JP5163450B2 - Steel manufacturing method - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a method of manufacturing a steel material that can easily and quickly optimize (correct) manufacturing conditions in order to maintain target characteristics when manufacturing steel materials used in automobiles, shipbuilding, civil engineering, and construction. is there.

When manufacturing primary steel products (hereinafter referred to as steel materials) such as thin steel plates, thick steel plates, steel bars and wires used as materials for automobiles, shipbuilding, civil engineering and construction, etc., they are adjusted to the prescribed components. The necessary shape and mechanical properties are obtained through hot working processes such as hot rolling and hot forging.
The hot working process includes, for example, a process of reheating and holding the material at about 1200 ° C., a process of reducing the thickness by several times of processing, a process of cooling to a predetermined temperature after processing, and a winding process of holding at a predetermined temperature for a certain time It consists of a plurality of processes such as processes, and each process needs to be optimized in order to obtain desired characteristics. Normally, in the actual machine manufacturing process, the manufacturing conditions in each process are optimized based on the result of laboratory investigation.

  However, high-grade steel grades such as high tensile steel often require narrow control of manufacturing conditions, and product quality may deteriorate due to slight fluctuations in conditions. In such a case, it is necessary to identify the cause of deterioration at an early stage and correct the control within the preferable manufacturing conditions, but there is no effective means at present. Therefore, it is difficult to achieve stable production of such high-grade steel types and yield improvement. Moreover, such a problem is conspicuous in high-grade steel types, but also exists in general steel types.

As mentioned above, especially in the production of high-grade steel grades, it is necessary to quickly identify the factors that deteriorate the product quality, quickly find the manufacturing conditions to be corrected, and provide appropriate feedback in order to minimize the decrease in yield. is necessary.
In addition, when manufacturing steel grades with many manufacturing processes, it is necessary to grasp the status of steel materials in the middle of the manufacturing process in more detail and perform accurate feedforward to the manufacturing conditions in the subsequent process based on that information to improve product yield. Is necessary for.

  The present invention has been made in view of such circumstances, and quickly finds manufacturing conditions to be corrected in order to maintain the target characteristics, and is a steel material capable of performing appropriate feedback or feedforward depending on various cases. The object is to provide a manufacturing method.

In order to solve the above-mentioned problems, the inventors diligently studied from the viewpoint of material deviation and material information due to variations in manufacturing conditions.
Conventional physical analysis methods are highly accurate, but it is difficult to obtain representative values in steel, and conventional chemical analysis methods are quick, but specific information that largely governs material properties can be selectively obtained. Have difficulty. For this reason, information such as the solid solution content of metal components in materials and the precipitation amount by size in metal carbides to quickly find the conditions that can be reflected and corrected in manufacturing conditions can be obtained using conventional physical analysis methods and Chemical analysis methods could not be obtained quickly and with high accuracy. As a result, proper feedback or feedforward could not be performed.
Therefore, in view of the above, the present inventors have corrected from various manufacturing conditions by quickly and accurately obtaining the solid solution content of the metal component in the material and the precipitation amount by size in the metal carbide. It was considered to quickly find out the power condition and perform appropriate feedback or feedforward according to various cases. And this invention came to be completed based on the said thought.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Manufacturing step A for manufacturing a steel material, information on the composition of precipitates and / or inclusions, information on the size of precipitates and / or inclusions, and elements of interest in the steel material manufactured in manufacturing step A An analysis step for obtaining one or more pieces of solid solution amount information, and when at least one of the analysis results based on the information obtained in the analysis step is out of a predetermined range, precipitates and / or A manufacturing condition correcting step for correcting at least one manufacturing condition in which one or more of the composition of inclusions, the size of precipitates and / or inclusions, and the solid solution amount of the element of interest changes;
A method for manufacturing a steel material, comprising: a manufacturing step B for manufacturing a steel material under the manufacturing conditions corrected in the manufacturing condition correcting step.
[2] In the above [1], in the analysis step, after the produced steel material is electrolyzed in an electrolytic solution, precipitates and / or inclusions adhering to the steel material are separated into a solution having dispersibility. Analysis of obtaining one or more of information on the composition of precipitates and / or inclusions, information on the size of precipitates and / or inclusions, and information on the solid solution amount of the element of interest Production method.
[3] In the above [2], in the analysis step, the precipitate and / or the inclusion is sized by filtering one or more stages of the dispersed solution containing the separated precipitate and / or the inclusion. Separating separately, The manufacturing method of the steel materials characterized by the above-mentioned.
[4] In any one of the above [1] to [3], the analyzing step analyzes an electrolytic solution after electrolyzing the steel material, and calculates a ratio between the concentration of the element of interest and the concentration of iron in the electrolytic solution. A method for producing a steel material, characterized by analyzing the solid solution amount of the element of interest by multiplying the obtained ratio by the total concentration of iron in the steel material.
[5] In any one of the above [1] to [4], the manufacturing step A includes a hot working process, and the manufacturing condition correcting step includes a solid solution of the element of interest obtained in the analyzing step. When the amount is less than the predetermined range, and / or when the amount of the element of interest contained in the precipitate obtained in the analysis step and / or the size of inclusions is more than 100 nm And reheating temperature in the hot working step is corrected.
[6] In any one of the above [1] to [4], the manufacturing step A includes a hot working step and a cooling step performed after the hot working step, and the manufacturing condition correcting step includes When the ratio of precipitates and / or inclusions having a size of 20 to 100 nm with respect to the total amount of precipitates and / or inclusions obtained in the analysis step is not less than a predetermined range, the cooling rate in the cooling step is set. A method of manufacturing a steel material, characterized by correcting.
[7] In any one of [1] to [4], the manufacturing step A includes a series of steps of hot working, cooling, and intermediate holding in a specific temperature range, and the manufacturing condition correcting step includes: The amount of the element of interest contained in precipitates and / or inclusions having a size of less than 20 nm obtained in the analysis step is not more than a predetermined range, and the precipitates having a size of 20 nm or more obtained in the analysis step and A method for producing a steel material, comprising: correcting an intermediate holding condition in the intermediate holding step when an amount of an element of interest contained in an inclusion is within a predetermined range.
[8] A method for manufacturing a steel material, which is manufactured by any one of the methods [1] to [7], and is subjected to heat treatment after shipment to adjust characteristics.
In the present invention, precipitates and / or inclusions are collectively referred to as precipitates.

