WO2005064029A1

WO2005064029A1 - STRUCTURAL Fe-Cr BASED STEEL PLATE AND METHOD FOR PRODUCTION THEREOF

Info

Publication number: WO2005064029A1
Application number: PCT/JP2004/019288
Authority: WO
Inventors: Hiroki Ota; Takumi Ujiro; Osamu Frukimi
Original assignee: Jfe Steel Corporation
Priority date: 2003-12-25
Filing date: 2004-12-16
Publication date: 2005-07-14

Abstract

A method for producing a structural Fe-Cr based steel plate having a tensile strength of 400 to 500 MPa, which comprises providing a steel slab containing, in mass %, C : 0.0025 to 0.010 %, N : 0.0025 to 0.010 %, C+N : 0.015 % or less, Si : 0.01 to 1.0 %, Mn : 0.01 to 0.50 mass %, P : 0.04 % or less, S : 0.03 % or less, Cr : 6 % or more and less than 10 %, Cu : 0.01 to 1.0 %, Ni : 0.01 to 1.0 %, V : 0.003 to 0.20 %, Al : 0.05 % or less, Nb : 0.01 to 0.15 %, and optionally further containing Mo : 0.03 to 1.0 %, heating the slab to a temperature of 1100 to 1280°C, completing the hot rolling of the slab at a temperature higher than 930°C, taking up the rolled plate at a temperature higher than 810°C, and cooling the plate at an average rate of 2°C/min or less for the cooling between 800 to 400°C; and a structural Fe-Cr based steel plate produced by the above method. The structural Fe-Cr based steel plate, as it is hot-rolled, exhibits a tensile strength in the range of 400 to 500 MPa over the whole length and the whole width of a coil having been taken up, and can be produced at a low cost in good yield.

Description

Specification

Structural Fe-Cr steel sheet and manufacturing method

The present invention is intended for civil engineering such as bridges and housing structures, which are required to have excellent strength such as corrosion resistance, durability, weldability, and welded properties, equivalent to that of SS 400 steel specified in JISG 3101 (1995). With regard to structural Fe-Cr steel sheets used for building structures, in particular, we propose structural Fe-Cr steel sheets that have small strength variations in the coil after winding and have excellent toughness in welds, and a method of manufacturing the same. Is what you do.

術 surgery

Civil engineering · Architectural structures are required to have strength, corrosion resistance and durability. Therefore, these applications have been conventionally specified in JISG 3101 (1 995) .Standard steel such as SS400 and SN400B specified in JI SG 3136 (1994), and JISG 3106 ( High-strength steels such as SM490 specified in 1 999), as well as materials obtained by applying these steel materials to painting, plating, and force-thion electrodeposition coating, are widely used. On the other hand, in recent years, with the diversification of designs and increasing awareness of environmental issues, the use of various materials is being considered. Among them, Fe-Cr steel, which is excellent in corrosion resistance and design, requires almost no maintenance work for rust such as plating, anti-corrosion coating, drilling and touch-up after welding, so its life cycle cost (LC It is a very attractive material in terms of C).

Among the above Fe-Cr steels, the materials most studied as structural materials for civil engineering and construction are the most used in terms of strength, corrosion resistance, ease of welding, weld toughness, versatility, etc. This is an austenitic stainless steel represented by S US 304 A specified in JI SG 4321 (2000). This austenitic stainless steel has properties such as strength, corrosion resistance, fire resistance and weld toughness that can be sufficiently satisfied as civil engineering and construction materials. However, austenitic stainless steel is much more expensive than ordinary steel because it contains a large amount of alloying elements such as Ni and Cr. Plating and painting It is difficult to use it as a substitute for general-purpose materials that have been treated, and there is a problem that the applicable range is extremely narrow.

To solve this problem, SUS410 and SUS410S (JISG4304 (19999)), which do not contain expensive Ni and have relatively low Cr content Improvement of martensitic stainless steel is being studied for use in civil engineering and construction materials. Martensitic stainless copper is free from σ brittleness and 475 ° C brittleness, which are problems with high Cr alloys, and stress corrosion cracking in chloride-containing environments, which is a problem with austenitic stainless copper. It also has the excellent property of not being. As an example of the above examination, for example, Japanese Patent Publication No. 51-013463 discloses that Cr: 10 to 18 wt%, Ni: 0.1 to 3.4 wt%, Si: 1.0 wt% or less, and Mn: 4.0 wt% or less. Further, C: 0.03% or less, N: 0.02. A martensitic stainless steel for a welded structure, in which the performance of the welded portion is improved by reducing the ratio to / ₀ or less and generating a martensitic martensite structure in the heat affected zone of the weld, is disclosed. Japanese Patent Publication No. 57-028738 contains Cr: 10-13.5 wt%, Si: 0.5 wt% or less, and Mn: 1.0-3.5 wt%, and C: 0.02 wt% or less. , N: Reduced to less than 0.02 wt%, and by further limiting Ni to less than 0.1 wt%, excellent pre- and post-weld pre- and post-heating toughness and workability of welds A structural martensitic stainless steel is disclosed. Also, Japanese Patent Application Laid-Open No. 2002-053938 discloses that a Fe—Cr alloy containing Cr in a range of more than 8 mass% and less than 15 mass%, in particular, by adding Co, V, and W in combination, is added. Technology to improve initial spouting resistance, workability and weldability without increasing the amount of Ni, Cu, Cr, Mo, etc., adding Ti and N, and without excessively reducing C and N. It has been disclosed. However, the steel materials disclosed in JP-B-51-013463 and JP-B-57-028738 have too high strength in the hot-rolled state, so that it is necessary to perform annealing after hot-rolling. Had left problems in terms of delivery. The technique disclosed in Japanese Patent Application Laid-Open No. 2002-053938 requires the complex addition of Co, V, and W, and recommends hot-rolled sheet annealing for softening.

