ES2753390T3 - Hot formed steel plate element - Google Patents

Abstract

Hot-formed sheet steel element with a chemical composition consisting of, in terms of% by mass: 0.08 to 0.16% C, 0.19% or less Si, 0.40 to 1 , 50% of Mn, 0.02% or less of P, 0.01% or less of S, 0.01 to 1.0% of Al sol., 0.01% or less of N, of 0, 25 to 3.00% Cr, 0.01 to 0.05% Ti, 0.001 to 0.01% B, 0 to 0.5% Nb, 0 to 2.0% Ni, 0 to 1.0% Cu, from 0 to 1.0% Mo, from 0 to 1.0% V, from 0 to 0.005% Ca, and a remainder consisting of Fe and impurities, where a fraction by total volume of martensite, annealed martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less, a mean grain size of the γ-prior grains is 10 μm or less, a number density of residual carbides present is 4 x 103 per mm2 or less, a value of the steel purity level specified by JIS G 0555 (2003) is 0.08% or less, where the value of the steel purity level is obtained by calculating the percentages of inclus surface ions based on A, based on B and based on C, and a degree of segregation α of Mn represented by formula (i) below is 1.6 or less, α = [maximum concentration of Mn (mass%) in a central part of a plate thickness] / [average Mn concentration (mass%) at a position 1/4 of the depth of the plate thickness from a surface] (i).

Description

DESCRIPTION

Hot formed steel plate element

Technical field

The present specification relates to a hot formed steel plate element by hot forming a steel plate.

Background of the Art

A high-strength steel plate that has high tensile strength has been widely applied to the automotive steel plate field in order to achieve both weight savings in order to improve fuel consumption and to improve fuel economy. collision resistance. However, the high strength causes a deterioration in the press formability of the steel plate, making it difficult to produce products with complicated shapes.

As a result, for example, a high strength of the steel plate disadvantageously causes a deterioration of ductility, which causes breaks in sites that have a high degree of processing, and disadvantageously causes a deterioration of the precision of dimensions or the like, due to to increased elastic recovery and warpage. Therefore, a high strength steel plate, particularly a tensile strength of 780 MPa or more, is not easily press formed into a product with a complicated shape.

In recent years, for example, as described in Japanese Patent Application Open to Public Inspection (JP-A) No. 2002-102980, a hot stamping technique has been adopted as a technique for press forming of a material that is difficult to form, such as a high strength steel plate. The hot stamping technique is a hot forming technique for heating and forming a proportioned material for forming. Because in this technique the steel plate is simultaneously formed and tempered, the steel plate is soft and has a favorable formability during forming, and the element formed after forming can have a higher strength than that of a plate steel for cold forming.

Japanese Patent Application Open to Public Inspection (JP-A) No. 2006-213959 describes a steel element with a tensile strength of 980 MPa.

Japanese Patent Application Open to Public Inspection (JP-A) No. 2007-314817 describes that a hot-formed steel sheet element with excellent tensile strength and robustness is obtained by reducing the level of purity and degrees of segregation of P and S.

Application No. US2014 / 0037980 A1 describes a hot stamping molded article that is used to produce energy absorption, containing the steel plate (in mass%), carbon (0.002-0.1), silicon (0, 01-0.5), mixture of manganese and chromium (0.5-2.5), phosphorus (0.1 or less), sulfur (0.01 or less), aluminum (0.05 or less), nitrogen (0.005 or less) and the rest, iron and unavoidable impurities. The hot stamping molded article has a metal structure containing martensite, bainite, and unavoidable mixing structures, or a metal structure containing bainitic ferrite, and inevitable mixing structures.

The metallic material described in JP-A No. 2002-102980 has insufficient hardenability during hot pressing, and consequently the metallic material has poor hardness stability. Steel sheets with excellent tensile strength and robustness are described in JP-A No. 2006-213959 and No. 2007-314817, although the local deformation characteristics continue to be improved.

An objective of the specification embodiments is to provide a hot-formed steel plate element with excellent hardness stability and local deformability. In many cases, the non-hot-formed steel plate element is not a flat plate but a formed body, and is referred to as a "hot-formed steel plate element", including the case where the Hot formed steel plate element is the body formed in the specification.

In accordance with one aspect of the present specification, a hot formed steel sheet element is provided with a chemical composition consisting of, in terms of% by mass, 0.08 to 0.16% C, 0.19% or less than Si, between 0.40 and 1.50% Mn, 0.02% or less P, 0.01% or less S, between 0.01 and 1.0% Al sol., 0, 01% or less N, between 0.25 and 3.00% Cr, between 0.01 and 0.05% Ti, between 0.001 and 0.01% B, between 0 and 0.50% Nb , between 0 and 2.0% Ni, between 0 and 1.0% Cu, between 0 and 1.0% Mo, between 0 and 1.0% V, between 0 and 0.005% Ca, and the rest consisting of Faith and impurities,

wherein the total volume fraction of martensite, temperate martensite, and bainite is 50% or more, and the volume fraction of ferrite is 3% or less,

the mean grain size of Y-prior grains is 10 pm or less, and

the density in number of residual carbides present is 4 x 103 per mm2 or less.

Brief description of the drawings

Fig. 1 is a schematic view showing a shape of a mold in hot forming in the Examples. Fig. 2 is a schematic view showing a shape of a shaped body obtained by hot forming in the Examples.

Fig. 3 is a schematic view showing a shape of a tensile test piece.

Description of embodiments

The present inventors have intensively carried out studies to provide a hot formed steel sheet element with excellent local hardness stability and deformability, and consequently have obtained the results described below.

(1) Fine Y-prior grains in the hot-formed steel plate element delay the appearance and connection of voids, providing an improvement in local deformability. Therefore, fine Y-prior grains are preferable. (2) In the case of the presence of various residual carbides in the hot-formed steel plate element, hardenability may deteriorate after hot-formed, causing deterioration of the hardness stability, and the residual carbides serve as hole production source, causing deterioration of local deformability. Therefore, the number density of residual carbides is preferably reduced.

