel plates cut into a length of 12 cm and a width of 7 cm. When cut out from a molded product or a steel pipe, even if it is not completely flat but slightly curved, this is used as a test piece and is called a steel plate. After degreasing, chemical conversion and electrodeposition coating, the local anode density is evaluated. The test condition is continuous immersion in a 7% NaCl aqueous solution at 55 ° C., which is taken out after 253 hours. The tape peel test is performed within 6 hours and 24 hours after removal. The tape should be a transparent pressure-sensitive adhesive tape with a width of 25 mm, and the two tapes should be attached side by side in the longitudinal direction, avoiding the area in the vicinity of the end face from the left and right end faces of the steel sheet to 1 cm inside. Subsequent tape peel test methods shall in principle conform to the description in 7.2.6 of JIS K5600-5-6. That is, rub the tape firmly with your fingertips to ensure proper contact with the paint, and within 5 minutes after confirming that the contact is neat from the color of the paint seen through the tape, make sure that the tape is as close to 60 ° as possible. Grasp the edge and pull it apart one by one in 0.5 to 1 second. After the tape is peeled off, the paint peeling portion generated on the painted surface is evaluated as follows. First, the peeled tape is stuck on a sheet of transparent film. Next, in the portion corresponding to 12 cm in the longitudinal direction of the test piece, the tape in the region from the upper and lower end faces to the inside of each 1 cm is cut and removed. In other words, the region from the top, bottom, left, and right end faces to 1 cm inside is not evaluated. This is because the electrodeposition film thickness is thin due to the influence of the edge in the vicinity of the end face, and the influence is eliminated. As a result, when the two tapes are combined, the area to be evaluated becomes an area of 10 cm in the longitudinal direction and 5 cm in the width direction of the steel sheet. Thereafter, the surface of the tape is observed with a magnifying glass or the like, and the number of paint peeling portions having a major axis of 0.5 mm or more attached to the tape is counted. Divide this by the area (50 cm ² ) to give a measure of the local anode density. Test 3 samples per steel grade and get the average value. If this value is less than 1 piece / cm ² , the local anode density generated on the steel sheet surface is small enough, and even in normal corrosion tests with cuts after coating, few blisters are generated away from the cuts. As a result, the tape peeling width becomes small. More preferably, it is less than 0.5 pieces / cm ² .

  Another effect of having a low local anode density is maintaining the aesthetics after painting in an actual vehicle. Although details will be described in Example 2, the wheel, which is one of the applications of the present invention, has a large area in the plane portion because of the structure, and the aesthetic maintenance after painting is very important.

The total of one or more of C, Si, Mn, P, S, Al, N, Cr of elements that are easily oxidized than iron (easily oxidizable elements) is defined as 0.5 mass% or more. This is because if it is less than 0.5 mass%, a high-strength hot-rolled steel material cannot be obtained.

The above ( 1 ) further defines the surface structure of a hot-rolled steel material having the performance described above . The present inventors have analyzed in detail how the local anode is formed on the steel surface after finding out the mechanism of deterioration of the corrosion resistance after coating due to the formation of the local anode. Consequently, (a) the concentration of the easily oxidizable element in the steel surface is higher region than steel concentrations, (b) low coverage of the surface oxide layer covering the grain of the base iron of the steel material surface, When these two conditions are met, it has been found that a local anode can be easily formed. If there is a region where the concentration of the easily oxidizable element is higher than the concentration in steel on the surface of the steel material, the surrounding iron is considered to be easily dissolved. And if the surface is not coat | covered with the oxide layer, it will be highly likely to become a local anode. In short, the more uneven the steel surface after hot rolling and pickling, the easier it is to form a local anode.

Therefore, intensive investigations were made on measures to reduce the local anode, and it was found that there are two methods. One of them is the surface structure defined in ( 1 ) above. Wherein (1) the concentration of oxidizable elements in (a) the steel surface density in the steel super Enoi, increasing the coverage of the surface oxide layer covering the grain of the base steel of (b) the steel surface The formation of the local anode is reduced under the two conditions. Below, the structure and manufacturing method are demonstrated in detail.

