JPH03107427A - Production of cr-ni stainless steel sheet excellent in mechanical property and surface characteristic - Google Patents

Abstract

PURPOSE:To stably produce a stainless steel sheet excellent in mechanical properties and surface characteristics by successively carrying out the synchronous continuous casting of a cast slab of Cr-Ni stainless steel, the coiling of the cast slab, cold rolling, and annealing treatment under respectively specified conditions. CONSTITUTION:A Cr-Ni stainless steel containing, by weight, 18%Cr-8%Ni as essential component is cast into a cast slab of >=6mm thickness by using a continuous caster of a type where the wall surface of a mold is moved synchronously with a cast slab while regulating cooling rate at the time of solidification to >=30 deg.C/sec. This cast slab is cooled down to 650 deg.C while securing >=20 deg.C/sec average cooling rate, coiled at <=650 deg.C, pickled, and cold-rolled at <=85% rolling reduction, and then, the resulting steel sheet is subjected to control where the relationship between temp. and time is changed in a temp. range of 1000-1300 deg.C and further to annealing where the average crystalline grain size of the material is regulated to grain size No.6-8. By this method, the Cr-Ni stainless steel sheet excellent in mechanical properties and surface characteristics can be stably obtained while obviating the necessity of hot rolling stage.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention uses a so-called synchronous continuous casting method in which there is no relative speed difference between the slab and the inner wall surface of the mold in the process of manufacturing Cr-Ni stainless steel sheets. The present invention relates to a manufacturing method in which mechanical properties and surface properties are improved by continuous casting, in which a slab is cast to a thickness closer to the product size, followed by cold rolling and annealing.

(Prior art) In the conventional continuous casting method, a slab with a thickness of 100 mm or more is manufactured while oscillating the mold, and then the surface is treated, heated to 1000"C or more in a heating furnace, and then rough rolled. A hot strip with a thickness of several (2) mm has been produced by hot rolling using a continuous rolling mill consisting of a continuous rolling mill consisting of a continuous rolling mill and a finishing rolling process. In order to obtain the required shape (flatness), material quality, and surface properties, hot-rolled sheets are annealed to soften the hot strip that has undergone intense hot processing to make it easier to cold-roll. It was necessary to remove scale, etc. that had formed on the hot strip by grinding after the pickling process.This conventional process required a large amount of hot rolling equipment, etc. It was difficult to say that it was the best manufacturing process in terms of productivity because it required a large amount of energy.Furthermore, because the final product was manufactured from material with a thickness of 100 mm or more, a texture developed, and its anisotropy was affected during processing. There were many restrictions on use, such as the need to take into account the following during processing.

In addition, in order to solve the fundamental problem in the prior art, that in order to roll slabs with a thickness of 100 mm or more into hot strips, a long hot rolling equipment and a large amount of energy and rolling power are required. (3) Research is underway on a process for obtaining slabs (strips) with a thickness equal to or close to that of hot strips during continuous casting. For example, “Tetsu to Hagane” 85”, 819
A process for obtaining hot strip directly by continuous casting is disclosed in the article featured in No. 7-85, No. 8, 256. In such a continuous casting process, the gauge of the slab to be obtained is 1 to 10.
When it is on the mm level, the twin drum method is exclusively applied, and the gauge of the slab is 20~b.

(Problems to be Solved by the Invention) As stated above, in the process of manufacturing Cr-Ni stainless steel mesh plates, steel plates are obtained using a long hot rolling facility that requires a large amount of heating energy and rolling power. This was a major obstacle that lowered productivity and increased costs. In addition, since steel plates were obtained from conventional slabs of 100 mm or more, problems also occurred during use, such as the large anisotropy that had to be taken into consideration when processing the product (4). . In addition, because the manufacturing process for thin slabs is simplified, the mechanical properties of the product and the surface properties required for stainless steel are greatly affected by the slab structure, and it is necessary to improve these issues, making it difficult to manufacture. This was a major problem.

(Means for solving the problem) For this reason, the present inventors have developed a method using Cr-N, which has excellent mechanical properties and surface properties.
In order to establish a stable manufacturing method for i-series stainless steel, we conducted research and established the manufacturing method as outlined below.