The specification embodiments are based on the results. According to one aspect of the embodiments,

(1) A hot-formed steel plate element with a chemical composition consisting of, in terms of% by mass, between 0.08 and 0.16% C, 0.19 or less Si, is provided between 0.40 and 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, between 0.01 and 1.0% of Al sol., 0.01% or less of N, between 0.25 and 3.00% Cr, between 0.01 and 0.05% Ti, between 0.001 and 0.01% B, between 0 and 0.50% Nb, between 0 and 2 0.0% Ni, between 0 and 1.0% Cu, between 0 and 1.0% Mo, between 0 and 1.0% V, between 0 and 0.005% Ca, the rest consisting of Fe and impurities, where the total volume fraction of martensite, temperate martensite, and bainite is 50% or more, and the volume fraction of ferrite is 3% or less,

the average grain size of Y-prior grains is 10 pm or less, and

the density in number of residual carbides present is 4 x 103 per mm2 or less, the value of the purity level of the steel specified in the JIS G 0555 (2003) standard is 0.08% or less, where the value of the level of Steel purity is obtained by calculating the surface percentages of A-based, B-based and C-based inclusions, and a degree of segregation a of Mn represented by formula (i) below is 1.6 or less, a = [maximum concentration of Mn (% by mass) in a central part of a thickness of the plate] / [average concentration of Mn (% by mass) at a position a% of the thickness of the plate from a surface] ... (i).

(2) In the hot formed steel plate element of (1), the chemical composition preferably includes one or more selected from the group consisting of, in terms of% by mass, between 0.003 and 0.50% Nb, between 0.01 and 2.0% Ni, between 0.01 and 1.0% Cu, between 0.01 and 1.0% Mo, between 0.01 and 1.0% V, and between 0.001 and 0.005% Ca.

(3) In any of the hot formed steel plate elements of (1) and (2), the steel plate element preferably has a surface on which a plating layer is formed.

(4) In any of the hot formed sheet steel elements from (1) to (3), the sheet steel element preferably has a tensile strength of 1.0 GPa or more.

Hereinafter, the embodiments are described in detail.

(A) Chemical composition

The reason why the content of each item is limited is described below. In the later description, the symbol “%” of the content of each element refers to “% by mass”.

C: between 0.08 and 0.16%.

C is an important element to improve the hardenability of the steel and to guarantee the resistance after the hardening. Because C is an austenite-forming element, it has the function of suppressing deformation-induced ferrite transformation during high-stress formation. This facilitates obtaining a stable hardness distribution on the steel plate element after hot forming. The C content of less than 0.08% makes it difficult to obtain a tensile strength of 1.0 GPa or more after hardening and obtaining the aforementioned effect. Therefore, the C content is fixed at 0.08% or more. A C content greater than 0.16% causes an excessive increase in resistance after quenching, causing deterioration of the local deformability.

Therefore, the C content is fixed at 0.16% or less. The C content is preferably 0.085% or more, and more preferably 0.9% or more. The C content is preferably 0.15% or less, and more preferably 0.14% or less.

Yes: 0.19% or less.

If it is an element that has the function of suppressing the formation of flakes during heating at high temperature for hot forming. However, the Si content greater than 0.19% causes a noticeable increase in the heating temperature necessary for the transformation of austenite during hot forming. This causes an increase in the cost necessary for heat treatment and insufficient quenching due to insufficient heating. Si is a ferrite forming element. In this way, an excessively high Si content is suitable for producing the deformation-induced ferrite transformation during high stress formation. This causes a local reduction in the hardness of the steel plate element after hot forming, making it difficult to obtain a stable distribution of hardness. Furthermore, a significant amount of Si causes deterioration of wettability in the event that a hot-dip plating treatment is carried out, which can lead to a lack of plating. Therefore, the Si content is set at 0.19% or less. The Si content is preferably 0.15% or less. In case the aforementioned effect is desired to be obtained, the Si content is preferably 0.01% or more.

Mn: between 0.40 and 1.50%.

Mn is a useful element to improve the hardenability of a steel plate and to stably guarantee resistance after hot forming. A content of Mn less than 0.40% makes it difficult to obtain the aforementioned effect. Therefore, the Mn content is fixed at 0.40% or more. A content of Mn greater than 1.50% produces thick MnS, which becomes a deterioration factor for local deformability. Therefore, the Mn content is fixed at 1.50% or less. The Mn content is preferably 0.80% or more, and preferably 1.40% or less.

P: 0.02% or less.

Since P is an element contained in the form of impurities, and has the function of making it possible to improve the hardenability of the steel and stably guarantee the strength of the steel after quenching, it is positive that there is a content of P. However , the P content greater than 0.02% causes a noticeable deterioration of the local deformability. Therefore, the P content is fixed at 0.02% or less. The P content is preferably 0.01% or less. Although the lower limit of P content is not particularly limited, an excessive reduction in P content causes a significant increase in cost. For this reason, the P content is preferably set to 0.0002% or more.

S: 0.01% or less.

Sn is an element contained in the form of impurities and that causes a deterioration of the local deformability. S content greater than 0.01% causes a noticeable deterioration of local deformability. Therefore, the content of S is set at 0.01% or less. Although the lower limit of S content is not particularly limited, an excessive reduction in S content causes a significant increase in cost. Therefore, the content of S is preferably set at 0.0002% or more.

In the sun: between 0.01 and 1.0%.

To the sun. It is an element that has the function of providing solidity to the steel by deoxidizing the molten steel. A content of Al sol. less than 0.01% causes insufficient deoxidation. Also, because the Al sun. It is also an element that has functions to improve the hardenability of the steel plate and that stably guarantees the resistance after hardening, it is positive that there is an Al sol content. Therefore, the content of Al sol. it is set at 0.01% or more. However, the content of Al sol. greater than 1.0% provides a small effect obtained by function and unnecessarily causes an increase in cost. For this reason, the content of Al sol. it is set at 1.0% or less. The content of Al sol. it is preferably 0.02% or more and preferably 0.2% or less.