First, (a) the concentration of oxidizable elements on the steel surface does not exceed the concentration in steel, but whether or not there is concentration of oxidizable elements on the steel surface is determined by glow discharge emission spectrometry (GDS). Judgment by An example is shown in FIG. This is a GDS analysis result of a slab having the composition C shown in Table 1 shown in an example described later as a hot rolled steel sheet having a thickness of 3.2 mm under different hot rolling and pickling conditions. The apparatus used is a glow discharge emission spectrometer manufactured by Rigaku. The measurement time was 10 sec, the measurement interval was 10 msec, the sputtering conditions were set power 20 W, Ar flow rate 250 mL / min. The elements to be measured are Fe, O, Si, and Mn. Among these, when looking at Si that is most easily oxidized, FIG. 1 (a) shows that there is no Si concentration peak on the steel surface within a sputtering time of 2 seconds. "the concentration of oxidizable elements in the surface of the steel material is a concentration in the steel super Enoi" which satisfies the condition say of. On the other hand, in FIG. 2 (b), there is a Si concentration peak on the surface of the steel material having a sputtering time of about 0.5 seconds, and the above conditions are not satisfied. Measurements are made at 5 locations on the steel at a distance of 100 cm or more, and among these, 3 or more locations are judged according to which type (a) or (b). The surface of the steel material with a sputtering time of 2 seconds or less here corresponds to a region having a depth of about 5 μm from the outermost layer of the steel material.

Next, the process for producing the hot rolled steel sheet for the concentration of oxidizable elements exceed not the concentration in the steel at the steel surface. In order to eliminate the concentration of easily oxidizable elements, for example, production should be performed under conditions where external oxidation is more advantageous than internal oxidation during the period from the start of finish rolling, which is considered to generate a secondary scale, to winding. Is effective. For this purpose, it is preferable to prevent oxygen from diffusing near the surface of the steel sheet by increasing the scale thickness after the final deske. In order to increase the scale thickness after the final deske, it is effective to select, for example, the following conditions in the process from the heating furnace to the finish rolling. (i) Increase the slab heating temperature and lengthen the soaking time to leave the primary scale even after the final deske. (ii) Increase the coarse bar thickness after rough rolling to ensure the scale thickness. iii) Set the final deske pressure lower, (iv) Increase the time from the final rough rolling pass to finish rolling to accelerate the start of secondary scale generation, (v) Suppress the rolling reduction immediately after the start of finish rolling By doing so, the generation of cracks in the scale layer is suppressed.

  Specifically, first, slab heating is preferably performed at a high temperature of 1250 ° C. or higher and a soaking time of 200 minutes or longer. In rough rolling, for example, if the slab thickness is 250 mm, the rough bar thickness is preferably 70 mm or more. The final deske water discharge pressure should be less than 9.8MPa. The time from the final rough rolling pass to the start of finish rolling should be 90 seconds or more. Also, the rolling reduction of the F1 roll immediately after the start of finish rolling is preferably less than 25%.

  Next, the pickling conditions may be any pickling conditions necessary and sufficient for removing so-called scales and black skin. For this purpose, it is preferable to add a peracid washing inhibitor after lowering the hydrochloric acid concentration and bath temperature and shortening the treatment time as compared with the conventional operation. Excess pickling as seen in Patent Documents 1 and 2 leads to non-uniform surface structure, which is counterproductive.

Next, a description will be given to increase the coverage of the surface oxide layer covering the grain of the base steel of (b) the steel surface. FIG. 2 shows the result of observing the surface oxide layer formed on the ground iron by SEM (scanning electron microscope) with the hot-rolled steel sheet inclined by 60 °. The end face is visible in the crystal grains of the ground iron, and the oxide layer is on the surface. Fig. 2 (a) shows an example of a steel plate with a large surface roughness coated with an oxide layer uniformly, and Fig. 2 (b) shows an oxide layer with a different thickness depending on the location on a steel plate with a small surface roughness. It is an example of what has. As can be seen from these, the surface oxide layer is a very thin layer having a thickness of several μm or less, and is different from a so-called scale or black skin. To confirm that the oxide layer is present on the surface, perform surface analysis of the sample by XPS (X-ray photoelectron spectroscopy), qualitative analysis of oxygen and iron state analysis, Is present on the surface, and it is sufficient to confirm that iron is in an oxide state.

