RU2522065C1 - Structural sheet stainless steel having excellent weld corrosion resistance, and its making method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: sheet is made from steel containing the following, wt %: 0.01 to 0.03 C, 0.01 to 0.03 N, 0.10 to 0.40 Si, 1.5 to 2.5 Mn, 0.04 or less - P, 0.02 or less - S, 0.05 to 0.15 Al, 10 to 13 Cr, 0.5 to 1.0 Ni, 4×(C+N) or more and 0.3 or less - Ti, Fe and inevitable impurities are the rest; with that, V, Ca and O are controlled in inevitable impurities: 0.05 or less - V, 0.0030 or less - Ca and 0.0080 or less - O. A sheet has an assessment parameter of microstructure of a heat affected zone at welding F and processibility index FFV meeting F=Cr+2×Si+4×Ti-2×Ni-Mn-30×(C+N)≤11, and FFV=Cr+3×Si+16×Ti+Mo+2×Al-2×Mn-4×(Ni+Cu)-40×(C+N)+20×V≤9.0.

EFFECT: sheet can be made at low costs and with high manufacturing capability, and weld parts from the sheet have excellent corrosion resistance.

6 cl, 2 dwg, 2 tbl, 1 ex

Description

FIELD OF THE INVENTION

The present invention relates to a structural stainless steel sheet having excellent corrosion resistance of welded parts, which is suitably used as a material for the bodies of railway cars intended, for example, for transporting coal or iron ore, and to a method for producing this structural stainless steel sheet.

State of the art

As the material for the bodies of railway wagons carrying coal or iron ore, stainless steel is usually used. Since coal mine has a high sulfur content, the material for the bodies of railway cars must have corrosion resistance to sulfuric acid and, in particular, resistance to intergranular corrosion of welded parts.

For example, Patent Document 1 discloses, as a stainless steel having both corrosion resistance and weldability, a Ti-containing ferritic stainless steel that exhibits excellent toughness in a weld. However, the technology disclosed in Patent Document 1 provides such components that the structure of the weldment contains a ferrite phase and, therefore, has the disadvantage associated with insufficient toughness of the joints and corrosion resistance of the welded parts.

On the other hand, Patent Document 2 and Patent Document 3 disclose a technology in which an appropriate amount of martensitic phase is formed in a welded part, which is achieved by adjusting the phase composition at high temperature, thereby improving the processability and corrosion resistance of the welded part. In addition, patent document 4 discloses stainless steel, which is suitable for a welding method using gaseous carbon dioxide. Further, one of the authors of the present invention previously proposed a structural stainless steel sheet that improves the corrosion resistance of welded parts by properly adjusting its composition using parameters capable of accurately predicting the structure of the welded part (Patent Document 5).

Sources of prior art

Patent Documents.

Patent Document 1: JP-A-3-249150;

Patent Document 2: JP-A-2002-167653;

Patent Document 3: JP-A-2009-13431;

Patent Document 4: JP-A-2002-30391;

Patent Document 5: JP-A 2009-280850.

SUMMARY OF THE INVENTION

Solved by the invention tasks.

However, studies of the optimal ranges of component content conducted in the disclosed patent documents 2-5 methods were not completely sufficient. In particular, manufacturability was almost not taken into account in these techniques. Accordingly, the appearance of cracks at the stage of slab production and the manifestation of a surface defect called “captivity” is noteworthy, and therefore it is difficult to avoid an increase in cost caused by a decrease in the yield of suitable products.

The present invention was made in the light of these circumstances, and the purpose of the present invention is to provide a structural stainless steel sheet that can be produced at low cost with high productivity, and the welded parts thereof have excellent corrosion resistance.

Ways to solve the problem.

One of the authors of the present invention conducted extensive research to overcome the aforementioned drawback and found that intergranular corrosion caused by Cr depletion in the vicinity of the grain boundary can be suppressed, and a heat-affected zone during welding, mainly formed by martensite, can be formed in the structure by regulating the content of chemical components, in particular, Mn and Ti, and maintaining a balance between the respective components within the appropriate ranges, and proposed a parameter (F value) presented in patent document 5. After that, the authors of the present invention, based on the discovery of this fact, continued detailed studies, in particular, manufacturability, and as a result of such studies, they found that the manifestations caused by the presence of inclusions of cracks in the slab and captivity (surface defects) can be significantly reduced by adding the proper amount of Al to the composition, reducing the content of V, Ca, O to specified ranges or lower and setting within a suitable range ins FFV, proposed as a new parameter, indicative of the determination of the suitability of this material for mass production, and have accomplished the present invention.

Thus, the present invention provides a structural stainless steel sheet providing excellent corrosion resistance of the welded parts, while this structural stainless steel sheet has a composition that contains from 0.01 to 0.03 wt.% C, from 0.01 to 0, 03 wt.% N, from 0.10 to 0.40 wt.% Si, from 1.5 to 2.5 wt.% Mn, 0.04 wt.% Or less P, 0.02 wt.% Or less S, from 0.05 to 0.15 wt.% Al, from 10 to 13 wt.% Cr, from 0.5 to 1.0 wt.% Ni, 4 * (C + N) or more and 0.3 wt.% or less Ti (C, N represent the content (in wt.%) of C and N), and Fe and the inevitable impurities the rest, when e om V, Ca and O in the inevitable impurities are regulated to 0.05 mass% or less of V, 0.0030 mass% or less of Ca and 0.0080 wt% or less O...; a sheet for which the F value and the FFV value, expressed by the following equations, satisfy the condition that the F value is 11 and the FFV value is 9.0.