N: 0.01% or less.

N is an element contained in the form of impurities, which causes the deterioration of robustness. A content of N greater than 0.01% forms thick nitride in the steel, which causes a notable deterioration in local deformability and robustness. Therefore, the N content is set to 0.01% or less. The N content is preferably 0.008% or less. Although the lower limit of the N content need not be particularly limited, an excessive reduction in the N content causes a significant increase in cost. For this reason, the N content is preferably set at 0.0002% or more, and more preferably at 0.0008% or more.

Cr: between 0.25 and 3.00%.

Cr is an element that has the function of improving the hardenability of the steel. Therefore Cr is an element particularly important in an embodiment where the Mn content is limited to 1.50% or less. Cr is an austenite-forming element and has the function of suppressing deformation-induced ferrite transformation during high-stress forming. Therefore, there is a Cr content, which facilitates obtaining a stable distribution of hardness in a sheet steel element after hot forming. The Cr content of less than 0.25% cannot sufficiently provide the aforementioned effect. Therefore, the Cr content is set at 0.25% or more. A Cr content greater than 3.00% causes a concentration of carbonates in the steel, which delays the solution of solid carbides in the heating stage if it is provided for hot forming, causing deterioration of hardenability . Therefore, the Cr content is set at 3.00 or less. The Cr content is preferably 0.30% or more, and more preferably 0.40% or more. The Cr content is preferably 2.50% or less, and more preferably 2.00% or less.

Ti: between 0.01 and 0.05%.

Ti is an element that has the function of suppressing the recrystallization of the austenite grains in the event that the steel plate for hot forming is heated to point Ac3 or more and is provided for hot forming. Furthermore, Ti has the function of forming fine carbides to suppress grain growth from austenite grains, thereby providing fine grains. For this reason, Ti has the function of greatly improving the local deformability of the hot-formed steel plate element. Because Ti preferentially binds N in steel, Ti suppresses the consumption of B due to the precipitation of BN, and as a consequence Ti has the function of improving hardenability due to B. Therefore, the content of Ti it is set at 0.01% or more. However, the Ti content greater than 0.05% causes an increase in the amount of TiC precipitation, causing C consumption, thereby causing a reduction in post-quenching strength. For this reason, the Ti content is set at 0.05% or less. The Ti content is preferably 0.015% or more. The Ti content is preferably 0.04% or less, and more preferably 0.03% or less.

B: between 0.001 and 0.01%.

B is an element with functions that allow it to improve the hardenability of steel and stably guarantee post-hardening resistance. Therefore, in an embodiment where the Mn content is limited to 1.50% or less, B is a particularly important element. A B content of less than 0.001% cannot sufficiently provide the aforementioned effect. Therefore, the content of B is set at 0.001% or more. A content of B greater than 0.01% causes saturation of the aforementioned effect and deterioration of the local deformability of a tempered part. Therefore, the content of B is set at 0.01% or less. The B content is preferably 0.005% or less.

The hot formed steel plate elements of the embodiments have a chemical composition consisting of elements C to B, the remainder consisting of Fe and impurities.

The "impurities" herein are elements that are mixed by various factors with raw materials, such as ore or waste, and in a production process when sheet steel is produced on an industrial scale, and a content of the same included within a range that does not allow the elements to exert any adverse influence on the performances.

The hot formed steel plate element of the embodiments may further contain one or more elements selected from the group consisting of Nb, Ni, Cu, Mo, V, and Ca in amounts indicated below, in addition to the aforementioned elements.

Nb: between 0 and 0.50%.

The Nb is an element with suppression functions of the recrystallization in the event that the steel plate for the hot forming is heated to the point Ac3 or more and is provided for the hot forming, as well as to form destined fine carbides to suppress grain growth, thereby providing fine grains of austenite. For this reason, Nb largely has the function of improving the local deformability of the hot-formed steel plate element. Therefore, if necessary, there may be a content of Nb. However, the Nb content greater than 0.50% causes an increase in NbC precipitation, causing C consumption and thus causing a reduction in post quenching strength. For this reason, the Nb content is set to 0.50% or less. The Nb content is preferably 0.45% or less. In case the aforementioned effect is desired to be obtained, the Nb content is preferably fixed at 0.003% or more, and more preferably at 0.005% or more.

Ni: between 0 and 2.0%.

Because Ni is an effective element to improve the hardenability of steel plates and to stably guarantee post-hardening resistance, there can be a Ni content if necessary. However, the Ni content greater than 2.0% provides a small effect, which unnecessarily causes an increase in cost. For this reason, the Ni content is set to 2.0% or less. The Ni content is preferably 1.5% or less. In the event that the aforementioned effect is desired to be obtained, the Ni content is preferably fixed at 0.01% or more, and more preferably at 0.05% or more.

Cu: between 0 and 1.0%.

Because Cu is an effective element to improve the hardenability of steel plates and to stably guarantee post-hardening resistance, there can be a Cu content if necessary. However, the Cu content greater than 1.0% provides a small effect, unnecessarily causing an increase in cost. For this reason, the Cu content is set to 1.0% or less. The Cu content is preferably 0.5% or less. In case the aforementioned effect is desired to be obtained, the Cu content is preferably fixed at 0.01% or more, and more preferably at 0.03% or more.

Mo: between 0 and 1.0%.

Mo is an element that has the function of forming fine carbides in the event that the steel plate for hot forming is heated to the point Ac3 or more and is provided for hot forming to suppress grain growth, thus providing fine grains of austenite. Mo also has an effect of greatly improving the local deformability of the hot-formed steel plate element. For these reasons, there may be Mo content if necessary. However, Mo content greater than 1.0% causes saturation of the effect, unnecessarily causing an increase in cost. Therefore, the Mo content is fixed at 1.0% or less. The Mo content is preferably 0.7% or less. In case the aforementioned effect is desired to be obtained, the Mo content is preferably fixed at 0.01% or more, and more preferably at 0.04% or more.

