JPH0739620B2 - Lightweight structural steels for building structures, their materials, and their manufacturing methods - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a lightweight structural steel for construction, which is suitable as a structural material for industrialized houses, and a hot-rolled steel sheet as a raw material thereof, and a method for producing the same. It is about.

<Prior art and its problems> Not only does the strong desire for single-family dwelling units diminish in recent years, despite the dramatic rise in land prices, but the trend toward high-class people is rising for 50 years or 100 “High-standard housing” aimed at long-term durability such as the year has become a quiet boom.

Therefore, each industrialized housing maker is enthusiastically responding to these requirements by "ceramic conversion of exterior housing wall panels" and "expansion of application range of ALC concrete". (Framework, columns, beams, etc.)
Since iron-based materials are most suitable for
There has been a growing interest in measures for further improving the durability of lightweight steels used as aggregates.

By the way, the aggregates currently used in industrial steel-framed houses are lightweight structural steel for general structures and JIS G 3350.
It is a welded lightweight H-section steel for general structures specified by JIS G 3350, and SS41 steel class carbon steel was generally used as the material. However, until now, most of the product forms delivered from material manufacturers to industrialized housing manufacturers were hot-rolled black leather materials, but the recent trend is that lightweight section steel made from Zn-plated materials is used. It has increased. These are: a) Aggregate corrosion progresses due to rainwater and sea salt particles entering through the gaps in the outer wall panel, b) Undesirable corrosion progress of aggregates due to water entering from the interior as a result of dew condensation, etc. The problem of corrosion over time around baths and kitchens is attracting attention.

However, after being delivered from a material manufacturer and completing the assembly as a frame by each industrialized house manufacturer, iron phosphate chemical conversion treatment or zinc phosphate chemical conversion treatment as a base treatment is performed, and then anion or cation electrodeposition coating or static coating is performed. The "light-weight steel", which is generally coated with electric powder and transported to the construction site, has the following problems in anticorrosion even if it is Zn-plated.
That is, (a) the plating film at the welded portion disappears due to welding performed at the construction site, so the rust-preventive effect of the plated film cannot be expected, (b) the throwing power of the plated film at the end is not good, Therefore, the corrosion resistance of this part is inferior, (c) The metal material surface is exposed in many bolt holes, and the corrosion resistance of this part is inferior. (D) If the plating film is damaged during construction, the scratched part Corrosion resistance deteriorates.

Therefore, some industrial housing manufacturers have begun to adopt lightweight section steels based on "plated steel sheets with higher corrosion resistance than conventional plated steel sheets" as a solution to these problems. However, no matter how high corrosion-resistant plated steel sheet is used as a base material for lightweight steel, as long as it is a plated steel sheet, the problems pointed out in (a) and (c) above are still unsolved, and high corrosion resistance It cannot be said that the adoption of plated steel sheets is a sufficient measure.

Further, some industrialized housing manufacturers have pointed out that the plating means is not necessarily a sufficient measure against corrosion over the years due to the problem of dew condensation.

Therefore, in order to cope with these points and the high-grade orientation of users, A) As a material for lightweight shaped steel, A) no deterioration of corrosion resistance at the welded part is seen, or very slight, B) at the bolt holed part. Corrosion resistance deterioration is not observed, or is extremely slight, C) Corrosion resistance deterioration at the damaged coating film that is likely to occur during construction is not observed or is extremely slight, etc. However, it was necessary to develop a steel material that has significantly better corrosion resistance than conventional materials. Moreover, in order to promote a wide range of applications to replace the conventional lightweight steel for industrialized houses, it was also necessary that the price level of the steel material to be developed should be sufficiently competitive with the conventional steel material.

<Means for Solving the Problems> From the viewpoints described above, the inventors of the present invention have not only excellent corrosion resistance, but also have no particular adverse effect on the performance depending on how they are handled, and also the conventional material in terms of price. As a result of various studies to provide a lightweight structural steel for building structures that can sufficiently counter the above, it is difficult to achieve the objective by the method that has been considered in the past, "higher corrosion resistance" or "high corrosion resistance plating". However, there are ordinary stainless steel plates (for example, SUS410) in the material steel plate.
L) with less Cr than in
If the other components in the raw steel sheet are properly adjusted, sufficient corrosion resistance can be ensured without being particular about the plating treatment, and a lightweight structural steel for building structures can be realized that can sufficiently meet the above requirements. I was able to obtain new knowledge.