According to the present invention, it is possible to quickly find a manufacturing condition to be corrected for maintaining a target property, and stably obtain a desired material property by performing appropriate feedback or feedforward according to various cases. It is possible to provide a method for producing a possible steel material.
For example, if you know the precipitation behavior of the element of interest that is highly related to material fluctuations in advance and then compare the precipitation information obtained by analysis with the precipitation behavior of the element of interest, you can correct any part of the manufacturing conditions. Whether it is good or not can be quickly determined, and stable material securing is achieved.
In addition, when manufacturing a steel type with a long manufacturing process, if the status of the steel material in the middle of the manufacturing process is outside the specified range so that the correction does not work, Analytical data for quickly ascertaining the cause of product quality degradation is required quickly with high accuracy. In such a case, the production method of the present invention is useful. And by using the manufacturing method of this invention, the continuation determination of the manufacturing after the following process is attained, and a manufacturing cost is significantly reduced.

Hereinafter, the present invention will be described in detail.
The chemical composition in steel is an important factor that has various effects on the structure formation of steel materials. It influences the final structure formation through the grain growth behavior, transformation phenomenon and precipitation phenomenon, and greatly affects the macro material. . In particular, in high-grade steel grades such as high tensile steel that effectively utilize precipitation strengthening due to carbide or the like, the control of precipitates in steel is very important in securing a stable material. Therefore, by obtaining detailed information on the behavior of precipitates in steel, it is possible to ascertain the cause of material fluctuation with high accuracy.

As a result of the above considerations, in the present invention, first, in order to quickly find out the production conditions to be corrected, a solid solution state in steel of the contained element (hereinafter referred to as the element of interest) that has a great influence on material variation Analysis to obtain information that can be reflected in the correction of manufacturing conditions, a new analysis method that can quickly and accurately grasp the quantitative values of precipitates and the quantitative values of precipitates by size, etc. I decided to use it as a method. Next, after obtaining information that can be reflected in the correction of the manufacturing conditions, based on this information, conditions to be corrected are quickly found out from various manufacturing conditions, and appropriate feedback or feedforward is performed.
The above are features of the present invention and are important requirements. The details will be described below.

  In the analysis step of obtaining one or more of information on the composition of precipitates, information on the size of precipitates, etc., and information on the solid solution amount of the element of interest in the steel material, the inventors have developed with high accuracy, A method of analyzing the composition and size of precipitates and the amount of solid solution of the element of interest will be used, and this will be described first.

  The steel material sample is electrolyzed under appropriate conditions, and the solid solution portion of the precipitation strengthening element is dissolved in the electrolytic solution together with the iron of the matrix to expose the precipitate and the like on the sample surface. At this time, the exposed precipitates and the like adhere to the surface of the sample, which is the anode, by electric attraction, so that the precipitates and the electrolytic solution (solid solution portion) can be separated. That is, almost all the precipitates and the like can be taken out from the electrolytic solution only by taking out the sample to which the deposits and the like are attached from the electrolytic solution. Then, the sample is immersed in a dispersible solution such as an aqueous polyphosphoric acid solution to apply ultrasonic waves, and precipitates and the like attached to the sample are peeled off from the sample. At this time, the separated precipitates and the like are given surface charges from the polyphosphate, repel each other, and are dispersed in a solution having dispersibility.