V: between 0 and 1.0%.

Because V is an effective element to improve the hardenability of steel plates and to stably guarantee post-hardening resistance, there can be a content of V if necessary. However, V content greater than 1.0% provides a small effect, unnecessarily causing an increase in cost. For this reason, the content of V is set to 1.0% or less. The V content is preferably 0.08% or less. In case the effect is desired to be obtained, the content of V is preferably set at 0.01% or more, and more preferably at 0.02% or more.

Ca: between 0 and 0.005%.

Because Ca is an element that has a grain refining effect of inclusions in steel, improving local deformation after quenching, there may be a Ca content if necessary. However, a Ca content greater than 0.005% causes saturation of the effect, which unnecessarily causes an increase in cost. Therefore, the Ca content is set to 0.005% or less. The Ca content is preferably 0.004% or less. In case the effect is desired to be obtained, the Ca content is preferably set to 0.001% or more, and more preferably 0.002% or more.

(B) Metal structure.

In the embodiments, in order to improve local deformability, variations in hardness in the metal structure are preferably suppressed after hot forming. Because the increased difference in hardness in the structure serves as a starting point for voids, the mixing of a low-temperature transformation structure, such as martensite or hard bainite, and a soft ferrite structure is preferably suppressed to the maximum extent. Therefore, it is preferable that the hot formed steel plate elements of the embodiments have a low temperature transformation structure and have a metal structure with a ferrite volume fraction of 3% or less.

The metal structure having primarily a low temperature transformation structure refers to a metal structure in which the fraction of the total volume of martensite, annealed martensite, and bainite is 50% or more. Martensite annealed herein refers to martensite transformed during quenching and annealed by automatic annealing, and martensite subjected to low temperature annealing, such as a post-quench baking process. The volume fraction of the structure transformed at low temperature into the metal structure is preferably 80% or more, and more preferably 90% or more.

Because residual austenite improves ductility according to the TRIP effect, there may be a residual austenite content without problem. However, the martensite transformed from austenite is hard, which serves as a starting point for gaps. Therefore, the volume fraction of the residual austenite contained in the metal frame is preferably 10% or less.

Degree of segregation a of Mn: 1.6 or less.

A = [maximum concentration of Mn (mass%) in a central part of a plate thickness] / [average concentration of Mn (mass%) at a position 1/4 depth of the plate thickness from a surface ] ... (i)

In the central part of the plate thickness section of the hot-formed steel plate element, produces central segregation, with concentration of Mn. Therefore, the MnS is concentrated in the center in the form of inclusions, which is adequate to cause the formation of hard martensite. This causes a difference in hardness between the hard martensite and the surrounding material, resulting in deterioration of local deformability. Particularly, in the case where the value of the degree of segregation a represented by the formula (i) exceeds 1.6, the local deformability deteriorates remarkably. Therefore, in order to improve local deformability, the value a of the hot formed steel plate element is set to 1.6 or less. In order to further improve local deformability, the value a is more preferably set to 1.2 or less.

The segregation of Mn in the steel plate is controlled mainly by the composition of the steel plate (particularly the content of impurities) and a continuous molding condition, and is not substantially modified after and before hot rolling and forming hot. Therefore, the inclusions and segregation situation of the hot forming steel plate are practically equal to that of the hot formed steel plate element made by hot forming of the steel plate for hot forming. Since the value a is not greatly modified by hot forming, the value a of the hot formed steel plate element can also be set to 1.6 or less by setting the value a of the steel plate to hot formed at 1.6 or less. The value a of the hot formed steel plate element can also be set to 1.2 or less by setting the value a to 1.2 or less.

The maximum concentration of Mn in the central part of the thickness of the plate is obtained by the following method. The central portion of the plate thickness of the steel plate is subjected to line analysis using an Electron Probe Microanalyzer (EPMA). Three measured values are selected in a higher order of the analysis results and their average value is calculated. By the following method, the average concentration of Mn is obtained at the position 1/4 depth of the thickness of the plate from the surface. Similarly, ten sites at 1/4 of the depth of the steel plate were analyzed by EPMA and their mean value was calculated.

Purity level: 0.08% or less.

In the case of the presence of A-based, B-based and C-based inclusions described in standard JIS G 0555 (2003) in large quantities in the steel plate element, the inclusions are suitable to serve break point initial. In the event that the inclusions are increased, the propagation of the cracks easily occurs, causing deterioration of the local deformability. Particularly, in the case of the hot-formed steel plate element with tensile strength of 1.0 GPa or more, the inclusions fraction is preferably suppressed to a low level. In the event that the value of the purity level of the steel specified in standard JIS G 0555 (2003) exceeds 0.08%, the amount of inclusions is large, making it difficult to obtain sufficient local deformability in practice. . Therefore, the purity level value of the steel plate for hot forming is set to 0.08% or less. The purity level value is more preferably set to 0.04% or less in order to further improve local deformability. The value of the purity level of the steel is obtained by calculating the surface percentages of the inclusions based on A, based on B and based on C.

Because the purity level value is not greatly modified by hot forming, the purity level value of the hot formed steel sheet element can also be set to 0.08% or less by fixing the purity level value of the hot forming steel plate at 0.08% or less. The purity level value of the hot formed steel plate element can also be set to 0.04% or less by setting the purity level value of the hot formed steel plate element to 0.04% or less.

In the embodiments, the purity level value of the hot-formed steel plate or the hot-formed steel plate element is obtained by the following method. The test materials are cut from five locations on the hot-formed steel plate or on the hot-formed steel plate element. In the event that the plate thickness of the hot-formed steel plate or the hot-formed steel plate element is defined as t, the purity level at each of the 1 / 8t positions is investigated, 1 / 4t, 1 / 2t, 3 / 4t and 7 / 8t in the direction of the thickness of the plate of each one of the test materials by means of standard n ° JIS-G-0555. The largest value (lowest purity property) of the purity level at each of the plate thicknesses is used as the value of the purity level of the test material.

