JP4844197B2 - Manufacturing method of steel material with excellent weather resistance and paint peeling resistance - Google Patents

Description

  The present invention manufactures a steel material with excellent weather resistance and excellent paint peeling resistance that can be used as a minimum maintenance material even in an environment where the amount of incoming salt is large, such as in a beach area or a region where snow melting salt is spread. It is about the method.

  Usually, when a weather-resistant steel material is exposed to an atmospheric corrosive environment, a protective rust layer is formed on the surface thereof, and subsequent steel material corrosion is suppressed. For this reason, weathering steel is used for structures such as bridges as a minimum maintenance steel that can be used as it is without being painted.

  However, in areas where the amount of incoming salt is high, such as in the coastal area or inland where snow melting salt is sprayed, a protective rust layer is not formed on the surface of the steel material, and the effect of suppressing corrosion is not observed. For this reason, it is not possible to use a bare weatherproof steel material without painting at the beach, and it is common to use plain steel that is coated with plain steel. However, in the case of such coating use, it is necessary to repaint every 10 years because of the deterioration of the coating film due to corrosion, and therefore the cost required for maintenance becomes enormous.

In recent years, weather resistant steel (JIS G 3114: weather resistant hot rolled steel for welded structure) standardized by the Japanese Industrial Standard (JIS) has an incoming salt content of 0.05 mg / dm ² / day (0. 05mdd) and above, that is, in the beach area, the loss of corrosion is large due to the occurrence of scale-like rust, layered rust, etc., so it cannot be used without painting (Ministry of Construction Public Works Research Institute, Steel Club) Japan Bridge Construction Association: Joint Research Report on the Application of Weatherproof Steel to Bridges (XX)-Design and Construction Guidelines for Uncoated Weatherproof Bridges (Revised Version-1993.3)).

  For this reason, in salty environments such as beach areas, ordinary steel materials are usually painted to deal with, but bridges constructed on coastal areas near river mouths and roads such as mountainous areas where snowmelt salt is spread are corroded. However, the current situation is that it must be repainted. Since these repainting takes a lot of man-hours, there is a strong demand for steel materials that can be used without painting. Recently, Ni-based high weathering steel to which about 1-3% of Ni has been added has been developed, but it has been found that application is difficult in regions where the amount of incoming salt exceeds 0.3 to 0.4 mdd.

  Since corrosion of steel materials increases as the amount of flying salt increases, weathering steel according to the amount of flying salt is required from the viewpoint of corrosion resistance and economy. Moreover, even if it is called a bridge, the corrosive environment of steel materials is not the same by the place and site | part used. For example, outside the girders, they are exposed to rainfall, condensed water and sunlight. On the other hand, inside the girder, it is exposed to condensed water, but there is no rain. In general, in an environment where the amount of incoming salt is large, it is said that the inside of the girders is more corrosive than the outside of the girders.

  In addition, in an environment where snow melting salt is sprinkled on the road, the salt is wound up on a running car and adheres to the bridge supporting the road, creating a severe corrosive environment. Moreover, even under the eaves a little away from the coast, it is exposed to a severe salt damage environment, and in such an area, the amount of incoming salt becomes a severe corrosive environment of 1 mdd or more.

  In order to cope with such problems, the development of steel materials that prevent corrosion in an environment with a large amount of incoming salt has been developed, and in Patent Document 1, the weathering steel material with an increased chromium (Cr) content is proposed. Further, Patent Document 2 proposes a weather-resistant steel material having an increased nickel (Ni) content. However, although Cr can improve the weather resistance in a region where the amount of incoming salt is below a certain level, Cr deteriorates the weather resistance in a severe salt environment exceeding that.