That is, a Cr-containing steel plate with an appropriate amount of Cr added and other components appropriately adjusted has a) a very small deterioration in corrosion resistance at the welded part, and a) a very small deterioration in corrosion resistance at the bolt hole. ) If necessary, conventional rust prevention measures can be taken, and even if the coating film is damaged, deterioration of corrosion resistance at the scratched part is guaranteed by the good corrosion resistance of the base metal, so it is exceptional. It has been clarified that it has features such as that it does not cause any inconvenience, and that the following items can be confirmed.

First, using a Cr-containing steel sheet whose composition has been adjusted as described above, a conventional structural welded lightweight H-section steel with mechanical properties equivalent to JIS G 3353 by the existing "line for forming lightweight section steel by electric resistance welding". Was investigated in order to produce a "welding defect called" penetrator ", which was often a problem when performing conventional electric resistance welding of lightweight H-section steel in the atmosphere (generated by heating during electric resistance welding). Weld defects caused by the oxide scale being caught and left inside the fusion-bonded portion) tended to be recognized even when using the Cr-containing steel sheet of the specific composition as a material, but for this problem, for example, As described in JP-A-60-199582, "the electric resistance welding in a state of being shielded by a non-oxidizing atmosphere or a reducing flame" can be sufficiently dealt with. " Obvious Further, the specific composition
According to various studies on the manufacturing conditions of Cr-containing steel sheet, "When manufacturing a hot-rolled steel sheet from a Cr-containing steel slab of the above composition obtained by the continuous casting method, the heating temperature of the slab, the hot rolling finishing temperature, and the finishing sheet thickness. Also, if the winding temperature is regulated to a specific condition, it becomes possible to extremely stably manufacture a highly corrosion-resistant steel plate that is sufficiently excellent in mechanical properties for structural lightweight steel production. ” I was able to. Moreover,
The Cr-containing steel sheet with the above specified composition can be manufactured by utilizing the conventional carbon steel production line, and the SS-41
Sufficiently able to withstand plating-type materials that use steel class carbon steel as a base material, and that lightweight section steels made from these materials can be formed without much modification to conventional lightweight section steel production lines. Was also confirmed.

The present invention has been made based on the above findings and the like, "a lightweight structural steel for a building structure formed by electric resistance welding, and a hot rolled steel sheet for producing a lightweight structural steel for a building structure, C: 0.001 to 0.1% (hereinafter,% indicating the component ratio is% by weight), Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.5%, P: 0.001 to 0.
15%, S: 0.0001 to 0.02%, Cr: more than 5.0% and less than 10.5%, N: 0.001
~ 0.10%, O: 0.0001 ~ 0.02%, Al: 0.001 ~ 0.1%, or 1 or 2 kinds of Cu: 0.001 ~ 0.8%, Ni: 0.001 ~ 0.8%, in addition to this, the balance Is composed of Fe and inevitable impurities '', and further, C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, or in addition to this, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8%, 1 or 2 A hot-rolled steel sheet that also contains seeds, the balance of which is composed of Fe and unavoidable impurities, is resistance-welded and molded in a state of being shielded by a non-oxidizing atmosphere or reducing flame to produce a lightweight structural steel for building structures. Manufacturing point '', C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, or in addition to this, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8%, 1 or 2 A continuous cast slab that also contains seeds and the balance consisting of Fe and unavoidable impurities is heated to 1000 to 1300 ° C and hot-rolled, and after finishing in a temperature range of 500 ° C or higher, immediately 400
It is also characterized in that it is rolled in the temperature range of ℃ or higher to produce hot rolled steel sheet for the production of lightweight shaped steel for building structures. "

The hot-rolled steel sheet for manufacturing lightweight structural steel for a building structure according to the present invention has a general structural lightweight H-shaped steel (excellent performance) obtained by cutting into an appropriate width and continuously forming by electric resistance welding ( Not only can it be the so-called "welded H" of JIS G 3353), but it can also be cut into a proper width and continuously molded as it is to be a lightweight structural steel for general structures (JIS G 3350, even if it has a shape equivalent to JIS G 3352). It goes without saying that it also has excellent performance as a deck plate.