Therefore, technology is being developed to reduce costs by reducing alloying elements and omitting hot-rolled sheet annealing. For example, Japanese Patent Application Laid-Open No. H11-302737 discloses that Cr: 8 to 16 wt%, Si: 0.05 to 1.5 wt%, Mn: 0.05 to 1.5 wt%, and C: 0.005 to 0. lwt%, N: 0.05% by weight or less, C + N: Steel slab reduced to 0.1% by weight or less is heated to 1100 to 1250 ° C, hot rolling is completed at 800 ° C or more, and wound at 700 ° C or more After that, a technique is disclosed in which the hot-rolled sheet annealing is omitted by cooling at an average cooling rate of 5 ° C / min or less to room temperature.

However, even in the technology disclosed in Japanese Patent Application Laid-Open No. H11-302737, no measures have been taken for hardening the coil end. For this reason, at the coil longitudinal end (leading end and trailing end) and the widthwise end (edge), the steel is excessively cooled during hot rolling and after winding, and becomes hardened. There is a problem that the yield is low and the yield is low. In order to soften the hard part at the end and eliminate variations in the material, it is effective to perform hot-rolled sheet annealing.However, this leads to an increase in manufacturing cost and, if the annealing temperature is too high, crystal grains And carbonitrides may be coarsened to lower toughness. As described above, in the Fe-Cr-based copper sheet manufactured by the conventional technology, when hot-rolled as it is, the front and rear ends of the hot-rolled coil and the ends in the width direction (edges) are remarkably hardened. The parts with large increases in strength had to be cut off and used, leading to a decrease in yield.

An object of the present invention is to solve the above-mentioned problems of the prior art, and to obtain a tensile strength of 400 to 400 mm over the entire length of the coil as hot rolled, that is, without hot-rolled sheet annealing.

An object of the present invention is to propose a structural Fe—Cr steel sheet that is in the range of 500 MPa and that can be produced at a low cost at a low yield. Disclosure of the invention

The present inventors have focused on Fe_Cr-based steel having a Cr content of 6 mass% or more and less than 10 mass%, which can provide low cost and sufficient corrosion resistance when used for civil engineering and building structures. A method for keeping the tensile strength of hot-rolled steel sheet within the range of 400 to 500 MPa over the entire length of the coil as hot-rolled was studied. As a result, the above problem can be achieved by reducing the cooling rate of the coil after hot rolling by any method to an average cooling rate of 800 ° C to 400 ° C over the entire length of 2 ° C / min or less. I found that. In addition, in the manufacturing method in which annealing is performed after winding at a high temperature, the toughness of the steel is reduced due to the coarsening of the crystal grains and carbonitrides as described above. Coil The present inventors have found that the problem of a decrease in the toughness of steel that occurs when the cooling rate is too slow can be avoided by adding Nb in an appropriate range, and completed the present invention.

The present invention developed on the basis of the above findings is as follows: C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1 0 mass%, Mn: 0.01 to 0.50 mass%, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 6 mass% to less than 10 mass%, Cu: 0.01 to 1.0 mass %, Ni: 0.01 to 1.0 mass%, V: 0.003 to

0.2 mass%, A1: 0.05 mass% or less and Nb: 0.01-0.15 mass%, with the balance being Fe and unavoidable impurities, with a tensile strength of 400-500 MPa Structural Fe-Cr steel sheet.

When high corrosion resistance is required, the steel sheet of the present invention preferably further contains Mo: 0.03% by mass to 1.0% by mass in addition to the above component composition.

In addition, the present invention provides: C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 0.01 to 0.50 mass%, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 6 mass% to less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, V: 0.003 to 0.20 mass%, A1:

A steel slab containing not more than 0.05 mass% and Nb: 0.01 to 0.15 raass% is heated to a temperature of 1100 to 1280 ° C, and after rough rolling, at a finish rolling end temperature of more than 930 ° C. Hot rolled and wound at a temperature above 810 ° C, then the coil is cooled at an average cooling rate between 800 and 400 ° C

We propose a method of manufacturing structural Fe-Cr steel sheets that can be controlled at 2 ° C / min or less.

When high corrosion resistance is required, the copper slab of the present invention further contains Mo: 0.03% by mass to 1.0% by mass in addition to the above composition of the steel slab. Is preferred.

Further, in the above-described production method of the present invention, it is preferable that at least one pass of the rough rolling is performed at a temperature of more than 1000 ° C. and a rolling reduction of 30% or more.

Further, in the manufacturing method of the present invention, the average cooling rate between 800 and 400 ° C. at all positions of the coil is set to 2 ° C./min or less. cover, it forces ^s preferably carried out using any of the heat-retaining box or Honetsuro. According to the present invention, by appropriately combining the composition of the steel sheet with the hot rolling conditions and the cooling conditions after hot rolling, the steel sheet has the same strength as SS400 steel in the hot rolled state, and has a total coil length. A soft Fe—Cr steel sheet for structural use can be obtained over the entire width. Therefore, the Fe—Cr-based steel sheet of the present invention can be used for the production of various section steels on the existing production line under the same conditions as the conventional material. Further, since the Fe—Cr-based steel sheet of the present invention can be processed by various kinds of welding, it can also be used for manufacturing a welded structural section steel. Furthermore, since the Fe—Cr steel sheet of the present invention has sufficient corrosion resistance and durability even when used for civil engineering and construction structures, life cycle costs can be reduced, and its industrial utility value is extremely large. . Brief Description of Drawings

Figure 1: Graph showing one example of the result of calculating the temperature history of the hot-rolled coil after winding. Fig. 2: Graph showing an example of the result of calculating the temperature history when the heat insulation cover is placed on the hot-rolled coil after winding.