On the other hand, when the Ni content is increased, the weather resistance is improved to some extent, but the cost of the steel material itself becomes high, and the material used for applications such as bridges becomes expensive. In order to avoid this, if the Ni content is reduced, the weather resistance will not be improved so much, and if the amount of incoming salt is high, a layered peeling rust will form on the surface of the steel material, resulting in significant corrosion and withstand long-term use. The problem of not being able to occur.
JP-A-9-176790 JP-A-5-51668

    Therefore, the present inventors have found that when Sn is added, the weather resistance is remarkably improved in an environment where the amount of incoming salt is large. However, in this case, it has been found that when Ni is added, weather resistance is not improved in an environment where the amount of incoming salt is large, but on the contrary, it may be deteriorated. The steel material described in Japanese Patent Application No. 2004-308446 was invented.

  It has been found that this steel material is also effective in improving paint peeling resistance in the marine field and the bridge field. For example, a ship such as a tanker or a freighter ships is loaded with seawater in a ballast tank so that the hull is stable even when empty. Seawater has a corrosive action on steel, and promotes corrosion of steel materials constituting the ballast tank. It is known that the corrosion of the steel materials that make up this ballast tank is not so much at the inner wall of the tank where the seawater injected and loaded into the ballast tank is in direct contact, but at the portion that is in contact with the space (gas phase) on the seawater surface. It has been. This is because the tank inner wall of the space portion is always in a wet state, and oxygen that causes corrosion (promotes) continues to be sufficiently supplied from the air. In order to prevent corrosion of the inner wall surface of the ballast tank, conventionally, tar epoxy paint is coated on the inner wall surface of the ballast tank with a relatively thick film thickness of about 200 μm to prevent corrosion. However, this method also has the disadvantages that the corrosive environment is severe, the life of the coating film is as short as about 10 years, and repair coating is required. This steel material is effective in extending the service life of the coating and greatly extending the repair coating interval, and even when applied to a bridge, the same effect can be obtained. In the present specification, the present invention will be described on the assumption that the corrosion resistance under such an environment is also considered as one type of weather resistance.

  However, these steel materials have a problem in that elements such as Cu and Sn called trump elements that are difficult to refining and removing are contained in the steel materials, and scratches are generated on the surface of the steel materials during rolling. When such a flaw occurs, corrosion expands from the flawed portion, and the peeling of the coating film is thereby promoted.

  In order to solve the above-mentioned problems included in steel materials having excellent weather resistance and paint peeling resistance, the present invention has excellent processing characteristics such as ductility, weather resistance that prevents surface scratches, and paint peeling resistance. It aims at providing the manufacturing method of the outstanding steel materials.

  In order to solve the above problems, the present inventors have studied the influence of the surface temperature during hot rolling of a steel material containing Cu and Sn. As a result, the surface scratches after rolling have a large relationship with the surface temperature, and when heated and rolled in a certain temperature range, a steel material that has no surface scratches and has excellent corrosion resistance, paint peeling resistance, mechanical properties, and weldability is obtained. It turned out to be obtained.

  That is, the surface temperature range is controlled to 1172 to 1325 ° C. during hot rolling, 70% or more of the total rolling reduction is finished in a temperature range of 900 ° C. or more, and rolling is finished at 700 ° C. or more. It has been found that can be prevented.

  Although this mechanism is unknown, it is presumed that a competitive reaction occurs between the generation of a low melting point substance of Cu and Sn and the generation and removal of scale from the surface due to oxidation, resulting in suppression of surface scratches.

In addition, when manufacturing the steel material according to the above-mentioned patent application, the heating temperature during rolling was low, and the finishing temperature was also as low as 650 ° C. , so there were quite a lot of rolling flaws, and the maintenance after rolling It took a considerable amount of man-hours and was considered a steel material with high production costs.

The present invention has been completed on the basis of such knowledge, and has the gist of the following methods (1) to (4) for producing a steel material.
(1) By mass%, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005 %: Cu: 0.05 to 1.0%, Ni: 0.01 to 0.5 %, Cr: 0.01 to 3.0%, Al: 0.003 to 0.2%, and N: 0 0.001 to 0.1%, O (oxygen) 0.005% or less, Sn: 0.03 to 0.20%, the balance being Fe and impurities, Ni / Cu After heating the surface temperature of the slab, which is 0.5 or less, to 1175 to 1325 ° C., the rolling of 70% or more of the total reduction amount is finished in the temperature range of 900 ° C. or more, and the rolling is finished at 700 ° C. or more. A method for producing a steel material, which is then cooled.