That is, the lightweight structural steel for a building structure according to the present invention is sufficiently satisfactory to be applied to an industrialized house requiring long-term durability such as 50 years or 100 years where market needs are increasing day by day. The hot rolled steel sheet according to the present invention exhibits excellent performance and is very suitable as a material for these lightweight shaped steels.

Next, in the present invention, the composition of the lightweight steel for building structure and the hot-rolled steel sheet for manufacturing lightweight steel for building structure, and the reasons for limiting the manufacturing conditions thereof as described above will be described in detail.

<Operation> A) Component composition C C in steel is an extremely important element for ensuring proper mechanical properties in the material of the present invention. However, when a large amount of C is present, it tends to combine with Cr in the steel to form Cr-based carbides and deteriorate corrosion resistance, while on the other hand, it promotes austenitization, martensite formation, and bainite formation. Causes a significant increase in hardness. From such a viewpoint, the upper limit of the C content is set to 0.1%. On the other hand, if the C content is made as low as possible, the manufacturing cost will be significantly increased, and it is necessary to reduce the C content to less than 0.001% from the viewpoint of improving the performance as a lightweight steel for industrialized housing. Therefore, the lower limit of C content is 0.001%
I decided.

Si Si, like Al, is an important component as a deoxidizing element,
If the content is less than 0.01%, the effect as a deoxidizing element cannot be obtained. On the other hand, if Si is contained in excess of 0.60%, the steel quality will be significantly hardened, so the upper limit of Si content is 0.60%.
I decided.

Mn in Mn steel is an important element in adjusting the austenite balance of the material of the present invention, but no significant change in performance is observed even if it is contained in an amount exceeding 2.5%, while its content is 0.
When it is less than 01%, S in steel becomes Fe-based sulfide, which causes deterioration of corrosion resistance. Therefore, the Mn content is 2.5 to 0.01%.
I decided.

P P in steel has the function of adjusting the austenite balance of the material, the function of increasing the strength, and the function of improving the bolt drilling workability, but if its content is less than 0.001%, the above effect can be sufficiently secured. On the other hand, on the other hand, if the content exceeds 0.15%, the above effect becomes too remarkable and rather harmful, so the P content was set to 0.001 to 0.15%.

S Since S in steel forms sulfides and leads to deterioration of the corrosion resistance of the material, it is necessary to reduce the content to 0.02% or less, but if it is less than 0.0001%, at the current industrial manufacturing level. It is difficult and causes a significant increase in cost. Also, S
Even if the content is reduced to less than 0.0001%, no further significant improvement effect is exhibited in ensuring the corrosion resistance of the material of the present invention. Therefore, the S content is set to 0.0001 to 0.02%.

Cr In steel, Cr is the most important alloying element that affects the durability of steel materials, especially the corrosion resistance, but if its content is 5.0% or less, it is necessary to secure the desired corrosion resistance as a structural steel material for building structures. On the other hand, if 10.5% or more of Cr is contained, "deterioration of coating film adhesion", which is a problem peculiar to stainless steel sheet, becomes remarkable, so the Cr content exceeds 5.0% and less than 10.5%. Specified. However, preferably, the Cr content is 8.
It is better to adjust it in the range of 0% to less than 10.5%.

N N in steel is an effective austenite balance adjusting element, but if it is contained in an amount exceeding 0.10%, not only will the steel quality be significantly hardened, but it will cause considerable difficulty in steelmaking at the current refining level. , The upper limit of the N content was set to 0.10%. On the other hand, the lower limit of the N content was set to 0.001% in consideration of its use as a general-purpose steel material.

O 2 O is an impurity element that is inevitably mixed into the steel and must be kept to 0.02% or less from the viewpoint of hot workability and toughness, but considering the industrially manufacturable limit, O The content was set to 0.0001 to 0.02%.

Al Al is an effective deoxidizing element of steel during refining, and 0.001% or more must be contained as a deoxidizing residue in order to perform sufficient deoxidizing, but exceeding 0.1% Even if it is included, it does not exert a remarkable improvement effect and causes a significant increase in cost. Therefore, the Al content is 0.001-0.1
Defined as%.

