JPH03197621A - Method for manufacturing non-oriented electrical steel sheet with excellent magnetic properties and surface properties - Google Patents

Abstract

PURPOSE:To improve the magnetic characteristics and surface characteristic of the electrical steel sheet by subjecting a continuous casting slab having a specific compsn. to hot rolling and descaling, then managing decarburization conditions in open coil annealing and conditions of homogenization annealing. CONSTITUTION:The continuous casting slab consisting, by weight %, 0.009 to 0.025% C, 1.3 to 3.0% Si, <=0.8% Al, <=3.2% (Si+Al), 0.2 to 1.2% Mn, <=0.03% P, <=0.005% S, <=0.005% N, and the balance Fe and unavoidable impurities is hot rolled directly or after heating to <=1200 deg.C. The steel is coiled at <=650 deg.C and is subjected to a descaling treatment. The steel sheet is then subjected to the decarburization treatment at 640 to 720 deg.C in the atmosphere of the range expressed by equation in an open treatment stage. The steel sheet is thereafter subjected to the homogenization annealing to hold the steel sheet for 30 minutes at 700 to 850 deg.C in the atmosphere of <=0.03 [PH2O/PH2], <=75% H2 concn. to decrease the concn. of the C in the steel to <=0.003%. The steel sheet is subjected to cold rolling and continuous annealing in succession to the above. The non-oriented electrical steel sheet which is improved in magnetic flux density and is suppressed in the generation of the ridging at the time of cold rolling is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a non-oriented electrical steel sheet with excellent magnetic properties and surface properties.

[Conventional technology]

Among the metallurgical factors that must be unavoidably considered in the manufacturing process of electrical steel sheets, one of the most important is the control of the carbon content in the steel.

In other words, deterioration of characteristics over time under the usage environment (magnetic aging)
In order to suppress residual solid solution carbon, which is the main cause, it is generally reduced to below SOPP (preferably 30p) at the final stage.
It is essential to reduce it to a level of p1 or less.

Conventionally, in the decarburization treatment of electrical steel sheets, a non-oxidizing and decarburizing atmosphere (generally 0.1≦(PH, 0/PH
1)≦0.2, however, PH,O: H,0 partial pressure, P)l
A method has been adopted in which decarburization annealing is performed for a short time in H, partial pressure (hereinafter the same). This is partly because it was difficult to stably reduce carbon in steel to about 30 ppm during the steel manufacturing stage.

In contrast, recent advances in mesh degassing technology have made it possible to reduce carbon in steel to 30 ppm or less during steel manufacturing, and there are many technologies that eliminate the need for decarburization during final continuous annealing. Disclosed.

Here, the technical advantages and disadvantages of the above-mentioned final decarburization method and the Wi-rust decarburization method are shown below.

(Final decarburization method) (1) Due to the presence of carbon in the steel, the Si content in particular is 1.5
%, the dynamic recovery and recrystallization of ferrite are promoted during hot rolling, and the development of streak-like ferrite, which causes writhing during cold rolling, is suppressed.

(2) Although degassing treatment is not required during steel manufacturing, the formation of subscale during the final decarburization annealing brings about an increase in iron loss value.

(Steel decarburization method) (1) Since the steel decarburization method controls the carbon content to a predetermined level at the material stage, it does not require decarburization treatment in the subsequent manufacturing process, and has excellent productivity.

(2) In the atmospheric decarburization treatment performed after forming the steel strip, the inevitable formation of a subscale layer (especially in high Si #i) becomes no longer a problem.

(3) However, when carbon is reduced to an extremely low level at the material stage, the solidified structure becomes coarse-grained, dynamic recovery and recrystallization during hot rolling become slow, and a streak-like ferrite structure tends to develop. Stiffness tends to become more pronounced in today's continuous casting and continuous hot rolling processes, and is a cause of rolling during cold rolling.

Therefore, to solve this problem, the amount of C in steel can be changed to Si, A
Although a technique (Japanese Patent Publication No. 48-49617) has been disclosed in which the lower limit value is regulated as a function of the amount of ll, the effect is not sufficient.

[Problem to be solved by the invention]

By the way, for the above-mentioned typical manufacturing process of electrical steel sheets, a technology for decarburizing hot-rolled sheets has also been disclosed (
Japanese Patent Publication No. 41-21967, Japanese Patent Publication No. 19766-1976, Japanese Patent Publication No. 66816-1973, etc.). However, these techniques are not disclosed as unique techniques that take into account the advantages and disadvantages of the two methods described above, but are merely disclosed as one method of decarburization treatment.