Next, the crystal grains of the ground iron will be described. The crystal grains of the base steel, when the surface subjected to SEM observation, refers to all those visible to clear particulate boundary. The size of the crystal grain, as in FIG. 2, can be confirmed by performing a simultaneous observation of the end face and the surface, depending on the steel type and the hot rolling conditions, there least about 10μm is usual. In addition to this, a small particle having a size of about several μm is defined as a crystal grain here. For example, in the SEM photograph of FIG. 3 (c), large grains around 10 μm in the vicinity of the central part and small grains about several μm in size that appear densely in the upper left are observed. Both are defined as crystal grains of ground iron. Size of several μm order of small crystal grains in the vicinity of the surface layer during rolling (normal size) as the grain is destroyed, or the one formed by etching (dissolution of the base steel) during pickling think We Ru. In any case, these are crystal grains of the base iron that are not covered by the surface oxide layer.

The coverage of the steel surface oxide layer is calculated as follows. First, a sample having a size capable of SEM observation, for example, a 30 mm square is cut out from the steel material. Then, SEM observation is performed on three random points separated by 5 mm or more from directly above (without tilting the steel plate). The observation is performed at an accelerating voltage of 15 kV and a field of view of 1000 times magnification. If the surface oxide layer almost covers the crystal grains of the ground iron, the score is 2, and the surface oxide layer and the ground iron crystal grains may be mixed. If the part where the crystal grains are exposed is dominant, the score is 0. An example photograph corresponding to each score is shown in FIG. If there is an intermediate one, the score closer to the example photograph is taken. The same evaluation is performed on a total of 10 samples collected from a position of 100 cm or more on the steel material, and the scores are totaled. For example, in the entire field of view, if the surface oxide layer substantially covers the crystal grains of the ground iron, the total score is 60 points. Then, the surface coverage is obtained using the following calculation formula. The numbers after the decimal point are rounded off.

Surface coverage (%) = total score x 100/60
In ( 1 ), there is no region where the concentration of the easily oxidizable element is higher than the concentration in steel on the surface of the steel material. In this case, if the coverage of the surface oxide layer is 50% or more, the formation of local anodes can be sufficiently suppressed. More preferably, it is 60% or more.

  Next, a method for controlling the coverage of the steel surface oxide layer will be described. This involves pickling and rinsing. As described above, the pickling condition is that the concentration of hydrochloric acid and the bath temperature are lower than the conventional operation, and the treatment time is shortened. This is advantageous in avoiding the formation of grains and increasing the coverage of the steel surface oxide layer. In addition, after adding 1 to 3 mass% phosphoric acid and 2 to 5 mass% boric acid in the rinse water after pickling, rinsing for 5 to 8 seconds with a bath temperature of 60 to 80 ° C, and a drying temperature of 80 to 150 ° C By drying with, the coverage of the surface oxide layer can be further increased.

The ( 2 ) defines another surface structure of the hot rolled steel material having the performance (1). This, (a) the concentration of the oxidizable element steel surface is densely generate region higher than steel concentrations, (b) the coverage of the surface oxide layer covering the grain of the base iron of the steel surface The two conditions of increasing the resistance of the steel material reduce the unevenness of the steel surface and reduce the formation of local anodes.

  Among these, first, (a) the formation of a region where the concentration of the easily oxidizable element is higher than the concentration in steel on the surface of the steel material will be described. The local anode is easily formed when the surface structure is not uniform. Concentration of easily oxidizable elements in the base iron also tends to form a local anode if it is sparsely present, but if it is dense on the surface, this can be said to be a uniform surface structure. And generation of local anodes can be suppressed.

  Whether or not the easily oxidizable element is concentrated on the steel surface is determined by glow discharge emission analysis (GDS) as shown in FIG. Thus, when it is determined that there is a concentration of easily oxidizable elements, the surface content (mass%) of the easily oxidizable elements is measured by fluorescent X-ray, and the content in steel (mass%) is measured. In accordance with the following formula, it is determined whether the concentration is dense or coarse.