Cu, and Ni Cu and Ni, in addition to the effect of improving the corrosion resistance of the material,
It is an austenite-forming element and has the function of adjusting the austenite balance. Further, Cu forms an intermetallic compound in the steel and is effective in improving strength, so one or two elements may be contained as necessary. However, if the content is less than 0.001%, the desired effect due to the above action cannot be obtained. On the other hand, all of these are expensive additive elements, and addition of a large amount leads to an increase in material cost, so the upper limit of each was set to 0.8%.

In addition, in the material according to the present invention, as impurities at the time of alloy addition, melting of refractory, residues of additive elements for reducing S in steel, residual deoxidizer, or impurities mixed from added scrap, etc. Needless to say, it does not matter even if a small amount of V, Ca, REM, Ti, Nb, Mo, etc. is contained.

B) Manufacturing conditions of hot-rolled steel sheet for manufacturing lightweight steel for building structure The heating temperature for hot rolling the slab manufactured by the continuous casting method is required to be 1000 to 1300 ° C.

This is because when the heating temperature of the slab during hot rolling is less than 1000 ° C, segregation during solidification is not homogenized, and Cr, Cu,
Since alloying elements such as Ni and impurities such as P and S do not diffuse sufficiently, the mechanical properties after hot rolling cannot be guaranteed stably,
On the other hand, if heating is performed at over 1300 ° C, homogenization due to diffusion of alloy components can be achieved, but oxidation on the slab surface becomes severe, and in addition, it causes coarsening.

Hot rolling must be completed in the temperature range of 500 ° C or higher. This is because in the steel having the composition according to the present invention, when the finishing temperature of hot rolling is lower than 500 ° C., the deformation resistance becomes remarkably large and rolling becomes conspicuously difficult. In addition, the hot rolling final temperature must be 500 ° C or higher in order to secure the required winding temperature: 400 ° C or higher.

The finished plate thickness at this time is preferably 1 to 10 mm.

The coiling temperature after hot rolling must be 400 ° C or higher.

That is, since the steel according to the present invention contains a relatively large amount of Cr, it has high hardenability, and if it is air-cooled as it is after the completion of rolling, it becomes a [bainite + martensite] mixed structure and high strength is obtained, but ductility is remarkably reduced. . However, since the structural member is required to have a predetermined ductility, the gradual cooling treatment is necessary. Therefore, the inventors of the present invention have variously studied the optimum slow cooling conditions after the completion of rolling, and after completion of rolling, the coil is wound around a coil at a temperature of 400 ° C. or higher and gradually cooled, which is a so-called “self-tempering effect”.
It has been empirically found that it is extremely effective to secure the above-mentioned condition, and that this is a suitable means for obtaining stable and excellent performance. Therefore, the winding temperature was set to 400 ° C or higher.

C) Manufacturing Conditions for Welded Light Section Steel for Building Structures When manufacturing a welded lightweight section steel for building structures according to one of the present invention, electric resistance welding is carried out in a non-oxidizing atmosphere or in a state of being sealed with a reducing flame. Is due to the following reasons.

For example, when manufacturing a welded lightweight H-section steel, the web is continuously resistance-welded in a direction perpendicular to the flange.
The biggest problem at this time is insufficient strength at the joint. This is caused by the "penetrator" in which the oxide scale generated by heating during electric resistance welding is caught and remains inside the fusion-welded portion. Therefore, preventing the performance deterioration due to the penetrator and ensuring the performance guarantee as a product is the most important technical issue for shaped steel manufacturers.

Various proposals have been made to prevent the generation of the above penetrators. The main ones are i) a method for stabilizing the welding phenomenon by an electric device or a structural device of the manufacturing facility, ii) nitrogen gas, etc. Welding with non-oxidizing gas shielding, iii) Welding with shielding with reducing flame. Of these, the method described in item (i) is a measure that focuses on the fact that a penetrator is easily generated when the welding heat input is excessive, but it is clear that such a measure is insufficient in the effect of preventing the penetrator generation. Therefore, there is a problem that the performance cannot be guaranteed against the generation of a minute perretator.

