WO2018123356A1

WO2018123356A1 - High-strength steel sheet and high-strength electrogalvanized steel sheet

Info

Publication number: WO2018123356A1
Application number: PCT/JP2017/041809
Authority: WO
Inventors: 潤一郎衣笠; 幸博内海; 航佑柴田
Original assignee: 株式会社神戸製鋼所
Priority date: 2016-12-28
Filing date: 2017-11-21
Publication date: 2018-07-05

Abstract

The present invention relates to a high-strength steel sheet that contains C: 0.10-0.35 mass%, Si: 0-0.6 mass%, Mn: greater than 0 mass% and not more than 1.5 mass%, Al: greater than 0 mass% and not more than 0.15 mass%, N: greater than 0 mass% and not more than 0.01 mass%, P: greater than 0 mass% and not more than 0.02%, and S: greater than 0 mass% and not more than 0.01 mass%, wherein the remainder is iron and unavoidable impurities; the area percentage of martensite in the overall structure is at least 95%; the amount of solid-solution C in the martensite is not more than 0.05 mass%; the numerical density of carbide having a major diameter of at least 200 nm is not more than 50 per μm³; and the tensile strength is at least 1270 MPa.

Description

High strength steel plate and high strength electrogalvanized steel plate

The present invention relates to a high-strength steel plate and a high-strength electrogalvanized steel plate.

In recent years, steel sheets for automobiles have been increased in strength in order to achieve both weight reduction and collision safety. An automotive steel plate is required to have excellent press formability in order to be processed into a skeletal component having a complicated shape. For example, automotive parts typified by bumpers are mainly formed by bending, so that they are particularly required to have excellent bending workability (hereinafter sometimes referred to as “bendability”) in press formability. It is done. Also, as one of the issues due to the increase in strength, there is a problem that the hydrogen generated due to the corrosion of the steel plate in use is more concerned about the occurrence of delayed fracture due to hydrogen embrittlement caused by intrusion into the steel. is there.

In addition, from the viewpoint of corrosion resistance, automotive steel parts have steel galvanized (hereinafter sometimes referred to as “EG”), hot dip galvanized, and alloyed hot dip galvanized steel sheets (hereinafter referred to as these). Are sometimes collectively referred to as “galvanized steel sheets”). These galvanized steel sheets also have a problem that, as with the high-strength steel sheets, there is an increased concern about the occurrence of delayed fracture due to hydrogen embrittlement due to the increase in strength.

Although this delayed fracture has been reported in bolts and various techniques for improving delayed fracture resistance have been studied, thin steel sheets are considered to be bolts in that large plastic strain is introduced by press forming. Is different. In general, it is said that plastic strain promotes delayed fracture, and there is a need for delayed fracture resistance improvement technology that takes into account the effect of plastic strain.

Steel material factors affecting delayed fracture include steel structure, strength or hardness, crystal grain size, various alloy elements, etc., but it is difficult to say that measures to suppress delayed fracture are still well established. It is. Therefore, attempts have been made to improve the delayed fracture resistance from various viewpoints.

As a technique for improving resistance to hydrogen embrittlement and improving delayed fracture resistance, for example, Patent Document 1 describes, “With bainite or martensite as the largest phase, an oxide of Nb, Cr, Ti, Mo in the grains, By controlling the morphology of the average particle size, density, distribution, etc. of any one or more of sulfide, nitride, composite crystallized product and composite precipitate, and exhibiting the function as a hydrogen trap site, A technique of “making a high-strength steel sheet excellent in hydrogen embrittlement resistance” has been proposed. However, the hydrogen trap by the above form control alone is not sufficient for exhibiting excellent delayed fracture resistance.

Further, Patent Document 2 is a steel sheet in which martensite occupies 95% by area or more of the entire structure, and defines the prior austenite grain size, the transition density, the solid solution C concentration in martensite, and the prior austenite length. A technique for improving delayed fracture resistance by defining the ratio of the length of carbides precipitated at the prior austenite grain boundaries with respect to the above by a predetermined relational expression has been proposed.

In the technique of Patent Document 2, delayed fracture resistance is evaluated by an SSRT test which is a low strain rate tensile test. In other words, this is a technology for improving delayed fracture resistance at a site where the introduced plastic strain is relatively small, and may not be sufficient to improve delayed fracture resistance of a steel plate into which large plastic strain is introduced. is there.

On the other hand, in Patent Document 3, as an example of a high-strength steel plate, “Ceq1 defined by Ceq1 = C + Mn / 5 + Si / 13 while adjusting the chemical component composition is 0.5% or less, and the steel structure is martensite. A high-strength steel sheet having a single-phase structure and excellent seam weldability having a tensile strength of 1180 MPa or more has been proposed. However, this technique has not been studied for improving delayed fracture resistance, and may not be sufficient from the viewpoint of delayed fracture resistance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet having excellent bendability and excellent delayed fracture resistance. Another object of the present invention is to provide a high-strength electrogalvanized steel sheet having an electrogalvanized layer on the surface of the high-strength steel sheet.

Japanese Patent No. 4167587 Japanese Patent No. 5662920 JP 2013-36112 A

The high-strength steel sheet according to one aspect of the present invention includes C: 0.10% by mass to 0.35% by mass, Si: 0% by mass to 0.6% by mass, Mn: more than 0% by mass and 1.5% by mass. %: Al: more than 0% by weight, 0.15% by weight or less, N: more than 0% by weight, 0.01% by weight or less, P: more than 0% by weight, 0.02% by weight or less, and S: more than 0% by weight 0.01% by mass or less, the balance being iron and inevitable impurities, the area ratio of martensite in the whole structure being 95% or more, the solid solution C of the martensite being 0.05% by mass % Or less, the density of carbides having a major axis of 200 nm or more is 50 pieces / μm ³ or less, and the tensile strength is 1270 MPa or more.

In order to solve the above-mentioned problems, the present inventors have made eager efforts to improve the bendability and delayed fracture resistance of martensite-based steel sheets that can achieve a high strength of 1270 MPa or higher. Repeated research.

