JP2007277585A - High-strength hot-rolled steel sheet excellent in chemical convertibility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet which stably exhibits excellent chemical convertibility, even when it contains Mo which is added for enhancing the strength thereof. <P>SOLUTION: The high-strength hot-rolled steel sheet having improved chemical convertibility satisfies the surface characteristics that the maximum height (Ry) of profile irregularities in the surface is not less than 10 μm and the mean spacing (Sm) of the profile irregularities is not more than 30 μm. Further, this high-strength hot-rolled steel sheet satisfies either, preferably both, of the following characteristics that the profile bearing length ratio (tp40) of the surface is not more than 20%, and the difference between the profile bearing length ratio (tp60) and the profile bearing length ratio (tp40) of the surfaces is not less than 60%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet having high strength and excellent chemical conversion treatment properties such as phosphate treatment.

  Recently, from the viewpoint of improving fuel economy and reducing exhaust gas due to the weight reduction of automobiles and the like, there has been a demand for further strengthening of steel materials, especially for cold-rolled steel plates, which are rapidly becoming high-tensile (high-strength). is made of. However, on the other hand, from the viewpoint of collision safety, there are many requests for thick high-strength steel sheets for strengthening rigidity. It is difficult to deal with such demands with cold-rolled steel sheets in terms of equipment and costs, and it is necessary to deal with hot-rolled steel sheets.

  Even when a hot-rolled steel sheet is used, since it is press-formed in the same way as a cold-rolled steel sheet when processed into a product shape, ductility such as elongation cannot be neglected even if the strength is increased. However, not only hot-rolled steel sheets, but as general properties of materials, the higher the strength, the lower the ductility and the worse the workability. Therefore, for steel materials, the addition of alloying elements that can increase the strength without reducing ductility has been studied.

  Among these, Mo is attracting attention as a useful element that increases strength without significantly reducing ductility. Moreover, Mo promotes the formation of a bainite structure that contributes to high strength by suppressing the formation of a ferrite structure that occurs during the cooling process after the end of hot rolling, and increases the slab heating temperature before hot rolling and employs low-temperature winding. Therefore, it is also attracting attention as an additive alloy element for high-strength hot-rolled steel sheets.

  However, when Mo is added as an alloy element, the chemical conversion property of the steel sheet is deteriorated, and as a result, the adhesion of the coated film after electrodeposition coating is adversely affected and the appearance and corrosion resistance of the finished product are adversely affected.

  On the other hand, in order to improve the chemical conversion treatment properties of steel plates, several methods for improving the surface properties (for example, microscopic unevenness pattern) of the steel plate materials have been proposed.

  For example, Patent Document 1 discloses a hot-rolled pickled steel sheet that has improved chemical conversion properties by defining a microscopic shape of the steel sheet surface. In this technique, the surface texture is adjusted by performing skin pass rolling using a roll (dull roll) having a concavo-convex pattern formed on the surface of a steel sheet with a high energy density beam and transferring the concavo-convex pattern on the surface of the roll to the surface of the steel sheet. However, this method cannot avoid a cost increase due to additional processes such as roll dull processing and skin pass rolling, and in particular, a satisfactory effect cannot be obtained for Mo-added steel.

  Patent Document 2 discloses a method for improving the chemical conversion treatment property by suppressing the average crystal grain size of the Ti-added hot-rolled high-tensile steel sheet to 3.0 μm or less and suppressing the surface roughness (Ra) to 1.5 μm or less. It is disclosed. However, this method also cannot achieve the intended effect on Mo-added steel.

Furthermore, Patent Document 3 discloses a technique for controlling a microscopic uneven pattern on the surface of a steel plate. However, this technique is intended to improve the paint sharpness and press workability of the steel sheet, and the uneven pattern controls the diameter of the protrusions in the range of 50 to 200 μm. Since it is much larger than the crystal size of zinc phosphate, which is noticed as a factor affecting the chemical conversion treatment property, it hardly contributes to the improvement of chemical conversion treatment property.
JP-A-2-187202 JP 2002-226944 A JP-A-5-293503

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is not only high-strength hot-rolled steel sheet not containing Mo, but also hot-rolled steel sheet to which Mo is added for higher strength. Even so, an object is to provide a high-strength hot-rolled steel sheet that can stably exhibit excellent chemical conversion properties.