The value of F is Cr + 2 × Si + 4 × Ti-2 × Ni-Mn-30 × (C + N).

The FFV value is Cr + 3 × Si + 16 × Ti + Mo + 2 × Al-2 × Mn-4 × (Ni + Cu) -40 × (C + N) + 20 × V.

In these equations, the corresponding element symbols represent the contents of these elements (in mass%).

In addition, the present invention provides a structural stainless steel sheet having excellent corrosion resistance of the welded parts, which is further characterized in that, in addition to the aforementioned components, 1.0 mass% or less of Cu,

In addition, the present invention provides a structural stainless steel sheet having excellent corrosion resistance of the welded parts, which further differs in that, in addition to the aforementioned components, 1.0 mass% or less of Mo.

In addition, the present invention provides a method of manufacturing a structural stainless steel sheet, wherein a steel slab having a composition that contains from 0.01 to 0.03 wt.% C, from 0.01 to 0.03 wt.% N, from 0.10 to 0.40 wt.% Si, from 1.5 to 2.5 wt.% Mn, 0.04 wt.% Or less P, 0.02 wt.% Or less S, from 0.05 to 0.15 wt.% Al, 10 to 13 wt.% Cr, 0.5 to 1.0 wt.% Ni, 4 × (C + N) or more and 0.3 wt.% Or less Ti ( C, N represent the content (in mass%) of C and N), and Fe and the inevitable impurities are the rest, while V, Ca and O are regulated in unavoidable impurities to 0.05 mass% or less than V, 0.0030 mass. % silt and less than Ca and 0.0080 wt.% or less than O, while the F value and the FFV value, expressed by the following equations, satisfy the condition according to which the F value is 11, and the FFV value is 9.0, heated to a temperature of 1100 up to 1300 ° C and then hot rolling is carried out, which includes rough hot rolling, in which rolling is performed in at least one pass or more with a reduction ratio of 30% or more in a temperature range exceeding 1000 ° C, or hot rolling is performed without annealing of the hot rolled sheet or after tzhig hot-rolled sheet at a temperature of from 600 to 1000 ° C. After that, the hot-rolled sheet or annealed hot-rolled sheet is etched.

The value of F = Cr + 2 × Si + 4 × Ti-2 × Ni-Mn-30 × (C + N).

The FFV value is Cr + 3 × Si + 16 × Ti + Mo + 2 × Al-2 × Mn-4 × (Ni + Cu) -40 × (C + N) + 20 × V.

In these equations, the corresponding element symbols represent the contents of these elements (in mass%).

In addition, the present invention provides a method for producing a structural stainless steel sheet having excellent corrosion resistance of welded parts, which further differs in that, in addition to the aforementioned components, 1.0 mass% or less of Cu.

In addition, the present invention provides a method for producing a structural stainless steel sheet having excellent corrosion resistance of welded parts, which is further characterized by a content of 1.0 wt.% Or less Mo in addition to the above-mentioned components.

Advantage of the invention

According to the present invention, it is possible to create a structural stainless steel sheet having excellent corrosion resistance of welded parts, which is produced at low cost and with high productivity, and is suitably used as a material for the bodies of railway cars carrying, for example, coal or iron ore.

Brief Description of the Drawings

FIG. 1 is a graph showing the relationship between the FFV value and the frequency of occurrence of surface defects.

FIG. 2 is a photograph taken with an optical microscope, showing an example of observing the cross section of a test sample after a corrosion test with sulfuric acid and copper sulfate when deep pitting is detected in the heat affected zone during welding.

The implementation of the invention

Further, the present invention is explained in more detail.

First of all, the composition of the present invention is explained. In the following explanation, all indicators are presented in mass percentages.

C: from 0.01 to 0.03%.

N: 0.01 to 0.03%.

To obtain the strength required for a structural stainless steel sheet, it is necessary that such a structural stainless steel sheet contain at least 0.01% or more C and 0.01% or more N. On the other hand, when the content of C, N exceeds 0.03 %, there is a tendency to precipitate Cr carbide or Cr carbonitride, as a result of which the corrosion resistance and, in particular, the corrosion resistance of the heat affected zone during welding are deteriorated. In addition, in the heat affected zone during welding, an increase in hardness occurs, which also impairs the toughness. Accordingly, the content values of both elements C and N are limited to ranges from 0.01 to 0.03%. The content of C is preferably limited to a value in the range of 0.015 to 0.025%, and the content of N is preferably limited to a value in the range of 0.012 to 0.02%.

Si: 0.10 to 0.40%.

Si is an element that is used as a deoxidizing agent, and in order to realize the benefits provided by Si, a Si content of 0.10% or more is required. On the other hand, when the Si content exceeds 0.40%, the toughness of the hot-rolled steel sheet deteriorates. Accordingly, the Si content is limited to a value that is within the range of 0.10 to 0.40%. The lower limit of the Si content is preferably set to 0.20%, and the upper limit of the Si content is preferably set to 0.30%.