(2) By mass%, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005 %: Cu: 0.05 to 1.0%, Ni: 0.01 to 0.5 %, Cr: 0.01 to 3.0%, Al: 0.003 to 0.2%, and N: 0 0.001 to 0.1%, O (oxygen) 0.005% or less, Sn: 0.03 to 0.20%, the balance being Fe and impurities, Ni / Cu After heating the surface temperature of the slab, which is 0.5 or less, to 1175 to 1325 ° C., the rolling of 70% or more of the total reduction amount is finished in the temperature range of 900 ° C. or more, and the rolling is finished at 700 ° C. or more. Then, it cools to the temperature of 500 degrees C or less with the average cooling rate of 30 degrees C / s or less, The manufacturing method of the steel materials characterized by the above-mentioned.

(3) After the rolling is finished at 700 ° C. or higher, the steel material is cooled to a temperature of 500 ° C. or lower and then heat-treated at 650 ° C. or lower. Method.
(4) The slab contains Cr: 0.36 to 3.0% , Ti: 0.3% or less, Nb: 0.1% or less, Mo: 1.0% or less, Co: 1 0.0 or less, W: 1.0% or less, V: 1.0% or less, Ca: 0.1% or less, Zr: 0.2% or less, B: 0.01% or less, Mg: 0.1% 1 or 2 or more types chosen from the group which consists of below and REM: 0.02% or less, The manufacturing method of the steel materials in any one of said (1)-(3) characterized by the above-mentioned.

  According to the production method of the present invention, it is possible to obtain a steel material that is excellent in weather resistance (corrosion resistance) and paint peeling resistance and has a good surface property, and can produce an economical steel material that does not require surface treatment after rolling. .

  The present invention relates to a method for producing a steel material having excellent corrosion resistance and anti-peeling resistance, and is widely used after production, in a bare state, or in a thick plate or building material used by painting, and has a shape. There is no particular limitation.

The present invention is described in detail below. In the following description, the alloy element content “%” means “mass%”.
The slab composition employed in the present invention is as follows.

C: 0.001 to 0.15%
C is an alloying element necessary for ensuring the strength of steel, but if contained in a large amount, the weldability of the steel material deteriorates. Therefore, the upper limit of the C content is 0.15%. Moreover, since predetermined intensity | strength cannot be ensured if it becomes less than 0.001%, a minimum is made into 0.001%. A desirable range is 0.005% to 0.15%.

Si: 2.5% or less Si is an alloy element necessary for deoxidation at the time of steelmaking, and is an element that improves weather resistance. If the content exceeds 2.5%, the toughness of the steel is impaired. Similarly, when Al which acts as a deoxidizer is contained, it may not be contained in the slab. Therefore, the content is 2.5% or less.

Mn: 0.5 to 2.5% or less Mn is an element having an effect of increasing the strength of steel at a low cost. However, it combines with S to form MnS, and this MnS becomes a starting point of corrosion and is corrosion resistant. In turn, the weather resistance is deteriorated. When the amount of S in steel is low, it generally has an effect of improving the weather resistance in a high flying salinity environment. However, the mechanism is unknown, but when it coexists with Ni, if it exceeds 2.5%, the weather resistance deteriorates. Therefore, the content is 2.5% or less. Desirably, it is 1.5% or less. In order to maintain the strength as structural steel, 0.5% or more is contained.

P: 0.03% or less P is an element which is contained as an impurity and significantly improves the weather resistance. However, if it is excessively contained, the weldability is deteriorated. Therefore, the content is 0.03% or less. Although the lower limit is not particularly defined, it is desirable to contain 0.005% or more in order to exhibit the effect of improving weather resistance.

S: 0.005% or less S is contained as an impurity, and forms MnS of non-metallic inclusions by combining with Mn to easily become a starting point of corrosion and deteriorate weather resistance. Therefore, it is necessary to reduce S as much as possible. Therefore, the upper limit is made 0.005%.