The high-strength hot-rolled steel sheet according to the present invention that has solved the above-mentioned problems is a high-strength hot-rolled steel sheet having a tensile strength of 390 MPa level or higher, and further 780 MPa level or higher, and has unevenness present on the steel sheet surface. In addition to satisfying the requirement that the maximum depth (Ry) is 10 μm or more and the average interval (Sm) of the irregularities is 30 μm or less,
1) The load length ratio (tp40) of the surface unevenness is 20% or less, and 2) The difference between the load length ratio (tp60) of the surface unevenness (tp40) is 60% or more,
It is a high-strength hot-rolled steel sheet that satisfies any one of the two requirements, and more preferably has excellent chemical conversion properties that simultaneously satisfies the requirements 1) and 2).

  The component composition of the steel sheet according to the present invention can be arbitrarily changed according to the required strength, but preferably, C: 0.03 to 1.0%, Si: 2% or less (including 0%) as basic components ), Mn: 0.3 to 4.0%, Al: 0.005 to 0.5%, preferably further Mo: 0.05 to 1.0% for higher strength, or Further, if necessary, Cr: 1.5% or less (not including 0%), Ti: 0.2% or less (not including 0%), Nb: 0.1% or less (not including 0%) V: 0.1% or less (not including 0%), Cu: 1.0% or less (not including 0%), Ni: 1.0% or less (not including 0%), B: 0.0. It contains at least one element selected from the group consisting of 002% or less (not including 0%), Ca: 0.005% or less (not including 0%), Parts are steel consisting of iron and inevitable impurities.

  In addition, the strength level of the high strength hot rolled steel sheet according to the present invention cannot be determined uniformly because it varies depending on the application and purpose, but the general strength level has a tensile strength of 390 MPa or more. Further, in order to meet the recent demand for higher strength of steel sheets, those having a tensile strength of 780 MPa or more are preferable. In this case, 0.05 to 1.0% of Mo is contained and 1.5% of Cr is added. In order to obtain a high-strength steel sheet having a tensile strength of 900 MPa or more, the Mo content is 0.05 to 1.0%, and the Cr content is 0.3 to 1.5. And 85% or more of the metal structure is preferably bainite.

  According to the present invention, the maximum depth (Ry) of the unevenness existing on the surface of the hot-rolled steel sheet and the average interval (Sm) of the unevenness are defined, and the load length ratio (tp40) of the surface unevenness and / or By specifying the difference between the load length ratio (tp40) and the same (tp60), the chemical conversion processability can be remarkably improved, and not only hot rolled steel sheets not containing Mo but also Mo which deteriorates the chemical conversion processability. Even a high-strength steel sheet containing an appropriate amount for high strength can guarantee excellent chemical conversion treatment, and can provide a high-strength hot-rolled steel sheet having both strength and chemical conversion treatment at low cost.

  Under the above-described problems, the present inventors have made extensive research to improve the problem of chemical conversion treatment deterioration due to the addition of Mo, particularly for hot-rolled steel sheets to which Mo has been added as a means for increasing the strength. I came.

  As a result, the maximum depth (Ry) of the unevenness on the surface of the hot-rolled steel sheet is specified as “10 μm or more”, the average interval (Sm) of the unevenness is specified as “30 μm or less”, and the load length ratio of the surface unevenness (tp40) Is adjusted to 20% or less and / or the difference [(tp60) − (tp40)] between the load length ratio (tp60) and the same (tp40) of the unevenness to 60% or more, the hot rolling not including Mo Not only steel sheets, but also hot-rolled steel sheets to which an appropriate amount of Mo has been added, a decrease in chemical conversion property is suppressed as much as possible, and a hot-rolled steel sheet having excellent chemical conversion property and strength can be obtained. stopped.