As a result, it was found that when the carbide size in the steel sheet is large and the aspect ratio is large, voids are formed around the carbide during bending, and the bendability deteriorates as a starting point of fracture. In addition, among the strengthening mechanisms of grain refinement, grain dispersion strengthening, dislocation strengthening and solid solution strengthening, which is known as the strengthening mechanism of steel sheets, It has been found that the solid solution C present in martensite, which lowers the toughness and remarkably deteriorates the delayed fracture resistance.

On the other hand, precipitation strengthening by carbides among particle dispersion strengthening, if the carbide size is fine, improves the balance between high strength and bendability and delayed fracture resistance without degrading bendability and delayed fracture resistance. It was clarified that it contributes to. In addition, dislocation strengthening was found to have little adverse effect on delayed fracture resistance.

The reason why the effect of dislocation strengthening is relatively small compared to the solid solution strengthening by C is considered to be because the correlation between the dislocation density on the surface of the steel sheet subjected to bending and the initial dislocation density is small. This is thought to be because dislocation growth is remarkable in the bent portion, but the dislocation density can be increased to a saturated state.

In other words, in order to improve delayed fracture resistance, it is important to control the dispersion state of carbides in martensite, the amount of dissolved C, and the dislocation density within an appropriate range, and the chemical composition of the steel sheet is appropriately controlled. In addition to adjusting, the inventors have found that it is necessary to properly control the quench-temper process.

Specifically, it is a single-phase structure mainly composed of martensite such that the area ratio of martensite in the entire structure is 95% or more, and the solid solution C amount in this martensite is 0.05 mass. It was found important to control the number density of carbides having a major axis of 200 nm or more to 50 pieces / μm ³ or less, and the present invention was completed.

That is, the high-strength steel sheet according to one aspect of the present invention includes C: 0.10% by mass to 0.35% by mass, Si: 0% by mass to 0.6% by mass, and Mn: more than 0% by mass. 5 mass% or less, Al: more than 0 mass%, 0.15 mass% or less, N: more than 0 mass%, 0.01 mass% or less, P: more than 0 mass%, 0.02 mass% or less, and S: 0 mass More than 0.01% by mass and the balance is iron and inevitable impurities, the area ratio of martensite in the entire structure is 95% or more, and the amount of dissolved C in the martensite is 0 The number density of carbides having a major axis of 200 nm or more is 50 pieces / μm ³ or less, and the tensile strength is 1270 MPa or more.

According to such a configuration, a high-strength steel sheet and a high-strength electrogalvanized steel sheet exhibiting high strength of 1270 MPa or more and excellent in bendability and delayed fracture resistance can be realized. Such high-strength steel sheets and high-strength electrogalvanized steel sheets are useful as raw materials for parts that require high strength, such as automotive parts such as bumpers, automobile structural materials, and reinforcing materials.

Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto.

The reason why the above requirements are specified in the high strength steel sheet of the present embodiment is as follows. In this embodiment, the “dislocation density” is not defined, but this is because it is difficult to measure the dislocation density of martensite in which C is present in a solid solution state, and the amount of C is 0.10. The mass range is not less than 0.35% by mass and is a martensite single phase structure, and the dispersion state of carbides is such that the number density of carbides having a major axis of 200 nm or more is 50 / μm ³ or less. In this case, it is considered that the dislocation density to be controlled is achieved.

[Martensite area ratio in all organizations is 95% or more]
The steel plate of the present embodiment is a high-strength steel plate having a higher strength, for example, a tensile strength of 1270 MPa or more, preferably 1360 MPa or more. Such high strength is required as a characteristic of parts applied to, for example, automobile steel plates, particularly bumpers and body frame members.

In achieving a high strength as described above, when the steel structure is rich in ferrite, the strength of the steel sheet tends to decrease, and it is necessary to add a large amount of alloying elements to ensure the strength. to degrade. In a structure containing a large amount of ferrite, when subjected to severe processing as performed on automobile steel sheets, processing strain tends to accumulate in the soft phase ferrite, and delayed fracture resistance may deteriorate. For these reasons, the steel sheet of this embodiment has a single-phase structure of martensite and suppresses the amount of alloy elements added.

In this embodiment, the single-phase structure of martensite means a structure in which the area ratio of martensite in the entire structure is 95% or more. The area ratio of martensite in the entire structure needs to be 95% or more in order to achieve a high strength of 1270 MPa or more. The area ratio of martensite is preferably 97% or more, and may be 100%.

In addition to the martensite, the steel plate of the present embodiment may contain a phase inevitably included in the manufacturing process, for example, a ferrite phase, a bainite phase, a retained austenite phase, and the like up to 5 area% or less. However, if the ratio exceeds 5 area%, the area ratio of martensite is relatively less than 95%, and a strength of 1270 MPa or more cannot be achieved.

[Solution amount of C in martensite: 0.05% by mass or less]
By reducing the solid solution C in martensite, the toughness can be improved and the delayed fracture resistance can be improved by reducing the strain of the structure itself. In particular, it is considered that excellent delayed fracture resistance can be achieved by setting the solid solution C amount in martensite to 0.05 mass% or less. The amount of solute C in martensite is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. However, the lower limit of the amount of solute C in martensite is approximately 0% by mass or more from the viewpoint that all the solute C precipitates as carbide when tempering proceeds.

[Number density of carbides whose major axis is 200 nm or more: 50 / μm ³ or less]
When the major axis of the carbide is increased, voids are formed around the carbide during bending, which becomes a starting point of fracture and deteriorates bendability. Therefore, the size and number of carbides are very important. Specifically, in this embodiment, the number density of coarse carbides having a major axis of 200 nm or more that deteriorates bendability is defined. A fine carbide having a major axis of less than 200 nm hardly affects the bendability.

If the number density of carbides having a major axis of 200 nm or more is large, the bendability is deteriorated, so that 50 pieces / μm ³ or less is necessary. The number density is preferably 40 pieces / μm ³ or less, more preferably 30 pieces / μm ³ or less, and most preferably 10 pieces / μm ³ or less.