On the other hand, the methods described in the items ii) and iii) are extremely effective methods for preventing the occurrence of the penetrator as disclosed in JP-A-60-199582 and the like. Even when the hot-rolled steel sheet raw material whose composition is specified in (3) is used, a remarkable effect is exhibited. Therefore, in the present invention, electrical resistance welding is limited to being performed in a state of being sealed in a non-oxidizing atmosphere or a reducing flame.

Examples of the above "non-oxidizing atmosphere" include a nitrogen gas atmosphere, an Ar gas atmosphere, a natural gas atmosphere, or an atmosphere of converter gas or blast furnace gas that can be easily obtained at a steel mill. As long as it is an atmosphere that can suppress the high temperature oxidation of the material as compared with the case of heating, it does not matter what kind.

On the other hand, a "reducing flame" is created by burning an organic liquid that produces a reducing atmosphere after vaporization. As the organic liquid, for example, lubricating oil, cutting oil, machine oil, rust preventive oil, kerosene, heavy oil, via, dimethyl phthalate, cellosolve acetate, oleic acid, alcohol or a mixture thereof, or high vapor pressure and reducing property after vaporization A solid powder capable of creating an atmosphere may be added to the above liquid.

Then, by using the hot-rolled steel sheet material whose components are defined in the present invention, and performing electric resistance welding while suppressing oxidation in a state of being sealed in the non-oxidizing atmosphere or reducing flame, a good welding lightweight type without a penetrator defect It is possible to manufacture steel while ensuring stable performance.

That is, in the case of manufacturing the welded lightweight shaped steel for a building structure according to the present invention, it is stable welding if the welding in the air or the water is performed using the hot-rolled black skin material as it is as in the conventional method. Although mechanical properties such as joint strength cannot be obtained sufficiently, the black skin near the joint is removed in advance before welding (mechanical removal,
If it is subjected to acid pickling, etc., and then sealed and welded in a non-oxidizing atmosphere or a reducing flame, a welded lightweight shaped steel for building structure having excellent corrosion resistance and mechanical properties can be realized very stably.

Next, the present invention will be described more specifically by way of examples.

<Example> Example 1 First, the sample steels A to I shown in Table 1 were melted using an AOD furnace of 60 tons and continuously cast into slabs. Then
After heating the slab to 1200 ° C., it was hot-rolled using a tandem rolling mill to obtain coils having a width of 1020 mm and plate thicknesses of 3.2 mm and 4.2 mm.

At this time, the final stand pass target temperature is 650 ℃, 500
The coiling was carried out aiming at coiling above ℃, but the coiling temperature confirmed by actual measurement was 520 ℃.

The hot-rolled coil thus obtained remains as a black skin.
It was slit into 3 mm width and 125 mm width and used as a material for structural lightweight H-section steel (hereinafter "welded H").

Then, some of these are subjected to a dry-wet repeated test and an atmospheric exposure test, and for the other, electrical resistance welding is performed to mechanically grind the joint part in advance to remove the black skin, and then perform normal welding. H Web thickness using on-site production line 3.
A light-weight weld H of 2 mm, flange thickness 4.5 mm, height 250 mm, width 125 mm was prototyped.

The repeated dry and wet test and the atmospheric exposure test were carried out as follows.

The test material was sufficiently degreased with an alkaline degreasing solution and then coated with an epoxy resin-based cationic electrodeposition coating to give a test material. The coating thickness was constant at 15 microns. Even though the epoxy resin-based electrodeposition coating is applied to urban areas where there is no problem of corrosion due to sea salt particles, such as in the coastal areas, industrialized housing structural members are brought into the field and rained until the construction and erection. This is because the corrosion-resistant material of the steel level of the present invention causes rusting. However, the rusting in this case is extremely slight, and there is no progress to the subsequent large corrosion, but it is considered that the red rusting is extremely regrettable from the viewpoint of the owner who is an amateur, and it is the cause of the complaint. Since it was judged that it would be, such conditions were set.