Average grain size of Y-prior grains: 10 pm or less.

In the event that the Y-prior grain size in the hot formed steel plate member is reduced, the local deformability is improved. In a steel plate containing mainly martensite, voids are produced at the Y-prior grain edges and at the edges of the bottom grain structures. However, refining the Y-prior grains can suppress the appearance of voids and improve local deformability to delay connection. In the event that the average grain size of the Y-prior exceeds 10 pm, this effect cannot be shown. Therefore, the average grain size of the Y-prior grains in the hot-formed steel plate member is set to 10 pm or less. In order to carry out grain refining of Y-prior grains, it is effective to reduce the heating temperature and delay the dissolution of carbides during heating in order to suppress grain growth.

The average size of Y-prior grains can be measured using a method specified in ISO643. That is, the number of glass beads is measured in a measured view. The average surface area of the crystal grains is obtained by dividing the measured surface area by the number of crystal grains, and calculating the crystal grain size in an equivalent circular diameter. At that time, it is preferable that the grain at the edge of view is measured as 1/2, and the magnification ratio is adjusted so that the number of glass grains is constantly 200 or more. Preferably a plurality of views are measured in order to improve accuracy.

Residual carbides: 4 x 103 per mm2 or less.

In the case of hot forming, sufficient hardenability can be guaranteed by solving the carbides generally present in the steel. However, a part of the carbides may remain unresolved. The residual carbides have the effect of suppressing the growth of the grains and during the maintenance of the heating in the hot formation by a fixing effect ("pinning", in English). Therefore, during the maintenance of the heating, the presence of residual carbides is desirable. As residual carbides are reduced after hot forming, hardenability improves, which can ensure high strength. Therefore, it is preferable that the number density of residual carbides can be reduced in the event that heating maintenance is completed.

In the presence of various residual carbides, hardenability may deteriorate after hot forming, and the residual carbides serve as a source of void production, causing deterioration of local deformability. Particularly, in the event that the density in number of residual carbides exceeds 4x103 per mm2, the hardenability after hot forming may deteriorate. Therefore, the density in number of residual carbides present in the hot formed steel plate element is 4x103 per mm2 or less.

(C) Coating layer.

The high strength hot formed steel sheet element according to the embodiments can have a surface on which a coating layer is formed in order to improve corrosion resistance or the like. The coating layer can be an electrogalvanizing layer and can be a hot dip coating layer. Examples of the electrogalvanizing layer include electrogalvanizing, Zn-Ni alloy electrical coating, and Zn-Fe alloy electrical coating. Examples of the hot-dip coating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, cast aluminum coating, cast Zn-Al alloy coating, alloy coating. Zn-Al-Mg cast and Zn-Al-Mg-Si alloy coating cast. A coating deposit amount is not particularly limited and can be adjusted within a general range.

(D) Method of manufacturing steel plate for hot forming.

The manufacturing conditions of the hot forming steel plate used to manufacture the hot forming steel plate element according to the embodiments are not particularly limited, although the hot forming steel plate can be conveniently manufactured by using a manufacturing method shown below.

Steel with the aforementioned chemical composition is melted in a furnace and a sheet is then produced by molding. In order to set the purity level of the steel plate to 0.08% or less, it is desirable to set the heating temperature of the molten steel to a temperature higher than 5 ° C or more than the temperature of the liquidus line of the steel in the case where molten steel is continuously molded and to suppress the amount of molten steel to be molded per unit time at 6 t / min or less.

In the event that the amount to be molded per unit time of the molten steel exceeds 6 t / min during continuous molding, the molten steel is rapidly stirred in a mold. In this way, inclusions can be trapped by a solidifying shell, causing an increase in inclusions in the slab. In the case where the heating temperature of the molten steel is lower than a temperature higher by 5 ° C than the liquidus line temperature, the viscosity of the molten steel is increased and, in this way, the inclusions are less likely to float in a continuous molding machine. Consequently, the inclusions in the slab are increased, allowing the purity property to deteriorate.

Molten steel is molded by setting the heating temperature of the molten steel to 5 ° C or more than the liquidus line temperature of the molten steel, and the amount of molten steel to be molded per unit time is set at 6 t / min or less, which is less likely to cause inclusions to enter the slab. Consequently, the amount of inclusions in the stage in which the slab is produced can be effectively reduced, allowing a purity level of the steel plate of 0.08% or less to be easily achieved.

In the case that molten steel is continuously molded, the heating temperature of the molten steel is more desirably set to a temperature higher than 8 ° C or more than the temperature of the liquidus line and the amount of molten steel to be molded per time unit is most desirably set at 5 t / min or less. Through setting the heating temperature of the molten steel to a temperature higher than 8 ° C or more than the liquidus line temperature, and setting the amount of molten steel to be molded per unit time at 5 t / min or less, the purity level is easily set at 0.04% or less, which is desirable.

In order to suppress the concentration of MnS that causes deterioration of local deformability, a central segregation reduction treatment is desirably carried out to reduce the central segregation of Mn. Examples of reducing treatment of central segregation include a method of discharging molten steel in which Mn is concentrated in a non-solidified layer before the slab is completely solidified.

Specifically, the molten steel in which the Mn is concentrated before fully solidifying can be discharged by a treatment such as electromagnetic stirring or reduction of the non-solidified layer. The electromagnetic stirring treatment can be carried out by stirring the non-solidified molten steel at 250 to 1000 Gauss, for example. The reduction treatment of the non-solidified layer can be carried out by reducing the last part to solidify, at a slope of about 1 mm / m, for example.

The slab obtained by the aforementioned method can be subjected to an immersion treatment, if necessary. By performing the immersion treatment, the segregated Mn is diffused, which can provide a reduction in the degree of segregation. A preferable immersion temperature in the event that the immersion treatment is carried out is 1,200 to 1,300 ° C, and a preferable immersion time is 20 to 50 hours.