Precipitates having a major composition of 200 μm or more and an aspect ratio of 2 or more, which are precipitated under the component composition and manufacturing conditions in the present embodiment, are substantially carbides mainly composed of Fe and C. Specifically, Fe And a carbide whose total content of C exceeds 90% by mass. Such carbides include not only Fe ₃ C but also carbides containing V, Nb, Mo, Ti, etc. in a proportion of less than 10% by mass in the entire carbide. Depending on the component composition and manufacturing conditions in the present embodiment, carbides not mainly composed of Fe and C, specifically, V, Nb, Mo, Ti and C may be formed. Compared to carbides mainly composed of, the amount is negligible and does not adversely affect bendability and delayed fracture resistance.

In the steel sheet of this embodiment, high strength is shown by the structure, excellent delayed fracture resistance is shown by controlling the amount of dissolved C, and excellent bendability is shown by adjusting the carbides. In order to ensure basic characteristics such as weldability, toughness, and ductility, the content of each element in the steel sheet must also be controlled as follows.

[C: 0.10% by mass to 0.35% by mass]
C is an element effective for enhancing the hardenability of the steel sheet and ensuring high strength. In order to exert such effects, the C amount needs to be 0.10% by mass or more. Preferably it is 0.12 mass% or more, More preferably, it is 0.15 mass% or more. However, when the amount of C becomes excessive, the weldability of the steel sheet deteriorates. Therefore, the amount of C needs to be 0.35 mass% or less, preferably 0.34 mass% or less, more preferably 0.33 mass% or less.

[Si: 0% by mass to 0.6% by mass]
Si is an element effective for improving the temper softening resistance of the steel sheet, and is also an element effective for improving the strength by solid solution strengthening. From the viewpoint of exerting these effects, it is preferable to contain 0.002% or more of Si. More preferably, it is 0.005 mass% or more, More preferably, it is 0.010 mass% or more. However, since Si is a ferrite-forming element, if the amount of Si is excessive, the hardenability is impaired and it is difficult to ensure high strength. Therefore, the amount of Si needs to be 0.6 mass% or less. Preferably it is 0.5 mass% or less, More preferably, it is 0.1 mass% or less, More preferably, it is 0.05 mass% or less.

[Mn: more than 0% by mass and 1.5% by mass or less]
Mn is an effective element for enhancing the hardenability of the steel sheet and ensuring high strength. Although such an effect increases as the content increases, it is preferable to contain 0.1% by mass or more in order to effectively exhibit the above effect. More preferably, it is 0.5 mass% or more, More preferably, it is 0.8 mass% or more. However, when the amount of Mn becomes excessive, delayed fracture resistance and weldability deteriorate. Therefore, the amount of Mn needs to be 1.5 mass% or less. Preferably, it is 1.4 mass% or less, More preferably, it is 1.3 mass% or less.

[Al: more than 0% by mass and 0.15% by mass or less]
Al is an element added as a deoxidizer and also has an effect of improving the corrosion resistance of steel. Although these effects increase as the content increases, in order to effectively exhibit the above effects, it is preferable to contain 0.04% by mass or more. More preferably, it is 0.06% or more. However, when the amount of Al is excessive, a large amount of inclusions are generated and cause surface defects, so the upper limit is made 0.15% by mass or less. Preferably it is 0.10 mass% or less, More preferably, it is 0.07 mass% or less.

[N: more than 0% by mass and 0.01% by mass or less]
N is an impurity that is inevitably mixed in. If the amount of N is excessive, the amount of nitride precipitation increases, which adversely affects the toughness of the steel sheet. Therefore, the N amount needs to be 0.01% by mass or less. Preferably it is 0.008 mass% or less, More preferably, it is 0.006 mass% or less. Note that the N content is approximately 0.001% by mass or more in consideration of the cost for steelmaking.

[P: more than 0% by mass and 0.02% by mass or less]
P is an impurity inevitably mixed in. When the amount of P becomes excessive, brittleness increases and the ductility of the steel sheet is lowered, so it is necessary to suppress it to 0.02% by mass or less. Preferably it is 0.01 mass% or less, More preferably, it is 0.006 mass% or less.

[S: more than 0% by mass and 0.01% by mass or less]
S is an impurity inevitably mixed in. S produces sulfide inclusions and degrades the workability and weldability of the steel sheet. Moreover, it combines with Mn to form MnS, which becomes a local corrosion starting point, and promotes hydrogen generation and hydrogen penetration, thereby deteriorating delayed fracture resistance. For this reason, the smaller the amount of S, the better. The content is preferably 0.005% by mass or less, and more preferably 0.003% by mass or less.

The chemical component composition defined in the present embodiment is as described above, and the balance is iron and inevitable impurities other than N, P, and S. As this inevitable impurity, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. can be allowed.

In addition to the above elements, if necessary, (a) at least one selected from the group consisting of Cr: more than 0% by mass and 1.0% by mass and B: more than 0% by mass and less than 0.01% by mass, (b ) Cu: more than 0% by mass and 0.5% by mass or less; Ni: at least one selected from the group consisting of more than 0% by mass and 0.5% by mass or less; (c) V: more than 0% by mass and 0.1% by mass Hereinafter, at least one selected from the group consisting of Nb: more than 0% by mass and 0.1% by mass or less, Mo: more than 0% by mass and 0.5% by mass or less, and Ti: more than 0% by mass and 0.2% by mass or less. And / or (d) it is also effective to contain at least one selected from the group consisting of Ca: more than 0% by mass and 0.005% by mass and Mg: more than 0% by mass and 0.005% by mass or less. Yes, the properties of high-strength steel sheets are improved according to the type of elements to be included. .

[Cr: at least one selected from the group consisting of more than 0% by mass and 1.0% by mass or less and B: more than 0% by mass and 0.01% by mass or less]
Cr and B are effective elements for improving the hardenability of the steel sheet and increasing the strength. Cr is an element effective for increasing the temper softening resistance of martensitic steel. In order to exhibit these effects effectively, Cr is preferably contained in an amount of 0.01% by mass or more, more preferably 0.05% by mass or more. Further, B is preferably contained in an amount of 0.0001% by mass or more, more preferably 0.0005% or more.