Fig. 3: The cooling curve of Fig. 2 and the cooling curve at 2 ° C / min are superimposed on the CCT diagram.

Figure 4: An example of a thermal cover. BEST MODE FOR CARRYING OUT THE INVENTION

An experiment that triggered the development of the present invention will be described.

The inventors have studied a method in which the strength is in the range of 400 to 500 MPa over the entire length of the wound coil while hot rolling is performed. First, to accurately determine the cooling rate of the coil, a thermocouple was attached to the hot-rolled coil, and the aging of the temperature at each position in the coil was measured. Then, based on this measurement result, the portion of the coil after winding, which has the slowest cooling, Tmax (hereinafter referred to as the “highest point. Normally, the coil thickness is near the center in the width direction”), and the coil cools fastest A prediction was made by calculating the temperature change of the portion Tmin (hereinafter referred to as the "coldest point". Normally, both edges in the width direction of the outermost winding of the coil). As an example, a coil after hot rolling with a weight of 12300 kg, a coil width of 1450 sq. And a diameter of 760 mm was wound at 850 ° C and then cooled under the condition of being allowed to cool in an air atmosphere of 20 ° C. Calculation of time The results are shown in FIG. As is evident from Fig. 1, at the coldest point Tmin of the coil, the temperature drops to about 400 ° C in only about 30 minutes, and it is as fast as about 13 ° C / min between 800 and 400 ° C. It turned out that it was cooling at a speed. For this reason, in the conventional copper plate, many hard phases such as a martensite phase and a bainite phase are generated at the front and rear ends of the coil (inner winding part や outer winding part) and in the width direction edge part where the cooling rate is high. It was thought that it was hardened.

Therefore, the inventors collected metallurgical data such as continuous cooling transformation curves (CCT diagrams) and isothermal transformation curves (TTT diagrams) for alloy steels with a Cr content of 6 mass% or more and less than 10 mass%. The transformation behavior when heat retention was performed on the way was examined. As a result, if the heat is kept by any means after winding, before the rear end of the coil or the edge in the width direction reaches a temperature of less than 400 ° C, the heat recovery effect due to the internal heat of the coil and the heat retention Due to the gradual cooling effect, the average cooling rate between 800 ° C and 400 ° C over the entire length of the coil can be kept at 2 ° C / min or less over the entire width of the coil even during hot rolling, and the target softening can be achieved. I found what I could achieve. The average cooling rate referred to in the present invention is a cooling rate obtained by dividing a temperature difference of 400 ° C from 800 ° C to 400 ° C by a total time required for cooling from 800 ° C to 400 ° C. It is not a temporary cooling rate during cooling.

In Fig. 2, the coil was wound 30 minutes after winding under the same conditions as in Fig. 1, and as one method of heat retention, a 100-mm-thick heat insulating cover lined with heat-insulating material was placed over the coil. It shows the results of calculating the time-dependent changes in temperature at the coil's highest point Tinax and coldest point Tmin. From Fig. 2, it can be seen that the use of the heat retention cover allows the cooling time from 800 ° C to 400 ° C at the coldest point Tmin of the coil with the fastest cooling rate to be 400 minutes or more, that is, the average cooling rate is 1 ° C It can be seen that it can be reduced to min or less.

FIG. 3 is a diagram in which the cooling curve of FIG. 2 and the cooling curve when continuously cooled at 2 ° C./min are superimposed on a CCT diagram. From Fig. 3, if the cooling time from 800 ° C to 400 ° C is 12000 seconds (200 minutes) or more, that is, the average cooling rate is 2 ° C / min or less, the bainite (B in the figure) It can be seen that a soft ferrite (in the figure: F) single-phase structure can be obtained without the formation of. Also, at the coldest point Tmin of the coil, by starting heat retention before the coil is cooled to less than 400 ° C, the formation of a hard martensite phase (M in the figure: M) is completely suppressed. The bainite produced by cooling before the start of heat retention It can be seen that it can be transformed into a tempered bainite or ferrite phase by the tempering effect due to reheating and can be softened.

As described above, after coil winding and before the temperature of the coil's coldest point Tmin is cooled to less than 400 ° C, some means of heat retention is applied to reduce the average cooling rate of the steel sheet to 2 ° C / min or less. As a result, it was found that a soft Fe—Cr-based steel sheet could be obtained over the entire length and width of the coil.

Next, the reason why the composition of the Fe—Cr-based copper plate of the present invention is set in the above range will be described.

C: 0.0025-0.010 mass%, N: 0.0025-0.010 mass% and C + N: 0.015 mass% or less

The weld heat affected zone of the steel of the present invention has a fine martensitic structure, but C and N greatly affect the hardness of the martensite phase. It is effective to reduce C and N to improve the toughness and workability of the heat affected zone and prevent weld cracking. However, excessive reduction of the C and N contents reduces the martensite forming ability of the heat-affected zone of the weld, but promotes the formation of coarse ferrite and significantly reduces the toughness of the weld. In addition, the cost of precision will increase. Therefore, the contents of C and N are each set to 0.0025 mass% or more. On the other hand, if the contents of C, N and C + N are excessively high, the hardness of the martensite phase formed in the heat affected zone becomes extremely high, and the brittleness deteriorates. Therefore, C: 0.010 mass% or less, N: 0.010 mass% or less, C +

N: Limit to 0.015 mass% or less. Preferably, C: 0.003 to 0.008 mass%, N: 0.003 to 0.006 mass%, C + N: 0.012 mass% or less.