The slab is then hot rolled. As hot rolling conditions, from the point of view of more uniform carbide production, it is preferable that the hot rolling starting temperature is set in a temperature region of 1,000 to 1,300 ° C, and the temperature is set at which hot rolling is completed at 850 ° C or higher. The winding temperature is preferably higher from the point of view of processability. However, in the event that the winding temperature is excessively high, the formation of flakes causes a reduction in performance and, thus, the winding temperature is preferably 500 to 650 ° C. A hot rolled steel plate obtained by hot rolling is subjected to a pickling treatment by chemical purity or the like.

In the embodiments, in order to carry out grain refining of the Y-prior grains after hot forming and to reduce the number density of residual carbides, the hot-rolled steel plate is subjected to pickling treatment it is preferably annealed to produce a hot rolled annealed steel plate.

In order to provide a fine Y-prior grain size after hot forming, grain growth is preferably suppressed by carbides in solution. However, in order to improve hardenability, to ensure high strength, and to suppress void production in the hot-formed steel plate member, the number density of residual carbides is preferably reduced.

In order to provide the fine Y-prior grain size in the hot-formed steel plate element and to reduce the number density of residual carbides, the shape of the carbides present in the steel plate before hot-formed and the degree of concentration of the elements in the carbides are important. It is desirable that the carbides are finely dispersed. However, because the carbides dissolve rapidly in this case, a grain growth suppressing effect cannot be expected. In the event that elements such as Mn and Cr are concentrated in the carbides, the carbides are less likely to dissolve. Therefore, it is desirable that the carbides in the steel plate before hot forming are finely dispersed and that the degree of concentration of the elements in the carbides is higher.

The shape of the carbides can be controlled by adjusting the annealing conditions after hot rolling. Specifically, it is preferable that the annealing temperature is set to an Ac1 point or less and the Ac-i point to -100 ° C or more, and the annealing time is 5 hours or less.

In the event that the post-hot roll winding temperature is set to 550 ° C or less, the carbides are likely to be finely dispersed. However, because the degree of concentration of the elements in the carbides is also reduced, annealing advances the concentration of the elements.

In the event that the winding temperature is 550 ° C or higher, perlite is generated, and the concentration of the elements in the carbides in the perlite advances. In this case, in order to divide the perlite to disperse the carbides, the annealing is carried out.

As the steel plate for the hot formed steel plate element in the embodiments, the above mentioned hot rolled annealed steel plate, the cold rolled steel plate obtained by cold rolling of a steel plate, can be used. hot-rolled annealed or cold-rolled annealed steel plate obtained by annealing a cold-rolled steel plate. A treatment step can be selected if appropriate according to the required level of precision of the product plate thickness, or the like. Because carbides are hard, the shape of the carbides remains unchanged even in the case of cold rolling, and the shape prior to rolling cold is maintained even after cold rolling.

Cold rolling can be carried out using a standard method. From the point of view of ensuring a favorable smoothness, the cold rolling reduction ratio is preferably set at 30% or more. In order to avoid excessive loading, the reduction ratio in cold rolling is preferably set to 80% or less.

In the case that a cold-rolled steel plate is annealed, it is desirable that the cold-rolled steel plate is preliminary subjected to a treatment, such as defatting. Annealing is preferably carried out at point A c or less, for hours or less, preferably for 3 hours or less in order to reduce the stress of the cold rolling.

(E) Method for forming a coating layer.

As described above, the hot-formed steel plate element according to the embodiments may have a surface on which a coating layer is formed in order to improve corrosion resistance, or the like. The coating layer is desirably formed on the steel plate prior to hot forming. In the event that a zinc-based coating is applied to the surface of the steel plate, preferably a zinc-based coating is applied to a continuous hot-dip galvanizing line from the point of view of productivity . In this case, annealing can be carried out prior to coating treatment on the continuous hot-dip galvanizing line. Only an annealing-free coating treatment can be carried out with a heating hold temperature set to a low temperature. An alloy cast zinc laminated steel sheet may be provided by performing an alloy heat treatment after hot dip galvanizing. The zinc-based coating can also be applied by electrogalvanizing. The zinc-based coating can be applied to at least part of the surface of the steel material. Generally, however, the zinc-based coating is applied entirely to one surface or both surfaces of the steel plate.

(F) Method for manufacturing hot formed steel plate element.

By hot forming the steel plate for hot forming, a high strength hot formed steel plate element can be obtained. From the standpoint of suppressing grain growth, the heating rate of the steel plate during hot forming is desirably 20 ° C / s or higher, and more preferably 50 ° C / s or higher. The heating temperature of the steel plate during hot forming is desirably set to a temperature higher than the Ac3 point and 1,050 ° C or lower. In the event that the heating temperature is equal to or less than the Ac3 point, the ferrite, perlite or bainite persists in the steel plate without providing a single phase state of austenite prior to hot forming. As a result, the desired hardness may not be obtained without providing the primarily martensite containing metal structure after hot forming. This causes not only an increase in the variation of the hardness of the hot-formed steel plate element, but also a deterioration of the local deformability.

In the event that the heating temperature exceeds 1,050 ° C, the austenite is thick, which can cause deterioration of the local deformability of the steel plate element. Therefore, the heating temperature of the steel plate during hot forming is preferably set to 1,050 ° C or less. In the event that the heating time is less than 1 min, the single phase of austenite may be insufficient, even if heating is carried out. Furthermore, because the dissolution of the carbides is insufficient, the density in number of the residual carbides increases, although the size of the grains is refined and. In the event that the heating time exceeds 10 min., The austenite is thick, which may cause deterioration of the local deformability of the hot formed steel plate element. Therefore, the heating time of the steel plate during hot forming is desirably set to between 1 and 10 min.

In the event that the hot forming start temperature is below the Ar3 point, the transformation of the ferrite begins. Therefore, even if forced cooling follows, a structure containing primarily martensite may not be provided. Therefore, the hot forming start temperature is desirably equal to point Ar3 or higher. Rapid cooling is desirably carried out at a cooling rate of 10 ° C / s or higher after hot forming, and rapid cooling is most desirably carried out at the rate of 20 ° C / s or higher speed . The upper limit of the cooling rate is not particularly specified.