However, when Cr is excessively contained, the delayed fracture resistance of the steel sheet deteriorates, so the upper limit is preferably 1.0% by mass or less, more preferably 0.7% by mass or less. Moreover, since the ductility of a steel plate will fall when B is contained excessively, it is preferable to make an upper limit into 0.01 mass% or less, More preferably, it is 0.008 mass% or less, More preferably, it is 0.0065 mass%. It is as follows.

[Cu: at least one selected from the group consisting of more than 0 mass% and 0.5 mass% or less and Ni: more than 0 mass% and 0.5 mass% or less]
Cu and Ni are effective elements for suppressing the generation of hydrogen involved in hydrogen embrittlement by improving the corrosion resistance of the steel sheet and improving delayed fracture resistance. In order to exhibit such an effect effectively, it is preferable to contain 0.01 mass% or more with Cu, More preferably, 0.05 mass% or more is contained. Moreover, in the case of Ni, it is preferable to contain 0.01 mass% or more, More preferably 0.05 mass% or more.

However, when the amount of Cu is excessive, the pickling property and chemical conversion property of the steel sheet deteriorate, so the upper limit is preferably 0.5% by mass or less, more preferably 0.4% by mass or less. Moreover, when the amount of Ni becomes excessive, the ductility and workability of the steel sheet decrease, so the upper limit is preferably 0.5% by mass or less, more preferably 0.4% by mass or less.

[V: more than 0% by mass and 0.1% by mass or less, Nb: more than 0% by mass and 0.1% by mass or less, Mo: more than 0% by mass and 0.5% by mass or less, and Ti: more than 0% by mass and 0.2% by mass % At least one selected from the group consisting of% or less]
V, Nb, Mo and Ti are all effective elements for improving the strength of the steel sheet and for improving toughness after quenching by refining austenite grains. In order to exhibit such an effect effectively, it is preferable to contain these elements 0.003% by mass or more. More preferably, it is 0.010 mass% or more, More preferably, it is 0.02 mass% or more.

However, when the above elements are excessively contained, precipitation of carbonitrides and the like increases and the workability of the steel sheet decreases. Therefore, when V or Nb is contained, it is preferable that both be 0.1% by mass or less, and more preferably 0.05% by mass or less. In Mo, it is preferable to set it as 0.5 mass% or less, More preferably, it is 0.4 mass%. In Ti, it is preferable to set it as 0.2 mass% or less, More preferably, it is 0.1 mass% or less.

[Ca: at least one selected from the group consisting of more than 0% by mass and 0.005% by mass and Mg: more than 0% by mass and 0.005% by mass or less]
Ca combines with S instead of Mn, and controls the form of MnS extending in the rolling direction. Moreover, since MnS is divided at the end face of the steel sheet, localization of the local corrosion starting point can be suppressed, and by suppressing the hydrogen generation and hydrogen intrusion by suppressing the pH drop at the local corrosion starting point, the delay resistance of the steel sheet is suppressed. It is an element that can improve destructive properties. In order to effectively exhibit such effects, Ca is preferably contained in an amount of 0.001% by mass or more. More preferably, it is 0.0015 mass% or more. However, when the amount of Ca becomes excessive, the workability of the steel sheet deteriorates. Therefore, the amount is preferably 0.005% by mass or less, and more preferably 0.003% by mass or less.

On the other hand, Mg combines with O to form MgO, thereby suppressing pH drop at the corrosion front and suppressing hydrogen generation and hydrogen intrusion by suppressing pH drop at the local corrosion starting point, similar to Ca. It is an element that can improve the delayed fracture resistance of the steel sheet. In order to exhibit such an effect effectively, it is preferable to contain Mg 0.001 mass% or more. More preferably, it is 0.0015 mass% or more. However, if the amount of Mg becomes excessive, the workability of the steel sheet deteriorates, so the content is preferably 0.005% by mass or less, more preferably 0.003% by mass or less.

The steel plate of the present embodiment is intended for a thin steel plate having a thickness of about 1 to 3 mm, but the product form is not particularly limited. For example, for hot-rolled steel sheets, cold-rolled steel sheets, or steel sheets that have been annealed after being hot-rolled or cold-rolled, various plating treatments such as chemical conversion treatment, electrogalvanizing, vapor deposition, etc. It is intended to include a surface-treated steel sheet that has been subjected to painting treatment, painting ground treatment, organic coating treatment, and the like.

In order to manufacture the high-strength steel sheet of the present embodiment as described above, the following procedure may be followed. Specifically, general conditions can be adopted except for the annealing treatment described later. For example, when performing an annealing treatment under the following conditions using a cold-rolled steel sheet, it is melted in accordance with a conventional method, and after obtaining a steel piece such as a slab by continuous casting, it is heated to about 1100 ° C. to 1250 ° C., and then It is hot-rolled, pickled and then cold-rolled to obtain a cold-rolled steel sheet. And it is recommended to perform the annealing process performed next on the following conditions.

[Annealing conditions]
In the continuous annealing line, it is preferable to heat in the temperature range of Ac ₃ transformation point to 950 ° C. and hold in this temperature range for 30 seconds or more. When this heating temperature is less than the Ac ₃ transformation point, the structure of the hot-rolled steel sheet, for example, the two-phase structure of ferrite-pearlite is taken over, and even after the quenching treatment, the structure is not mainly composed of martensite. It becomes difficult to achieve a strength of 1270 MPa or more.

Also, holding at an excessively high temperature such that the heating temperature exceeds 950 ° C. is not preferable because it is inferior in equipment load and economically. The upper limit of the heating temperature is more preferably 930 ° C. or less. The residence time at the heating temperature may be appropriately determined according to the heating temperature. However, in order to complete the austenite transformation of the steel sheet, it is preferable to hold by heating for 30 seconds or more. More preferably, it is 100 seconds or more. However, if the residence time becomes too long, the crystal grain size becomes coarse, which is disadvantageous in terms of strength and toughness. More preferably, it is 900 seconds or less, More preferably, it is 800 seconds or less. Moreover, high temperature and long time holding are economically disadvantageous.

[Quenching treatment]
Next, a martensite single phase structure is obtained by quenching with water cooling from a temperature range of 600 ° C. or higher. The average cooling rate during the water cooling is approximately 50 ° C./second or more. If the cooling rate is slower than this, ferrite precipitates during cooling, a martensite single phase structure cannot be obtained, and a tensile strength of 1270 MPa or more cannot be secured. The lower limit of the average cooling rate during water cooling is more preferably 100 ° C./second or more.