Si: 0.01 to 1.0 mass%

Si is an element added as a deoxidizing agent and as a strengthening element. Content

If the content is less than 0.01 mass%, a sufficient deoxidizing effect cannot be obtained.On the other hand, an excessive addition exceeding 1.0 mass% reduces the toughness and the workability that causes a reduction in workability. Let it. Therefore, the amount of Si is limited to the range of 0.01 to 1.0 ma _SS %. Preferably, it is in the range of 0.1 to 0.5 mass%.

Mn: 0.01 to 0.50 mass% Mn is an austenite (γ) phase stabilizing element, and makes the structure of the heat affected zone a fine martensite structure, contributing to the improvement of the toughness of the weld. If added excessively, the ratio of the hard phase as hot rolled increases, and the target tensile strength (400 to 500 MPa) cannot be obtained. In addition, when welding, the hardness of the martensite generated in the two-phase zone heating section is increased, and the strength MnS, which causes brittleness, is formed, thereby reducing the corrosion resistance. Therefore, the upper limit of the amount of added Mn is limited to 0.50 mass%. On the other hand, Mn is useful as a deoxidizer like Si, so the lower limit is set to 0.01 mass%. Preferably, it is in the range of 0.10 to 0.50 mass%.

P: 0.04 mass% or less

P is an element that not only reduces hot workability, formability, and toughness but also is harmful to corrosion resistance. In particular, if the content exceeds 0.04 mass%, the adverse effect becomes remarkable, so P is limited to 0.04 mass% or less. It is preferably at most 0.030 mass%. S: 0.03 mass% or less

S combines with Mn to form Mn S, which reduces corrosion resistance and durability. S is also a harmful element that segregates at the grain boundaries and promotes grain boundary brittleness, so it is desirable to reduce S as much as possible. In particular, when the content is more than 0. 03MA _SS%, because the adverse effect becomes significant, the content of S is limited to not more than 0. 03mass%. Preferably it is 0.008 mass% or less.

Cr: mass% W on 10raass% 7) 5, ¾

Cr is an element effective in improving the corrosion resistance, and if it is less than 6 ma _SS %, it is difficult to secure sufficient corrosion resistance for civil engineering and building structures. On the other hand, if Cr is added in an amount of 10 ma _SS % or more, the cost is increased, and it is difficult to obtain a desired strength while hot rolling. Therefore, the amount of Cr added is in the range of 6 mass% or more and less than 10 mass%. When importance is attached to corrosion resistance, the range is preferably not less than 8 ma _SS % and less than 10 mass%.

Cu: 0.01 to 1.0 mass%,

Cu is an element effective for improving corrosion resistance, and is added for the purpose of prolonging the life of civil engineering and building structures. And then force, 0. In the hydrogenation mosquito卩less than 01Mass% rather poor in the effect of addition, whereas, 1. excessive addition of more than 0 mA _SS% is force ho lead to increase in cost, to increase hot cracking sensitivity thermal Embrittlement may occur during rolling. Therefore, Cu is 0.01 1. The range is 0ma _SS %. From the viewpoint of achieving both corrosion resistance, hot cracking resistance, and workability, Cu is preferably in the range of 0.1 to 0.7 mass%.

Ni: 0.01 to 1.0 mass%

Ni is an element effective for improving ductility and toughness. In the present invention, it is particularly added to improve the toughness of the heat affected zone and improve the heat resistance. In addition, Ni is also effective in preventing brittle cracking during hot rolling caused by the addition of Cu. However, when the content is less than 0.01 mass%, the effect of addition is poor. On the other hand, when the content is more than 1.0 mass%, the effect of addition is not only saturated, but also the material becomes harder and the cost increases. Therefore, Ni is limited to the range of 0.01 to 1.0 mass%. A preferred range for satisfying both the strength of the base material and the toughness of the heat affected zone is 0.05 to 0.4 mass%.

V: 0.003 to 0.20 mass%

By adding an appropriate amount of V, it is possible to prevent embrittlement of the heat affected zone during welding and also prevent ferrite crystal grains from becoming coarse. However, the addition amount was 0.003ma _SS Q /. If it is less than the above, the above effect is not sufficient. On the other hand, if it exceeds 0.20 mass%, the martensite forming ability of the heat affected zone is significantly reduced, and the toughness of the welded portion is lowered. Also, it becomes difficult to obtain a desired tensile strength (400 to 500 MPa) while hot rolling. Therefore, V is added in the range of 0.003 to 0.20 mass%. Preferably, it is 0.005 to 0.15 mass%.

A1: 0.05 mass% or less

A1 is an additive element useful as a deoxidizing agent, and also effectively contributes to improving the bending workability of a steel sheet. To obtain the effect, it is preferable to add 0.0003 mass% or more. However, when the A1 content exceeds 0.05 mass%, the amount of inclusions increases and mechanical properties deteriorate. Therefore, A1 is limited to 0.05 mass% or less. A1 may not be particularly contained if oxygen in steel can be sufficiently reduced by deoxidation with other components such as Si and Mn.