For example, Japanese Patent Publication No. 41-21967 is a technology disclosed for rimmed steel, and is related to a general laminated steel sheet with 2.02% SiS, and subscale formation during decarburization treatment is not so much of a problem. Moreover, this technology requires subsequent temper rolling. In addition, although Japanese Patent Publication No. 19766 discloses a technology for high-Si steel, it only stipulates the C content of the final product to be 0.005% or less, and especially in relation to the properties, it does not specify the initial carbon content or Regulations regarding hot rolling conditions are not disclosed. In addition, this technology requires a step to remove the internal oxidation layer after decarburizing the hot rolled sheet. Furthermore, in JP-A No. 53-66816, a technology is disclosed in which a silicon steel hot-rolled steel strip with o, ooa%C is decarburized in the atmosphere, but with such an initial carbon content, high In Si steel, it is difficult to sufficiently control the hot rolling structure.

In view of the above-mentioned prior art, the present invention discloses a technology that requires decarburization at the hot-rolled sheet stage regarding the decarburization treatment of electrical steel sheets, and its purpose is to perform the final decarburization process. This solution solves the problem of subscale layers formed by high-temperature annealing in steel mills, and the operational and production efficiency problems associated with wet annealing in continuous annealing furnaces. The purpose is to solve the problems caused by jing and disadvantages in magnetic flux density.

[Means to solve the problem]

For this reason, in the invention, when decarburizing a hot rolled sheet,
In addition to strictly controlling the amount of carbon during the steelmaking stage, descaling is required to suppress the nitriding reaction caused by scale formed during hot rolling, and by strictly controlling the decarburization conditions using open coils. The main point of this method is to omit the subsequent internal oxidation removal step.

That is, in the present invention, C: 0.009 to 0.009% by weight.
026%, SL: 1.3-3.0%, All1:i
i: 0.8%, (Si+AM)≦3.2%, Mn:
0.2-1.2%, P≦0.03%, S≦0.005%
, N≦0.005%, balance Fe and unavoidable impurities, directly or after heating to below 1200°C, hot rolling and coiling at below 650”C, and descaling the steel strip. After that, by open coil annealing, first, 0.05≦(PH20/PH,)≦−0,014C5i
+AA)"+0.01(SL+AR) After decarburizing at a temperature of 640 to 720"C in an atmosphere in the range of +0.16, subsequently (pu, o/ pHz)≦0
．． 03. H, 700-85 in an atmosphere with a concentration ≧75%
By performing homogenization annealing at a temperature of 0°C for 30 minutes or more, the amount of C in the steel is reduced to 0.003% or less,
It is characterized in that the hot rolled steel strip is subjected to cold rolling and continuous annealing.

Hereinafter, the details of the present invention will be explained together with the reasons for its limitations.

In the present invention, by decarburizing hot-rolled sheets, it is possible to make maximum use of the effect of carbon in steel on microstructure formation during hot rolling. The amount of carbon in the steel was determined to quickly complete decarburization while preventing scale formation, and to improve ridging resistance by suppressing the development of streak-like ferrite structures during hot rolling with the minimum necessary amount of carbon. , hot rolling conditions, and decarburization annealing conditions are limited as follows.

(Components in m) C: Defined at 0.009 to 0.026%. Figure 1 shows the relationship between the initial carbon content and the sliding height during cold rolling.If the C content is less than the above lower limit, silicon steel containing Si≧1.3% will have a streak height during hot rolling. It is difficult to sufficiently suppress the formation of tissue, and there are also disadvantages regarding the level of magnetic flux density (B S.). On the other hand, C
If the amount exceeds the upper limit, the amount of residual C will be 30% in a low-temperature, non-oxidizing decarburization atmosphere that suppresses the formation of subscale and can be subjected to cold rolling and subsequent processes without going through a subscale removal process. ppm, which is undesirable in some cases.Also, as shown in Figure 1, from the viewpoint of improving ridging and magnetic flux density, even if C exceeding the upper limit is contained, it is significant in suppressing ridging and improving magnetic flux density. This method has no effect and requires a long decarburization process, which poses a problem of increased iron loss due to the formation of subscales.