CrN whose basic components are 18% Cr-8% Ni by weight%
Using a continuous casting machine in which the mold wall moves in synchronization with the slab, I-series stainless steel is cooled at a cooling rate of 30% during solidification.
'' (: /sec or more, cast a slab with a thickness of 6 or less, ensure an average cooling rate of 20℃/SeC or more, and
After cooling to 1000°C and rolling it up at a temperature of 650°C or less, it is pickled, followed by cold rolling with a rolling reduction of 85% or less, and further at a temperature of 1000°C to (5) (5) 1300°C. Cr-Ni stainless steel with excellent mechanical properties and surface texture, characterized by annealing to bring the average crystal grain size of the material to 6 to 8 in terms of grain size number by controlling the temperature and time relationship in the range. Method of manufacturing steel plates.

Furthermore, in order to improve castability, as components, SiS 2.5%, MnS 2.0%, S≦0.008%,
N≦0.18%, 15×S(%)+N(%)×0.
This invention is characterized by satisfying 18.

The manufacturing method of the present invention will be explained in detail below.

A small mesh block of Cr-Ni stainless steel with 18Cr-8Ni as the main component is melted in a laboratory, and the slab thickness is 10 mm.
The following slabs were cast, subjected to various cooling rates, then coiled, pickled, cold rolled, annealed, and then subjected to material tests to evaluate mechanical properties.

In addition, for evaluation of anisotropy, tensile test pieces were taken from the product sheet parallel to the rolling direction (L direction), perpendicular to the rolling direction (C direction), and at 45 degrees (D direction), and a 15% tensile test was performed. The Lankford value (r value) was determined and (6) Δr-((r,-+rc+2rn)/2)l was determined.

Table 1 shows the composition (wt%), slab thickness, coiling temperature, and average cooling rate to 650° C. or the coiling temperature of the test steel.

Furthermore, Table 2 shows the process conditions and material test results when the slab was pickled, directly cold rolled and annealed, and the material quality was evaluated.

The cooling rate after casting of the method of the present invention in Table 2 is 20°C/se.
C or higher, the winding temperature is 650℃ or less, the cold rolling rate is 85% or less, the annealing temperature and time are controlled, and the grain size is 6 to 8.The strength and ductility are comparable to the current process. The anisotropy was small and the surface quality was good.

However, among the comparative methods, the cooling rate after casting was 20℃/se.
The surface properties of Nl, N2, and Pl below c are poor, and this is because the cooling rate after casting is slow, resulting in intergranular oxidation during cooling and precipitation of Cr carbides, which causes intergranular corrosion during pickling. This is due to deterioration of gloss. In particular, Nl, N2, and N3, which had a winding temperature of 800'C, caused severe intergranular corrosion and significantly deteriorated gloss. Furthermore, even if the cooling rate is 20°C/(7) sec or more, if the winding temperature is 650°C or higher -J- as in Ml, 01, Cr carbide will precipitate during cooling after winding. The surface quality has deteriorated.

In addition, A3. For A4 and 2,
Due to its high strength and particularly high yield strength, it becomes a problem during processing.

This is because the annealing temperature is too low or the annealing time is too short.In the slab direct cold rolling process like the method of the present invention, δ-ferrite (hereinafter referred to as δ The process of annihilating -Fe, abbreviated as
This is because δ-Fe does not disappear during annealing unless the annealing conditions are appropriate, resulting in grain refinement due to grain growth suppression and increased strength due to the presence of δ-Fe. It is.

E2. The grain size increased due to the annealing conditions and the grain size number became less than 6. Regarding F3, orange beer was generated on the surface during stretching and processing, resulting in poor surface quality. In addition, N3 is cold-rolled with a cold-rolling reduction of over 85%.
It was found that Δr is large in PI (8) and the anisotropy is almost on par with current process materials.

As shown above, when producing Cr-Ni stainless thin steel sheets, such as 18Cr-8Ni, directly from thin slabs through the cold rolling process, it is necessary to It is necessary to construct a process that satisfies both properties, and it is necessary to control the process conditions from cooling after solidification to final annealing, as in the method of the present invention.

Next, the reasons for limiting the process conditions of the method of the present invention will be explained.

The average cooling rate from immediately after casting to 650°C is 20°C/s.
The reason for setting it above ec is that cooling is slow in this temperature range.
Due to the coarsening of austenite (hereinafter abbreviated as T) grains in the slab, fine undulations (roving) occur on the surface of the cold-rolled sheet during cold rolling, and grain boundary oxidation and precipitation of Cr carbides occur. This is because intergranular corrosion occurs during the previous pickling process, resulting in rough skin and poor gloss in the product due to intergranular corrosion.To prevent this, the average cooling rate is set to 650°C (9). It is necessary to set the degree to 20° (:/sec or more).