(Surface content / content in steel) ≥ 1.5
If (surface content / in-steel content) <1.5, the surface content (mass%) of the easily oxidizable element by crude fluorescent X-ray is measured as follows. First, a standard sample having a known content of easily oxidizable elements in steel is prepared. Taking the case where the easily oxidizable element is Si as an example, A, B, D, and E shown in Table 1 of Examples to be described later, from a low to a high Si content (mass%) in steel A series of 4 to 5 samples should be prepared. These may be cut out from a slab or may be produced by vacuum melting in a laboratory. After cutting this to a size that can measure fluorescent X-rays, for example, 33 mm in the width direction, 33 mm in the longitudinal direction, and 50 mm in thickness, cut the thickness 10 mm or more from the surface to correctly measure the content in steel. Remove and surface grind with a grinder. Next, the strength of the easily oxidizable element is measured by fluorescent X-rays, and a relational expression between the steel content (mass%) and the strength, a so-called calibration curve is obtained. When the above preparation is completed, after cutting the steel material after hot rolling and pickling as a test material into a 33 mm square, the strength of the oxidizable element is measured by fluorescent X-ray without grinding the surface, This is converted into the content of easily oxidizable elements (mass%) using the calibration curve created earlier. This value is defined as the surface content (mass%) of the easily oxidizable element in the steel material after hot rolling and pickling.

  Next, a manufacturing method for densely generating a region where the concentration of the easily oxidizable element is higher than the concentration in steel on the steel surface will be described. In order to cause the concentration of easily oxidizable elements to occur densely, for example, on the condition that internal oxidation is more advantageous than external oxidation during the period from the start of finish rolling, which is considered to generate a secondary scale, to winding. It is effective to manufacture a hot-rolled steel sheet. For this purpose, it is preferable to make the diffusion of oxygen easily to the vicinity of the steel sheet surface layer by reducing the scale thickness after the final deske. In order to reduce the scale thickness after the final deske, it is effective to select, for example, the following conditions in the process from the heating furnace to the finish rolling. (I) By reducing the slab heating temperature and shortening the soaking time, the primary scale does not remain after the final deske. (II) The scale bar thickness is reduced by reducing the thickness of the rough bar after rough rolling. (III) Set the final deske pressure higher, (IV) Shorten the time from the final pass of the rough rolling to the finish rolling, and start the finish rolling while generating less secondary scale. (V) Finishing A crack is generated in the scale layer by reducing the pressure immediately after the start of rolling.

  Specifically, first, the slab heating is preferably performed at 1170 ° C. or less and within a soaking time of 100 minutes. In rough rolling, for example, when the slab thickness is 250 mm, the rough bar thickness is preferably 35 mm or less. The final discharge water discharge pressure should be 24.5MPa or more. The time from the final pass of rough rolling to finish rolling should be less than 50 seconds. The finish rolling F1 roll should have a rolling reduction of over 45%.

  The pickling conditions may be any pickling conditions that are necessary and sufficient for removing the so-called scale and black skin and maintaining the underlying internal oxide layer. For this purpose, it is preferable to add a peracid washing inhibitor after lowering the hydrochloric acid concentration and bath temperature and shortening the treatment time as compared with the conventional operation.

Then, although increasing the coverage of the surface oxide layer covering the grain of the base steel of (b) the steel surface, which is as already mentioned above (1). However, in ( 2 ), the required coverage of the steel surface oxide layer differs depending on whether the concentration of the easily oxidizable element is dense or coarse. If the concentration of the easily oxidizable element is dense, the formation of the local anode can be effectively suppressed if the coverage of the steel surface oxide layer is 70% or more. On the other hand, when the concentration of the easily oxidizable element is rough, the coverage of the steel surface oxide layer needs to be 80% or more.

In the above ( 1 ) and ( 2 ), among the elements that are more easily oxidized than iron, the elements that are most easily oxidized are defined, but the order of the ease of oxidation may be determined according to the Ellingham diagram. However, elements with extremely low contents (for example, 0.1 mass% or less) should be excluded. Table 1 of Examples described later exemplifies elements that are most easily oxidized when the steel components are different.

Wherein (3), as a likely element to be oxidized than the iron, Si, Mn, Al, among the four kinds of elements of Cr, a definition that one or more. These elements are suitable for obtaining inexpensive and high-performance high-strength steel materials.

The above ( 4 ) defines the total content of elements that are more easily oxidized than iron as described in any one of (1) to ( 3 ), and is intended to specifically have a strength class of 440 MPa or more. . If the total content is less than 1.5 mass%, a high-strength steel material of this class cannot be obtained, and if it exceeds 4.0 mass%, the possibility of deterioration in workability, weldability, etc. increases.

  In the following, the present invention will be described in a non-limiting manner using examples.