Mn: 1.5 to 2.5%.

Mn is an element suitable for use as a deoxidizing agent, and also as a reinforcing element providing the strength required for a structural stainless steel sheet, in addition, Mn is also an element that stabilizes austenite at high temperatures. In addition, in the present invention, Mn is an important element for controlling the microstructure of the heat affected zone during welding in terms of providing the required volume fraction of the martensitic structure. In order to be able to implement such a function, it is necessary to set the Mn content to 1.5% or more. On the other hand, when the Mn content exceeds 2.5%, this not only does not satisfy the conditions for realizing the benefits provided by Mn, but an excess Mn content also worsens the toughness of the steel sheet, adversely affects surface quality, impairing descaling at the production stage, and increases the cost of the alloy. Accordingly, the Mn content is limited to a value that is within the range of 1.5 to 2.5%. Preferably, the Mn content is limited to a value that is within the range of 1.8 to 2.5%. More preferably, the Mn content is limited to a value that is within the range of 1.85 to 2.0%.

P: 0.04% or less.

The content of P from the point of view of ensuring suitability for hot processing is preferably set small, and the permissible upper limit of the content of P is defined as 0.04%. More preferably, the upper limit of the P content is set to 0.035% or less.

S: 0.02% or less.

The content of S from the point of view of ensuring suitability for hot working and corrosion resistance is preferably set to be small, and an acceptable upper limit of the content of S is set to 0.02%. More preferably, the upper limit of the S content is set to 0.005% or less.

Al: 0.05 to 0.15%.

Although Al is an element that is typically added to the composition as a deoxidizing agent, the inventors have found that Al improves processability and, in particular, is effective in suppressing the appearance of cracks in the slab stage; according to the present invention, Al is added in amounts that allow Al to exhibit such a function. In order to suppress the appearance of cracks in the slab, in addition to the presence of Al, a decrease in the content of V, Ca, and O is necessary, as well as optimization of the FFV value, as described below. Although the mechanism for suppressing the appearance of cracks in the slab due to the presence of Al is not fully understood, it is determined that such an improvement is achieved with proper regulation of the phase composition and control of the morphology of inclusions. To provide such an advantage, it is necessary to set the Al content to 0.05% or more. On the other hand, when the Al content exceeds 0.15%, large-sized inclusions based on aluminum are formed, and thereby surface defects are caused. Accordingly, the Al content is limited to a value that is within the range of 0.05 to 0.15%. Preferably, the Al content is limited to a value that is within the range of 0.080 to 0.150%. More preferably, the Al content is limited to a value that is within the range of 0.085 to 0.120%.

Cr: 10 to 13%.

Cr is an element that forms a passivating film and is required to ensure corrosion resistance, in particular, corrosion resistance of the heat affected zone during welding. To obtain this advantage, it is necessary to set the Cr content to 10% or more. On the other hand, when the Cr content exceeds 13%, not only does the cost increase, but it is also difficult to ensure a sufficient austenite phase in the welded part at high temperatures and, therefore, it becomes difficult to achieve the fraction required for the heat affected zone during welding after the martensitic structure is welded. The result is a deterioration in resistance to intergranular corrosion in the heat affected zone during welding. Accordingly, the Cr content is limited to a value that is within the range of 10 to 13%. Preferably, the Cr content is limited to a value that is within the range of 10.5 to 12.5%.

Ni: 0.5 to 1.0%.

To ensure strength and toughness, the Ni content is set to 0.5% or more. On the other hand, Ni is an expensive element and therefore, from an economic point of view, the upper limit of the Ni content is set to 1.0%. Ni, as well as Mn, is an element that stabilizes austenite at high temperature, and, therefore, Ni is suitable for controlling the microstructure of the heat-affected zone when welding into a martensitic structure with the required volume fraction. However, this advantage can be sufficiently ensured by the addition of Mn and, therefore, it is advisable to limit the Ni content to a value that is in the range from 0.5 to 1.0%. Preferably, the Ni content is limited to a value that is within the range of 0.60 to 1.0%. More preferably, the Ni content is limited to a value that is within the range of 0.60 to 0.90%.

Ti: 4 × (C + N) or more and 0.3% or less.

Ti is an important element that provides in the present invention excellent corrosion resistance of the welded part, and is an element especially required to improve the resistance to intergranular corrosion of the heat affected zone during welding. Ti has the advantage that Ti precipitates and binds C, N in steel in the form of Ti carbide, nitride or carbonitride (hereinafter, three types of compositions consisting of carbide, nitride and carbonitride are collectively referred to as carbonitride or the like), such thus preventing the formation of Cr carbonitride or the like. In the present invention, in the heat-affected zone when welding a steel sheet which has a structure formed by ferrite and martensite, from the point of view of corrosion resistance, it becomes a problem to deteriorate the corrosion resistance of the part formed by the ferrite phase, which during the cooling process precipitates carbonitride or the like. In a steel sheet according to the present invention, carbonitride or other similar Cr compounds are deposited during welding in the heat affected zone, so that Cr depletion occurs near grain boundaries, a disadvantage associated in particular with a deterioration in the resistance to intergranular corrosion of the part represented by the ferrite phase can be overcome thanks to the presence of Ti. To enable Ti to demonstrate such a function, it is necessary to set the Ti content to 4 × (C + N) or more (C, N represent the content (in mass%) of C and N). On the other hand, even when the Ti content exceeds 0.3%, this does not satisfy the conditions of not only the benefits provided by Ti, but also the formation of a large amount of carbonitride or other similar Ti deposits in steel, thereby causing a deterioration in the toughness of the steel sheet.