Even if the average cooling rate at the time of water cooling is excessively high, there is no problem on the material, but excessive capital investment is required, so the average cooling rate is preferably about 1000 ° C./second or less. More preferably, it is 500 ° C./second or less.

The cooling end temperature is approximately 100 ° C. or less because the cooling method is water cooling. The lower limit of the cooling end temperature is not limited at all, but it is substantially lower than room temperature because it is economically expensive to set it to room temperature or lower.

[Tempering treatment]
Tempering is performed after the quenching. This tempering treatment is preferably performed in two stages. By appropriately setting the temperature range of this tempering treatment, the precipitation and growth of carbides can be controlled. In order to prevent carbides from growing and coarsening in the martensite that is the base material of the steel sheet, the first tempering temperature T1 is higher than the second tempering temperature T2, that is, T1. It is preferable to control so as to satisfy the relationship of> T2. In the tempering treatment, first, in order to actively produce carbide precipitation nuclei, it is preferable to perform a heat treatment in which the tempering temperature T1 is set to a temperature range of 200 to 240 ° C. and is maintained for 50 seconds or more.

When the first-stage tempering temperature T1 is less than 200 ° C., the formation of carbide precipitation nuclei becomes insufficient. On the other hand, when it exceeds 240 ° C., carbide precipitation becomes excessive, and the bendability of the steel sheet decreases. The tempering temperature T1 is more preferably 210 ° C. or higher and 230 ° C. or lower.

When the holding time t1 at the tempering temperature T1 is less than 50 seconds, the formation of carbide precipitation nuclei becomes insufficient, and the discharge of the solid solution C of the martensite structure becomes insufficient. descend. The holding time t1 is more preferably 70 seconds or more, and further preferably 100 seconds or more.

On the other hand, if the holding time t1 at the tempering temperature T1 is too long, the carbide growth becomes excessive, and the toughness of the steel sheet and the bendability are lowered. Moreover, since the carbide | carbonized_material will grow excessively for long time holding | maintenance at high temperature, it is preferable to set it as 200 seconds or less. The holding time t1 is more preferably 180 seconds or less, and even more preferably 160 seconds or less.

Next, it is preferable to perform the second tempering treatment in a temperature range where the tempering temperature T2 is 100 ° C. or higher and 210 ° C. or lower and the holding time t2 is 300 seconds or longer. By making the tempering temperature T2 lower than the tempering temperature T1, the growth rate of the carbide nucleated at the tempering temperature T1 can be reduced as compared with the case where the tempering temperature T2 is held at the tempering temperature T1. By controlling in this way, it is possible to precipitate a large amount of fine carbides with a small adverse effect on bendability, to reduce the number density of coarse carbides, and to reduce C dissolved in martensite. Promoted.

When the tempering temperature T2 exceeds 210 ° C., the growth rate of the carbide becomes too large, so that coarse carbide is generated and the bendability of the steel sheet is lowered. More preferably, it is 190 degrees C or less. The lower limit of the tempering temperature T2 is preferably set to 100 ° C. or higher in order to sufficiently discharge the solute C. More preferably, it is 150 degreeC or more.

When the holding time t2 at the heating temperature T2 is less than 300 seconds, the toughness of the steel sheet may be reduced because the discharge of the solid solution C does not occur sufficiently. The holding time t2 is more preferably 320 seconds or more, and further preferably 350 seconds or more.

On the other hand, if the holding time t2 at the tempering temperature T2 becomes too long, the precipitation and growth of carbides become excessive, and the bendability of the steel sheet decreases. Moreover, since holding for a long time at a high temperature is economically disadvantageous, it is preferably set to 1000 seconds or less. More preferably, it is 800 seconds or less, More preferably, it is 600 seconds or less.

The cooling rate from the tempering temperature T1 to the tempering temperature T2 is preferably 0.1 ° C./second or more. More preferably, it is 0.12 degreeC / second or more, More preferably, it is 0.15 degreeC / second or more. The upper limit of the cooling rate from the tempering temperature T1 to the tempering temperature T2 is preferable because it is preferable. However, in consideration of the temperature range to be cooled, the industrial upper limit is about 100 ° C./second.

The high-strength steel plate according to this embodiment has been described above.

As described above, the high-strength steel plate of this embodiment may have an electrogalvanized layer on the surface thereof. That is, the present invention also includes an electrogalvanized steel sheet (hereinafter sometimes referred to as “EG steel sheet”) having an electrogalvanized layer on the surface of the high-strength steel sheet.

The EG steel sheet as described above is obtained by subjecting the high-strength steel sheet according to the present embodiment obtained after cooling to room temperature after tempering to electrogalvanization according to a conventional method.

The electrogalvanization is formed, for example, by conducting an electrogalvanization process by energizing the high-strength steel sheet while being immersed in a zinc solution at 50 to 60 ° C. The amount of plating adhesion at this time is not particularly limited, and may be, for example, about 10 to 100 g / m ² per side.

The high-strength steel sheet of the present invention further contains at least one selected from the group consisting of Cr: more than 0% by mass and 1.0% by mass and B: more than 0% by mass and less than 0.01% by mass as necessary. It may be. Thereby, the characteristic of a high-strength steel plate is further improved according to the kind of element to contain.

The high-strength steel sheet may further contain at least one selected from the group consisting of Cu: more than 0% by mass and 0.5% by mass or less and Ni: more than 0% by mass and less than 0.5% by mass. Good. Thereby, the characteristic of a high-strength steel plate is further improved according to the kind of element to contain.

Further, in the high-strength steel sheet, V: more than 0% by mass and 0.1% by mass or less, Nb: more than 0% by mass and 0.1% by mass or less, Mo: more than 0% by mass and 0.5% by mass or less, and Ti: At least one selected from the group consisting of more than 0% by mass and 0.2% by mass or less may be contained. Thereby, the characteristic of a high-strength steel plate is further improved according to the kind of element to contain.