Si: Specified at 1.3 to 3.0%. Below the lower limit, the increase in solidification temperature not only suppresses the development of columnar crystals during casting, but also reduces γ during the subsequent slab cooling and reheating process.
Due to the two-α transformation, the slab structure becomes fine-grained, making it difficult for a streak-like structure to develop during hot rolling, and the occurrence of ridging becomes less of a problem. Therefore, the lower limit of the Si level that can maximize the effects of the present invention is set at 1.3%. - On the other hand, if the amount of Si exceeds the upper limit, even if the amount of C at the upper limit is added, the occurrence of ridging cannot be sufficiently suppressed, and a subscale layer due to preferential oxidation of Si during decarburization annealing is formed. , subscale removal is inevitable in subsequent steps.

Mn: Specified at 0.2 to 1.2%. Below the lower limit, sufficient precipitation of MnS cannot be promoted even if S is defined to be 0.005% or less. Also.

Like C, Mn is an austenite stabilizing element and suppresses the formation of streak structure during hot rolling, but at 1.2%
Even if it is added in an amount exceeding 100%, no significant effect commensurate with the added amount is observed.

P: P is intentionally added in some cases from the viewpoint of increasing hardness and reducing iron loss, but in the present invention, P is added during hot-rolled sheet annealing.
The upper limit is set to 0.03% because it is possible to suppress the segregation of Si into the ferrite grain boundaries and to reduce iron loss without the aid of P in steel with 20.3% Si.

S: The upper limit of S is set to 0.005% from the viewpoint of preventing fine dispersion of MnS.

All: An plays a role similar to that of Si, and is generally treated as (Si+^a) in some cases. In the steel containing 20.3% Si as in the present invention, An is not an essential element in order to exhibit its effects, and there is no need to restrict the lower limit. In most cases, AQ is added to boat fishing with the aim of improving the level of magnetic properties itself, and in particular, its upper limit is regulated at 0°8% from the viewpoint of preferential oxidation prevention of All.

Si+A1 An important technical feature of the present invention is that an internal oxidation layer removal step is not required after hot-rolled sheet decarburization annealing. This is achieved by the process conditions specified in the present invention and by limiting (Si+A11) to 3.2% or less.

N: N is 0.0 with the aim of suppressing the amount of AnN precipitation.
The upper limit shall be 0.05%.

Now, as a means to solve the problems of the conventional technology,
In the present invention, in addition to the above-mentioned specification of the component system, the hot rolling conditions and the decarburization treatment conditions of the hot rolled sheet are specified as follows.

(Hot rolling conditions) Slab heating temperature: Continuously cast slabs can be directly hot rolled (direct rolling) after casting, but if the slab is reheated and hot rolled, the upper limit of the slab heating temperature is 120
The temperature shall be 0°C.

This avoids the problem of scale melting during heating, suppresses the development of a streak-like structure during hot rolling due to coarsening of the structure during heating of the slab, and prevents the remelting of MnS and Al1N and the subsequent annealing process. This is to suppress the increase in iron loss value due to fine precipitation.

Winding temperature: The winding temperature shall be 650°C or less. This is aimed at improving the descaling properties before decarburization annealing, but at the same time, a uniform fine recrystallized structure is formed during the decarburization annealing process of the hot rolled sheet, which improves the quality of the high temperature rolled material. Compared to this, the grain boundary diffusion of C is promoted and the decarburization time is shortened. Furthermore, it is effective in suppressing scale remaining and preventing nitridation during decarburization annealing.

(Decarburization treatment conditions) As the most important component of the present application, a hot rolled steel strip that has undergone descaling treatment is subjected to decarburization annealing and homogenization annealing in a series of steps under the following conditions.

That is, in an open coil shape, first, 0.05≦(P
H, O/ PH2)≦-0,014(Si+Aλ)2+
Decarburization is carried out at a temperature of 640 to 700° C. in an atmosphere satisfying the condition of 0.01 (Si+A consideration)+0.16. Figure 2 shows P)l, 0/
This shows the relationship between PH and decarburization, and the influence on subscale layer formation in the relationship with Si + AA. When PH, O/P) 1 m is less than 0.05,
If the decarburization temperature is lowered to 640° C. in consideration of subscale formation, sufficient decarburization will not occur.

On the other hand, regarding the amount of Si and All in steel, -0,014 (S
i+^11)” +0.01 (Si 10 mouths) +0.16
If the temperature exceeds 700° C., a subscale layer is formed during decarburization at 700° C., and descaling treatment after annealing becomes inevitable.

Furthermore, in the present invention, following the decarburization annealing, it is essential to perform annealing at a temperature of 700 to 850° C. for 30 minutes or more with the aim of improving magnetic properties. In particular, in the present invention, the atmosphere during annealing is [PH, O/P) I
, )≦0.03. )l, concentration ≧75%.