Accordingly, the Ti content is limited to 4 × C + N or more and 0.3% or less. More preferably, the Ti content is limited to a value that is within the range of 0.180 to 0.230%. Thus, it is optimal for the steel sheet to have a decrease in C, N such that the Ti content condition corresponding to a value of 4 × (C + N) or more is simultaneously satisfied.

In the present invention, in order to increase productivity (yield) or manufacturability and, in particular, to suppress the formation of captures (surface defects) that appear due to cracks or inclusions at the slab stage, it is important to reduce V, Ca and O, as this is described later.

V: 0.05% or less.

It often happens that V is added to the steel sheet as an impurity to the starting material Cr or the like, and there may be times when V is inadvertently added to the steel sheet. However, in order to, in particular, suppress the appearance of cracks at the slab stage, strict regulation of the V content is necessary. From this point of view, it is necessary to limit the V content to 0.05% or less. More preferably, the V content is limited to 0.03% or less. Even more preferably, the V content is limited to less than 0.03%. Although by limiting the V content to values of 0.01% or less, a more significant crack suppression effect can be achieved, the selection of starting materials and the like becomes necessary, and as a result, such a limitation of the V content becomes economically disadvantageous.

Ca: 0.0030% or less.

Calcium forms inclusions with a low melting point and, as a result, Ca causes surface defects, the occurrence of which is attributed precisely to the presence of inclusions. Accordingly, in the present invention, a strict restriction of the Ca content is required, and the upper limit of the Ca content is set to 0.0030%. Preferably, the Ca content is as low as possible, and preferably the Ca content may be limited to 0.0010%, more preferably it may be limited to 0.0002% or less. However, it becomes necessary to select starting or other similar materials and, therefore, such a restriction of the Ca content becomes economically disadvantageous.

O: 0.0080% or less.

It is necessary to reduce the O content to suppress the formation of oxide-based inclusions, thus ensuring high productivity, in this regard, the upper limit of the O content is set equal to 0.0080%. More preferably, the upper limit of the O content is set to 0.060% or less.

In addition, in the present invention, corrosion resistance and performance can be greatly improved by setting the F values and FFV values described below within appropriate ranges.

The value of F≤11.

The value of F is expressed as Cr + 2 × Si + 4 × Ti-2 × Ni-Mn-30 × (C + N) (the corresponding element symbols represent the values of the content of these elements (in mass%)) and during welding it is an evaluation parameter microstructures of the heat affected zone during welding. More precisely, the value of F is a parameter for estimating the volume fraction of the martensitic structure (residual fraction of the ferritic structure). In one part of the steel sheet, such as the heat affected zone during welding, which is exposed to high temperatures, part of the zone is converted to austenite (or a portion of this part is further converted to 8-ferrite (delta ferrite)), and these phases are converted during the cooling phase in martensite. This proportion is determined by the quantitative balance between stabilizing ferrite elements (ferrite-forming elements) and elements stabilizing austenite (austenite-forming elements). In the above equation expressing the value of F, elements with a positive sign (Cr, Si, Ti) are ferrite-stabilizing elements, and elements with a negative sign (Ni, Mn, C, N) are austenite-stabilizing elements. Thus, the larger the F value, the more likely the ferrite structure will remain (that is, the larger the volume fraction of the ferrite structure becomes, the smaller the volume fraction of the martensitic structure), while the smaller the F value, the more it decreases the probability of maintaining the ferrite structure (that is, the smaller the volume fraction of ferrite becomes, the higher the volume fraction of the martensitic structure is).

In Patent Document 5, an attempt was made to optimize the compositions according to the results of a study of the relationship between the F value and the volume fraction of the martensitic structure of the heat affected zone during welding, as well as by assessing the corrosion resistance of the region near the heat affected zone during welding using a corrosion test with sulfuric acid and copper sulfate. Also in this embodiment, in the same manner as in the aforementioned patent document 5, in order to increase the corrosion resistance of the heat affected zone during welding, the aforementioned F value is limited to 11 or less (volume fraction of martensite 40% or more). The above value of F is preferably limited to 10.5 or less (volume fraction of martensite 60% or more) and more preferably limited to 10 or less. From the viewpoint of corrosion resistance in the weldment, the lower limit of the value of F is here preferably set to 5.0 or more, and more preferably set to 6.0 or more.

The value of FFV≤9.0.