Further, the winding temperature needs to be 650° C. or lower in order to prevent precipitation of Cr carbides during winding.

The reason why the cold rolling rate was set at 85% or less is that if the slab is subjected to a cold rolling rate higher than this, the cold rolling texture will develop and the anisotropy will increase. This is because the anisotropy is equivalent to that of a product manufactured from a thick slab of =. When manufacturing from thin slabs, the anisotropy decreases as the cold rolling reduction ratio decreases.In particular, when manufacturing products with small anisotropy, the cold rolling reduction ratio should be 70% or less. desirable.

Regarding the annealing of cold-rolled sheets, the reason why the grain size number is specified as 6 to 8 by controlling the temperature/time relationship in the temperature range of 1000 to 1300°C is because hot rolling is performed from thin slabs as in the method of the present invention. In the process of omitting direct cold rolling and annealing, there is no step to eliminate δ-Fe present in the slab, such as heating hot-rolled thick slabs as in the conventional method, so δFe is eliminated by final annealing. This is because it becomes necessary to satisfy the mechanical properties of the steel, and the combination of the component and annealing (10) in order to eliminate δ-Fe becomes an important technique.

Especially in terms of the disappearance of δ-Fe, δF below 1000℃
Since long-time annealing is required for the disappearance of e, and the recrystallization of γ grains does not proceed, the lower limit of the annealing temperature for cold-rolled sheets is set to 10
The temperature was 00°C. On the other hand, when the temperature exceeds 1300°C, the recrystallization of γ grains progresses, but in this temperature range, the δFe phase also becomes stable, and the δFe phase precipitates instead of becoming a T single phase during annealing, and the δ-Fe phase becomes stable.
The upper limit of the annealing temperature of the cold-rolled sheet was set to 1300°C because the presence of , causes an increase in strength, a decrease in elongation, and a deterioration in workability. At this time, the average crystal grain size of one grain was set in the range of 6 to 8 in terms of particle size number.This is because if the grain size of the product plate becomes coarse grains with a particle size number of 6 or less, roughness of the surface becomes noticeable during processing. This is because orange peel will occur, which will greatly impair the aesthetic appearance, and if the grain size number is 8 or more, a fine structure will increase the strength, resulting in a decrease in elongation and deterioration in workability.

In order to prevent this, it is necessary to make the annealing time in the above annealing temperature range so that the grains are not too coarse or too fine. Control is performed to change the temperature/time relationship in the temperature range of ℃, and the particle size number is 6 to 6.
8.

As a result of expanding the range of ingredients and considering the above, it was found that the ingredient system shown in the batter was established.

In addition, the components are shown in weight%.

C: 0.005-0.1OSi: 2% or less Mn: 3%
Below P: 0.050% or below S:
0.010% or less Cr: 15.0-30.
ONi: 5.0-15.0 Mo: 3.5%
Below Cu: 3.0% or less A1: 0.1% or less 0
: o, o1% or less N : 0.25% or less T
i: 0.6% or less Nb: 1.0% or less Ca:
The reasons for limiting the ingredients to 0.01% or less will be described.

CTC is an element harmful to the corrosion resistance of stainless steel.

If it is less than 0.005%, the manufacturing cost will increase to 0.10%.
If it exceeds 0.005, the corrosion resistance will be significantly deteriorated.
It was set to 0.10%.

Si: Si is an element that improves the corrosion resistance of stainless steel (12) and is also effective for oxidation resistance, but it is set to 2.0% or less because it reduces ductility at high temperatures.

Mn: Mn can be added as a substitute for the expensive Ni, and at the same time increases the solid solubility of N, but if it exceeds 3.0%, cracking during casting becomes noticeable, so it is kept below 3.0%. In terms of properties, the smaller the content, the better, so the content was set to 0.050% or less. If it exceeds this value, corrosion resistance and castability will deteriorate.

In terms of corrosion resistance and castability, less SO3 is better.
．． 0.010% or less. If it exceeds this value, corrosion resistance and castability will deteriorate.