Nb: 0.01 to 0.15 mass%

Nb is a very important element in the present invention. The average cooling rate of 800-400 ° C is 15 ° C / hx (0.25 ° C / min) or less due to excessive heat retention after the Fe-Cr-based steel sheet of the present invention is wound around a hot-rolled coil. The ferrite grains and carbonitrides become significantly coarser May occur and the toughness may be significantly reduced. In order to prevent this decrease in toughness, it is effective to add an appropriate amount of Nb, and the addition of Nb can completely prevent the decrease in toughness. This is considered to be due to the pinning effect of fine and stable Nb (C, N) precipitated during hot rolling, which prevented the coarsening of crystal grains and carbonitrides during heat retention. However, if the addition amount is less than 0.01 mass%, the effect is poor.On the other hand, if it exceeds 0.15 mass%, the strength increases, and the tensile strength becomes 400 to

Not only will it not be possible to obtain 500 MPa, but also the workability will decrease. In addition, a martensite structure cannot be obtained in the heat affected zone of welding, and the toughness decreases. From the balance between strength, toughness and weldability, the preferred addition range is 0.02 to 0.10 mass%.

In the present invention, Mo can be added in the following range in addition to the above essential components.

Mo: 0.03mass% ~ l. 0mass%

Mo is an element effective for improving corrosion resistance, and in the present invention, Mo can be added as needed. To obtain the effect, it is preferable to add 0.03 ma _SS % or more. However, if it is added in excess of 1.0 mass%, the workability will be significantly reduced and the desired tensile strength (400-500 MPa) will not be obtained while hot rolling, so the added amount will be _{1.0 mAsS} Q / _o. It is preferable to limit to the following. From the viewpoint of the balance between corrosion resistance, strength and workability, the range of 0.1 to 0.5 mass% is more preferable.

In addition, in addition to the above components, the steel sheet of the present invention may contain Co and Z or W, Co: 0.01 to 0.5 mass%, W: 0.001 to 0.00, in order to improve the initial heat resistance. It can be contained in the range of 05 mass%.

Next, the strength characteristics of the Fe—Cr-based steel sheet according to the present invention will be described.

The steel sheet of the present invention needs to have a tensile strength in the range of 400 to 500 MPa over the entire length of the hot-rolled coil. The reason for setting the tensile strength in the range of 400 to 500 MPa is that the section steels conventionally used for civil engineering and building structures are manufactured by processing SS400 steel class steel materials. In order to utilize the steel as it is, it is necessary to have the same strength and workability as the SS400 steel. In other words, if the tensile strength exceeds 500 MPa, the processing load on the section steel production line increases, the equipment needs to be strengthened, and the workability deteriorates, which is not preferable. On the other hand, 400MPa If it is less than this, excessive deformation occurs when forming into a section steel, and the strength required for structural materials cannot be obtained. The reason why the range in which the above tensile strength can be obtained is defined as the entire length of the coil is that it is necessary to remove and use a part that does not satisfy this requirement, which causes a decrease in yield.

Further, the steel sheet of the present invention is applied to civil engineering and architectural structures, and is required to have sufficient impact resistance even in use in extremely cold regions. In consideration of such a case, the steel sheet of the present invention, one 50 ° absorption E energy V of Sharubi one impact test in C E_ _5. Is preferably 100 J / cm ² or more.

Next, a method for producing an Fe—Cr-based steel sheet according to the present invention will be described.

After the steel adjusted to the above composition is melted by a commonly known method such as a converter or electric furnace, vacuum degassing (RH) method, VOD (Vacuum Oxygen Decarburization) method, AOD (Argon Oxygen Decarburizat ion) method Secondary scouring is performed by a known method such as that described above, and then a copper slab (steel material) is produced by a continuous casting method or an ingot-integral block rolling method. In order to reduce the level of unavoidable impurities, it is desirable to select appropriate raw materials, such as scrap sorting. The thickness of the steel slab is preferably 100 bun or more in order to secure a reduction ratio in hot rough rolling described below.

Next, the steel slab is heated to a temperature of 1100 to 1280 ° C and hot-rolled to obtain a hot-rolled steel sheet. The above-mentioned slab heating temperature is preferably as high as possible to achieve softening by self-annealing after winding the hot-rolled coil, but if it exceeds 1280 ° C, dripping of the slab becomes remarkable, and the crystal grains become coarse and It is not preferable because the toughness of the rolled steel sheet is reduced. On the other hand, if the heating temperature is lower than 1100 ° C, it is difficult to make the finish temperature (FDT) of the finish rolling of hot rolling higher than 930 ° C. The preferred slab heating temperature range is 1100 to 1250 ° C.

In the rough rolling step of hot rolling, it is preferable to perform at least one or more passes of rolling at a rolling reduction of 30% or more in a temperature range exceeding 1000 ° C. This high-pressure rolling can refine the crystal structure of the hot-rolled steel sheet, which is caused by heat retention after coil winding, which will be described later. It can compensate for the decrease in toughness. In addition, the upper limit of the rolling reduction per pass in the rough rolling is preferably 60% or less because there is a risk of deteriorating the surface properties. Further, the above-described heavy rolling under rough rolling is also effective in improving the toughness of a portion heated to a two-phase region of ferrite (α) + austenite (γ) during welding. This is because the martensite generated in the heat-affected zone of the weld heated to the two-phase region is formed at the ferrite crystal grain boundaries of the copper plate, but when this martensite becomes excessively hard, it becomes the starting point of cracking and the brittleness decreases. descend. Therefore, if the ferrite structure serving as the matrix is made finer and the toughness of the ferrite phase is improved, the propagation of cracks can be suppressed and brittleness can be suppressed. Although the steel sheet of the present invention has a single austenite phase at a temperature exceeding 1000 ° C, the formation site of the ferrite phase is formed by performing at least one pass of rolling at a rolling reduction of 30% or more in rough rolling. The crystal grains can be refined by increasing the number. The reason why the temperature of the rough rolling is preferably higher than 1000 ° C is also to make the finish rolling finish temperature exceed 930 ° C.