The FFV value is expressed as Cr + 3 × Si + 16 × Ti + Mo + 2 × Al-2 * Mn-4 × (Ni + Cu) -40 × (C + N) + 20 × V (the corresponding element symbols represent the contents of these elements (in wt.%)). FFV is introduced in the present invention as an indicator for the display of manufacturability. The FFV value is set based on a consideration of the phase state during hot rolling. By adjusting the components as described above, in particular by adjusting the Al content and the upper limits of V, Ca, O and then setting a lower FFV, a noticeable reduction in the manifestation of surface defects caused by cracks in the slab stage or inclusions can be achieved. Significant from a technical point of view, the present invention is to achieve a significant reduction in the volume of production of defective products caused by manifestations of surface defects by optimizing a new parameter that takes into account the amount of Al that was not taken into account when introducing the value F. Although the mechanism for improving manufacturability as a result of optimization the FFV value is not completely clarified, nevertheless, manufacturability is significantly improved by limiting the FFV value to 9.0 or e and FFV value is set to 9.0 or less. Preferably, the FFV value is set to 8.5 or less. Although an effective way to reduce the FFV is to decrease the amount of Cr or increase the amounts of C, N, it is likely that a decrease in the amount of Cr or increase the amounts of C, N will lead to a deterioration in corrosion resistance. Accordingly, it is preferable to set the lower limit of the FFV value to 5.0 or more, and more preferably to set the lower limit of the FFV value to 6.0 or more.

For the steel sheet of the present invention, which is applied in the form of a hot rolled sheet or a hot rolled annealed sheet, control of inclusions and crack formation in the slab stage is important to reduce the occurrence of surface defects. Therefore, with regard to the manifestation of surface defects, areas such as those with cracks or foams that significantly reduce the output of suitable products not only worsen the appearance, but also become the starting point for the appearance of rust, therefore, areas in which during the shipment of the steel sheet, cracks or captures appear as a product, it is necessary to trim. Although the above equation of magnitude FFV includes Mo, V, Cu, there may be cases where these components are not added to the steel. When they are not added to steel, the FFV value is calculated by setting the content of components not contained in steel to 0%.

FIG. 1 shows the relationship between the FFV value and the frequency of occurrence of surface defects. The frequency of occurrence of surface defects is calculated based on the length of the area on which defects appear, taken in reference to the total length of the roll. It is understood that by limiting the FFV value within the range of 9.0 or lower, the occurrence of surface defects can be markedly suppressed.

In the present invention, when necessary, in addition to the above components, the steel may contain Cu within the following range.

Cu: 1.0% or less.

Cu is an element that enhances corrosion resistance, and is an element that, in particular, reduces crevice corrosion. Accordingly, Cu can be added when it is required that the steel possess high corrosion resistance. However, when the Cu content exceeds 1.0%, its suitability for hot working is deteriorated, and the phase balance is violated at high temperatures and, therefore, makes it difficult for the zone to obtain a thermal influence during welding of the desired microstructure. Accordingly, when Cu is added to the composition, the upper limit of the Cu content is set to 1.0%. From the point of view of allowing Cu to demonstrate a sufficient effect of improving corrosion resistance, it is effective to establish a Cu content of 0.3% or more. More preferably, the Cu content is limited to a value that is within the range of 0.3 to 0.5%.

Mo: 1.0% or less.

Mo is an element that improves corrosion resistance and can be added to the composition when it is required that the steel sheet has particularly high corrosion resistance. However, when the Mo content exceeds 1.0%, the suitability for cold working deteriorates, and surface roughness also manifests itself during hot rolling, so the surface quality is very much deteriorated. Accordingly, when Mo is added to the composition, the upper limit of the Mo content is set to 1.0%. From the point of view of allowing Mo to exhibit sufficient corrosion resistance, it is effective to establish a Mo content of 0.03% or more. More preferably, the Mo content is limited to a value that is within the range of 0.1 to 1.0%.

In the present invention, in addition to improving the corrosion resistance obtained through the addition of 1.0% or less Cu or Mo described above, based on well-known information about the improvement in ductility or other similar properties with the addition of 0.005% or less B, other elements can be added. Also in this case, it is important to take into account the phase balance at high temperatures. Nb is a strong stabilizing element and significantly upsets the phase balance by combining with C or N, so Nb is not added in the present invention. The rest, in addition to the elements described above, is composed of Fe and inevitable impurities.

In the steel sheet of the present invention, when establishing, in order to improve the corrosion resistance of the heat affected zone during welding, the aforementioned F value of 11 or less, the volume fraction of martensite in the heat affected zone during welding reaches 40% or more. With the preferred setting of the aforementioned F value to be 10.5 or less, the martensite fraction in the heat affected zone during welding reaches 60% or more. With an even more preferred setting of the aforementioned F value to be 10 or less, the volume fraction of martensite in the heat affected zone during welding in this case reaches 80% or more. Also in the steel sheet according to the present invention in terms of volume fractions of 50% or more of the matrix steel (base material) is formed by a ferritic structure. The remaining structure is formed, in particular, in a hot-rolled state by a structure in which the martensite phase and the residual γ phase are present, and partially contains carbonitride or the like. As regards, in particular, the structure of the hot-rolled annealed sheet, which is manufactured as described below, so that the contents of the components of the composition are within the proper range, and the annealing of the hot-rolled sheet is carried out under appropriate annealing conditions, almost 100% of the volume fraction of such a structure is represented ferrite phase and therefore the structure has excellent machinability.