This annealing is aimed at growing the ferrite grains in the hot rolled sheet to a size that is favorable for magnetic properties.
Even if decarburization annealing is controlled to the optimal conditions specified in the present invention, the absorption of nitrogen in the atmosphere by the oxidized layer on the extreme surface layer that is inevitably formed (- H2 + N used for boat fishing)
, in a mixed gas), a sufficiently low dew point and 75% or higher 11. This is to homogenize the concentration. Figure 3 shows H2 in homogenization annealing.
This shows the influence of concentration on nitride layer formation, and steel-3 in Table 1 is under severe conditions regarding subscale formation.
For °4, [PH2O/PH2] = 0.10.
The thickness of the nitrided layer was investigated when decarburization was performed under the conditions of I+□ concentration = 10% and 700°C for 2 hours, and then homogenization annealing was performed at 840°C for 3 hours while changing the ■2 concentration. As is clear from the figure, it can be seen that the formation of a nitrided layer is suppressed when the H2 concentration is 75% or more.

In the present invention, by adopting the above configuration, the formation of a streak-like ferrite structure during hot rolling is suppressed due to the presence of carbon in the steel at the slab stage, and due to the significant difference in the formation of the texture. , the occurrence of rolling during cold rolling is prevented, and the magnetic flux density of the final product is improved. Furthermore, in the present invention, by performing decarburization treatment at the hot-rolled sheet stage under controlled conditions, it is possible to reduce carbon in the steel to a level where magnetic aging is not a problem, and homogenize it under sufficient temperature and time. By adding annealing, excellent low iron loss and high magnetic flux density can be obtained, and since wet annealing is not required in the final continuous annealing stage, troublesome operations such as dew point control are eliminated. It becomes easier to configure a product mix that includes decarburized steel types.

〔Example〕

Table 1 shows the chemical composition of the test steel.

All of these steels were melted in a converter, subjected to RH degassing treatment and carbon content adjustment by carburization, and then continuously cast. After heating a 200 mt continuously cast slab to 1140°C, it was rough rolled and finished rolled to become a 2 Orm t hot rolled sheet.
It was wound up at 610°C. Some of the steel strips were also rolled up at 680°C.

After winding, the steel strip is pickled and made into a loose coil.
Decarburization treatment in an atmosphere with a HzO/PH ratio (Fragrances 1 to 6 are 20% H, +N2 atmosphere, dew point: 15℃, Perfumes 7 to 10 are 25% H2 + N, dew point: 10℃) After that, homogenization annealing was performed in atmospheres having various pH, 0/PH, ratio, and hydrogen concentration. After the annealing, the hot-rolled steel strip was cold-pressed to 0.5 mat, and then continuously annealed at the optimum temperature for each component system. The ridging height, final carbon content, magnetic properties, etc. of the steel strips thus obtained are shown in Tables 2 to 4 together with the hot rolled plate annealing conditions.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the initial carbon content in steel and the sliding height during cold pressing. Figure 2 shows the relationship between PI (, O/PH) and decarburization in the hot-rolled plate decarburization annealing atmosphere, and the relationship between Si
12 is a graph showing the influence of P)1.0/PH2 on subscale layer generation in relation to ten mouths. FIG. 3 is a graph showing the influence of H2 concentration on the formation of a nitrided layer during homogenization annealing of a hot rolled sheet. First 1I4Pj! Amount of happiness (%)

Claims (1)

[Claims] In weight %, C: 0.009 to 0.026%, Si: 1.
3-3.0%, Al≦0.8%, (Si+Al)≦3.
2%, Mn: 0.2-1.2%, P≦0.03%, s≦
A continuous casting slab consisting of 0.005%, N≦0.005%, balance Fe and unavoidable impurities is directly or
After heating to 00°C or lower, hot rolling and coiling at 650°C or lower, the steel strip is descaled and then subjected to open coil annealing to obtain 0.05≦[PH_2O/PH_2]≦−0.014(
After decarburizing at a temperature of 640 to 720°C in an atmosphere in the range of Si + Al)^2 + 0.01 (Si + Al + 0.16), [PH_2O/PH_2]
700~ in an atmosphere with ≦0.03, H_2 concentration ≧75%
The C content in the steel is reduced to 0.003% or less by performing homogenization annealing at a temperature range of 850°C for 30 minutes or more, and the hot rolled steel strip is subjected to cold rolling and continuous annealing. A method for manufacturing non-oriented electrical steel sheets with excellent magnetic properties and surface properties.