The above-mentioned maximum depth (Ry) of the surface irregularities defined in the present invention means the distance between the highest peak (Rt) and the deepest valley bottom (Rb) of the surface roughness curve as shown in FIG. For example, as shown in FIG. 2, the average interval (Sm) is a point (S ₁ , S ₂ ...) From a change point to the next change point, with a change point from a peak to a valley of the average line in the surface roughness curve. ... means the average value of S _n ). The load length ratio (tp) is, for example, the length of the cut portion (l ₁ , l ₂ ... L _n ) when the surface roughness curve is cut at a certain cutting line level (p) as shown in FIG. It means the percentage with respect to the measurement length (L), and when the cutting line level (p) is the highest peak (Rt), it is 0 (zero) and (tp0) is the deepest valley (Rb). tp100). The percentage of the cut portion length (l ₁ + l ₂ + l ₃ +... _{N n} ) when the cut line level (p) is “40” or “60” with respect to the measured length (L) is It is a value represented by (tp40) or (tp60).

  And the maximum depth (Ry) of the surface irregularities is “10 μm or more”, the average interval (Sm) is “30 μm or less”, and the load length ratio (tp40) of the surface irregularities is 20% or less, and / or Alternatively, the difference [(tp60) − (tp40)] between the same load length ratio (tp60) and (tp40) is 60% or more includes not only steel materials not containing Mo but also an appropriate amount of Mo. It has been confirmed that even hot-rolled steel sheets can stably exhibit excellent chemical conversion properties.

  In the present invention, as described above, as the maximum depth (Ry) of the surface unevenness is relatively deep and the average interval (Sm) of the unevenness is relatively small, the surface unevenness is fine and deep zinc phosphate. It is considered that the function as a nucleation site of the crystal is enhanced, and the zinc phosphate crystal is easily formed and grown on the entire surface, so that the chemical conversion treatment property is enhanced.

  Further, the load length ratio (tp40) of the surface unevenness is “20% or less” (that is, relatively small), which means that the area (area) of the recessed portion recessed from the protruding portion protruding on the surface is relatively This means that the concave portion similarly becomes a nucleation site of the zinc phosphate crystal, promotes the formation and growth of the zinc phosphate crystal, and further has the same load length ratio (tp60) (tp40). Difference [(tp60) − (tp40)] is “60% or more” (that is, the difference between tp60 and tp40 is relatively large) means that the slope extending from the top of the convex portion to the bottom of the concave portion is in the bottom direction. It represents that it is depressed on the bay instead of having a linear inclined surface, and the formation of zinc phosphate crystals by the sloped portion recessed in the bay functioning as a crystal precipitation site, Promotes growth and contributes to further improvement in chemical conversion processability It is considered that.

  In any case, in the present invention, the maximum depth (Ry) of the surface irregularities is set to “10 μm or more” and the average interval (Sm) is set to “30 μm or less”, as will be clarified in the examples described later. The load length rate (tp40), which has never been recognized from the viewpoint of chemical conversion treatment, is “20% or less” and / or the difference between the load length rate (tp60) and the same (tp40) [(tp60) − By setting (tp40)] to “60% or more”, it was possible to stably obtain excellent chemical conversion properties.

  More preferably, the average interval (Sm) is 20 μm or less, the load length ratio (tp40) is 15% or less, and the difference in load length ratio [(tp60) − (tp40)] is 70 in order to improve the chemical conversion processability. % Or more. Although the value of the additional length ratio (tP60) is not particularly defined, it is preferably 60% or more, more preferably 70% or more in order to improve the chemical conversion treatment property.