Cr: Cr is a basic component of stainless steel, and was set at 15.0 to 30.0% in consideration of the balance with Ni. 15.0
If it is less than 30.0%, corrosion resistance will be poor, and if it exceeds 30.0%, it will be expensive.

Ni: Ni is a basic component of stainless steel along with Cr, and is added as a gamma stabilizing element, and is added in an amount of 5.0 to 15.0% in balance with the amount of Cr.

Mo: Mo is an element that improves the corrosion resistance of stainless steel, and is particularly effective in suppressing local corrosion (13), and can be added in an amount of 3.5% or less if necessary.

Cu: Cu is an element that improves the corrosion resistance of stainless steel, and can be added in an amount of 3.0% or less if necessary.

Al: A1 is added in an amount of 0.1% or less as a strong deoxidizing agent. Exceeding this will deteriorate corrosion resistance and castability.

0: Less O is better in terms of corrosion resistance and castability;
．． 0.010% or less. If it exceeds this value, corrosion resistance and castability will deteriorate.

N: N is a strong T stabilizing element and also improves corrosion resistance, and can be added in an amount of 0.25% or less. 0.
If it exceeds 25%, cracks during casting become noticeable.

Ti: Ti is an element that fixes C, improves corrosion resistance, coexists with Ca, fixes 0, and prevents the appearance of oxides of S L Mn, and can be added in an amount of 0.6% or less, if necessary.

Nb: Nb is an element that fixes C and improves corrosion resistance, and can be added in an amount of 1.0% or less if necessary.

(14) Ca: Ca is effective as a strong deoxidizing and desulfurizing agent.
It can be added at 1% or less. Exceeding this will result in poor surface quality.

The above invention is based on Cr-N which has excellent mechanical properties and surface properties.
The purpose of this invention is to manufacture i-series stainless steel directly from thin slabs through a process of cold rolling and annealing, but by incorporating measures to prevent slab cracking into the above invention, it has improved operability during casting, and It is possible to create a process that is even better in terms of yield. For this reason, the inventors have conducted research in the direction of imparting ductility to the material (slab) immediately after solidification from the viewpoint of the composition as a means of preventing cracking of thin slabs that are placed for rapid cooling. For various alloys, a round bar tensile test piece was electrically heated, the temperature was raised until the center of the parallel part started melting, and the melting was continued while measuring the temperature. Then, it was rapidly cooled at 20 ° C / 5 ecT and held at a temperature just below the melting point. A tensile test was conducted, and the reduction of area (%) and tensile strength of the test piece until breakage were measured. We conducted research on the alloy composition, paying particular attention to the temperature at which the aperture becomes 50x or more. The investigated alloy composition is Cr-Ni stainless steel and has the following composition (15). Compositions are expressed in weight percent.

C: 0.005-0.10 Mn: 0-3.0 S: 0.0003-0.04 Ni: 5.0-15.0 Cu: O-3.0 0: 0.002-0.011 Ti: O~0.6 Ca: O~0.01 As a result of these studies, it was found that there are components that have a very significant effect on the ductility of the alloy immediately after solidification.

The main components of stainless steel, such as Cr, Ni, and Mo, do not have much of an effect, but Si, Mn, and S
,N.

The influence of P, O, etc. is significant. Figure 3 is patent application No. 6216.
No. 7633 (Japanese Unexamined Patent Publication No. 11925/1983), tensile tests were carried out at various temperatures just below the melting point after melting in the melt Greeple test, and the temperature at which the reduction of area reached 50% was shown. It is. Most of the considerations in the figure are ■
18Cr-8Ni-0,6SiSi: 0 to 4 shown in
．． 0P: 0.001~0.050Cr: 1
5. (1-30,0 Mono ~ 3.5 A1: 0 ~ 0.5 N: 0.005 ~ 0.25 Nb: O ~ 0.8 (16) An alloy containing 1,2Mn as the basic component was investigated. As shown in ■, in these component systems, the temperature that reaches 50% just below the melting point is around 1330℃, and as shown in ■, it changes greatly when S is changed, and the temperature is 1340℃ for low S and 1300℃ for high S.
It drops to C. Note that this temperature is the surface temperature of the Greeple test piece. The crack in the center of the specimen propagated along the dendrite interface and the remaining liquid layer, and is thought to be caused by liquid film embrittlement. However, 18Cr 8N shown in ■
In i, Si is 0.2%, Mn is 0.2%, and S is 0.0.
In the case of the basic component system of 0.01%, the above temperature is 13%.
It was found that the temperature exceeds 50"C and approaches 1370-1380°C. Thus, the alloy (■) has high ductility just below the melting point and is extremely difficult to crack. As shown in (■), the Si
, disclosed that this temperature fluctuates greatly in a component system in which the amount of Mn is changed.