  First, when the precipitates are not classified by size, a solution having dispersibility containing the precipitates is analyzed by a dynamic light scattering spectroscopic analysis method, and the average particle size and particle size distribution of all the precipitates are determined. Ask.

  Next, when the precipitates are separated by size, the following procedure is used. A solution having dispersibility in which precipitates are dispersed is sequentially filtered using a filter having filter pore sizes Y and Z (where Y> Z). At this time, the residue on the filter with a pore size Y is a precipitate of size Y or larger, the residue on the filter with a pore size Z is a precipitate of size Z or larger and less than Y, and the filtrate that has passed through the filter with a pore size Z Includes precipitates having a size less than Z. Next, the precipitate and the filtrate on the filter after filtration are analyzed by inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc., size Y or more, size Z or more and less than Y, The content of the element of interest in precipitates and the like of size Z is determined. Alternatively, the filtrate after filtration is analyzed by a dynamic light scattering spectroscopic analysis method, and the average particle size and particle size distribution of precipitates having a size less than Z are obtained.

  In this way, by using a filter having a plurality of filter pore diameters, the precipitates and the like can be separated according to size. In addition, when a solution having dispersibility containing precipitates and the like is filtered with a filter having a predetermined filter pore size, it is divided into those collected by the filter according to the size of the precipitates and those passing through the filter. At this time, the filter pores are blocked by relatively large precipitates and the like, and precipitates having a size that should pass through may be collected without passing through the filter. In such a case, the analytical value of the precipitate collected in the filter becomes higher than the correct value, and conversely, the analytical value of the filtrate becomes lower than the correct value. However, if a filter that is a straight hole and has a porosity of 4% or more is used as a filter, it is possible to analyze precipitates by size more accurately without collecting precipitates smaller than the filter pore diameter. Become. Here, the straight hole refers to a filter hole penetrating with a certain opening shape.

  In order to obtain the solid solution amount of the element of interest, it is necessary to measure the absolute amount of the element of interest in the electrolytic solution separated from precipitates and divide by the electrolytic weight of the steel material sample. However, a general electrolytic solution is an organic solvent mainly composed of methanol, and has a high volatility and a liquid amount of several hundred milliliters. Therefore, it is not easy to measure the content of the element of interest. Therefore, after collecting an appropriate amount of 1/10 or less from a large amount of electrolyte and drying it, dissolve it in an appropriate solution to make an aqueous solution, and then measure the element of interest and iron by an appropriate solution analysis method. By multiplying (ie, the measured concentration of the element of interest / the measured concentration of iron) by the iron content in the steel material sample, the solid solution amount of the element of interest in steel was determined. As a method for analyzing the aqueous solution, ICP emission spectroscopy, ICP mass spectrometry, and atomic absorption spectrometry are suitable. In addition, methods for determining the iron content in steel samples include spark discharge emission spectroscopy (JIS G1253), X-ray fluorescence analysis (JIS G1256), ICP emission spectroscopy, ICP mass spectrometry, etc. A method of subtracting the total value of elements other than iron obtained by the method from 100 mass% is appropriate.

  In the method for producing a steel material of the present invention, at least one piece of information (analysis result) among the composition, size, and solid solution amount of the element of interest obtained in this way is reflected in the modification of the production conditions. In the case where only the result of the composition of the precipitate or the like is used, for example, it can be presented by either the type of the element of interest constituting the precipitate and the content thereof. Further, when only the result of the size of the precipitate or the like is used, for example, the average particle size or the particle size distribution obtained by the above-mentioned dynamic light scattering spectroscopic analysis method can be presented. When using the results of both the composition and the size of the precipitates, for example, it can be presented by the content in the precipitate of the element of interest for each size of the precipitates. It should be noted that the content of the element of interest in the precipitates, for example, for the element of interest, how much the element is present as a precipitate, etc., (a) the content of the steel material as a whole, (b) the element of interest The ratio to the total amount, (c) the ratio to the solid solution amount of the element of interest, etc. can be presented as necessary. In addition, the solid solution amount of the element of interest is, for example, how much the element is present in the solid solution state in terms of (d) the content ratio with respect to the entire steel material, and (e) the ratio with respect to the total amount of element of interest. (F) The ratio to the content of the element of interest in the precipitate or the like can be presented as necessary.

Next, when at least one of the analysis results based on each information obtained in the analysis step is out of a predetermined range, the composition of the precipitates, the size of the precipitates, and the solid solution amount of the element of interest Modify at least one production condition that changes more than one. And steel materials are manufactured with the corrected manufacturing conditions.
Based on the above, embodiments of the present invention will be exemplified below.
First, the difference between the present invention and the conventional method will be described with reference to FIGS. FIG. 1 is a flowchart showing a conventional manufacturing condition determination process. Conventionally, production conditions have been optimized in the production of actual machines by prototyping actual machines based on production conditions that have been narrowed down to some extent at the laboratory stage, and guaranteeing differences between the actual machines and the laboratory level. Therefore, when the final material test value does not satisfy the desired characteristics, there is a need to perform a factor analysis each time as shown in FIG. 1 and to reexamine various manufacturing conditions.