  By adopting the surface properties as described above, the phosphate crystals deposited on the steel sheet surface by the chemical conversion treatment become finer, and the P ratio, which is an indicator of the soundness of phosphate, namely Phosphophyllite (phosphoferrite) : P) and Hopeite (Hopeite: H) ratio (P / P + H) is closer to 1, and chemical conversion treatment is improved. In addition, in Mo-added steel, the chemical potential deteriorates because the natural potential proceeds in a noble direction in the chemical conversion solution. However, if the surface properties as described above are used, the deterioration of chemical conversion properties due to Mo is more than compensated. Excellent chemical conversion processability can be obtained.

The method for obtaining the hot-rolled steel sheet having the surface texture as described above is not particularly limited, but according to the experiments by the present inventors, it was confirmed that the surface texture can be approximated by tightening the pickling time. is doing. That is, pickling for removing oxides (so-called scales) generated on the surface of the steel sheet in the hot rolling process is usually performed at about 50 to 85 ° C. for about 10 to 30 seconds using about 10 to 20% hydrochloric acid aqueous solution. However, the above-mentioned surface properties intended in the present invention can be achieved by increasing the concentration of hydrochloric acid in the pickling solution, increasing the pickling temperature, or setting the pickling time longer. More specifically, when the hydrochloric acid concentration of the pickling solution is A (%), the pickling temperature is B (° C.), and the pickling time (immersion time) is C (seconds), these are the following formula (I) (A / 100) × B ² × C ≧ 40000 …… (I)
(For example, 11% HCl-75 ° C.-80 seconds, 15% HCl-80 ° C.-50 seconds, 16% HCl-85 ° C.-40 seconds, etc.), more preferably further in the pickling bath If the pickling solution is supplied to the surface of the traveling steel plate at a flow rate of about 1.0 to 5.0 m / second, or the pickling solution is blown from the nozzle, and the pickling solution is brought into a high-speed turbulent state on the steel plate surface, It has been confirmed that the surface texture as described above can be easily obtained.

  Next, the reason why the preferable component composition of the steel material used in the present invention is determined is as follows.

C: 0.03-1.0%
C is an important element for increasing the strength of the hot-rolled steel sheet, and if it is less than 0.03%, most of C is dissolved in ferrite. The formation of cementite, which is an iron carbide, or carbides such as Nb, Ti, V, etc., which may be added as necessary, is insufficient, and it is difficult to obtain the strength intended by the present invention. More preferably, it is 0.05% or more. However, if the amount is too large, the moldability is deteriorated and the weldability is adversely affected. Therefore, it is preferable to keep it at most 1.0% or less, more preferably 0.23% or less.

Si: 2% or less (including 0%)
Si is an element that effectively acts as a deoxidizing element when melting steel, and also contributes to increasing the strength of steel, but if too much, not only deteriorates formability but also causes surface defects. Since it tends to occur and adversely affects pickling properties and paintability, it is preferable to keep it at most 2%, preferably 1.5% or less.

Mn: 0.3 to 4.0%
Mn has an effect of increasing strength, and is also an important element for fixing S, which is mixed into steel and causes embrittlement, as MnS. In order to effectively exhibit these actions, it is preferable to contain at least 0.3% or more, preferably 0.5% or more. However, if the amount is too large, not only the ductility is lowered and workability is adversely affected, but also the weldability is lowered. Therefore, it is preferable to keep it at most 4.0%, preferably 2.5% or less.

Al: 0.005 to 0.5%
Al is an indispensable element as a deoxidizing element, and in order to exert its effect effectively, it must be contained in an amount of 0.001% or more, preferably 0.005% or more. However, if the amount is too large, the toughness deteriorates due to an increase in the amount of oxide inclusions, and surface defects are liable to occur. Therefore, it is preferable to keep it at most 0.5%, preferably 0.3% or less.