Cu: 0.05 to 1.0%
Cu is a basic element that improves the weather resistance, and when 0.05% or more is contained, the weather resistance is improved. However, even if the content exceeds 1.0%, the effect is not only saturated, but also causes embrittlement. Therefore, the content is made 0.05 to 1.0%.

Ni: 0.01 to 0.5% (Ni / Cu: 0.5 or less)
Ni has been attracting attention as an element that remarkably improves the corrosion resistance (seaside weather resistance) in an environment with a large amount of incoming salt, and has been developed and put into practical use as a Ni-based weathering steel. However, although the reason is not clear, when it is added in combination with Sn, there is no effect of improving the corrosion resistance, but rather the adverse effect of reducing the effect of improving the weather resistance by Sn appears. This adverse effect of Ni becomes significant when the Ni content exceeds 0.5% or exceeds 1/2 the Cu content. However, in order to prevent deterioration of hot workability due to Cu addition, so-called Cu embrittlement, addition of 0.01% or more of Ni is necessary. Therefore, in the present invention, the Ni content is limited to a very small amount of 0.01 to 0.5%, and Ni is added so that the Ni / Cu mass ratio is 0.5 or less. It is economically advantageous to reduce the expensive Ni content. Preferably, Ni: 0.01% or more and less than 0.4%.

Cr: 0.01 to 3.0%
Cr can be expected to improve the corrosion resistance due to protective rust in an environment where the amount of incoming salt is not so high, but it promotes the anodic dissolution reaction of steel in an environment where the amount of incoming salt is high and deteriorates the weather resistance. However, when Sn or Sb is contained, the effect of improving weather resistance due to the Cr content is exhibited even in an environment with a large amount of incoming salt. This effect is exhibited when the content is 0.01% or more. However, if it exceeds 3.0%, the local corrosion sensitivity increases and the weldability deteriorates. Therefore, the Cr content is set to 0.01 to 3.0%. In addition, the desirable range of content is 0.5 to 3.0%.

Al: 0.003-0.2%
Al is an element added as a deoxidizer, and 0.003% or more is necessary to obtain the effect with certainty. On the other hand, if it exceeds 0.2%, the weldability decreases. For this reason, Al content was made into 0.003-0.2%.

N: 0.001 to 0.1%
N exists as an impurity. N is an important element in the present invention. Like C, N binds to Cr and precipitates as a nitride such as CrN, and reduces the concentration of solid solution Cr effective for corrosion resistance to deteriorate seawater resistance. Not toughness. For this reason, the smaller the N content, the better. However, the economical cost is high to make it less than 0.001%, and if it is 0.1% or less, its harmfulness is small and acceptable. For this reason, the N content is limited to 0.001 to 0.1%. A preferable upper limit is 0.006%, and a more preferable upper limit is 0.004%.

O (oxygen): 0.005% or less O exists as an impurity. O is an important element in the present invention, as is the case with the above N, and forms corrosion-resistant non-metallic inclusions such as CaO, MgO, and Al ₂ O ₃ that tend to cause pitting corrosion in Cr-containing steels. Deteriorate. Excessive O also deteriorates toughness and weldability. However, if the content is up to 0.005%, the influence is small, so the content was made 0.005% or less. It is preferably 0.004% or less, and more preferably 0.003% or less.

Sn: 0.03-0.20%
Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, by reducing Fe ³⁺ quickly and reducing the concentration of Fe ³⁺ as an oxidizing agent, the corrosion promoting action of Fe ³⁺ is suppressed, so that the weather resistance in a high flying salinity environment is improved.

  Sn also has the effect of suppressing the anodic dissolution reaction of steel and improving the corrosion resistance. Furthermore, by containing Sn, the effect of improving the weather resistance of Cr is exhibited even in an environment with a large amount of incoming salt. These effects become prominent when the content is 0.03% or more, and if it exceeds 0.2%, mechanical properties and weldability are deteriorated. Therefore, the content is made 0.03% to 0.2%. In addition, the desirable range of content is 0.03 to 0.15%.