Further, the high-strength steel sheet may contain at least one selected from the group consisting of Ca: more than 0 mass% and 0.005 mass% or less and Mg: more than 0 mass% and 0.005 mass% or less. . Thereby, the characteristic of a high-strength steel plate is further improved according to the kind of element to contain.

The present invention includes a high-strength electrogalvanized steel sheet having an electrogalvanized layer on the surface of the high-strength steel sheet.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples as a matter of course, and is appropriately modified within a range that can meet the gist of the following. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

(Test Example 1)
Steel types A to Y having the chemical composition shown in Table 1 were melted. Specifically, desulfurization was performed in a ladle after primary refining in a converter. Moreover, the vacuum degassing process by RH method was implemented after the ladle refining as needed. In the chemical composition shown in Table 1, the balance is iron and inevitable impurities other than N, P, and S. The Ac ₃ transformation point of each steel type shown in Table 1 below is the following (1) referring to the formula VII-20 described on page 273 of “Leslie Steel Materialology: 1985, William C. Leslie”. It is the value calculated by the formula.

Ac ₃ transformation point (° C.) = 910−203 × [C] ^1/2 −15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [ W] −30 × [Mn] −11 × [Cr] −20 × [Cu] + 700 × [P] + 400 × [Al] + 120 × [As] + 400 × [Ti] (1)
However, the above [C], [Ni], [Si], [V], [Mo], [W], [Mn], [Cr], [Cu], [P], [Al], [As] And [Ti] mean mass% in the steel sheet of C, Ni, Si, V, Mo, W, Mn, Cr, Cu, P, Al, As, and Ti, respectively.

After that, continuous casting was performed by a conventional method to obtain a slab. Then, after hot rolling, pickling and cold rolling were sequentially performed by a conventional method to obtain a steel plate having a thickness of 1.0 mm. Subsequently, continuous annealing was performed.

Test No. in Table 2 below. After annealing at the annealing temperature and annealing time shown in 1 to 49, it was cooled to the quenching start temperature shown in Table 2 at a cooling rate of 10 ° C./second. Next, quenching was performed by quenching from the quenching start temperature to room temperature at the cooling rate shown in Table 2 below, and further the treatment was performed under the tempering conditions shown in Table 2. Here, the cooling rate from the first tempering temperature T1 to the second tempering temperature T2 was set to about 0.2 to 0.5 ° C./second. The conditions for the hot rolling are as follows.

[Hot rolling conditions]
Heating temperature: 1250 ° C
Finishing temperature: 880 ° C
Winding temperature: 620 ° C
Finished thickness: 2.6mm

Using the steel sheet obtained as described above, various characteristics were evaluated under the following conditions.

[Measurement of area ratio of steel structure]
A test piece of 1.0 mm × 20 mm × 20 mm was cut out, a cross section parallel to the rolling direction was polished, and after nital corrosion was performed, a ¼ part of the plate thickness was subjected to a scanning electron microscope (SEM: Scanning Electron Microscope). The observation was performed at 1000 times.

At this time, the size of one field of view is 90 μm × 120 μm, and in 10 arbitrary fields of view, 10 lines are drawn at equal intervals in the vertical and horizontal directions, and the intersection points are the number of intersection points that are martensite structures, ferrite structures, etc. Each of the number of intersections that is a structure other than martensite was divided by the total number of intersections to obtain the area ratio of the martensite structure and the area ratio of the structure other than martensite. The results are shown in Table 3 below.

[Evaluation of tensile properties]
The tensile strength (TS) was measured in accordance with the method specified in JIS Z 2241: 2011 by taking a JIS No. 5 tensile test piece from the steel plate so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction. And the thing whose tensile strength is 1270 Mpa or more was evaluated as high intensity | strength. The results are shown in Table 3 below. At this time, for reference, the yield strength (YS) of the steel sheet (corresponding to 0.2% proof stress: However, if the structure contains ferrite and indicates the yield point, the yield point (YP) is measured), and the total elongation ( The mechanical properties such as EL) are also shown in Table 3 below.

[Evaluation of bendability]
The bendability of the steel sheet was evaluated by the following procedure. A test piece having a width of 30 mm × a length of 35 mm with a major axis in the direction perpendicular to the rolling direction was prepared, and a bending test was performed by a V block method in accordance with JIS Z 2248: 2014. The bending radius at that time is variously changed from 0 to 7 mm, the minimum bending radius that can be bent without breaking the material is obtained, and this is defined as the limiting bending radius R (mm) / the limiting bending radius R (mm) / The plate thickness (mm) was calculated. Then, a test piece having a limit bending radius R (mm) / plate thickness (mm) of 4.0 or less was evaluated as being excellent in bendability, and a test piece exceeding 4.0 was evaluated as being inferior in bendability. In Table 3 below, test examples with excellent bendability are indicated by “◯”, and test examples with inferior bendability are indicated by “x”.

[Measurement of solute C content]
From each steel plate obtained above, a rod-shaped test piece having a diameter of 0.3 mm × length of 30 mm was taken so that the rolling direction was long, and the amount of solid solution C was measured by a high-intensity X-ray diffraction method. . At this time, the inter-lattice distance of martensite was analyzed from the waveform obtained by the high-intensity X-ray diffractometry, and the B.A. described in “Ata Mat. Vol. 59: 2011, 5845-” was analyzed. The amount of solid solution C was determined with reference to the equation of Hutchinson et al.

[Measurement of carbide size and number density]
Disc-shaped flakes having a diameter of 3 mm × thickness: 0.1 mm were collected from a quarter of the plate thickness of each steel plate obtained above by cutting, polishing (thinning), and punching. Then, the thickness of the sample is polished to 0.1 μm or less by an electrolytic thin film method, and observed with a transmission electron microscope (TEM: Transmission Electron Microscope, trade name “H-800”, manufactured by Hitachi, Ltd.). The number density of carbides was measured. The TEM observation was performed with 5 visual fields of 60000 times (volume per visual field: about 0.33 μm ³ ). In the image obtained by this observation, the observed second phase (precipitate) having an aspect ratio of 2 or more is a carbide, and the number of carbides having a major axis of 200 nm or more is analyzed using image analysis software (ImageJ). The number density of the carbide was calculated by converting per μm ³ .