On the other hand, FIG. 2 is a flowchart showing a manufacturing condition determination process which is one embodiment of the present invention. In the present invention, as shown in FIG. 2, when the material is out of the predetermined range and the material is deviated, at least of the information on the composition of the precipitate, the information on the size of the precipitate, and the information on the solid solution amount of the element of interest. By using one, it is possible to quickly narrow down the manufacturing conditions to be changed and to feed back to the narrowed manufacturing conditions.
For example, paying attention to a particularly important metal element that greatly affects the material, the solid solution amount of the element is obtained. Then, the value of this solid solution amount is compared with the standard value determined in advance in the laboratory in order to obtain the target quality and performance, or the standard value when appropriately processed in the actual machine, and the standard value is calculated. It is confirmed whether it is within a permissible range (hereinafter, this permissible range is referred to as a predetermined range). And when it remove | deviates from the predetermined range, winding temperature is reset.
For example, when the element of interest exists in a precipitated state, particularly when it exists as a carbide or nitride, obtain at least one of composition information such as precipitates and size information such as precipitates, and this value Is compared with a predetermined range. And based on the result, slab reheating conditions and the cooling rate after hot working are corrected.
For example, when producing a steel sheet with a strength of 780 MPa or more, if insufficient strength occurs, the conventional production method lacks the absolute amount of fine precipitates that contribute to strengthening because the slab heating conditions are not appropriate. However, it is difficult to easily determine whether the re-deposited precipitates are coarse because the cooling conditions leading to the winding process after hot rolling are not appropriate. Therefore, it was necessary to reconfirm the conditions of each process considered to be a factor, and to specify the cause of insufficient strength while changing these variously. However, if the steel material manufacturing method of the present invention is used, at least one of the solid solution amount of the element of interest in the final product or the composition information such as precipitates and the size information of the precipitates can be obtained quickly and accurately. By comparing with a predetermined range, it is possible to efficiently narrow down the cause of insufficient strength. As a result of comparison with the specified range, when the composition of the steel material is appropriate, but most of the elements of interest contained in the steel material have a size of 100 nm or more that is not effective at all, the slab heating temperature The main factor is that the manufacturing conditions are insufficient, and it is possible to immediately correct manufacturing conditions such as increasing the heating temperature and securing the heating time. In addition, as a result of comparison with the predetermined range, if there are more precipitates of about 20 nm or more and less than 100 nm compared to the predetermined range, it can be said that the controlled cooling conditions after hot rolling were inappropriate. Feedback manufacturing conditions to correct the conditions. In addition, when the amount of precipitation less than 20 nm effective for strengthening is not secured despite the fact that there are few coarse precipitates of 100 nm or more, it can be seen that the winding conditions corresponding to the precipitation treatment are fluctuating. In this case, it is possible to immediately feedback the manufacturing conditions by correcting the winding conditions.

In the present invention, the product yield can be improved by feedforward.
In the case of producing a steel type having a very long production process, if the final product is out of specification, a great amount of time and energy are lost. Therefore, in order to obtain the target characteristics when manufacturing such a steel type, it is important to correct the manufacturing conditions of the subsequent process after examining the state in the steel sheet during the manufacturing process.
For example, in step A, the size and the amount of precipitates of specific metal elements in the steel sheet are monitored. If the value of the monitoring satisfies a predetermined range with respect to the value of the precipitate of 20 nm or more that is known in advance by a laboratory experiment or the like, the condition of the subsequent process B is not changed. On the other hand, if the monitoring value is out of the predetermined range, the manufacturing conditions in the process B (for example, heat treatment time and temperature) are appropriately changed according to the value, and the characteristics of the final product plate are controlled within the specification range. .
A method of manufacturing using such feedforward has not been conventionally performed. This is because it has conventionally been difficult to obtain the precipitation state (information) in Step A necessary for finely adjusting the conditions of Step B quickly and with high accuracy. However, if the steel material manufacturing method according to the present invention is used, size information such as a solid solution state, a precipitation state, or a precipitate of a specific metal element can be obtained quickly and accurately. It becomes possible.
Further, according to the steel material manufacturing method of the present invention, the precipitate state during the process greatly deviates from the predetermined range to the extent that it cannot be corrected even if the subsequent process is corrected based on the information of the precipitate state during the process. In the event that it has occurred, it is possible to perform an action of not performing the subsequent steps. Such a process makes it possible to determine whether the product is good or bad at an early stage and to obtain an industrial merit that no wasteful manufacturing is performed. In particular, in the steel type with a long manufacturing process, the merit is great.