The finish temperature in finish rolling following hot rough rolling must exceed 930 ° C, and the coiling temperature of the coil after hot rolling must exceed 810 ° C. This is because, in the present invention, after the hot-rolled coil is wound up, softening is promoted by utilizing self-annealing. For this purpose, the coil winding temperature must be higher than 810 ° C, and in order to secure this coil temperature, the finish rolling finish temperature must be higher than 930 ° C. Further, by setting the finish rolling end temperature to be higher than 930 ° C, it is possible to prevent the introduction of processed ferrite by rolling in the α + γ2 phase region. Furthermore, the reason for setting the coil winding temperature to be higher than 810 ° C is that by keeping the temperature inside the coil after winding high, the recuperation effect by heat retention is enhanced. This is also to raise the temperature of all parts to 400 ° C or more, and to promote the softening by the self-annealing more effectively. The end-of-rolling temperature and winding temperature are preferably less than 110 ° C and less than 950 ° C, respectively, in order to prevent a decrease in toughness due to grain coarsening.

Next, in the present invention, the average cooling rate inside the coil is set to 2 ° C / min or less, that is, the cooling time from 800 to 400 ° C in the coil after winding is set to 200 minutes or more. is necessary. By setting this average cooling rate, the steel sheet can be made into a ferrite single phase (partially carbonitride), a tempered bainite single phase or a tempered bainite + ferrite structure, and can be made of a hard martensite. It is possible to completely suppress the formation of phases, and eventually obtain the desired uniform copper plate strength (TS: 400 to 500 MPa). Can. Here, the cooling rate inside the coil means a cooling rate at a central portion in the longitudinal direction of the coil and at a position 50 ° or more inside the edge portion in the plate width direction. The most reliable method of measuring the cooling rate of this part is to insert a thermocouple into the coil, but it can also be estimated by calculation from the temperature outside the coil.

By the way, setting the average cooling rate of the coil after winding to 2 ° C./min or less can be achieved relatively easily within the coil. The average cooling rate tends to be faster than 2 ° C / min at the leading end (inner winding) and the rear end (outer winding) of the coil and at the edge in the width direction of the coil. The bainite-martensite phase is formed and hardens. For this reason, this part of the coil has been cut and used in the past, causing a decrease in yield.

As a countermeasure against this problem, the present invention starts the heat retention by some means before the coldest point of the coil after winding is cooled to less than 400 ° C. Utilizing this method, we propose a method to set the cooling time between 800 and 400 ° C at substantially all positions in the coil to 200 minutes or more and the average cooling rate to 2 ° C / min or less. By performing this heat retention, the coldest point of the coil can be sufficiently tempered, so that the desired strength can be achieved over the entire length and width of the coil. Preferably, the average cooling rate at all positions in the coil is set to l ° C / min or less. The coldest point of the coil is generally a portion corresponding to both widthwise edges of the outermost winding of the coil. Therefore, the cooling rate can be measured by attaching a thermocouple to this portion by welding or the like. Alternatively, the temperature may be measured using a radiation thermometer.

As a method of keeping heat, for example, as shown in Fig. 4, a method of covering a coil with a heat insulating cover lined with heat insulating material inside an iron box, digging a pit-shaped hole, and attaching heat insulating material to the inner wall Various methods can be applied, such as a method of placing in a heat retention box, or a heat retention furnace having a heating function, and a preferable heat retention means can be adopted according to the manufacturing equipment that the practitioner has. . In addition, it is preferable to arrange a heat insulating material on the lower surface in consideration of cooling from the lower part of the coil. In addition, heating means such as induction heating may be used in combination for the front and rear ends and the width wedge of the coil where cooling is remarkable. In the heat retention of the present invention, if the cooling time between 800 and 400 ° C at all positions of the coil after winding is 200 minutes or more and the average cooling rate is 2 ° C / min or less, Cooling of the coil is not specifically defined. Therefore, the cooling may be continued as it is. However, from the viewpoint of shortening the cooling time and improving the productivity, after cooling with a heat retention device at an average cooling rate between 800 to 400 ° C of 2 ° C / min or less and cooling for 200 minutes or more, It is possible to reduce the cooling time by removing the heat retention device and allowing it to cool before it cools below 400 ° C.

By employing the above heat retention method, it is possible to keep the tensile strength in the range of 400 to 500 MPa over the entire length of the coil, without performing hot-rolled sheet annealing and keeping hot rolling. As a result, it is possible to suppress the yield reduction due to the truncation of the coil front and rear ends and the trimming of the width direction edge portions, which are problems in the conventional technology, and it is possible to reduce costs. In addition, since the tensile strength can be made equivalent to that of the SS400 steel, it is possible to perform processing such as bending and drilling using the existing production line as it is.

In the past, when the coil after winding was kept warm and cooled slowly, there was a problem that the toughness was remarkably reduced due to the coarsening of ferrite crystal grains and carbonitrides. However, since an appropriate amount of Nb is added as a material component, this decrease in toughness can be completely prevented. Therefore, the toughness of the steel sheet of the present invention does not decrease even if the cooling rate becomes excessively low due to heat retention and becomes 15 ° CZhr (0.25 ° C / min) or less. As a result, it is not necessary to precisely control the temperature during heat retention, and the effect of increasing the degree of freedom in designing the heat insulation device can be obtained.