The following explains a method of manufacturing a steel sheet according to the present invention.

A method of manufacturing a stainless steel sheet of the present invention can be carried out in accordance with this method and is not particularly limited. However, as a method by which a stainless steel sheet according to the present invention can be manufactured with high efficiency, a method is recommended in which the molten steel having the above composition is continuously cast or otherwise formed into a slab, the slab is converted into a hot rolled coil, a hot rolled coil if necessary, it is annealed and after descaling (by shot-blasting, etching, etc.), a stainless steel sheet is thus produced whether.

Next, the method of the present invention is explained in more detail.

First, using a widely used melting furnace, such as a steel furnace or electric furnace, molten steel is prepared with a composition adjusted in accordance with the present invention, after which the molten steel is purified by known refining methods such as vacuum degassing method (RH method), VOD method ( vacuum oxygen decarburization) or the AOD method (argon-oxygen decarburization), and the molten steel is molded into a steel slab (starting steel material) continuously by casting or by a method of manufacturing castings and rolling on a blooming. In terms of productivity and quality, it is preferable to use as a continuous casting method. In addition, to ensure the degree of compression described below during hot roughing, the thickness of the slab can preferably be set to 100 mm or more. More preferably, specifying a slab thickness within a range of 200 mm or more.

Then the steel slab is heated up to a temperature of 1100 to 1300 ° C and then hot rolled, which results in a hot-rolled steel sheet. Setting a high temperature for heating the slab is necessary in order to enhance the resistance of the hot-rolled sheet to the formation of surface roughness or its ability to withstand the appearance of waviness after annealing during cold rolling. However, when the heating temperature of the slab exceeds 1300 ° C, slag flows become noticeable, and the crystalline grains coarsen and, thus, the toughness of the hot-rolled sheet deteriorates. On the other hand, when the heating temperature of the slab is lower than 1100 ° C, the forces during hot rolling increase and as a result, the surface roughness during hot rolling becomes noticeable, in addition, the recrystallization during hot rolling is insufficient and, thus, the toughness of the hot rolled sheet also deteriorates .

In the rough hot rolling step, it is preferable to perform rolling with a reduction ratio of 30% or more in a temperature range exceeding 1000 ° C. in at least one pass or more. Due to such rolling with a high degree of compression, the granular (crystalline) structure of the steel sheet becomes finely dispersed with an increase in the toughness of the steel sheet. After rough hot rolling, in accordance with this method, the final hot rolling is performed (in the mode of normal finish hot rolling).

Such a hot rolled hot rolled sheet having a thickness of from about 2.0 to 8.0 mm is used as a structural material directly or after etching without annealing. Etching can be applied to the hot rolled sheet after annealing the hot rolled sheet at a temperature of from 600 to 1000 ° C. When the heating temperature of the hot-rolled sheet during annealing is lower than 600 ° C, it is possible that the martensite phase or the residual γ-phase, which may exist in the hot-rolled state, is preserved and, therefore, the volume fraction of the ferritic structure becomes 50% or lower, resulting in a steel the sheet cannot acquire sufficient machinability. On the other hand, when the heating temperature during annealing exceeds 1000 ° C, coarsening of the grain becomes noticeable and, therefore, the toughness of the hot-rolled sheet deteriorates. The annealing of the hot-rolled sheet can preferably be performed so that the hot-rolled sheet is held for 1 hour or more at a predetermined temperature of 600 to 1000 ° C. during the so-called annealing in containers. In addition, when the heating temperature during annealing becomes excessively high, there is a situation where the hot-rolled sheet reaches the temperature at which the γ transition occurs, and therefore, an excessively high temperature is not preferred. Accordingly, it is necessary to regulate the composition within the appropriate range and select the correct temperature range appropriate for the composition. In the composition range of the steel compositions of the present invention, when the annealing heating temperature is mainly set to a value in the range of 600 to 900 ° C., almost 100% (in terms of volume fraction) of the hot-rolled sheet becomes a ferritic phase and therefore it is preferable to set the temperature heating during annealing within this temperature range.

When welding a stainless steel sheet according to the present invention, all conventional welding methods are applicable to the steel of the present invention, including arc welding such as TIG welding (gas tungsten arc welding) or MIG welding (arc welding with a consumable electrode in inert gas), seam welding, electric welding, such as spot welding, laser welding, etc.

Example

Stainless steel, having the composition shown in table 1, is molded in the form of slabs with a thickness of 200 mm using a steelmaking converter, VOD (vacuum oxygen decarburization) and continuous casting. These slabs are heated at a temperature of 1180 ° C and then, using hot rolling, the slab is converted into a roll-shaped hot-rolled sheet with a thickness of 5.0 mm. The temperature of the completion of hot rolling (exhaust) is set equal to 900 ° C, and the temperature of the winding into a roll after hot rolling is set to 700 ° C. The obtained hot-rolled steel sheet is annealed for 10 hours at a temperature of 690 ° C and after that the scale is removed from the hot-rolled steel sheet by shot-blasting and pickling.