  In addition to the above alloy elements, the steel material of the present invention further includes one or more selected from the group consisting of Ti, Nb, Mo, Co, W, V, Ca, Zr, B, Mg, and REM. You may contain. The reason why the contents of these elements are limited as described above is as follows.

Ti: 0.3% or less Ti forms TiC to fix C, suppresses the formation of chromium carbide and improves the weather resistance, and suppresses the formation of MnS as a starting point of corrosion due to the formation of TiS. However, if it exceeds 0.3%, such an effect is saturated, and the cost of the steel material increases. This effect appears when the content is 0.01% or more. Therefore, when Ti is contained, its content is 0.3% or less, preferably 0.01 to 0.3% or less.

Nb: 0.1% or less Nb, like Ti, has the effect of forming NbC and suppressing the formation of chromium carbides to improve weather resistance. When it exceeds 0.1%, it is saturated. This effect appears when the content is 0.01% or more. For this reason, when Nb is contained, the content is 0.1% or less, preferably 0.01 to 0.1% or less.

Mo: 1.0% or less Mo is an element that dissolves and adsorbs to rust in the form of oxyacid ion MoO ₄ ²⁻ , suppresses transmission of chloride ions in the rust layer, and improves corrosion resistance. When the content in steel exceeds 1.0%, not only the effect is saturated, but also the cost of the steel material increases. This effect is acquired when it becomes 0.01% or more. Therefore, when Mo is contained, its content is 1.0% or less, preferably 0.01 to 1.0%.

Co: 1.0% or less Co dissolves and is present in rust, and densifies the rust, thereby improving protection and improving corrosion resistance. Moreover, there is an effect of suppressing the progress of peeling due to the protective property at the time of peeling the coating film. If it exceeds 1.0%, not only will it be saturated, but the cost of the steel will increase. This effect appears when the content is 0.01% or more. Therefore, when Co is contained, its content is 1.0% or less, preferably 0.01 to 1.0%.

W: 1.0% or less W, like Mo, dissolves and exists in the form of oxyacid ions, suppresses permeation of chloride ions in the rust layer, and improves corrosion resistance. If it exceeds 1.0%, not only will it be saturated, but the cost of the steel will increase. This effect appears when the content is 0.01% or more. Therefore, when W is contained, its content is 1.0% or less, preferably 0.01 to 1.0%.

V: 1.0% or less V, like Mo and W, dissolves and exists in the form of oxyacid ions, suppresses permeation of chloride ions in the rust layer, and improves corrosion resistance. When it exceeds 1.0%, it is saturated. This effect appears when the content is 0.01% or more. Therefore, when V is contained, its content is 1.0% or less, preferably 0.01 to 1.0%.

Ca: 0.1% or less Ca is present in the form of oxide in steel, and has an action of suppressing the promotion of corrosion by suppressing the decrease in pH of the interface in the corrosion reaction part. When it exceeds 0.1%, it is saturated. This effect can be obtained by adding 0.0001% or more. Therefore, when Ca is contained, its content is 0.1% or less, preferably 0.0001 to 0.1% or less.

Zr: 0.2% or less Zr, like Ti, forms ZrS by bonding with S in steel, and plays a role of so-called shape adjustment that suppresses the formation of MnS that becomes a starting point of corrosion. When the content exceeds 0.2%, not only the effect is saturated, but also the cost of the steel material increases. This effect appears when the Zr content is 0.01% or more. Therefore, when Zr is contained, its content is 0.2% or less, preferably 0.01 to 0.2%.

B: 0.01% or less B is an element contained for the purpose of increasing the strength of the steel material. When the content exceeds 0.01%, the toughness of the steel material deteriorates. However, if the B content is less than 0.0003%, the effect of increasing the strength of the steel material is not sufficiently observed. Therefore, when B is contained, its content is 0.01% or less, preferably 0.0003% to 0.01%.