Sulfuric acid was used as the dipping solution for the dry and wet repeated test.
A 300 ppml Cl ^- containing saline solution having a pH adjusted to 3.5 was used. This is because normal rainwater contains carbon dioxide as well as SOx and NOx in the atmosphere.
By absorption, a pH of around 5 to 3.5, and Cl from the sea as sea salt ^- Cl in 30~50ppm absorb ions ^-
This is because it contains ions. 2km from the sea
Industrialized houses located within the following location conditions are suffering from sea salt particle problems, and the corrosion problem of structural members is a major problem especially in the coastal region of the Sea of Japan exposed to strong winds from the sea in winter. . Therefore, from the viewpoint of concentration problems associated with drying and appropriate corrosion acceleration, the Cl ^- ion concentration in the test solution for immersion is 6 to 10 times the Cl ^- ion concentration in rainwater, which is 300 ppm.
And

The condition of the repeated dry and wet conditions was such that immersion was repeated for 3 hours-air drying for 3 hours, and the test solution temperature during immersion was 35 ° C. The number of repeated dry and wet cycles is 50.

The results of these tests are shown in Table 2.

As is clear from the results shown in Table 2, in the steel sheet satisfying the conditions specified in the present invention, various corrosion resistance and coating film conditions are sufficiently excellent, while the composition of the steel sheet is in the present invention. It can be seen that if the specified conditions are not satisfied, the corrosion resistance is not sufficient or the coating film condition is poor.

That is, in Table 2, in the case of using A steel that does not contain Cr in steel (test number 1, 2), the effect of the chemical conversion treatment as the coating base is swelling in the healthy part of the coating film in the dry and wet repeated test. It is recognized as the presence or absence of corrosion, but red rust is generated in the scratched portion of the coating film, the bolt hole formation portion and the welded portion, and the coating film swells in the processed edge coating sound portion, and it cannot be said that the corrosion resistance is sufficient.

Also, in the case of using B steel having a base metal Cr concentration of 3.45% (Test No. 3), the chemical conversion treatment as the coating base treatment cannot be performed due to the Cr content, and therefore the coating film swells in the coating film sound part. ,
Further, remarkable red rust similar to Test No. 2 is found in the scratched portion of the coating film, the bolt hole, and the welded portion. Further, although not shown in Table 2, in Test No. 3, large corrosion progress accompanied by peeling of the coating film was observed after 50 cycles.

In the case of using C steel with a Cr concentration of 5.07% (Test No. 4), swelling was not seen in the coating film sound part, and the degree of rusting in the bolt hole was remarkably slight, and at the processing edge part. No swelling of the coating film was observed. The same tendency was observed in test number 5 using steel D having a higher Cr concentration than in test number 4.

In the case of using F steel with a Cr concentration of 10.1% (Test No. 7), no blister of the coating film is observed in the damaged portion of the coating film and the processed edge coating sound part, but H steel containing 11.6% of Cr is used. The swelling of the coating film or the peeling of the coating film was observed at the scratched portion of the coating film in the case of using the steel I (test number 9) and the steel I containing 16.8% Cr (test number 10).

As described above, it can be confirmed from this test result that the Cr content must be adjusted to more than 5.0% and less than 10.5%, but here, the swelling of the coating film is closely related to the adhesion of the coating film. Therefore, the coating film adhesion in the corrosion resistant steel of the present invention steel level will be further described.

Light-weight shaped steel made from carbon steel of SS-41 steel class is subjected to chemical conversion treatment to improve coating adhesion and corrosion resistance.When applying the current shaped steel as a structural member to industrialized houses, For this reason, rust prevention and chemical conversion treatment are indispensable.

Initially, the inventors considered applying the present method as it is to the steel of the present invention and studied the application of chemical conversion treatment, but the reactivity between the chemical conversion treatment liquid and the material does not progress in the steel containing Cr even in a small amount. It was easily confirmed. Therefore, the steel of the present invention is based on the fact that no chemical conversion treatment is carried out. However, the fact that an effective chemical conversion treatment method or undercoating method applicable to Cr steel sheets in the future has been developed and the adhesion of the coating film and the corrosion resistance under the coating film have been improved is basically the structural material of the steel of the present invention. It does not have any negative impact on performance, but rather should be greatly welcomed.

By the way, Table 3 shows the adhesion of a similar material when under-coating corrosion is involved, and is generally performed after 100 cycles of the same dry and wet repeated test as shown in Table 2. It is the result of evaluating the adhesion of the coating film by the cellophane tape coating film peeling test. From Table 3, the following can be seen.

That is, when the content is 10.5% Cr or more, the generation of red rust on the base material is suppressed, but the problem of coating film peeling occurs.