After descaling from the steel sheet, flat T-shaped test specimens were cut out, each of which was formed by a lower plate and a vertical plate, the samples were collected and single-pass fillet welding was applied to both sides of the T-shaped specimen (metal electrode welding in gas atmosphere, protective atmosphere: 98 vol.% Ar + 2 vol.% O ₂ , flow rate: 20 l / min), thus, resulting in three samples welded with a fillet weld. As an electrode, MGS-309LS manufactured by Kobe steel limited was used, and the input heat of welding was set to a value in the range from 0.4 to 0.8 kJ / mm. From these samples obtained by fillet weld welding, corrosion test samples were selected that were subjected to a corrosion test with sulfuric acid and copper sulfate (modified Strauss test in accordance with ASTM method A262 E and ASTM method A763 Z, test fluid: Cu / 6% CuSO ₄ / 0.5% H2SO ₄ , the test specimen with polished end surfaces is immersed for 20 hours in a boiling test fluid), and the state of the area in close to the heat affected zone during welding in relation to its corrosion.

FIG. 2 is an optical micrograph showing an example of observing the cross section of a sample after a corrosion test with sulfuric acid and copper sulfate. Grade “C” is given in cases where intergranular corrosion is observed or pitting corrosion is observed in the heat affected zone during welding, much deeper than intergranular corrosion, as shown in the photograph. The rating "B" is given in cases when slight corrosion is observed in the heat affected zone during welding. A rating is given when corrosion is not observed when examined under an optical microscope. In addition, the surface condition of the hot-rolled annealed sheet after etching is examined along the entire length of the sheet. Using as an indicator the ratio of the length of the hot-rolled annealed sheet, on which surface defects caused by inclusions or cracks in the slab are observed, to the entire length of the hot-rolled annealed sheet, an assessment was made in which the score “a” was assigned in cases when the frequency of manifestation of defects was 3 % or less, “b” - in cases where the frequency of manifestation of defects was 3% or more and 30% or less, and “c” - in cases where the frequency of manifestation of defects was more than 30%. These results are presented in table 2.

As a result, it is clear that Examples Nos. 1-5, 10-13, and 15, which are encompassed by the scope of the present invention, demonstrate suitable corrosion resistance of the welded part, and the surface condition of the welded part is also very acceptable. On the contrary, in comparative examples Nos. 9 and 14, in which the values of F are outside the scope of the present invention, the amount of martensite formed in the heat affected zone during welding is small and, therefore, these examples show resistance to intergranular corrosion, obviously lower compared with resistance to intergranular corrosion of examples of the present invention. In addition, in comparative example No. 6, in which the Si content exceeds the range of Si content of the present invention, and the Al content is lower than the range of Al content of the present invention, as well as comparative examples No. 7, 8, 9 and 14, in which the FFV values are outside the range of FFV values of the present invention, when considering the surface made after hot rolling and annealing, there were a large number of cracks associated with the quality of the slab, and a large number of captures attributed to the presence of inclusions.

Since the steel of the present invention is used in the form of a hot-rolled sheet or a hot-rolled annealed sheet, the formation of captures significantly reduces the output of suitable products. Since the portions with the films not only have an unsatisfactory appearance, but also become the starting point for corrosion, it is therefore necessary to trim the sections corresponding to the films during the shipment of the hot rolled sheet or hot rolled annealed sheet as a product.

Table 1 No. Chemical composition (wt.%) Velichina F FFV value FROM Si Mn R S Al Cu Ni Cr Ti V N ABOUT Ca one 0,022 0.24 1.87 0,034 0.005 0.105 0.65 11,2 0.194 0.01 0.0150 0.0052 0.0010 8.2 7.6 steel of the present invention 2 0,025 0.30 1,53 0,029 0.001 0,120 0.85 12.6 0.180 0.01 0,0242 0.0050 0,0005 9.2 8.4 steel of the present invention 3 0.015 0.28 1.90 0,031 0.004 0.119 0.70 11,4 0.210 0.01 0.0195 0.0065 0.0001 8.5 8.1 steel of the present invention four 0,020 0.21 1,64 0,034 0.003 0,082 0.40 0.80 12.0 0.192 0.01 0.0165 0.0055 0.0024 8.9 6.5 steel of the present invention 5 0.018 0.24 1.95 0,030 0.003 0.103 0.60 11.0 0.185 0,03 0.0145 0.0050 0.0001 8.1 7.9 steel of the present invention 6 0.018 0.45 1.70 0,030 0.010 0.013 0.91 11,2 0.240 0.01 0.0130 0.0062 0.0001 8.6 8.3 comparative steel 7 0,022 0.40 1.70 0,025 0.002 0.005 0.40 11.1 0,200 0.01 0.0140 0.0054 0.0010 9.1 9.3 comparative steel 8 0,020 0.40 1.9 0,030 0.006 0.014 0.91 11,2 0.251 0.10 0,0100 0.0055 0.0001 8.4 9.9 comparative steel 9 0.01 0.50 1.20 0,029 0.002 0.004 0.30 11.9 0,200 0.01 0.0120 0.0057 0,0002 11,2 12.3 comparative steel 10 0,020 0.29 1.91 0,026 0.002 0.113 0.86 11.5 0.221 0.008 0.0171 0.0054 0,0004 8.2 7.5 steel of the present invention eleven 0.019 0.40 1.81 0,030 0.001 0.107 0.45 0.95 13.0 0.298 0.02 0.0171 0.0054 0,0004 10,2 8.9 steel of the present invention 12 0,025 0.19 1.95 0,031 0.002 0.150 0.95 10.1 0.194 0.04 0.0198 0.0054 0,0004 6.1 5,4 steel of the present invention 13 0,022 0.22 1.89 0,031 0.002 0.122 0.80 12.1 0.205 0.01 0.0178 0.0049 0,0002 8.7 7.9 steel of the present invention fourteen 0,025 0.38 1.12 0,034 0.003 0.250 0.60 13.0 0.297 0,03 0.0193 0.0056 0,0005 11.3 13.6 comparative steel fifteen 0,023 0.25 1.85 0,030 0.002 0,110 0.71 11.5 0.216 0.02 0.0154 0.0051 0,0005 8.4 8.4 steel of the present invention