The steel sheet of the present invention that has been gradually cooled by heat retention may be used as it is in hot rolling.However, if necessary, shape correction by skin pass rolling or descaling by shot blasting, pickling, or the like is performed. Alternatively, it may be used after being adjusted to a desired surface property by polishing or the like. In order to improve corrosion resistance, mechanical scale removal by shot blasting was performed, and then the steel sheet surface under the scale steel sheet interface was pickled by 10 / zm or more to completely remove the dechromized layer. Removal is preferred. Further, if necessary, it is possible to use after applying a fire retardant or the like. When pickling is performed, hot-rolled sheet annealing may be additionally performed for the purpose of modifying the scale and the dechromized layer present on the steel sheet immediately below the scale to improve pickling properties. Since the steel sheet of the present invention obtained by the above-mentioned manufacturing method has excellent workability and toughness in a hot-rolled state, it is used for shaped steels of various shapes manufactured by bending and roll forming. It is suitable for use in civil engineering and architectural structural materials, especially for structural steel for residential structures. Further, the steel sheet of the present invention also has an excellent property that embrittlement of the heat affected zone does not occur. Therefore, it can be used as a material for shaped steel assembled by welding such as arc welding, and is a lightweight H-shaped steel welded by electric resistance welding that is formed by electric resistance welding using induction heating or direct current heating. It is also suitable as a material for welding (ERW) pipes and square pipes. Further, the steel sheet of the present invention can be used as a material for various structures such as a container, a coal wagon, and a bus frame by utilizing the above characteristics. Example 1

Copper having the component composition shown in Table 1 was melted through a converter-secondary scouring step, and was made into a 200 mm thick slab by a continuous manufacturing method. After reheating these slabs to 1170 to 1220 ° C, as shown in Table 2, the roughing of a total of 7 passes, with the rolling reduction of the sixth pass being 20 to 40% and the rolling reduction of other passes being less than 30% Rolling, and then finish rolling in 7 passes with a finish rolling finish temperature of 950 to 50 ° C. 6. To make a hot-rolled steel sheet with an Oram thickness, wind up at a temperature of 815 to 910 ° C. A coil was used. The rolled hot-rolled coil is transported to a heat-insulating pad covered with heat-insulating material, and covered with a heat-retaining power bar lined with heat-insulating material with a thickness of 150 as shown in Fig. 4 to keep heat. Cooled slowly. The measurement of the cooling rate of the coil was performed by welding a thermocouple near the outermost end of the coil (edge portion) and inside the coil. For some coils, the cooling rate was changed by adjusting the weight of the coil or by changing the thickness of the heat insulating material of the heat insulation cover. Furthermore, for some coils, after the heat retention was started, the heat retention cover was removed and the coils were allowed to cool before the temperature of each part of the coil reached less than 400 ° C.

A test sample was taken from the outermost turns of the various hot-rolled coils obtained as described above. A piece was cut out and subjected to a tensile test in accordance with JISZ2241 to measure 0.2% power resistance, tensile strength and elongation. In addition, for the outermost winding plate in the width direction 1 to 4 parts, a 2-stroke V-notch impact test specimen (sub-size test specimen with a test specimen width of 5 mm) in accordance with JISZ 222 was collected. A Charpy impact test at 150 ° C. was performed in accordance with 222, and the absorbed energy V E — ₅₀ (J / cm ² ) was measured.

The results of the above measurements are also shown in Table 2. From these results, according to the present invention, according to the present invention, the steel sheet of the invention example which was gradually cooled by covering the heat retention power had a tensile strength of 400 to 500 MPa comparable to SS400 steel or SN400B, Hardening hardly occurs even at the coldest point near the outermost winding of the coil, which is the coldest point, in the width direction, and the desired strength is obtained. Good toughness is obtained within the desired strength range. On the other hand, although the component composition range is within the present invention, in the comparative examples No. 10 and 11 in which the cooling conditions are faster than the range of the present invention, softening of the outermost winding portion of the coil is not obtained. . No. 14 is a comparative example where C and C + N are high, No. 15 is a comparative example where N and C + N are high, and No. 16 is a comparative example where C + N is high. The winding part has been strengthened. Also, its toughness is low. Furthermore, No. 17 is a comparative example having a high Cu content, No. 18 is a high V content, and No. 19 is a comparative example having a high Mn content. The value is higher than the steel sheet strength. Also, its toughness is low. Further, in No. 20, since the Nb content was less than the range of the present invention, the toughness after the heat retention and slow cooling was remarkably reduced. Not obtained. On the other hand, No. 21 has a high strength and a low toughness even after slow cooling because the Nb content exceeds the range of the present invention. No. 24 is within the scope of the present invention, but the rolling reduction of the rough rolling was less than 30% in all passes, so that the toughness was lower than the other invention examples. . Industrial applicability

The steel having the component of the present invention can be applied to various steel materials used in the field of civil engineering and construction, such as thick steel plates, section steels, and steel bars, in addition to hot-rolled copper sheets produced by hot rolling. Steel Chemical composition (mass _S %)

Note Remarks C Si Mn P S Al Cr N Cu Ni V Nb Mo C + N

A 0.0046 0.20 0.24 0.028 0.004 0.010 9.42 0.0060 0.52 0.18 0.05 0.05-0.0106 Invention steel

B 0.0038 0.21 0.26 0.025 0.005 0.008 9.97 0.0067 0.40 0.30 0.005 0.10-0.0105 Invention steel

C 0.0098 0.18 0.06 0.025 0.008 0.008 6.41 0.0025 0.45 0.20 0.03 0.03-0.0123 Invention steel

D 0.0044 0.21 0.30 0.028 0.005 0.041 9.40 0.0040 0.66 0.20 0.01 0.15-0.0084 Invention steel

E 0.0048 0.33 0.48 0.032 0.006 0.010 9.14 0.0045 0.35 0.14 0.18 0.06-0.0093 Invention steel