The inventors further investigated Si and Mn and found that the change in ductility just below the melting point depends largely on the amount of Si, but not on the amount of Mn, as shown in Figure 1. . From FIG. 1 (17), when Si is set to 0.45% or less, Mn is allowed up to 2%. Furthermore, as shown in Figure 2, when we organized the relationship between S and N, we found that as S and N increase, the ductility at high temperatures tends to decrease.
．． When it exceeds 18, the ductility decreases significantly. 18Cr-8
In the Ni basic component system, Si, Mn, S,
For Si, Mn, S, and N, which have a large effect on ductility due to the influence of N, ○, etc., the range of components to prevent deterioration of surface properties due to cracking due to lack of ductility just below the melting point is determined as follows. Ta.

Si: 0-0.5% Mn: O~2.0% S: 0-0.008% N: 0-0.18% Also 10x S + N <0.18 (Example) The present invention will be explained in more detail with reference to Examples.

Table 3 (1
8) An alloy having the components shown in the following was melted. These alloys were cast in a twin roll casting machine with internal water cooling to a thickness of 2.4 mm and 800 m.
Continuously cast into m-wide slabs and heated up to 1000℃ by water cooling.
Cooling was carried out at 20°C/sec and an average cooling rate of 45°C/sec up to 650°C, and winding was performed at 550°C. Afterwards, this slab was subjected to pickling, cold rolling, and annealing under the conditions shown in Table 4 to evaluate its material quality, and the material shown in Table 4 was obtained. −
Ni-based stainless steel was successfully manufactured.

came.

(Effects of the Invention) According to the present invention, the CrNi stainless steel is economically superior by omitting the hot rolling process by using a slab with a shape as close as possible to the product shape, and is also excellent in material and surface properties. Since steel can be manufactured stably, the present invention is a manufacturing method of extremely great industrial value.

(19) Unexamined Publication Hei 3 107427 (8) Unexamined Publication Hei 3 107427 (11)

[Brief explanation of drawings]

Figure 1 shows the results of a post-melting tensile test to investigate the influence of Mn and St on ductility just below the melting point in the 18Cr-8Ni 0.003S system, and the temperature at which the reduction of area becomes 50% is 1350°C or higher. The temperature below 011350°C is indicated by ×. Figure 2 shows 18Cr -8Ni -0,3Si -
It is a figure showing the results of examining the influence of S and N on ductility just below the melting point in the 0,4Mn system by a tensile test after melting.
02135 The temperature at which the aperture becomes 50% is 1350℃ or higher.
Temperatures below 0°C are indicated by x. FIG. 3 is a diagram showing the temperature at which the reduction of area reaches 50% in a tensile test after melting various alloys. (25) Procedural amendment (method) % formula % Display of case 1999 Patent Application No. 245272 2゜ Title of invention Method for manufacturing Cr Ni stainless steel sheet with excellent mechanical properties and surface properties 3゜ Amendment Relationship with the incident

Claims (1)

[Claims] 1. C whose basic components are 18% Cr-8% Ni by weight%
Cast r-Ni stainless steel into slabs with a thickness of 6 mm or less using a continuous casting machine in which the mold wall surface moves in synchronization with the slab, with a cooling rate of 30 ° C / sec or more during solidification. , ensuring an average cooling rate of 20°C/sec or more
After cooling to 50℃ and rolling it up at a temperature of 650℃ or less,
Pickling is performed, followed by cold rolling with a rolling reduction of 85% or less, and the temperature/time relationship is controlled in a temperature range of 1000°C to 1300°C to determine the average grain size of the material. A method for manufacturing a Cr-Ni stainless steel sheet with excellent mechanical properties and surface properties, which comprises annealing to a number of 6 to 8. 2. The Cr-Ni stainless steel whose basic components are 18% Cr-8% Ni by weight, further contains Si≦0.5%
, Mn≦2.0%, S≦0.008%, N≦0.18%
2. The method according to claim 1, wherein the method satisfies the following relationship: 15×S(%)+N(%)<0.18.