  A steel material consisting of the steel composition shown in Table 1 (only the main composition other than Fe is shown) is melted, and the resulting slab is heated to 1250 ° C, followed by hot rolling at a hot rolling end temperature of 890-920 ° C. Then, it was cooled at a cooling rate of 25 to 30 ° C./s to around 600 ° C. (winding temperature + 10 ° C.). Subsequently, it wound up at coiling temperature (CT) 575-675 degreeC, and manufactured the hot-rolled sheet AD of 3 mm in thickness with an actual hot rolling machine.

Next, for steels A, B, C, and D, the solid solution state and precipitation state of Ti and Mo, which are important as precipitation strengthening elements, were examined by the analysis method described above.
Specifically, the quantification of Ti and Mo precipitates by size was performed as follows. A test piece of appropriate size was cut out from the hot-rolled sheet and subjected to constant current electrolysis by 0.2 g at a current density of 20 mA / cm ² in 10% AA electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol). , Take out the test piece with deposits etc. on the surface from the electrolyte, immerse it in 500mg / l of sodium hexametaphosphate aqueous solution (hereinafter SHMP aqueous solution), and apply ultrasonic vibration to test the deposits etc. It peeled from the piece and separated into an aqueous SHPM solution. This SHMP aqueous solution containing precipitates and the like is filtered using a filter with a filter pore size of 100 nm, and further filtered using a filter with a filter pore size of 20 nm. The residue on the filtered filter and the filtrate are subjected to ICP emission. The analysis was performed using a spectroscopic analyzer, and the residue on the filter after filtration and the absolute amounts of Ti and Mo in the filtrate were measured. Next, the absolute amounts of Ti and Mo were divided by the electrolytic weight, and the contents of Ti and Mo contained in precipitates classified into a size of 100 nm or more, a size of 20 nm or more and less than 100 nm, and a size of less than 20 nm were determined. The electrolytic weight was determined by measuring the weight of the test piece after separation of deposits and the like and subtracting it from the weight of the test piece before electrolysis. The contents of Ti and Mo are values when the total composition of the test steel is 100 mass%.
Further, the Ti and Mo contents in the obtained precipitate were totaled for each Ti and each Mo, and the amount of precipitated Ti and the amount of precipitated Mo were calculated. The calculated amount of precipitated Ti and amount of precipitated Mo were each used as the denominator, and the ratio obtained from the content by size was defined as the ratio by size of the precipitate.
In addition, the determination of the solid solution amount of Ti and Mo was performed as follows. The electrolytic solution after the electrolysis was used as an analysis solution, and the concentrations of Ti, Mo, and Fe as a comparative element in the solution were measured using ICP mass spectrometry. Based on the obtained concentration, the concentration ratios of Ti and Mo to Fe were calculated, respectively, and the solid solution amount of Ti and the solid solution amount of Mo were obtained by multiplying the Fe content in the sample. Note that the Fe content in the sample can be obtained by subtracting the total of composition values other than Fe shown in Table 1 from 100 mass%. The solid solution amounts of Ti and Mo are values when the total composition of the test steel is 100 mass%.
Then, from the obtained solid solution amounts of Ti and Mo, a ratio with respect to the total content of Ti or Mo shown in Table 1 was obtained to obtain a solid solution ratio. About this solid solution ratio, it is not much different from the case where the difference between the amount of precipitated Ti or precipitated Mo calculated from the result of quantification as a precipitate and the total content of Ti or Mo is divided by the total content. I have confirmed that.
In the above analysis, a sample was taken from the L cross section of the hot-rolled sheet in consideration of non-uniformity in the thickness direction. The obtained results are shown in Table 2 and Table 3.

From the results shown in Tables 2 and 3, the following points became clear.
It is clear that there is a large difference in the precipitation amounts of Ti and Mo between steel A and steels B, C and D. Of the precipitated Ti, the ratio of coarse precipitates with a size of 100 nm or more in Steel A is about the same as that of precipitates with a size of less than 20 nm. However, this is because the precipitation amount of the steel A itself is small, so that the influence of slightly existing insoluble precipitation is strong. In addition, the ratio of the intermediate region in which the size of the precipitate or the like is 20 nm to less than 100 nm is not necessarily high. From these results, it is not considered that Steel A had an abnormality in the slab heating temperature, the hot rolling finishing temperature, and the subsequent cooling rate among the important conditions of the hot rolling process. In steel A, it is conceivable that the temperature is insufficient in the winding process for positively obtaining precipitates having a size of less than 20 nm effective for strengthening, which directly causes a lack of strength.

For steel D, the precipitation amount of Mo is almost the same as that of steels B and C. However, when attention is paid to Ti, the precipitation amount of Ti slightly increases and the amount of Ti in precipitates of size less than 20 nm slightly decreases. This is a result of the high coiling temperature and the promotion of precipitation, coarsening, and recovery of the matrix structure.
In steel B and C, there is no significant difference in the precipitation state of Ti and Mo, indicating that the manufacturing conditions are appropriate.

  Next, the hot-rolled sheet obtained above was pickled, and then a JIS No. 5 tensile test piece was collected and examined for mechanical properties. The results obtained are shown in Table 4. The mechanical properties were performed according to JIS Z2241.

