HEAT TREATED STEEL PRODUCT HAVING HIGH STRENGTH AND EXCELLENT CHEMICAL CONVERSION COATING ABILITY AND METHOD OF PRODUCTION OF SAME

Abstract

A steel product bent by heating to 600° C. or more, specifically a heat treated steel product having high strength and excellent chemical conversion coating ability which has scale with FeO content of 90% or more, having a thickness of 1 μm or less on the surface.

Claims

1. Heat treated steel product comprising a scale with FeO content of 90% or more, having a thickness of 1 μm or less on the surface.

2. The heat treated steel product according to claim 1, wherein the steel has a structure consisting of martensite, or martensite and tempered martensite.

3. The heat treated steel product according to claim 1, wherein the steel product is a hollow member having a closed horizontal cross-sectional shape.

4. The heat treated steel product according to claim 1, wherein a maximum value and a minimum value of the thickness of the scale are within ±10% of an average value of the thickness.

5. A method of producing a heat treated steel product using a working apparatus having a gas chamber, a heating device, and a cooling device from an upstream side, said method of producing the heat treated steel product comprising introducing an inert gas into a gas chamber and filling the inert gas into a space including the heating device and the cooling device while making the working apparatus move relative to a steel material so that the steel material is locally heated by the heating device then the steel material is cooled by the cooling device, wherein a time period during which the steel material dwells in a 600° C. or more temperature region is less than 1 second and between the heating and cooling, a bending operation is performed at a portion of the steel material greatly dropping in deformation resistance due to heating.

6. The method of producing a heat treated steel product according to claim 5, wherein in the step of cooling, a time period during which the steel material dwells in a 600° C. to 300° C. temperature region is within 3 seconds.

7. The heat treated steel product according to claim 2, wherein the steel product is a hollow member having a closed horizontal cross-sectional shape.

8. The heat treated steel product according to claim 2, wherein a maximum value and a minimum value of the thickness of the scale are within ±10% of an average value of the thickness.

9. The heat treated steel product according to claim 3, wherein a maximum value and a minimum value of the thickness of the scale are within ±10% of an average value of the thickness.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0025] FIG. 1 is a view showing one example of a working apparatus able to be used in the present invention.

DESCRIPTION OF EMBODIMENTS

[0026] The reasons for limitation of the heat treatment use product and the method of production of the same according to the present invention will be explained below.

[0027] The heat treated steel product of the present invention is produced using as a material a steel material which has not been plated. The surface of the product after heat treatment has a very thin scale (oxide film). The thickness has to be 1 μm or less.

[0028] If the thickness of the scale exceeds 1 μm, a lot of scale will remain without being dissolved at the time of chemical conversion, the supply of iron ions will become insufficient, and the chemical conversion coating ability will become degraded. Further, if the scale becomes thicker, even if a chemical conversion coating is formed on the scale, the scale and base iron will easily peel apart and the coating adhesion will become inferior. Therefore, the thickness of the scale is 1 μm or less, preferably 0.5 μm or less.

[0029] Further, scale has to include FeO: 90% or more. This ratio can be found by finding the X-ray intensities of FeO, Fe.sub.3O.sub.4, and Fe.sub.2O.sub.3 by analyzing the product surfaces by X-ray diffraction and calculating the ratio of the X-ray intensity of FeO with respect to the total of the X-ray intensities of FeO, Fe.sub.3O.sub.4, and Fe.sub.2O.sub.3.

[0030] If the ratio of FeO is less than 90%, a lot of scale will remain without dissolving at the time of chemical conversion, the supply of iron ions will become insufficient, and the chemical conversion coating ability will become inferior. The reason is not necessary clear, but is believed to be like the following:

[0031] In scale, first, FeO is formed at a high temperature. Along with the progression of oxidation, Fe.sub.3O.sub.4 is formed or, during the cooling process, some of the FeO undergoes eutectoid transformation and Fe.sub.3O.sub.4 is formed. If, in the product, the ratio of FeO in the scale decreases and the ratio of Fe.sub.3O.sub.4 increases, the chemical conversion coating ability deteriorates since Fe.sub.3O.sub.4 is harder to dissolve in a chemical conversion coating solution compared with FeO.