In order to obtain a hot-formed steel plate element having a mainly martensite-containing metal structure exhibiting less variation in hardness, the steel plate after hot forming desirably cools rapidly to the surface temperature of the steel plate reaches 350 ° C or is lower. The final cooling temperature is preferably set at 100 ° C or less, and more preferably, at room temperature.

Hereinafter, the embodiments are described more specifically, with reference to Examples, although the present invention is not limited thereto.

Examples

Steel having the chemical components shown in Table 1 was cast in a test transformer and subjected to continuous molding in a continuous molding test apparatus, each producing slabs having a width of 1,000 mm and a 250 mm thickness. The symbol * used in Table 1 refers to the departure from the composition range of the embodiments. Under the conditions shown in Table 2, the heating temperature of the molten steel and the amount of molten steel intended to be molded per unit time were adjusted. The cooling rate of each of the slabs was controlled, while the amount of water in a secondary cooling spray strip was modified. A central segregation reduction treatment was carried out by carrying out a smooth reduction with a slope of 1 mm / m using rollers in a solidified terminal part and discharging the concentrated molten steel from a last solidified part. Subsequently, a part of the slabs was subjected to a soaking treatment under the conditions of 1,250 ° C and 24 hours.

Table 1

Table 2

The slabs obtained were hot rolled with a hot rolling testing apparatus, producing 3.0mm thick hot rolled steel sheets. Each of the hot rolled steel plates was wound and then subjected to chemical purity and additional annealing. A part of the steel plates were further cold rolled with a cold rolling test apparatus, producing 1.5 mm thick cold rolled steel sheets. In addition, a part of the cold rolled steel sheets was annealed at 600 ° C for 2 h, obtaining cold rolled annealed steel sheets.

Then, as shown in fig. 1 and in fig. 2, the hot forming steel plates 1 were subjected to hot pressing (hot forming) with a mold (punch 11, die 12) using a hot pressing testing apparatus, obtaining the steel plate elements hot-formed 2. Steel plates were heated to various surface temperatures ranging from 820 ° C to 1,100 ° C in a heating oven, held at temperature for 90 seconds, then removed from the heating oven, immediately subjected They were hot pressed with the mold with a cooling device, and they were subjected to a tempering treatment simultaneously with the forming. The hot formed steel plate elements were evaluated as follows. The evaluation results are shown in Table 2. In Table 2, "hot rolled" refers to 3.0mm thick hot rolled steel sheets subjected to hot rolling, and "cold rolling" Refers to a 1.5mm thick cold rolled steel sheet obtained by additional cold rolling of hot rolled steel sheets. The symbol * refers to the departure from the range of the embodiments.

<Evaluation of the mechanical characteristics of the hot-formed steel plate element>

JIS # 5 tensile test specimens were obtained from the right-angle lamination of the hot-formed steel plate elements and subjected to a tensile test in accordance with JIS No. Z2241 (2011) to measure the strength tensile (RT).

metal structure identification>

The hot formed steel plate elements were cut into samples so that the central part of the plate thickness in sections parallel to the rolling direction of the hot formed steel plate elements were the viewing surfaces, and then samples were mirror polished. The samples were then corroded with Nital and the metal structures of the five views of each of the samples were observed using a scanning electron microscope (magnification ratio: 2000). The ferrite surface fraction was obtained by subjecting the obtained photomicrograph to an image treatment. It was used as the volume fraction of ferrite. The volume fraction of residual austenite in the metal structure was obtained by X-ray diffraction (XRD). The balance of the same was calculated as the volume fraction of a low temperature transformation structure. The volume and residual fraction were obtained from the ratio of intensities between the diffraction intensity Ia (200) of (200) ferrite, the diffraction intensity Ia (211) of (211) ferrite, the diffraction intensity Iy (220) of (220) austenite and diffraction intensity Iy (311) of (311) austenite according to X-ray diffraction using a Mo lamp after chemical polishing of 1/8 of the inner layer thickness of the plate from the surface of each of the steel plates.

Vy (% by volume) = 0.25 x {Iy (220) / (1.35 x Ia (200) Iy (220)) Iy (220) / (0.69 x Ia (211) Iy (220)) Iy (311) / 1.5 x Ia (200) Iy (311)) Iy (311) / (0.69 x Ia (211) Iy (311))}

<Assessment of the purity level>

The test materials were cut from five sites of the hot-formed steel plate elements. The purity level in each of the 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t and 7 / 8t positions was investigated with respect to the plate thickness t of each of the test materials by a method point counting. The highest value (lowest purity property) of the purity level at each of the plate thicknesses was used as the purity level value of the test material.

<Measurement of the degree of segregation a of Mn>

The central part of the plate thickness of the hot formed steel plate element was subjected to line analysis by EPMA. Three high-order measured values of the analysis results were measured, and then the mean value thereof was calculated to obtain the maximum concentration of Mn in the central part of the thickness of the plate. Ten sites were analyzed by EPMA at the position 1/4 the depth of the plate thickness from the surface of the hot-formed steel plate element, obtaining the mean value thereof. The average concentration of Mn was obtained at the position of 1/4 of the depth of the thickness of the plate from the surface. The degree of segregation a of Mn was obtained by dividing the maximum concentration of Mn in the central part of the thickness of the plate by the average concentration of Mn at the position 1/4 of the depth of the plate thickness from the surface.

<Measurement of mean grain size of Y-prior grains>

The mean grain size of the Y-prior grains in the hot-formed steel plate element was obtained by measuring the number of crystalline grains in a measured view, by dividing the surface of the view measured by the number of crystalline grains, obtaining the average surface of the crystalline grains and calculating a crystalline grain size in an equivalent circular diameter. At that time, the grain at the edge of view was measured as 1/2 and the observation magnification ratio was adjusted, if necessary, so that the number of crystalline grains was set to 200 or more.