From the results in Table 4, Steel B and Steel C have tensile strength (TS): 800 MPa or more, total elongation: (EL) 20% or more, both strength and elongation are good, and clear the target characteristics. It was.
In Steel A, both strength and elongation were insufficient.
In Steel D, the total elongation (EL) was secured and good, but a decrease in tensile strength (TS) was observed.

From the results of Table 2, Table 3, and Table 4, the following can be said.
Steel A had insufficient strength and elongation due to insufficient temperature in the winding process.
In Steel D, the coiling temperature is high, the precipitation is accelerated, the coarsening, and the recovery of the matrix structure progresses. Therefore, the total elongation (EL) is secured and good, but the tensile strength (TS) is reduced. It was.
From this analysis result, it can be derived that the optimum winding temperature that achieves both higher tensile strength TS and total elongation EL is the steels B and C having the highest Ti content in precipitates of size less than 20 nm.
From the above, it can be seen that in the steel type using the precipitation strengthening mechanism, the dependency of the mechanical properties on the coiling temperature can be supported by detailed information such as precipitates.

Next, based on the above results, the manufacturing conditions of Steel D were fed back according to the analysis flow of FIG.
From the analysis result information of precipitates by size, the cause of deterioration in steel D due to impure elongation was that the coiling temperature was high, the precipitation was accelerated, the coarsening, and the recovery of the matrix structure proceeded. Conceivable. Moreover, in this steel, the winding temperature in steels B and C is considered optimal. Therefore, the coiling temperature was modified to be around the coiling temperature (630 ° C.) in steels B and C, and the other compositions and production conditions were the same as for steel D, and steel D ′ was manufactured. And the mechanical characteristic was measured by the method similar to the said steel AD. Moreover, the solid solution state and precipitation state of Ti and Mo were analyzed by the same method as the steels A to D described above. The results obtained are shown in Table 5. The winding temperature of Steel B was 625 ° C, and the winding temperature of Steel C was 650 ° C.

D ′ is the same component as D, and is manufactured by controlling the coiling temperature to an appropriate value based on the analysis result of D. According to Table 5, steel D ', which was modified to lower the coiling temperature, had good characteristics comparable to steel B. By appropriately correcting the coiling temperature, the same precipitation state as steel B was obtained. I was able to reconfirm.
As described above, the relationship between at least one of the obtained precipitation information and solid solution information, mechanical properties, and manufacturing conditions is held as a database, and standard values of precipitation information and solid solution information are set in the database. In other words, it is possible to immediately correct the material deviation due to the coiling temperature.

  A steel material composed of the steel composition shown in Table 6 (only the main composition other than Fe is shown) is melted, and the resulting molten slab is heated to 1050 to 1200 ° C and then heated at a hot rolling end temperature of 900 to 920 ° C. The steel sheet was hot-rolled and cooled at a cooling rate of 25 to 30 ° C./s to around 600 ° C. (winding temperature + 10 ° C.). Subsequently, it wound up at coiling temperature (CT) 600-625 degreeC, and manufactured the hot-rolled sheet PS of 3 mm in thickness with an actual hot rolling machine.

Next, for steels P, Q, R, and S, the solid solution state and precipitation state of Ti and Mo important as precipitation strengthening elements were examined by the same analysis method as in Example 1.
The obtained results are shown in Table 7 and Table 8.

From the results of Tables 7 and 8, the following points became clear.
In steel R and steel S, the solid solution ratios of both Ti and Mo tend to decrease slightly, but as a clearer change, the ratio of Ti size of 100 nm or more is clearly increased compared to the other two steel types, As a result, the decrease in the ratio at a size less than 20 nm is remarkable. This indicates that an increase in coarse precipitates due to the low slab heating temperature leads to insufficient strength.

  Next, the hot-rolled sheet obtained above was pickled, and then a JIS No. 5 tensile test piece was collected and examined for mechanical properties. Table 9 shows the obtained results.

From the results in Table 9, steel P and steel Q have tensile strength (TS): 1000 MPa or more, total elongation: (EL) 18% or more, both strength and elongation are good, and clear the target characteristics. It was.
Steel R and Steel S were clearly inferior in tensile strength (TS) to Steel P and Steel Q, although there was not much difference in total elongation (EL).

The following can be said from the results of Tables 7, 8 and 9.
In Steel R and Steel S, tensile strength (TS) was insufficient due to an increase in coarse precipitates due to low slab heating temperature.
It can be derived from this analysis result that the optimum slab heating temperature that achieves both higher tensile strength TS and total elongation EL is the slab heating temperature of steel P and steel Q.
From the above, it can be seen that the dependence of the mechanical properties on the slab heating temperature can be supported by detailed information such as precipitates.