[0032] The steel product of the present invention has to have the high strength obtained by heat treatment while being excellent in chemical conversion coating ability, so the steel structure is comprised of martensite. However, depending on the required strength and performance, part of the martensite may also be replaced with tempered martensite. Further, carbides and residual austenite which unavoidably remain in the process of heat treatment may also be contained.

[0033] Note that, the steel is not limited in structure in the non-heat treated parts provided anywhere in a heat treated steel product as needed and the boundary region between a heat treated part and non-heat treated part. Such a part may be provided at part of the product.

[0034] The heat treated steel product of the present invention is not particularly limited in shape, but a hollow member having a closed horizontal cross-sectional shape is suitable. A heat treated steel product can be produced for example by hot 3D bending. Hot 3D bending is suitable for obtaining a high strength, high rigidity hollow member having any bent shape.

[0035] The heat treated steel product of the present invention is produced using a working apparatus having a gas chamber, heating device, and cooling device from the upstream side. Below, this will be explained more specifically using FIG. 1.

[0036] FIG. 1 shows one example of the working apparatus used in the present invention. The steel material 11 is made to move with respect to the working apparatus 10 to work it. The working apparatus has a gas chamber 12, heating device 13, and cooling device 14 from the upstream side. In FIG. 1, for assisting understanding of the structure, the cross-section is drawn, but the gas chamber 12, heating device 13, and cooling device 14 are provided so as to cover the entire circumference of the steel material 11.

[0037] Inside the gas chamber 12, argon, nitrogen, or another inert gas is introduced. The inert gas is filled in the space containing the heating device 13 and cooling device 14. The steel material 11 is heated locally by the heating device 13 (11a), then is cooled by the cooling device 14. Here, in the process of heating and cooling, the time period during which the steel material 11 dwells in the 600° C. or more temperature region is made less than 1 second.

[0038] If performing the heat treatment in the state where air is contained in the space around the heated part of the steel material, thick scale is formed and the chemical conversion coating ability and corrosion resistance after coating deteriorate. On the other hand, even if blowing and filling the inert gas in the space around the heated part, if the dwell time in the 600° C. or higher temperature region where the steel material rapidly oxidizes exceeds 1 second, the scale is formed thickly or the scale advances in degree of oxidation and the ratio of Fe.sub.3O.sub.4 increases, so the chemical conversion coating ability deteriorates.

[0039] Therefore, in the present invention, a working apparatus provided with a gas chamber at an upstream side of a heating device is used, the inert gas is introduced into the gas chamber, and the space around the heated part and cooled part of the steel material including the space around the steel material before heating is filled with the inert gas. Furthermore, in the heating and cooling process, the time period during which the steel material dwells at 600° C. or more is made less than 1 second, preferably is made 0.5 second or less.

[0040] Furthermore, in the process where the steel material is cooled, the time period during which the steel material dwells at 600° C. to 300° C. in temperature region is preferably made within 3 seconds. If scale is formed at a high temperature, then becomes near 600° C. or less in the cooling process, the FeO undergoes eutectoid transformation and Fe.sub.3O.sub.4 is formed. For this reason, making the steel material quickly pass through the 600° C. to 300° C. temperature region where the reaction easily proceeds so as to suppress the formation of Fe.sub.3O.sub.4 and return the steel material to a low temperature with the FeO as is preferable for obtaining an excellent chemical conversion coating ability.

[0041] Furthermore, in the present invention, by sufficiently filling the inert gas around the heated part of the steel material, it becomes possible to make the thickness of the scale uniform. Preferably, the maximum value and minimum value of thickness of the scale can be made ±10% or less of the average value of the thickness.

[0042] It is also possible add tempering or other heat treatment in accordance with the strength and performance required from the product. In this case, it is effective to make the dwell time at 600° C. or more through all of the heat treatment less than 1 second, more preferably make the dwell time from 600° C. to 300° C. less than 3 seconds.