In test No. 15 using steel C having a Cr concentration of 5.07%, red rusting and coating swelling proceeded in the coating film flawed portion, and the coating film peeling percentage in the cellophane tape coating film peeling test increased. .

When the Cr concentration is 8.97% or more, the progress of red rust is more noticeable than when 50 cycles of the dry and wet repeated test, but the cellophane tape coating peeling property is not deteriorated and the Cr in the rust layer is not deteriorated.
It is speculated that the rust layer becomes dense due to the formation of the rust layer containing the corrosion product of the system, resulting in the improved adhesion of the coating film.

Therefore, among the materials according to the present invention, the most desirable proper Cr concentration from the viewpoint of undercoat corrosion progress and coating adhesion is 8.
It can be said that it is 0% or more and less than 10.5%.

On the other hand, it was confirmed that the fractured portion in the welded portion tensile test of the trial welded lightweight H-section steel was the fracture of the base material of the web.

Example 2 The sample steels A to I shown in Table 1 were melted by using a 60 ton AOD furnace and were continuously cast into slabs. Next, after heating the slab to 1200 ° C. or higher, hot rolling was performed using a tandem rolling mill to obtain coils having a width of 1020 mm and plate thicknesses of 3.2 mm and 4.2 mm.

At this time, the final stand pass target temperature is 650 ℃, 500
Winding was carried out with the goal of winding above ℃.

Table 4 summarizes the slab heating temperature, hot rolling end-to-end temperature, winding temperature confirmed by actual measurement, final plate thickness, and the accuracy values of the mechanical properties of the test pieces taken from the coil ends.

The hot-rolled coil thus obtained remains as a black skin.
Slit processing was performed on the width of 3 mm and the width of 125 mm to make the material of welding H.

Then, the portion to be joined by electrical resistance welding is mechanically ground in advance to remove the black skin, and a web thickness of 3.2 mm, a flange thickness of 4.5 mm, and a height of 250 m are obtained by using an ordinary welding H site production line.
A lightweight welding H with an m and a width of 125 mm was made as a prototype.

Next, the mechanical properties of the obtained light-weight weld H were measured, and the results are also shown in Table 4.

As is clear from the results shown in Table 4, the light-weight weld H (Test No. 25) manufactured under the conditions specified in the present invention.
(29 ~)) fully meet the JIS standard values of conventional structural light-weight structural steel, and it is clear that there is no problem in application as a lightweight structural steel.

On the other hand, in the test numbers 23 and 24 in which the component composition is out of the regulation of the present invention, although good mechanical properties can be obtained if the production conditions are within the prescribed range of the present invention, There was a problem with corrosion resistance.

On the contrary, even if the composition of the test steel is within the specified range in the present invention, the test conditions 30 and 31 in which the production conditions are out of the specified in the present invention are for the hardenability improving effect of Cr. It can be seen that the extraordinarily high strength is obtained by the introduction of the low temperature transformation product, which is caused, and the ductility is further remarkably reduced.

The fractured portion of the trial welded lightweight H-section steel in the weld tensile test was the fracture of the base material of the web.

Example 3 The test steel D shown in Table 1 was melted by using a 60 ton AOD furnace and continuously cast into a slab. Then, the slab was heated to 1200 ° C., and then hot rolled using a tandem rolling mill to form a coil having a width of 1020 mm and plate thicknesses of 3.2 mm and 4.2 mm.

The target temperature for passing the final stand at this time is set to 800 ° C and 650
The coiling was carried out with the goal of coiling above ℃, but the coiling temperature confirmed by actual measurement was 670 ℃.

The hot-rolled coil thus obtained remains as a black skin.
It was used as a material for welding H, which was slitted into a width of 3 mm and a width of 125 mm and continuously electric resistance welded to form.

Then, the portion to be joined by electrical resistance welding is mechanically ground in advance to remove the black skin, and a web thickness of 3.2 mm, a flange thickness of 4.5 mm, and a height of 250 m are obtained by using an ordinary welding H site production line.
A lightweight welding H with an m and a width of 125 mm was made as a prototype.