Mg: 0.1% or less Mg, like Ca, suppresses the decrease in pH at the interface in the corrosion reaction part and improves the corrosion resistance. When it exceeds 0.1%, it is saturated. This effect can be obtained by adding 0.0001% or more. Therefore, when Mg is contained, its content is 0.1% or less, preferably 0.0001 to 0.1%.

REM: 0.02%
REM is contained for the purpose of improving the weldability of steel. If it exceeds 0.02%, the effect is saturated. The effect is exhibited when the content is 0.0001% or more. For this reason, when it contains REM, the content is 0.02% or less, Preferably you may be 0.0001 to 0.02%.

The slab containing the above composition is melted by a conventional method. For example, the slab is manufactured by a continuous casting method.
In performing hot rolling, the surface temperature of the slab is set to 1175 ° C. or higher in order to balance the formation and removal of the Cu—Sn low melting point component and the scale on the steel surface. On the other hand, if the surface temperature of the slab exceeds 1325 ° C., the shape of the surface due to scale drop cannot be maintained, and scale loss increases, which is not economical. Therefore, the surface temperature of a slab shall be 1175-1325 degreeC.

  The surface temperature can also be measured with general equipment such as a radiation thermometer. Although it is possible to predict and control the surface temperature by calculation in advance by the time (in-furnace time = pre-tropical zone, heating zone, soaking zone) and time during slab heating, the in-furnace time when heating in the furnace is It can be carried out in a normal range of 20 minutes to 4 hours.

  After the slab is heated, hot rolling is performed. Rolling is performed in a temperature range of 900 ° C. or higher so that 70% or more of the total rolling reduction is completed. When the rolling amount at 900 ° C. or higher is less than 70%, surface scratches occur during rolling, and the corrosion resistance and paint peel resistance deteriorate. At this time, the amount of reduction per pass is desirably 10% or less. Further, the rolling is finished at 700 ° C. or higher. That is, the rolling finishing temperature is set to 700 ° C. or higher. When the rolling finishing temperature is less than 700 ° C., surface scratches are generated, and the corrosion resistance and paint peeling resistance tend to deteriorate. Desirably, rolling is finished at 750 ° C. or higher.

  Thereafter, the steel material obtained by rolling is cooled. At this time, the cooling method is not particularly limited, such as air cooling or water cooling. However, the steel material is preferably cooled to a temperature of 500 ° C. or less at an average cooling rate of 30 ° C./s or less. This is because when cooling is performed at a temperature exceeding 500 ° C. and a cooling rate exceeding 30 ° C./s, precipitation of Ti and Nb carbides becomes insufficient, and corrosion resistance tends to deteriorate.

  The steel material obtained by the above process is preferably heat-treated at 650 ° C. or lower after being cooled to a temperature of 500 ° C. or lower. Residual stress applied to the steel material can be removed by heat treatment.

Further, known treatments such as blast treatment, primary rust prevention treatment, and painting can be performed depending on the application.
In order to further improve the weather resistance, the steel material produced according to the present invention is preferably covered with a corrosion-resistant coating. The anticorrosion film specified here is a rust stabilization treatment film, Zn plating, Al plating, Zn-Al plating, etc., Zn spraying, metal spraying such as Al spraying, vinyl butyral, epoxy, urethane, It means a coating applied for the purpose of corrosion protection of steel, such as general anti-corrosion coatings such as phthalic acid. Even if any anticorrosion film is applied, it has excellent weather resistance and can exhibit high anticorrosion performance.

  As described above, the steel material of the present invention exhibits excellent beach weather resistance in an environment with a large amount of incoming salt, so it is necessary to paint structures such as bridges in the beach area and areas where snow melting salt is sprayed. Can not be used as minimum maintenance material.

  Slabs having chemical compositions indicated by slab symbols a to o in Table 1 were produced in a 150 kg vacuum melting furnace. The slab was heated while measuring its surface temperature with a radiation thermometer, and after reaching a predetermined surface temperature, rolling was started to form a steel plate having dimensions of about 1000 mm long × 150 mm wide × about 4 mm thick. . The steel plates were cooled by cooling, air cooling or water cooling, and then some of the steel plates were heat treated. Table 2 shows the manufacturing conditions of the steel sheet at this time.