F 0.0058 0.10 0.27 0.010 0.001 0.011 8.99 0.0040 0.45 0.20 0.09 0.04-0.0098 Invention steel

G 0.0066 0.22 0.22 0.014 0.003 0.008 8.83 0.0036 0.41 0.20 0.06 0.04 one 0.0102 Invention steel

H 0.0094 0.45 0.14 0.028 0.004 0.001 9.40 0.0056 0.30 0.03 0.10 0.01-0.0150 Invention steel

I 0.0051 0.20 0.24 0.030 0.005 0.012 9.12 0.0046 0.05 0.19 0.04 0.02 0.28 0.0097 Invention steel

J 0.0054 0.20 0.26 0.027 0.005 0.023 9.22 0.0039 0.28 0.20 0.05 0.05 0.08 0.0093 Invention steel κ 0.0108 0.22 0.25 0.035 0.007 0.009 8.89 0.0047 0.33 0.20 0.03 0.04-0.0155 Comparative steel

_L 0.0058 0.22 0.26 0.035 0.005 0.009 9.10 0.0105 0.44 0.20 0.04 0.04-0.0163 Comparative steel

M 0.0094 0.30 0.14 0.031 0.006 0.012 9.30 0.0066 0.31 0.01 0.03 0.03-0.0160 Comparative steel

N 0.0090 0.20 0.23 0.029 0.004 0.012 9.25 0.0045 1.20 0.20 0.05 0.03-0.0135 Comparative steel

0 0.0088 0.24 0.22 0.028 0.006 0.010 9.33 0.0052 0.40 0.30 0.30 0.08-0.0140 Comparative steel

_P 0.0065 0.23 0.55 0.033 0.005 0.013 8.94 0.0059 0.32 0.20 0.08 0.04-0.0124 Comparative steel

_Q 0.0045 0.20 0.25 0.026 0.005 0.009 9.40 0.0063 0.40 0.15 0.03 0.001-0.0108 Comparative steel

_R 0.0048 0.20 0.24 0.029 0.005 0.010 9.22 0.0070 0.45 0.20 0.05 0.18-0.0118 Comparative steel

S 0.0025 0.20 0.23 0.027 0.005 0.010 9.40 0.0093 0.49 0.20 0.05 0.05-0.0118 Invention steel

T 0.0047 0.02 0.24 0.028 0.004 0.009 9.39 0.0061 0.51 0.19 0.05 0.05-0.0108 Invention steel

U 0.0048 0.65 0.40 0.029 0.004 0.011 9.45 0.0059 0.50 0.18 0.05 0.05 one 0.0107 Invention steel

V 0.0055 0.15 0.02 0.024 0.028 0.008 9.05 0.0054 0.55 0.64 0.04 0.04-0.0109 Invention steel

W 0.0055 0.20 0.25 0.027 0.003 0.012 9.44 0.0060 0.01 0.33 0.05 0.05-0.0115 Invention steel

X 0.0035 0.22 0.08 0.028 0.005 0.009 9.32 0.0044 0.95 0.48 0.05 0.05-0.0079 Invention steel

Y 0.0055 0.20 0.24 0.028 0.004 0.010 9.41 0.0055 0.52 0.18 0.004 0.07-0.0110 Invented steel indicates that it is outside the present invention.

Table 2 (continued)

Note: The underlined parts indicate that they are outside the scope of the present invention. * Indicates the case where the heat retention cover was removed after 300 minutes and allowed to cool.

Claims

The scope of the claims

1. C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 0.01 ~ 0.50 mass%, P: 0.04 mass% or less,

S: 0.03 mass% or less, Cr: 6 mass% to less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, V: 0.003 to 0.20 mass% , Al: 0.05 mass% or less and

Nb: Structural Fe-Cr steel sheet containing 0.01 to 0.15 mass%, the balance being Fe and unavoidable impurities, and having a tensile strength of 400 to 500 MPa.

2. The structural Fe-Cr steel sheet according to claim 1, further comprising Mo: 0.03 mass% to 1.0 mass% in addition to the above component composition.

3. C: 0.0025-0.010 mass%, N: 0.0025-0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01-11.0 mass%, Mn: 0.01 ~ 0.50 mass%, P: 0.04 mass% or less,

S: 0.03 mass% or less, Cr: 6 mass% to less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, V: 0.003 to 0.20 mass% , Al: less than 0.05 mass%

Nb: A steel slab containing 0.01 to 0.15 mass% is heated to a temperature of 1100 to 1280 ° C, and after rough rolling, hot rolled at a finish rolling end temperature of more than 930 ° C, 810 A method for producing structural Fe-Cr steel sheets in which the coil is wound at a temperature higher than ° C and the average cooling rate of the coil between 800 and 400 ° C is controlled to 2 ° C / min or less.

4. The production method according to claim 3, further comprising Mo: 0.03 mass% to 1.0 mass% in addition to the component composition of the steel slab.

5. The production method according to claim 3, wherein at least one pass of the rough rolling is performed at a temperature of more than 1000 ° C. and a rolling reduction of 30% or more.

6. The method according to any one of claims 3 to 5, wherein the average cooling rate at 800 to 400 ° C at all positions of the coil is 2 ° C / min or less.

7. The manufacturing method according to claim 6, wherein the cooling of the coil is performed by using any one of a heat retaining power par, a heat retaining box and a heat retaining furnace.

8. The absorbed energy VE- ₅ in a Charpy impact test at 50 ° C of one of the steel sheets according to claim 1 or 2. Is a Fe-Cr steel sheet for structural use with 100 J Zcm ² or more.