The tensile strength of the light-weight welding H prototyped was 45 kgf / mm ² , and the yield point was 2.
With 9 kgf / mm ² and elongation of 35%, it satisfied JIS G 3353 standards for conventional SS-41 steel class materials in terms of mechanical properties. The fractured portion of the trial welded lightweight H-section steel in the weld tensile test was the fracture of the base material of the web.

Example 4 The sample steel C shown in Table 1 was melted by using a 60 ton AOD furnace and was continuously cast into a slab. Then, the slab was heated to 1200 ° C., and then hot rolled using a tandem rolling mill to form a coil having a width of 1020 mm and plate thicknesses of 3.2 mm and 4.2 mm.

At this time, the final stand pass target temperature is 650 ℃, 500
The coiling was carried out aiming at coiling above ℃, but the coiling temperature confirmed by actual measurement was 520 ℃.

The hot-rolled coil thus obtained remains as a black skin.
Slit processing was performed on the width of 3 mm and the width of 125 mm to make the material of welding H.

Next, the portion to be joined by electric resistance welding of the above welding H material is mechanically ground in advance to remove the black skin, and electric resistance welding is performed in a state of being shielded by a non-oxidizing atmosphere or reducing flame. Web thickness 3.2mm, Flange thickness 4.5mm, Height 250mm, Width
A 125mm lightweight welding H was prototyped. Also, as a comparative example,
A similar lightweight weld H was also manufactured by electric resistance welding in the atmosphere as usual.

At this time, Ar gas was used as the non-oxidizing atmosphere, and natural gas flame, lubricating oil burning flame, kerosene burning flame, and cutting oil burning flame were used as reducing flames.

With respect to the obtained light weight weld H, Table 5 shows the results of investigating the number of penetrators in the welded portion.

As is clear from the results shown in Table 5, compared to conventional electric resistance welding in the atmosphere, the number of penetrators is remarkably higher when electric resistance welding is performed in a state shielded by a non-oxidizing atmosphere or reducing flame. You can see that there is a phenomenon.

<Summary of Effects> As described above, according to the present invention, the deterioration of corrosion resistance at the welded portion is very small, and the deterioration of corrosion resistance at the bolt holed portion is also extremely small, which is significantly higher than that of the conventional material. It is possible to provide a lightweight shaped steel material with excellent performance to
Currently, the market needs are increasing every day for 50 years or 10 years.
It is possible to provide lightweight structural steels for structures for industrialized houses that require long-term durability such as 0 years at a low cost, which is extremely useful industrially and socially.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/18

Claims (6)

[Claims] 1. A weight ratio of C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Construction made by electrical resistance welding forming of hot-rolled steel sheet containing less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1% and the balance Fe and inevitable impurities. Lightweight structural steel. 2. By weight ratio, C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8% , Lightweight structural steel for building structure, which is formed by electric resistance welding forming a hot-rolled steel sheet with the balance being Fe and unavoidable impurities. 3. A weight ratio of C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, or in addition to this, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8%, 1 or 2 A hot-rolled steel sheet that also contains seeds, the balance of which is composed of Fe and unavoidable impurities, and is formed by electrical resistance welding in a state of being shielded by a non-oxidizing atmosphere or reducing flame, For manufacturing lightweight steel for automobiles. 4. By weight ratio, C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, the balance being Fe and unavoidable impurities, for manufacturing lightweight steel for building structures Hot rolled steel sheet. 5. By weight ratio, C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8% The balance is Fe and unavoidable impurities, and a hot-rolled steel sheet for manufacturing lightweight structural steel for building structures. 6. A weight ratio of C: 0.001 to 0.1%, Si: 0.01% to less than 0.60%, Mn: 0.01 to 2.
5%, P: 0.001-0.15%, S: 0.0001-0.02%, Cr: 5.0% over 10.
Less than 5%, N: 0.001 to 0.10%, O: 0.0001 to 0.02%, Al: 0.001 to 0.1%, or in addition to this, Cu: 0.001 to 0.8%, Ni: 0.001 to 0.8%, 1 or 2 A continuous cast slab containing seeds and the balance consisting of Fe and unavoidable impurities is heated to 1000 to 1300 ° C and hot-rolled, and immediately after finishing in a temperature range of 500 ° C or higher, 400
A method for producing a hot-rolled steel sheet for producing lightweight steel for building structures, which comprises winding in a temperature range of ℃ or higher.