Samples of 15 mm × 60 mm were cut out from a total of three places in the center in the width direction and two places in the longitudinal direction from the steel sheet, and the samples were evaluated for cracking with an optical microscope. Specifically, the cross section after polishing of each sample was observed with an optical microscope over 180 mm (= 60 mm × 3). At this time, each sample having a length of 10 mm was regarded as one sample, and an average value of crack depths of three samples having a large crack depth among the 18 samples was evaluated as the maximum crack depth. Further, the sample was coated with a modified epoxy paint (Banno 200: manufactured by China Paint) by air spray so that the dry film thickness was 150 μm. A crosscut was made at a depth reaching the steel substrate, and then evaluated by SAE J2334 test. In addition, SAE J2334 test is wet: 50 ° C., 100% RH, 6 hours; salt adhesion: immersed in an aqueous solution of 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ , 0.25 Time: Drying: accelerated corrosion test with 60 ° C., 50% RH, 17.75 hours as one cycle (24 hours). (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida: Environmental Materials Science, Kyoritsu Publishing (2004), p. 74). The results are shown in Table 2.

  The steel sheets (test numbers 1 to 15) produced by the production method of the present invention had a maximum crack depth of about 10 μm, and no flaws were recognized even by visual observation. On the other hand, the one that started rolling while the surface temperature of the slab was low (test numbers 16 to 18) had an extremely large maximum crack depth, and clear cracks could be confirmed by visual observation. In addition, the sample not subjected to the reduction of not less than 70% at 900 ° C. or higher (test number 19) and the rolling end temperature less than 700 ° C. (test number 20) are not as large as the samples of test numbers 16 to 18. The maximum crack depth was increased. Further, the one with the increased cooling rate after the end of rolling (test number 21) had a small maximum crack depth, but as a result of the SAE J2334 test, the coating film peeling was large and the coating peeling resistance was difficult.

Claims (4)

  In mass%, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005% or less, Cu: 0.05-1.0%, Ni: 0.01-0.5%, Cr: 0.01-3.0%, Al: 0.003-0.2% and N: 0.001- 0.1%, O (oxygen) is 0.005% or less, Sn: 0.03 to 0.20%, the balance is Fe and impurities slab, Ni / Cu is 0.5 After heating the surface temperature of the slab, which is below, to 1175 to 1325 ° C., after rolling at a temperature range of 900 ° C. or higher, 70% or more of the total reduction amount is finished, and after finishing rolling at 700 ° C. or higher, A method for producing a steel material, characterized by cooling.   In mass%, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005% or less, Cu: 0.05-1.0%, Ni: 0.01-0.5%, Cr: 0.01-3.0%, Al: 0.003-0.2% and N: 0.001- 0.1%, O (oxygen) is 0.005% or less, Sn: 0.03 to 0.20%, the balance is Fe and impurities slab, Ni / Cu is 0.5 After heating the surface temperature of the slab, which is below, to 1175 to 1325 ° C., after rolling at a temperature range of 900 ° C. or higher, 70% or more of the total reduction amount is finished, and after finishing rolling at 700 ° C. or higher, A method for producing a steel material, characterized by cooling to a temperature of 500 ° C. or lower at an average cooling rate of 30 ° C./s or lower.   3. The method for producing a steel material according to claim 1, wherein after rolling at 700 ° C. or higher, the steel material is cooled to a temperature of 500 ° C. or lower and then heat-treated at 650 ° C. or lower. The slab contains Cr: 0.36 to 3.0%,
Further, Ti: 0.3% or less, Nb: 0.1% or less, Mo: 1.0% or less, Co: 1.0 or less, W: 1.0% or less, V: 1.0% or less, Ca : 0.1% or less, Zr: 0.2% or less, B: 0.01 % or less, Mg: 0.1% or less, and REM: one or two selected from the group consisting of 0.02% or less It contains the above, The manufacturing method of the steel materials in any one of Claims 1-3 characterized by the above-mentioned.