Next, based on the above result, according to the analysis flow of FIG.
From the analysis result information of the precipitates by size, it can be considered that in Steel R, the tensile strength is impure and the cause of deterioration is that the slab heating temperature is low and coarse precipitates of 100 nm or more are increased. Moreover, in this steel, it is thought that the slab heating temperature in steel P and Q is optimal. Therefore, the slab heating temperature was changed to the vicinity of the slab heating temperature (1200 ° C.) in the steels P and Q, and the other compositions and production conditions were the same as the steel R, and the steel R ′ was manufactured. And the mechanical characteristic was measured by the method similar to the said steel PS. Moreover, the solid solution state and precipitation state of Ti and Mo were analyzed by the same method as the steels P to S described above. Table 10 shows the obtained results. The slab heating temperature of steel P was 1150 ° C, and the slab heating temperature of steel Q was 1200 ° C.

R ′ is the same component as R, and is manufactured by controlling the slab heating temperature to an appropriate value based on the analysis result of R. From Table 10, steel R 'modified to a higher SRT has good properties comparable to steel Q, and the same precipitation state as steel Q can be reconfirmed by properly modifying SRT. I was able to.
As described above, the relationship between at least one of the obtained precipitation information and solid solution information, mechanical properties, and manufacturing conditions is held as a database, and standard values of precipitation information and solid solution information are set in the database. In other words, it is possible to immediately correct the material deviation caused by SRT.

  As described above, in the present invention, hot rolling conditions (rolling pass schedule, end temperature, etc.), cooling conditions after hot rolling, annealing conditions after cold rolling, etc., precipitation / solid solution state of elements in steel It can be applied to all manufacturing parameters that greatly affect the material.

Next, an application example of feedforward in which the average particle diameter of the precipitate is rapidly evaluated and the heat treatment conditions in the next process are changed will be described.
Specifically, a method for manufacturing a steel material for a grain-oriented electrical steel sheet is described. An important characteristic in grain-oriented electrical steel sheets is magnetic characteristics, and an important factor related to the magnetic characteristics is the size of the crystal grain size. Furthermore, since the state of the precipitate greatly affects the determination of the crystal grain size, evaluating the size of the precipitate quickly and with high accuracy has a great merit in the process. That is, if the precipitate size can be evaluated quickly and with high accuracy in a certain process, the crystal grain size in the next process when manufactured under the same heat treatment conditions can be predicted. When the evaluated precipitate size is out of the predetermined range, the heat treatment conditions for the next step can be changed. Also, if the precipitate size evaluated so as to be impossible to correct is significantly different from the predetermined range, the processing after the next step should be stopped to prevent wasteful processing after the next step for defective materials. Is also possible.

  In order to achieve this, it is necessary to be able to evaluate the size of precipitates as a highly correlated index with the crystal grain size quickly and with high accuracy. Was examined.

C: 0.055 mass%, Si: 3.15 mass%, Mn: 0.05 mass%, S: 0.02 mass%, and Sn: 0.02 mass%, the balance comprising a silicon steel slab made of Fe and inevitable impurities After heating at 1380 ° C. for 30 minutes and hot rolling to a sheet thickness of 2.2 mm, based on the process shown in FIG. 3, the annealing process and rolling process for 1 minute at around 1000 ° C. are performed twice, A sample having a thickness of 0.23 mm was manufactured. Here, in order to intentionally change the precipitate particle size, the temperatures of the annealing steps 1 and 2 were changed at six levels shown in Table 11.
(Comparative example)
With respect to Samples A, C, and F obtained as described above, the surface was subjected to electrolytic etching, and the result of observation by SEM was subjected to image analysis processing to obtain the particle size distribution. Based on the particle size distribution, the accuracy (1σ) when the average value and repeated evaluation were performed was obtained. The obtained results are shown in Table 11 together with the annealing temperature.
(Invention example)
Samples A to F obtained above were cut to an appropriate size, and about 0.2 g in a 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) had a current density of 20 mA / cm. Constant current electrolysis was performed at ² . After the electrolysis, remove the sample piece with deposits on the surface from the electrolyte and immerse it in an aqueous solution of sodium hexametaphosphate (500 mg / l) (hereinafter referred to as the SHMP aqueous solution) to apply ultrasonic vibration. The precipitate was peeled from the sample piece and separated into an aqueous SHMP solution. Next, the SHMP aqueous solution containing precipitates was measured with a dynamic light scattering type particle size distribution measuring device, and the average particle size value of the included precipitates and the accuracy (1σ) when repeated evaluation was performed Asked. The obtained results are also shown in Table 11.

  Table 11 also shows the results of evaluating the average crystal grain size of each sample that has undergone the annealing process 3 by observation with an optical microscope. In the optical microscope observation, the plate surface of each sample that passed through the annealing step 3 was etched with a 3% nitric acid / methanol solution, and five visual fields were observed at an observation magnification of 50 times, and an average crystal grain size was determined by a cutting method.