[Evaluation test for delayed fracture resistance of steel sheet]
From each steel plate obtained above, a test piece of width: 150 mm × length: 30 mm was cut out and the cut end face was milled, and then a hole through which a bolt for applying stress was passed was formed at the end of the test piece. Further, U-bending is performed with a punch / die at a bending radius of 10 mm, a strain gauge is attached to the top of the bending head, and the bending test piece is tightened with bolts and nuts to apply a stress corresponding to the yield strength YS of the steel sheet. Granted. This U-bending test piece was immersed in a 0.1N-HCl solution for 200 hours, and a test piece in which no crack was generated in each of the three tests was evaluated as having excellent delayed fracture resistance. However, the specimens with cracks were evaluated as having poor delayed fracture resistance. In Table 3 below, a test example excellent in delayed fracture resistance is indicated by “◯”, and a test example inferior in delayed fracture resistance is indicated by “x”.

From these results, it can be considered as follows. First, test no. 1 to 5, 17 to 21, and 33 to 44 are test examples that use steel sheets that satisfy the chemical component composition defined in the present invention, are manufactured under appropriate manufacturing conditions, and satisfy the requirements defined in the present invention. It can be seen that the tensile strength is a high strength of 1270 MPa or more, and the bendability and delayed fracture resistance are excellent.

On the other hand, in the test examples that do not satisfy any of the requirements specified in the present invention, the tensile strength, the bendability, and the delayed fracture resistance are deteriorated. That is, test no. 6-17, and test no. Nos. 22 to 32 satisfy the chemical component composition specified in the present invention, but were not manufactured under preferable manufacturing conditions. Therefore, any of the structure, the amount of solute C, and the number density of carbides is within the range specified in the present invention. And at least one of tensile strength, bendability and delayed fracture resistance is deteriorated.

Specifically, test no. 5 and test no. No. 22 had a low quenching start temperature, and excessive ferrite was formed on the martensite substrate, so that the delayed fracture resistance deteriorated. In addition, Test No. 6 and test no. No. 23 has a low annealing temperature and is annealed in a two-phase region, and has a two-phase structure of ferrite-martensite structure. Therefore, the tensile strength and delayed fracture resistance decreased.

Test No. 8 and test no. No. 24 is an example in which the holding time at the annealing temperature is shortened, and since the structure is not austenite single phase and has a two-phase structure of ferrite-martensite structure, the tensile strength of the steel sheet decreases. Delayed fracture resistance has deteriorated.

Test No. 9 and test no. No. 25 is an example in which the first-stage tempering temperature T1 is low. Since the amount of dissolved C in the martensite becomes excessive, the toughness is lowered and the delayed fracture resistance is deteriorated. Test No. 10 and test no. No. 26 is a test example in which the first-stage tempering temperature T1 is high. Since a large amount of coarse carbides is generated, the tensile strength of the steel sheet decreases and the delayed fracture resistance deteriorates.

Test No. 11 and test no. No. 27 is a test example in which the holding time t1 in the first tempering is shortened, and since the amount of dissolved C in the martensite becomes excessive, the toughness of the steel sheet is lowered and the delayed fracture resistance is deteriorated. . Test No. 12 and test no. No. 28 is a test example in which the holding time t1 in the first tempering is long, and a large amount of coarse carbides is generated, so that the bendability of the steel sheet is deteriorated.

Test No. 13 and test no. 29 is a test example in which the second-stage tempering was not performed. Since the amount of dissolved C in the martensite was excessive, the toughness of the steel sheet was lowered and the delayed fracture resistance was deteriorated. Test No. 14 and test no. 30 is a test example in which the second-stage tempering temperature T2 is low. Since the amount of dissolved C in the martensite is excessive, the toughness of the steel sheet is lowered and the delayed fracture resistance is deteriorated.

Test No. 15 and test no. No. 31 is an example in which the holding time t2 in the second stage of tempering is shortened, and since the amount of dissolved C in the martensite becomes excessive, the toughness of the steel sheet is lowered and the delayed fracture resistance is deteriorated. . Test No. 16 and test no. No. 32 is a test example in which the second stage tempering temperature T2 is high, and a large amount of coarse carbides is generated, so that the bendability of the steel sheet is deteriorated.

On the other hand, test no. 45 to 49 are test examples using steel types U to Y that are manufactured under preferable manufacturing conditions but do not satisfy the chemical composition defined in the present invention, and at least one of tensile strength and delayed fracture resistance is selected. It has deteriorated.

Specifically, test no. No. 45 is a test example using a steel type U with insufficient C amount, because the hardenability is insufficient and the ferrite-martensite structure has a two-phase structure, so that the tensile strength is insufficient. Furthermore, since the processing strain concentrates on a large amount of ferrite, the delayed fracture resistance of the steel sheet is deteriorated. Test No. No. 46 is a test example using a steel type V having an excessive amount of C. Since the amount of dissolved C in the martensite is excessive, the delayed fracture resistance is deteriorated.

Test No. No. 47 is a test example using a steel type W with an excessive amount of Si, and the hardenability of the steel sheet is insufficient, and since it has a two-phase structure of ferrite-martensite structure, the tensile strength of the steel sheet is high. The resistance to delayed fracture is deteriorated.

Test No. 48 is a test example using the steel type X having an excessive amount of Mn, and it is expected that a large amount of MnS is generated in the center segregation part, and the delayed fracture resistance of the steel sheet is deteriorated. Test No. No. 49 is a test example using steel type Y with an excessive amount of S. MnS is expected to be produced in a large amount, and the delayed fracture resistance of the steel sheet is deteriorated.

(Test Example 2)
Test No. shown in Table 2 above. 4, 5, 18, 41, and no. 48 high-strength steel sheets (cold-rolled steel sheets) were electrogalvanized under the following conditions, and various EG steel sheets No. 50-54 were obtained.