Mo: 1.0% or less Mo is an important element for increasing the strength of a hot-rolled steel sheet by solid solution strengthening, and the effect is effectively exhibited by containing 0.05% or more. However, when the required strength is less than the 390 MPa level, it is not necessary to include Mo. Although the amount of Mo depends on the required strength level of the hot-rolled steel sheet, it is 0.1% or more that the effect is more reliably exhibited. However, if it exceeds 1.0%, the adverse effect on ductility (workability) is more significant than the contribution to increasing the strength, and the strength-elongation balance deteriorates rapidly, so the upper limit was set to 1.0%. . More preferably, it should be suppressed to 0.5% or less. As described above, the present invention has the greatest feature in that the chemical conversion treatment performance deteriorated by the addition of Mo is supplemented by the improvement of the surface properties. High strength hot rolled steel sheet that does not contain steel is also effectively exhibited.

Cr: 1.5% or less Cr has an effect of increasing the strength of the hot-rolled steel sheet with a small addition amount, and particularly when a tensile strength of 780 MPa level or more is required, it is contained at least about 0.1% or more. In the case where a tensile strength of 900 MPa level or more is required, it is preferable to contain 0.3% or more. However, if the Cr content is too large, ductility (workability) is greatly deteriorated more than the contribution to high strength like Mo, so at most 1.5% or less, preferably 1.0% or less. It is good.

  If the required tensile strength is in the range of 390 to 780 MPa, the target tensile strength can be obtained without adding Cr by simply adjusting the content of C, Si, Mn, and Mo among the above elements. Can do. However, even when a hot-rolled steel sheet having such a strength level is obtained, the addition of Cr is extremely effective in practice because the strength can be easily controlled only by finely adjusting the Cr addition amount. From such a viewpoint, a more preferable Cr content is 0.1% or more and 1.5% or less.

  The constituent elements of the steel used in the present invention are as described above, and the balance is substantially Fe. Here, “substantially” means that the inclusion of inevitable impurity elements that can be mixed in the steel raw material or its manufacturing process is allowed, or the amount of other elements is small in a range that does not hinder the action and effect of each component element described above. It may be included. Examples of such inevitable impurity elements include P, S, N, and O, and examples of other elements include Ti, Nb, V, Cu, Ni, B, and Ca. However, if these elements are too large, the ductility and surface properties are deteriorated to a greater or lesser extent and adversely affect the chemical conversion properties. Therefore, Ti is 0.2% or less, Nb is 0.1% or less, and V is 0. .1% or less, Cu is 1.0% or less, Ni is 1.0% or less, B is 0.002% or less, and Ca is 0.005% or less.

The strength of the hot-rolled steel sheet according to the present invention can be set at any level of 390 MPa level or higher, 780 MPa level or higher, or 900 MPa level or higher by changing the content of C, Si, Mn, Mo, Cr, etc. according to the application. Although it is possible to adjust the strength, in order to obtain a high strength hot rolled steel sheet of 900 MPa level or higher, Cr is also essentially added as a strengthening element other than Mo, and heat treatment conditions are devised (for example, hot rolling finishing temperature) (Ac ₃ points) or more, and then the cooling rate is 30 ° C./second or more, and the steel structure is bainite rich (preferably 85% or more). .

  Although the present invention is configured as described above, the chemical conversion treatment property is improved for high-strength steel sheets, and the surface properties are appropriately adjusted even for high-strength hot-rolled steel sheets to which Mo useful as a strengthening element is added. By controlling, it was possible to prevent the deterioration of chemical conversion treatment, which had been pointed out as a practical problem with the addition of Mo, and to provide a hot-rolled steel sheet having both high strength and excellent chemical conversion treatment. .

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Examples Steel materials 1 to 15 having chemical components shown in Table 1 below were melted and slabs were produced by casting. This slab was reheated to Ac ₃ points or more, and hot rolled under the conditions shown in Table 2 to obtain a hot rolled steel sheet having a thickness of 3.2 mm. Table 2 shows the mechanical properties of the obtained hot-rolled steel sheet and the bainite area ratio in the longitudinal sectional structure.