<Density in number of residual carbides>

The surface of the hot-formed steel plate element was corroded using a stained glass solution and the size was enlarged 2,000 times with a scanning electron microscope. A plurality of views were observed. During this time, the number of views with the presence of carbides was counted in order to calculate the number per 1 mm2

<Measurement of local deformability>

Local deformability was measured according to a notched tensile test. A tensile test specimen had a parallel part width of 16.5 mm and a parallel part length of 60 mm, and was obtained in a longitudinal rolling direction. A 2 mm deep V-notch was made in the central part of the length of the tensile test specimen and the processed tensile test specimen was used as a notched tensile test specimen. The thickness of the notched specimen was set at 1.4 mm. The shape of the notched tensile test specimen is shown in fig. 3. The tensile test was carried out using the notched tensile test specimen and the elongation of the notch was measured by breaking the tensile test specimen notched into a V-notched part, in order to evaluate the local deformability. A reference point distance of 5 mm was set and the draw speed (crosshead speed) during the pull test was set to 0.5 mm / min.

<Variation of hardness>

The following test was carried out in order to evaluate the stability of the hardness. Steel plates for hot forming were heated at 10 ° C / s to 900 ° C with a heat treatment simulator and then held at 150 s. The steel plates for hot forming were then cooled at cooling rates of approximately 80 ° C / s and 10 ° C / s to room temperature. Each of the samples was subjected to a Vickers hardness test at the% position of the section plate thickness. The hardness measurement was carried out based on the JIS Z 2244 (2009) standard at five points with a test force set at 9.8 N and the average of the measurements was obtained. The mean values of the hardnesses at the cooling rate of approximately 80 ° C / s and the mean values of the hardnesses at the cooling rate of 10 ° C / were defined as HS80 and HS10, and the AHv difference was used as the hardness stability index.

In order to assess the stability and local hardness and deformability of each of the samples, it was determined that the samples that had an AHv of 50 or less and a notch elongation of 6% or more were favorable.

As shown in Table 2, test number 2 had a steel composition that satisfied the range of embodiments, although the amount of molten steel for molding per unit time was large. In this way, the purity level value exceeded 0.08%, which resulted in poor local deformability.

Because test number 3 had not undergone either a central segregation reduction treatment or a soaking treatment, the degree of Mn segregation exceeded 1.6, resulting in poor local deformability.

Because test number 5 had a low heating temperature of the molten steel, the purity level value exceeded 0.08%, resulting in poor local deformability.

Because test number 6 had a low hot forming temperature, the volume fraction of ferrite exceeded 3% after hot forming, which resulted in low hardness stability. Furthermore, the number density of residual carbides was also up to 8.0x103 per mm2, which resulted in poor local deformability.

Because test number 9 exhibited a high heating temperature during heat forming, the Y-prior grain size was increased, resulting in poor local deformability.

Because test number 11 had a high winding temperature after hot rolling, the residual carbide density was increased, resulting in poor local deformability.

Because test number 14 had an elevated annealing temperature after hot rolling and a long annealing time, the volume fraction of ferrite exceeded 3% after hot forming, resulting in poor stability of the hardness. Insufficient dissolution of carbides caused an increase of the density of the residual carbides, which resulted in poor local deformability.

Because test number 16 had an S content that exceeded the upper limit value of the range of the embodiments, the purity level value exceeded 0.08%, resulting in poor local deformability. Because test number 17 had an Mn content that exceeded the upper limit value of the range of the embodiments, the degree of Mn segregation exceeded 1.6, resulting in poor local deformability.

Because test number 18 had a Si content that exceeded the upper limit value of the range of the embodiments, point A3 was increased and the volume fraction of ferrite exceeded 3% after hot forming, which gave as poor hardness stability resulted.

Test number 19 had a C content that exceeded the upper limit value of the range of the embodiments, resulting in poor local deformability.

Test number 20 had a lower Cr content than the range of the run, which resulted in poor hardness stability.

Test numbers 1, 4, 7, 8, 10, 12, 13, and 15 satisfying the range of embodiments were excellent in both hardness stability and local deformability.

As described above, the various typical embodiments have been described, although the invention is not limited to these embodiments. The scope of the invention is limited only by the following claims.

Claims (4)

1. Hot-formed steel plate element with a chemical composition consisting of, in terms of% by mass: 0.08 to 0.16% C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, 0.01 to 1.0% Al sol., 0.01% or less of N, 0.25 to 3.00% Cr, 0.01 to 0.05% Ti, 0.001 to 0.01% B, from 0 to 0.5% of Nb, 0 to 2.0% Ni, from 0 to 1,0% Cu, 0 to 1.0% Mo, 0 to 1.0% of V, 0 to 0.005% Ca, and consisting of a residue of Faith and impurities, where a total volume fraction of martensite, annealed martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less, an average grain size of Y-prior grains is 10 pm or less, a density in number of residual carbides present is 4 x 103 per mm2 or less, a steel purity level value specified by JIS G 0555 (2003) is 0.08% or less, where the steel purity level value is obtained by calculating the surface percentages of inclusions based on A, based on B and C-based, and a degree of segregation a of Mn represented by formula (i) below is 1.6 or less, a = [maximum concentration of Mn (mass%) in a central part of a plate thickness] / [mean concentration of Mn (mass%) at a position 1/4 the depth of the plate thickness from a surface] (i). 2. The hot-formed steel plate element according to claim 1, wherein the chemical composition comprises one or more selected from the group consisting of, in terms of% by mass: 0.003 to 0.50% of Nb, 0.01 to 2.0% Ni, 0.01 to 10% Cu, 0.01 to 1.0% Mo, from 0.01 to 1.0% of V and 0.001 to 0.005% Ca. 3. The hot-formed steel plate element according to claim 1 or 2, wherein the steel plate element has a surface on which a coating layer is formed. 4. The hot-formed steel plate element according to any of claims 1 to 3, wherein the steel plate element has a tensile strength of 1.0 GPa or more.