[Electrogalvanizing treatment conditions]
The cold-rolled steel sheet was immersed in a galvanizing bath at 55 ° C., subjected to an electrogalvanizing treatment, then washed with water and dried to obtain an electrogalvanized steel sheet. The electrogalvanization process at this time was performed with a current density of 40 A / dm ² . The amount of galvanized coating was 40 g / m ² per side. In the electrogalvanizing treatment, an EG steel plate No. 1 having an electrogalvanized layer on the surface of the cold-rolled steel plate is appropriately subjected to washing treatment such as immersion in alkaline aqueous solution, degreasing, water washing and pickling. 50-54 were produced.

[Corrosion resistance evaluation test]
The EG steel sheet obtained above was subjected to the following corrosion resistance evaluation test to evaluate the corrosion resistance (test Nos. 50 to 54 in Table 4 below). At this time, the same evaluation was performed also about the cold-rolled steel plate which is a base steel plate of each EG steel plate (test No. 4, 5, 18, 41, 48).

A test piece having a width of 70 mm and a length of 150 mm was cut out, a combined cycle corrosion test described later was performed for one week, and the corrosion weight loss of the steel sheet (mass loss per unit area of the steel sheet due to corrosion) was measured to evaluate the corrosion resistance. Here, corrosion loss: 500 g / m ² or less is indicated by “◎” as a test example that is particularly excellent in corrosion resistance, and corrosion weight loss: a test sample that is more than 500 g / m ² and is 1000 g / m ² or less is indicated as “○ Corrosion weight loss: those exceeding 1000 g / m ² are indicated by “x” as test examples inferior in corrosion resistance.

[Delayed fracture resistance test]
Further, the obtained EG steel sheet was subjected to delayed fracture resistance test under the following conditions to evaluate delayed fracture resistance (test Nos. 50 to 54 in Table 4 below).

A test piece having a width of 150 mm and a length of 30 mm was cut out and the cut end face was milled, and then a hole for passing a bolt for applying stress was formed in the end of the test piece. Further, U-bending is performed with a punch / die at a bending radius of 10 mm, a strain gauge is attached to the top of the bending head, and the bending test piece is tightened with bolts and nuts to apply a stress corresponding to the yield strength YS of the steel sheet. Granted. Furthermore, in order to prevent contact corrosion between different base metal plates and bolts, the bolts and holes were masked with a silicon sealant (Shin-Etsu Silicone KE-45 manufactured by Shin-Etsu Chemical Co., Ltd.).

The above specimen was subjected to the combined cycle corrosion test described later for 4 weeks, and the presence or absence of delayed fracture was investigated. A test example in which delayed fracture (perforation resistance, cracking) did not occur after the combined cycle corrosion test was indicated by “◯”, and a test example in which delayed fracture occurred was indicated by “x”.

(Composite cycle corrosion test)
Salt spray (5% by mass-NaCl, 35 ° C. × 2 hours) → Dry (relative humidity: 20-30%, 60 ° C. × 4 hours) → Wet (relative humidity: 95% or more, 50 ° C. × 2 hours) 1 The cycle (one cycle is 8 hours) was performed three times a day. As described above, this was performed for 1 week for the corrosion resistance evaluation test and for 4 weeks for the delayed fracture resistance test.

The results are shown in Table 4 below. In addition, the delayed fracture resistance evaluation in the cold-rolled steel plates (test Nos. 4, 5, 18, 41, and 48) shown in Table 4 shows the results in Table 3 above.

From these results, it can be considered as follows. First, test no. 50, 51, 52, and 53 are test examples of high-strength electrogalvanized steel sheets obtained by electrogalvanizing high-strength steel sheets manufactured under appropriate manufacturing conditions using steel sheets that satisfy the chemical composition. , And delayed fracture resistance was found to be excellent. In addition, the above test No. It was found that the cold-rolled steel sheets (test Nos. 4, 5, 18, 41, and 48) that were the base steel sheets in 50 to 53 were also excellent in corrosion resistance.

In contrast, test no. 48 is an example using steel type X with an excessive amount of Mn, and the delayed fracture resistance of the cold-rolled steel sheet was originally deteriorated (Table 3), but electrogalvanizing was applied to such cold-rolled steel sheet. It was found that even if it was applied, sufficient delayed fracture resistance was not exhibited (Test No. 54).

This application is based on Japanese Patent Application Japanese Patent Application No. 2016-255466 filed on Dec. 28, 2016 and Japanese Patent Application No. 2017-183608 filed on Sep. 25, 2017. The contents thereof are included in the present application.

In order to express the present invention, the present invention has been described appropriately and sufficiently through the embodiments with reference to the drawings and the like. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that it can be done. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not limited to the scope of the claims. To be construed as inclusive.

The present invention has wide industrial applicability in the technical field of steel sheets and plated steel sheets.

Claims

C: 0.10% by mass to 0.35% by mass,
Si: 0 mass% or more and 0.6 mass% or less,
Mn: more than 0% by mass and 1.5% by mass or less,
Al: more than 0% by mass and 0.15% by mass or less,
N: more than 0% by mass and 0.01% by mass or less,
P: more than 0% by mass and 0.02% by mass or less, and S: more than 0% by mass and 0.01% by mass or less, with the balance being iron and inevitable impurities,
The area ratio of martensite in the entire organization is 95% or more,
The solid solution C of the martensite is 0.05% by mass or less,
The density of the carbide having a major axis of 200 nm or more is 50 pieces / μm 3 or less, and
A high-strength steel sheet having a tensile strength of 1270 MPa or more.
The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Cr: more than 0% by mass and 1.0% by mass or less and B: more than 0% by mass and 0.01% by mass or less.
The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Cu: more than 0% by mass and 0.5% by mass or less and Ni: more than 0% by mass and 0.5% by mass or less.
Furthermore, V: more than 0% by mass and 0.1% by mass or less, Nb: more than 0% by mass and 0.1% by mass or less, Mo: more than 0% by mass and 0.5% by mass or less, and Ti: more than 0% by mass and 0.2% by mass The high-strength steel sheet according to claim 1, comprising at least one selected from the group consisting of mass% or less.
The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Ca: more than 0% by mass and 0.005% by mass or less and Mg: more than 0% by mass and 0.005% by mass or less. .
A high-strength electrogalvanized steel sheet having an electrogalvanized layer on the surface of the high-strength steel sheet according to claim 1.