  Each obtained hot-rolled steel sheet is pickled under the conditions shown in Tables 3 and 4 (note that the flow rate of the pickling solution is blown toward the steel sheet surface from a slit-like jet nozzle disposed at the entrance and exit of the pickling tank. The surface properties of the pickled steel sheet obtained were observed with a laser microscope (model number “1LM21W” manufactured by Lasertec Co., Ltd.) using a 50 × objective lens and randomly selected. By scanning an area of 0.16 mm × 0.22 mm per location for 10 locations, the average spacing (Sm), maximum depth (Ry), load length ratio (tp40) and (tp60) of surface irregularities While calculating | requiring each value and its difference as an average value, the chemical conversion treatment property was evaluated by the following method. In addition, about some samples, after the pickling process, the skin pass was performed with the predetermined rolling reduction, and chemical conversion treatment property was evaluated. The results are collectively shown in Tables 3 and 4.

Chemical conversion:
After chemical conversion treatment of the surface of each test steel plate under the following conditions, the steel plate surface was observed by SEM at a magnification of 1000 times, and the adhesion status of zinc phosphate crystals was examined for 10 randomly selected fields, and chemical conversion treatment was performed according to the following criteria. Sex was evaluated.
Chemical conversion treatment liquid: Uses Palbond L3020 chemical conversion treatment liquid manufactured by Nihon Parkerizing Co., Ltd. Chemical conversion treatment process: Degreasing (120 degree at 45 ° C. using degreasing liquid “Fine Cleaner” manufactured by Nihon Parkerizing Co., Ltd.) → Water washing (30 seconds) → Surface adjustment (immersion for 15 seconds in surface preparation liquid “preparen Z” manufactured by Nihon Parkerizing Co., Ltd.) → Chemical conversion treatment (immersion in the above chemical conversion liquid at 43 ° C. for 120 seconds)

Evaluation Criteria Skete: Uniformly adhered in all 10 fields of view: ◎ Skews of 5% or less in 10 fields of view were observed in 3 fields or less: (◯), others: (x) .
Particle size: 10 large particles are selected from each field of view, and the average particle size is evaluated.
10 μm or more: ×, 7 μm or more and less than 10 μm: ○,
4 μm or more to less than 7 μm: ◎ <4 μm: ●.
P ratio: Peaks corresponding to phosphoferrite (p) and hopite (H) are measured by X-ray diffraction on the steel sheet surface after chemical conversion treatment, and evaluated by the ratio (P / P + H) (average value of n = 5). . P ratio = P / (P + H), less than 0.85: x, 0.85 or more and less than 0.93: ◯, 0.93 or more and less than 0.96: ◎, 0.96 or more: ●.

Judgment was comprehensively evaluated from the above-mentioned scale, particle size and P ratio as follows.
If the scale is ◎, the particle size is ●, and the P ratio is ●, the total is ● (best),
Scale is ◎, particle size and P ratio is more than ◎, other than the above, ◎ (excellent),
Suke, particle size, P ratio is more than ○, and other than the above, ○ (good),
If any of the scale, particle size, and P ratio is x, it is x (bad) in total.

  From the above Tables 1 to 4, it can be considered as follows.

  Sample No. Nos. 1, 5, 9, and 14 are comparative examples in which the surface properties deviate from the requirements of the present invention, and all have poor chemical conversion properties. Sample No. 13 is sample No. Although the skin pass treatment was performed after the pickling treatment of No. 11, the surface treatment was out of the specified range of the present invention by the skin pass, so that the chemical conversion treatment property was deteriorated.

  Sample No. No. 16 has a slightly mild pickling condition, so the surface properties defined in the present invention are not obtained, and the chemical conversion treatment property cannot be said to be sufficient.

  Sample No. No. 20 has a relatively low pickling temperature and a relatively short immersion time, and the flow rate of the pickling liquid sprayed toward the steel sheet is relatively slow and the surface properties are not sufficiently improved. The processability is slightly insufficient.

  Sample No. In No. 24, since the maximum depth (Ry) was out of the specified range in the skin pass after the pickling treatment, the chemical conversion treatment performance was slightly insufficient.