[0043] Note that, the positioning devices 21a, 22b, industrial robot 32, chuck 33, etc. drawn in FIG. 1 show preferable examples of a working apparatus able to be used in the present invention. The present invention is not limited by this drawing needless to say. Further, while not shown, it is also possible to provide a shield plate at the downstream side of the cooling device 14 to make it easier for the inert gas to fill the space including the gas chamber 12, heating device 13, and cooling device 14.

EXAMPLES

[0044] To confirm the effects of the present invention, rectangular cross-section electric resistance welded steel tubes having the chemical composition shown in Table 1 (40 mm×40 mm×thickness 1.6 mm) as materials were prepared.

TABLE-US-00001 TABLE 1 (mass %, balance: Fe and unavoidable impurities) C Si Mn P S sol. Al N Cr Ti Nb B 0.22 0.20 0.75 0.014 0.003 0.04 0.004 0.30 0.030 0.025 0.015

[0045] These steel tube materials were heat treated under the conditions shown in Table 2 using the hot 3D bending apparatus shown in FIG. 1 to obtain heat treated steel products. Note that, No. 3 of Table 2 was tempered by control of the cooling process of the hot 3D bending apparatus.

[0046] The obtained heat treated steel products were examined for cross-sectional structure after Nital etching using a scanning electron microscope at powers of 500× for four fields to confirm the steel structure.

[0047] Further, the surfaces of the steel tubes were measured for thickness of the scale by X-ray photoelectron spectrometry. X-ray diffraction was used for analysis of the scale composition. The X-ray intensities of the FeO, Fe.sub.3O.sub.4, and Fe.sub.2O.sub.3 were found and the ratio of the X-ray intensity of FeO with respect to the total of the X-ray intensities of the FeO, Fe.sub.3O.sub.4, and Fe.sub.2O.sub.3 was calculated. This was used as the FeO ratio in the scale.

[0048] Here, the ratio of the X-ray intensity evaluates the X-ray diffraction peaks of FeO, Fe.sub.3O.sub.4, and Fe.sub.2O.sub.3 at the time of an X-ray source of CuKα (40 kV-50 mA) by the Rietveld method.

[0049] Further, the obtained heat treated steel products were formed with similar chemical conversion coatings as above, then were formed with electrodeposition coatings by a PN-110 made by Nippon Paint aiming at a coating thickness of 20 μm to obtained coated products. The coated products were evaluated for cross-cut tape peeling after immersion in 40° C. warm water for 240 hours as a coating film adhesion test. Further, they were evaluated for rust and blistering of the cut parts after 180 cycles of a JASO cyclic corrosion test.

[0050] In the coating film adhesion test, samples where there were no greatly peeling pieces and where small peeling of the cut cross parts accounted for 5 area % or less were judged as “good”. In the evaluation of rust and blistering of the JASO test, samples with a maximum width of rust or blistering at the two sides of the cuts of 12 mm or less were judged as “good”.

[0051] The results are shown together in Table 2. Note that, in the column of “Steel structure” of Table 2, “M” indicates martensite, while “TM” indicates tempered martensite. In the results of evaluation of the corrosion resistance after coating, good is indicated by “G”, while poor is indicated by “P”.

TABLE-US-00002 TABLE 2 Production conditions Corrosion Dwell Dwell resistance time at time at Scale after coating Heated Heating 600° C. 600 to FeO Coating part temp. or more 300° C. Steel Thickness ratio adhesion JASO No. atmosphere (° C.) (sec) (sec) structure (μm) (%) test test Class 1 Blown 1000 0.4 1 M 0.3 98 G G Inv. ex. 2 nitrogen 1000 0.9 1 M 0.8 90 G G Inv. ex. 3 1000 0.4 2.8 M+ 0.4 92 G G Inv. ex. partial TM 4 1000 2.0 1 M 1.5 85 G P Comp. ex. 5 1000 1.0 10 M 1.0 80 G P Comp. ex. 6 Air 1000 0.4 1 M 3.0 70 P P Comp. ex.

[0052] As shown in Table 2, it could be confirmed that by satisfying the ranges prescribed in the present invention, a heat treated steel product excellent in chemical conversion coating ability and in turn excellent also in corrosion resistance after coating is provided even if used for a chemical conversion coating step without going through a shot blasting or other scale removal step.