(Example 1)
(1) Composition of test steel Table 1 shows the composition of nine types of steel slabs used as test materials.

(2) Hot Rolling / Pickling Conditions Table 2 shows the manufacturing conditions in the hot rolling / pickling process. The slab thickness was 250 mm. Final descaling was performed after slab heating, rough rolling and before finish rolling. In the examples of the present invention, Hibiron AS-20C manufactured by Sugimura Chemical was added as an acid pickling inhibitor to the pickling solution, and 1.5 mass% phosphoric acid and 2.5 mass% boric acid were added to the rinsing water. In the comparative example, no peracid washing inhibitor was added to the pickling solution, and neither phosphoric acid nor boric acid was added to the rinse water.

(3) Performance Evaluation of Pickling Plate Table 3 shows the results of performance evaluation using the slab shown in Table 1 as a pickling plate under the manufacturing conditions shown in Table 2. Element concentration of the steel sheet surface layer, its density, and oxide layer coverage were determined by GDS (FIG. 1), fluorescent X-ray measurement, and surface SEM observation (FIG. 3) as described above.

  The pickling plate was cut into 7 cm (width direction) × 12 cm (longitudinal direction), degreased with alkali, and then subjected to chemical conversion treatment and electrodeposition coating under the following conditions.

Surface adjustment: PREPAREN-X (manufactured by Nihon Parkerizing)
A 3 g / L aqueous solution was prepared and immersed at room temperature for 25 seconds.

Chemical treatment: Palbond-SX35 (manufactured by Nihon Parkerizing)
According to the prescription, free acid 0.8 points, total acid 24 points,
Adjust the accelerator concentration to 3.8 points and bath temperature at 35 ℃
It was immersed for 120 seconds, then washed with water, washed with pure water and dried.

Electrodeposition painting: Powernics PN330 gray (made by Nippon Paint)
Electrodeposition for 180 seconds at a voltage of 240V, drying plate temperature of 165 ℃,
It was baked to have a soaking time of 15 minutes. Film thickness 15-20
μm. In addition, any level
There were no coating film defects.

For each level, eight samples were prepared, and two of them were processed up to chemical conversion, and no electrodeposition coating was performed. For one sheet, the chemical conversion treatment was dissolved in a chromic acid aqueous solution, and the amount of the chemical conversion treatment film deposited was measured from the mass change before and after dissolution. The other sheet was used for the evaluation of P ratio and scale (the scale is the part without the chemical conversion coating). The P ratio is measured according to a conventional method, and the intensity of the (100) plane of the hopite (H) and the (110) plane of the phosphophyllite (P) is measured by X-ray diffraction, and the P ratio = (P) / ((P ) + (H))
Calculated by As a result, in any sample, the P ratio was as good as 0.9 or more. In addition, the skeins were observed by SEM at a magnification of 1000 times, but none of the samples had skeins.

Six sheets for each level were subjected to both chemical conversion treatment and electrodeposition coating in this order. In order to evaluate the formation density of the local anode, three of these were immersed in a 7% NaCl aqueous solution at 55 ° C. for 253 hours without cutting as described in the description of (1) above. The coated surface was peeled off using the procedure described above, and the number of paint peeled parts with a major axis of 0.5 mm or more was counted, and this was divided by the sample area (50 cm ² ) excluding the vicinity of the top, bottom, left and right end faces. Density.

  The remaining three sheets were subjected to chemical conversion treatment and electrodeposition coating, and a cut wound reaching the base iron was inserted in a length of 5 cm in the longitudinal direction of the surface and immersed continuously in a 7% NaCl aqueous solution at 55 ° C. for 253 hours. After completion of the immersion, the sample was taken out and lightly blown dry, and then the periphery of the cut part was peeled off with a tape having a width of 30 mm, and the maximum peel width on one side from the cut was measured.

  The evaluation results are summarized in Table 3. In any of the hot-rolled steel sheets of the present invention, the formation of local anodes is suppressed. Further, as a result, the cut plate peeling width is as small as that of a mild steel cold-rolled steel plate, indicating that the post-coating corrosion resistance is very excellent. On the other hand, the hot-rolled steel sheet of the comparative example has a high P ratio high-quality chemical conversion coating formed uniformly without any scale, and the electrodeposition coating has high local anode density even though there are no coating defects such as blisters. . As a result, the cut plate peeling width is larger than that of the present invention, and the post-coating corrosion resistance is inferior.