  Sample No. No. 28 is an example in which the average interval (Sm) of the surface irregularities is particularly out of the preferred range in the skin pass after the pickling treatment, and the chemical conversion treatment property is poor.

  Sample No. No. 31 does not have an appropriate surface property because the pickling treatment conditions are inappropriate, and the chemical conversion treatment is slightly insufficient.

  Sample No. No. 36 is suitable for pickling treatment and has good surface properties, but because the steel material used contains Mo in an amount exceeding the specified value, the chemical conversion treatment property is poor.

  In contrast, sample no. 2 to 4, 6 to 8, 10 to 12, 15, 17 to 19, 21 to 23, 25 to 27, 29, 30, 32 to 35, and 37 are examples that satisfy the prescribed requirements of the present invention. In all cases, excellent chemical conversion properties are obtained.

  Sample No. Nos. 16, 20, 24, and 31 do not satisfy the requirements of the present invention regarding the load length ratio of the surface, but Mo is not contained in the steel, so that a good chemical conversion treatment property can be obtained. ing.

It is a figure for demonstrating the definition of the maximum depth (Ry) of the unevenness | corrugation which exists in the steel plate surface. It is a figure for demonstrating the definition of the average space | interval (Sm) of the unevenness | corrugation which exists in the steel plate surface. It is a figure for demonstrating the definition of the load length rate (tp40) of the unevenness | corrugation which exists on the steel plate surface, (tp60).

Claims (8)

  The maximum depth (Ry) of the unevenness present on the steel sheet surface is 10 μm or more, the average interval (Sm) of the unevenness is 30 μm or less, and the load length ratio (tp40) of the surface unevenness is 20% or less. A high-strength hot-rolled steel sheet with excellent chemical conversion properties.   The maximum depth (Ry) of the unevenness existing on the steel sheet surface is 10 μm or more, the average interval (Sm) of the unevenness is 30 μm or less, and the difference between the load length ratio (tp60) of the surface unevenness (tp40) is 60 % High-strength hot-rolled steel sheet with excellent chemical conversion properties.   The maximum depth (Ry) of the unevenness present on the surface of the steel sheet is 10 μm or more, the average interval (Sm) of the unevenness is 30 μm or less, the load length ratio (tp40) of the surface unevenness is 20% or less, and the surface A high-strength hot-rolled steel sheet excellent in chemical conversion property, characterized in that the difference between the load length ratio (tp60) and (tp40) is 60% or more.   Steel: C: 0.03 to 1.0% (in the case of chemical components, means mass%, the same shall apply hereinafter), Si: 2% or less (including 0%), Mn: 0.3 to 4.0 %, Al: 0.001 to 0.5% is satisfied, the balance is substantially iron, and the tensile strength is 390 MPa or more. High-strength heat excellent in chemical conversion treatment according to any one of claims 1 to 3 Rolled steel sheet.   The high-strength hot-rolled steel sheet excellent in chemical conversion treatment property according to claim 4, wherein the steel contains Mo: 0.05 to 1.0%.   The high-strength hot-rolled steel sheet excellent in chemical conversion property according to claim 5, wherein the steel contains Mo: 0.05 to 1.0% and has a tensile strength of 780 MPa or more. Steel is another element,
Cr: 1.5% or less (excluding 0%),
Ti: 0.2% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%),
Cu: 1.0% or less (excluding 0%),
Ni: 1.0% or less (excluding 0%),
B: 0.002% or less (excluding 0%),
Ca: 0.005% or less (excluding 0%),
The high-strength hot-rolled steel sheet excellent in chemical conversion property according to any one of claims 4 to 6, comprising at least one element selected from the group consisting of:   The steel according to claims 4 to 7, wherein the steel contains Mo: 0.05 to 1.0% and Cr: 0.3 to 1.5%, 85% or more of the metal structure is bainite, and the tensile strength is 900 MPa or more. A high-strength hot-rolled steel sheet according to any one of the above.