From Table 11, in the comparative example, about 100 fields of view were observed in a field of view of about 2 μm × 3 μm, and the particle size distribution was calculated by image analysis processing for precipitates existing in the field of view. Although it is a measurement, the variation (1σ) is large, and it is judged that the result is not representative of the entire sample. Also, from the viewpoint of rapidity, it is considered that it is not sufficient for application to process feedforward.
On the other hand, in the example of the present invention, it can be seen that the average particle size of precipitates in steel when the heat treatment conditions are changed in each step and the repeatability can be evaluated more accurately than the particle size measurement method used in the comparative example. .

  Next, for samples C, D and F, based on the average particle size measurement results of the precipitates after rolling step 2, the conditions of annealing step 3 were changed, and samples C'D 'and F' were newly added. The precipitate particle size, the precipitate particle size accuracy, and the crystal particle size after the annealing step 3 were determined by the same method as described above. The results obtained are shown in Table 12.

From Table 12, it can be seen that the crystal grain size of each sample after the annealing step 3 is controlled to about 20 μm as in the case of other steels.
Thus, if the size of the precipitates present in the steel after performing the rolling step 2 according to the present invention can be repeatedly evaluated with high accuracy, an appropriate crystal grain size can be obtained by changing the heat treatment conditions in the next step. It is shown that it can be secured.

Potential for industrial use

  In the steel material manufacturing method of the present invention, since desired material characteristics can be stably obtained by performing optimization (correction) of manufacturing conditions easily and quickly, it can be used as a material for automobiles, shipbuilding, civil engineering and construction. It can be used suitably.

It is the figure which showed an example of the conventional method in a manufacturing condition determination process. It is a figure which shows an example of the figure which showed an example of this invention method in the manufacturing condition determination process which concerns on this invention. It is a figure which shows an example of the figure which showed an example of this invention method in the manufacturing condition determination process which concerns on this invention.

Claims (6)

Manufacturing step A for manufacturing a steel material, information on the composition of precipitates and / or inclusions, information on the size of precipitates and / or inclusions, and solid solution of the element of interest in the steel material manufactured in manufacturing step A An analysis step for obtaining quantity information, and a composition of precipitates and / or inclusions, a precipitate when at least one of analysis results based on the information obtained in the analysis step is out of a predetermined range And / or a manufacturing condition correcting step for correcting at least one manufacturing condition in which one or more of the inclusions and the solid solution amount of the element of interest change, and a steel material according to the manufacturing condition corrected in the manufacturing condition correcting step. It has a manufacturing step B to produce the analysis step is to electrolyze produced steel in an electrolytic solution of an organic solvent system, after the electrolysis, immersed in ultra-sound solution having a dispersibility After separating the precipitates and / or inclusions adhering to the steel material by applying a wave into a solution having dispersibility, the filter hole penetrates the solution having dispersibility in a certain opening shape. The precipitates and / or inclusions are separated according to size by filtering two or more stages through a filter having a porosity of 4% or more, and information on the composition of the precipitates and / or inclusions, precipitates And / or an analysis for obtaining information about the size of inclusions . The analysis step analyzes an organic solvent-based electrolytic solution after electrolyzing the steel material, determines a ratio between the concentration of the element of interest and the iron concentration in the electrolytic solution, and determines the ratio of the iron of the steel material to the determined ratio. The method for producing a steel material according to claim 1, wherein the solid solution amount of the element of interest is analyzed by multiplying the total concentration. The production step A includes a hot working process, and the production condition correction step is performed when the solid solution amount of the element of interest obtained in the analysis step is equal to or less than a predetermined range and / or the analysis step. When the amount of the element of interest contained in the precipitate and / or the inclusion having a size of inclusions of more than 100 nm is a predetermined range or more, the reheating temperature in the hot working step is corrected. The method for producing a steel material according to claim 1 or 2, characterized in that The manufacturing step A includes a hot working process and a cooling process that is performed subsequently after the hot working process, and the manufacturing condition correcting step includes the entire precipitate and / or inclusions obtained in the analyzing step. The steel material according to claim 1 or 2 , wherein a cooling rate in the cooling step is corrected when a ratio of precipitates and / or inclusions having a size of 20 to 100 nm with respect to the amount is not less than a predetermined range. Manufacturing method. The manufacturing step A includes a series of processes of hot working, cooling, and intermediate holding in a specific temperature range, and the manufacturing condition correcting step includes precipitates having a size of less than 20 nm obtained in the analysis step and / or Alternatively, the amount of the element of interest contained in the inclusion is within a predetermined range, and the amount of the element of interest contained in the precipitate and / or the inclusion having a size of 20 nm or more obtained in the analysis step is within the predetermined range. 3. The method for manufacturing a steel material according to claim 1, wherein the intermediate holding condition in the intermediate holding step is corrected in some cases. A method for producing a steel material, characterized in that the steel material is produced by the method according to any one of claims 1 to 5 and is subjected to heat treatment after shipment to adjust characteristics.

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