(Example 2)
Wheels for automobiles are actually manufactured from the pickling plates of Invention Examples 2, 4, 6, 10 and Comparative Examples 14, 15, 19, and 20 used in Example 1, and chemical conversion treatment is performed by an automobile parts manufacturer. After applying the paint, an actual vehicle running test was conducted. Two actual cars are prepared, and one wheel (1500CC class) has a wheel with a thickness of 3.2 mm, i.e., numbers 31, 33 (invention example) and numbers 35, 36 (comparative example) in Table 4. Install the other one (3000CC class, pickup truck) with a wheel with a thickness of 6.3 mm, that is, No. 32, 34 (invention example) and No. 37, 38 (comparative example) in Table 4. Installed. And on a rough road simulation course of a car manufacturer consisting of a mud splash section, a salt water section, a stone splash section, etc., the wheel was attached without a cap and ran for 10,000 km. After the test, the aesthetics of the wheel were observed and evaluated as ◯ when the initial aesthetic was maintained, and x when the aesthetic was impaired due to the occurrence of blisters on the flat surface.

  The results are shown in Table 4. The hot-rolled steel sheet of the present invention has a low local anode density, so that the appearance of the painted surface is maintained when it is molded into a wheel and subjected to an actual vehicle running test.

FIG. 4 is a diagram showing an example of distribution measurement of oxidizable elements in the vicinity of a steel surface layer by GDS, (a) an example without concentration (Example 25), (b) a diagram with an example of concentration (Comparative Example 17). is there. It is a SEM photograph (60 ° inclination) of the steel surface oxide layer formed on the base iron, (a) Example of uniform coating of the base iron with the surface oxide, (b) Covering the base iron with the surface oxide Is an SEM photograph of a non-uniform example. FIG. 2 is a photograph showing an example of the coverage score of a ground crystal grain by a surface oxide layer (photographed at an acceleration voltage of 15 kV), showing (a) score 2, (b) score 1, and (c) score 0, respectively.

Claims (4)

A hot-rolled steel material containing a total of 0.5 mass% or more of one or more of elements C, Si, Mn, P, S, Al, N, and Cr that are more easily oxidized than iron, and more easily oxidized than iron. Among the elements that are most easily oxidized, the concentration on the surface of the steel material does not exceed the concentration in steel, and there is an oxide layer that covers the steel grain on the surface of the steel material. The coating area ratio is 50% or more. Further, a phosphate film having a P ratio of 0.9 or more is formed on the surface of the hot-rolled steel by a dipping-type zinc phosphate treatment, followed by cation electrodeposition coating. This is characterized by the fact that the number of coating peeling parts with a major axis of 0.5 mm or more generated by detaching the coating surface after tape immersion in a 7% NaCl aqueous solution at 55 ° C. for 253 hours is less than 1 per 1 cm ^2. Hot rolled steel with excellent corrosion resistance and aesthetics after painting. A hot-rolled steel material containing 0.5 mass% or more of one or more of elements C, Si, Mn, P, S, Al, N, and Cr, which are more easily oxidized than iron, and more easily oxidized than iron Among these, although there is a region where the concentration on the surface of the steel material is higher than the concentration in the steel although it is most easily oxidized, and there is an oxide layer covering the crystal grains of the steel on the surface of the steel material, the oxide layer Cationic electrodeposition after forming a phosphate film having a P ratio of 0.9 or more on the surface of the hot-rolled steel material by dipping-type zinc phosphate treatment on the surface of the hot rolled steel material. painted, after which 253 hours immersion in 7% NaCl aqueous solution of 55 ° C., that the number of long diameter 0.5mm or more paint peeling unit that generated by the coated surface to the tape peeling is ² per less than one 1cm Hot rolled steel with excellent corrosion resistance and aesthetics after painting. Hot rolling said as likely element to be oxidized than iron, excellent Si, Mn, Al, the corrosion resistance and appearance after coating according to claim 1 or 2, characterized in that the one or more of Cr Steel material. The hot-rolled steel material excellent in corrosion resistance and aesthetics after coating according to any one of claims 1 to 3, wherein the total content of elements that are more easily oxidized than iron is 1.5 mass% or more and 4.0 mass% or less.