table 2 No. Test results with sulfuric acid and copper sulfate Surface quality one BUT but steel of the present invention 2 BUT but steel of the present invention 3 BUT but steel of the present invention four BUT but steel of the present invention 5 BUT but steel of the present invention 6 BUT b comparative steel 7 BUT b comparative steel 8 AT b comparative steel 9 FROM from comparative steel 10 BUT but steel of the present invention eleven BUT but steel of the present invention 12 BUT but steel of the present invention 13 BUT but steel of the present invention fourteen FROM b comparative steel fifteen BUT but steel of the present invention

A: lack of corrosion;

B: slight corrosion;

C: intergranular corrosion or deep pitting corrosion;

a: defect rate of 3% or less;

b: the level of occurrence of defects exceeds 3% and reaches 30% or less;

c: the level of occurrence of defects exceeds 30%.

Claims (6)

1. A structural stainless steel sheet having a composition comprising from 0.01 to 0.03 wt.% C, from 0.01 to 0.03 wt.% N, from 0.10 to 0.40 wt.% Si, from 1.5 to 2.5 wt.% Mn, 0.04 wt.% or less than P, 0.02 wt.% or less than S, from 0.05 to 0.15 wt.% Al, from 10 to 13 wt.% Cr, from 0.5 to 1.0 wt.% Ni, 4 × (C + N) or more and 0.3 wt.% or less Ti, with C, N representing the content of elements C and N in wt. .%, and Fe and inevitable impurities, the rest, while in the composition of inevitable impurities 0.05 wt.% or less than V, 0.0030 wt.% or less Ca and 0.0080 wt.% or less O, and the values of F and FFV satisfy the conditions:
the value of F = Cr + 2 × Si + 4 × Ti-2 × Ni-Mn-30 × (C + N) is ≤ 11, and
the value of FFV = Cr + 3 × Si + 16 × Ti + Mo + 2 × Al-2 × Mn-4 × (Ni + Cu) -40 × (C + N) + 20 × V is ≤ 9.0,
where the corresponding symbols of the elements represent the content of these elements in wt.%. 2. The sheet according to claim 1, additionally containing 1.0 wt.% Or less Cu. 3. The sheet according to claim 1 or 2, additionally containing 1.0 wt.% Or less Mo. 4. A method of manufacturing a structural stainless steel sheet, in which a steel slab having a steel composition containing from 0.01 to 0.03 wt.% C, from 0.01 to 0.03 wt.% N, from 0.10 to 0.40 wt.% Si, from 1.5 to 2.5 wt.% Mn, 0.04 wt.% Or less P, 0.02 wt.% Or less S, from 0.05 to 0.15 wt. Wt.% Al, from 10 to 13 wt.% Cr, from 0.5 to 1.0 wt.% Ni, 4 × (C + N) or more and 0.3 wt.% Or less Ti, with C, N reflect the content of elements C and N in wt.%, and Fe and unavoidable impurities the rest, while in the composition of unavoidable impurities 0.05 wt.% or less V, 0.0030 wt.% or less Ca and 0.0080 wt.% or less than O, and the values of F and FFV satisfy Lyrics:
the value of F = Cr + 2 × Si + 4 × Ti-2 × Ni-Mn-30 × (C + N) is ≤ 11, and
the value of FFV = Cr + 3 × Si + 16 × Ti + Mo + 2 × Al-2 × Mn-4 × (Ni + Cu) -40 × (C + N) + 20 × V is ≤ 9.0,
where the corresponding symbols of the elements represent the content of these elements, in wt.%,
heated to a temperature of 1100 to 1300 ° C and then hot rolling is performed, which includes rough hot rolling, in which rolling is performed in at least one pass or more with a reduction ratio of 30% or more in a temperature range exceeding 1000 ° C, or hot rolling is performed without annealing the hot rolled sheet or after annealing the hot rolled sheet at a temperature of 600 to 1000 ° C, after which the hot rolled sheet or annealed hot rolled sheet is etched. 5. The method according to claim 4, in which the slab further comprises 1.0 wt.% Or less Cu. 6. The method according to claim 4 or 5, in which the slab further comprises 1.0 wt.% Or less Mo.

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